Electroless Copper Deposition: A Sustainable Approach

by

Marika Renée Kutnahorsky

A thesis submitted in conformity with the requirements

for the degree of Masters of Applied Science

Department of Materials Science and Engineering

University of Toronto

© Copyright by Marika Renée Kutnahorsky (2009)

ii

Electroless Copper Deposition: A Sustainable Approach Masters of Applied Science (2009)

Marika Renée Kutnahorsky

Department of Materials Science and Engineering

University of Toronto

Abstract

A sustainable electroless copper coating process was developed for plating automotive

fasteners shaped from AISI 9255 low carbon, high silicon steel. The objective was to

minimize the ionic and organic species present in each step of the plating process. A

sulfuric acid solution inhibited with quinine was defined to clean the steel prior to

plating. The corrosivity of the solution was examined through electrochemical and

weight loss measurements to evaluate the efficiency of the cleaning process at high

temperatures and high acid concentrations. An electroless copper coating process was

then developed using a simple copper sulfate chemistry inhibited with quinine to extend

the possible operating window. Finally, benzotriazole was evaluated as a possible anti-

oxidant coating. Accelerated thioacetamide corrosion tests were used to evaluate the

corrosion inhibition of benzotriazole on copper coatings.

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Acknowledgments

I would like to thank Professors Donald Kirk and Steven Thorpe, without whose

advice and guidance this project would not have been possible. This process was a very

valuable learning experience.

Thanks to all of the group members in the Surface Engineering and

Electrochemistry Group. Your support and encouragement kept me going through the

rough patches.

Thank you to the MSE department for all of their aid and advice.

To Matt Olmstead – this thesis wouldn’t have been possible without your love

and support. You always had my back, and provided me with strength, encouragement,

coffee, backrubs and occasionally poutine. Thank you.

Wolf and Francine Kutnahorsky, thanks for the encouragement, love, support and

patience. G.I.R. Unit and Darwin, thanks for the snorgles.

To all of my friends who supported me during my writing phase, your love and

support is appreciated more than you could know.

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Table of Contents

ABSTRACT .................................................................................................................................. II 

ACKNOWLEDGMENTS ........................................................................................................... III 

TABLE OF CONTENTS ............................................................................................................ IV 

LIST OF TABLES ..................................................................................................................... VII 

LIST OF FIGURES ..................................................................................................................... IX 

LIST OF APPENDICES ............................................................................................................. XI 

1.  INTRODUCTION .................................................................................................................. 1 

1.1.  OBJECTIVES .......................................................................................................................... 3 

2.  BACKGROUND ..................................................................................................................... 4 

2.1.  METHODS OF CLEANING IRON ............................................................................................. 4 2.1.1.  ACID CLEANING ................................................................................................................. 4 2.1.1.1.  HIGH-CONCENTRATION ACID CLEANING (PICKLING) .................................................... 4 2.1.1.2.  LOW-CONCENTRATION ACID CLEANING ........................................................................ 5 2.1.1.3.  ORGANIC ACID CLEANING .............................................................................................. 6 2.1.2.  ELECTROLYTIC CLEANING ................................................................................................. 7 2.1.3.  ALKALINE CLEANING ......................................................................................................... 7 2.2.  DISSOLUTION OF IRON IN ACIDIC MEDIA ............................................................................ 8 2.2.1.  MECHANISM OF IRON DISSOLUTION ................................................................................... 8 2.2.1.1.  ELECTRODE POTENTIAL .................................................................................................. 8 2.2.1.2.  KINETIC DISSOLUTION PARAMETERS ............................................................................. 10 2.2.1.3.  IRON DISSOLUTION PATHWAY ....................................................................................... 12 2.2.1.4.  CORROSION RATES ........................................................................................................ 15 2.2.1.5.  POLARIZATION RESISTANCE .......................................................................................... 15 2.2.1.6.  TAFEL EXTRAPOLATION ................................................................................................ 17 2.2.1.7.  COMPARISON OF WEIGHT LOSS VERSUS ELECTROCHEMICAL METHODS ...................... 18 2.2.2.  EFFECT OF TEMPERATURE ON CORROSION RATE .............................................................. 19 2.2.3.  EFFECT OF ANION TYPE .................................................................................................... 20 2.2.3.1.  HYDROCHLORIC ACID ................................................................................................... 20 2.2.3.2.  SULFURIC ACID ............................................................................................................. 21 2.2.4.  EFFECT OF ACID CONCENTRATION ................................................................................... 21 2.2.5.  EFFECT OF FLUID VELOCITY ............................................................................................. 22 2.3.  SMUT FORMATION ............................................................................................................. 22 2.3.1.  DEFINITION OF SMUT ........................................................................................................ 22 

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2.3.2.  CHEMISTRY ....................................................................................................................... 23 2.3.3.  FACTORS AFFECTING SMUT FORMATION .......................................................................... 23 2.3.4.  METHODS OF CONTROLLING SMUT FORMATION .............................................................. 24 2.4.  INHIBITORS ......................................................................................................................... 24 2.4.1.  DEFINITION AND CLASSIFICATION OF INHIBITOR TYPES FOR FE ...................................... 25 2.4.2.  ANODIC INHIBITORS ......................................................................................................... 25 2.4.3.  CATHODIC INHIBITORS ..................................................................................................... 27 2.4.4.  MIXED INHIBITORS ........................................................................................................... 28 2.4.5.  ORGANIC INHIBITORS ....................................................................................................... 28 2.4.6.  QUININE ............................................................................................................................ 29 2.4.6.1.  MECHANISMS OF QUININE INHIBITION .......................................................................... 30 2.4.6.2.  TEMPERATURE STABILITY OF QUININE .......................................................................... 32 2.4.6.3.  MEASUREMENT OF QUININE IN PROCESS CONTROL ....................................................... 33 2.4.7.  OTHER POSSIBLE INHIBITORS ........................................................................................... 33 2.4.8.  BENZOTRIAZOLE AS A COPPER INHIBITOR ........................................................................ 35 2.5.  COPPER COATING TECHNOLOGIES .................................................................................. 37 2.5.1.  FUNDAMENTALS OF ELECTROLESS COPPER PLATING ....................................................... 37 2.5.2.  FUNDAMENTALS OF IMMERSION PLATING ........................................................................ 41 2.5.3.  FUNDAMENTALS OF MECHANICAL PLATING ..................................................................... 41 2.5.4.  FUNDAMENTALS OF BARREL PLATING ............................................................................. 42 2.5.5.  CURRENT SYSTEMS USED IN INDUSTRY ............................................................................ 43 2.5.6.  LIMITATIONS AND CHALLENGES ...................................................................................... 44 2.5.7.  PROPOSED SYSTEM ........................................................................................................... 44 

3.  EXPERIMENTAL METHODOLOGY ............................................................................. 47 

3.1.  FASTENERS USED ............................................................................................................... 47 3.2.  CLEANING CIRCUIT ........................................................................................................... 47 3.2.1.  BEAKER TESTS .................................................................................................................. 47 3.2.2.  SMUT RATING ................................................................................................................... 49 3.2.3.  BARREL PLATING TESTS ................................................................................................... 50 3.2.4.  DIGITAL IMAGING ............................................................................................................. 50 3.2.5.  ADHESION TESTS .............................................................................................................. 51 3.2.6.  WEIGHT LOSS TESTS ........................................................................................................ 51 3.2.7.  ELECTROCHEMICAL TESTS ............................................................................................... 51 3.3.  COPPER PLATING ............................................................................................................... 52 3.3.1.  BEAKER TESTS .................................................................................................................. 52 3.3.2.  BARREL PLATING TESTS ................................................................................................... 53 3.3.3.  ADHESION TESTS .............................................................................................................. 54 3.4.  ANTI-OXIDIZING AGENT .................................................................................................... 54 3.4.1.  BEAKER TESTS .................................................................................................................. 54 3.4.2.  BATCH TESTS .................................................................................................................... 55 3.4.3.  COPPER TARNISHING TESTS ............................................................................................. 56

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4.  RESULTS AND DISCUSSION ........................................................................................... 57 

4.1.  HYDROCHLORIC ACID CLEANING SOLUTIONS ................................................................ 57 4.1.1.  UNINHIBITED HYDROCHLORIC ACID SOLUTIONS ............................................................ 58 4.1.1.1.  ACID CONCENTRATION VARIATION .............................................................................. 58 4.1.1.2.  TEMPERATURE VARIATION ........................................................................................... 60 4.1.2.  INHIBITED HYDROCHLORIC ACID CLEANING SOLUTION ................................................. 61 4.1.2.1.  TEMPERATURE VARIATION ........................................................................................... 61 4.2.  SULFURIC ACID CLEANING SOLUTION ............................................................................. 63 4.2.1.  UNINHIBITED SULFURIC ACID SOLUTION ........................................................................ 63 4.2.1.1.  TEMPERATURE VARIATIONS .......................................................................................... 63 4.2.1.2.  ACID CONCENTRATION VARIATIONS ............................................................................ 68 4.2.2.  INHIBITED SULFURIC ACID SOLUTION .......................................................................... 71 4.2.2.1.  INHIBITOR CONCENTRATION VARIATION ...................................................................... 71 4.2.2.2.  TEMPERATURE VARIATIONS .......................................................................................... 75 4.2.2.3.  ACID CONCENTRATION VARIATIONS ............................................................................ 77 4.3.  COPPER FLASH ................................................................................................................... 77 4.3.1.  PRELIMINARY CLEANING SOLUTION ADHESION TESTS ..................................................... 77 4.3.2.  BEAKER TESTS .................................................................................................................. 80 4.3.2.1.  ACID CONCENTRATION VARIATION .............................................................................. 80 4.3.2.2.  COPPER SULFATE CONCENTRATION VARIATION .......................................................... 81 4.3.3.  INHIBITED COPPER FLASH TESTS ..................................................................................... 82 4.3.4.  BARREL PLATING TESTS ................................................................................................... 83 4.3.4.1.  COPPER SULFATE CONCENTRATION VARIATIONS .......................................................... 83 4.3.4.2.  INHIBITOR CONCENTRATION VARIATIONS ..................................................................... 85 4.4.  ANTI-OXIDIZING AGENT .................................................................................................... 88 4.4.1.  BEAKER TESTS .................................................................................................................. 88 4.4.2.  BATCH TESTS .................................................................................................................... 91 4.5.  DEVELOPED SYSTEM.......................................................................................................... 92 

5.  CONCLUSIONS ................................................................................................................... 94 

6.  REFERENCES ..................................................................................................................... 95 

APPENDICES ............................................................................................................................. 99 

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List of Tables Table 1  Criteria for improving the sustainability of electroless copper plating processes ............................................................................................................. 2 Table 2  Calculated anodic Tafel slopes for the proposed reaction mechanisms in steady-

state and nonsteady-state reactions. .................................................................. 14 Table 3  Tafel data for iron in various inhibited and uninhibited acidic media .............. 14 Table 4  Corrosion data for low carbon steel (wt%: 0.1 C, 0.29 Mn, 0.07 Si, 0.021P) in a 1 M uninhibited hydrochloric acid solution at various temperatures ......... 20 Table 5  Electrochemical data for iron dissolution in hydrochloric acid solutions of

varying concentration, performed at room temperature on Armco iron [33] ... 22 Table 6  Inhibitors commonly used for industrial cleaning applications ........................ 25 Table 7  Electrochemical parameters for quinine at various temperatures and

concentrations in 1 M hydrochloric acid [31]. .................................................. 30 Table 8  Environmentally friendly inhibitors researched as quinine alternates .............. 33 Table 9  Potentiodynamic polarization parameters for the corrosion of iron in acidic

media with varying inhibitor concentration and type. ...................................... 34 Table 10  Corrosion rate and inhibition efficiency data obtained by Kahled [58] from

weight loss measurements for copper in 0.5 M HCl solutions in the absence and presence of various concentrations of BTA at 30°C .................................. 36 Table 11  Process flow for electroless copper deposition with outline of environmental

concerns associate with each step ..................................................................... 40 Table 12  Process outline for MacDermid SC-G Copper Flash solution .......................... 43 Table 13  Comparison of main coating options (from [41]) ............................................. 45 Table 14  Parameters for initial development of cleaning solution inhibited with quinine ............................................................................................................. 48 Table 15  Evaluation of smut by appearance .................................................................... 49 Table 16  Summary of solutions subjected to electrochemical testing ............................. 52 Table 17  Copper sulfate solution parameters, uninhibited solutions and solutions inhibited with 0.003 g/L quinine ....................................................................... 53 Table 18   Summary of test parameters for barrel plating tests of uninhibited and inhibited

with 0.003g/L quinine copper flash solutions ................................................. 54 Table 19   Process outline for beaker testing of benzotriazole anti-oxidant coating ......... 55 Table 20 Smut formation in varying acid concentration in an uninhibited hydrochloric

acid solution .................................................................................................... 58 Table 21  Timeframe determinations for hydrochloric acid cleaning solution at room

temperature ...................................................................................................... 59 Table 22  Temperature variations for uninhibited hydrochloric acid cleaning solutions. ......................................................................................................... 60 Table 23  Hydrochloric acid (10 vol%) cleaning solutions inhibited with 0.03 g/L

quinine ............................................................................................................. 62 Table 24 Summary of inhibited and uninhibited sulfuric acid cleaning solutions tested ............................................................................................................... 63 Table 25  Uninhibited sulfuric acid cleaning solutions at room temperature and elevated temperatures ...................................................................................... 64

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Table 26  Electrochemical data for uninhibited 1vol% sulfuric acid cleaning solution ............................................................................................................ 65 Table 27  Variation of acid concentration in uninhibited sulfuric acid cleaning

solutions at 25°C ............................................................................................. 69 Table 28  Low acid cleaning using 1 vol% sulfuric acid and various quinine

concentrations, cleaned samples (top row) and copper coated samples (bottom row) ................................................................................................................. 72 

Table 29   Electrochemical data for variations in quinine concentration at 25°C ........... 74 Table 30   Electrochemical data for temperature variations in uninhibited and 0.03 g/L

quinine inhibited sulfuric acid solution .......................................................... 76 Table 31  Initial copper flash trials used to prove suitability of inhibited sulfuric acid

cleaning solution. ............................................................................................ 79 Table 32  Sulfuric Acid concentration variations in a beaker set-up with 15g/L copper

sulfate copper flash solution ............................................................................ 80 Table 33  Copper sulfate concentration variations in a 0.1 vol% sulfuric acid copper

flash solution prior to adhesion tests with adhesion test results ...................... 82 Table 34  Samples and adhesion results for variations in copper sulfate concentrations of copper flash solutions inhibited with quinine ............................................. 83 Table 35  Batch tests of copper sulfate concentration variations in an uninhibited copper sulfate flash solution ............................................................................ 84 Table 36  Quinine concentration variations in 5 g/L copper sulfate copper flash batch

tests .................................................................................................................. 85 Table 37   Copper sulfate concentration variations and rpm variations in inhibited copper

flash solution ................................................................................................... 86 Table 38  Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-

oxidant solutions without preliminary alkaline dip ......................................... 89 Table 39  Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-

oxidant solutions with preliminary alkaline dip .............................................. 90 Table 40  Corrosion test results for benzotriazole dip batch tests. .................................. 92 Table 41  Summary of electroless copper flash system developed for a room

temperature barrel plating system ................................................................... 93 Table 42   Raw electrochemical data ............................................................................ 102 Table 43  Complete uninhibited hydrochloric acid test results ..................................... 106 Table 44  Temperature and acid variations in uninhibited sulfuric acid cleaning

solutions ........................................................................................................ 107 Table 45  Inhibited sulfuric acid solution samples ........................................................ 108 Table 46  Complete summary of beaker tests in copper flash development ................. 109 Table 47  Samples from each batch of varying copper sulfate concentration copper flash

batch tests ...................................................................................................... 110 Table 48  Copper sulfate flash solutions with 1 vol% sulfuric acid inhibited with

varying concentrations of quinine, flashed for various times ....................... 111 

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List of Figures

Figure 1   Proposed continuous plating line schematic ..................................................... 1 Figure 2   Concentration gradient in the solution near a surface controlled by

concentration polarization ............................................................................... 11 Figure 3   Polarization resistance data at small applied current densities in 0.52N H2SO4

at 30°C [From Graydon, J. W. "Linear polarization." Lecture.] ..................... 16 Figure 4   Polarization curve showing experimental and extracted Tafel lines for 1080

steel in deaerated 1N H2SO4. βc= -98mV, βa= 38 mV (derived from cathodic data), icorr = 1180μA/cm2 [29] ......................................................................... 18 

Figure 5   Evans diagram showing the effect of anodic inhibitors on corrosion current. ............................................................................................................ 26 Figure 6   Evans diagram for the effect of cathodic inhibitors on corrosion current ...... 27 Figure 7   Evans diagram for effect of mixed inhibitors on corrosion current ................ 28 Figure 8   The quinine molecule [31]. ............................................................................. 29 Figure 9    Inhibition efficiency at different concentrations of quinine for low carbon

steel in 1 M HCl at different temperatures: (1) 20°C, (2) 30°C, (3) 40°C and (4) 50°C. ....................................................................................................... 31 

Figure 10  The benzotriazole molecule [63] ................................................................... 36 Figure 11  Technic "Mini Electroless Copper Line" system ........................................... 46 Figure 12  Singleton “Mini Barrel” system ..................................................................... 46 Figure 13   Tinnerman automotive fastener used for this thesis ..................................... 47 Figure 14   Linear sweep data for uninhibited 1 vol% sulfuric acid cleaning solutions at

25°C, 50°C, and 75°C .................................................................................... 65 Figure 15  Evans diagram illustrating the effect of temperature increase ....................... 66 Figure 16   Weight losses converted to corrosion current density over time for various

uninhibited sulfuric acid cleaning solutions ................................................. 69 Figure 17   Corrosion current density (by converted weight loss) data for various

quinine concentrations in barrel-cleaned 1 vol% sulfuric acid cleaning solution .......................................................................................................... 73 

Figure 18  Evans diagram illustrating the effect of quinine on iron dissolution ............. 74 Figure 19   Temperature dependence of corrosion rate in 0.03 g/L quinine inhibited

sulfuric acid solution ..................................................................................... 75 Figure 20  Linear sweep data performed in inhibited solutions at 25°C, 50°C and 75°C ........................................................................................................ 77 Figure 21  SEM images at (a) 250X and (b) 500x magnification of samples as well as

(c) surface mapping of samples plated under the following conditions: 1) uninhibited, 5.0 g/L CuSO4 for 60s, 2) inhibited, 2.5 g/L CuSO4 for 60s, 3) inhibited, 5.0 g/L CuSO4 for 60s and 4) inhibited, 5.0 g/L CuSO4 for 90s at slow RPM ...................................................................................................... 87 

Figure 22   Polarization resistance for iron in 0.03 g/L quinine inhibited sulfuric acid solution at room temperature ......................................................................... 99 

Figure 23  Tafel extrapolation data for a 0.03 g/L inhibited sulfuric acid cleaning solution at 25°C ........................................................................................... 100 

Figure 24   Cathodic Tafel area from Figure 22 ............................................................ 100 

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Figure 25   Anodic Tafel area for Figure 23 ................................................................. 101 Figure 26  Corrosion tests set up prior to addition of filter paper and chemicals ......... 104 Figure 27  Bird’s eye view of sample holder without lid for corrosion testing ............ 105 

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List of Appendices Appendix 1    Corrosion Current Density Sample Calculation ....................................... 99 Appendix 2    Raw Electrochemical Data ...................................................................... 102 Appendix 3    Amercoat™ 90 Epoxy Application Procedure ....................................... 103 Appendix 4    Corrosion Test Set Up............................................................................. 104 Appendix 5    Uninhibited Hydrochloric Acid Solution Samples ................................. 106 Appendix 6 Uninhibited Sulfuric Acid Samples ........................................................ 107 Appendix 7    Inhibited Sulfuric Acid Solution Samples .............................................. 108 Appendix 8   Complete Copper Bath Beaker Test Results ........................................... 109 Appendix 9    Variation of copper sulfate in batch copper flash tests ........................... 110 Appendix 10  Summary of inhibited copper flash batch tests....................................... 111 

1

1. Introduction

Copper coatings are widely used as a baseline in plating technologies [1]. They

provide a conductive base for further plating, and an inexpensive corrosion resistant

coating for end-of-line products. Since the copper coating process is additive, that is

additional chemicals are added to the baths at each stage, the chemistry becomes complex

and must be discarded after use. This causes an unnecessarily large amount of waste and

is costly. By installing a continuous processing line (Figure 1) and recycling and

recovering from the flow streams, there is an opportunity to minimize the environmental

impact from the process streams. This would be highly advantageous in any industry

requiring copper coatings that are environmentally regulated.

Figure 1 Proposed continuous plating line schematic

The process must be a low cost coating process that is applicable to parts having

complex geometries and must be able to be completed in a reasonable time frame. A set

of useful criteria is outlined in Table 1.

An initial feasibility test was conducted by M. Stemp [2] to develop conditions for

a new cleaning solution, plating solution and anti-oxidizing agent. The final goal of this

project is to develop an effective copper coating system that will avoid the use of

expensive proprietary formulations and that will be suitable for components with

complex geometries. The system should provide a high gloss coating, with uniform

thickness, and controllable porosity that may be coated in a reasonable time using the

least aggressive cleaning and plating solutions possible.

Various plating techniques are possible; however initial testing determined that a

barrel coating set up should be used for the scope of this project. This will allow for

integration of the cleaning and copper flash processes developed as a preliminary step for

Cleaning

Rinse

Copper Plating

Rinse

Brightening Agent

2

Stage Criteria Importance

Cleaning

Minimize effluent Decrease the total concentration of cleaning

agent in waste stream

Robust time frame Allows for a time insensitive window for

complete cleaning of multiple parts

Environmentally

conscious chemicals

Avoid use of hazardous chemicals

Plating

Minimize effluents Decrease the chemical concentration where

possible to minimize waste

Compatible chemistry

with cleaning solution

Allow for recycling of cleaning solution into

plating solution or vice-versa

Good throwing power Required for plating complex parts

Low cost reagents Improve economic feasibility

Moderate deposition rate Must be able to deposit uniform coating on all

samples within adequate time frame

Good adhesion Coating must not delaminate or flake off

Simple apparatus and

procedures

Decrease the start-up costs

Re-useable solution Improve economic and environmental

effectiveness relative to commercial systems

Brightener

Minimize Effluent Decrease waste

Environmentally

conscious chemicals

Avoid hazardous chemicals

Maintain bright copper

coating color

Ensure copper plated parts can be stored for a

reasonable time before delivery to customer

Table 1 Criteria for improving the sustainability of electroless copper plating processes

barrel plating systems. Barrel plating is the most efficient method to finish bulk parts

that do not require individual handling. Furthermore, the system allows for easy transfer

3

of components in a multi-step cleaning and plating system and would allow for easy

recycling of the streams.

1.1. Objectives

The objective of this project is to develop a replacement for the

mechanical batch copper plating process on heat-treated steel fasteners currently used in

the automotive industry. For this project there were three stages; cleaning, copper

flashing and application of anti-oxidant coating. The operating window – or allowable

immersion time in solution before any adverse effects arise – should be maximized for

each step, and the ionic and organic species present and concentrations of each species

should be minimized to make each step as economically feasible as possible.

The goals for the cleaning stage were to determine the acidic cleaning medium

and inhibitor. The concentration of all acids used was to remain at a minimum to

minimize the effluents in the waste stream. If possible, the species used in the cleaning

step should contain some of the same species as those used in subsequent steps to allow

recycling of the solution. An environmentally friendly inhibitor for the cleaning solution

needed to be identified and the efficiency of the inhibitor over a range of concentrations

and temperatures had to be determined. The inhibitor was required to minimize corrosion

of the parts and provide for a time insensitive processing window.

The goal of the copper flash step was to provide a uniform visible coating while

minimizing the concentrations of all the required chemicals in a simple copper sulfate

bath. To do so, it was necessary to determine the role of each component in the solution.

Ideal plating parameters were then determined by varying plating conditions.

The final goal of this thesis was to determine an environmentally friendly anti-

oxidizing agent to be applied to the flashed samples. The chemical to be used, as well as

plating parameters were determined. A tarnishing test was evaluated and the corrosion

resistances of the coatings were determined.

4

2. Background

2.1. Methods of cleaning iron

A cleaning step will be necessary to remove an oxide layer left after heat

treatment of the fastener. Below is a summary of the most commonly used methods of

cleaning iron and ferrous alloys.

2.1.1. Acid Cleaning

Various acids may be used to clean ferrous alloys. If inorganic acids are used in

low concentrations and at low temperatures, the cleaning process is referred to as acid

cleaning, while pickling typically refers to high temperature, higher acid concentration

solutions.

2.1.1.1. High-Concentration Acid Cleaning (Pickling)

Acid pickling is a fast, reliable method of cleaning oxides from an iron substrate.

It is economical and easily scaled up, making it ideal for batch processes. Unlike acid

cleaning, pickling is typically conducted at high temperatures, and while the exact

temperature will vary depending on the acid type, it usually ranges from 60-90°C [3-5].

A variety of pickling solutions are available, the most commonly used and inexpensive

solutions for ferrous alloys are hydrochloric acid and sulfuric acid solutions.

Sulfuric acid solutions are the most commonly used pickling solution. They

typically use 10-15 vol% and operate at temperatures ranging from 65-83°C.

Hydrochloric acid solutions are frequently used for continuous operations. The cleaning

solution parameters are 20-40 vol% at ambient temperature to 55°C.

Advantages of pickling include the wide range of operating parameters of the

pickling solution which may be varied to meet specific requirements (i.e. for a specific

alloy composition, a specific temperature and acid concentration may be more effective),

an inexpensive set up, easily scalable, the low cost of materials and the process may be

adapted for continuous use.

5

Disadvantages to pickling include high amounts of metal loss due to the

aggressive nature of the solution. There is a potential source of hydrogen embrittlement,

especially in high-carbon steels and the process is likely to deposit smut on the substrate.

Further, excessive pitting may occur in cast irons and steels.

2.1.1.2. Low-Concentration Acid Cleaning

Low concentration acid cleaning is frequently used for final or near-final

preparation of substrates before plating [3, 4]. Typically low concentration cleaning

solutions are used on samples that are not heavily oxidized or contaminated with

organics.

It is less aggressive than acid pickling but may use the same acids, namely

sulfuric and hydrochloric acids. The cleaner composition may be limited to one or two

acids, or may contain various additives such as surfactants and inhibitors. The

application methods are typically immersion, spray or rotating-barrel methods.

Important process parameters for low acid cleaning include the temperature of the

solution, acid concentration, agitation of parts, and rinsing. Low acid cleaning, unlike

pickling, is a room temperature process. Although the efficiency of the cleaning solution

increases with increasing temperature, high temperature solutions can lead to aggressive

chemical attack on the surface of the metal. Furthermore, at high temperature many

additives such as inhibitors and surfactants risk breaking down due to poor thermal

stability. Surfaces emerging from high temperature solutions are likely to dry and streak.

Finally, the life of the cleaning solution and equipment decreases significantly in high

temperature cleaning solutions. Further discussion of temperature effects can be found in

section 2.2.2.

Acid concentration will affect the aggressiveness of the solution in terms of both

scale and oxide removal and attack on the metal substrate. The effect of acid

concentration is further discussed in section 2.2.4

Agitation is a crucial parameter in cleaning. It helps to remove surface oxides,

removes dissolved metal ions from the surface of the metal and helps prevent local pH

6

changes. Changing the degree of agitation will have a large effect on cleaning. This is

further discussed in section 2.2.5.

Rinsing plays an important role in acid cleaning by removing cleaning

byproducts. Cold water or water at room temperature is frequently used if the purpose of

cleaning is simply to remove metal oxides. If waxes or heavy greases are being removed,

then a hot water soak is used to avoid resolidification of residues.

2.1.1.3. Organic Acid Cleaning

Organic acids are used as an environmentally friendly alternative in various metal

cleaning operations. Typically acids used include citric acid, acetic acid, EDTA, formic

acid, gluconic acid and hydroxyacetic acid. These acids may be used alone or formulated

with bases and other additives such as inhibitors, surfactants and chelating agents [5].

A major advantage in organic acid cleaning is that organic acids may have

multiple functions and act as acids, buffers or chelating agents [6]. In solution, they will

release one or more hydronium ions, and typically have multiple dissociation constants

providing some buffering capacity. If there are two or more complexing sites, they may

act as chelating agents.

Organic acids remove oxides by reacting with the metal to produce salts such as

citrates and acetates, as well as dissolved metal ions and other byproducts. As these

reactions occur, hydrogen gas is produced and accumulates under the oxide layer. The

force of hydrogen gas evolution can lift oxides off the surface of the metal. The organics,

acting as chelating agents, will then complex the metal ions and remove them from the

surface into the bulk solution.

Further advantages include low corrosivity to the base metal compared to mineral

acids such as sulfuric and hydrochloric acids. Organic acid cleaning solutions typically

operate at a higher pH than mineral acids. This further reduces the corrosivity of the

cleaning solution, and makes the solutions safer to handle.

Disadvantages of organic acid cleaning include longer cleaning times due to the

decreased corrosivity of the solution, higher temperature requirements and higher costs of

the acids compared to mineral acid cleaning solutions.

7

Waste disposal of organic solutions can be achieved through a variety of methods.

Biodegradation, chemical treatment and incineration are common disposal methods.

Spent solutions may also be recovered through ion exchange and reduction of metal ions

via reducing agents.

2.1.2. Electrolytic Cleaning

Electrolytic cleaning is frequently used to remove oxides from steels that are to be

electroplated in continuous processing lines [3, 7]. It is especially valuable for removing

oxides such as Cr2O3 and FeCr2O4, which are difficult to remove through acid pickling

alone. The process is typically performed at room temperature and at low sulfuric acid

concentrations (5-10 vol%). The sample is charged cathodically for 15-20 seconds at

current densities typically less than 1000A/m2 [7]. To combat smut formation if the

sample is cleaned anodically, a short cathodic charge is applied before the sample is

removed from solution.

Sulfuric acid is most commonly used for electrolytic cleaning. Hydrochloric acid

is never used as the anode reaction produces toxic chlorine gas.

2.1.3. Alkaline Cleaning

If hydrogen embrittlement is of concern, a highly caustic alkaline solution may be

used as an alternative to acidic media [7,8]. Alkaline cleaners are ideal for removal of

oils, greases, waxes, metallic oxides and dirt. The solutions contain three main

components [9]:

i. Builders: These are the alkaline salts in the solution. Cleaners may include a

blend of phosphates, silicates, carbonates and borates.

ii. Additives: These may be solvents that aid in cleaning, such as glycols and gycol

ethers, corrosion inhibitors to prevent metal oxidation and chelating agents to

soften the water and counteract any metal ion activity.

8

iii. Surfactants: These organics are the main force in removal of soils on the sample.

They act by lowering the surface tension of the cleaner at the sample’s surface,

thus allowing uniform coverage of the sample.

Alkaline cleaners are typically operated at very high alkaline salt concentrations

in the order of 120-360 g/L and at temperatures above 90°C. Barrel cleaning is quite

commonly used with an alkaline cleaner. However, it is possible to over clean a sample,

causing corrosion of the sample and localized pitting. Since the samples in this work

arrive directly from a high temperature furnace, no organic residues are expected.

2.2. Dissolution of iron in acidic media

2.2.1. Mechanism of iron dissolution

2.2.1.1. Electrode Potential In order for iron dissolution or oxidation to occur, a reduction reaction must occur

simultaneously to provide a balance of charge with respect to the exchange of electrons

[10]. In acidic media, this will typically be the hydrogen evolution reaction shown in

equation {1}. The pathway of iron dissolution is discussed in section 2.2.1.3 below. The

overall reaction may be summarized as a simple oxidation, as seen in {2}. The

summation of the half reactions {1} and {2} give the overall dissolution reaction of iron

in acidic media, {3}.

{1} 2H + + 2e− → H2

{2} Fe → Fe 2+ + 2e−

{3} 2H + + Fe → Fe2+ + H2

The Gibbs free energy associated with the overall reaction is related to the voltage

by the thermodynamic equation:

9

{4} ΔG = −nFE

ΔG ≡ Gibb’s free energy (J/mol)

n ≡ number of electrons transferred in reaction

F ≡ Faraday’s constant; 96486 C/mole

E ≡ potential (V)

Note the sign convention is such that a spontaneous reaction will have a positive

potential and a negative free energy. Half cell potentials (given for reduction reactions)

are measured against a standard hydrogen electrode, which is assigned a reference

potential of 0 volts. The potential of the overall reaction is taken as the difference

between the cathodic and anodic half-cell potentials, described in {5}.

{5} E = ec − ea

E ≡ overall cell potential (V)

ea ≡ anodic half-cell reduction potential (V)

ec ≡ cathodic half-cell reduction potential (V)

It is important to note that the half-cell potentials are all taken using the reduction

reaction of the species involved. Standard half-cell potentials are easily found in

reference tables [10] while half-cell potential in non-standard solutions may be calculated

through the use of the Nernst equation {6}.

{6} e = eo −2.3RT

nFlog

aprodi[ ]

areactj[ ]

e ≡ half-cell potential (V)

eo ≡ equilibrium half-cell potential (V)

R ≡ gas constant; 8.314 J/K•mol

T ≡ absolute temperature (K)

aprod ≡ activity of products

areact ≡activity of reactants

i ≡ stoichiometric coefficients of products

j ≡ stoichiometric coefficients of reactants

The half-cell potential is a measure of the oxidizing power of a species. The

species with the more active (negative) potential will proceed via an oxidation reaction

10

while the species with the more noble (higher) potential will proceed via a reduction

reaction.

2.2.1.2. Kinetic dissolution parameters Since electrochemical reactions produce or consume electrons, the flow of

electrons to or from a reacting surface can give a measure of the reaction rate [11]. This

proportionality is described in Faraday’s law {7};

{7} nFItam =

m ≡ mass reacted (g)

I ≡ electron flow, or current (A)

t ≡ time (seconds)

a ≡ atomic weight (g/mol)

n ≡ number of equivalents

F ≡ Faraday’s constant; 96 486 (Coulombs/mol)

The corrosion rate, or corrosion current density is determined by dividing through

by time and the surface area, giving {8};

{8} nFia

tAmr ==

r≡ corrosion rate (g/cm2s) i ≡ current density (A/cm2) A≡ area (cm2)

{9} ηc = βc logicio

{10} ηa = βa logiaio

ηc or ηa ≡cathodic or anodic overpotential (V)

ic or ia ≡ current density of given reaction (μA/cm2)

io ≡ exchange current density (μA/cm2)

βc or βa ≡ Tafel slope of cathodic or anodic reaction (V/decade)

11

At equilibrium in the absence of an external bias, the forward and reverse reaction

rates are equal, and the current is referred to as io, the exchange current density. The

overpotential, η, is the voltage relative to equilibrium required to achieve a given current

due to polarization losses in the cell and is given by η = eapplied - e. For cathodic

polarization at steady state, the overpotential can also be expressed as the well-known

Tafel equation {9} and for anodic polarization it is represented as {10}.

If the overvoltage becomes sufficiently large then diffusional rates start to limit

the reaction and a linear concentration gradient develops at the electrode surface in the

solution. Assuming 1st order diffusion according to Fick’s Law, the change in potential

becomes {11} illustrated in Figure 2, with limiting current density {12};

{11} ⎥⎦

⎤⎢⎣

⎡−=

Lconc i

inF

RT 1log3.2η

{12} iL =DznFCB

δ

iL ≡ limiting current density (μA/cm2)

Dz ≡ diffusivity of reacting species (cm2/s)

CB ≡ concentration of the uniform bulk solution (mol/cm3)

δ ≡ thickness of concentration gradient in solution (cm)

Figure 2 Concentration gradient in the solution near a surface controlled by concentration polarization

Distance

CB

Concentration Linear approximation Actual concentration gradient

δ

Co = 0

12

Finally, the corrosion current density, icorr, exists when the current density of the

anodic reaction is equal to the current density of the cathodic reaction {13}. The

corrosion potential, Ecorr is simply the potential associated with the corrosion current

density.

{13} ic = ia = ⏐icorr⏐

2.2.1.3. Iron dissolution pathway

The overall iron dissolution reaction shown in {3} does not indicate the

mechanism by which iron corrodes. Many authors agree [12, and references therein] that

the dissolution reaction shows no dependence on ferrous ion concentrations, but on a

reaction catalyzed by hydroxyl ions. This holds true even in highly acidic solutions.

Hilbert et al. [12] suggested that there is a possibility of hydroxyl generation from

water molecules by deprotonation. The cause of this reaction is claimed to be the

positive surface charge and tendency of transition metals to form complexes.

Heusler [13] and Bockris [14] proposed two separate theories for the anodic

dissolution mechanism of iron. Heusler proposed a mechanism of iron dissolution

whereby the precipitation of FeOHads would act as a catalyst for further Fe dissolution.

Bockris proposed an alternative mechanism with an intermediary species of FeOH+

produced from the FeOH taking part in the dissociation mechanism.

Lorenz [15] showed that both proposed mechanisms may occur, and that it

depends on the purity and surface morphology of the iron surface. High purity iron with

low surface activity follows the mechanism proposed by Bockris , while highly active

iron with higher surface activity follows the method proposed by Heusler.

Below are the two proposed pathways for iron dissolution. Both dissolution

pathways begin with iron reacting with water to form an adsorbed iron species a

hydroxide ion and an electron being released:

{14} Fe + H2O ↔ Fe(H2O)ads

{15} Fe(H2O)ads ↔ Fe(OH−)ads + H +

13

{16} Fe(OH−)ads ↔ (FeOH)ads + e−

For an overall first reaction step:

{17} Fe + H2O↔ (FeOH)ads + H + + e−

In high purity iron the dissolution pathway follows the mechanism proposed by

Bockris:

{18} (FeOH)ads → FeOH+ + e−

{19} FeOH+ + H+ ↔ Feaq2+ + H2O

In iron with a higher density of crystal imperfections, the dissolution pathway

follows the mechanism proposed by Heusler:

{20} Fe + (FeOH)ads → [Fe(FeOH)]

{21} [Fe(FeOH)]+ OH− → FeOH+ + (FeOH)ads + 2e−

{22} FeOH+ + H+ ↔ Feaq2+ + H2O

Both mechanisms result in the same final step, equations {19} and {22}. The

“catalyzed” mechanism, as proposed by Heusler uses the surface catalyst [Fe(FeOH)].

This catalyst likely forms at the kinks of the metal crystals or at adatoms in their vicinity

[12]. The “noncatalyzed” mechanism features two individual charge transfers, while the

“catalyzed” mechanism features one charge transfer in which two charge units are

transferred. This discrepancy in the charge transfer method is reflected in the predicted

Tafel slopes for each mechanism, as seen in Table 2. These Tafel slopes were derived

from the kinetic equations of each reaction pathway [12]. Data for steady-state could be

likened to advanced stages of cleaning, whereas nonsteady-state would be more

representative of initial reaction rates.

14

Reaction mechanism Predicted Tafel slope

Steady-state reactions Nonsteady-state reactions

Noncatalyzed 40 mV 40 mV

Catalyzed 30 mV 60 mV

Table 2 Calculated anodic Tafel slopes for the proposed reaction mechanisms in steady-state and nonsteady-state reactions. Electrochemical data derived by Tafel extrapolation for the dissolution of iron

from various sources in a variety of acidic media is provided in Table 3. Both inhibited

and uninhibited solution data is provided. The effect of inhibitors will be discussed in

section 2.4.

Source Medium Ecorr vs.

AgCl

icorr

(μA/cm2)

βa

(mV/dec)

βc

(mV/dec)

A.C Makrides

[16]

0.52N H2SO4 -0.476

–

-0.487

50 – 60 95-105

L. Cavallaro

[17]

1N HCl -0.460 67 50 130

1N H2SO4 -0.442 68 70 112

+ 10-4M thiourea -0.446 8.5 84 136

+ 10-3M thiourea -0.436 4.7 92 147

+ 0.02%

polybutanimine

-0.424 20 57 118

Table 3 Tafel data for iron in various inhibited and uninhibited acidic media

The dissolution of iron oxides in various acidic media will be discussed in section 2.2.3.

15

2.2.1.4. Corrosion rates

Corrosion testing is used for a variety of reasons [18,19], including determining

service life of equipment or a coating, screening available metals and evaluating the

corrosivity of a solution. Higher corrosion rates will indicate a highly aggressive

cleaning solution, which is to be avoided for the specified system.

For this thesis, corrosion current densities (icorr in μA/cm2) were determined

electrochemically by Tafel extrapolation in accordance with ASTM G3 - 89(2004)

Standard Practice for Conventions Applicable to Electrochemical Measurements in

Corrosion Testing [20]. The theory behind two different techniques for determining icorr,

polarization resistance and Tafel extrapolation, are discussed in sections 2.2.1.5 and

2.2.1.6 respectively. Another method of determining corrosion rate, by weight loss

analysis, is described in detail in section 2.2.1.7.

2.2.1.5. Polarization resistance

Polarization resistance provides information on the corrosion rates of a material

close to equilibrium (i.e. at small overpotentials) based on the idea that the degree of

polarization for a given system will be greater for a system with lower corrosion rates

[21-26]. Regions of linearity are observed at overpotentials within a few millivolts of

Ecorr. Polarization resistance is defined as the slope of this region:

{23} Rp =Δε

Δiapp

Rp ≡ polarization resistance (Ω/cm2)

Δε ≡ overpotential (V)

Δiapp ≡ applied current density (A/cm2)

Measurements of Rp are determined using a controlled potential. Typically the

voltage applied is ±10 mV from Ecorr, as seen in the polarization resistance curve shown

in Figure 3.

16

Rp is then used in conjunction with the Stern-Geary [22] equation to find the

corrosion current density:

{24} icorr =βaβc

2.3Rp βa + βc( )

βa ≡ anodic Tafel slope (mV/decade)

βc ≡ cathodic Tafel slope (mV/decade)

RP ≡ polarization resistance (Ω)

icorr ≡ corrosion current density (μA/cm2)

Figure 3 Polarization resistance data at small applied current densities in

0.52N H2SO4 at 30°C [From Graydon, J. W. "Linear polarization."

Lecture.]

Some automatic estimation software assigns a default value of βa = βc = 0.1.

While this will give a maximum error of a factor of two [22] a more accurate way of

determining the Tafel slope is discussed in section 2.2.1.6.

17

2.2.1.6. Tafel extrapolation

The Tafel region is defined as the linear region of a semi log polarization curve.

Conventionally, it is measured 60 mV away from Ecorr in either the cathodic or anodic

direction. For the iron system, the Tafel behavior is typically limited to one decade of

current density [22,27].

Tafel slopes indicate the rate determining step of the reaction mechanisms

occurring in a system. Changes to the cathodic and anodic Tafel slopes due to the

addition of an inhibitor may indicate the nature of an inhibitor through a change in the

mechanism of an oxidation or reduction reaction.

The corrosion potential and current density may also be determined using an

extrapolation of the Tafel slope to the Ecorr value. Typically the cathodic Tafel slope is

used for such calculations [22, 27, 28] with the Tafel line being traced back to the

determined Ecorr value. The point where Ecorr and the Tafel lines meet will give icorr, the

corrosion current density. Only the cathodic Tafel slope is used when the anodic

polarization curve does not show well-defined Tafel behavior due in part to the

dissociation of the anode (i.e. metal ions). Rapid dissociation of ions may contaminate

the solution before the anodic polarization measurements are complete or the surface may

be changed as ions are liberated.

As the potential is pushed cathodically or anodically from Ecorr, there must be a

charge conservation associated with the current. If the overpotential is negative (i.e.

moving to the cathodic reaction side), the increase in cathodic reduction rate must cause a

decrease in the anodic oxidation rate. The applied current may then be described as

follows:

{25} iapp,c = ic − ia

ic ≡ cathodic current (A)

ia ≡ anodic current (A)

iapp ≡ applied current (A)

Using formula {25}, the derived anodic Tafel slope may be calculated as the

difference between the applied cathodic current and the extrapolated Tafel slope. The

18

extrapolated Tafel slope is simply a continuation to Ecorr of the Tafel region on the

polarization diagram. An example is shown in Figure 4.

The point of intersection between the derived cathodic Tafel slope and the

experimental anodic Tafel slope will be Ecorr, with a corresponding icorr, giving the

corrosion current density.

Figure 4 Polarization curve showing experimental and extracted Tafel lines for

1080 steel in deaerated 1N H2SO4. βc= -98mV, βa= 38 mV (derived

from cathodic data), icorr = 1180μA/cm2 [29]

2.2.1.7. Comparison of weight loss versus electrochemical

methods

Weight loss data may be converted to corrosion current densities by Faraday’s

law {7}, which states proportionality between current density and mass reacted. Thus, if

19

the current density is known, the corrosion rate in terms of weight loss may be calculated

and vice-versa.

The most prominent form of expressing corrosion rates are mils per year (mpy) or

mm/yr [30]. Corrosion current density may be converted to corrosion rate by the

following:

{26} aCrnicorr

ρ=

icorr ≡ corrosion current density (μA/cm2)

a ≡ atomic weight (g/mol)

n ≡ the number of equivalents

ρ ≡ density (g/cm3)

C ≡ conversion factor equal to 0.129 for mpy and 0.00327 for mm/yr

r ≡ corrosion rate (mpy or mm/yr)

2.2.2. Effect of temperature on corrosion rate

Hudson [8] showed that for sulfuric acid and hydrochloric acid pickling solutions,

the cleaning (corrosion) rate increased with increasing temperature.

From equation {6}, it can be seen that an increase in temperature will lead to an

increase in potential. Thus, as temperature increases, Ecorr will increase. Eo is defined at

standard conditions but is also temperature dependent and will increase as temperature

increases.

Awad [31] performed Tafel extrapolation measurements on low carbon steel in

aerated hydrochloric acid solutions with and without an inhibitor. His data showed no

significant increase in potential, but a dramatic increase in corrosion current density with

increasing temperature, as seen in Table 4.

20

T (°C) Ecorr (mV

vs. SCE)

βa

(mV/dec)

βc

(mV/dec)

icorr

(mA/cm2)

20 576 125 126 3.66

30 568 125 106 6.00

40 572 136 96 16.49

50 564 143 98 19.90

Table 4 Corrosion data for low carbon steel (wt%: 0.1 C, 0.29 Mn, 0.07 Si, 0.021P) in a 1 M uninhibited hydrochloric acid solution at various temperatures

2.2.3. Effect of anion type

Various mechanisms exist for iron and iron oxide dissolution in acidic media [12-

16, 32]. For the purpose of this thesis, the two main acids of interest were hydrochloric

and sulfuric acids.

2.2.3.1. Hydrochloric Acid The dissociation of the base metal will form ferrous chloride and hydrogen gas, as

follows:

{27}

If an oxide film is present, dissociation in hydrochloric acid involves a direct

attack on the oxide film. The film will dissociate by one of the following two reactions:

{28}

{29}

21

2.2.3.2. Sulfuric Acid

The dissociation of iron oxides in sulfuric acid involves the acid penetrating the

oxide layer through cracks. The substrate is then dissolved and hydrogen gas is formed

by the following reaction:

{30}

The hydrogen gas present, along with the dissolution of FeO will aid in removal

of the remaining metal oxides, which will then dissociate in solution. The dissociation

reactions for the oxides are as follow:

{31}

{32}

2.2.4. Effect of acid concentration

For both sulfuric and hydrochloric acid cleaning solution, the corrosion rate

increases with increasing acid concentration [3-8, 33]. Reaction rate is a function of

concentration (by the rate law) regardless of the order of the reaction. Thus increasing

the concentration of the acid will increase the overall reaction rate. Since the reaction

rate is a measure of corrosivity of a solution, it follows that the corrosivity of a solution

will increase with increasing acid concentration. This may be seen in the iron dissolution

data for varying concentrations of hydrochloric acid in Table 5.

22

HCl

(mol/L)

Ecorr (mV

vs. SCE)

βa

(mV/dec)

βc

(mV/dec)

icorr

(mA/cm2)

0.1 542 79 117 1.51

0.5 569 68 119 2.51

1 553 65 122 3.80

5 525 120 130 16.60

Table 5 Electrochemical data for iron dissolution in hydrochloric acid solutions of varying concentration, performed at room temperature on Armco iron [33]

2.2.5. Effect of fluid velocity

Cathodic reduction reactions on the surface of the corroding metal will deplete the

nearby solution of H+, limiting the maximum possible current density. A schematic of

the depletion of H+ in solution is shown in Figure 2.

The maximum reaction rate that cannot be surpassed due to depletion is called the

limiting current density and is defined in {12}. By increasing solution velocity, one may

decrease δ and increase iL. If the limiting current density is increased, the maximum

possible reaction rate is also increased and the corrosion rate can increase.

For cleaning, a high flow is required both to decrease cleaning time (by increasing

corrosion rate) and to aid in removal of oxides by mechanical means [34, 35]. If the flow

is too low, there will be insufficient replenishment of hydrogen required for oxide

dissolution. However, if the flow is very high erosion may occur, which is undesirable.

2.3. Smut Formation

2.3.1. Definition of smut

A major problem with acid cleaners is the formation of smut. Smut is a term used

to denote an accumulation of reaction products on metal surfaces [36,37]. Typically

23

caused by “over cleaning” smut occurs more frequently in high-carbon and high-silicon

steels. Smut formation is a significant concern in developing the cleaning system since it

can decrease the adhesion of metal coatings by up to 50% [38].

2.3.2. Chemistry

Smut is an amorphous, highly adhesive oxide coating. Baun [36] showed that the

composition of smut on silicon containing steel was almost entirely graphite and silicon

oxide. He found that ferrous alloys and steels containing silicon are more likely to form

smut in large quantities. Further, he found that on steels and ferrous alloys contain

silicon, the smut was primarily silicon oxides and trace amounts of other metallic and

non-metallic oxides. Smuts of this nature were considerably more difficult to remove

from the substrate than simple carbon-based smut. Since the alloy used for this thesis has

a silicon content of 1.5-1.8 vol%, it is likely that it would form an adhesive silicon-based

smut. Thus smut formation should be avoided.

2.3.3. Factors affecting smut formation

Various factors affect smut formation. The most prominent is changes in

temperature. As discussed in section 2.2.2 above, reaction rate increases with

temperature. This will cause an increase in the rate of smut formation as well. Allen et al

[37] showed that for acid solutions, smut formation will increase with increasing

temperature and acid concentration. The atmosphere also has a large effect on smut

formation. Highly aerated solutions will have faster smut formation due to the

availability of oxygen. Finally, since smut is commonly dominated by silicon oxide,

higher silicon content in steels will lead to greater smutting.

24

2.3.4. Methods of controlling smut formation

Smut may be lightly bonded, in which case it may be removed by simply rinsing

or light mechanical work (for example, rubbing with a cloth). However, some smut

particularly smut with a high silicon oxide content is highly adherent, requiring

aggressive chemical treatment for removal, such as chromic acid dips [36] or highly

concentrated sulfuric or hydrochloric acid solutions [37]. Thus it is important to avoid the

formation of smut. Should it occur in the system being investigated in this work, removal

by mechanical means would be impractical due to the large number of components being

processed, and chemical removal would result in additional environmental waste

products.

Smut formation may be minimized by operating at low temperatures, with high

acid concentrations and in deaerated atmospheres. Another means of controlling smut is

by decreasing the amount of silicon in the steels used for a given application.

As smut formation increases with time [37], to avoid smut formation, a short

cleaning time frame is needed. However, the time frame should be long enough to ensure

sufficient oxide removal. As such, it is likely that an inhibitor will be needed to slow

corrosion of the metal surface and smut formation.

2.4. Inhibitors

A wide variety of inhibitors are available to reduce the corrosion rate of iron in

both hydrochloric and sulfuric acid solutions [39-56]. These inhibitors slow down the

rate of chemical attack on the surface of the metal by one of two means: either by

slowing the rate determining step of the reaction mechanism or by blocking active sites

on the surface of the metal. A minimum concentration of the inhibitor must be present,

however an excess of the inhibitor may lead to a more aggressive solution than the

uninhibited solution. Good circulation should always be present to avoid stagnant areas

where the inhibitor may be depleted.

25

Most inhibitors are considered hazardous to the environment. Only

“environmentally friendly” non-hazardous inhibitors were considered in this thesis,

limiting the type and range of inhibitors that could be successfully employed in this

study.

2.4.1. Definition and classification of inhibitor types for Fe

Inhibitors may be classified as either organic or inorganic inhibitors. Typically in

acidic solutions, sulfur-containing compounds are used for sulfuric acid solutions, while

nitrogen-containing compounds are used for hydrochloric acid solutions [39]. A list of

commonly used inhibitors is provided in Table 6. Inhibitors act by bonding to the surface

of the compound and serving as a barrier to corrosion or they may alter the anodic and/or

cathodic behavior of the surface and slow the oxidation reaction, reduction reaction or

both reactions involved in the dissolution of iron.

Acid Media Commonly used inhibitors

Sulfuric acid Phenylthiourea, di-ortho-totyl-thiourea, mercaptans, sulfides

Hydrochloric acid Pyridine, quinoline, various amines, decylamine,

phenylthiourea, dibenzylsulfoxides

Table 6 Inhibitors commonly used for industrial cleaning applications

There are three basic inhibitor types: anodic, cathodic and mixed inhibitors. A

brief description of each type is summarized below.

2.4.2. Anodic Inhibitors

Anodic inhibitors form a protective passive film at the anode, increasing

the potential of the anode and slowing corrosion. Increasing the anode potential will

cause Ecorr to increase and icorr to decrease, as seen in Figure 5. These inhibitors enhance

chemisorption of dissolved oxygen, which aids in passivating the metal. A high dose of

anodic inhibitor is typically necessary to initiate film formation in the system.

26

Since the inhibitors mask the anodic site, it is critical that they are not depleted

since below a critical limit, pitting may occur due to area effects. Area effects occur

when a large amount of anodic sites are blocked, the corrosion current stays the same, but

the unprotected areas must support a higher corrosion current density, thus increasing the

local corrosion rate.

Figure 5 Evans diagram showing the effect of anodic inhibitors on corrosion current. Anodic inhibitors have varying effectiveness in varying pH. Common anodic

inhibitors and the medium in which they are used are listed below [38]:

i. Chromate and nitrite (neutral solutions) – these inhibitors catalyze the reaction

between the metal and oxygen, creating a passive film. Chromate and nitrite are

the only anodic inhibitors that function in the absence of oxygen.

ii. Molybdate and orthorphosphate (neutral and acidic solutions) – these inhibitors

act in a similar fashion as those listed above, however they require oxygen to be

efficient.

27

2.4.3. Cathodic Inhibitors

Cathodic inhibitors are less effective than anodic inhibitors [39], however they

will not lead to excessive pitting. They form a visible film along the surface of the

cathode, which restricts the access of oxygen to the metal substrate. The film will also

Figure 6 Evans diagram for the effect of cathodic inhibitors on corrosion current block hydrogen evolution sites create a limiting current density for the reaction since it

may no longer occur uniformly over the surface of the substrate. This will decrease Ecorr

and icorr, as seen in Figure 6.

As with anodic inhibitors, cathodic inhibitors will have varying efficiency in

acidic, alkaline and neutral solutions. Common cathodic inhibitors and the media in

which they are used are listed below:

i. Zinc hydroxide and zinc phosphate (neutral and alkaline solutions)

ii. Calcium carbonate and calcium phosphate (neutral solutions)

28

2.4.4. Mixed Inhibitors

Mixed inhibitors suppress both cathodic and anodic electrochemical reactions.

They may be a combination of cathodic and anodic inhibitors or simply an inhibitor that

adsorbs to the entire surface of the substrate, causing a barrier between the substrate and

the corrosive solution. Mixed inhibitors will cause a decrease in Ecorr and icorr for the

system, as seen in Figure 7.

Figure 7 Evans diagram for effect of mixed inhibitors on corrosion current

2.4.5. Organic Inhibitors

Organic inhibitors must be adsorbed to the substrate in order to be effective [40].

Adsorption is facilitated by polar groups on the molecule, which can attach to the

substrate. The most effective polar groups include sulfur, nitrogen and hydroxyl species.

Different organics will have varied effectiveness in any given solution. Typically

mixtures of organics are used, and derived proprietary organic compounds are common.

29

2.4.6. Quinine

Quinine is an organic compound obtained from the bark of the cinchona tree

common to South America. It is used in tonic water, and as a medicinal treatment for

muscle cramps and malaria. It is an odorless white powder that has very low solubility in

water, but is soluble in acidic solutions. The structure of quinine is shown in Figure 8.

Figure 8 The quinine molecule [31].

Quinine has been proved as an effective inhibitor in the corrosion of low-alloy

steel in hydrochloric acid solutions. Awad [31] used electrochemical impedance

spectroscopy and potentiodynamic polarization to show that the inhibitive action of

quinine was due to the mechanism of physically blocking sites on the metal surface. The

adsorption mechanism of quinine is physisorption, as proven by Awad through

calculating the magnitude of various adsorption parameters, such as ∆ , ∆ , and

∆ .

Despite evidence against quinine’s effectiveness in sulfuric acid solutions [31],

early work by M. Stemp [41] proved that quinine in low concentrations is an equally

effective inhibitor in sulfuric acid solutions at low temperature. Advantages to using

quinine are its low environmental impact and its easy detection through fluorescence

[42], which should allow for a method of monitoring the quinine concentration in the

cleaning solution and also for detection of any quinine carry-over in subsequent

solutions.

30

2.4.6.1. Mechanisms of quinine inhibition

Awad [31] found that quinine generally acts as a mixed inhibitor in hydrochloric

acid solutions. He completed polarization curves at various temperatures and quinine

concentrations. An excerpt of his results is summarized in Table 7, with θ representing

T (°C) C (mM) Ecorr (mV) βa

(mV/dec)

βc

(mV/dec)

icorr

(mA/cm2)

θ

20

0 576 125 126 3.66 N/A

0.08 563 112 90 1.39 0.62

0.8 568 99 71 0.15 0.96

30

0 568 125 106 6.00 N/A

0.08 574 114 95 2.70 0.55

0.8 559 114 63 0.66 0.89

40

0 572 136 96 16.49 N/A

0.08 575 127 91 8.44 0.49

0.8 589 125 147 3.30 082

50

0 564 143 98 19.90 N/A

0.08 574 138 87 11.34 0.43

0.8 574 147 97 5.30 0.75

Table 7 Electrochemical parameters for quinine at various temperatures and

concentrations in 1 M hydrochloric acid [31].

the degree of surface coverage, which is also a measure of inhibitor efficiency. The

surface coverage was defined as the fraction of corrosion inhibition in the system:

{33}

i1 ≡ corrosion current densities in the absence of quinine (μA/cm2)

i2 ≡ corrosion current densities in the presence of quinine (μA/cm2)

31

By examining the cathodic and anodic Tafel data, it is apparent that for

any given temperature they both change as a function of quinine. This indicates that both

reactions are being inhibited. Thus Awad concluded that the inhibition method of

quinine must be by simply blocking the surface of the substrate. The corrosion potential

data is also indicative of a mixed inhibitor, as there is no significant change over the

quinine concentration range.

Awad calculated the inhibition efficiency as related to surface coverage by the

following equation [31]:

{34}

i1 ≡ corrosion current densities in the absence of quinine (μA/cm2)

i2 ≡ corrosion current densities in the presence of quinine (μA/cm2)

His findings are summarized in Figure 9.

Figure 9 Inhibition efficiency at different concentrations of quinine for low

carbon steel in 1 M HCl at different temperatures: (1) 20°C, (2) 30°C,

(3) 40°C and (4) 50°C.

Figure 9 shows that the inhibitor efficiency increases with increasing

concentration of inhibitor. The degree of surface coverage also increases with increasing

2

32

quinine concentration, thus confirming that the method of inhibition is in fact adsorption

to the surface of the substrate.

2.4.6.2. Temperature stability of quinine

As can be seen in Table 7 and Figure 9, the effectiveness of quinine decreases as

temperature increases for a given quinine concentration. This can be observed both by an

increase in corrosion current density and a decrease in surface coverage for a given

quinine concentration.

Awad studied the thermodynamic parameters for quinine adsorption and found a

free energy of adsorption equal to ~ -20 kJ/mol, a magnitude consistent with a

physisorption process. The adsorption equilibrium constants (K) of the system were

determined where:

{35} K =1

55.5exp

−ΔGadso

RT

⎛

⎝ ⎜

⎞

⎠ ⎟

K ≡ adsorption equilibrium constant (unitless)

ΔGadso ≡ free energy of adsorption (J/mol)

R ≡ gas constant ( J

K ⋅ mol)

T ≡ temperature in K

The equilibrium constant was found to decrease with increasing temperature, thus

reinforcing the theory that at high temperature the inhibitor loses efficiency by desorption

from the substrate.

33

2.4.6.3. Measurement of quinine in process control

A major concern with organic inhibitors is that their concentration in solution may

be difficult to determine analytically, especially in real time. While titrations are

appropriate for inorganic inhibitors, they are an inefficient means of determining organic

substances due to the complexity of organics.

Quinine has an advantage over other organic inhibitors as it will fluoresce in

dilute acid solutions at excitation wavelengths of 250 and 350 nm [42-44]. Through

simple fluorescence, quinine concentrations in solution may be determined. This process

is described in ASTM standard: ASTM E 579 (2004) [45]. This property could be of

great use in determining the lifetime of cleaning solutions and the quantity of inhibitor

carry-over in any subsequent plating or brightening solutions.

2.4.7. Other possible inhibitors

Inhibitor Media Reference

Jojoba oil 1M HCl [47]

Rosemary oil 2M H3PO4 [48]

Thiourea derivatives Various concentrations H2SO4 [49]

Butindiol derivatives Various concentrations H2SO4 [49]

Artemisia oil 1M HCl [50]

Natural honey 0.5M NaCl [51]

Eugenol derivatives 1M HCl [52]

Mixed halides and thiols Various concentrations H2SO4 [53]

Extract of Nypa fruticans Various concentrations HCl [54]

Extract of hella leaves Various concentrations HCl [55]

Tetramethyl-dithia-octaaza-

cyclotetradeca-hexaene

Hot, 20% H2SO4 [56]

Table 8 Environmentally friendly inhibitors researched as quinine alternates

34

Various other environmentally friendly inhibitors were examined but not tested.

These inhibitors and the media in which they were originally tested are summarized in

Table 8 .

Of the alternate inhibitors researched, the most promising ones included

rosemary, artemisia and jojoba oil. Rosemary has been found to be an effective inhibitor

in phosphoric acid [48], however the inhibitor concentrations required (up to 10 g/L)

make it a poor choice when compared to the low concentrations of quinine required.

Concentration

(g/L)

Ecorr

(mV vs. SCE)

βc

(mV/Dec)

icorr

(μA/cm2)

Picorr

(percent)

Rosemary Oil – 2M H3PO4, 298K [48]

Uninhibited -465 188 533 N/A

10 -485 162 145 73

4 -480 166 217 59

1.5 -478 187 329 38

0.6 -472 185 445 17

Artemisia Oil – 1M HCl, 298K [50]

Uninhibited -440 153 66 N/A

19 -465 140 16 76

1.9 -470 136 31 53

1.3 -480 135 40 39

0.95 -475 137 55 17

Jojoba Oil – 1M HCl, 298K [47]

Uninhibited -500 230 936 N/A

0.515 -487 270 19 98

0.386 -480 280 50 94

0.129 -475 260 95 88

0.005 -482 259 392 58

Table 9 Potentiodynamic polarization parameters for the corrosion of iron in

acidic media with varying inhibitor concentration and type.

35

Artemisia oil as an inhibitor showed inhibitive properties in hydrochloric acid

[50]. Artemisia oil acts as a cathodic inhibitor and the inhibition efficiency was shown

to increase with increasing temperature. However, like rosemary oil, high concentrations

(up to 19 g/L) of oil were required to inhibit the solution.

Jojoba oil, in small quantities, is reported to be an effective inhibitor in

hydrochloric acid solutions [47]. Less than 0.4 g/L was required to give significant

corrosion inhibition. Should an environmentally friendly alternate to quinine prove

necessary, this could be a viable alternative.

A summary of the electrochemical properties of the above mentioned inhibitors is

provided in Table 9, with Picorr being the inhibitor efficiency as calculated in section

2.4.6.1. The most promising of the alternative inhibitors appears to be jojoba oil, which

gives high inhibition efficiency (Picorr) at low concentrations. Of the examined inhibitors,

both quinine and jojoba oil have similar achievable Picorr values, but the a much lower

concentration of quinine is required to reach high Picorr values (mM) than jojoba oil (M).

2.4.8. Benzotriazole as a copper inhibitor A second inhibitor will be required in the overall plating system to act as a

protective, corrosion resistant coating to the applied copper flash. The inhibitor

investigated was benzotriazole.

Benzotriazole (BTA) is a well-established inhibitor for copper surfaces in acidic

media [58-62]. The structure of benzotriazole is shown in Figure 10. As with most

organic inhibitors, it contains a polar group with nitrogen atoms. These nitrogen atoms

act as an electron donor, enhancing chemisorption on the surface of the metal.

36

Figure 10 The benzotriazole molecule [63] The inhibition mechanism is by adsorption onto the surface of the metal. The

benzotriazole has been shown to form a polymeric surface film of variable thickness, in

the range of 40-140 Å [60-62]. For copper corrosion inhibition, there are two possible

structure of polymeric film, depending on the nature of the copper oxide present.

Cuprous oxides yield Cu(I) BTA, which consists of linear polymeric chains. Cupric

oxides yield Cu(II) BTA, which is a network structure. This polymeric film inhibits

dissolution of copper by acting as a barrier to cathodic reactions.

Concentration of BTA (M) Corrosion rate (mg/cm2h) Picorr

Blank 1.38 N/A

10-5 0.671 51.37

10-4 0.531 61.52

5 x 10-4 0.381 72.39

10-3 0.291 78.91

5 x 10-3 0.231 83.26

10-2 0.1301 90.57

5 x 10-2 0.0501 96.36

Table 10 Corrosion rate and inhibition efficiency data obtained by Kahled [58] from weight loss measurements for copper in 0.5 M HCl solutions in the absence and presence of various concentrations of BTA at 30°C

37

Kahled used weight loss measurements for copper in 0.5 M HCl solutions in the

absence and presence of various concentrations of BTA to determine the inhibitor

efficiency [58]. His results are summarized in Table 10. Inhibitor efficiency was

calculated from {34}.

Bellakhal and Dachraoui [64] found that benzotriazole is an equally effective

inhibitor in humid air plasma. They immersed copper foils in a 0.1 M benzotriazole

solution, and exposed the surface to a strongly oxidizing gliding arc plasma in humid air.

They found that the inhibitor limits the production of copper oxides and that under very

harsh oxidizing conditions the inhibitor effect remains unmodified for 40 minutes.

Beyond that time, they observed a decrease in the efficiency of the inhibitor as marked by

increased formation of copper oxides.

2.5. Copper Coating Technologies

Copper coatings are very frequently applied as an inexpensive, corrosion-resistant

coating to bulk components. It is a low-cost coating that is easily applied and acts as a

good end-of-process coating or as a conductive substrate for subsequent metallic

coatings.

A review of common copper plating processes and plating techniques are

provided in the following sections, including the thermodynamic fundamentals of the

process commonly used.

2.5.1. Fundamentals of electroless copper plating

Electroless plating is a well-established process that lends itself to the coating of

small, complex parts. The process involves a metal substrate that is brought in contact

with an aqueous alkaline solution containing copper ions. The copper in solution is

reduced onto the surface of the substrate by the following reaction (involving

formaldehyde as the reducing agent):

{36} Cu2+ + 2HCHO+ 4OH− → Cu + 2HCOO− + 2H2O+ H2

38

This is a complex, autocatalytic reaction with highly sensitive bath chemistry and

complex, multi-step mechanisms. While bath chemistries are typically proprietary and

will vary for individual use, almost all baths will include the following [65-69]

components:

i. A metal ion source

The most commonly used metal ion source for copper plating baths is copper (II)

sulfate (CuSO4) specifically, the pentahydrate copper sulfate (CuSO4⋅5H2O) salt.

ii. A reducing agent

The anodic reaction is facilitated by the selection of a reducing agent such that the

potential is more negative than the potential for copper deposition. Selection of the

reducing agent should also ensure that the copper deposits on the substrate and does not

“plate out” in solution.

iii. A complexant

As the copper plates onto the substrate, the solution becomes depleted of ions,

causing local pH variations. These local variations may lead to precipitation of the metal

in solution. Complexants prevent precipitation by depressing the free metal ion activity.

This will also allow the bath to operate at higher pH ranges. Weak or insufficient

complexant concentration will lead to precipitation and too strong or too high a

concentration of complexant will lead to insufficient metal ions for deposition.

iv. A buffer

A buffer is required to stabilize the pH of the solution, which will vary due to the

formation of H+ as the reducing agents are oxidized. Variations in pH over time can lead

to inconsistent coating properties (composition and surface roughness) and deposition

rate.

v. A stabilizer

Stabilizers adsorb to impurities in the solution and prevent unwanted copper

precipitation and decomposition of the bath. Stabilizers act by adsorbing to the active

nuclei and shielding them from the reducing agent in the plating solution.

39

vi. An exaltant (accelerator)

Complexants and stabilizers may reduce the plating rate to unprofitable levels.

Accelerators are added to the bath to increase the plating rate by driving the oxidation of

the reducing agent.

Bath parameters such as the copper concentration, iron concentration, bath

temperature and plating time will all affect the coating’s properties. Increased copper

concentration will result in higher deposition rate, but too high a concentration will cause

precipitation to occur. Higher iron concentration in the plating solution will lead to a

lower deposition rate. Both increasing the bath temperature and plating time will

increase the deposition rate.

The electroless plating process is a multi-step process, involving various cleaning

and rinse steps before plating. Table 11 shows a flow chart of a typical electroless copper

plating system [70]. Since each cleaning solution is of a different pH and composition,

there are a large number of waste streams produced, with a large variety of species in

each.

Overall, there are numerous acidic and alkaline waste streams as well as five rinse

water streams, all of which will require waste treatment. Because of the variations in

waste type (i.e. alkaline and acidic), and the variety of ions in the waste stream there is

little room for recycling of the wastes within the system. There will also be a large

variety of species in the waste that will require treatment. By reducing the amount of

species in the waste stream there is room to minimize the amount of waste treatment

required. Furthermore, by using the same species in the cleaning and flash steps, there is

room for recycling solutions within the system.

40

Solution Type Purpose Environmental Concerns

Cleaner/

conditioner

Alkaline or

Acid

Removes organics

and impurities from

substrate

May contain a variety of species

(surfactants, chelating agents,

etc.) which need to be treated in

waste stream

Rinse Water Removes residues Contaminated water, more waste

processing

Pickling Acid

Removes

impurities, prepares

surface for plating,

promotes good

adhesion

High acid concentration makes

for difficult handling and high

concentration of ions in waste

stream

Rinse Water Removes residues Contaminated water, more waste

processing

Electroless

Copper Alkaline Deposits copper

Many bath components make for

complex and costly waste

management

Rinse Water Removes residues Contaminated water, more waste

processing

10% Sulfuric

Acid Acidic

Neutralizes the

alkalinity of the

electroless copper

Another waste stream to process,

containing acids and remnants of

copper plating solution

Rinse Water Removes residues Contaminated water, more waste

processing

Anti-oxidant Neutral Prohibits oxidation Another waste stream to process

Rinse Water Removes residues Contaminated water, more waste

processing

Table 11 Process flow for electroless copper deposition with outline of environmental concerns associate with each step

41

2.5.2. Fundamentals of immersion plating Immersion, galvanic or displacement plating is one of the simplest forms of

electroless plating [57,58, 71]. It is typically an inexpensive process, and has excellent

throwing power. This process may only be used to deposit noble metals.

This process involves the deposition of one metal in solution on to a dissimilar

substrate. The more electropositive metal will be reduced, or deposited, while the more

electronegative metal will oxidize, or dissolve. In this process, the substrate acts as the

reducing agent, so unlike electroless deposition, no reducing agents are required in

solution.

For copper plating on an iron substrate, the reaction will proceed as follows:

{37} Fe + Cu2+ → Cu + Fe2+

where: EFe

o = -0.447 V vs. SHE ECu

o = +0.337 V vs. SHE Immersion plating solutions have very simple chemistries, with the most basic

consisting of a metal ion source, such a copper sulfate, and acid or base to aid in

dissociation of the substrate. Complexing agents may be used to increase the solubility

of the metal ions in solution and inhibitors may be used to slow the overall reaction.

The maximum achievable thickness is limited with immersion plating processes,

because once the substrate is covered, there is no further driving force and the deposition

reaction stops. As such, the maximum achievable thickness is approximately 1μm.

Plating rate may be increased by increasing the concentration of depositing metal ion in

solution and by stirring. The finish of the final coating may be matte or shiny.

2.5.3. Fundamentals of mechanical plating A mechanical plating system is frequently used to plate small components with

copper. The process that this thesis is attempting to replace is a mechanical plating

process described in further detail in sections 2.5.5 and 2.5.6. A basic explanation of

mechanical plating is outlined below.

42

Mechanical plating is a kinetic process where powdered metals are cold welded to

a substrate [72,73]. Typically ferrous-based parts are plated with ductile metals, such as

copper, lead, indium, zinc, cadmium and tin. This process is most suitable to small parts,

and is frequently used on screws, bolts, nuts, washers, etc. Parts are tumbled, typically in

rubber-lined barrels with water, glass bead impact media and various chemicals, which

are added in sequence.

A surface conditioner is added first. This removes oxides present on the parts.

Surface conditioners contain a mix of mineral acids, wetting agents, and inhibitors. The

conditioner also provides the proper pH for the plating reactions.

Following the conditioner is a coppering solution, which provides an adherent

copper flash. This flash acts as an adhesive base for further coatings, provides a heavier

barrier to hydrogen and gives a uniform substrate for further plating.

Promoter chemicals are then added. These will eventually clean the metal

powder, as well as control the size of metal powder agglomerates that form. It also acts

as a catalyst to the coating process.

Finally, metal powder is added. The amount of powder added controls the

thickness and weight of the final deposit. The appearance of the finished coating is

matte.

2.5.4. Fundamentals of barrel plating

Barrel plating is a method of plating components that involves a rotating vessel

that is immersed in a variety of cleaning and plating solution tanks [74, 1]. Current is

applied to interior cathodic electrical contacts in the barrel, charging the components to

be plated and allowing for current to flow for electrodeposition reactions.

Barrel plating is most commonly used for corrosion protective coatings on bulk

components and it is the most cost-effective way to plate threaded parts and fasteners.

Benefits include fast, large volume productions, easily automated processes, and

components may remain in the same vessel from one solution to the next. Further, the

rotation of the barrel agitates the solution, eliminating the need for external pumps. The

43

rotation also keeps the parts in motion, thus yielding more uniform cleaning and metal

deposits.

Various types of coatings may be accommodated in barrel plating, including zinc,

cadmium, tin, copper and precious metals.

2.5.5. Current systems used in industry

Mechanical plating is very commonly used in the automotive industry to apply a

copper flash to small components, such as fasteners and bolts. For the scope of this

thesis, a popular commercial flash system was used as a baseline for comparison of a

developed process. The flash used is the MacDermid SC-G Copper Flash.

Solution (additive) Tumble time Function

Macuguard SCF +

Foamout

12 minutes Cleans and prepares surface for subsequent

coating, Foamout reducing foaming of solution

Macuguard XL 40 8 minutes Mechanically applies copper flash

Drain

Water and polish 1-2 minutes Brightens copper flash and creates corrosion-

resistant coating

Water Rinse Dilutes polish solution for subsequent draining

Dump

Dry

Table 12 Process outline for MacDermid SC-G Copper Flash solution

This is an additive liquid process conducted in a large, non-metallic vessel [75].

The parts to be plated are added to the vessel where they are initially mixed with water,

glass impact media and a surface conditioner. The samples are tumbled at room

temperature for 12 minutes and an anti-foaming agent is added. MacDermid SC-G copper

flash solution is then added to the vessel and the samples are tumbled for an additional 5

minutes. Finally, the samples are drained, and the vessel is filled with water and a

44

polishing solution. Samples are rinsed and dried and the used solution in disposed of. A

process outline is shown in Table 12.

2.5.6. Limitations and challenges

The current system used has a few limitations. Firstly, it is a batch process, which

limits the rate of part production to the size of the vessel and the plating time. It is a

single-use, additive process meaning there is a large amount of waste produced and

copper or iron from the waste solution may not be recovered. When coating small,

intricate parts, the used mechanical media may become trapped in crevices of the parts,

causing non-uniformity of the coating and impurities on the surface of the part.

It is an electroless plating system, which has a large number of additives such as

inhibitors, accelerators, buffers and chelating agents that may be harmful to the

environment. Furthermore, since the solution is proprietary, the waste treatment is

considerably more complex as the exact chemicals and quantities of said chemicals are

unknown. As such, considerable challenges in waste management arise – such as

separation of unknown chemicals. All of these factors increase the cost of the coating

both in terms of solution cost and waste processing cost.

To reduce the cost of the plating process, a scalable, continuous, re-usable

solution should be developed. This will enable longer service life of the cleaning and

plating solutions and easier waste treatment of any effluents, as the solution’s chemistry

will be known.

2.5.7. Proposed system

The following is a brief comparison of the various plating systems described

above (from [41]).

45

Electroless Plating Immersion

Plating

Mechanical

Plating

Barrel Plating

Max. Thickness > 75μm ~ 1μm >75μm >75μm

Rate of Deposition 20-25μm films

achievable in 20 hours

or more

Minutes

(but only up to

1μm)

20-25μm films

achievable in 20-

45 minutes

20-25μm films

achievable in one

hour or less

Coating Uniformity

(thickness variation

over sample)

Excellent Excellent Good, but

dependent on

part geometry

Good, but dependent

on part geometry

Coating Adhesion Good Poor to good Good Good

Environmental health

and safety concerns

Related to cleaning

products and bath

chemistries

Limited to

cleaning

products

Limited to

cleaning

products

Related to cleaning

products and bath

chemistries

Power Supply

Required

No No No Yes

System Cost Higher, full line

required (pre-treat,

plating and rinse baths)

Higher, full

plating line

required (pre-

treat, plating and

rinse baths)

Low, due to

single barrel for

all steps

Highest, full plating

line required (pre-

treat, plating, and

rinse baths) and

power supply

Coating Cost Highest, due to use of

additional chemicals

Low Low Higher, due to use of

additional chemicals

(mitigated somewhat

by power supply)

Impact Media

Required

No No Yes No

Temperature <80°C Room to 70°C <80°C <80°C

Waste Treatment:

species to be removed

base metal, copper

source, reducing agent,

complexant, buffer,

stabilizer, accelerator

base metal ions,

other additives

TBD

proprietary

chemicals with

each batch

base metal ions,

other additives TBD

H Embrittlement Low Low No Low

Easily Scalable Yes Yes Yes Yes

Table 13 Comparison of main coating options (from [41])

For this thesis, selection of the plating technology is governed by a number of

criteria including:

46

i. Coating quality, and appearance

ii. Throwing power (coating uniformity)

iii. Deposition rate

iv. Adhesion of the deposit to the substrate

v. Cost of the plating system set up

vi. Environmental considerations

Each of the above systems meets some of the above criteria, with immersion

plating being the most feasible. Since the parts are quite small and complex, a Technic

“Mini Electroless Copper Line” system (Figure 11) with a Singleton Mini Barrel (Figure

12) was purchased as a vessel to conduct the cleaning, plating and brightener solution

development.

Figure 11 Technic "Mini Electroless Copper Line" system

Figure 12 Singleton “Mini Barrel” system

47

3. Experimental Methodology

Below is a summary of the parts used during this experiment as well as

experimental methodology used for each circuit: cleaning, plating and anti-oxidant. All

the water used for mixing solutions and rinses was ultra-pure (Type I -18M-ohm) water,

and shall henceforth be simply referred to as water.

3.1. Fasteners Used

For the span of this thesis, a spring steel alloy (AISI 9255) commonly used for

fasteners in the automotive industry was selected as the base substrate for subsequent

development of the copper flash process. Spring steels are low alloy, medium carbon

steels with high yield strength. The key component in most spring steels is silicon. The

nominal composition of the steel used was 0.52-0.6 wt% C, 1.5-1.8 wt% Si, and 0.7-1

wt% Mn with the residual being iron.

The geometry of the fastener is quite complex, as shown in Figure 13. The

surface area of the component, as provided by Tinnerman, is approximately 10.03 cm2.

That value was used as the default surface area whenever applicable.

Figure 13 Tinnerman automotive fastener used for this thesis

3.2. Cleaning Circuit

3.2.1. Beaker Tests Initial experiments were run on a beaker scale. 100mL samples of each solution

were mixed and heated on a Corning PC-101 hot plate to the desired temperature.

1.15 cm

48

Solutions were maintained within ± 5°C of the desired temperature. Prior to cleaning, as-

received samples were ultrasonically washed in water for 5 minutes. This process was

done to remove the debris on the samples accumulated from shipping and handling the

parts. Samples were then immersed in the cleaning solution, which was agitated with a

Acid concentration

(vol %)

Temperature

(°C)

Immersion Time

(s = seconds or

m= minutes)

Inhibitor

Concentration

(g/L)

Hydrochloric Acid Solution

5 25, 50, 75 10s, 30s, 1m, 2m,

5m, 15m, 30m Uninhibited

10 25, 50, 75 10s, 30s, 1m, 2m,

5m, 15m, 30m

Uninhibited and

0.03

20 25, 50, 75 10s, 30s, 1m, 2m,

5m, 15m, 30m Uninhibited

Sulfuric Acid Solution

1 25 2m, 5m, 10m, 15m Uninhibited;

0.003-0.03

5 25, 40 2m, 5m, 10m, 15m Uninhibited;

0.0003-0.3

10 25, 40 2m, 5m, 15m Uninhibited and

0.03

Table 14 Parameters for initial development of cleaning solution inhibited with quinine small Teflon stir bar at a low setting, for the specified time interval and rinsed twice by

dipping the sample in water before being air dried in an air stream. The parts were then

examined visually to determine the extent of scale removal and smut formation and

compared against samples cleaned by the MacDermid SCF cleaner and as-received

samples to determine the extent of cleaning.

49

Digital imaging was performed on the sample in accordance with the procedures

described in section 3.2.4.

A summary of the parameters used for the initial cleaning solution tests are shown in Table 14.

3.2.2. Smut rating

Degree of Smutting Rating Sample image

No smut 0

Very light smut 1

Light smut 2

Grey smut 3

Dark smut 4

Heavy dark smut 5

Very heavy dark smut 6

Very heavy white smut 7

Table 15 Evaluation of smut by appearance Smutted samples were categorized based on the color and amount of smut formed

on the substrate surface. Based on the evaluation of smut by appearance summarized by

50

Allen [37], samples were assigned a rating from 0-7 indicating the level of smut

formation. Criteria for and examples of each level are summarized in Table 15.

3.2.3. Barrel Plating Tests Once the process was ready to scale up, a small barrel set up was used. A

Technic “Mini Electroless Copper Line” system with a 6” basket made from injection-

molded integral-mesh polypropylene was used to set up a 6L tank of cleaning solution

and a 6L water rinse, as shown in Figure 11 and Figure 12. For initial copper coatings, a

6L copper plating solution and a second 6L water rinse was also set up. The barrel was

rotated at 20 rpm.

Each barrel run consisted of 50 samples (filling the barrel to approximately 50%)

with 5 samples tagged using nylon monofilament. These samples were weighed both

before and after cleaning using a Mettler AE260 scale (accuracy ± 0.1 mg) and the

weight loss measurements were used as a means to determine the extent of chemical

attack on the sample.

3.2.4. Digital Imaging Digital photographs were taken of each sample using a Nikon D40X camera with

AF Micro Nikkor 60mm lens. The samples were placed in a Medalight Digital imaging

box with all light sources (front, back, left, right and bottom of the box) at the highest

setting to ensure uniform lighting. The camera settings were as follow: F8, ISO 200,

direct sunlight light balance, high sharpening, medium high tone composition, SRGB

color mode, and normal saturation.

Once images had been captured, the exposure was adjusted as necessary to ensure

a white background to each image. Images were cropped and straightened using iPhoto

’08 version 7.1.5.

51

3.2.5. Adhesion Tests

A peel test was used to determine the adhesion of copper coatings to the surface

of the substrate. The face of the sample was covered with masking tape, and firm

pressure was applied using a palette knife. The masking tape was peeled off and the

surface of the tape and the sample was examined. If there was any transference of the

coating onto the masking tape, the sample adhesion was considered poor, while very poor

indicated full transference of the sample coating to the tape. If there was no transference,

the adhesion was considered adequate.

3.2.6. Weight Loss Tests Weight loss measurements were taken for each batch cleaned with the barrel

plating system. Ten percent of each batch (five of fifty samples) was tagged using

monofilament and each of the samples tagged was weighed after being ultrasonically

cleaned in isopropanol for 5 minutes. Samples were weighed on a Mettler AE260 scale

(accuracy ± 0.1 mg). Samples were then mixed into the batch, cleaned under the desired

parameters and separated once again. After cleaning, they were dried in an air stream

and weighed once more. The weight loss for each system was taken as the average

weight loss of the five samples measured.

3.2.7. Electrochemical Tests Polarization resistance and Tafel measurements were performed on samples in a

variety of sulfuric acid cleaning solutions. 500 mL glass electrochemical cells were used

for the tests, with a CH Instrument potentiostat, using CHI 660c analysis software. A

three-electrode set-up was used using a silver-silver chloride reference electrode in a

luggin capillary and a platinum counter electrode.

All solutions were aerated, agitated with a stir bar operating at 200 rpm on a

Variomag Electronicrührer Poly 15 stir plate, and all heated samples were heated in a

water bath. Temperature was monitored using a thermometer in the solution. The sweep

52

rate was 10mV/minute in accordance with ASTM standards G3-89(2004), G5-97(2009)

and G5 - 94(2004) [20, 26, 28].

A summary of the solutions tested is included in Table 16. A summary of how

resistance polarization and Tafel measurements were conducted may be found in

Appendix 1 and Appendix 2 , respectively.

Temperature

(°C)

Quinine Concentration

(g/L)

Acid Concentration

(vol %)

25 0, 0.3, 0.03, 0.003 1

50 0, 0.03 1

75 0, 0.03 1

Table 16 Summary of solutions subjected to electrochemical testing Anodes were prepared by soldering pieces of steel automotive fasteners to a

copper wire. Any unwanted exposed metal was masked with Amercoat™ 90HS epoxy.

Amercoat™ application methods are detailed in Appendix 3. Anode areas were

calculated through imaging, using Image J software.

3.3. Copper Plating

3.3.1. Beaker Tests Each sample was ultrasonically cleaned for 5 minutes in isopropyl alcohol and

then rinsed with water before testing. Samples were immersed in 200mL cleaning

solution (1 vol% H2SO4 + 0.003g/L quinine, bulk water) in a 250mL beaker being

agitated with a Teflon stir bar (approximately 300 rpm) for two minutes. Samples were

rinsed twice in water to remove any cleaning solution residue before being immersed in

80mL of various stirred (300 rpm, Teflon stir bar) copper plating solutions for various

time intervals. All samples were stirred on a Variomag Electronicrührer Poly 15 stir

plate. Once the sample had been immersed for the desired amount of time, they were

rinsed in water, rinsed with isopropyl alcohol to speed up the drying time and dried in an

53

air stream. Pictures of all samples were taken and catalogued. The imaging process is

described in section 3.2.4.

Inhibited copper sulfate solutions were tested using the above method with 0.003

g/L quinine.

The variables for the copper sulfate solution used are described in Table 17

below.

Sulfuric Acid

Concentration

(vol %)

Copper Sulfate

Concentration

(g/L)

Immersion time

(seconds)

0.01 5, 15, 30 30, 60, 90

0.1 5, 15, 30 30, 60, 90

1 5, 15, 30 30, 60, 90

3 5, 15, 30 30, 60, 90

0.1 + quinine 15, 30 30, 60, 90

Table 17 Copper sulfate solution parameters, uninhibited solutions and solutions inhibited with 0.003 g/L quinine

3.3.2. Barrel Plating Tests The best results from the beaker tests were then scaled up to the Technic “Mini

Electroless Copper Line” system. To conserve samples, anywhere from 10-40 samples

were used in each test. Samples were loaded into the barrel and rotated in a 1 vol %,

0.03g/L quinine solution for two minutes, rinsed in ultra pure water for one minute, and

then immersed in the copper plating solution for the specified time. All tumbling was

done at 20 rpm, save a final test conducted at 5 rpm. Once the coating process was

complete, samples were rinsed in ultra pure water for ten seconds before being air-dried.

Pictures of a selection of samples from each batch were taken and catalogued in the

manner described in section 3.2.4. A summary of the parameters tested is outlined in

Table 18.

54

Sulfuric Acid

Concentration

(vol %)

Copper Sulfate

Concentration

(g/L)

Immersion time

(minutes)

0.1

2.5 1, 3

5 1, 3

10 1,3

15 1,3

0.1 + quinine

2.5 1, 1.5, 2

5 1, 1.5, 2

15 1,2

Table 18 Summary of test parameters for barrel plating tests of uninhibited and inhibited with 0.003g/L quinine copper flash solutions

3.3.3. Adhesion Tests

Adhesion tests were performed in the same manner as described in section 3.2.5.

3.4. Anti-oxidizing Agent

The anti-oxidizing agent used for the development of the final step in the copper

flash system was benzotriazole.

3.4.1. Beaker Tests Samples were first tested in a beaker-scale plating set-up. Samples were cleaned,

rinsed, copper flashed, rinsed again and immersed in various concentrations of

benzotriazole for varying times. The process, including all variables is outlined in Table

19. All solutions were stirred at 200 rpm using a Teflon stir bar and a Variomag

Electronicrührer Poly 15 stir plate for the duration of the procedure.

55

Solution Immersion time

Purpose Composition Volume (mL)

Cleaning

1 vol% H2SO4

0.003 g/L quinine

water

200 2 minutes

Rinse Water 50 10 seconds

Copper flash

0.1 vol% H2SO4

5 g/L CuSO4

0.003 g/L quinine

water

200 1 minute

Rinse Water 50 10 seconds

Anti-oxidant

Variable:

0.1 or 0.01 M

benzotriazole in water

40

Variable:

30 seconds,

60 seconds, or

90 seconds

Rinse Water 50 10 seconds

Table 19 Process outline for beaker testing of benzotriazole anti-oxidant coating

3.4.2. Batch Tests

The process for batch testing included cleaning and copper flash solutions

identical to those outlined in Table 19. Only the 0.01 M benzotriazole solution was

scaled up to a 6L bath. The immersion times for those tests were 60 and 90 seconds. A

batch of 10 samples was used for each test run. Those samples were rotated at 5 rpm in

the barrel plating chamber for the duration of the copper flash circuit.

56

3.4.3. Copper Tarnishing Tests

Copper tarnishing tests were performed on each of the samples produced in the

beaker tests and on a sampling of the batch tests to confirm that scaling up did not affect

the anti-oxidant coating.

Samples were tested in batches of three, in accordance with ISO 4538-1978

thioacetamide corrosion test [76]. A 1L glass desiccant container was modified for the

purpose of these experiments. Images of the set up may be found in

Appendix 4. A glass dish was placed at the base of the container to flatten the bottom.

Fisher brand filter paper was cut to size so it would sit flush against the sides of the

container. A high-walled glass dish was used to hold the thioacetamide and as a support

for the sample rack, which was constructed out of Lego. Finally, a stand for the dish and

rack was also constructed out of Lego. The samples were hung above a glass container,

which had been covered with a fine layer of thioacetamide.

A solution of three parts sodium acetate to one part water (by weight) was mixed

such that it would moisten the filter paper along the walls of the vessel and have an

excess of approximately 1 cm of solution along the bottom of the vessel. For the vessel

used, 60 g of sodium acetate and 20 g of water were mixed together to yield the required

amount of salt solution.

All samples tested were photographed in the light box setup prior to corrosion

testing, as outlined in section 3.2.4.

Samples were suspended with nylon monofilament approximately 1 cm above the

thioacetamide and approximately 2 cm apart. They were sealed in a gas-tight chamber

and left for 3 hours. At the end of that time, the samples were removed and once again

photographed.

The final parameter developed for batch testing was subjected to a longer

corrosion tests – both a 3 hour and a 6 hour test was performed on the samples plated in

0.01 M benzotriazole for 60 seconds, using the test methodology described above.

57

4. Results and Discussion

If this electroless barrel plating system were to be used in industry to apply a

copper flash, thousands of parts would be plated in each load. Thus it is important that

each step have an operating window wide enough to ensure total cleaning/plating of each

part without any parts becoming over-cleaned or the integrity of the coating being

compromised. One of the major goals of each step was to widen the operating window as

much as possible while maximizing the throughput.

The first step in the process to be optimized was the cleaning step. To be

consistent with the goal of broadening the operating window, samples should all be

thoroughly cleaned in a time frame of 2-5 minutes. Solution parameters were varied until

clean samples, as defined by samples that are free of both oxides and smut, were

produced. However, smut should be minimized even at longer cleaning times to allow

for a wide margin of error. As such, longer time frames were also used in the initial

development of the cleaning solutions.

4.1. Hydrochloric Acid Cleaning Solutions

Being one of the most common, readily available and inexpensive acids used in

the industry, hydrochloric acid was chosen as the first acid to examine during the

development of the cleaning circuit. Initial tests used a broad spectrum of conditions to

determine potentially useful concentration and time values. Preliminary tests were

conducted in the beaker set up described above for 5, 15 and 30 minutes. These values

were chosen to assess the extent of scale removal and smut formation over an extended

period of time. This would give an idea of the maximum achievable levels of smut

formation and the maximum possible operating window. To assess a wide variety of

parameters - various temperatures and acid concentrations were used. A summary of the

parameters used is shown in Table 14.

58

4.1.1. Uninhibited Hydrochloric Acid Solutions

4.1.1.1. Acid Concentration Variation

The acid concentration was varied from 5-20 vol%. A summary of the cleaning

conditions of samples cleaned at room temperature is shown in Table 20 below.

Temperature HCl Acid

Concentration

(vol%)

Time

5 minutes 15 minutes 30 minutes

25°C

5

Level 1

Level 2

Level 3

10

Level 2

Level 2

Level 3

20

Level 2

Level 3

Level 4

Table 20 Smut formation in varying acid concentration in an uninhibited hydrochloric acid solution The solution is quite aggressive at room temperature, and smutting occurred at all

acid concentrations within 5 minutes. By the smut index assigned to each sample, it was

found that for a given time, smut formation increases with increasing acid concentration.

Similarly, for a given acid concentration, smut formation increases with time.

At 5 minutes cleaning time, there was very little difference in the extent of

smutting between the samples cleaned in 5 vol% hydrochloric acid, and those cleaned in

20 vol% hydrochloric acid. This information indicates that a low acid concentration may

59

be used – allowing for easier waste management. It should be noted that none of the

above solutions are acceptable cleaning solutions, as all acid concentrations and all

cleaning times presented in Table 20 produced smutted samples.

To assess the viability of a hydrochloric acid cleaning solution, it was necessary

to develop a timeframe where no smutting occurred. Keeping all solutions at room

temperature, samples were cleaned for 10 seconds, 30 seconds, 1 minute and 2 minutes in

the three acid concentrations used above. The results are shown in Table 21.

HCl Acid

Concentration

(vol%/M)

Time

10 seconds 30 seconds 1 minute 2 minutes

5

No smutting

Level 1

Level 1

Level 1

10

No smutting

Level 1

Level 1

Level 2

20

Level 1

Level 2

Level 2

Level 2

Table 21 Timeframe determinations for hydrochloric acid cleaning solution at room temperature

For all of the concentrations, a thin layer of smut is formed between 10 and 30

seconds. For a beaker set up, this is an unacceptably low working time. Thus an

inhibitor should be added to the cleaning solution to obtain a wider operating window.

60

4.1.1.2. Temperature Variation

High temperature tests were performed for all acid concentrations at temperatures

of 50°C and 75°C. The results for the 5 vol% cleaning solution are summarized in Table

22 below, the remainder of the results may be found in Appendix 5.

Temperature HCl Acid

Concentration

(vol%)

Time

5 minutes 15 minutes 30 minutes

25°C 5

Level 1

Level 2

Level 3

50°C 5

Level 3

Level 4

Level 5

75°C 5

Level 5

Level 6

Level 6

75°C 20

Level 6

Level 7

Level 7

Table 22 Temperature variations for uninhibited hydrochloric acid cleaning solutions.

Increasing the temperature of a cleaning solution leads to higher cleaning rates [3-

8], so it is expected that smutting will occur more rapidly at high temperatures. As

predicted, the smut formed for a given time increases with increasing temperature. At

61

high temperatures, a dark smut begins to form at shorter times, as seen by comparing the

level of smut formation in 5 minutes at 25°C, 50°C and 75°C.

As the temperature increases, the concentration of the acid plays a greater role in

smut formation. Unlike the samples cleaned at room temperature, there is a significant

difference between the samples cleaned for any given time in 5 vol% and 20 vol%

hydrochloric acid at 75°C. At that temperature, samples cleaned in the low acid solution

did not smut as rapidly (within half a minute), and maintained a thin, black smut layer

even at longer times. However, samples cleaned in the high acid solution smutted very

rapidly (within seconds) and developed a thicker, lighter colored smut.

The best samples produced were those cleaned at low temperature. However,

even the samples cleaned at the lowest acid concentration formed a thin layer of smut on

the surface during the operating window of 2-5 minutes. This smut layer is detrimental

as it will interfere with adhesion of the subsequent copper coating, and should be reduced

as much as possible.

4.1.2. Inhibited Hydrochloric Acid Cleaning Solution Ideally, an uninhibited solution should be developed (as it would minimize the

organic species in the waste stream). However, no suitable results were obtained with

uninhibited hydrochloric acid solutions, so 0.03 g/L quinine was added to slow down the

cleaning mechanism. This inhibitor and concentration were chosen based on Awad’s and

Stemp’s work [31 and 41].

4.1.2.1. Temperature Variation

Quinine was reported to be an effective inhibitor in small quantities and at low

temperatures in hydrochloric acid solutions [31]. To test the viability of a quinine-

inhibited solution, a 10 vol% hydrochloric acid solution was mixed with 0.03 g/L quinine

and the temperature was varied with times of 5, 15 and 30 minutes. The results are

summarized in Table 23.

62

Temperature Time

5 minutes 15 minutes 30 minutes

25°C

Level 1

Level 2

Level 3

50°C

Level 3

Level 4

Level 5

75°C

Level 5

Level 6

Level 7

Table 23 Hydrochloric acid (10 vol%) cleaning solutions inhibited with 0.03 g/L quinine At 25°C, quinine was an effective inhibitor at short times – reducing the level of

smut formation from level 2 to level 1. However, it did not eliminate smut altogether.

All other samples were smutted, especially those at high temperature. This is partially

due to the expected breakdown of quinine at high temperature as reported by Awad [31]

and a greater rate of smut formation in acid cleaning solutions at high temperature (as

noted earlier in section 4.1.1.2). While hydrochloric acid could work as a potential

cleaning solution, it is unadvisable because of the likelihood of carryover of chloride ions

to the copper flash circuit, which is based on a sulphate chemistry. An inhibited

hydrochloric acid solution would risk contaminating the copper flash solution with both

chloride ions and inhibitor molecules and of producing a mixed chloride/sulphate media

that could be more aggressive than solutions containing each ion separately. Due to the

high risk of contamination, sulfuric acid cleaning solutions were examined as an

alternative to an inhibited hydrochloric acid solution.

63

4.2. Sulfuric Acid Cleaning Solution

A summary of the acid concentrations, temperatures and inhibitors used for the

sulfuric acid cleaning trials is summarized in Table 24 below. A laboratory grade

sulfuric acid was used in preparing the solutions.

Acid concentration

(vol %)

Temperatures tested

(°C)

Quinine

concentration

5% 25, 50 0 - 0.3 g/L

10% 25, 40 0 - 0.03 g/L

Table 24 Summary of inhibited and uninhibited sulfuric acid cleaning solutions

tested

4.2.1. Uninhibited Sulfuric Acid Solution

4.2.1.1. Temperature Variations Due to the short cleaning time required for the hydrochloric acid cleaning

solution, the initial trials for the sulfuric acid cleaning solutions were modified. Samples

were immersed for the times of 10s, and 30s, and 1, 2, 5, 15 and 30 minutes. Since the

results were poor for hydrochloric acid at high temperatures, the temperatures used for

sulfuric acid were lowered. Once again, the beaker set up was used for cleaning the

samples and all samples were imaged after cleaning.

Samples cleaned in 5 vol% sulfuric acid at 50 °C were heavily smutted. The

highest temperature to be tested was thus lowered for 10 vol% sulfuric acid to 40°C.

Once again, the components cleaned in uninhibited high temperature solutions were

heavily smutted and smutting began at very short time periods (i.e. within 30 seconds).

Images of samples cleaned at 10s, 30s, 1m and 2m are shown in Table 25, showing the

extent of smutting at short times. All images, including those for samples cleaned at 5

and 15 minutes are found in Appendix 6.

64

Temp Acid

Conc.

(vol%)

Time

10 seconds 30 seconds 1 minute 2 minutes

25°C

5

No smut

No smut

No smut

Level 1

10

No smut

No smut

Level 1

Level 2

40°C 10

No smut

No smut

No smut

Level 4

50°C 5

No smut

Level 1

Level 1

Level 5

Table 25 Uninhibited sulfuric acid cleaning solutions at room temperature and elevated temperatures

Tafel measurements were taken for samples cleaned in 1 vol% sulfuric acid and

the corrosion potential, Tafel slopes and corrosion current density were found. The data

is summarized in Table 26 and Figure 14. Based on the steady-state anodic Tafel slopes

calculated, the dissolution mechanism of iron in sulfuric acid is consistent with that

proposed by Heusler’s two-electron exchange “catalyzed” pathway described in section

2.2.1.3. It should be noted that the cathodic Tafel values are anomalously high, and as

such they may not be accurately analyzed from a mechanistic perspective.

65

Temperature

(°C)

Ecorr

(mV)

βa

(mV per

decade)

βc

(mV per

decade)

icorr

(mA/cm2)

25 -450 26 ± 5 220 ± 5 2.56

50 -439 30 ± 10 200 ± 10 20.3

75 -419 55 ± 10 240 ± 10 233

Table 26 Electrochemical data for uninhibited 1vol% sulfuric acid cleaning solution

Figure 14 Linear sweep data for uninhibited 1 vol% sulfuric acid cleaning solutions at 25°C, 50°C, and 75°C By the Nernst equation {6}, the potential should increase as temperature

increases. The kinetics would also increase, so an increase in io would also be expected.

The data from Table 25 for 25°C and 75°C was plotted in an Evans diagram; the results

are shown in Figure 15.

66

Figure 15 Evans diagram illustrating the effect of temperature increase As predicted, the corrosion potential and corrosion current density increased with

increasing temperature, seen in Figure 14 and Figure 15. An interesting characteristic on

the linear sweeps is at elevated temperatures there is a secondary peak in the cathodic

region of the reaction. Cavallaro [17] noted this type of behavior for iron in sulfuric acid,

but failed to explain it.

The peak causes a change in the Tafel behavior of the sweep. At room

temperature, the cathodic reaction has a Tafel slope of approximately 220 mV/decade.

Despite this value being anomalously high, the magnitude of the change in Tafel value

was examined. The cathodic reaction at that temperature is hydrogen evolution. At high

temperature, the cathodic Tafel slope changes from a value of approximately

220mV/decade to approximately 40mV/decade at potential values of -0.53V to -0.45V

(vs. Ag/AgCl). This is indicative of either a change in the cathodic reaction or a change

in the cathodic reaction mechanism.

At high temperatures, smut formation occurs very rapidly. While setting up the

Tafel extrapolation measurements, there was a brief period where the test specimen was

immersed in solution without any current flowing. It would be vulnerable to smut

67

formation during that time. Thus at high temperatures, there will be a higher smut

(silicon oxide) concentration on the surface of the test specimen than at room

temperature, where there is no bump. This change in surface composition could account

for the peak by changing the reaction mechanism of hydrogen evolution or by changing

the cathodic reaction from hydrogen evolution to oxygen reduction.

Bockris [14], and references therein] described two possible hydrogen evolution

paths for iron in acid. The first is a coupled discharge with the following two steps:

{38} H3O+ + e− → Hads + H2O

{39} H3O+ + Hads + e− → H2(g )

The rate determining step for this reaction mechanism is determined by the

cathodic Tafel value. If the cathodic Tafel is -2RT/F, then {38} is the rate determining

step. If the cathodic Tafel is -RT/2F then {39} is the rate determining step. Thus the rate

determining step changing from reaction {38} to reaction {39} would result in a decrease

of Tafel slopes by a factor of 4.

The second hydrogen evolution mechanism consists of an atomic combination

step:

{40} 2H3O+ + 2e− → 2Hads + 2H2O

{41} 2Hads → H2(g )

Reaction {40} is just reaction {38} doubled, thus the Tafel slope for that rate

determining step will still be -2RT/F. If reaction {41} is the rate determining step, the

Tafel slope will be -2RT/3F. The rate determining step changing from reaction {40} to

reaction {41} would result in a decrease of Tafel slopes by 3.

A final possibility is that due to the increase in silicon oxide on the surface of the

test specimen, the cathodic reaction may change altogether. Since the test cell was

aerated, it’s possible that the hydrogen evolution reaction occurring at low potentials

switched to an oxygen reduction reaction as the potential increased toward Ecorr. Bockris

[14] found that for surface oxides, and adsorbed oxygen in sulfuric acid, there is a

likelihood for the oxygen reduction reaction to occur at potentials greater than 0V.

68

However, this is unlikely to be the case, as the peak occurs at potentials between -0.53V

and -0.45V.

The most likely explanation is a change in the rate determining step of the

hydrogen evolution reaction as shown in reactions {38} and {39}. The change in the

magnitude of Tafel slopes found experimentally is approximately consistent with that

seen in the aforementioned model.

Further study of this phenomenon is beyond the scope of this thesis.

For Tafel extrapolations at high temperatures, the higher Tafel value was used in

determining the corrosion current density as it had a more clearly defined Tafel region.

Since high corrosion rate relates to rapid smut formation, high temperature

uninhibited solutions are clearly not desirable since smutting will occur very rapidly.

4.2.1.2. Acid Concentration Variations Images were taken of samples cleaned at 1, 2 and 5 minutes to determine the

extent of smut formation in the desired operating window. As with samples cleaned at

high temperature for hydrochloric acid, high levels of smutting was seen at higher

temperatures. As such, only the samples produced at 25°C are shown and described in

the summary of results in Table 27.

The low acid samples showed promising results, and an even lower acid

concentration was hypothesized to be a potential candidate for the final cleaning solution.

The system was thus scaled up and a large batch of samples was cleaned using the barrel

plating apparatus in a 1 vol% sulfuric acid solution for various time intervals. Weight

loss data was taken for 10% of each batch (five samples) and compared against the

weight loss data from samples cleaned in 5 vol% sulfuric acid.

An extensive weight loss study was performed for times ranging from 10 seconds

to 30 minutes for uninhibited 5 vol% sulfuric acid cleaning solutions. Using Faraday’s

law {7}, the weight loss data was converted to a corrosion current, and subsequently to a

corrosion current density. The results, shown in Figure 16, give a model for smut

formation on the surface of the fasteners.

69

Time Acid Concentration

5 vol% 10 vol%

1 minute

No smut

Level 1 smutting

2 minutes

Level 1 smutting

Level 2 smutting

5 minutes

Level 3 smutting

Level 4 smutting

Table 27 Variation of acid concentration in uninhibited sulfuric acid cleaning

solutions at 25°C

Figure 16 Weight losses converted to corrosion current density over time for

various uninhibited sulfuric acid cleaning solutions

70

Initial measurements were conducted in triplicate, in a beaker set-up with a 5

vol% sulfuric acid solution. The agitation for these samples was not as high as samples

produced in the barrel-plating set-up for 1 vol% samples. The barrel was loaded with 50

samples, and tumbled at 20 rpm for the designated time. The variation in samples and the

weight loss profile for uninhibited acid solutions is shown in Figure 16. The data shown

for 1 vol% is an average weight loss value for the 5 samples measured, and is given as a

approximate basis for comparison.

The comparison of 5 vol% to 1 vol% can not be directly made due to the different

cleaning conditions (barrel vs. beaker set-up). To confirm that there was in fact a greater

weight loss in the higher acid solution, data for uninhibited samples produced at 5 and 10

minutes in the barrel-plating set-up were added to Figure 16. Since the cleaning would

ultimately be performed in a barrel, it was essential that any tests involving mass transfer

were performed in both beaker and barrel scale.

From Figure 16, a cleaning profile may be derived. Initially, the corrosion current

density was very high. This is representative of the initial removal of oxides from the

substrate, and the dissolution of iron into solution. As time increases, the slope begins to

decrease. This is the period during which smut begins to form. Finally, at long times

there is a plateau in the corrosion current density due to the protective barrier of smut

reducing further dissolution of iron.

It can be seen that the corrosion current densities in the barrel set up for 1 vol%

are less than those for 5 vol% acid cleaning solutions in the barrel set up. For a given

time, weight loss is greater in higher acid concentrations. It has been noted that there is a

co-relation between weight loss, cleaning rate and smut formation [38]. As weight loss

rates increase, cleaning rates will increase and the onset of smut formation will occur

more rapidly. It is thus beneficial to use a lower acid concentration in the cleaning

solution, both to minimize the effluents in the waste stream and to offset smut formation.

By comparing the beaker and barrel results for 5 vol% acid, it is apparent that

agitation plays an important role in the cleaning rate. Increasing the agitation – such as

going from beaker set up (low agitation) to barrel set up (high agitation) causes an

increase in the cleaning rate of a given solution.

71

4.2.2. Inhibited Sulfuric Acid Solution To avoid smut formation, inhibited solutions were used. The inhibitor of interest

was quinine, as earlier work [41] had shown it to be effective in sulfuric acid.

4.2.2.1. Inhibitor Concentration Variation A summary of the quinine concentrations used may be found in Table 16. The

samples were cleaned for 5 and 10 minutes, with the goal of determining a timeframe for

the onset of smutting. These times were chosen because 5 minutes was the outer limit of

the optimum timeframe and 10 minutes was well past the optimum timeframe determined

for the initial 5 vol% inhibited cleaning solution. As an additional test of the degree of

cleaning, cleaned samples were copper plated and an adhesion test was performed.

In keeping with the desire to have a low-concentration waste stream, an attempt

was made to develop a low-acid concentration cleaning cycle. Using 1 vol % sulfuric

acid and three different concentrations of inhibitor, samples were cleaned for 2, 5, 10 and

15 minutes. Samples were then flashed with a Cu coating for 1 minute after being

cleaned for the desired amount of time. The results are summarized in Table 28 below.

The adhesion of the copper flash on these samples was measured qualitatively by

performing a peel test. The peel test evaluated the cleaning of the samples, and

confirmed that highly smutted samples have poor coating adhesion.

Uninhibited low acid samples (1 vol% acid) had smutting occurring at 5 minutes.

However, adhesion of the copper flash was adequate for samples cleaned up to 5 minutes,

even in the presence of thin layers of smut. While all the inhibited samples were

completely clean for the 2 and 5 minute cleaning duration, some minor smutting occurred

at 10 minutes for both inhibitor concentrations, causing minor adherence problems and at

15 minutes all samples were smutted and failed adhesion tests.

72

Quinine

Concentration

2 minutes

cleaning

5 minutes

cleaning

10 minutes

cleaning

15 minutes

cleaning

Uninhibited

No smutting

Level 1

Level 2

Level 3

Adhesion Adequate Adequate Mediocre Very poor

0.003 g/L

No smutting

No smutting

Level 1

Level 3

Adhesion Adequate Adequate Mediocre Poor

0.03 g/L

No smutting

No smutting

Level 1

Level 3

Adhesion Adequate Adequate Adequate Very poor

Table 28 Low acid cleaning using 1 vol% sulfuric acid and various quinine

concentrations, cleaned samples (top row) and copper coated samples

(bottom row)

73

It is interesting to note that higher quinine concentrations did not have better

inhibitive properties. Rather, the higher quinine concentration appeared to attack the

surface of the sample at higher cleaning times, causing very poor adhesion. It is possible

that poor adhesion is due to the inhibition mechanism of quinine, which adsorbs to the

substrate [31]. Further evidence that high concentrations of quinine were not desirable

(in relation to the acid concentration) can be found in the converted weight loss and

electrochemical data below.

Once again, weight loss measurements were conducted in a barrel set-up. The

weight loss measurements were converted to corrosion current density to give a cleaning

profile. All concentrations of quinine added decreased the corrosion current density

when compared to uninhibited solutions. However, past 0.003 g/L quinine appears to

behave as an aggressive species.

Figure 17 Corrosion current density (by converted weight loss) data for various

quinine concentrations in barrel-cleaned 1 vol% sulfuric acid cleaning

solution

When compared to an uninhibited solution the addition of quinine in any amount

lowered icorr and increased Ecorr, as illustrated in Figure 17 and Table 29.

74

Quinine

concentration

(g/L)

Ecorr

(mV)

βa

(mV/decade)

βc

(mV/decade)

icorr

(mA/cm2)

Uninhibited -450 26.3 ± 5 223 ± 10 2.56

0.003 -447 22.3 ± 5 175 ± 10 0.84

0.03 -438 15.8 ± 5 180 ± 10 1.23

0.3 -421 22.0 ± 5 216 ± 10 1.54

Table 29 Electrochemical data for variations in quinine concentration at 25°C

Figure 18 Evans diagram illustrating the effect of quinine on iron dissolution The inhibitive action of quinine appears to be limited to concentrations no greater

than 0.003 g/L. The mechanism is by simple adsorption, which will have the effect of

blocking active sites on the anode. This will cause a decrease in the exchange current

density (io) of both the cathode and anode. As seen in Table 29, the anodic Tafel slope

does not change significantly and the cathodic Tafel slope is slightly decreased in the

presence of quinine. Once again, the Tafel values are anomalously high; however the

trend in Tafel shift will be examined. The net result will be an increase in the corrosion

75

potential (Ecorr) and a decrease in the corrosion current density (icorr), as shown

schematically in Figure 18.

At concentrations above 0.003 g/L, the efficiency of quinine as an inhibitor in

sulfuric acid solutions decreases. While all quinine concentrations tested showed

inhibition of the cleaning solution and reductions in weight loss (relative to uninhibited

solutions), lower inhibitor concentrations were found to be more effective in reducing icorr

than higher inhibitor concentrations.

4.2.2.2. Temperature Variations Preliminary tests conducted by M. Stemp showed that quinine is not an effective

inhibitor at elevated temperatures [41]. To verify this finding, Tafel measurements and

weight loss measurements were performed on 0.03 g/L quinine inhibited solutions at

room temperature, 50°C and 75°C solutions. The corrosion current density was

calculated from weight loss measurements for each temperature in both inhibited and

uninhibited solutions, with results shown in Figure 19. The corrosion data is summarized

in Table 30.

Figure 19 Temperature dependence of corrosion rate in 0.03 g/L quinine inhibited sulfuric acid solution The overall effect of temperature increase on an inhibited sulfuric acid cleaning

solution is similar to that of uninhibited cleaning solutions shown in Figure 15; both Ecorr

76

and icorr increase with increasing temperature. From Figure 19 it can be seen that quinine

does inhibit the solution at high temperatures as well as low temperatures.

Temperature

(°C) Inhibited?

Ecorr

(mV)

βa

(mV per

decade)

βc

(mV per

decade)

icorr

(mA/cm2)

25 No -450 26.3 ± 5 223 ± 10 2.56

Yes -438 15.8 ± 5 180 ± 10 1.23

50 No -439 30.2 ± 5 197 ± 10 20.3

Yes -443 15.6 ± 5 106 ± 10 9.38

75 No -419 54.5 ± 5 241 ± 10 233

Yes -421 27.3 ± 5 138 ± 10 86.1

Table 30 Electrochemical data for temperature variations in uninhibited and 0.03 g/L quinine inhibited sulfuric acid solution

Awad [31] found that in hydrochloric acid solutions at 50°C (Table 7) there was

little change in the Tafel behavior of inhibited and uninhibited solutions. He concluded

that the addition of quinine does not change the reaction mechanism of hydrogen

evolution or iron dissolution and that the inhibition mechanism was by simple blocking

of the surface by physisorption.

In sulfuric acid, the method of inhibition is also by physisorption, as supported by

the calculated activation energy value of ~ 10kJ/mol, which falls in the physisorption

range. The quoted activation energy value was calculated based on the plotted slope of

log icorr vs. 1/T. Contrary to Awad’s observations, in inhibited sulfuric acid solutions at

high temperature both the cathodic and anodic Tafel slopes change compared to

uninhibited solutions.

77

Figure 20 Linear sweep data performed in inhibited solutions at 25°C, 50°C and 75°C

As with the uninhibited solutions, a secondary peak appeared in the

cathodic reaction of samples tested at high temperature for inhibited solutions, as seen in

Figure 20.

4.2.2.3. Acid Concentration Variations

Similar to uninhibited solutions, the corrosion rate increased with increasing acid

concentration. Adequate cleaning was achieved at considerably low acid concentrations

– as low as 1 vol%. Since cleaning had already been proved possible at those

concentrations no further testing on high acid concentration was performed.

4.3. Copper Flash

4.3.1. Preliminary cleaning solution adhesion tests In order to test the feasibility of the inhibited 5 vol% sulfuric acid cleaning

solution, a copper flash was plated onto cleaned samples and the coating was compared

78

to the MacDermid process in terms of appearance and adhesion. If the copper coating had

adequate adhesion to the cleaned substrate and a uniform appearance, then the cleaning

process was deemed acceptable to be scaled up to a 6 L tank, and no further development

of the cleaning process would be necessary. Results are discussed below.

Since the MacDermid process used as a baseline for comparison is an additive

process, the copper flash step could not be isolated. It was thus necessary to develop a

preliminary copper flash solution.

The copper flash solution used was a 5 vol% sulfuric acid + 15g/L copper sulfate.

This chemistry had been proven to yield acceptable results in previous work completed

by M. Stemp [41]. Initially, the samples were cleaned for 4 minutes followed by a 5-

minute immersion in the copper flash. The flash time was chosen to match that of the

copper flash step in the MacDermid process. Adhesion to the substrate was very poor,

with heavy smutting occurring underneath the copper coating and a highly stressed

coating that flaked off during the drying process. To test whether the smutting was due

to the cleaning process or the flash, an as-received sample was immersed in the copper

flash solution for 5 minutes. Once again, heavy smutting occurred on the substrate

resulting in poor adhesion. The copper flash solution chosen was yielding highly stressed

coatings, with acid undercutting of the substrate – causing smutting to occur after the

cleaning process.

However, since the solution had been proven to yield adequate coatings, the

timeframe of the copper flash was lowered. A new sample was cleaned in the inhibited

sulfuric acid solution, and then immersed in the copper flash solution until the sample

was completely coated with copper (approximately 30 seconds). The adhesion of the

copper flash was greatly improved by short flash times. Longer flash times resulted in

acid undercutting and smutting of the substrate, as well as highly stressed coatings that

would flake off during drying, all of which contributed to poor adhesion.

All results may be seen and are discussed in Table 31, below.

The inhibited sulfuric acid cleaning solution proved suitable as a cleaning step

before copper plating. The copper flash solution used yielded highly stressed, non-

adherent samples both on cleaned and as-received samples, suggesting that the flash

solution was likely problematic, not the cleaning solution. By decreasing the flash time

79

from 5 minutes to 30 seconds, the adhesion and appearance of the coating greatly

improved.

Plating Condition and Results Resulting coating

MacDermid – no polish

The sample is bright and shiny, with a uniform Cu coating.

MacDermid – polished

The sample is not as bright as the unpolished sample. This may

be due to unclear processing instructions provided by

MacDermid. The amount of polish required was not specified.

MacDermid – polished (second trial)

A second polished sample was produced. Once again, it is not

as bright as the unpolished sample.

H2SO4 + Quinine (4min) + CuSO4 flash (5min)

The substrate was smutted by the Cu flash solution, causing a

highly stressed coating with very poor adhesion.

CuSO4 flash (5min)

Without cleaning, the Cu flash solution still caused smutting of

the underlying substrate, a highly stressed sample, and poor

adhesion.

H2SO4 + Quinine (4min) + CuSO4 flash (30 seconds)

By decreasing the time of the Cu flash, it was possible to plate a

bright, uniform coating with good adhesion.

Table 31 Initial copper flash trials used to prove suitability of inhibited sulfuric

acid cleaning solution.

80

4.3.2. Beaker Tests

For complete beaker test results, see Appendix 8. A selection of the results will

be presented in the sections below.

4.3.2.1. Acid Concentration Variation Shown in Table 32 are the acid concentration variation results for 15 g/L copper

sulfate. While a range of copper sulfate concentrations and acid concentrations were

tested the results show the overall trend of acid variation.

Table 32 Sulfuric Acid concentration variations in a beaker set-up with 15g/L copper sulfate copper flash solution All 15 g/L copper sulfate samples had a uniform copper coating at 30 seconds. At

longer immersion times, there appeared to be undercutting of the substrate, causing very

Copper Sulfate

Concentration

(g/L)

Acid

Concentration

(vol%)

Time

30 seconds 60 seconds 90 seconds

15

0.01

Adhesion Poor Poor Very poor

0.1

Adhesion Adequate Adequate Poor

1

Adhesion Adequate Poor Very poor

81

poor adhesion of the copper coating and smutting of the substrate. In some cases, the

coating flaked off during drying. The adhesion of each sample was tested using a peel

test, and the adhesion was rated as adequate (no transfer of copper coating to tape), poor

(some transfer) and very poor (nearly complete transfer of copper coating to tape).

Both low and high acid concentration had poor adhesion. The role of the acid in

this solution is to maintain the pH at a range in which both copper and iron ions will

dissolve (pH = 0 - 4). Too low a concentration will not be in the ideal pH range, while

too high an acid concentration will result in an aggressive attack on the cleaned substrate

during copper plating. The best samples were produced at 0.1 vol% sulfuric acid. This

concentration was chosen for scaling up to the barrel plating process.

4.3.2.2. Copper Sulfate Concentration Variation As seen in Table 33, an increase in copper sulfate concentration increased the

plating rate of the copper coating. By the Nernst equation {6}, it can be seen that as the

concentration of copper sulfate (reactant) increases, the potential of the reaction

increases. Higher reaction potentials have a higher driving force than lower reaction

potentials, therefore the reaction has a higher driving force at high copper sulfate

concentrations. As the surface coverage begins to increase (i.e. the copper deposits), the

potential will decrease due to a lower availability of iron from bare steel. The reaction

will begin to slow down, and continue to slow until equilibrium is reached. As such,

immersion coating will never yield a coating with complete surface coverage.

Samples produced at low copper sulfate concentrations (5 g/L) took up to 90

seconds to plate evenly while samples with higher copper sulfate concentrations (30 g/L)

took as little as 30 seconds to plate evenly. At longer plating times, the undercutting of

the substrate was still occurring, so for this system the ideal plating time would be no

longer than 30 seconds. This is an unacceptably short time frame for bulk production,

however in developing the cleaning solution it was noted that the tumbling motion of

barrel plating extended the operational window from beaker plating to barrel plating and

it is likely that operational window for copper plating time will be similarly extended. To

82

increase the plating time, the lowest copper sulfate concentration solution should be used.

As such, barrel tests began with a copper sulfate concentration no greater than 5 g/L.

Acid

Concentration

(vol%)

Copper Sulfate

Concentration

(g/L)

Time

30 seconds 60 seconds 90 seconds

0.1

5

Adhesion Adequate Adequate Poor

15

Adhesion Adequate Poor Poor

30

Adhesion Adequate Poor Poor

Table 33 Copper sulfate concentration variations in a 0.1 vol% sulfuric acid copper flash solution prior to adhesion tests with adhesion test results

4.3.3. Inhibited Copper Flash Tests The above copper sulfate solutions had a very narrow operating window. In an

effort to increase the possible plating time, an inhibitor was used. Since quinine is

already present in the cleaning solution, it was the first choice for an inhibitor in the

copper plating step. An initial concentration was chosen to match the ideal concentration

of the cleaning solution, so 0.003g/L quinine was used for preliminary testing. Results

are summarized in Table 34.

The addition of quinine to the copper flash solution helped keep the coating from

flaking off during drying and appeared to have no negative impact on the integrity of the

83

coating. The adhesion of the coating was improved at the low copper sulfate

concentration, but was worse at the high concentration. An inhibited solution should be

tested in the barrel set-up for the final plating process to evaluate the inhibitor’s ability to

increase the operating window of the copper flash step.

Table 34 Samples and adhesion results for variations in copper sulfate concentrations of copper flash solutions inhibited with quinine

4.3.4. Barrel Plating Tests

4.3.4.1. Copper sulfate concentration variations

To assess the viable operating window of the system, various copper sulfate

concentrations were tested at one and three minutes. A sample of each batch is shown in

Table 35 and a larger sample selection may be found in Appendix 9. Solutions

containing 2.5, 5, 10 and 15 g/L were tested.

A concentration of 5 g/L copper sulfate gave the most uniform coating after one

minute, however the operating window for that concentration was very narrow. At three

minutes, all of the samples showed poor adhesion with areas that had been knocked off

Quinine

Concentration

(g/L)

Copper Sulfate

Concentration

g/L

Time

30 seconds 60 seconds 90 seconds

0.003

15

Adhesion Adequate Adequate Very Poor

30

Adhesion Poor Very Poor Very Poor

84

by the parts tumbling in the barrel. Upon drying, the highest copper sulfate

concentrations used (10 and 15 g/L) showed complete delamination of coatings plated for

3 minutes and some delamination of coatings plated for 1 minute. To increase the

operating window of the copper flash, an inhibited solution was tested.

Copper Sulfate

Concentration

(g/L)

Flash time

1 minute 3 minutes

2.5

5

10

15

Table 35 Batch tests of copper sulfate concentration variations in an uninhibited copper sulfate flash solution

85

4.3.4.2. Inhibitor concentration variations Quinine was used as an inhibitor in a copper flash solution with 0.1 vol% sulfuric

acid and 15 g/L copper sulfate. Two inhibitor concentrations were tested at one and three

minutes. Results are summarized in Table 36 with expanded results in Appendix 10.

At low concentrations (0.003 g/L) quinine improved the adhesion of the coating

when compared to uninhibited, barrel plated copper coatings. The extent of coating

knocked off by the tumbling of parts against each other was decreased with the addition

of quinine in low concentrations.

As seen in the development of the cleaning solution, past a threshold value

quinine became an aggressive ion. This holds true for quinine in copper sulfate plating

solutions as well. At concentrations of 0.03 g/L, the amount of coating knocked off by

tumbled parts was comparable to uninhibited flash solutions and greater than solutions

inhibited with 0.003 g/L quinine.

Quinine

Concentration

(g/L)

Flash time

1 minute 3 minutes

0.003

0.03

Table 36 Quinine concentration variations in 5 g/L copper sulfate copper flash batch tests

86

The addition of low concentrations of quinine decreased the amount of coating

knocked off, and had a beneficial effect on the adhesion of the coating. However, the

operating window of the copper flash was not extended past 1 minute.

To obtain a wider operating window, an inhibited solution with lower copper

sulfate concentrations was used. At very low concentrations – 2.5 g/L – the operating

window ranged from 60 to 90 seconds, with the ideal time being 60 seconds. The

adhesion was adequate up to 60 seconds, and at 90 seconds there were minimal areas

where the coating had been knocked off the substrate.

Table 37 Copper sulfate concentration variations and rpm variations in inhibited copper flash solution

Copper Sulfate

Concentration

g/L

Time

30 seconds 60 seconds 90 seconds 120 seconds

2.5

NA

Adhesion Adequate Adequate Poor

5

NA

Adhesion Adequate Poor Very Poor

5 – slow rpm NA NA

Adhesion Adequate Adequate

87

1. a) b) c)

2. a) b) c)

3. a) b) c)

4. a) b) c)

Figure 21 SEM images at (a) 250X and (b) 500x magnification of samples as well as (c) surface mapping of samples plated under the following conditions: 1) uninhibited, 5.0 g/L CuSO4 for 60s, 2) inhibited, 2.5 g/L CuSO4 for 60s, 3) inhibited, 5.0 g/L CuSO4 for 60s and 4) inhibited, 5.0 g/L CuSO4 for 90s at slow RPM Further tests were conducted in 5 g/L copper sulfate solution at a lower rpm (5

rpm) for the barrel plater to see if knock-off could be further minimized. This was

Fe substrate

Fe substrate

Fe substrate

Fe substrate

88

successful in increasing the possible operating window. Samples had no knock off at 90

seconds, and very minimal knock off at two minutes. From Table 37, it can be seen that

the optimal plating parameter is at a slow rpm and 5 g/L copper sulfate. SEM images

confirmed that inhibited copper flash solutions yield coatings with smooth morphology

and good surface coverage.

The above SEM images show the surface morphology of uninhibited and

inhibited samples. The images in part c) of each sample had their contrast increased to

emphasize the steel substrate areas, which appear as darker patches on the image.

As seen in Figure 21 1c-4c the surface coverage was lowest for the sample plated

in the uninhibited solution for 60 seconds. This poor coverage is likely due to knock-off

from the tumbling in the barrel plating system. As observed earlier, when an inhibitor is

added to the plating solution, the knocking off of the copper coating is reduced. This is

confirmed by the high surface coverage observed in Figure 21 2c and 3c. The sample

coated using a slow rpm, 4a-c, had a rougher surface morphology and slightly less

surface coverage than samples 2 and 3. However, adhesion tests showed that it had

adequate adhesion, and it the operating window for that condition is higher than that of

previous conditions (i.e. samples 1-3).

Based on the adhesion and surface morphology results, the slow rpm (5rpm vs. all

others plated at 20 rpm) and inhibited solution have successfully widened the operating

window to 60-120 seconds.

4.4. Anti-oxidizing agent

4.4.1. Beaker Tests

Beaker tests were performed for initial determination of an anti-oxidant coating

that would provide resistance to general atmospheric corrosion.

The anti-oxidant of interest was benzotriazole, and solutions of two different

concentrations were mixed. Many stepwise plating procedures use a neutralizing dip

(alkaline for an acidic plating solution and acidic for an alkaline plating solution) before

the anti-oxidant step [58, 66]. Two batches of samples were produced: samples rinsed in

89

water and then dipped in anti-oxidant and samples dipped in an alkaline solution

followed by an anti-oxidant dip.

These samples were then tested in the corrosion chamber as per the test outline

described in section 3.4.3 and

Appendix 4 to determine the effectiveness of the anti-oxidant coating. Images of the

coatings before and after corrosion testing were examined for any indication of corrosion

products. The results of the coatings without an alkaline dip are shown in Table 38

below and with an alkaline dip are shown in Table 39.

Immersion time in anti-oxidant

(seconds)

Corrosion testing

Anti-oxidant solution concentration

No coating 0.01 M 0.1 M

Without alkaline dip

30

Before

After

60

Before

After

90

Before

After

Table 38 Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-oxidant solutions without preliminary alkaline dip

90

Immersion time in anti-oxidant

(seconds)

Corrosion testing

Anti-oxidant solution concentration

No coating 0.01 M 0.1 M

With alkaline dip, pH = 9

30

Before

After

60

Before

After

90

Before

After

Table 39 Summary of corrosion test results for 0.01M and 0.1M benzotriazole anti-oxidant solutions with preliminary alkaline dip As seen in Table 38 and Table 39, the samples without the alkaline dip proved to

have better corrosion resistance than those that had been dipped prior to anti-oxidant

treatment. The alkaline dip prior to the benzotriazole dip appeared to cause minor

oxidation of copper coating leading to a decrease in the luster of the coating.

Of the two different concentrations of benzotriazole used, both concentrations

were equally effective. Increased immersion time lead to better corrosion results,

however the luster of the coating was compromised, and the surface finish was not as

91

glossy when dipped in the benzotriazole rinse for 90 seconds as compared with samples

dipped for 60 seconds.

Since one of the goals of this thesis is to minimize the ionic and organic species

present, the ideal conditions are 0.01 M benzotriazole dip, no alkaline rinse and 60

second immersion time. Those parameters will be used in scaling up to a 6L bath for

final line testing.

4.4.2. Batch Tests

Using 6L basins of each solution, the cleaning, copper flash and benzotriazole dip

was applied to a batch consisting of 10 samples and rotated at 5 rpm. The process is

summarized in section 3.4.2.

Two batches of samples were produced at a dip time of 60 seconds, and one batch

was produced at a dip time of 90 seconds. The batches produced with a shorter dip time

had a more uniform finish. Accelerated corrosion tests were performed as per section

3.4.3 and

Appendix 4 on coated samples for 3 and 6 hours, with results shown in Table 40.

The samples produced at with a 60 second benzotriazole dip had very good

corrosion resistance at a 3 hour test period when compared to samples without the dip.

At a 3 hour test period, there was considerable localized corrosion, but not as uniform as

the corrosion of the untreated sample. Samples with a 90 second benzotriazole dip also

had good corrosion resistance, but the non-uniformity of the surface before the corrosion

testing made it less desirable than the 60 second dip.

After 6 hours of testing, there was considerable corrosion on both as-plated

(without dip) and anti-oxidized samples. At that testing time, samples with the

benzotriazole dip still had better corrosion resistance than the as-plated samples.

92

Benzotriazole dip time

Corrosion test

Test time Before testing After testing

60 seconds 3 hours

90 seconds 3 hours

NO DIP 3 hours

60 seconds 6 hours

NO DIP 6 hours

Table 40 Corrosion test results for benzotriazole dip batch tests.

4.5. Developed System

A summary of the parameters for the developed system compared to the

MacDermid process is outlined in Table 41.

93

Solution

Step

Developed System MacDermid Process

Composition

(bulk water)

Immersion

time

Solution

added

Tumbling

time

Cleaning 1 vol% H2SO4

0.003 g/L quinine 2-5 minutes

Macuguard

SCF +

Foamout

12 minutes

Rinse Water 10-30 seconds N/A N/A

Copper flash

0.1 vol% H2SO4

5 g/L CuSO4

0.003 g/L quinine

1-2 minutes Macuguard

XL 40 8 minutes

Rinse Water 10-30 seconds Water rinse

and drain N/A

Anti-oxidant 0.01 M benzotriazole 60 seconds Relubro165

polisher 1-2 minutes

Rinse Water 10-30 seconds Water rinse

and drain N/A

Table 41 Summary of electroless copper flash system developed for a room temperature barrel plating system All steps are conducted at room temperature, with a 50% barrel loading (barrel is

half-full) and a barrel rotation speed of 5 rpm. The total plating time of the developed

system was approximately 5-10 minutes while the MacDermid process was 21-22

minutes, not including the time taken to drain the solutions between plating and polishing

steps. Furthermore, all solutions developed are re-useable, as opposed to the single use

solutions in the MacDermid step. Thus by implementing this sustainable process, it is

possible to increase the throughput of parts by 2-4 times and the life of the solution is

extended.

94

5. Conclusions

An electroless copper coating system was developed with the intent of

minimizing the ionic and organic concentrations of each step in the process. A cleaning,

plating and anti-oxidizing process was developed using only four species, and in the

minimum possible concentrations. This process also allowed for the widest possible

operating window for each step.

Contrary to claims that quinine was an ineffective inhibitor in sulfuric acid [31],

through weight loss and electrochemical measurements it was found to be an effective

inhibitor in low concentrations. At high concentrations, it became an aggressive ion.

Quinine was also found to aid in the coating integrity and expand the operational

window when added in small concentrations to copper sulfate flash solutions.

Benzotriazole proved to be an effective inhibitor against atmospheric corrosion

for copper coatings, even in concentrations as low as 0.01M.

95

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99

Appendices

Appendix 1 Corrosion Current Density Sample Calculation

Polarization Resistance:

A range of ± 10 mV from Ecorr was used to calculate polarization resistance. The

raw data was plotted in that range and the polarization resistance was calculated by

equation {23}

Rp =Δε

Δiapp

Figure 22 Polarization resistance for iron in 0.03 g/L quinine inhibited sulfuric

acid solution at room temperature

0.43 — 0.4389 1

0.0088.9

8.99 Ω/

100

The cathodic and anodic Tafel slopes were calculated;

Figure 23 Tafel extrapolation data for a 0.03 g/L inhibited sulfuric acid cleaning solution at 25°C

Figure 24 Cathodic Tafel area from Figure 23 0.60 — 0.573 5

0.150 /

101

The anodic Tafel slope was calculated in a similar manner;

Figure 25 Anodic Tafel area for Figure 23 0.410 — 0.424

8 1

0.017 /

Then icorr was determined by applying equation {24}

2.3

0.017 0.152.3 8.9 0.017 0.15

0.00165 /cm2

1.65 /cm2

icorr calculated automatically by CHI: 1.23 mA/cm2

102

Appendix 2 Raw Electrochemical Data

Sample

RP

(Ω/cm2)

βa

(mV/dec)

βc

(mV/dec)

Ecorr

(mV)

Icorr

(mA)

Anode

area

(cm2)

icorr

(mA/cm2)

25°C

uninh 176 26.3 223 -450 0.196 0.0766 2.56

25°C

0.003 532 22.3 175 -447 0.0253 0.0302 0.84

25°C

0.03 356 15.8 180 -438 0.0804 0.0654 1.23

25°C 0.3 273 22.0 216 -421 0.126 0.0817 1.54

50°C

uninh 21.6 30.2 197 -439 1.29 0.0635 20.3

50°C inh 47.2 15.6 106 -443 0.580 0.0620 9.38

75°C

uninh 1.79 54.5 241 -419 12.9 0.0552 233

75°C inh 4.89 27.3 138 -421 5.64 0.0655 86.1

Table 42 Raw electrochemical data

103

Appendix 3 Amercoat™ 90 Epoxy Application Procedure

The following procedure was taken from the Procedure Manual found in the Surface

Engineering and Electrochemistry Lab, MB 209, University of Toronto. It was initially

prepared by Dave Anthony for S.J. Thorpe. All mixing and application of Amercoat™

must be performed in a fumehood with proper gloves and eye protection as it contains

Xylene as a solvent. Xylene vapor is both toxic and flammable.

1. Prepare a mixture of resin (white) and curing agent (translucent) in a ratio of 4:1. A

quantity of 2 mL resin and 0.5 mL curing agent is sufficient to coat 10 small anodes.

2. Open the Amercoat™ resin container and thoroughly mix the resin by hand, until no

further separation of the solvent is apparent. Once the resin is mixed, the stir bar

should be cleaned with a paper towel that is to be discarded in the fumehood.

3. Using a syringe, extract 2 mL of resin and expel into a clean weigh boat. Discard the

syringe into the waste basket in the fumehood.

4. Repeat steps 2 and 3 with the Amercoat™ curing agent, taking care to extract the

correct amount of curing agent to meet the 4:1 ratio requirements.

5. Once the curing agent and resin have been added to the weigh boat, use the end of

one of the syringes to mix the two components thoroughly.

6. Using a fine bristle brush, apply a thin coating of epoxy to the sample. Make sure to

coat all conductive surfaces that are not to be tested.

7. Using a stereomicroscope, examine the sample 1 hour after plating and re-touch if

necessary.

8. After coating the sample, it must be left in the fumehood for 24 hours to ensure the

Amercoat™ 90HS is completely cured.

9. Clean the brush thoroughly with isopropyl alcohol (IPA) after use.

10. Repeat steps 1 through 9 until 3 separate coats of Amercoat™ 90HS have been

applied to each sample. A minimum of 24 hours drying time is required between

each coating.

104

Appendix 4 Corrosion Test Set Up Corrosion tests were performed in accordance with the ISO standard ISO 4538-

1978: thioacetamide corrosion test.

This test specifies a chamber with absorbent paper along the bottom and sides.

For that purpose, Fisher brand filter paper was used to line the bottom and sides with a

3:1 (by weight) sodium acetate solution. The filter paper is not shown in the images

below, so that the framework may be seen.

Figure 26 Corrosion tests set up prior to addition of filter paper and chemicals

105

Figure 27 Bird’s eye view of sample holder without lid for corrosion testing

Samples

106

Appendix 5 Uninhibited Hydrochloric Acid Solution Samples

Temperature Acid

Concentration

(vol%)

Time

5 minutes 15 minutes 30 minutes

25°C

5

10

20

50°C

5

10

20

75°C

5

10

20

Table 43 Complete uninhibited hydrochloric acid test results

107

Appendix 6 Uninhibited Sulfuric Acid Samples Temp Acid

Conc.

(vol%)

Time

10

seconds

30

seconds

1

minute

2

minutes

5

minutes

15

minutes

25°C

5

No smut

No smut

No smut Level 1

Level 3

Level 4

10

No smut

No smut

Level 1

Level 2

Level 4

Level 5

50°C 5

No smut

Level 1

Level 1 Level 5

Level 6

Level 6

40°C 10

No smut

No smut

No smut Level 4

Smut formation very

high at 2 minutes. No

further tests

conducted

Table 44 Temperature and acid variations in uninhibited sulfuric acid cleaning solutions

108

Appendix 7 Inhibited Sulfuric Acid Solution Samples

Quinine

Concentration

Cleaning Only 1 min Cu Flash

5 min 10 min 5 min 10 min

No quinine

0.0003 g/L

0.003 g/L

0.03 g/L

0.3 g/L

Table 45 Inhibited sulfuric acid solution samples

109

Appendix 8 Complete Copper Bath Beaker Test Results

Acid

Concentration

(vol%)

Copper Sulfate

Concentration

(g/L)

Time

30 seconds 60 seconds 90 seconds

0.01

5

15

30

0.1

5

15

30

1

5

15

30

Table 46 Complete summary of beaker tests in copper flash development

110

Appendix 9 Variation of Copper Sulfate in Batch Copper Flash Tests Copper sulfate

concentration

(g/L)

Time

(minutes) Samples

2.5

1

3

5

1

3

10

1

3

15

1

3

Table 47 Samples from each batch of varying copper sulfate concentration copper flash batch tests

111

Appendix 10 Summary of Inhibited Copper Flash Batch Tests

CuSO4 Quinine Time Samples

2.5 g/L 0.003 g/L

60s

90s

120s

5 g/L 0.003 g/L

60s

90s

120s

15 g/L

0.003 g/L

60s

180s

0.03 g/L

60s

180s

Table 48 Copper sulfate flash solutions with 1 vol% sulfuric acid inhibited with varying concentrations of quinine, flashed for various times