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EFFECT OF CORROSION INHIBITOR AND LASER SURFACE TREATMENT ON CORROSION BEHAVIOR OF STEEL USED IN CHILLING SYSTEM by LEONG HOI SAN Master of Science in Electromechanical Engineering 2011 Faculty of Science and Technology University of Macau
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EFFECT OF CORROSION INHIBITOR AND LASER
SURFACE TREATMENT ON CORROSION
BEHAVIOR OF STEEL USED IN CHILLING SYSTEM
by
LEONG HOI SAN
Master of Science in Electromechanical Engineering
2011
Faculty of Science and Technology
University of Macau
EFFECT OF CORROSION INHIBITOR AND LASER SURFACE
TREATMENT ON CORROSION BEHAVIOR OF STEEL USED IN
CHILLING SYSTEM
by
LEONG HOI SAN
Master of Science in Electromechanical Engineering
Faculty of Science and Technology
University of Macau
2011
Approved by _________________________________________________
Supervisor
_________________________________________________
_________________________________________________
_________________________________________________
Date _________________________________________________________
In presenting this thesis in partial fulfillment of the requirements for a Master's degree
at the University of Macau, I agree that the Library and the Faculty of Science and
Technology shall make its copies freely available for inspection. However,
reproduction of this thesis for any purposes or by any means shall not be allowed
without my written permission. Authorization is sought by contacting the author at
Address:
Telephone: 853-66314890
Fax: N/A
E-mail: [email protected]
Signature _____________________
Date _________________________
University of Macau
Abstract
EFFECT OF CORROSION INHIBITOR AND LASER SURFACE
TREATMENT ON CORROSION BEHAVIOR OF STEEL USED IN
CHILLING SYSTEM
by Leong Hoi San
Thesis Supervisor: Prof. Kwok Chi Tat
Electromechanical Engineering
Chilled water is a popular cooling medium in HVAC system, which is always distributed to
air handling units through the pipes. However, a seriouscorrosion problem has been found for
the piping system if no preventative measure is taken. In order to mitigate the corrosion of
pipes, the application of corrosion inhibitor and laser surface melting on black steel were
attempted.
Black steel samples were immersed in tap water medium at various temperatures (5 and 13
oC), and various concentrations of sodium nitrite-based corrosion inhibitors for investigation.
This study concerns the assessment of the inhibition effect by using potentiodynamic
polarization method and microstructure analysis. In this thesis, sodium nitrite-based corrosion
inhibitor was found to be significantly reduced the corrosion rate of specimens in tap water
and formed a protective coating in-situ by the reaction between the solution and the steel
surface. In addition, it was observed that this improvement was related to the concentration of
inhibitor and optimum concentration was evaluated.
On the other hand, influence of laser surface melting of black steel on both hardness and
corrosion behavior was also studied. The microstructures of the as-received and laser-treated
samples were characterized by using optical and scanning electron microscopy with hardness
testing techniques, in various experimental conditions: with different laser beam density and
scanning speed. Furthermore, investigation was also carried out on the corrosion
characteristics of laser-treated specimens in 0.9 wt% NaNO2 solution and tap water at 13 oC
In the case of laser surface melting, most of specimens show improved corrosion resistance
and refinement of microstructure with hardness increment. In fact, the hardness and corrosion
characteristic of all laser-treated specimens are strongly dependent on the scanning speed and
power density of the laser beam, which in turn result in different microstructures.
i
TABLE OF CONTENTS
List of Figures ................................................................................................................v
List of Tables ............................................................................................................. viii
List of Abbreviations ................................................................................................... ix
Chapter 1: INTRODUCTION........................................................................................1
1.1 Overview of Chiller System.....................................................................................1
1.2 Piping System of Chilled Water ..............................................................................5
1.2.1 Working Condition of Chilled Water Piping System .....................................5
1.2.2 Materials of Chilled Water Piping System .....................................................5
1.3 Corrosion in Chilled Water Piping System ..............................................................6
1.4 Corrosion Prevention and Control ...........................................................................7
1.5 Laser Surface Modification ......................................................................................7
1.6 Objectives ................................................................................................................8
Chapter 2: LITERATURE REVIEW...........................................................................10
2.1 Corrosion Principle ................................................................................................10
2.1.1 Costs of Corrosion ........................................................................................10
2.1.2 Corrosion of Metallic Materials ....................................................................11
2.1.3 Electrochemical Nature of Aqueous Corrosion ............................................11
2.1.4 Electrochemical Reactions ............................................................................12
2.1.5 Corrosion Potential .......................................................................................14
2.1.6 Passivity ........................................................................................................15
2.1.7 Forms of Corrosion .......................................................................................16
2.2 Corrosion Rate Determination ...............................................................................17
2.2.1 Corrosion Rate ..............................................................................................17
2.2.2 Mixed Potential Theory ................................................................................18
2.2.3 Principle of Corrosion Test ...........................................................................19
2.2.4 Corrosion Rate Measurements ......................................................................20
2.3 Corrosion Forms in Chilled Water Piping System ................................................21
2.3.1 General Corrosion .........................................................................................21
ii
2.3.2 Pitting Corrosion ...........................................................................................22
2.3.3 Erosion Corrosion .........................................................................................23
2.3.4 Galvanic Corrosion .......................................................................................24
2.3.5 Corrosion Under Insulation (CUI) ................................................................27
2.4 Corrosion Inhibitors in Corrosion Control .............................................................29
2.4.1 Corrosion Inhibitor........................................................................................30
2.4.2 Applications on Chilled Water Piping System .............................................31
2.5 Laser Surface Modification ....................................................................................33
2.5.1 Laser Induction .............................................................................................33
2.5.2 Application of Laser Surface Modification ..................................................34
2.5.2.1 Laser Transformation Hardening (LTH) .............................................36
2.5.2.2 Laser Surface Melting (LSM) ..............................................................39
2.5.3 Constituents of Laser ....................................................................................42
2.5.4 Laser Surface Melting for Improving Corrosion Resistance ........................43
Chapter 3: EXPERIMENTAL DETAILS ...................................................................46
3.1 Material and Specimen Preparation .......................................................................46
3.2 Corrosion Inhibitor Preparation .............................................................................46
3.3 Laser Surface Treatment ........................................................................................47
3.3.1 Laser System .................................................................................................47
3.3.2 Laser Surface Melting of Black Steel ...........................................................48
3.3.3 Laser Surface Melting Processing ................................................................49
3.4 Corrosion Test ........................................................................................................50
3.4.1 Instrumentation and Tools Preparation .........................................................50
3.4.2 Open Circuit Potential Test ...........................................................................52
3.4.3 Polarization Scan ..........................................................................................53
3.4.3.1 Anodic Scan .........................................................................................53
3.4.3.2 Cathodic Scan ......................................................................................55
3.4.4 Corrosion Rate Calculation ...........................................................................56
3.5 Microstructure and Metallographic Examination ..................................................57
3.6 Micro-hardness Examination .................................................................................57
3.7 Summary of the Test ..............................................................................................58
iii
Chapter 4: RESULTS AND DISCUSSION I: EFFECT OF CORROSION
INHIBITOR ON CORROSION BEHAVIOR OF BLACK STEEL .....................59
4.1 Corrosion Behavior at 5℃ .....................................................................................59
4.1.1 Open Circuit Potential Measurements ..........................................................59
4.1.2 Polarization Behavior....................................................................................63
4.2 Corrosion Behavior at 13℃ ...................................................................................68
4.2.1 Open Circuit Potential Measurements ..........................................................68
4.2.2 Polarization Behavior....................................................................................71
4.3 Corrosion Behavior Comparison between 5℃and 13℃ .......................................74
4.4 Corrosion Morphologies after Corrosion Test .......................................................76
4.4.1 Black Steel with Immersing in Tap Water ....................................................76
4.4.2 Black Steel with Corrosion Inhibitor Solution at Low Temperature ............77
4.4.3 Black Steel with Corrosion Inhibitor Solution at High Temperature ...........78
Chapter 5: RESULTS AND DISCUSSION II: EFFECT OF LASER
SURFACE TREATMENT ON CORROSION BEHAVIOR OF BLACK
STEEL....................................................................................................................80
5.1 Microstructure and Metallographic Analysis.........................................................80
5.2 Hardness Profile .....................................................................................................83
5.3 Corrosion Behavior ................................................................................................88
5.3.1 Laser-Treated Steel with Tap Water .............................................................88
5.3.2 Laser-Treated Steel with Corrosion Inhibitor ...............................................90
5.4 Corrosion Morphology of Laser-Treated Steel with Corrosion Inhibitor
after Corrosion Test ...............................................................................................94
Chapter 6: CONCLUSIONS ........................................................................................99
6.1 Corrosion Inhibitor on Chilled Water Piping System ............................................99
6.2 Laser Surface Melting on Chilled Water Pipes ....................................................100
6.3 Corrosion Inhibitor on LSM Chilled Water Piping System.................................100
6.4 Perspectives for Future Work ..............................................................................101
6.4.1 Research on Corrosion Inhibitor .................................................................101
6.4.2 Application for Laser Surface Melted Specimens ......................................102
6.4.3 Promotion on Laser Surface Melting for Piping System ............................102
iv
REFERENCES ..........................................................................................................103
v
LIST OF FIGURES
Number Page
Figure 1.1 Water cooled chiller ...........................................................................................1
Figure 1.2 Basic chiller loop with water cooled chiller .......................................................3
Figure 1.3 Refrigerant cycle of YK centrifugal vapor-compression chiller ........................4
Figure 1.4 Corrosion of inner part of chilled water pipe .....................................................6
Figure 2.1 Schematic diagram of Zinc dissolution in hydrochloric acid solution .............13
Figure 2.2 Typical Polarization curve for an active / passive metal ..................................15
Figure 2.3 Anodic and cathodic half-cell reactions present simultaneously on
corroding Zinc surface .....................................................................................19
Figure 2.4 Erosion corrosion on chilled water pipe ...........................................................23
Figure 2.5 Cavitation on check valve ................................................................................24
Figure 2.6 Galvanic Series of various metals / alloys in sea water....................................25
Figure 2.7 Galvanic corrosion in chilled piping system ....................................................26
Figure 2.8 Corrosion under insulation on condenser .........................................................28
Figure 2.9 Comparison without and with corrosion inhibitor ............................................30
Figure 2.10 Chilled water chemical dosing system ...........................................................32
Figure 2.11 Schematics of various laser surfacing processes ............................................35
Figure 2.12 Laser transformation hardening of a shaft ......................................................37
Figure 2.13 Iron-iron carbide phase diagram .....................................................................37
Figure 2.14 Schematic of hardening using (a) conventional treatment and (b) laser
treatment with melting marked by broken line, and without melting
marked by solid line .........................................................................................38
Figure 2.15 Microstructure of laser surface melting on aluminum alloy ..........................40
Figure 2.16 Laser surface melting process (MZ: melting zone; HAZ: heat affected
zone) .................................................................................................................40
Figure 2.17 Basic construction of laser..............................................................................43
vi
Figure 2.18 Microstructure of black steel pipe (1000X)....................................................45
Figure 3.1 Diode laser system and computer controlled XYZ table ..................................48
Figure 3.2 Argon was used to be shielding gas in laser surface melting ...........................49
Figure 3.3 VersaStat II potentiostatic/galvanostatic system ..............................................51
Figure 3.4 Corrosion test set up .........................................................................................52
Figure 3.5 Theoretical anodic polarization scan ................................................................54
Figure 3.6 Theoretical cathodic polarization scan .............................................................56
Figure 3.7 Micro-hardness tester .......................................................................................58
Figure 4.1 Plots of OCP vs. time for black steel in different concentration of
NaNO2 solutions at 5℃ ...................................................................................61
Figure 4.2 Pourbaix diagram for iron at 25℃ ....................................................................63
Figure 4.3 Potentiodynamic polarization curves of black steel in different
concentration of NaNO2 solution at 5℃ ..........................................................66
Figure 4.4 Pitting occurs with dissolution of surface layer at Ebd .....................................67
Figure 4.5 Plots of OCP vs. time for black steel in different concentration of
NaNO2 solutions at 13℃ .................................................................................71
Figure 4.6 Potentiodynamic polarization curves of black steel in different
concentration of NaNO2 solution at 13℃ ........................................................72
Figure 4.7 Corrosion behavior of black steel in different concentration of NaNO2
solution .............................................................................................................75
Figure 4.8 Microstructure of black steel specimen proceed at corrosion test
without inhibitor at 5℃ ....................................................................................77
Figure 4.9 Corrosion morphology of black steel after corrosion test with 1.0%
NaNO2 solution at 5℃at different magnification ............................................78
Figure 4.10 Corrosion morphology of black steel after corrosion test with 1.0%
NaNO2 solution at 13℃at different magnification ..........................................79
Figure 5.1 Microstructure examination on laser surface melted specimens ......................82
Figure 5.2Hardness profiles along the depth of cross section of laser melted
specimens .........................................................................................................85
vii
Figure 5.3Plot of OCP vs. time of LSM black steel in different laser parameters at
13℃..................................................................................................................89
Figure 5.4 Potentiodynamic polarization curves of LSM black steel in different
laser parameters at 13℃...................................................................................89
Figure 5.5Plot of OCP vs. time of LSM black steel with 0.9% sodium nitrite-based
solution .............................................................................................................91
Figure 5.6 Potentiodynamic polarization curves of LSM black steel with 0.9%
sodium nitrite-based solution ...........................................................................92
Figure 5.7 Metallographic examination on LSM specimens with corrosion
inhibitor ............................................................................................................95
Figure 5.8Microstructure examination on LSM specimens with corrosion inhibitor ........97
viii
LIST OF TABLES
Number Page
Table 2.1 Annual cost of corrosion in GDP .......................................................................10
Table 3.1 Nominal compositions of black steel .................................................................46
Table 3.2 Composition of original solution as corrosion inhibitor ....................................47
Table 3.3 Water Examination of Tap Water ......................................................................47
Table 3.4 Laser surface melting parameters for black steel ...............................................50
Table 3.5 Summary of various tests in black steel specimens ...........................................58
Table 4.1 OCP of black steel in different concentration of sodium nitrite at 5℃ .............61
Table 4.2 Corrosion parameters of black steel with different concentration of
NaNO2solution at 5℃ ......................................................................................67
Table 4.3 OCP of black steel in different concentration of sodium nitrite at 13℃ ...........69
Table 4.4 Corrosion parameters of black steel with different concentration of
NaNO2solution at 13℃ ....................................................................................73
Table 5.1 Hardness profiles along the depth of cross section of laser melted
specimens .........................................................................................................86
Table 5.2 Corrosion parameter of laser surface melted black steel ...................................88
Table 5.3 Corrosion parameters of LSM black steel in 0.9% sodium nitrite-based
solution .............................................................................................................91
ix
LIST OF ABBREVIATIONS
PRV.Pre-Rotation Vanes
GDP. Gross Domestic Product
emf. Electromotive Force
CUI. Corrosion under Insulation
Laser.Light Amplification by Stimulated Emission of Radiation
LTH. Laser Transformation Hardening
LSM. Laser Surface Melting
MZ. Melted Zone
HAZ. Heat Affected Zone
OM. Optical Microscopy
SEM. Scanning Electron Microscopy
x
ACKNOWLEDGMENTS
The author wishes to acknowledge the support he has gained from the enthusiasm
of many professors and colleagues. This thesis would not have been possible to finish
without the help and support of the kind people around me.
I am heartily thankful to my supervisor at University of Macau: Prof. Kwok Chi Tat,
for reading my thesis and offering valuable advice. For every discussion, he always
shows his patience and leads me to develop my idea. Many of these were offered the
opportunity to me to enter a new domain of corrosion world, and to help me to handle
the problems in my job; to the laboratory technician, Ms. Ivy Wong, for assisting me
to finish the experiments and taking the SEM micrographs of the specimens.
Many thanks also to my colleagues at Dafoo Facilities Management Company
Limited: Thomysant Tagulao, plant engineer, for offering and sharing his valuable
information and own experience in chiller plant operation; to Nathaniel Tagulao,
chemical engineer, for helping me to get the raw material and idea for mixing the
corrosion inhibitor.
Lastly, but by no means least, I offer my regards and blessings to all of those who
supported me in any respect during the completion of the project.
For any errors or inadequacies that may remain in this work, of course, the
responsibility is entirely my own.
xi
DEDICATION
The author wishes to dedicate this thesis to his family. Each member of the family
deserves a share of this achievement. Their support and encouragement made the
finishing of this study possible.
And of course, to all of his colleagues and friends who dedicate their related
working experience of corrosion.
1
CHAPTER 1: INTRODUCTION
1.1 OVERVIEW OF CHILLER SYSTEM
In Macao, there are many casinos and buildings choose chiller system as their main
components of HVAC systems. It is the most efficient and flexible way to cool the
building in the world. The use of chillers allows the engineers or operators to produce
chilled water in a central building location or even on the roof and distribute the
chilled water economically to anywhere of the building.
Figure 1.1 Water cooled chiller
Chiller system is a system which can remove heat from liquid (usually is water) via
a vapor-compression cycle or absorption refrigeration cycle. Normally, chiller can be
water cooled and air cooled. It consists of five major components: compressor,
evaporator, condenser, circulating pumps and circuit pipes. In this paper, YK
centrifugal vapor-compression water cooled chiller will be chosen as our topic. It is
2
relatively simple and compact. Figure 1.2 shows a basic chiller loop with water
cooled chiller. There are four processes during one cycle: evaporating, compressing,
condensing and reducing.
The refrigerant cycle of YK centrifugal vapor-compression chiller is shown
schematically in Figure 1.3. First, refrigerant liquid moves from condenser to
evaporator, and then it absorbs the heat from water. Due to the extreme vacuum of the
shell, refrigerant liquid will be boiled to refrigerant vapor (about 3.9℃), creating the
refrigerant effect. At the same time, water (around 13℃) will also become chilled
water (around 5℃ to 7℃) which is typically distributed to air handling units through
the circulating pipes. In this way, air can be cooled by the chilled water and used to
cool the building. Then, the chilled water will be re-circulated back to the chiller to be
cooled again (Closed Loop).
On the other hand, refrigerant vapor will go through mist eliminators to compressor
to become high pressure vapor. There are some pre-rotation vanes inside the
compressor which can be partially opened or closed by the building loads.
Then, the refrigerant vapor will move to condenser tube for condensing to become
refrigerant liquid again. At the same time, the heat will be removed from refrigerant
vapor by the condensing water which dissipates to the atmosphere by cooling tower.
Before the refrigerant liquid moves from condenser to evaporator, it should go
through an expansion valve for reducing its pressure. The chiller cycle is now
completed and the process will start over again.
3
Figure 1.2 Basic chiller loop with water cooled chiller
4
Figure 1.3 Refrigerant cycle of YK centrifugal vapor-compression chiller
Moreover, control of chiller is also typically based on temperature of returning chilled
water. Suchtemperature indicates the cooling load in the facility at any given time.
The warmer the temperature of returning chilled water, the larger cooling load of the
facility. Occasionally, the chiller is controlled by the temperature of leaving chilled
water (supply). This is a typical process for chilled water applications. In this case,
PRV will usually respond best and will provide modulating control to meet the load.
5
1.2 PIPING SYSTEM OF CHILLED WATER
1.2.1 WORKING CONDITION OF CHILLED WATER PIPING SYSTEM
In chiller cycle, chilled water (around 5℃ to 7℃) will become warmer (around 10
℃ to 13℃) after absorbing the heat from air. Then, it will be re-circulated back to the
chiller to be cooled again.
Chilled water piping system is a closed loop that is not open to the atmosphere. Due
to the high-rise applications, circulating pumps (chilled water pumps) must be used
for “lifting” the water from low level to high level of the building. Therefore, the
static pressure is existed and become considerable (may exceed the pressure rating of
chiller components and piping systems).
As mention before, chilled water undergoes temperature changing and pressure
changing in the whole process. Thus, the pipes must have a property that can resist
these kinds of working condition.
1.2.2 MATERIALS OF CHILLED WATER PIPING SYSTEM
Nowadays, the chilled water pipes are usually made of steels or copper. Among
them, black steel pipe is the first choice for chilled water piping system due to its
mechanical properties and excellent characteristics. It has been used for pipelines in
oil and petroleum industries and for water, gas and sewage purposes for many years.
Its name comes from a black oxide scale formed on finished surface of the steel after
forging. On the other hand, it also smears with protective oil because the steel is
susceptible to rusting or corrosion. That makes it will not easily rust for a long time
and reduces maintenance frequency.
6
Corrosion scale
1.3 CORROSION IN CHILLED WATER PIPING SYSTEM
Black steel is widely applied in chilled water piping system, but there is a big
problem –corrosion of inner surface of the pipes. Although some pipes have been
made of copper, the corrosion problem still exists. Corrosion can cause a serious
problem in the chiller system. Sometimes water leakage with ice can be found on the
pipe. That means the bore of the pipe may be blocked with corrosion scale (as shown
in Figure 1.4), which is a very spongy structure to be scour away and may slow the
chilled water down resulting in decrease in efficiency of the system. Finally, the
refrigerant will freeze the water and ice will burst the pipe since the flow rate of
chilled water is too low.
Figure 1.4 Corrosion of inner part of chilled water pipe
The consequences of corrosion are very common, such as rust on iron, tarnish on
silver and green patina on copper. In fact, most of them can lead to failures in plant
infrastructure and machines which are usually costly to repair, costly in terms of lost
or contaminated product, in terms of environmental damage, and possibly causing in
human safety. It has been estimated that approximately 5% GNP of USA is spent on
corrosion prevention and maintenance or replacement of products lost or
contaminated as a result of corrosion reaction.
7
1.4 CORROSION PREVENTION AND CONTROL
Nonetheless, black steel still has many advantages that make it can use as major
material for chilled water piping system. Compare to other materials, it is available in
a wide range of sizes, wall thicknesses, lengths, and grades. However, it is also more
economical than other non-ferrous materials, resulting in lower material costs and
requires no special handling.
Moreover, high tensile strength and internal pressure make it can resist high water
pressure. However, the only limitation is that the black steel is not a suitable in acidic
water or when long-term durability is desired. Therefore, corrosion prevention and
control must be employed to protect the pipes in case of any water leakage which is
caused by corrosion.
The most widely used corrosion prevention in closed loop piping system is water
quality control. By monitoring and using corrosion inhibitors, the corrosion rate will
decrease due to formation of a passive layer on the internal surface. However, it may
consume more time for chemical concentration adjustment, in order to get the desired
effect.
1.5 LASER SURFACE MODIFICATION
Nowadays, laser has become one of the most versatile and powerful tool for
materials processing. In industrial applications, laser can simply be regarded as a
device for producing a finely controlled, easily manipulated heating source of an
extremely high power density. With this controlled source of heat, materials can be
machined or surface treated with exceptionally high rates of heating and cooling.
Moreover, highly localized treatments, hardening of components complex in shape
8
and size, and a reduction in post heat treatment machining operations are allowed.
Compared with other surface treatments, there are some advantages of laser surface
modification from different aspects:
1) No wearing of mechanical tools;
2) No limitation on a wide variety of materials;
3) Little distortion of product can be easily obtained;
4) Never introduce any impurities into the processed material;
5) Under computer numerical control, automated process can be achieved.
Therefore, it is not surprising that laser surface modification can always offer a
wide range of possibilities to achieve desired surface properties.
1.6 OBJECTIVES
Corrosion is one of the severe damage for chiller system. It can cause enormous
damage to the system and its related equipment. Thus, it is not surprising to see that,
there are many building owners, operators, and plant engineers who stay in the
building all the time to provide corrosion control and monitoring of water quality.
In order to minimize any fatal incidents and economic losses in chilled water piping
system, corrosion control and prevention are required to consider. In general,
corrosion inhibitor is a widely used method in chiller plants. However, it can lead to
concern other serious problems, such as pollution and additional preventive
maintenance cost.
Laser surface modification has been reported to be a feasible tool for enhancing the
hardness and wear properties of metallic materials. However, studies related to the
effect of laser surface melting on corrosion behavior of the black steel are scare less
9
than other laser materials processing methods. Owing to this reason, the objectives of
the present study including:
1) To find the effect and most suitable concentration of corrosion inhibitor for chilled
water piping system;
2) To investigate the feasibility of LSM for improving hardness and corrosion
resistance of chilled water pipes without altering their overall compositions.
10
CHAPTER 2: LITERATURE REVIEW
2.1 CORROSION PRINCIPLE
Corrosion is a deterioration of metal by reaction with its environment. It can also be
applied to the degradation of many nonmetallic materials, such as ceramics, polymers.
To the great majority of people, corrosion means rust, an almost universal object of
hatred. However, tarnishing is also a type of corrosion, mainly on copper, silver and
brass. All of these can cause enormous damage and degradation to building, bridges,
ships and cars, etc. Therefore, the economic costs of corrosion damage and corrosion
related service failures have been paid can be very high. Moreover, it may cause some
fatal accidents if no any prevention is taken.
2.1.1 COSTS OF CORROSION
The annual cost of corrosion worldwide is estimated to exceed $U.S. 1.8 trillion,
which approximately 3% to 4% of the Gross Domestic Product (GDP) of
industrialized countries1:
Country Annual Corrosion Cost
United States of America 2.7% GDP
United Kingdom 3.5% GDP
Germany 3% GDP
China 5% GDP
Table 2.1 Annual cost of corrosion in GDP
The data given above are only the direct economic costs of corrosion. The indirect
costs resulting from actual or possible corrosion are more difficult to evaluate but are
11
probably even greater, such as plant downtime, loss of product, loss of efficiency and
contamination, etc.
2.1.2 CORROSION OF METALLIC MATERIALS
Corrosion of metals is a chemical or electrochemical process in which surface
atoms of a solid metal react with a substance in contact with the exposed surface.
Usually the corroding medium is a liquid substance, but gases and even solids can
also act as corroding media.
It can disfigure appearance under service conditions, or it can reduce strength to a
level at which failure will occur. In general, most metallic materials are subject to
corrosion in a wide variety of environments. There is a net decrease in free energy in
going from metallic to oxidized states. Consequently, essentially all metals occur in
nature as compounds, except Au and Pt which exist in nature in the metallic state.
2.1.3 ELECTROCHEMICAL NATURE OF AQUEOUS CORROSION
Corrosion of metallic material is usually electromechanical in nature. During this
process, atoms of metal are oxidized and form ions; electrons flow from the anode to
the cathode where they may take part in a reduction process. In electrochemistry,
oxidation is the loss of electrons. The atom that loses the electrons becomes an ion.
Conversely, phenomena which ion converts back to metal by putting back the
electrons, is called reduction.
From the standpoint of this electrolytic theory, the intent of metallic corrosion
processes is an electronic transferring in aqueous solutions. Thus, it is necessary to
discuss the electrochemical nature of corrosion briefly before continuing with
discussion of corrosion in chilled water piping system.
12
2.1.4 ELECTROCHEMICAL REACTIONS
Consider a common example of electrochemical reactions in rusting of steel when
exposed to a moist atmosphere:
322 4364 OHFeOOHFe
The product of this rusting reaction is an insoluble ferric hydroxide. When the part
is removed from the water corrodent, there is an opportunity for drying, and this
corrosion product changes and forms to familiar red-brown rust:
OHOOHFe 2323 3Fe→2
In principle, corrosion is consisted from one oxidation and one reduction reaction,
and they are often combined on a single piece of metal. In this way, charge transfer
or exchange of electrons occurs.
As shown in Figure 2.1, a piece of Zinc immersed in hydrochloric acid solution is
undergoing corrosion. At some points on the surface, Zinc is transformed to Zinc ions.
At the same time, the electrons that produced from this reaction pass through the solid
conducting metal to other sites on the metal surface and reduce hydrogen ions to
hydrogen gas. The related electrochemical reactions are listed as follows:
Anodic (oxidation) reaction: _+2 2+→ eZnZn
Cathodic (reduction) reaction: 2
_+ →2+2 HeH
13
Figure 2.1 Schematic diagram of Zinc dissolution in hydrochloric acid solution
Briefly then, for corrosion to occur there must be a formation of ions and release of
electrons at an anodic surface were oxidation or deterioration of the metal occurs.
There must be a simultaneous reaction at the cathodic surface to consume the
electrons generated at anode. The anodic and cathodic reactions must go on at the
same time and at equivalent rates.
Normally, the anodic reaction occurring during corrosion can be written in the general
form:
_→ neMM n
That is, the corrosion of metal M results in the oxidation of metal M to an ion with
a valence charge of n+ and the release of n electrons.
Similarly, the reduction reaction can also be found during the corrosion of metals:
OHeOHO 442 22 (Neutral or basic solutions)
14
OHeHO 22 244 (Acid solutions)
Moreover, it can also be observed that, water will be reduced in the absence of
oxygen:
OHHeOH 222 22
2.1.5 CORROSION POTENTIAL
As mentioned before, electrochemical corrosion reaction must be found
simultaneously on anode and cathode. In a pure metal, an anode may be a grain
boundary, and the grain can be the cathode. Note that all of these reactions are stand
for the process of electron or charge transfer, resulting in producing a voltage between
anode and cathode. This is an important concept in corrosion because electrochemical
reaction cannot occur if this potential is unavailable.
In other words, voltage might be produced while two metals were coupled together
in a conducting fluid. In here, a galvanic couple is formed when two different metals
are used. Therefore, a change in electrochemical potential or the electron activity has
a profound effect with the tendency of corrosion.
However, a new problem was created – it is inconvenient to measure and compare
the electrode potential with each other among different galvanic couples. Because of
this, one metal in the couple is always assigned to a special electrode called standard
half-cell, which produces a standard reference potential. By this definition, many
different metals can be coupled to it for measuring their tendency to corrode in a fluid.
The potentials measured are called corrosion potentials.
15
2.1.6 PASSIVITY
Most commercially available corrosion resistant alloys depend on passive films that
inhibit electrochemical action between the metal and their corrosive environment.
This phenomenon is termed passivity, is displayed by chromium, nickel, titanium,
aluminum and many of their alloys. It is defined as a condition of corrosion resistance
due to formation of thin surface films under oxidizing conditions with high anodic
polarization. Figure 2.2 explains how the polarization curve grows in passivity of
metals or alloys.
Figure 2.2 Typical Polarization curve for an active / passive metal
At relatively low potential characteristic of deaerated acid solutions, corrosion rate
measured by anodic current density is high and increase further with potential in the
active state. Above the primary passive potential, Epp, the current density suddenly
decreases to very low value that remains independent of potential. This is termed the
passive range, passive film is formed that acts as a barrier to the anodic dissolution
16
reaction. However, at even higher potential, the passive film breaks down and the
current density increases again with potential in the transpassive region.
In many metallic materials, stainless steels are highly resistant to corrosion in a
rather wide variety of atmosphere as a result of passivation. They contain at least 12%
chromium, which as a solid solution alloying element in iron, can minimize the
formation of rust. Although chromium cannot be used alone due to its brittleness, it is
still a key alloying element forming resistant passive oxide films on the surface.
In fact, passive film is not perfect because it is always thin and often fragile, its
breakdown can result in unpredictable localized forms of corrosion, which accelerates
the corrosion rate and leads to catastrophic failure of many engineering components.
2.1.7 FORMS OF CORROSION
There are two fundamental types of corrosion, the first type is uniform or general
corrosion and the second one is localized corrosion including the following forms:
i. Galvanic corrosion
ii. Crevice corrosion
iii. Pitting corrosion
iv. Intergranular corrosion
v. Dealloying or selective etching
vi. Stress corrosion
vii. Corrosion fatigue
viii. Erosion corrosion
ix. Cavitation
In fact, uniform corrosion accounts for the greatest loss of metal. It is an expected
17
mode of corrosion because it is predictable and thus preventive work can usually be
planned to avoid some fatal accidents. Yet the other localized forms of corrosion are
more insidious and difficult to predict and control. Therefore, even if localized
corrosion doesn’t consume so much material, penetration and failure are often existed
more rapidly.
In many cases, identifying a corrosion problem and source is a simply question of
looking in the right. Quite often, no problem is known about, nor even suspected, until
some incidents are found. Under the worst case, such problem may exist for years and
exhibit no indication to the property owner or plant operators, then directly causing
the fatal accident. Here, five main corrosion forms that affect chilled water piping
system will be discussed in detail in a later chapter (in Section 2.3).
2.2 CORROSION RATE DETERMINATION
2.2.1 CORROSION RATE
As discussion earlier, corrosion is an electrochemical reaction either produces or
consumes electrons. Thus, the rate of electron flow to or from a reacting interface may
be the measurement of corrosion rate.
The corrosion rate r can be expressed in terms of current density i (current per unit
area of material corroding) and determined using the following expression:
nF
ia
tA
mr
Where
a = atomic weight (g)
n = the number of electrons (or equivalents) exchanged in the reaction
18
F = Faraday’s constant, 96500 C/mol
However, this rate can also be expressed as CPR (corrosion penetration rate), which
in terms of weight loss of material per unit area per unit time:
EWKiCPR corr= )•/ daym(g 2
Where
icorr = current density (current per unit area of material corroding) (μA/cm2)
K = constant = 8.95 × 10-3 daymμAcmg 22 ••/•
EW = NEQ-1
= 1/Σ(fini/ai)
ni = number of electrons feed by the corrosion reaction of ith
alloy element
fi = mass fraction of ith
alloy element
ai = atomic weight of ith
alloy element
2.2.2 MIXED POTENTIAL THEORY
For corrosion reaction, it is the fact that both oxidation and reduction reaction of
corrosion would occur simultaneously on anode and cathode. There is no net
accumulation of charges on corroding surface, resulting in equilibrium and same rate
at each side. Therefore, it can be expressed as:nF
irr oxidred
0==
In this way, same current density can also be found between anodic oxidation and
cathodic reduction. In general, two different half-cell electrode potentials cannot
coexist separately on an electrically conductive surface. As illustrated in Figure 2.3,
each of them must polarized or change potential to a common intermediate value,
Ecorr, which is called free corrosion potential. It is a combination or mixture of the
half-cell electrode potentials. At this point, the rates of anodic and cathodic reaction
19
are equal and the rate of anodic dissolution, ia, is always identical to the corrosion rate
icorr, in terms of current density: icorr = ia = ic.
Figure 2.3 Anodic and cathodic half-cell reactions present simultaneously on corroding Zinc surface
2.2.3 PRINCIPLE OF CORROSION TEST
A common method of measuring corrosion rates is simply to expose a weighed
piece of specimen to corrosive environment for a known length of time, remove and
weigh again, and calculate the weight loss of coupon specimen. However, that is not
always convenient in industrial application because of the difficulty in placing and
removing in the plant.
In order for an electrochemical process to take place, there must be an anode,
cathode, as well as both an ionic and electrical conduction path between the two.
When performing a polarization scan (will be discussed in Section 3.4.3), the ionic
conduction path is provided through the solution separating the working and counter
electrodes, while the electrical conduction is provided through the potentiostat. When
20
the working and counter electrodes are connected together to form a galvanic cell, or
where a corrosion cell is created due to differential conditions existing at a metal
surface, a potential difference exists which causes a net current to flow. Then,
potentiostat is used to control the driving force for electrochemical reactions taking
place on the working electrode. The magnitude of this driving force in turn dictates
which electrochemical processes actually take place at the anode and cathode, as well
as their rate.
2.2.4 CORROSION RATE MEASUREMENTS
As mentioned before, current represents the rate with which the anodic or cathodic
reactions are taking place on the working electrode. Typically, current is expressed in
terms of current per unit area of the working electrode, or current density i. Numerous
variables will influence the rate of a given electrochemical reaction, including
temperature, surface condition of the surface being interrogated, as well as the
chemical environment in which the experiment is performed.
Nowadays, a computer electrochemical system (such as VersaStat II) can control
the voltage difference between a working electrode and a reference electrode in an
electrochemical cell. It implements this control by injecting current into the cell
through a counter electrode and measures the current flow between the working and
counter electrodes. The controlled variable in a potentiostat is the cell potential and
the measured variable is the cell current. As a result, corrosion behavior and corrosion
rate of materials can then be evaluated.
21
2.3 CORROSION FORMS IN CHILLED WATER PIPING SYSTEM
It is not difficult to find that, a rust build-up at the cooling tower, fouled drift
eliminators, pipe tubercles, and flakes of scale and rust caught in the chilled water
pipes. All of these are obvious indications of a corrosion problem. In this way,
efficiency of chiller system and related equipment will always get worse if no any
measures are taken.
There are a large number of parameters can influence corrosion rate;
(1) Structural factors - composition, residual stresses, and dissolved gases.
(2) Environmental factors - concentration, temperature, corrosion inhibitors and
applied stress.
The interior part of chilled water pipes can show a wide range of corrosion
characteristics. However, corrosion problems don’t appear overnight, and are
generally the result of a failure to provide good chemical inhibitor protection over a
period of time. Moreover, other factors and failure to take certain preventative
measures may also reasons.
While there are so many exceptions, but it can classify into five generalizations:
2.3.1 GENERAL CORROSION
Electrochemical reaction occurs at more or less, the same rate over the entire
surface. It is the well distributed and low level attack against the entire metal surface
with little or no localized penetration. It is the least damaging of all forms of corrosion.
General corrosion usually occurs in environments in which the corrosion rate is
inherently low or well controlled – such as for chemically treated closed circulating
systems, and in some open water systems.
22
It is the only form of corrosion whereby weight loss or metal loss data from
ultrasonic testing can be used to accurately and reliably estimate corrosion rates and
future pipe life expectancy. The corrosion current cacorr iii == gives an indication
of how much metal loss occurs as a function of time: nF
ir =
Where
r = corrosion rate
n = the number of electron associated with the ionization of each metal atom
F = 96500C/mol
2.3.2 PITTING CORROSION
This is a localized, deep penetration of the metal surface with little general
corrosion in the surrounding area, resulting in the formation of pits. The pits may be
deep, shallow, or undercut. Due to surface deposits, electrical imbalance or some
other initiating mechanism, all existing corrosion potential attacks a select number of
individual sites.
In most cases, pitting corrosion is extended throughout the entire metal surface,
creating it an irregular or very rough surface profile. In other instances, pits are
concentrated in specific areas, leaving the majority of the metal surface in like new
condition.
Pitting corrosion is the most common form of corrosion found where there are
surface scratch, incomplete chemical protective films and insulating or barrier
deposits of dirt, iron oxide, organic, and other foreign substances at the pipe surface.
It is very common at galvanized steel pipe, where any failure of the galvanizing
invokes a pitting condition. Actually, pitting corrosion may also include: crevice
23
corrosion, water-line attack, under deposit attack, impingement or erosion corrosion
attack, and concentration-cell corrosion.
Stainless steels, which are highly resistant to corrosion due to forming a passive
film, are especially susceptible to pitting by local breakdown of the film at isolated
sites. As a result, the pit can be accelerated to corrode and leaded to catastrophic
failure of the components.
2.3.3 EROSION CORROSION
Erosion corrosion is the result of combination of a corrosive fluid and high flow
velocity. This is the gradual and selective deterioration of a metal surface due to
mechanical wear and abrasion. It is attributed to entrained air bubbles, suspended
particles under a flow rate of sufficient velocity.
Erosion corrosion is similar to impingement attack, and is primarily found at
elbows and tees of pipes, or in those areas where the water sharply changes direction.
Softer metal such as copper and brass, are inherently more susceptible to erosion
corrosion than steel.
Figure 2.4 Erosion corrosion on chilled water pipe
24
In fact, the same stagnant or slow flowing fluid will also cause a low or modest
corrosion rate, but rapid movement of the corrosive fluid physically erodes and
removes the protective corrosion product film, exposes the reactive alloy beneath, and
accelerates corrosion. For example, low strength alloys that depend on a surface
corrosion product layer for corrosion resistance are always suffered in this situation.
Figure 2.5 Cavitation on check valve
Cavitation is a special case of erosion corrosion. High fluid velocity with high
differential pressure can be produced by a pump. It can cause the local pressure
rapidly falls below vapor pressure of the water. Thus, bubbles are created in these
regions. Then, all of them will be collapsed or implode when the pressure increases
again. As a result, an intense shockwave is created to remove metal or oxide from the
metal surface. The attack takes the form of roughened pits, which may eventually
result in penetration, as shown in Figure 2.5.
2.3.4 GALVANIC CORROSION
This is an aggressive and localized form of corrosion due to the electrochemical
reaction often found between two dissimilar metals in an electrically conductive
25
environment. It results in one of them is preferentially corroded (more active) while
the other is protected from corrosion (more noble). In general, these two dissimilar
metals have differing corrosion potential Ecorr.
Galvanic Series of various metals / alloys in sea water is given in Figure 2.6. Any
metals will be preferentially corroded when coupled to another metals with a more
positive or noble potential in Galvanic Series. Meanwhile, the more noble metal is
protected from corrosion.
Figure 2.6 Galvanic Series of various metals / alloys in sea water
26
The most common example of such corrosion activity, widely found throughout the
chiller system and process plant operations, is the direct connection of brass gate
valves to black steel pipe, or between copper tubing and black steel pipe. In this way,
steel serves as the anode to corrode. Although black steel pipe is coated with
protective oil for improve the corrosion resistance, it still shows the highest rate of
corrosion in its interior part for lack of galvanic insulator – usually developing over
many years.
Figure 2.7 Galvanic corrosion in chilled piping system
In most cases, the severity of pipe loss due to galvanic activity is often found
relative to the general corrosion activity of the piping system itself – with little or no
galvanic activity found where extremely low general corrosion rates exist. However,
galvanic losses often become aggressive while the pipe is suffered from high general
corrosion rate activity.
Black steel pipe
Galvanic corrosion
Copper pipe
27
2.3.5 CORROSION UNDER INSULATION (CUI)
It is a threat to any piping system which operates at lower temperature in humid
environments. In the absence of an effective moisture barrier and a protective metal
coating, moisture will penetrate fiberglass or foam insulation to condense at the cold
metal surface.
Moisture can often accumulate sufficiently to immerse the insulation and cause its
total deterioration. It creates an untreated water condition at the outer pipe surface,
which leading to corrosion problem.
In outdoor environments, moisture, rain, snow, and ice can also penetrate the
insulation due to physical damage, wear, or by the failure to install sealants at the
overlap of the hard metal outer shell. However, CUI usually remains hidden until
severe damage has occurred to the pipe, producing discoloration at the insulation
itself, or failure. In many cases, CUI can exceed the degree of physical damage
caused by internal corrosion of piping system.
It is the fact that CUI is commonly found at cold water domestic piping system,
free cooling condenser water systems, and especially in chilled water piping system
(most severe at the chilled water supply side). The degree of CUI depends upon a
combination of pipe temperature and humidity. For example, CUI will occur on
typically warm condensing water pipes even the humidity is high. Conversely, the
extremely low temperature of condenser with refrigerant liquid can create substantial
exterior pitting even from a relatively dry atmosphere.
28
Figure 2.8 Corrosion under insulation on condenser
As we mentioned before, all of these corrosions can also cause a serious problem in
chilled water piping system. First, pipe mass is lost through oxidization to dissolve
iron species. Second, scales can accumulate and to induce large tubercles that increase
corrosion rate and decrease water capacity. Third, the quality of water will decrease
due to releasing particulate of scale. Finally, the pipe may be burst because the bore of
the chilled water pipe is blocked with corrosion scale and the refrigerant freeze the
water, which may create loss of service and cost.
Nowadays, corrosion control in chilled water piping system has become a serious
Room Temperature = 20.4℃
Condenser = 10.6℃
Humidity = 49.2%
29
and concerned problem to many facility managers and plant engineers. Moreover, the
corrosion rate is also an important indicator to replace some piping systems. At that
time, an enormous damage will be caused, such as loss of service, equipment damage,
excessive maintenance demands, high energy cost and overall unnecessary expense.
To reduce this cost burden, improvements in materials selection, methods of
protection, design and in-service monitoring should be taken. However, it will lead to
another complicated problem - cost and efficiency. Therefore, compromise must be
found to face such of these problems.
2.4 CORROSION INHIBITORS IN CORROSION CONTROL
There are many examples have been shown that the formation of corrosion either
by electrochemical action or through solution by weak acids, is an essential step in the
formation of rust in chilled water piping systems. In this way, if means for stopping
this part of reaction are established, then rusting could be retarded or perhaps
prevented.
Corrosion inhibitors are one of the most efficient of these corrosion preventions. In
general, corrosion inhibitors are chemical compounds that deposit on exposed metal
surfaces from corrosive environment. As a result, they may form a uniform and
passive film, which likes a coating, acts as a physical barrier. Then, corrosion can be
controlled and minimized in high efficiency if the correct inhibitor and quantity is
selected.
30
As illustrated in Figure 2.9, a bar of iron was immersed in two different solutions –
with and without corrosion inhibitor added.
Figure 2.9 Comparison without and with corrosion inhibitor
It is apparent that, the right one was still shiny and smooth with compared to the
left one. However, the phenomenon is not rare to other material or system which with
this water treatment. Therefore, a positive evaluation is often obtained after corrosion
inhibitor is used.
2.4.1 CORROSION INHIBITOR
Nowadays, many chemical water treatment suppliers agree that corrosion inhibitor
is the most common and effective method to corrosion of chilled water piping systems,
which is adding chemical compound into the chilled water to decrease the corrosion
rate of metal or alloy. In a sense, inhibitor forms a protective coating in situ by
reaction of the solution with the corroding surface. An inhibiting compound in small,
but critical quantities reduces the corrosivity of the environment, and inhibits
oxidation or reduction reactions by removing reactants. In fact, minimum
concentration of inhibiting compound must be present to maintain the inhibiting
Tap water With corrosion inhibitor
31
surface film. Good circulation and absence of any stagnant areas are necessary to
maintain inhibitor concentration.
2.4.2 APPLICATIONS ON CHILLED WATER PIPING SYSTEM
Corrosion inhibitor finds greatest use in re-circulating systems, such as chilled
water piping system. However, the efficiency of corrosion inhibitor is a function of
many factors like: compound composition, inhibitor concentration, operating
temperature and the type of corrosion. If the correct inhibitor and quantity is selected,
then the higher efficiency will be possible to achieve. Moreover, it can be easily
observed that, combinations of inhibitors in commercial formulations often give
synergistic reductions and become more effective for several metals or alloys in the
system.
In fact, many inhibiting compounds are toxic, and recent environmental regulations
have limited their use. The solution should be covered with absorbent or contained
and sealed the container for disposal due to their exciting smell. Nevertheless, they
are still playing a critical role in numerous corrosion control strategies.
According to the principle of corrosion inhibitor, many chemical water suppliers
would prefer to offer their own proprietary corrosion inhibitors for treating closed
water looping system. Sodium nitrite is a known primarily ingredient in corrosion
inhibitor due to its primary effect on reducing the rate of anodic reaction. Moreover, it
is also a relatively inexpensive material and is not toxic in the quantities used.
The inorganic nitrite anion is known as an anodic inhibitor. That is, it can develop a
protective oxide film on ferrous metal similar to that which occurs naturally on
aluminum. It was found that the protective film can be formed on the surface of iron
32
in aerated solutions by reaction with dissolved oxygen. However, in deaerated
solution the inhibitors, due to their oxidizing character, react directly with iron to form
protective films. The overall reaction is:
9Fe(OH)2 + NO2- = 3Fe3O4 + NH4+ + 2OH
- + 6H2O (4-1)
Indeed, it is not surprised that the
effectiveness of corrosion inhibitors
varies with different solution
corrosivity, pH, temperature and
composition of solution. In most
cases, corrosion inhibitors are
intertwined with pH control agents
due to their performance
characteristics. They are generally
functional only in certain pH ranges. For sodium nitrite, pH of solution should be
controlled above 7.0 to maintain its inhibiting ability. However, it only has little effect
on corrosion control if pH is higher than 10.0. Therefore, sodium hydroxide is always
as pH control agent added into sodium nitrite to increase the efficiency of corrosion
inhibitor.
Chiller system is a very different cooling system from typical residential air
conditioner where a refrigerant is pumped through an air handler to cool the air.
Regardless of who provides it, chilled water (usually between 5℃ to 7℃) is pumped
through an air handler, which absorbs the heat from the air, then disperses the air
throughout the building to be cooled. Then, chilled water (around 10℃ to 13℃) will
Figure 2.10 Chilled water chemical dosing system
33
be re-circulated back to chiller to cool again. In other words, corrosion inhibitor will
also undergo this temperature changing in the whole process.
Incidentally, flow velocity of chilled water should be another considered factor
which relates to corrosion inhibitor. It seems that increasing flow velocity can
increase corrosion rate, but it can also hasten the precipitation of a protective layer. At
that time, a denser protective layer can be formed at higher chilled water flow rate.
Also, if the velocity is too high, chilled water may scour away the protective scale due
to its porous structure. Thus, a proportion rate to actual chiller system should be
considered. However, the actual chilled water flow rate is usually adjusted with
chilled water supply temperature. In this way, flow velocity is not defined as a factor
in this paper.
To determine the desired concentration of sodium nitrite solution for improving
corrosion resistance in chilled water piping system, samples of black steel were
immersed in different concentrations of sodium nitrite solution.
2.5 LASER SURFACE MODIFICATION
2.5.1 LASER INDUCTION
A laser is a device which transforms energy from one form into electromagnetic
(EM) radiation through a process of optical amplification based on the stimulated
emission of photons. The term “laser” is an acronym of “Light Amplification by
Stimulated Emission of Radiation”.
Nowadays, laser is applied in diverse areas with different output power. Many
lasers are designed for a higher peak output with an extremely short pulse. While
continuous wave (constant output), laser is used in communication or cutting.
34
However, all of these application areas can roughly fall into three groups: optical uses,
power uses as in material processing and ultra-power uses for atomic fusion.
In principle, laser is a light which has all the properties of incandescent light, such
as reflection, refraction, diffraction and polarisation. It is also notable for its certain
unique properties which are not present in other EM radiation.
They are:
Coherence
Monochromaticity
Directionality
The combination of these properties gives laser radiation many advantages, like
achieving very high power densities, which is not available from other sources.
Therefore, applied energy can be placed precisely on the surface and carried out the
surface treatment with very high quality.
2.5.2 APPLICATION OF LASER SURFACE MODIFICATION
As a versatile source of pure energy in a highly concentrated form, laser has
emerged as an attractive and powerful tool in material processing due to its unique
properties. It can be consider that, laser material processing is a kind of method to add
or improve the properties of materials through laser beam.
For laser surface modification, ease of automation can reduce labor cost and
increase the productivity. It can also improve the quality of product with low
distortion and small heat-affected zone. It is true that, local temperature rises always
occurs in treated area. Therefore, either melting or vaporization or a combination of
both occurs, producing a wide range of applications in lase surface modification.
35
The use of laser is to modify the metallurgical structure of material surface and to
tailor the surface properties without adversely affecting the bulk properties. In general,
there are four common types of laser surfacing technique as shown in the below:
(a) Laser transformation hardening (LTH) (b) Laser surface melting(LSM)
(c) Laser surface alloying (LSA) (d) Laser cladding (LC)
Figure 2.11 Schematics of various laser surfacing processes
The first is laser transformation hardening (LTH) in which the surface is heated so
that thermal diffusion and solid state transformation take place. The second is laser
surface melting (LSM), which results in a refinement of the structure due to the rapid
36
solidification for melting. The third is laser surface alloying (LSA), in which alloying
elements are added to the melting pool to change the composition of the surface. The
last one is laser cladding (LC), which overlay a layer of material without altering the
substrate of the metal. All of them have made a great improvement and impact on
modern industry. Here, only a briefly discussion of laser transformation hardening
(LTH) and laser surface melting (LSM) is provided due to its relationship with later
experiment.
2.5.2.1 Laser Transformation Hardening (LTH)
Laser transformation hardening is the use of laser radiation absorbed in a metal to
change the microstructure of ferrous alloys and produce a hard surface by controlled
heating and cooling. In general, the hard surface results in high resistance to wear and
improved fatigue resistance, giving material a long life time when used in high wear
situation. However, this hardening process can only be applied to particular types of
steel, so it might seem limited, but these types of steel are widely used in industry,
such as stainless steels, cast irons, and carbon steels, etc.
In laser transformation hardening, no external quenching medium is required.
Material is self-quenched by the unheated substrate through conduction. In fact, this
quenching rate is a critical parameter which determines the treated microstructure will
transform to martensite or other phases. This is a consequence of all laser energy
being absorbed at the surface, which limits the allowable heating time in order to
avoid melting. Moreover, it is also notable that the cooling rate must be rapid enough
to achieve the desired transformation.
For instance, a laser beam is defocused and scanned over the surface of hardenable
steel, as illustrated in Figure 2.12. Here, surface only heats above A3 can become
37
Figure 2.13Iron-iron carbide phase diagram
hardened zone.
Figure 2.12 Laser transformation hardening of a shaft
During the process, deposited energy will elevate the surface temperature well into
the austenitizing temperature
(above the upper critical
temperature A3 or Acm) without
melting the surface. Then,
phase transformation occurs.
On the other hand, the treated
surface will start to cool while
the laser beam passing through
of it. Due to the heat conduction, the cooling rate is very fast that there is not enough
time to diffuse back to grain boundaries and carbon is locked in the lattice, thus
causing lattice distortion and enhancing hardness.
In general, the fraction of laser beam power absorbed by the materials is controlled
by their reflectivity. Since the reflectivity of some bare metals is very good, it is
helpful to coat them to increase their absorption.
38
To compare with conventional treatment, scanning laser beam can produce a very
high heating temperature (over austenitizing temperature A3) in very small fractions of
a second, imparting little or no heat into the adjacent material. Therefore, the cooling
material under the heat layer will provide a path for rapid cooling. In this way, this
rapid cooling style can cool the material from austenitizing temperature and down to
the critical temperature Ms, produces a totally martensite structure, as shown in Figure
2.14. It is a harder structure than other crystalline structure, improving the mechanical
properties of the surface layer.
Figure 2.14Schematic of hardening using (a) conventional treatment and (b) laser treatment with
melting marked by broken line, and without melting marked by solid line
39
2.5.2.2 Laser Surface Melting (LSM)
Laser surface melting is a laser surface modification for refining the microstructure
of surface material. In fact, the experimental arrangement is similar to that for laser
transformation hardening, except that in this case a focused or near focused beam is
used. After that, a thin surface layer can be melted and rapid solidification occurs.
This generates a refined microstructure which may have improved properties, such as
greater hardness, greater homogeneity and reduced porosity. However, it can also
create additional fast diffusion paths on alloys or stainless steels, resulting from
enhancing the formation of a protective oxide scale and improving the corrosion
resistance.
If the heat input from the laser (102 ~ 10
4W/mm
2) is sufficient to promote melting,
the resulting microstructure depends on temperature gradient and solidification rate
which are controlled by the laser energy density and interaction time. In this process,
structure refinement varies from coarse to extremely fine textures, from dendrites to
martensites, to metastable or, in some cases with specific compositions, amorphous
structures. All of these can also improve the corrosion resistance. In fact, LSM can
harden alloys that cannot be hardened by LTH, such as, ferritic malleable gray iron
and tool steels. Melting can enhance the diffusion of carbon and ensure rapid quench
for producing a hardened region. Typically, melt depths vary from 10μm ~ 0.3mm.
Surface finish of around 25μm are fairly easy to obtain and can be reduced after
processing. Moreover, an inert gas is usually used for shrouding the melted surface for
avoiding oxidation.
40
Figure 2.15 Microstructure of laser surface melting on aluminum alloy
When a laser beam with high irradiance is rapidly scanned over a metal surface,
melting and solidification will also proceed in a thin layer of the surface. In fact,
almost all the energy is used for melting, and only a small amount is lost to subsurface
heating. Because of this, a large temperature gradient is existed between the molten
metal and substrate. In this way, the cold substrate will become a conducting source
for rapid quenching of molten material, resulting in producing fine near homogeneous
structures.
Figure 2.16 Laser surface melting process (MZ: melting zone; HAZ: heat affected zone)
41
For laser surface melting, the melted depth D can be estimated by the concept of
energy balancing. It is apparent that, the input energy is equated to the total energy
required to raise that depth to the vaporization temperature. Therefore,
Ftv = ρD[c(Tv-T0)+Hf]
Where
F = power per unit area (W/m2)
tv = interaction time while reaching vaporization temperature (s)
D = melted depth (m)
ρ = density (kg/m2)
c = specific heat capacity (J/kg•K)
Tv = vaporization temperature (K)
T0 = initial temperature (K)
Hf = latent heat of fusion (J/kg)
Consequently, the properties of material can be enhanced with idealized surface and
no additional material is needed. To compare with traditional material processing,
there are some advantages for laser surface modification:
(1) Very high accuracy in the final processed products that can be obtained without
the need for polishing.
(2) Laser beam can be focused to a very small area, producing high energy density
where it is needed, without affecting the neighboring areas of materials.
Therefore, small heat affected zone with little distortion of the products can be
easily obtained.
(3) Never introduce any impurities into the processed material.
42
(4) Under computer numerical control, automated process can be achieved to
reduce manpower, processing cost, and increase the productivity.
(5) No wearing of mechanical tools. During the working process, mechanical tools
are only needed to change their dimensions and constant measurements and
feedback to adapt their position to original plan in computerized
instrumentation.
(6) No limitation on a wide variety of materials even if the extremely hard, abrasive,
or brittle materials.
2.5.3 CONSTITUENTS OF LASER
A basic laser system consists of an active medium inside a highly reflective optical
cavity, as well as a means to supply energy to the active medium. In general, active
medium is a material with properties that allow it to amplify light by the mechanism
of stimulated emission (the process after which the laser is named - Light
Amplification by Stimulated Emission of Radiation). In its simplest form, the cavity
consists of two mirrors which are placed parallel to each other to form an optical
oscillator, which is a chamber in which light would oscillate back and forth between
the mirrors, each time passing through the active medium. Therefore, light of a
specific wavelength that passes through the active medium is amplified (with
increasing in power).
43
Figure 2.17 Basic construction of laser
1: Active medium, 2: Laser pumping energy, 3: High reflector, 4: Output coupler, 5: Laser beam
As the excitation mechanism, there are some systems for raising the atoms into
their excited state (in the active medium) to create population inversion, such as flash
lamps, DC power. The optical arrangement of laser is shown in Figure 2.17.One of the
two mirrors is partially transparent (10~99%) to allow some of the oscillating power
to emerge as the operating laser beam. The other mirror is 100% reflecting, so all the
radiation coming toward the mirror is reflected back to the active medium. In addition,
this mirror is also usually curved to reduce the divergence loss of the oscillating
power and make it possible to align the mirrors without undue difficulty.
2.5.4 LASER SURFACE MELTING FOR IMPROVING CORROSION RESISTANCE
In the recent years, as laser beam processing has many technological and economic
advantages for high precision, reliability, efficiency and productivity. Laser beam
44
processing has become an attractive method in the world. Typically, it can provide a
permanent or long period of improvement on the product.
To compare with other surface engineering technologies, laser surface modification
has a little expensive to the user. But they are still more cost effective because of their
precision and speed. Among them, laser surface melting is one of the laser surface
modifications that no additional material is required. On the other hand, it is
noticeable that a rapid self-quenching (can be up to 105 to 10
8℃/s) always happens on
the localized heating surface for producing a wide range of desirable microstructure.
Consequently, LSM has been considered as the simplest and most efficient technology
for improving the surface properties of materials, which are widely used in industry.
LSM is a kind of laser surface engineering by rapid melting and subsequent
solidification, resulting in homogenous and refined microstructure. In this way, most
of the surface defects can be removed from the materials, such as porosity, folds, laps,
scars or inclusions. In fact, the melt depth is varied from 10μm ~ 0.3mm.
As mentioned before, black steel pipe is a major material for chilled water piping
system due to its mechanical properties and excellent characteristics. Although it is a
forging part, it may still consists of an inhomogeneous structure of ferrite and graphite
in various forms (flakes, or spheres etc), as shown in Figure 2.1829
. Therefore,
corrosion will occurs from these defective surface areas. The overall effect is to
reduce the surface and underlying regions of the pipe to become a brittle structure
with much reduced mechanical strength, leading to possible broken under normal
working pressure.
45
Figure 2.18 Microstructure of black steel pipe (1000X)
In addition, laser surface melting can obtain a hardening effect - structure
refinement varies from graphite to cementite and austenite to martensite. However,
such value of the hardness depends on the extent of the carbon dissolution from the
graphite, giving a variation of hardness and structure with processing speed. As a
result, a very hard surface can be produced on the cheaper metals.
Nowadays, corrosion control and protection are especially valuable via prolonging
the service life of chilled water piping system for minimizing any economic losses. As
a feasible tool for enhancing hardness and other properties of materials, it is worthy to
study the corrosion behavior of black steel with laser surface melting treatment.
46
CHAPTER 3: EXPERIMENTAL DETAILS
In order to perform corrosion tests in a particular metal/solution system, a number
of components must be assembled and appropriately prepared:
3.1 MATERIAL AND SPECIMEN PREPARATION
Black steel extracted from the pipe of chilled water system was selected in this
study. The nominal compositions of black steel BS1387 are shown in Table 3.1.
Black
Steel
Composition C Mn Si P S Fe
Weight (%) 0.20 1.20 0.10 0.045 0.045 98.41
Table 3.1 Nominal compositions of black steel
Specimens were cut from the chilled water pipe which was not used before. Prior to
the corrosion tests, specimens were embedded in epoxy resin and exposed with an
area of 1 cm2. Subsequently, the surface of specimens were grinded progressively
with SiC papers from 60 grit, 240 grit, 400 grit and then finally 800 grit, in order to
produce a constant surface roughness before the test. After finishing the surface of
specimens, the edges were also sealed by epoxy resin to avoid crevice corrosion. Then,
a threaded stainless steel rod was screwed into a drilled and tapped hole in the
specimen as the working electrode.
3.2 CORROSION INHIBITOR PREPARATION
As mentioned before, different concentration of sodium nitrite-based solution was
used as the corrosion inhibitor. The composition of original solution of corrosion
inhibitor consists of:
47
Ingredient Sodium Nitrite
(NaNO2)
Borax
(Na2B4O7．10H2O)
Sodium
Hydroxide
(NaOH)
Tap Water
(H2O)
Weight (%) 31% 10% 3% 56%
Table 3.2 Composition of original solution as corrosion inhibitor
Here, tap water examination was shown in the following table, which provided by
IACM Lab.
Date Chloride
(mg/L)
Sodium
(mg/L)
Potassium
(mg/L)
Magnesium
(mg/L) pH
value
Fluoride
(mg/L)
Total
hardness (mg/L CaCO3)
2010-Aug 26.1 12.5 2.33 6.72 7.3 0.18 131
2010-Oct 33.3 12.7 2.36 7.66 7.2 0.2 144
2010-Dec 37.1 10.7 2.44 6.14 7.2 0.18 117
2011-Mar 40.0 15.1 2.68 5.48 7.3 0.2 103
Table 3.3 Water Examination of Tap Water
3.3 LASER SURFACE TREATMENT
3.3.1 LASER SYSTEM
Laser surface melting of received specimens were carried out using a 2.3 kW CW
fiber-couple diode laser, as illustrated in Figure 3.1.During the process, a laser beam
with near infra wavelength (λ=980nm) and necessary energy is provided. Then, it was
transmitted by an optical fiber and focused onto the desired place of specimens by a
lens of specified focal length.
In fact, such manipulation or motion system was controlled by a CNC unit. It can
provide a relative movement between laser beam and work piece. In this way, laser
beam can be controlled to follow a planned scanning path for desired processing
48
task.
Figure 3.1 Diode laser system and computer controlled XYZ table
Prior the laser processing, scanning path (or scanning sequence) is usually required
to design with the aid of drawing software, such as Auto CAD and Corel Draw. Then,
the drawing is converted to a CNC program that can be recognized by the controller
of CNC unit. Though adjusting the desired power level and scanning speed of the
laser machine, laser beam can be controlled to finish the processing task by CNC unit.
3.3.2 LASER SURFACE MELTING OF BLACK STEEL
For laser surface melting, a relationship is always existed between melted depth
and power density of laser beam, as mentioned in Section 2.5.2.2. In this way, surface
refinement and corrosion resistance improvement were done by choosing a suitable
Processing head
Processing gas
Optical fibre
49
power density of laser beam. In order to obtain the desired effect of such laser
processing, different power of laser beam energy were tested for laser surface melting.
3.3.3 LASER SURFACE MELTING PROCESSING
To decrease the adverse effect which brings from the curved specimens, surface
grinding with SiC papers must be used to provide a “desired” condition for laser
treatment. Here, embedding is not necessary before the laser treatment is completed.
As mentioned before, a scanning sequence is required to design prior the laser
processing. Here, the laser beam is dithered to produce a zig-zag trace, in order to
generate uniform heat distribution across the metal surface. On the other hand, a flow
of argon was preferred to use at the irradiated surface for protecting the lens against
contamination and overheating and to minimize specimen oxidation under the laser
beam.
Figure 3.2 Argon was used to be shielding gas in laser surface melting
Besides laser beam power density, there is another important parameter to the
process – scanning speed. It is the fact that a suitable scanning speed can give
sufficient time for promoting melting and heat conduct. Therefore, different laser
powers and scanning speeds were needed to choose for optimizing the result of LSM.
Argon
50
The detail laser processing parameters are shown in Table 3.4.
Specimens (Black Steel)
Laser Energy
Power (W)
Laser Beam
Diameter
(mm)
Laser Power
Density (W/mm2)
Scanning
Speed
(mm/s)
LSM01 500 1 636.9 25
LSM02 800 1 1019.1 25
LSM03 1000 1 1273.9 25
LSM04 1000 1 1273.9 50
LSM05 1500 1 1910.8 50
Table 3.4 Laser surface melting parameters for black steel
3.4 CORROSION TEST
3.4.1 INSTRUMENTATION AND TOOLS PREPARATION
Electrochemical polarization scans were carried out using a potentiostat/galvanostat
system – Princeton Applied Research VersaStat II (as shown in Figure 3.3). It is a
powerful tool for measuring electrochemical properties of material. It can provide an
inexpensive instrument to perform corrosion and basic research electrochemistry
experiments. The maximum current output and compliance voltage provide the power
and other requirements for many routine applications. Moreover, a small current
range (full scale) also gives VersaStat II very good sensitivity with nano-ampere
resolution.
51
Figure 3.3VersaStat II potentiostatic/galvanostatic system
Potentiodynamic scan was carried out by controlling the potential, and hence the
driving force, available for reaction to occur. The rate of the available reactions will
vary based on the magnitude of the driving force and the nature of the reaction itself.
Furthermore, it can also measure the current applied by the potentiostat to achieve the
desired degree of polarization.
The investigation of corrosion test is according to ASTM standard G5-94(2004)10
.
The working electrode is centrally located in the cell with a pair of auxiliary
electrodes on either side for better current distribution. In general, measurement of
cell potential, E, is necessary to determine the driving force of an electrochemical cell.
As mentioned before, any electrochemical cell should consist of two half-cells. For
convenient to the study and measurement, one of the half cells is always made known
or reference half-cell. Here, a saturated calomel electrode (SCE) was used as a
reference electrode, REF, was placed outside the cell, and the potential of the working
52
electrode is measured through the Luggin probe and solution bridge with respect to
the reference electrode. Probe tip should be placed near the working electrode surface
in order to minimize ohmic resistance interferences. A distance of around 1mm is
often recommended for experiment.
Figure 3.4 Corrosion test set up
Then, the below corrosion test can be performed.
3.4.2 OPEN CIRCUIT POTENTIAL TEST
An electrochemical reaction in a voltaic cell (with open circuit) stops when the
opposing electric field at each electrode is strong enough to arrest the reactions. It is
apparent that electric charge has been separated to create an electric potential
difference between these electrodes. The magnitude of potential difference is called
open circuit potential (OCP). It is notable that potential at which the total anodic
current is equivalent to the total cathodic current.
53
3.4.3 POLARIZATION SCAN
3.4.3.1 Anodic Scan
A schematic anodic polarization curve is illustrated in Figure 3.5. As can be seen in
the figure, the scan starts from point 1 and progresses in the positive (potential)
direction until termination at point 2. There are a number of notable features on the
curve. Qualitatively, the current-potential plot can be divided into active, passive and
transpassive regions. The active region is the region where corrosion current increases
with increasing applied potential. The passive region represents the state in which an
alloy can form a passive film to inhibit electrochemical reaction. The transpassive
region is that part of the plot where the applied potential is large enough to cause
breakdown of the oxide / passivation layer.
The open circuit potential Ecorr, is located at point A. At this potential, the sum of
anodic and cathodic reaction rates on the electrode surface is zero. As a result, the
measured current will be close to zero. This is due to the fact that the potentiostat only
measures the current which it must apply to achieve the desired level of polarization.
As the potential increases, current density will move into region B, which is the active
region. In this region, metal oxidation is the dominant reaction taking place. Point C is
known as the passivation potential, and as the applied potential increases above this
value the current density is seen to decrease with increasing potential (region D) until
a low, passive current density is achieved (passive region – region E).
54
Figure 3.5 Theoretical anodic polarization scan
Once the potential reached a sufficiently positive value (point F, sometimes termed
the breakdown potential Ebd or pitting potential Epit), the applied current rapidly
increases (region G). This increase may be due to a number of phenomena, depending
on the metal / environment combination. For some systems (e.g. aluminum alloys in
salt water), this sudden increase in current may be pitting (localized breakdown of
passive layer), while for others it may be transpassive dissolution.
In general, Ebd (or Epit) is the least noble potential where pitting or crevice corrosion,
or both, will initiate and propagate. This knee shape in Figure 3.5 means that for a
small increment in applied potential there is a large increase in the measured current,
signifying a breakdown of the surface oxide or passive layer. The potential at which
the reverse scan crosses over the forward scan is called protection potential (Eprot). It
is the noblest potential where pitting or crevice corrosion will not propagate. If Ebd
and Eprot are the same, there will be no tendency to pit. If Eprot is nobler than Ebd, there
55
will be no pitting. If the Eprot is more active than Ebd, pitting may occur. Moreover, it
can also be observed that the regeneration ability of passive film will be increased
with the decreasing area EGF (which is produced by Eprot and Ebd).
3.4.3.2 Cathodic Scan
A schematic cathodic polarization scan is illustrated in Figure 3.6. Overview is only
provided in this section because the following experiment does not include this one.
In a cathodic potentiodynamic scan, the potential is varied from point 1 in the
negative direction to point 2. The open circuit potential represents the potential at
which the sum of anodic and cathodic reactions occurring on the electrode surface is
zero, is located at point A. Depending on pH and dissolved oxygen concentration in
solution, region B may represent the oxygen reduction reaction. Since this reaction is
limited by how fast oxygen may diffuse in solution, there will be an upper limit on the
rate of this reaction, known as the limiting current density.
Further decreases in the applied potential result in no change in the reaction rate
(region C). Eventually, the applied potential becomes sufficiently negative with
driving force for another cathodic reaction to begin taking place, such as illustrated at
point D.
56
Figure 3.6 Theoretical cathodic polarization scan
As the potential and driving force become increasingly large, this reaction may
become dominant in region E. In addition, an increase in current may also be
observed if sufficient driving force exists to reduce the oxide present on the electrode
surface.
3.4.4 CORROSION RATE CALCULATION
From Section 2.2.1, CPR (corrosion penetration rate) can be expressed in terms of
weight loss of material per unit area per unit time: CPR = KicorrEW (g/m2•day)
Where,
EW = 18.3,
K= 8.95×10-3
g．cm2/μA．m
2．day,
icorr = current density which derive from experiment data,
Thus CPR = 0.163785icorr (g/m2．day)
57
3.5 MICROSTRUCTURE AND METALLOGRAPHIC EXAMINATION
The microstructure and phases of specimens were analyzed by optical microscopy
(OM) and scanning electron microscopy (SEM). Compared with OM, SEM has many
advantages, such as higher magnification, larger depth of focus and greater resolution.
It is a type of electron microscope that images a sample by scanning with a high
energy beam of electrons in a raster scan pattern. Here, electrons are generated from
an electron gun that entered the surface of specimen and many low energy secondary
electrons are produced. In fact, the information of the specimen’s surface topography
is contained in the intensity of these secondary electrons. Therefore, an image of the
specimen’s surface can be constructed by measuring secondary electron intensity as a
function of the position of the scanning primary electron beam. In addition, EDS
analysis was used for compositional analysis.
To determine surface corrosive status in different stages of experiment, raw
specimens and laser-treated specimens are analyzed by these examinations. For
laser-treated specimens, they are required to section, polish and etch with acidified
ferric chloride solution. Then, microstructure of the specimens can be analyzed
through OM and SEM.
3.6 MICRO-HARDNESS EXAMINATION
As mentioned before, a hardening effect can always be obtained from laser surface
melting. Consequently, specimens were polished and etched for a micro-hardness
examination. By using a Vickers hardness tester MHV2000 with 0.9807 N load and
10 seconds loading time (as shown in Figure 3.7), micro-hardness of the specimens
was determined. During the process, a small pyramidal indenter was indented into the
58
surface of specimen and diagonals (d1 and d2) of the indentation were obtained. Then,
Vickers hardness was calculated automatic by the tester with d1and d2.
Figure 3.7 Micro-hardness tester
3.7 SUMMARY OF THE TEST
Specimens were investigated by various tests as shown in below table:
Specimens LSM
Corrosion Test Micro Hardness
Test
SEM / OM
Examination Tap water Sodium Nitrite
(Corrosion Inhibitor)
Black
Steel
X X
Table 3.5 Summary of various tests in black steel specimens
59
CHAPTER 4: RESULTS AND DISCUSSION I: EFFECT OF CORROSION
INHIBITOR ON CORROSION BEHAVIOR OF BLACK STEEL
To determine the desired concentration of sodium nitrite (NaNO2) solution for
improving corrosion resistance in chilled water piping system, corrosion behavior of
the black steel with different concentration of corrosion inhibitor were investigated.
As mentioned before, chiller is a high efficient system, which can produce chilled
water (around 5℃ to 7℃) to cool the air for the serving area. Then, chilled water will
become warmer (around 10℃ to 13℃) and re-circulate to the chiller for cooling
again. Therefore, two different conditions are created and may be resulted in different
corrosion behavior of the black steel.
4.1 CORROSION BEHAVIOR AT 5℃
4.1.1 OPEN CIRCUIT POTENTIAL MEASUREMENTS
The plots of OCP against time for the black steel specimens in solution with
different concentration of NaNO2at 5℃ are shown in Figure 4.1. After two hours, the
OCP became stable and the steady values are summarized in Table 4.1.
60
(a)
(b)
61
Figure 4.1 Plots of OCP vs. time for black steel in different concentration of NaNO2 solutions at 5℃
Specimen Weight of
Corrosion Inhibitor (%) pH of Solution OCP (V)
Black
Steel
0 pH 6 -0.383
0.1 pH7 -0.307
0.15 pH8 -0.332
0.20 pH8 -0.391
0.25 pH8.5 -0.297
0.30 pH9 -0.383
0.35 pH9 -0.325
0.40 pH9 -0.410
0.45 pH9 -0.394
0.50 pH9.5 -0.333
0.60 pH10 -0.436
0.70 pH10 -0.430
0.80 pH10 -0.407
0.90 pH10 -0.339
1.0 pH10 -0.377
Table 4.1 OCP of black steel in different concentration of sodium nitrite at 5℃
(c)
62
As shown in the above data, OCP tends to increase to a noble level when corrosion
inhibitor is added. Among them, the OCP of black steel in 0.25% NaNO2 solution is
the noblest, indicating higher thermodynamic stability. As mentioned in Section 3.1,
iron is the major composition of black steel. It can exist in two oxidation states, +2 or
+3. According to the Pourbaix diagram for iron in the presence of water or humid
environments at 25℃ (Figure 4.2), ferrous ion Fe2+
is the stable substance at lower pH
region. This indicates that iron will corrode under this condition. On the other hand, it
can be seen that corrosion of iron produces ferric ions Fe3+
, ferrous hydroxide
Fe(OH)2, magnetite Fe3O4 and other form of products, can be found in different
regions of the Pourbaix diagram. The presence of a relatively large immunity region
in Figure 4.2, where corrosion products are solid and possibly, indicates that iron may
corrode much less under these potential / pH conditions. Therefore, a protective oxide
film is always found on iron surface in nearly neutral or alkaline solutions. Because of
these, the field of oxide stability is substantially greater at elevated pH, and iron is
more corrosion resistant in alkaline solutions. Contributing to the overall resistance of
iron are the generally nobler half-cell electrode potentials for the anodic dissolution
reactions which lower the driving force for corrosion reactions.
63
Figure 4.2 Pourbaix diagram for iron at 25℃
For the present results, the values of OCP become nobler in the range of pH 8.5 to
9.5 than the others. On the other hand, it is notable that the difference between the
stability line for water and iron decreases significantly as pH further increases. Thus,
the corrosion rate drops also, although the actual rate cannot be predicted from the
diagram.
4.1.2 POLARIZATION BEHAVIOR
The cyclic potentiodynamic polarization curves of the black steel in different
concentration of NaNO2 solution at 5℃ are shown in Figure 4.3 and the current
64
density icorr are listed in Table 4.2. In solution with corrosion inhibitor, the current
density icorr decreases and corrosion potential Ecorr increases (consistent with OCP)
with increasing pH value. Among them, the current density icorr of the black steel in
0.90% NaNO2 solution is the lowest, indicating the lowest of corrosion rate.
Moreover, obvious active-passive transition region can be observed in most
polarization curves except the one without NaNO2, showing that anodic dissolution
was not always in activation regime. As a result, stable surface passive layer was able
to form for inhibiting further corrosion.
In addition, the anodic current densities in various polarization curves are lower
and such of these in the passive regions are broad enough to inhibit corrosion. In this
way, the corrosion rates drop in such period. Meanwhile, an oxide layer has been
formed on the surface to inhibit the corrosion. The overall reaction is:
9Fe(OH)2 + NO2- = 3Fe3O4 + NH4
+ + 2OH
- + 6H2O
As mentioned before, a significant passivity was always found on the polarization
curves with NaNO2.Among them, the curves with adding 0.35%, 0.90% and 1.0%
NaNO2 solution are more superior than others due to their border passive region and
low current density. For modestly oxidizing conditions, the curves exceed the critical
current density for passivation and achieve to the stable passive condition.
On the other hand, a breakdown point (is called pitting potential Ebd or Epit) of
curves can be found which makes current density rapidly increases once the potential
reached a sufficiently “positive” value. Then pitting started to occur with dissolution
of surface layer as shown in Figure 4.4. To compare with each other, the related
current density of Ebd of these three curves is both very low and near whereas the
65
different degree of passive region. Among them, the passive region of polarization
curve with 0.35% sodium nitrite solution is a little bit narrow that cannot provide a
good corrosion resistance in the passive state. Consequently, there may be little
difference between solution with mixing 0.9% and 1.0% NaNO2 because their
corrosion rates are both very low and similar, and either may be adequate for
applications in chilled water piping system.
(a)
66
Figure 4.3 Potentiodynamic polarization curves of black steel in different concentration of NaNO2
solution at 5℃
(b)
(c)
67
Specimens
Weight
of
Corrosion
Inhibitor
(%)
OCP
(V)
Ecorr
(V) icorr(µA/
cm2) ipass(µA/
cm2) Epit(V)
CPR
(g/m2day)
Eprot
(V)
Black
Steel
0 -0.383 -0.385 3.260 - - 0.534 -
0.1 -0.307 -0.313 0.187 5.900 0.283 0.031 -0.007
0.15 -0.332 -0.345 3.230 19.300 0.327 0.529 0.047
0.20 -0.391 -0.386 0.351 5.000 0.309 0.057 0.032
0.25 -0.297 -0.324 0.065 4.500 0.313 0.011 0.063
0.30 -0.383 -0.382 0.156 4.400 0.340 0.026 0.050
0.35 -0.325 -0.343 0.069 4.600 0.334 0.011 0.044
0.40 -0.410 -0.417 0.281 6.880 0.408 0.046 -0.022
0.45 -0.394 -0.414 1.750 3.750 0.345 0.287 0.048
0.50 -0.333 -0.331 0.039 3.560 0.345 0.006 0.045
0.60 -0.436 -0.444 1.930 15.800 0.664 0.316 0.064
0.70 -0.430 -0.430 1.690 28.600 0.719 0.277 0.049
0.80 -0.407 -0.408 0.226 4.210 0.512 0.037 0.052
0.90 -0.339 -0.355 0.032 4.100 0.451 0.005 0.061
1.0 -0.377 -0.396 0.116 4.290 0.375 0.019 0.075
Table 4.2 Corrosion parameters of black steel with different concentration of NaNO2solution at 5℃
Figure 4.4 Pitting occurs with dissolution of surface layer atEbd
In addition, it is not difficult to found that re-passivity (discussed in Section 3.4.3.1)
Pitting
Surface layer dissolution
68
was existed when corrosion inhibitor is added. Among them, the protection potential
Eprot of black steel which is immersed in 1.0% NaNO2 solution is the noblest with
small value relative to its pitting potential Epit, indicating its strong re-passivity ability
of passivation layer. However, pitting still occurs in all the curves because Eprot is
more active than Ebd.
4.2 CORROSION BEHAVIOR AT 13℃
According to the corrosion principle, temperature is one of the variable
environmental factors on corrosion. Therefore, corrosion rate will enhance with
increasing temperature. As mentioned before, chilled water will become warmer after
absorbing the heat from the air. In this way, corrosion rate may increase in chilled
water return side after the chilled water passed through the heat exchanger.
4.2.1 OPEN CIRCUIT POTENTIAL MEASUREMENTS
The plots of OCP against time for black steel in different concentration of NaNO2
solution at 13℃ are shown in Figure 4.5. After two hours, the OCP became stable
and the steady values are summarized in Table 4.3.
There is significant shift of OCP in the active direction for the black steel in the
solution without NaNO2 at 13℃ as compared with the one at 5℃ (as shown in
Figure 4.5). Similar finding can be obtained for the black steel in the NaNO2 solution
at this higher temperature. In this way, corrosion will become serious in chilled water
return side. Nevertheless, these OCP can still shift to a noble level when corrosion
inhibitor was added. Among them, the OCP of the black steel in 0.7% NaNO2
solution is the noblest, indicating higher thermodynamic stability.
69
On the other hand, it is not difficult to find the relationship between OCP and pH
value of the solution. As described in Section 4.1.1, a protective oxide film can be
formed and becomes stable on the surface of iron at elevated pH. Thus, the overall
corrosion resistance of iron is generally noble in solution with mixing sodium
nitrite-based corrosion inhibitor.
Specimen Weight of
Corrosion Inhibitor (%) pH of Solution OCP (V)
Black
Steel
0 pH 6 -0.643
0.1 pH7 -0.330
0.15 pH8 -0.327
0.20 pH8 -0.332
0.25 pH8.5 -0.300
0.30 pH9 -0.375
0.35 pH9 -0.330
0.40 pH9 -0.365
0.45 pH9 -0.373
0.50 pH9.5 -0.320
0.60 pH10 -0.362
0.70 pH10 -0.135
0.80 pH10 -0.367
0.90 pH10 -0.396
1.0 pH10 -0.227
Table 4.3 OCP of black steel in different concentration of sodium nitrite at 13℃
70
(b)
(a)
71
Figure 4.5 Plots of OCP vs. time for black steel in different concentration of NaNO2 solutions at 13℃
4.2.2 POLARIZATION BEHAVIOR
The potentiodynamic polarization curves of black steel in different concentration of
NaNO2 solution at 13℃ are shown in Figure 4.6 and the anodic current density icorr
are listed in Table 4.4.
(c)
(a)
72
Figure 4.6 Potentiodynamic polarization curves of black steel in different concentration of NaNO2
solution at 13℃
(b)
(c)
73
Specimens
Weight
of
Corrosion
Inhibitor
(%)
OCP
(V)
Ecorr
(V) icorr
(µA/cm2) ipass
(µA/cm2)
Epit
(V)
CPR
(g/m2day)
Eprot
(V)
Black
Steel
0 -0.643 -0.655 30.500 - - 5.000 -
0.1 -0.330 -0.336 0.293 4.900 0.279 0.048 -0.031
0.15 -0.327 -0.322 0.181 4.400 0.323 0.030 0.003
0.20 -0.332 -0.334 0.112 3.510 0.366 0.018 0.026
0.25 -0.300 -0.309 0.126 4.200 0.339 0.021 0.009
0.30 -0.375 -0.362 0.161 4.500 0.484 0.026 0.034
0.35 -0.330 -0.331 0.123 5.100 0.360 0.020 0.010
0.40 -0.365 -0.359 0.273 6.600 0.355 0.045 0.015
0.45 -0.373 -0.394 1.660 16.000 0.356 0.272 -0.104
0.50 -0.320 -0.317 0.069 4.100 0.369 0.011 0.029
0.60 -0.362 -0.364 0.290 6.500 0.307 0.047 0.047
0.70 -0.135 -0.139 0.080 9.100 0.425 0.013 0.035
0.80 -0.367 -0.373 0.150 5.300 0.423 0.025 0.053
0.90 -0.396 -0.406 0.233 5.240 0.394 0.038 0.734
1.0 -0.227 -0.238 0.629 11.300 0.433 0.103 0.063
Table 4.4Corrosion parameters of black steel with different concentration of NaNO2solution at 13℃
From the above data, it is apparent that such solution temperature variation can
have significant effects on anodic polarization curves. In other words, this
polarization behavior is different from the one that is running at lower temperature.
Compare with tap water experimental data, current density icorr decreases
significantly in adding corrosion inhibitor. In general, the trend is almost the same as
the previous experimental data. Among them, the current density icorr of the black
steel in 0.50% NaNO2 solution is the smallest, indicating the slowest of corrosion rate.
In fact, all the polarization curves are superior to that one without any corrosion
inhibitor because they have lower corrosion densities in the active region.
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Moreover, different degree of Tafel region can be observed in polarization curves
with different concentration of corrosion inhibitor solution. For modestly oxidizing
conditions, the polarization curves which under 0.3%, 0.35%, 0.5%, 0.8% and 0.9%
NaNO2 solution would be recommended to apply because their reduction curves
exceed the critical current densities for passivation. Therefore, the passive layer is
formed.
Although the mentioned curves also have the passive region, solution with 0.5%,
0.8% or 0.9% NaNO2 is more superior to others due to existing lower current density
increment in anodic dissolution reaction. Nevertheless, one always conservatively
chooses the suitable environmental factor is according to the borderline passivity. To
compare only these three superior cases, the passive region in 0.5% sodium
nitrite-based solution is not broad enough to ensure good corrosion resistance in the
passive state. However, the performance on polarization curves for the remaining two
cases are so approximate (their profiles and corrosion rates are very near) that solution
with 0.8% and 0.9% NaNO2 can be adequate to use in chilled water piping system at
“high temperature”.
In addition, the protection potential Eprot of black steel which is immersed in 0.9%
NaNO2 is the noblest with small “negative” value relative to its pitting potentialEpit,
indicating its stronger passivity ability of passivation layer as compared with others.
4.3 CORROSION BEHAVIOR COMPARISON BETWEEN 5℃AND 13℃
Temperature has the dominant effect on corrosion behavior in the present study. As
mentioned before, chilled water piping system undergoes temperature changing (Δt ≈
8~10℃)in the closed re-circuiting loop. Accelerated corrosion can be observed as
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temperature increases. The corrosion behavior of related specimens in different
concentration of NaNO2 was shown in Figure 4.7.
To compare with OCP in tap water, the OCP at high temperature is more active than
low temperature whereas corrosion rateis also higherat high temperature. Both
indicatemore corrosive conditions at elevated temperature. Despite of these, such
difference becomes smaller while corrosion inhibitor is added. Furthermore, the
current density also becomes lower and passive layer may form on the surface to
inhibit corrosion.
Figure 4.7Corrosion behavior of black steel in different concentration of NaNO2 solution
Pourbaix diagram in Figure 4.2 (in Section 4.1.1) for iron shows that iron can form
a protective oxide film in nearly neutral solutions and keep it stable at elevated pH.
That accords with the experimental data (as shown in Figure 4.7). The effect of
corrosion inhibitor is not proportional to the concentration of the corrosion inhibitor.
76
In fact, the best performance always shows on solution with mixing 0.8% to 1.0%
NaNO2 (After mixing with this quantity of corrosion inhibitor, pH is around 10).With
different temperature but same concentration in corrosion inhibitor, specimen shows
different character in its corrosion behavior. Lower current density always stays on
low-temperature condition, indicating temperature is a main parameter on corrosion
behavior.
4.4 CORROSION MORPHOLOGIES AFTER CORROSION TEST
In order to obtain any other corrosion information from the specimens, it is worthy
to investigate their microstructure with SEM.
4.4.1 BLACK STEEL WITH IMMERSING IN TAP WATER
The microstructure of specimen is shown in Figure 4.8. Filiform corrosion
randomly developed as a shallow grooving of the surface. Typically, that is a special
form of crevice corrosion occurring beneath a surface layer. It is a common
phenomenon on the steel and depends on the relative moisture of the air.
(a) (b)
77
Figure 4.8 Microstructure of black steel specimen proceed at corrosion test without inhibitor at 5℃
On the other hand, heavy oxide particles were also found on the corroded surface of
oxidized specimen. It can observe that iron oxide was the dominant chemical
composition on the surface of the specimen.
4.4.2 BLACK STEEL WITH CORROSION INHIBITOR SOLUTION AT LOW TEMPERATURE
As discussed in Section 4.1.2, a high corrosion resistance can be achieved for the
black steel when certain concentration of inhibitor is added. As a result, it forms a
protective coating in situ by reaction between the solution and the surface. In here,
1.0% NaNO2-based corrosion inhibitor will only use as analyzing sample. That is, it
has a primary effect on reducing the rate of the anodic dissolution reaction of the
black steel.
(c) (d)
(a) (b)
78
Figure 4.9 Corrosion morphology of black steel after corrosion test with 1.0% NaNO2 solution at 5℃at
different magnification
To compare previous discussion with Figure 4.9, less serious corrosion attack was
found on the specimen’s surface. Instead, shallow pits and low corrosion rate were
detected. Such corrosion morphology reflected that the black steel can resist the
corrosive environment for certain of time because a passive layer was formed.
4.4.3 BLACK STEEL WITH CORROSION INHIBITOR SOLUTION AT HIGH TEMPERATURE
With the same concentration of NaNO2but at higher temperature, the effectiveness
of corrosion inhibitor against corrosion was decreased because of corrosion kinetics.
Therefore, the degree of corrosion attack of the surface became more serious than that
at low temperature as shown in Figure 4.10. However, the corrosion inhibitor can still
perform its function in this condition. Furthermore, iron oxide scales were observed
and can be the early initiation sites for pitting attack.
(c) (d)
79
Figure 4.10 Corrosion morphology of black steel after corrosion test with 1.0% NaNO2 solution at 13
℃at different magnification
(a) (b)
(c) (d)
80
CHAPTER 5: RESULTS AND DISCUSSION II: EFFECT OF LASER SURFACE
TREATMENT ON CORROSION BEHAVIOR OF BLACK STEEL
In order to study the effect of laser surface treatment for improving corrosion
resistance of the black steel used in chilled water piping system, different power
densities and scanning speed of the laser beam were attempted. Besides, the corrosion
performance of laser-treated specimens will also be discussed in this chapter.
As discussed in Chapter 4, corrosion inhibitor can be used as chemical treatment
for producing a protective layer on the steel surface. By this way, a significant
improvement in corrosion resistance is obtained when suitable concentration of the
inhibitor is used. For this reason, the corrosion behavior of the laser-treated specimens
in the corrosion inhibitor will be investigated in this chapter.
5.1 MICROSTRUCTURE AND METALLOGRAPHIC ANALYSIS
Prior to the examination, the laser-treated specimens were sectioned, polished and
etched with acidified ferric chloride solution. By using scanning electron microscopy
(SEM), the shape and microstructure of the laser-melted specimens were observed.
Microstructure of the laser melted specimens was illustrated in Figure 5.1. It can be
seen that surface of the laser-melted specimens was melted and re-solidified resulting
in refined microstructure. In general, observation of longitudinal cross-section of
laser-melted specimens showed that it can be divided into three distinct zones from
top to bottom: melted zone (MZ) where completed melting occurred; heat affected
zone (HAZ) where no melting was found but with heating and resulting in phase
transformation and altering properties; and substrate where no altering the original
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microstructure and properties.
(a)1 (a)2
(b)1 (b)2
(c)2 (c)1
MZ HAZ
Substrate
Substrate
MZ
HAZ
82
Figure 5.1 Microstructure examination on laser surface melted specimens
a) Untreated; (b): LSM01; (c): LSM02; (d): LSM03; (e): LSM04; (f): LSM05
1: Longitudinal cross-section of LSM specimen; 2: Microstructure HAZ of LSM specimen
From Figure 5.1, grain refinement can be observed in the laser-melted specimens
when laser surface melting (LSM) was performed. Microstructure near the surface
became more homogeneous and exhibited finer grains than that of the as-received
(untreated) black steel. Meanwhile, HAZ with hardening effect was also attached due
(d)1 (d)2
(e)1 (e)2
(f)1 (f)2
Substrate
Substrate
Substrate
MZ
MZ
HAZ
HAZ
MZ
HAZ
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to LSM and rapid self-quenching. From the standpoint of practical corrosion
resistance, LSM can homogenize the compositions of the black steel and hence
improve its corrosion resistance.
For laser-melted specimens, owing to the difference in laser processing parameters,
different microstructures in melted zone were obtained. Martensite is present as the
primary phase in all LSM specimens, which is brighter in contrast as shown in Figure
5.1 (b ~ f). Besides, austenite was also observed as secondary phase. In fact, such of
these structures were caused by rapid solidification by LSM. During LSM, surface of
specimens was heated into its melting point (always exceeds the austenitization
temperature) with moving laser beam. By scanning over the metal surface, melting
and solidification occurred. Moreover, a large temperature gradient exists between
conducting source for rapid self-quenching of molten material, resulting in producing
very fine martensite as the major phase and austenite as the minor phase. Thus, a
harder surface layer with negligible distortion was formed.
By the way, the relationship always exists between carbon content of the steel and
amount of martensite. Thus, laser processing parameters were the important factors in
varying the microstructure of the laser-melted black steel.
5.2 HARDNESS PROFILE
It has been found that the hardness of the laser-melted surface was enhanced
because a finer martensitic microstructure was formed in the melted zone. The
hardness profiles along the depth of cross section of laser-melted specimens are
showed in Figure 5.2. The average hardness values are summarized in Table 5.1.
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(a)
(b)
85
Figure 5.2 Hardness profiles along the depth of cross section of laser melted specimens
(c)
(d)
86
Specimens Depth of MZ
(mm)
Depth of
HAZ (mm)
Average hardness
of MZ (HV)
Average
hardness of
HAZ (HV)
Untreated - - 170 (Surface)
LSM01 0.18 0.08 185 180
LSM02 0.15 0.11 190 189
LSM03 0.21 0.18 192 185
LSM04 0.28 0.15 187 180
LSM05 0.21 0.23 190 180
Table 5.1 Hardness profiles along the depth of cross section of laser melted specimens
Among the laser melted specimens, LSM03 (specimen with 1kw laser energy
power and 25mm/s scanning speed) has the highest average hardness on MZ and with
around 12% increase as compared with the untreated specimen. Although this value is
still little lower than the hardness of tool steels, it fulfills most of the requirements in
piping system.
As discussed before, a typical structure of the laser-melted specimens (MZ, HAZ,
and substrate) was always found in the transverse cross-section view. During LSM,
melting temperature achieved at the specimen surface and smaller grains of martensite
was formed on the surface of the laser-melted specimen as a hardened layer due to
rapid quenching (high cooling rate).
Despite of these, it can also be observed that phase transformation was occurred in
HAZ. With residual temperature or laser beam energy from MZ, it is possible to have
martensite there. However, partial structure would still keep in austenite. Thus,
hardness value was found to increase within these zones. Moreover, the resistance to
erosion-corrosion of the laser-melted steel in water circulation system may also
increase because of the hardened layer. It was strengthened by solid solution
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hardening, dislocations and small grains, and presence of martensite.
In general, hardness increment is only found on MZ and HAZ of the laser-melted
specimens with different laser processing parameters. All of these are depended on
temperature gradient and solidification rate which are controlled by laser energy
density and interaction time. Therefore, hardness and microstructure change will be
varied under different laser processing conditions.
When a laser beam with high irradiance is scanned over the specimen’s surface, it
produces thin layer of molten material near the surface. Indeed, this input energy can
always heat the treated substrate into or above the austenitizing temperature
instantaneously. With different laser energy input, different penetration may be
obtained resulting in different depth of molten layer (MZ). Moreover, such function
would also affect an underlying layer with heating. Therefore, a positive hardening
effect and microstructure change will be occurred if rapid quenching is provided.
By using a certain scanning path and different scanning speed, uniform heating was
occurred on the overall surface. In this way, rapid quenching can be done by a large
temperature gradient which can conduct the heat into the substrate of the specimen
after the end of passing laser beam. As a result, scanning speed becomes a serious
problem to the hardness increment. In this paper, laser beam energy and scanning
speed in LSM03 can obtain a highest average hardness on MZ.
For the laser-treated specimens, structure of affected substrate was varied into a
more homogeneous structure. Therefore, hardness distribution would become more
uniform and remain constant along the melt depth or hardened depth (as shown in
Figure 5.2).
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5.3 CORROSION BEHAVIOR
5.3.1 LASER-TREATED STEEL WITH TAP WATER
OCP against time and potentiodynamic polarization curves of untreated and
laser-melted specimens in tap water at 13℃ are shown in Figure 5.3 and Figure 5.4.
The related data is summarized in Table 5.2.
Specimens Black Steel
LSM Parameter
OCP
(V)
Ecorr
(V)
icorr
(µA/cm2)
ipass
(µA/cm2)
Epit
(V)
CPR
(g/m2day)
Laser
Energy
Power
(W)
Scanning
Speed
(mm/s)
Untreated - - -0.643 -0.655 30.500 Active Active 5.000
LSM01 500 25 -0.635 -0.648 16.400 Active Active 2.686
LSM02 800 25 -0.498 -0.508 32.100 Active Active 5.26
LSM03 1000 25 -0.414 -0.428 0.256 25.300 0.274 0.042
LSM04 1000 50 -0.461 -0.463 9.890 Active Active 1.620
LSM05 1500 50 -0.636 -0.638 11.800 Active Active 1.933
Table 5.2 Corrosion parameters of laser surface melted black steel
No obvious passivation is observed in all laser-melted specimens corroded in tap
water (around 13℃). To compare with previously experiment performance (only
treated with corrosion inhibitor), the OCP of these laser-melted specimens is more
active than which immersing in corrosion inhibitor.
In general, OCP is a key performance of index for reflecting the “activity” of metal
under a particular corrosive environment. As shown in Table 5.2, OCP values of LSM
specimens are higher than untreated specimen, indicate that there is a tendency for
shifting these specimens into a noble direction.
89
Figure 5.3 Plot of OCP vs. time of LSM black steel in different laser parameters at 13℃
Figure 5.4 Potentiodynamic polarization curves of LSM black steel in different laser
parameters at 13℃
90
On the other hand, it can also be observed that the corrosion resistance of the
laser-melted specimens is improved, as reflected by a reduction in corrosion current
density icorr. Among them, LSM03 has the lowest icorr. (0.256µA/cm2), reduces around
by a factor of 120 as compared with untreated specimen (Ecorr = -0.655V; icorr =
30.5µA/cm2). That is mainly caused by the refinement of microstructure and
homogenization of composition. Based on the magnitude of the icorr, the corrosion
resistance of the laser-melted specimen is ranked in descending order as follows:
LSM03 > LSM04 > LSM05 > LSM01 > LSM02 ~ Untreated specimen
This ranking shows that corrosion resistance was undoubtedly arising with LSM.
However, this result generally has a relationship with different groups of laser
processing parameters (laser beam energy, scanning speed, interaction time, etc.). The
higher scanning speed or lower laser power density, the less the total volume is melted,
the higher the quenching rate and the finer structure can be obtained, as a
consequence of removal of defects such as sulfide inclusions with corrosion resistance
improvement. Therefore, a desired effect is obtained if suitable laser processing
parameters are selected. Here, the most significant improvement is observed in
LSM03 which possesses the lowest icorr. As a consequence, this specimen could be
characterized as promising material for high performance under corrosive conditions.
5.3.2 LASER-TREATED STEEL WITH CORROSION INHIBITOR
OCP against time and potentiodynamic polarization curves of untreated and
laser-melted specimens in 0.9 wt% sodium nitrite-based solution at 13℃ are shown
in Figure 5.5 and Figure 5.6. The related data is summarized in Table 5.3.
91
For convenient comparison, the specimen preparation and laser processing
parameters are chosen the same as Section 5.3.1.
Specimens
Black
Steel
LSM Parameter
OCP
(V)
Ecorr
(V) icorr(µA/c
m2) ipass(µA/c
m2) Epit
(V) CPR
(g/m2day)
Eprot
(V) Laser
Energy
Power
(W)
Scanning
Speed
(mm/s)
Untreated - - -0.396 -0.406 0.233 5.240 0.394 0.038 0.734
LSM01C 500 25 -0.361 -0.364 0.193 6.000 0.558 0.032 0.078
LSM02C 1000 25 -0.271 -0.273 0.074 5.700 0.471 0.012 0.071
LSM03C 1000 50 -0.389 -0.390 0.493 5.000 0.644 0.081 0.744
LSM04C 1500 50 -0.392 -0.388 0.163 7.200 0.437 0.027 0.027
Figure 5.5Plot of OCP vs. time of LSM black steel with 0.9% sodium nitrite-based solution
Table 5.3 Corrosion parameters of LSM black steel in 0.9% sodium nitrite-based solution
92
Figure 5.6 Potentiodynamic polarization curves of LSM black steel with 0.9% sodium nitrite-based
solution
As previously analysis in corrosion inhibitor, active-passive transition is found in
all polarization curves of laser-melted specimens in sodium nitrite-based solution at
13℃.Above the passivation potential, the current density suddenly decreased or kept
into a constant value with increasing potential. For that moment, curves are achieved
to a stable passive status and passive layer is formed. In addition, such passive region
in the curves is also broad enough to ensure good property to inhibit corrosion. In this
way, the corrosion rate was decreased. Nevertheless, it can be seen that all the starting
passive points are very near but with different degree of passive region. Among them,
the passive region in LSM01C (laser treated with 500W and 25mm/s scanning speed)
and LSM03C (laser treated with 1000W and 50mm/s scanning speed) are broader than
others that can provide a good corrosion resistance to their specimens. Despite of this,
93
LSM02C (laser treated with 1kW and 25mm/s scanning speed) is recommended to
choose for chilled water piping system due to its lowest current density and boarder
passive region.
To compare the corrosion behavior of the laser-melted specimens in the solutions
with and without corrosion inhibitor, the former is more superior due to its improved
corrosion potential Ecorr and lower corrosion current density icorr. All of them are
shifting the curves into a noble direction and thus lower corrosion tendency.
For minimizing the weight loss in corrosion, an important item – CPR shows that both
specimens with 1kW laser energy power and 25mm/s scanning speed in these two
groups have the lowest corrosion density and would be recommended to apply. Their
corrosion densities are 0.074µA/cm2 and 0.256µA/cm
2. From this result, a big
improvement on corrosion inhibitor with 3.5 times in corrosion current density
reduction was observed. Consequently, corrosion inhibitor can improve the corrosion
resistance not only for raw material, and also for laser-melted material. It is
significantly reflected that why such improvement can be commonly used in many
closed loop piping system. Despite of this, the breakdown point still existed because
Eprot is more active than Ebd which makes current density rapidly increases once the
potential reached a sufficiently value Epit. Then, pitting was starting to occur.
Based on the magnitude of the corrosion current density, the corrosion resistance of
the specimens can be ranked as:
LSM02C > LSM04C > LSM01C > Untreated specimen > LSM03C
The improvement in corrosion resistance in this Section obviously results from the
passive effect of corrosion inhibitor, material’s refinement in microstructure and
94
homogenization of chemical compositions by rapid solidification. Moreover, it can be
observed that stability with regeneration area was existed in all these curves, which
was different from “zero” laser treatment groups. (For details, please refer to Section
4.1 and Section 4.2)
5.4 CORROSION MORPHOLOGY OF LASER-TREATED STEEL WITH
CORROSION INHIBITOR AFTER CORROSION TEST
Corrosion morphologies of the laser-melted and untreated steel with corrosion
inhibitor are compared in Figure 5.7 and Figure 5.8. As mentioned before, LSM helps
in dissolving precipitated and segregated phases, and finely redistributing or removing
inclusion and impurities.
(a)1 (a)2
(b)1 (b)2
95
Figure 5.7 Metallographic examination on LSM specimen with corrosion inhibitor
(1: 200X; 2: 500X) (a) Untreated; (b): LSM01; (c): LSM02; (d): LSM03; (e): LSM04; (f): LSM05
(c)2 (c)1
(d)1 (d)2
(f)2 (f)1
(e)1 (e)2
96
(a)1 (a)2
(b)1 (b)2
(c)2 (c)1
(d)1 (d)2
97
Figure 5.8 Microstructure examination on LSM specimens with corrosion inhibitor
(1: 100X; 2: 1000X) (a) Untreated; (b): LSM01; (c): LSM02; (d): LSM03; (e): LSM04; (f): LSM05
Refinement of microstructure was achieved by LSM resulting in decrease in icorr.
The surface of specimens after LSM were smooth and without porosities or cracks. In
this way, corrosion resistance was increased. From the standpoint of the economy,
maintenance and repairing programs can be reduced during its lifetime.
In real situations, all corrosion cannot be fully resolved in piping systems. Similarly,
corrosion in here can only be minimized but not be completely eliminated. According
to the corrosion morphology, filiform corrosion is the mostly existing corrosion form
on the surface of specimens. In fact, that is a common phenomenon on the steel within
high humidity environment and gives the surface the appearance akin to that of a lawn
riddled with mole tunnels. Fortunately, the damage to the material tends to be limited
(f)2 (f)1
(e)1 (e)2
98
but the effect on appearance tends to have little effect.
Moreover, the similar result can be obtained from such examination compared with
corrosion test data. Corrosion resistance of LSM03, LSM04 and LSM05 are higher
than LSM01 and LSM02. Severe corrosion with unpleasant appearance was observed
in latter, and is mainly caused by the incomplete microstructure refinement.
Nevertheless, corrosion resistance on all of these specimens can still be improved
after LSM. It is apparent that this treatment has a direct effect on the specimens. With
different combinations of laser parameters (laser beam energy, scanning speed, and
etc.), a satisfactory result was obtained. Here, the best improvement is LSM03 with
smallest icorr.
Besides, the growing trend of anodic scan in the laser-melted black steel is
becoming slow and stable. By this way, the corrosion rate would keep slow even if the
potential (or driving force) is further increased. As a result, more time is needed to
consume for completing the whole process (polarization scan).
In fact, there is a big difference in the result between these two methods – corrosion
inhibitor and laser surface melting. Refer to Chapter 4, passive layer can be formed on
the surface of specimen by corrosion inhibitor, resulting in decrease in corrosion rate
to further inhibit the corrosion. At that time, passive region and transpassive region
are existed. However, such case didn’t happen on the LSM specimens due to their
different principle of corrosion protection. It is mainly depended on the refinement of
its microstructure and homogenization of composition. Therefore, passivation didn’t
exist, and slow with stable corrosion rate would substitute for passive effect.
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CHAPTER 6: CONCLUSIONS
By decreasing the corrosion rate and prolonging the lifetime in piping system,
corrosion inhibitor and laser surface melting (LSM) have been used as improvement
tools in corrosion resistance. Their effects on corrosion characteristics, microstructure
changes and surface hardness were investigated.
6.1 CORROSION INHIBITOR ON CHILLED WATER PIPING SYSTEM
As mentioned before, water quality is one of the affected factors to corrosion in the
piping system. It was shown that improved corrosion resistance was obtained by
adding sodium nitrite-based corrosion inhibitor.
The corrosion rate was reduced with forming a protective layer on the surface by
addition of corrosion inhibitor at various concentrations and various temperatures. In
the present study, it was found that this improvement is related to the concentration of
corrosion inhibitor. Therefore, heavy oxide particles and high corrosion rate was
existed on the specimen if no inhibitor was used.
To compare with these experimental specimens, such corrosion inhibitor can
perform its best effectiveness if pH is around 9.0 to 10.0. In a sense, the corrosivity of
environment can be controlled by certain quantities of inhibitor. Indeed, this corrosion
improvement is not only used in low temperature system, but also used in high
temperature system (normally indicates chilled water return side) due to its low
limitation. For this reason, chemical dosing system is always installed at one or
several places for concentration maintain. Thus, it is not surprising to see that, there
100
are many casinos and buildings choose this method to deal with corrosion problems in
Macao. Although some of them are toxic, they are still playing a critical role in
numerous corrosion control strategies because of their easy operation.
6.2 LASER SURFACE MELTING ON CHILLED WATER PIPES
For huge chilled water consumption, most of the internal diameters of chilled water
pipes are designed from Φ30mm toΦ1000mm. Therefore, laser surface melting (LSM)
is recommended to modify the materials for enhancing corrosion resistance due to its
convenience and high cost-effective.
By using laser surface melting, surfaces with homogenization of composition and
refinement of microstructure were obtained. In most cases, laser affected region can
be divided into two distinct zones: melt zone and heat affected zone. The melt zone
geometry depends mainly on the laser processing parameters, especially power
density and interaction time. On the other hand, such melting, always followed by a
rapid solidification may induce an attractive hardening of the surface, which is
attributable to the presence of martensite and retained austenite.
For corrosion behavior, laser surface melting also gave an enhanced base to the
materials by defect removing and microstructure refinement. It is apparent that
various degrees of corrosion resistance improvement were observed on specimens
with different laser processing parameters.
6.3 CORROSION INHIBITOR ON LSM CHILLED WATER PIPING SYSTEM
To minimize the maintenance cost with desired corrosion activity of piping system,
analysis of corrosion characteristic on laser-treated specimens in corrosion inhibitor
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was performed. Here, a big improvement with large decreasing corrosion current
density was obtained.
In fact, corrosion inhibitor has been considered to be an effective tool for corrosion
resistance. However, this effect is not only for raw material, and also for laser-treated
material. For example, the corrosion activity in Section 5.3.2 had become nobler
compared with specimens in Section 5.3.1. This is partly because the desirable
properties of an inhibitor usually extend beyond those simply related to metal
protection. On the other hand, laser surface melting also brings a fine structure to their
treated materials for enhancing their own properties.
With these two preventive measures, it is much more efficient to prevent corrosion
into an attractive level for chilled water piping system.
6.4 PERSPECTIVES FOR FUTURE WORK
In this project, investigation on improvement of corrosion resistance of chilled
water piping system has been done through corrosion inhibitor and laser surface
melting. Although the result can fulfill the requirement on this piping system, further
work is still recommended as follows:
6.4.1 RESEARCH ON CORROSION INHIBITOR
Nowadays, many scientific and technical corrosion literatures have descriptions and
lists of numerous chemical compounds that exhibit inhibitive properties. In the
present study, corrosion inhibitor has a very attractive performance on corrosion
resistance in piping system. Therefore, it is famous to be chosen for real application
due to its simple operation.
However, it is still facing a series of problems, such as toxicity, availability and
102
environmental friendliness. It is suggested to do more research on such areas for the
improvement. For this reason, component of inhibitor should be mixed and
established by a process of trial and error. In this way, corrosion inhibitor will become
a trustworthy partner on water piping system.
6.4.2 APPLICATION FOR LASER SURFACE MELTED SPECIMENS
As mentioned in Section 2.4.2, flow velocity of chilled water was ignored in this
paper due to its variable speed. In fact, cavitation test is recommended to carry out for
laser surface melted specimens.
It is noticeable that hardness was increased after laser treatment. As a result,
improvement on erosion resistance should also be obtained from this modification.
Although cavitation erosion is not a common phenomenon at chiller plant, it may
happen at somewhere by chance. For minimizing the operation loss before using, the
feasibility of laser treatment for enhancing erosion resistance is worthy to be studied.
6.4.3 PROMOTION ON LASER SURFACE MELTING FOR PIPING SYSTEM
In the present study, corrosion resistance has been found to improve with laser
surface melting by producing fine near homogeneous structure and hardening effect.
However, big difference may be existed between experimental and manufacturing
arrangement.
Due to the limitation of internal condition of water pipes, process head may be hard
to access such long distance and narrow space. Moreover, processing time is also
needed to consider because of its small laser beam diameter.
To solve all of these limitations, study is recommended to carry out by overcoming
such problems and promoting this technology for widely application.
103
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VITA
LEONG HOI SAN
University of Macau
2011
Name of Author: Leong Hoi San
Place of Birth: Macao
Date of Birth: February 10, 1987
Undergraduate and graduate schools attended:
South China University of Technology
University of Macau
Degrees Awarded:
Bachelor of Electromechanical Engineering, 2008, South China University of
Technology
Working Experience:
Assistant Engineer, Dafoo Facilities Management Company Limited, since 2009
