Rinse Water Quality The Importance for Metal Finishing

December 19, 2011

Water Quality: What does this mean ? Water is arguably the most important component in the pre-treatment process. Surprisingly, it is a component that is often ignored and overlooked in the metal finishing process. Very few finishers completely understand the role that water plays in a companies operations. This article will outlines everything you need to know about rinse water

quality in the metal finishing industry. It will cover the following topics:

1. The Chemistry of Water 2. How Rinse Water is Used in Metal Finishing 3. Total Dissolved Solids (TDS) 4. Why its Important ? 5. Types and Options of Water 6. Research Data

What is Water ? At its most basic, water is a molecule with one oxygen atom and two hydrogen atoms, bonded together by shared electrons. It is a V-shaped polar molecule, which means that it's charged positively near the hydrogen atoms and negatively near the oxygen atom. Water molecules are naturally attracted and stick to each other because of this polarity, forming a hydrogen bond.

x Appearance = Colorless, transparent, with a slight hint of blue. x pH = 7 (Neutral) x Melting Point = 0 Celcius or 32 F x Boiling Point = 100 Celcius or 32 F x Viscosity: 0.001 Pa s at 20 Celcius

For a full explaination of the properties and features of water: Click Here

Rinse Water in Metal Finishing No matter what type of chemical pre-treatment system your company utilizes, rinsing comes into play. The most common sequences and uses of rinse water are seen in the following scenarios:

5 stage wash example Alkaline clean / Rinse / Phosphate / Rinse / Final Sealer

4 stage wash example Alkaline clean / Rinse / Phosphate / Rinse

3 stage wash example Iron Phosphate / Rinse / Rinse or final sealer

2 stage wash example Any Chemical Treatmen / Rinse The primary function of rinse water in metal finishing is to remove all chemical from the substrate and prepare the surface profile with paint bonding characteristics.

Total Dissolved Solids (TDS) and Contaminants What are Total Dissolved Solids ? Total Dissolved Solids (TDS) are the total amount of mobile charged ions. This includes: minerals, salts or metals dissolved in a given volume of water. TDS is expressed in Parts Per Million (PPM) or in units of mg per unit volume of water (mg/L). TDS is directly related to the purity and overall quality of the water. Several minerals are commonly found in city rinse water:

x Calcium x Sodium x Potassium x Flouride x Magnesium x Bicarbonate x Nitrate x Chloride x Copper

Example of water TDS's

x Deionized Water (DI) = 0 PPM

x Reverse Osmosis Water (RO) = 5 - 15 PPM x Calgary Water = Approximately 180 - 200 PPM x Edmonton Water = Approximately 150 - 250 PPM x Greater Vancouver Area = Approximately 20 - 60 PPM x Rural Alberta, Saskatchewan and Manitoba = Approximately 1000+ PPM

Why is this Important ?

Water Spotting, Blistering, Adhesion Loss and Pre-Mature Corrosion The higher the TDS, the poorer the coating performance. When rinse water dries onto your part, the minerals in the water are left on the surface. If the TDS is high, there will be a significant amount of minerals that will dry on the surface. This often leads to water spotting.

The white residue that is left on the surface are the minerals in the water and can cause several issues for a painted products. This can often show through the paint and cause pre-mature blistering and corrosion. See image below:

Water spots can be seen through the paint Most of the primary minerals in water are alkaline by nature. Alkaline minerals will draw moisture in and hold it underneath your paint.

Over time, this moisture can cause loss of adhesion, premature blitering, corrosion and rust spots. See examples below:

Blistering occurs on a painted product

Loss of adhesion on painted product

Poor corrosion resistance on painted product Other Issues 1) Scale on Burner Tubes - the minerals in the water deposit on burner tubes over time and coat them with scale. This scale builds up and becomes quite thick. The burner may take longer and may need to be set higher to achieve required temperatures. Eventually, a chemical or mechanical descale would need to be done. 2) Increased Overall Maintentance of Equipment - as mentioned above, water minerals deposit on equipment and build over time. Increased maintenance needs to be done to keep application equipment functioning properly. 3) Poor Cleaning and Decreaed Bath Life - the level of minerals in the water can greatly effect any chemicals cleaning ability and bath life. Certain minerals actually react with the active ingredients in the chemical and cause them to be less effective. 4) Poor Rinsing - water spotting, poor adhesion and poor corrosion resistance. Understanding, selecting or using the appropriate rinse water is critical for any metal finishing company. Depending on the performance requirements for your company a variety of options are available for rinse water. They are as follows:

1. Plant Water 2. Reverse Osmosis (RO) 3. De-ionized (DI)

What are the rinse water options ? Plant Water All rinse water will be provided from the plant/city water source. No special equipment or requirements are necessary. The quality of the plant water will vary depending on the geographic location. Reverse Osmosis (RO) Water Reverse osmosis uses a membrane that is semi-permeable, allowing pure water to pass through it, while rejecting the contaminants that are too large to pass through the tiny pores in the membrane. Reverse osmosis (RO) units remove substantial amounts of most inorganic chemicals. They are effective at removing minerals and metals such as aluminum, arsenic, barium, cadmium, chloride, chromium, copper, fluoride, magnesium, iron, lead, manganese, mercury, nitrate, selenium, silver, sulfate, and zinc. Typically, RO systems will control TDS levels within the 5 - 15 PPM range; Usually 10 PPM. RO water can improve coating performance and reduce pretreatment chemical usage when used as make-up water for chemical baths. However, DI water is preferable as a final rinse, because the typical purity of RO water falls short of levels required for best coating performance. For a detailed explanation of how Reverse Osmosis (RO) is created;Click Video Below:

De-Ionized (DI) Water The deionization process removes ions from water via ion exchange. Positively charged ions (cations) and negatively charged ions (anions) are exchanged for hydrogen (H+) and hydroxyl (OH-) ions, respectively, due to the resin's greater affinity for other ions. The ion exchange process occurs on the binding sites of the resin beads. Once depleted of exchange capacity, the resin bed is regenerated with concentrated acid and caustic which strips away accumulated ions through physical displacement, leaving hydrogen or hydroxyl ions in their place.

Typically, DI systems will control TDS levels within the 0 - 15 PPM range; Usually 0 PPM. Using DI water in a pre-treatment system is the best way to ensure consistent results and maximum coating performance.

Performance Data The Effect Of Rinse Water Quality On Powder Coating Corrosion Resistance A study was conducted to compare the variation in performance using a variety of water sources. The following is a summary of the experiement and the findings. To facilitate the experiment, 20 steel test panels were prepared by ACT Laboratories Inc. as per the Bonderite 1000®, 6 P-60 process outlined below.

1. alkaline clean 2. hot rinse 3. hot rinse 4. cool rinse 5. iron phosphate (B-1000) 6. ambient rinse 7. chrome sealer 8. deionized water rinse 9. dry off

The panel matrix consisted of the following panels and processes:

All panels were placed in a salt spray cabinet (ASTM B117) and a copper accelerated salt spray (GM 4476P). These results were reported at 312 hours:

Pa

Tpeac C

anels B, C and D

The study concerformance. chieve the hig

Click Here to

C

D all failed drasti

cluded that thit was recom

ghest quality

o view the c

ContacEmailWeb:

Fin

ically at 144 Hou

he higher themmended that

paint perform

complete do

ct We: info@: www

nd Wes

urs with comple

e water TDS, tt Deionized (Dmance.

ocument

estChe@west

w.westc

stChe

ete delamination

the worse theDI) water is r

em Totchemchem.

m On

of the coating.

e coating ecommended

oday m.ca

ca

:

d to