the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
By Dave Wright [deceased]
- Mequon, Wisconsin
With sadness we note Dave's passing on Oct. 11, 2013. His longtime friend Anne Goyer offers a tribute to him in The Finishing Touch, Vol. 23 No. 4.
Excerpts from 'The Finishing Touch',
the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
by Dave Wright
Senior Technical Representative
Texo Corporation
Send your Questions to davewrit@execpc.com
Q. We are currently using zinc phosphate and oil for small parts. We have a requirement of 96 hours of salt spray. Can we get out of using zinc?
A. Most likely. It is the oil that is providing most of the corrosion resistance. If you can get the surface to hold the oil, you should get the resistance. There are a number of newer "heavy iron" phosphates on the market that will provide nearly the absorbing effect of a light zinc phosphate. Coating weights of up to 200 milligrams per square foot may be obtained. You will not get the same galvanic resistance as the zinc phosphate as you are no longer applying a sacrificial metal to the surface. This makes the oil selection even more critical. There are some high performance products that are capable of much higher salt sprays than 96 hours. Most require a low temperature forced air dry to facilitate a curing cycle.
Q. We have just started to show zinc in our effluent analysis, but use iron phosphate. Where is it coming from?
A. The information you provided gives very few clues, but I will try to provide some direction to focus your efforts. You have to assess all the avenues that zinc could be entering your washer (spray washer is assumed). If you are not using zinc containing chemicals, the likely avenues are the parts, the water, the washer itself. The parts are the most likely culprits. Do any of your lubes or oils contain zinc bearing compounds? Are you running galvanized, die cast or plated parts? Are your steel parts made from a product (like extruded steel rod) where zinc phosphate or a zinc containing lubricant was used as part of the process? Does your water contain low levels of zinc that is concentrated as the bath evaporates and is replenished? Has maintenance recently worked in the washer adding pipe? Was it galvanized? In summary, your detective work is just starting. Once you find the source, you can take steps to either rid or minimize the accumulation of the metal.
Q. Can you simply explain why we titrate and do a pH check on our iron phosphate bath? Remember I am not a chemist!
A. I'll try!. The titration is a procedure that uses an indicator solution (usually phenolphthalein) that changes color at a specific point of acid concentration. Taking a standard, consistent, quantity
(i.e. 10 milliliters) and using a standard titrating solution (i.e.
0.1N Sodium Hydroxide) you can measure the concentration of the chemical. When you check pH, you are measuring concentration of hydrogen ions in the bath or the activity of the bath. These are over simplifications of what is being done, but I hope they answer your question.
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