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. Which is better, Iron or Zinc phosphate? A.G. Dallas, TX
A. Whew, that's a loaded question! You have to define "better". Zinc is generally regarded as better for corrosion resistance to iron. Usually the substrate, specifications and existing equipment will dictate which material you should or can use. Zinc phosphate requires at least a five stage washer with a phosphate stage made of either stainless or one of the new composites. Zinc costs more to purchase and to operate. With Zinc you will automatically be dealing with a regulated metal and substantially more sludge (a normal byproduct of any phosphate operation). I have seen a number of conversions recently from zinc to some of the newer iron formulations with no degradation in performance at all. Let your parts and processes talk to you, that will make a decision easy.
Q. We clean parts with a five stage washer that contains an alkaline cleaner and iron phosphate prior to welding. The weldment is then painted prior to assembly. Many times the parts rust well before they are painted or even welded. What's happening? E.S. Chicago, IL
A. One of the clues you provide is that the parts are rusting before welding. Iron phosphates are not very good rust inhibitors, without a topcoat. The purpose of the phosphate is to act as an anchoring point for the topcoat. Iron phosphate can even accelerate rusting if not topcoated immediately. If you need to use the washer for pre-cleaning of parts, consider adding an aqueous rust inhibitor. There are a number of good products on the market made by a variety of chemical vendors that should be do the job. Many of them can be painted. This might allow you to add it to your washers last rinse stage without adding an extra operation. Most of these compounds are used in the
1-5% by volume range and usually pose no discharge problem.
Q. We really like our powdered cleaner, but it is a problem to feed into the tank. Would a liquid be better? Any ideas on feeding a powder? T.J. Phoenix, AZ
A. The use of cleaners in a liquid or powder form presents a number of problems and opportunities for creative thinking. Liquids are typically a little more costly, and easier to use. Liquids lend themselves to automated dispensing, resulting in less operator exposure. Consider premixing the powdered cleaner with warm water making a concentrated liquid slurry. This will help to get all the cleaner into solution more quickly.
Q. I am having a debate with our plant manager over rinse water usage. We have a five stage washer with stages #2 and #4 being overflowing fresh water rinses running at about 15 gpm each. I say we need to maintain the rinses and he says we can reduce the flow. What do you think? A.R. Saginaw, MI
A. I think you are both right. Rinsing is a very important element in any pretreatment system. The rinse water must be kept "clean" enough to sufficiently dilute the clinging film of process solution, from the previous stage. This helps to stop chemical reactions and minimize contamination of subsequent stages. This is not to say you need to run water flows wide open to achieve a quality rinse. Rinsing is a topic that I could devote a book to rather than a one paragraph answer. But there are a number of very cost effective things you can do to minimize water use in a spray washer. Here is what I would do first. Get a hold of a conductivity meter that reads in Mhos. (micro mhos, the inverse of an ohm). These devices are available at a reasonable cost or see if you can borrow one (your chemical vendor should have one). Test your fresh, incoming plant water. It should typically read between 200 - 900 Mhos. Record this number, this it the best that your rinse water can possibly read, today. Now test a rinse tank. If it reads the same, or nearly the same as your incoming water, back off the water flow (a little at a time) until the reading starts to rise. Do it slowly and cautiously, watching the parts carefully. As a rule of thumb, I would have no problem doubling your incoming waters reading for this stage. Using techniques such as this one, counter flowing and counter filling, you can maintain rinse capability while maintaining peace with the plant manager.
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