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 have a two stage washer. We have some new parts that we had to slow the line down to get a good cure. Since that time, everything wants to flash rust. All the parameters in the washer are within specifications, and we have even dumped and recharged to no avail. Do you think it is the steel?
A. It could be, but more than likely it is the decrease in line speed that is hurting you. You didn't say how much you slowed the line down, but could you have slowed it enough that parts have an excessive dwell time in between stages, or are even drying down? I would check this first. If this seems like the culprit, try installing misting nozzles in between the stages or extending a riser from the rinse or first stage into the "dead" area. This may help keep the parts wetted out. Another option might include trying to increase the cure temperature so that you can bring the line speed back up.
Q. It is difficult to maintain pH in the specified range of our five stage, iron phosphate washer. The pH keeps creeping back up. If we add phosphoric acid as our chemical supplier recommends, we get lousy looking parts. What is happening?
A. I'll bet you are on well water that is very "hard". The reason for this is the alkaline nature of the makeup water. As the water evaporates and is made up with fresh water, you are continually adding alkaline salts (primarily Calcium and Magnesium) which tend to elevate and "buffer" the pH level. As this buffering increases, it will take more and more acid to maintain pH. The easiest and most cost effective solution is to provide low solids (deionized or reverse osmosis), make up water. Low solids water has very little of the alkaline salts of normal (good drinking) water. It will not buffer the pH to the alkaline side nearly as much. Other benefits can include reduced sludge volume and increased corrosion resistance. Although a "system" will be less costly to operate, a replaceable cartridge system, is perfectly acceptable and an inexpensive way to prove it out. If you would like the details of how to set one up, let me know and I will describe it to you in more detail than we have space for here.
Q. We process steel and now occasionally some aluminum through our seven stage washer. When we run aluminum, the parts sometimes come out with a dark gray looking material that you can wipe off. Although we have no real salt spray requirements for the aluminum, adhesion of our powder is significantly worse than the steel. Any ideas?
A. Did you tell your chemical vendor that you were (or are) going to run aluminum? I'll bet not. Sounds to me like you are using a cleaner that is not "multi-metal" safe. By that we mean that it is probably a Sodium or Potassium hydroxide based cleaner that is not inhibited against attacking aluminum or zinc. Talk to your chemical vendor and let them know your plans. They should be able to come up with a
"safe" cleaner that is either inhibited against attacking aluminum or one that is caustic fee that will do the job. While you are at it, ask about a phosphate that contains fluoride to etch the aluminum for better adhesion. You really don't deposit an appreciable amount of iron phosphate on aluminum, but that is another topic altogether!
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