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. For many years our iron phosphate has had a fairly uniform gray color with periodic spots of bright blue, about 4-12" in diameter on parts that are 4'x2' sheet metal. We have always been told this is normal. We looked at some equipment at another plant that does similar parts. They don't have these spots, their parts are a uniform color throughout. Any ideas as to why we don't have the uniform appearance? T.H. Chicago, IL
A. It sounds like you have a condition that goes by a number of names. They are called hot spots, cats eyes, windows, etc. It occurs at sites on the part where the phosphate doesn't react well. This can be due to a number of reasons, but the important thing to know is that it is usually easily corrected. The primary cause is commonly too little acid content in the bath. Contact your pretreatment service person and ask the question. If they don't respond or don't know, try reducing the pH of the bath a little bit at a time with phosphoric acid. This should solve the problem.
Q. What is an oil splitting cleaner and how do I know if I need one? M.W. St. Paul, MN
A. In very simple terms, two types of cleaners exist. Oil splitting or rejecting cleaners, allow free oil to float to the surface of a cleaner bath while oil emulsifying cleaners hold the oil in suspension. In a spray washer, the oil splitting cleaners are generally preferable as they allow oil to accumulate on the surface of the bath where it can later be skimmed off for disposal or recycling. In dip operations, emulsifying cleaners have typically been used so that you don't pull the cleaned parts back through a layer of oil on the surface, recontaminating them. Equipment design in a dip tank can incorporate a weir system that allows the surface to be continuously skimmed or swept for free oil. Since the primary reason cleaners are "dumped" is oil contamination, keeping the oil out of solution can greatly prolong bath life. Most of the new
"closed loop" type of systems depend upon oil splitting cleaners for oil removal.
Q. We run a five stage washer that uses a Sodium Hydroxide based cleaner that has typically run at a pH of about 12-12.5. Since a major cleaning and boilout of the system, the pH falls off to about
9.9-10.5, resulting in dramatically increased chemical consumption, and a perception of loss of cleaning power. Any ideas? A.P. Memphis, TN
A. You didn't say how you heat the tank, but I will assume it is done by a gas firetube. Check your firetube for leaks, and they maybe small ones. Small pinholes in the tube can cause a leak of exhaust gases into the bath when the burner is on, yet not allow sufficient liquid in the tube to prevent startup. These exhaust gases are acidic in nature and can dramatically reduce pH forming a well buffered solution around a pH of 10. You can see for yourself how dramatic this can be. Take a jar or Erlenmeyer flask and fill it about half way with fresh water. Add 3-4 drops of Phenolphthalein indicator and then add one drop at a time of 0.1N Sodium Hydroxide titrating solution until it turns a stable pink. Now the magic, take a deep breath, hold it for a second, and exhale directly into the container, cap it and agitate it. If nothing happens exhale into it again, cap it and agitate. The solution will soon turn clear due solely to the Carbon Dioxide content of your exhaust gases. With this in mind, it is not too hard to imagine what a burner can do, leaking directly into the tank!
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