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
Q. We use hot rolled steel that we pickle in a 15% Hydrochloric bath prior to phosphatizing. This is an all immersion process (clean, rinse, pickle, rinse, rinse, then to either zinc phos and oil or oil) with the exception of a spray rinse after the pickle. Some parts go on from there to be painted. The problem is a surface smut, usually black to gray, which appears on the surface after the pickling stepIt doesn't happen with all parts. This smut interferes with the subsequent finishing steps. Can you tell me what it is and how to either prevent it or remove it? T.L. Chicago, IL.
A. This soil actually originates in the steel itself. Grades of steel range from high-carbon low-alloy, to ultra low-carbon, high-alloyed steels. It is the high carbon grades which will exhibit this problem upon pickling. Quite simply, the acid soluble oxides and iron are dissolved from the surface during pickling, but the acid-insoluble components like carbon and metal hydrolysis products, are concentrated on the surface. It is often difficult to prevent, as most Hydrochloric used for pickling is already inhibited and if you inhibit it too much, you won't attack the oxides you want to remove. These unwanted surface soils are not chemically bonded to the surface, and so can usually be removed by physical means. These include high pressure sprays and other abrasive techniques. There are a couple of things you can try to improve the removal of the smut in your current process. You can add a wetting agent, or rinse aid, to the spray rinse after pickling. This can lower the surface tension and release more of the soil from the metal surface, effectively enhancing the rinsing efficiency. A more aggressive means can be applied to remove the soil. Since the soils are insoluble in acid, a strong caustic solution may be helpful in cleaning them. I have had success in removing soils like this with a caustic product following the rinse after pickle.
Q.Our paint (high solids polyester bake) is not curing properly in some spots on our parts. This is exhibited by a still soft to the touch surface that may be adjacent to a perfectly hard finish. Our paint vendor blames it on pre-treatment but hasn't offered a satisfactory explanation and the pretreatment vendor says "no way" is it his problem. I am tired of the finger pointing, any ideas? D.B. Los Angeles, CA
A. I really don't have enough information to give you a conclusive answer, but it is possible the pretreatment may be at fault, especially if it is spotty. Many paint/powder cure cycles are inhibited drastically by a high (alkaline) pH. Check to see that your rinses are working properly or if you have a conveyor or washer drip of an alkaline material. If you can actually see spots on the parts prior to painting, touch them with a piece of pH paper to see if they are strongly alkaline.
Back to finishing.com Home Page