E-Coating Stainless Steel
Q. We are electrocoating small assemblies made up of multiple metals. One of the parts has a small cap made of 306 SS. It is possible to electrocoat stainless steel parts with good quality and performance? W.M.
A. An electrocoat system can coat any objects or metals as long as they have enough electrical conductivit in the 0-400-v range. The electrically charged paint will attach or deposit to any surface in the bath that is electrically opposite in charge. With this in mind, electrocoat systems can coat copper, aluminum, stainless steel, zinc products, iron, nickel, gold, silver, chrome, etc.
The issue with running stainless steel through an electrocoat system is not the electrocoat but the pretreatment ahead of the e-coat. Pretreatment includes all the processes that clean and condition the workpieces prior to painting and curing.
Most electrocoat systems use alkaline cleaners for oil and grease removal. For hard metals, the cleaners must be strong in alkalinity to be able to remove the cutting and working oils used in manufacturing workpieces. However, strong cleaners could excessively etch soft metals like copper or zinc, so milder cleaners must be used for those applications. Stainless steel is a hard metal, therefore any cleaner will do.
After cleaning, an additional layer called conversion coating is applied. This layer must provide excellent adhesion to the clean substrate surface as well as promote adhesion to the electrocoat and be chemically compatible with it. Typical electrocoat systems incorporate a zinc phosphate layer as the conversion coating between the metal substrate and the electrocoat. In typical operations, the electrocoat material is deposited over this phosphate or conversion coating and not on the metal. Some applications use direct-to-metal processes.
The critical stage of stainless steel processing is selecting the proper conversion coating system, as typical phosphate conversion coatings do not work well with stainless steels. Most stainless steels have very good chemical resistance and fend off any chemical attacks very well. It is for this reason that it is hard to deposit conversion coatings on stainless steels surfaces.
Stainless steel could be coated by just cleaning and then depositing the electrocoat directly over the cleaned metal. If necessary, a light mechanical blast or peen can be used for improved adhesion of the electrocoat paint.
This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 12, 2012.
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