Parts Cleaning Clinic: Phosphate Coating Problems

Q. We electrocoat sheet metal, iron castings (machined and non-machined) and aluminum castings (machined and non-machined surfaces) and are having problems with poor phosphating on aluminum substrates. We maintain the free acid at 0.7-1.1 points and total acid at 20-24 points, and the total fluoride at 1,500 ppm. Bath temperature is 40°C to 45°C. Any suggestions?

A. I was wondering whether you have a phosphating bath formulated for aluminum, since that can be a difficult metal to coat. When I came upon your comment with regard to the fluoride concentration, I knew that you at least had a formulation intended for phosphating of aluminum. I appreciate the detail regarding the operating conditions, but without more information on the specified operating range, it is a little difficult to diagnose what may be the problem.

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One item that does stand out is that the free acid amount seems quite low in relation to your total acid value. In order for the fluoride to work effectively in the bath, it will need to be at a high enough concentration that it will attack the aluminum to allow the phosphate reaction to occur. Your supplier’s technical service representative should be able to provide you with the proper process control parameters, control procedures and titrations as well as the correct operating ranges and corrective action if the tank is found outside recommended ranges. They can also warn you if there is buildup of cross contaminants because of the mixed metal pretreatment. If your supplier cannot provide this level of support, it is time to look for a new one.

Originally published in the November 2016 issue.