Cleaning Q&A: Rust on Untreated Zinc Phosphate Surfaces

Q. We are doing zinc phosphating for WCB grade cast iron valve bodies. The external surface is either painted or powder coated. However, the internal phosphated surface rusts if no rust preventive oil is applied within two or three days. Is there any solution?

A. The zinc phosphate pretreatment system is generally not intended to be a corrosion-resistant coating by itself. This conversion coating reacts with the base steel (or iron casting in this case) to precipitate an inorganic zinc phosphate crystalline structure on the iron surface where contacted by the phosphating solution. This zinc phosphate coating is intended as a paint base to improve the coating or powder adhesion and provide some additional corrosion resistance.

However, the zinc phosphate coating is not intended to have any significant corrosion resistance alone. In your case, fogging the internal surfaces with a rust preventative oil is the right step. There could be additional process parameter improvements pushing out your ambient environment corrosion protection (increased zinc phosphate coating weight, improved rinsing, improved seal coat performance), but it seems the best alternative is to ensure the internal valve body is coated with the rust preventative. This could be incorporated into the step where the operator is removing the valve body from the paint line.

 

Originally published in the June 2016 issue.