Powder Coating Q&A: Corrosion Resistance Options

Q. We have a chance to do a job that involves some large steel enclosures that will be used in an outdoor location near the seacoast. Most of the product we coat has modest requirements for corrosion resistance, but this job will need a very tough film that will not break down in one of the toughest environments. We have a 5-stage iron phosphate washer and we are not sure that it will provide enough corrosion protection. Any suggestions on how to get better outdoor performance would be appreciated.

A. The options for corrosion protection can take many forms. The important thing is to remember that coating a part is an engineered process and all of the parts of the process add up to the final performance. Pretreatment with iron phosphate will remove oils and similar contamination so the bond is good but the iron phosphate will not do much to protect the steel if the coating breaks down. If you were using zinc phosphate or chrome you would get some protection and that would put less pressure on the coating layer. With iron phosphate there is a lot of pressure on the coating layer. You will need to add a primer, preferably a zinc rich primer. The zinc rich primer will provide another barrier to protect against moisture penetration. The zinc content is a reactive, sacrificial barrier. The topcoat adds additional protection from moisture and ultraviolet rays. The combination makes up for the limited protection of the iron phosphate pretreatment. The zinc primer will work best over a blasted surface. The rough surface helps to set up the proper anodic electrochemical relationship between the steel and the zinc content in the primer. So your process should be as follows: blast the surface with a medium-grit aluminum oxide or garnet (not steel and not spherical shot), and run the parts through your 5-stage process if they will fit. If they do not fit you can blow off the dust and grit with clean, dry compressed air. Apply a generous layer of zinc rich primer and partially cure the primer layer; apply the topcoat and fully cure the part. This should provide the results you need.

Originally published in the November 2015 issue.