Corrosion Resistance

Q. We need some reasonable corrosion protection for out parts, and our powder coating is failing in salt spray testing after nearly 300 hours. What should we change to enhance our salt spray results? B.P.

A. First, let’s be clear on the difference between salt spray results and corrosion resistance in field use. Salt spray is a measurement of how well the coating holds up in a salt spray cabinet. It cannot predict the real resistance to a particular environment. It is useful for comparison of different treatment and coating options, but it does not predict field life. If you need to understand the potential field life you should use some type of cyclic testing.

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When a coating does not have good resistance in a corrosion test it can be from several different causes:

• The surface that the powder is applied to is not clean enough or lacks a sufficient conversion coating. For example, iron phosphate on steel with a single coat of powder will usually provide around 250 to 500 hours of salt spray resistance; zinc phosphate can increase that to between 500 and 1,000 hours; a primer coat and topcoat combination can give you well over 1,000 hours of salt spray.
• Your coating may not have very good performance properties. If you need corrosion resistance, be sure to let your powder supplier know it.
• The coating could be applied too thin. Be sure you have at least 3 mils of coating if you need corrosion resistance.
• The coating may be under-cured. Be sure that the film is fully cured.

If you have a satisfactory cleaning and pretreatment setup, a good powder, and good coverage at the necessary thickness you should be able to meet your salt spray requirements.