Minimizing Rust, Part 2

Question:

I read your clinic in the December issue and would like to point out factual errors in the section titled, “Minimizing Rust.” At the end of your response, you have given the parameters necessary to achieve a particular result in the salt spray test. The pretreatment requirements that you listed are part of the “urban legend” of the metal pretreatment business.

In general, there is no correlation between phosphate coating weight and corrosion resistance. To say that a part that has 60 mg/ft2 of iron phosphate coating is somehow worse than one that has 70 mg/ft2 and thus will not achieve 500 hr before failing in a salt spray test is unfounded. Corrosion resistance is directly related to coating quality, not the amount of coating. The protective quality of an iron phosphate coating will vary, according to the accelerator system used in the product.

It is far more important that a phosphate coating (iron or zinc phosphate, doesn’t matter) provide uniform and complete coverage on the metal substrate. Voids in the coating will be more likely to cause the early onset of corrosion than a coating weight that is slightly below some arbitrary standard.

I have been in the metal pretreatment business since 1984, and I remain astounded at the extent to which the myth “more coating weight is better” continues to be promoted. Your reply is a further promulgation of that myth and needed to be corrected. Best regards. G.G.

Answer:

Wow!! That’s what I get when I try to be too specific. I used ³ (greater than or equal to) when I should have used » (approximately) when referring to how much iron or zinc phosphate should be on a steel substrate. I am not sure it rises to the level of “factual error,” though. Leave it to a chemist to notice the faux pas of using the wrong math symbol. However, these values are not an “urban legend.” Rather, I pulled the information from the textbook “Powder Coating, The Complete Finisher’s Handbook” as published by The Powder Coating Institute where no less than 51 authors agreed to this information before it was published.

You are absolutely right that an even phosphate coating is very important in corrosion resistance and that 60 mg/ft2 may provide sufficient corrosion resistance for my examples. We all know that “more is not better” when applying iron phosphate, especially when it goes much beyond 70 mg/ft2. This is because too much iron phosphate will leave a powdery residue on the substrate, which will cause all sorts of finishing defects (poor adhesion, and surface contamination to name a few).

I do disagree with one of your points: “There is no correlation between phosphate coating weights and corrosion resistance.” It is proven that to achieve the specific threshold of corrosion resistance I list in my examples that you need a minimum coating weight that is closer to 70 mg/ft2 than it is to 30mg/ft2. So coating weight does matter!

Thanks for reading my column, and I will be more careful when using mathematical symbols in the future.