Iron Phosphate High Temperature Performance

Q. I read the question and answer in the Dec. 2005, Painting Clinic titled “Hot Dry-Off Oven Causing Problem”. My company paints carbon steel parts with a baking enamel. Before painting, these parts are processed through an immersion paint pretreatment consisting of alkaline cleaning, iron phosphate and seal, with rinses between each step. After painting, these parts are cured in an oven at 300°F for 15–30 minutes. My question is will curing the parts at 300°F adversely affect the iron phosphate coating? Thank you for your response. T. K.

A. In the referenced question, K. C. observed flash rusting of iron phosphated steel parts in a too hot (350ºF) dry-off oven. I answered that a dry-off oven whose temperature is greater than 250ºF can have a disastrous effect on iron phosphate treated steel parts. At these high dry-off oven temperatures and owing to the low phosphate film thickness, the steel substrate has been known to oxidize (flash rust) without affecting the iron phosphate itself. This is indicated by a shift in the color of the iron phosphated surface from blue to gold. Because of this oxide, the corrosion resistance of painted articles can be greatly reduced.

Atmospheric oxygen must contact the phosphated steel substrate for this oxidation process to occur. Since your parts are painted before heating above the critical temperature of 250ºF, the paint eliminates atmospheric oxygen contacting the substrate and it will not oxidize. Don’t worry, you are safe.