I recently read somewhere that operating a dry-off oven at temperatures in excess of 250°F can cause flash rusting to appear on iron phosphated steel articles. I am one who has observed this phenomenon on and off over the years but who has never associated its occurrence with high dry-off oven temperatures and adhesion problems. Could you please indicate the technical reasons for the development of the flash rusting in this manner as I am sure this information needs to be more widely known? Thank you for any help you can provide. K. C.
Thank you for reading my column and for writing to Paintings Clinic. Although not too much happens to a zinc phosphate except expulsion of the water of hydration, a dry-off oven whose temperature is greater than 250°F can have a disastrous effect on iron phosphates. At these high dry-off oven temperatures and owing to the low coating film thickness, the steel substrate has been known to oxidize without affecting the iron phosphate itself. This is indicated by a shift in the color of the iron phosphated surface from blue to gold. That’s the good news. The bad news is that because of this oxide, the corrosion resistance of painted articles is greatly reduced. By reducing the dry-off temperature, an operation can not only reduce manufacturing costs in these times of increasing energy-costs, it can also reduce rework and customer complaints.