Flash Rusting

Q. I run the paint shop at my plant and am faced with a never-ending problem. We manufacture large tanks that are painted inside and out. Before painting, we treat the steel with a zinc phosphate pretreatment. We are using an immersion process. Our process begins with an alkaline cleaner and then a double water rinse. This is followed by a muriatic acid pickle. Our problem is that right after the acid pickle, the tanks flash rust severely before we can get them into the zinc phosphate solution. Since the tanks are so large, and the layout of our plant is odd, it takes about 15 min to move them from the acid and rinse baths to the phosphate baths. Is there an additive that can be put into either the acid baths or the rinse bath to prevent this from happening? K.G.

 

A. From the description of your process, it sounds like you are pretreating rusty and oily steel before painting. The sequence seems to be right, removing oily soils before pickling, but the fifteen minute lag time between cleaning and phosphating solutions is longer than normal in a pretreatment line. This accounts for the flash rusting.

To solve the flash rusting problem, you have the following two choices: 1) Rearrange your pretreatment stations by moving the cleaning stages closer to the phosphate stage. This change would probably be an expensive use of time, materials and labor. 2) Add an anti-rust compound to your water rinse after the pickling stage. Your pretreatment chemical supplier can recommend a compound that is compatible with his materials.