Q. My company produces cold rolled steel equipment boxes for both interior and exterior use. Indoor boxes are fabricated from cold rolled steel and outdoor boxes are fabricated from hot dip galvanized steel. After we upgraded our finish system from alkyds to polyesters, we had problems on the hot dipped galvanized boxes in the form of blisters. Our metal pretreatment process for all products has the following five stages: hot alkaline cleaner, cold water rinse, hot 10% phosphoric acid, cold water rinse, warm water final rinse. The blistering only occurs on the hot dipped galvanized steel, not the cold rolled steel. What is causing our problem? C.F.
A. I suspect your five-stage pretreatment process does not passivate the zinc on the surface of the hot dipped galvanized steel, allowing it to react with some component of your polyester coating. The reactivity of metallic zinc and the passivating effects of phosphate pretreatments are well known. I prefer using a zinc phosphate pretreatment for zinc surfaces, although a properly formulated iron phosphate will provide a passivated paintable surface. If you must use an iron phosphate pretreatment, you will probably eliminate the blistering problem by replacing the 10% phosphoric acid in your third stage with a commercially available iron phosphate chemical specifically formulated to pretreat both cold rolled and galvanized steel.
You may also consider using a non-chromate acidulated rinse to replace the warm water final rinse. Although this will increase your finishing costs, it will improve your finish system and decrease your reject rate, thereby increasing productivity.