Pickling and phosphating are the base coats before powder coating or baking finish; poor pretreatment makes even a thick coating peel off in sheets—a detail many overlook. Coating life is half surface, half pretreatment; the order cannot be reversed.
Complete pretreatment usually has five steps: degreasing to remove oil, water rinse, pickling for rust removal or shot blasting, phosphating film formation, and drying. After phosphating, the workpiece surface forms an even gray phosphate film that chemically bonds with the coating, giving several times better adhesion than direct painting.
Typical consequences of inadequate pretreatment: surface oil not fully removed gives poor coating adhesion, and it peels in sheets within half a year; rust not fully removed continues to blister under the coating, peeling from within after a year. These issues are invisible at the factory and only surface after use.
On acceptance, scrape a small patch of coating and look at the cross-section: an even phosphate film and firmly adhered coating means it passes; if the coating peels up in sheets on a scratch and shows oily sheen underneath, pretreatment was skimped. A good pretreatment line also has independent water rinse tanks and drying temperature records; incomplete drying leaves moisture that causes coating blistering.
Selection advice: ask the manufacturer whether pretreatment is phosphating or simple painting, and whether a pickling step exists. Nanjing Tongnuo Warehouse Equipment Manufacturing Co., Ltd. uses complete phosphating pretreatment before powder coating, with coating adhesion reaching grade 0 by cross-hatch test.