Phosphating is the most preferred surface treatment process used for auto body sheet panel before painting due to its low-cost, easy production process, good corrosion resistance, and excellent adhesion with subsequent paint layer. There are different phosphating processes used for ferrous metal like zinc phosphating, iron phosphating, di-cationic & tri-cationic phosphating, etc. Among these phosphate coatings, the best corrosion resistance and surface adhesion are achieved by tri-cationic phosphate coatings (zinc-nickel-manganese phosphate). Many new technologies of phosphating are evolving. Key drivers for this evolution are increasing demand for higher corrosion resistance, multi-metal car body processing in same phosphating bath and sustainability initiatives to reduce the carbon footprints.
We have evaluated two of these recent technologies. First technology being evaluated is low temperature phosphating in which phosphate bath temperature is reduced by 10°C and second is liquid activation chemicals for phosphating in which liquid phase chemical is used for activation instead of conventionally used chemical in powder form which resulted in increased activation bath life.
Characterization of the phosphating layer formed with these technologies on different sheets like cold rolled steel (CRS), galvanized (GI) and galvannealed (GA) steel is done using SEM and EDS techniques [5]. This study shows that there is significance difference in phosphate crystal size and shape formed, and this has a bearing on the surface protective performance of the coating.