Optimization of Darkgreen Passivation Process for Galvanized Steel Sheets

Darkgreen passivation film was prepared on galvanized steel sheets by dipping in solution made of four types of acids incorporated with stabilizer and toner. The effects of bath composition, temperature, pH value and dipping time on performance of passivation film were investigated, and the optimized passivation parameters were determined. It was found that the optimal bath composition was determined as 20-30 g/L CrO3, 10～13 ml/L H3PO4, 2～6 mL/L H2SO4, 3～5 mL/L HNO3 ,3～5 mL/L stabilizer and 2～10 mL/L toner. The optimal passivation parameters were suggested as pH value of 1.0～ 1.5, temperature of 20～30 ℃ and passivation time of 40～60 s. The resultant passivation film was darkgreen and had gentle luster, with endurance to neutral salt spray of 400 h and scratch times of 100 in adhesion tests, far exceeding 10 times specified in relevant national standard. The established process had been well applied in industry.