Study on Electrogalvanizing Technology and Chromium-Free Passivation Treatment of 08Al Plate

An environment-friendly electrogalvanizing technology of 08Al cold rolled steel sheet (CRS) was developed. 08Al CRS sheet was placed as cathode in the electroplating solution containing zinc salt 1-10g/L, organic acid 3-24g/L, complexing agent 0.2-0.8g/L, and Hot-dip galvanized alloyed sheet (GA) as anode. The relationship between the current density and the plating mass per unit area was studied by applying 0.15-2A direct current at room temperature, current density 0.19-2.6A/dm 2 , and plating time 35min. The relationship between plating mass per unit area and anode thinning per unit area was linear regression by introducing current density as intermediate quantity. The effect of electric power range on the coating mass per unit area was studied under the condition of different concentration of main salt. The chromium-free passivation treatment was also carried out on the surface of electrogalvanizing layer. 08Al electrogalvanized sheet and GA sheet were placed in chromium-free passivation solution containing molybdate 1-10g/L, hydrogen peroxide 1-5g/L, fluorine ion complexing agent 1-10g/L, potassium hydroxide 1-2g/L. The temperature and reaction time of chromium-free passivation solution and the corrosion resistance of chemical conversion film were studied, and the experimental conditions of chromium-free passivation were optimized. The adhesion and corrosion resistance of the coating were tested by scratching test and neutral salt spray test (NSS). The results show that after scratching test, the adhesion of the electrogalvanizing layer on 08Al surface reaches grade 0, and the adhesion of the electrogalvanizing and chemical conversion film composite coating on 08Al surface reaches grade 0. The NSS test showed that the corrosion resistance increased with the increase of the thickness of the galvanizing layer. The corrosion resistance time of the galvanizing layer with the thickness of 15um exceeded 216h. Salt spray resistance time of electric galvanizing layer about 10um is 72-96 h; The salt spray resistance time of electrogalvanizing layer less than 5um is less than 48 h. The salt spray corrosion time of GA plate (galvanized layer thickness is 10-11um) is less than 96 h. After chromium-free passivation treatment, the corrosion resistance of GA plate is more than doubled to 192h. The salt spray corrosion resistance of the composite coating (10-11um thickness) on 08Al with electrogalvanizing and chemical conversion film can reach more than 216h.