Corrosion behaviour of galvanised steel in alkaline sodium chloride solution

Corrosion performance of galvanised steels with coating thickness 5.6, 8.4 and 19.2 mu m, named as T-1, T-2 and T-3, respectively, was evaluated by immersion test and potentiodynamic polarisation in S-1(0.025 M NaOH + 0.17 M NaCl) and S-2(0.05 M NaOH + 0.34 M NaCl) solutions having pH 11.13 and 11.69, respectively. Corrosion rates calculated from immersion test were found to increase with the increase in coating thickness in both the solutions. For T-1, T-2 and T-3, the corrosion rate decreased by 25.4%, 18.7% and 6.6%, respectively, in solution S-2 compared to S-1. FTIR studies of corrosion products showed that the absorption bands for ZnO at around 470 cm(-1) and Zn-5(OH)(8)Cl-2.H2O (simonkolleite) at 902 cm(-1)was higher for samples exposed to S-2 solution. The relative amount of the various phases in each solution was not affected by the coating thickness. Morphology of the immersion tested samples revealed that the corrosion product was thicker in S-2 solution. Higher alkalinity and chloride concentration environment resulted in two passive regions in the potentiodynamic polarisation curves and decrease in passive current. Pitting under the corrosion product was observed at higher anodic potentials in the presence of higher alkalinity and chloride concentrations.