Corrosion behaviour of galvanised steel in alkaline sodium chloride solution

2

ABSTRACT

Corrosion performance of galvanised steels with coating thickness 5.6, 8.4 and 19.2 µm, named as T₁, T₂ and T₃, respectively, was evaluated by immersion test and potentiodynamic polarisation in S₁(0.025 M NaOH + 0.17 M NaCl) and S₂(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 T₁, T₂ and T₃, the corrosion rate decreased by 25.4%, 18.7% and 6.6%, respectively, in solution S₂ compared to S₁. FTIR studies of corrosion products showed that the absorption bands for ZnO at around 470 cm⁻¹ and Zn₅(OH)₈Cl₂.H₂O (simonkolleite) at 902 cm⁻¹was higher for samples exposed to S₂ 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 S₂ 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.

KEYWORDS:

• Galvanised steel
• weight loss
• potentiodynamic polarisation
• SEM
• FTIR

Disclosure statement

No potential conflict of interest was reported by the author(s).