Comparative electrochemical and thermodynamic study of cold-rolled steel, Al alloy AA5754, and Zn corrosion in fluoride and chloride solutions

This study investigates the effects of fluoride and chloride ions on the corrosion of cold-rolled steel, aluminium alloy AA5754, and zinc, using ammonium bicarbonate as a buffer at pH 4 and compares electrochemical findings with thermodynamic calculations of E-pH diagrams. The distinct electrochemical behaviours of fluoride and chloride ions were confirmed, with fluoride-induced corrosion leading to the significant complex formation on cold-rolled steel and AA5754, the latter leading to a narrowing of the Al passive region. However, a comprehensive analysis of all ionic species in the solution (F-, Cl-, NH4+ and HCO3- ) and their complex equilibria reveals that zinc's corrosion is primarily influenced by the overall ionic strength of the solution and further complexation with the buffering agent at higher pH levels, should these conditions occur during corrosion. In addition, this analysis also highlights the subtle differences in the stabilization effects of open circuit potential on cold-rolled steel and AA5754. The results underscore the importance of considering the overall solution equilibrium in thermodynamic analyses and provide a foundation for further research on the role of F-and Cl-ions in zirconium conversion coatings.