In situ infrared reflection spectroscopy studies of the initial atmospheric corrosion of Zn–Al–Mg coated steel

• Investigation of initial atmospheric corrosion of Zn–Al–Mg coated steel.
• In situ studies using infrared reflection absorption spectroscopy.
• Initial corrosion products were Mg/Al and Zn/Al based layered double hydroxides.
• Initial dissolution of Mg and Al reduce the pH at cathodic sites.
• This prevent formation of ZnO and reduce the rate of the oxygen reduction reaction.

NaCl induced atmospheric corrosion of ZnAl2Mg2 coated, electrogalvanised (EG) and hot dipped galvanised (HDG) steel was studied using in situ infrared reflection absorption spectroscopy, XRD and SEM. Initial corrosion leads to the formation of Mg/Al and Zn/Al layered double hydroxides (LDHs) on ZnAl2Mg2, due to the anodic dissolution of Zn–MgZn2 phases and cathodic oxygen reduction on Zn–Al–MgZn2, Al-phases and on zinc dendrites. In contrast to EG and HDG, were no ZnO and Zn5(OH)8Cl2⋅H2O detected. This is explained by the buffering effect of Mg and Al which inhibit the ZnO formation, reduce the cathodic reaction and corrosion rate on ZnAl2Mg2.