The effect of 0.4 wt.% Mn addition on the localized corrosion behaviour of zinc in a long-term experiment

The corrosion performances of pure Zn and Zn–0.4Mn alloy exposed in marine environment (Qingdao, east of China) have been studied after exposure for three years. Pitting corrosion occurs on both pure Zn and Zn–0.4Mn with fewer and smaller pits on Zn–0.4Mn alloy as revealed by confocal scanning laser microscope (CSLM), which indicates that the addition of Mn can reduce the localized corrosion of Zn. Corrosion products formed on pure Zn and Zn–0.4Mn have been characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe micro-analyzer (EPMA). The main crystalline phases identified by XRD are Zn5(CO3)2(OH)6, Zn4SO4(OH)6 and NaZn4Cl(OH)6SO4·6H2O, respectively. Small amounts of ZnO and Zn(OH)2 are also found which mainly present on top surface of corrosion films as revealed by XPS. Combined results of XRD and EPMA disclose that Zn4SO4(OH)6 and NaZn4Cl(OH)6SO4·6H2O are the main compounds inside the pits with Zn4SO4(OH)6 locating at the pit bottom. The formation mechanism of corrosion products inside pits and the role of small Mn addition on localized corrosion of Zn have been discussed.

► Addition of 0.4 wt.% Mn can significantly reduce localized corrosion of Zn after exposure in marine environment for three years.
► The mechanism of Mn addition to improve the atmospheric corrosion performance of zinc is that Mn can reduce the acidity inside droplets and pits.
► Zn4SO4(OH)6 and NaZn4Cl(OH)6SO4·6H2O are the main compounds inside the pit with Zn4SO4(OH)6 locating at the pit bottom.