Evolution of corrosion degradation in galvanised steel bolts exposed to a tropical marine environment

The time-dependent degradation of galvanised steel bolts subjected to marine atmospheric exposure for a maximum period of 2 years in the Wenchang marine environment of Hainan province, South China, was investigated. The results showed that the zinc coating used as a protective surface finish deteriorated quickly due to the harsh environmental conditions. The deterioration of the zinc coating resulted in premature corrosion of the underlying steel substrate as early as 6 months. The main Zn corrosion products found at the initial exposure time were simonkollite (Zn5(OH)8Cl2 center dot H2O), hydrozincite (Zn5(OH)6(CO3)2), and zincite (ZnO). At subsequent exposure times, major steel corrosion products such as lepidocrocite (gamma-FeOOH), akaganeite (13-FeOOH), goethite (alpha-FeOOH), magnetite (Fe3O4), and maghemite (gamma-Fe2O3) were detected. Fe3O4/gamma-Fe2O3 was consistently found in the rust formed at both the exposed and crevice thread regions between 6 and 24 months of exposure. Based on the geometry of the bolt fastener, the corrosion rate and phase composition distribution varied between the two regions. Differences in oxygen exposure levels, corrosion kinetics, and electrochemical stability of the corrosion products were primary factors in these distinctive corrosion behaviours.