Atmospheric corrosion performance of structural steel bolts in the Wenchang marine environment, South China

The atmospheric corrosion performance of three different grades of hexagonal head structural steel bolts with distinctive surface finishes (304 stainless steel (plain), galvanized steel (Zn coated), and 40Cr steel (black coated)) was examined in this study using surface analysis and characterization techniques. The bolts were exposed in the Wenchang marine environment for two years. Results showed that a combination of factors such as alloying content, microstructure, surface finishing, and environmental conditions impacted the corrosion performance of the examined bolts. The corrosion mechanisms and patterns varied between the different bolt materials; 304SS bolts exhibited localized corrosion, while galvanized steel and 40Cr steel bolts exhibited uniform corrosion. In terms of coating performance in the marine environment, the zinc coating provided minimal protection to the underlying steel substrate when compared to the black oxide coating which failed prematurely, rendering it unsuitable for use in the marine atmosphere. Furthermore, despite the formation of a dense rust layer on the galvanized and 40Cr steel substrates after 2 years of marine atmospheric exposure, the fastener degradation was critical, suggesting that the rust layer is not beneficial in the threaded fasteners due to the high susceptibility to exfoliation that may eventually lead to thread stripping. This poses a serious risk to bolted assemblies.