The Role of Surface Roughness on Pitting Corrosion Initiation in Nickel Aluminium Bronzes in Air

Nickel Aluminium Bronze (NAB) continues to be used extensively in the marine industry and is most commonly seen in valves and propellers due to its strength and corrosion resistance. However, components used for marine applications often endure long exposure to air during storage periods prior to installation and NAB is vulnerable to pitting corrosion in air. To investigate the potential condition of stored NAB, this study measured the height, width and percentage coverage of corrosion pits observed in NAB samples after 4 months of air exposure at room temperature in a laboratory.

Four different types of NAB samples were considered; off-the-shelf (sample 1a), polished (sample 1b), laser beam welded (sample 1c) and failed in service (samples 2a and 2b) and the microstructure of the samples was characterised using a mixture of microscopic techniques including surface roughness characterisation, SEM, EBSD, EDX and neutron diffraction.

Examples of pitting corrosion were found on each sample type after 4 months of exposure to air. The results show significant correlation between surface roughness of the pre-corroded sample and pitting coverage, and this is thought to be due to the development of crevice corrosion during the 4 months of air exposure. It also highlights the inaccuracies in using Ra values alone to measure surface roughness in samples where localised corrosion pits are suspected due to high surface roughness or long periods of air exposure.

The results of this study bring into question the pitting corrosion that would be found in components that are stored for longer period before being installed onto vessels. This has concerning safety implications as the initial surface characteristics of the metal may influence the residual load-bearing capacity of components.