Corrosion Mechanisms and Fatigue Behaviour of Marine Biofouled Plasma Welded Nickel Aluminium Bronze

Abstract

Nickel Aluminium Bronze (NAB) is used extensively by the marine industry. It is often used in welded components that, if they fail, could cause loss of life or vessel. NAB is vulnerable to seawater corrosion, however the interaction between corrosion mechanisms and real-world factors including biofouling, Heat Affected Zone (HAZ) microstructure and residual stress (RS) are poorly understood. As marine structures typically experience cyclic loading, fatigue life prediction is vital. To enable failure prediction, this research investigates the combined effects of welding, marine biofouling and corrosion on the fatigue behaviour of NAB.

Using long term natural seawater and simulated seawater immersion tests of welded NAB coupons, this work demonstrates that previously published corrosion rates underestimate the corrosion of natural seawater immersed welded NAB. The results show that, whilst Selective Phase Corrosion (SPC) of the β’ phase occurred in the HAZ, SPC of the κIII phase occurred in RS affected areas just outside the HAZ and in biofouled areas where crevice corrosion was observed. As the κIII phase has a continuous geometry, this SPC resulted in deep corrosion cracks.

Corrosion pits developed at the weld toe of all natural seawater immersed coupons where the SPC of the β’ and κIII phases, stress concentration generated by pit geometry and welding induced RS acted cumulatively. These factors, seldom simulated in physical tests, severely impact NAB’s fatigue life. S-N curves were generated experimentally using fatigue samples taken from different corroded cases. Using these, a knockdown factor of 11 was assigned for the natural seawater corroded case.

A short crack microstructural model successfully predicted the fatigue strength of corroded welded NAB based on weld toe corrosion pit geometry. This model could be used to schedule preventative maintenance of NAB components exposed to natural seawater and could therefore prevent premature failure.

Date of Award	1 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Harry Coules (Supervisor) & Nicolas O Larrosa (Supervisor)