Welding processes used with Nickel-Aluminium Bronze components for service in the marine environment

Nickel-Aluminium Bronze (NAB) is widely used in seawater-contacting marine components such as external fixings, propellers and valves due to its good corrosion resistance and biofouling resistance. However, the development of welding processes that produce a favourable microstructure and mechanical properties in this alloy system is challenging, so welds in NAB are much more vulnerable to corrosion and Stress Corrosion Cracking (SCC) than the wrought material. This limits the maintainability of NAB parts and makes expensive replacement programmes necessary.
In this project, the structural properties and corrosion resistance of NAB welds produced using different welding processes will be analysed, along with those of steel welds for comparison. NAB and steel welds will be exposed to marine environments while being simultaneously stressed and both the corrosion rate and community of biofouling organisms will be monitored. We will link this information to analysis of the material's final state via optical and electron microscopy and X-ray diffraction, and to results from mechanical tests which will be performed on welds post-exposure. This will enable us to identify welding processes which produce favourable weld microstructure and crystallographic in this material, to determine the dominant processes involved in NAB weld degradation, and to predict the effects of corrosion on the long-term structural integrity of NAB welds. During the environmental tests, we will also investigate the link between marine biofouling and corrosion damage. This will allow us to develop more practical, biologically-based corrosion inspection methods for marine welds.
This work aligns with the Materials Engineering (Metals & Alloys) area of EPSRC's Manufacturing the Future theme. We will improve welding methods both for manufacturing NAB components and for NAB component repair by developing a greater understanding of how weld microstructures relate to corrosion-based degradation mechanisms. This will lead to economic benefits to operators of vessels and offshore structures, and to the UK marine maintenance industry. A direct practical outcome of this work will be to inform the welding processes used in Repair and Overhaul (R&O) by the industrial project partner Babcock International Group. It will also act as an enabling technology for the use of additive manufacturing (esp. Wire + Arc Additive Manufacturing) of NAB as an alternative to casting for creating complex-shaped components, which is currently under development by several major manufacturers. We also aim to link microstructure to biofouling for the first time, which will enable provide techniques for corrosion detection in NAB components, benefitting the safety and maintainability of offshore infrastructure