Cavitation of some copper alloys for naval propellers: electrolyte effect

Abstract

Efficient shipping is becoming more challenging, not only because of the increasing demand for this means of transport, the performance requirements and the optimisation of resources, but also because of the environmental constraints. Cavitation is a harmful phenomenon that can affect different systems of a ship such as pumps, valves, impellers, pipes, etc., but is particularly known for its effect on propellers, associated with loss of power and even impairment of structural integrity. The mechanical effect of cavitation on metallic materials is in general superimposed by corrosion and may thus lead to a synergistic degradation phenomenon. In this work, the cavitation behaviour and the potential synergistic effect with corrosion was evaluated for three widely used copper-based alloys, NAB (Nickel Aluminium Bronze), MAB (Manganese Aluminium Bronze), and a brass (low lead content) using three types of water: natural seawater, synthetic seawater and synthetic brackish water. Experiments were carried out in an ultrasonic bath, followed by structural surface characterisation. The main result is that MAB is most susceptible to both, general surface damage and deep localized attack. This is attributed to its poor ability in regenerating the protective layer, the area ratio of their phases and the high hardness of the κ-phase, in combination with high mechanical stresses during impacts affecting the grain boundaries. Moreover, natural seawater was found to increase cavitation, attributable to the dispersion of micro-particles, while brackish water with its content of sulfidic species was found to promote the strongest synergy between corrosion and cavitation.