Laser surface treatment of a propeller bronze for improving cavitation erosion and corrosion resistance

Abstract

Manganese-nickel aluminium bronze (MAB) and nickel aluminium bronze (NAB) are two most commonly employed alloys for the fabrication of marine screw propellers. MAB is superior to NAB in casting properties and is lighter, but it is inferior to NAB with respect to cavitation erosion resistance. Marine propeller is highly susceptible to cavitation erosion. The primary objective of the present study is an attempt to enhance the cavitation erosion and corrosion resistance of MAB through laser surface modification. Two types of surface modification, laser surface melting (LSM) and laser surface alloying (LSA) using aluminium as the alloying material were attempted. The microstructure of the modified layer was analyzed by optical microscopy, SEM, EDS and XRD. The pure cavitation erosion behavior (in deionized water), corrosion behavior and cavitation erosion-corrosion behavior (both in 3.5 wt% NaCl) were studied. The ultrasonic vibratory technique was used to study erosion, while immersion test and electrochemical methods were employed to study corrosion. The cavitation erosion resistance Re and cavitation erosion-corrosion resistance Rec of laser surface treated samples recorded a remarkable improvement. The LSM process improved both Re and Ree by 8.9 and 5.3 folds, respectively. On the other hand, Re and Rec increased by 32 and 15.8 times through LSA. The observed increment was mainly attributed to a highly refined microstructure consisting of single 帣-phase in body centered cubic structure with significantly reduced grain size, and also to a higher surface hardness. It was shown in our study that structural integrity played a vital role in cavitation erosion resistance. By studying the erosion mechanism of as-received MAB, which is a multi-phased, heterogeneous alloy, erosion started at the KI-phase and the 帢/帣 phase boundaries, and then propagated rapidly into the 帢-phase resulting in ductile rupture. On the contrary, the single-phase structure only exhibited slight boundary attack, being initiated from triple junctions. In particular, the higher Al content in the LSA samples produced a harder 帣-phase and relatively larger grains than LSM did, thus assisting tremendously in fencing off the attack of intense pressure pulses. The corrosion resistance, on the other hand, recorded a lower improvement compared with that in erosion due to the already high corrosion resistance of untreated MAB. Laser surface treatment produced a homogeneous microstructure and thus eliminated the electrochemically active phases which would otherwise aggravate the corrosion loss through selective phase attack and galvanic coupling. Generally speaking, the corrosion behavior improved as evidenced by a noble shift in the open-circuit potential, and a lower corrosion rate. Corrosion rate was suppressed by 1.5 and 2.1 folds in surface melted and alloyed MAB, respectively. The galvanic corrosion test also revealed that the laser-treated samples were all cathodic to as-received MAB. The reported study illustrates the feasibility of laser surface modification for improving the cavitation erosion and corrosion resistance for the propeller bronze MAB. It is expected that further work in LSA via tailor-making a surface layer of optimum composition would lead to even higher performance.