Microstructural modification of nickel aluminium bronze against erosion corrosion

Nickel aluminum bronze (NAB) used in marine applications usually undergoes erosion and corrosion from hard particle impingement and seawater corrosion. In materials viewpoint, the deterioration is caused by microstructural features, e.g. phases, and interfaces between phases. This work achieved heat treatment by hardening at 900 and 1000oC for 2 hours then water quenching to homogenize microstructure for lifetime extension of the alloy. An erosion corrosion test was performed in 3.5 wt.% NaCl solution containing 30 g/L of 300-500 μm SiO2 sand particles at a rotational speed of 1500 rpm for 6 hours. The results suggested that heat treatment could improve microstructural uniformity by dissolving k phases, hence reducing corrosion initiation sites, interfaces between a/k, reduced. Furthermore, hardness was enhanced by the formation of new phases, Al3Cu, Al7Cu4Ni and Al5FeNi, including changes in microstructure. The HT-1000 showed a superior hardness value because all of k precipitates dissolved and fine needle-shaped a phases formed. While HT-900 kIII, k_IV dissolved, and a  phase remained in the as-received condition. SEM micrographs indicated the least damage in the hardest HT-1000, whereas the most severe damage was found in the as-received sample. Heat treatment in this work not only improved hardness against impingement of SiO2 but reduced corrosion initiation sites; therefore, erosion corrosion resistance was enhanced.