The detection of selective phase corrosion in cast nickel aluminium bronze by acoustic emission techniques

Abstract

Cast nickel aluminium bronze (NAB) can suffer severe selective phase corrosion in seawater by a dealuminification process similar to dezincification in brasses. During the process, which occurs particularly in the heat-affected zones of weld repairs, copper is redeposited at the corroded areas thus making the phenomenon extremely difficult to detect by conventional non-destructive testing techniques. It was considered likely that the different fracture modes of sound material and corroded material would result in differing forms of acoustic emission during deformation. Acoustic emission monitoring was therefore carried out on tensile specimens of as-cast NAB, welded NAB and corroded NAB. The failed specimens were then examined using optical and scanning electron microscopy. It was apparent from the results that there were significant differences in the acoustic emission behaviour of the three types of material. This was revealed in characteristic event counts, event amplitudes and r.m.s. energy outputs and by normalized amplitude analysis of the data. These differences in behaviour have been correlated with the varying structures of NAB and their behaviour under stress.