Failure of a nickel aluminium bronze (NAB) canned motor pump impeller working under polluted sea water - Influence of material selection, section thickness dependent microstructure and temper annealing heat treatment

In the present study, a damaged pump impeller working under highly polluted sea water was taken up for investigation on probable cause(s) for its failure. Major steps in the investigation were visual examination, thorough microstructural observation of the affected (surface and sub-surface) and unaffected portions of the base material at different locations on the impeller having varying section thickness. An effort has been made for correlating unaffected microstructural attributes to corroded features with respect to various section thicknesses. Failure of the impeller is found to be due to combined effect of a corrosion susceptible microstructure and highly polluted corroding medium that resulted in selective phase corrosion (SPC) of certain phases in the microstructure leading to surface roughening and perforation, predominantly through pitting mechanism governed by dealuminification initiated at phase interfaces. Networked alpha plus kappaIII eutectoids, Cu-O type of precipitates, dendritic Cu-Al phase and retained beta have been found to be corrosion susceptible microstructure. Principle corroding features in thick sections were found to be kappaII and alpha plus kappaIII eutectoids, while that in thin sections include severe cracking involving Cu-S-O rich coarse precipitate (of 15 μm approx size), corrosion along interfaces of dendritic Cu-Al phase and retained beta with alpha matrix. Presence of alpha plus kappaIII eutectoids and retained beta has been the result of combined effect of improper selection of material (aluminium) composition, complex cast shape of the impeller and ineffective temper annealing treatment, while dendritic Cu-Al phase and high level of porosity are casting defects.