Stress cracking behaviour of dissimilar friction stir weldment in chloride solution: Dissolution assisted or hydrogen induced?

Abstract

The environment assisted cracking behaviour of a dissimilar weldment composed of two age hardenable aluminium alloys (AA 7075 and AA 6056) and produced by the friction stir welding (FSW) process, was assessed using slow strain rate tensile (SSRT) tests in chloride solutions. It was observed that the region comprising the thermo-mechanically affected zone (TMAZ)/heat affected zone (HAZ) of the AA 7075 aluminium alloy in the weldment is susceptible to stress assisted environmental cracking. During tests in which chromate had been added as an inhibitor to the chloride solution, the same region of this dissimilar metal weldment was found to fail in a premature fashion, despite the inhibitor effect. The same weldment, however, tested in chloride solution under cathodically polarised conditions (potentiostatic at −1100 mV versus Ag/AgCl) exhibited a behaviour similar to that observed in the SSRT tests in air. Analysis of the results suggests that cracking of this weldment in chloride solutions is governed by dissolution rather than hydrogen embrittlement.