Sequence of corrosion attack of machining induced flow zone produced on some AI alloys that leads to rapid intergranular penetration

Abstract

Machined surfaces of the aluminium alloy 7010-T736 suffered rapid superficial corrosion when immersed in sea water. The attack advanced as a series of expanding sectors removing an outer layer of the machining induced deformation zone about 300 nm deep. This process was frequently repeated, a further layer of similar thickness being removed beneath thefirst one, although removing the overlying tightly adherent corrosion product formed with the first ‘wave’ of attack prevented further corrosion of this nature. When the flow zone proper, i.e. where total obliteration of the original grain boundary network had occurred, was consumed by this form of corrosion, the less heavily sheared material beneath was then attacked, particularly down the compression bands that were a characteristic feature of the lower level. Eventually, when less disturbed material was reached, the attack became intergranular. Any mechanical damage, provided it had a shear component, when inflicted on either machined or polished surfaces, exacerbated the attack that has been described and thus promoted early intergranular corrosion.