Combined anodic/cathodic transient currents within nucleating pits on Al–Fe alloy surfaces

Pit nucleation and metastable growth on microelectrodes of aluminium and some Al alloys in chloride solution below the pitting potential are described by recurring current transients. Pit nucleation is a sharp, violent event and is observed repeatedly on all materials tested: pure aluminium, titanium and iron binary aluminium alloys, and alloy 1050A. At the imposed electrode potential, current transients due to pit nucleation recorded for pure aluminium and a solid solution titanium binary alloy are exclusively anodic. Those from alloys containing iron intermetallics sometimes exhibit both anodic and cathodic wings. The cathodic wing is due to transient hydrogen evolution at sites which border iron-rich intermetallic particles: these are preferred pit nucleation sites.