Evolution of Pitting Corrosion in a 2024-T3 Aluminum Alloy

Localized corrosion (pitting) was quantitatively assessed for a 2021 T3 (bare) aluminum alloy that was immersed in aerated 0.5M NaCl solution at 5, 20, 40 and 65°C for up to 192 h. Pitting corrosion was found to be associated with constituent particles. Type A (anodic) particles tended to dissolve in NaCl solutions, while Type C (cathodie) particles induced dissolution of the adjacent alloy matrix. The density of particles in the rolling (LT) plane was estimated to be more than 3,000 particles/mm2, and clustering, as well as banding of particles along the rolling direction of the alloy, were evident. The kinetics and mechanisms for pit nucleation and growth are described in relation to the initial population of constituent particles, as well as the 15 largest pits and the corresponding, original particle clusters. The degree of pitting damage increased with increasing exposure time and solution temperature, principally due to the coalescence of pits induced by cathodie particles. The pitting corrosion process appeared to be thermally activated, and followed a constant-volumetric-rate (or Faradaic) growth model, with an activation enthalpy of 30 to 40 kJ/mol. The influence of the distribution and nature of constituent particles on pitting corrosion, and its rate controlling process are considered, and a conceptual, mechanistic model for particle induced pit growth is proposed.