Influence of micro-structure on the corrosion behaviour of glass-bead blasted AA6061-B4C MMC in H3BO3 solution

The micro-structure, residual stresses, and micro-distortions generated near the surface in the AA6061-B4C metal matrix composite by glass-bead blasting are investigated and compared with those of the bulk. In contrast to the bulk, the sublayer is under compression and contains an extremely low density of B4C and a high density of dislocations. Corrosion behaviour was studied in H3BO3 after immersion at the OCP and after accelerated tests. In the sublayer, initial rapid oxidation (formation of a homogeneous oxide film) is followed by slow oxidation (average corrosion rate: 2 µm/year); the oxidation front moves parallel to the surface. In the bulk, oxidation takes place along preferential paths defined by the distribution of B4C. The driving force is the galvanic coupling between B4C and the matrix. Once corrosion proceeds, corrosion products (oxidation of the sublayer) become weakly adherent, peeling off locally. The bulk is then directly exposed to the solution, promoting corrosion.