On the Under-Anodic Film Filiform-Like Corrosion of Anodised Al–Cu–Li Alloy

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Under-anodic film corrosion of an Al–Cu–Li alloy anodised in tartaric–sulphuric acid (TSA) was investigated, with emphasis on the role of microstructural heterogeneity in corrosion initiation and propagation. Micrometric defects at the anodic oxide/alloy interface, arising from the dissolution of Cu-rich constituent intermetallic particles, act as initiation sites for corrosion. These sites evolve into circular cavities that propagate laterally beneath the anodic film, forming an advancing corrosion front that transitions to filament growth when it intersects grains enriched in the T1 (Al 2 CuLi) precipitates. The resulting thread-like surface morphology, combined with lateral propagation beneath the anodic film, is therefore described as filiform-like corrosion (FFC-like). Preferential crystallographically influenced dissolution, promoted by local galvanic coupling and hydrolysis within the confined electrolyte, contributed to the development of an aggressive local environment at the advancing corrosion front. The results indicate that propagation was controlled predominantly by anodising-induced interfacial defects, confined electrolyte chemistry, and microstructural heterogeneity, rather than by a sustained large-scale differential-aeration cell.