Characterisation of pitting corrosion in sealed anodic films formed in sulphuric, sulphuric/molybdate and chromic media

Specific attention was focused on the characterisation of pitting initiated in different anodic films formed on aluminium 2024 alloy. Polarisation test in NaCl solution was used to initiate pitting corrosion in anodic layers produced in chromic acid, sulphuric acid and sulphuric acid containing molybdate species. Characteristics of pits were investigated by optical microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and image analysis. From optical and SEM micrographs, morphology of pits initiated in the different anodic layers was qualitatively described. The image analysis allowed to characterise pits from quantitative point of view. From these results, corrosion performances of chromic, sulphuric and sulphuric/molybdate anodic layers could be discussed.