Atmospheric corrosion of bare and anodized aluminium in a wide range of environmental conditions. Part I: Visual observations and gravimetric results

This study compares the behaviour of aluminium in the bare condition and when protected with anodic films of approximately 7, 17 and 28 μm thickness for 42 months of exposure in 11 atmospheres of very different aggressivities, with salinity values ranging from between 2.1 and 684 mg Cl− m−2 d−1. The anodic films, obtained in a sulfuric acid bath and sealed for 60 min in boiling deionized water, were characterised before and after different exposure times by means of: visual inspection; observation with a magnifying glass; and using classic gravimetric techniques. Optical and electron microscopy were occasionally used with some specimens that showed symptoms of localised corrosion. Aluminium behaves as a passive material in atmospheres of low salinity but exhibits pitting corrosion after 1 year at chloride pollution levels of ≥50 mg Cl− m−2 d−1 and after 2 years at levels of ≥10 mg Cl− m−2 d−1. Anodising and sealing prevents the risk of pitting corrosion even in the most aggressive atmospheres, except in the case of the coatings of the lowest thickness.