Understanding the effect of wet and dry cycles on the atmospheric corrosion of aluminium alloys

Atmospheric corrosion involves repeated cycles of wet and dry periods. For instance, condensation of moisture and deliquescence of salts can occur during the night, and evaporation from the material surface usually occurs during the day. Rain causes salts to be washed away and can affect the wetting of the surface during the diurnal cycle. For aircraft, particular flight paths may cause condensation or increase the likelihood of salt contaminants. The present work examined the effect of cycling the relative humidity from high to low values, in order to produce wet and dry periods, respectively. The mass gained during wet periods can be measured and, thus, the volume of the NaCl droplet, and its concentration can be calculated. In addition, X-ray tomography was carried out on 2 mm diameter aluminium alloy pin specimens to visualise the corrosion damage after a salt droplet was placed on the surface, and the specimen was dried, and re-wet. Re-wetting resulted in corrosion reinitiating within or adjacent to the existing corrosion site, or initiated in a new site altogether. The effect of wet and dry cycles on damage accumulation will be discussed based on measurements of the corrosion depth and volume with periods of wetness.