The influence of CO2, AlCl3·6H2O, MgCl2·6H2O, Na2SO4 and NaCl on the atmospheric corrosion of aluminum

Abstract The influence of salt deposits on the atmospheric corrosion of high purity Al (99.999%) was studied in the laboratory. Four chloride and sulfate-containing salts, NaCl, Na 2 SO 4 , AlCl 3  · 6H 2 O and MgCl 2  · 6H 2 O were investigated. The samples were exposed to purified humid air with careful control of the relative humidity (95%), temperature (22.0 °C), and air flow. The concentration of CO 2 was 350 ppm or 2 , the corrosivity of the chloride salts studied increases in the order MgCl 2  · 6H 2 O  3  · 6H 2 O  2  · 6H 2 O is explained by the inability of Mg 2+ to support high pH values in the cathodic areas. In the presence of carbon dioxide, the corrosion induced by the salts studied exhibit similar rates. Carbon dioxide strongly inhibits aluminum corrosion in the presence of AlCl 3  · 6H 2 O and especially, NaCl, while it is slightly corrosive in the presence of MgCl 2  · 6H 2 O. The corrosion effects of CO 2 are explained in terms of its acidic properties and by the precipitation of carbonates. In the absence of CO 2 , Na 2 SO 4 is less corrosive than NaCl. This is explained by the lower solubility of aluminum hydroxy sulfates in comparison to the chlorides. The average corrosion rate in the presence of CO 2 is the same for both salts. The main difference is that sulfate is less efficient than chloride in causing pitting of aluminum in neutral or acidic media.