The Chemical Nature of Aluminum Corrosion: I Corrosion of Aluminum Surfaces by Aluminum Salts

The corrosion of aluminum (alloys 1199-H14 and 7075-T6) by solutions of Al(NO3)3, AlBr3, AlCl3, Al2(so4)3, al(clo4)3, all3 and NaCl of various concentrations has been measured. Experiments were also conducted in saturated solutions of aluminum acetate, benzoate, phosphate, chromate, and fluoride. The dissolution rate of the alloys varies considerably with the salt, the nitrate and iodide being most aggressive. The corrosion rate in sulfate solutions is much less than the rate in bromide, chloride, or perchlorate. The pH of the various salt solutions is not the determining parameter, but for a given salt (AlCl3) the corrosion rate increases with decrease in pH. The hydrolysis of the salts was considered from the viewpoint of assessing the significance of the hydrolysis reaction on aluminum corrosion. Salts such as AlCl3, Al(NO3)3, Al2 (SO4)3, and AlBr3 are treated as the salts of the respective strong acid and the weak base Al(OH)+ + with a pkb, of 8.67. The conjugate acid of this weak base is the hydrated aluminum ion. An attempt to correlate the hydrolysis, i.e., generation of hydrogen ion, with the ability of the salt to corrode aluminum (1199 and 7075) leads to the conclusion that it is not the hydrolysis, per se, that is the governing factor but rather the other chemical species in solution.