Effect of minor alloying elements on anodic polarisation behaviour of aluminium in carboxylic acid solutions containing chloride,nitrite,and nitrate ions

Abstract

The effect of chloride, nitrite, and nitrate ions and their binary mixtures on the anodic behaviour of pure aluminium and alloys of composition Al-0·07 Fe-0·04Si and Al-0·32Fe-0·17Si in deaerated 0·05M oxalic, tartaric, citric, and malic acid solutions at pH 4·5 has been studied by the potentiostatic technique. In oxalic acid solution, the alloys exhibited a more stable passive state than pure aluminium. The presence of 10^-4 -10^-2M Cl^- ions caused passivity breakdown for the pure metal and the alloys. Nitrite ions appreciably affected the passive state only for pure aluminium. The presence of 10^-3 M NO₃ ^- enhanced passivity breakdown of both pure aluminium and the alloys in oxalic acid solution and the alloys were found to be more susceptible to pitting attack in solutions containing chloride ions. In tartaric, malic, and citric acids, the presence of > 10^-4 M Cl^- caused passivity breakdown for pure aluminium and the alloys; the value of the pitting potential was found to depend on chloride ion concentration and on the impurity content of the metal. Addition of 10^-4 -10^-2 M NO² _- or N0³ _- did not markedly affect the anodic behaviour. This was ascribed to the presence of adsorbed carboxylate ions at the electrode surface. The presence of 10^-2 M Cl^- + 10^-2 M NO³ _- inhibited pitting attack and improved the passive state in all carboxylic acids examined. Although the addition of nitrite in combination with chloride ions enhanced attack of the alloys in tartaric acid, attack was inhibited for pure aluminium and the alloys in both citric and oxalic acids.