Corrosion rates and negative difference effects for Al and some Al alloys

The rotating disc-ring technique with a Pt ring was used to quantitatively detect hydrogen evolved on an Al-disc electrode during spontaneous corrosion at open-circuit potential or during anodic polarization of an Al disc to study the unusual increase of H-2 evolution with increase in anodic current (the so called negative difference effect). Experiments were made with high purity Al (99.999%), technical grade Al (99.5%) (Al-T) and Al-In alloy (0.074% In) in deaerated 0.5 M aqueous NaCl solution (pH similar to 6.5) and NaCl solutions acidified with HCl to pH values of 3.0, 2.0 and 1.0. It was shown, in neutral solutions, that the corrosion rates of all three materials are controlled by the rate of water molecule dissociation or in more acid solutions, by the combined water molecule dissociation rate and H+ ion discharge rate. The corrosion rates of all three materials in the range of 3-10 mu A cm(2) was the lowest for 99.999% Al. The impurities in Al-T and Al-In increase the hydrogen evolution rate at cathodic polarizations in a similar manner, but very differently affect the negative difference effect. High purity Al and Al-T have a similar negative difference effect (8-15%), while the presence of In decreases it to only about 1%. No pitting and no negative difference effect were observed at potentials more negative than E-pit. Therefore, the negative difference effect is connected with the anodic pitting dissolution mechanism. Several aspects of this problem are discussed in more detail.