Comparative studies of the electrochemical behaviour of aluminium in 1 M H 2SO 4/water and in 1 M H 2SO 4/dimethylformamide (anhydrous)

Abstract

Anodic current potential curves of Al (99.999%) in H 3O/1 M H 2SO 4 are quite different from that in anhydrous DMF/1 M H 2SO 4. Al is active in DMF/H 2SO 4, and at a potential of 0.4 V (SCE) gives a constant current-time curve, where in H 2O/H 2SO 4 the current decreases in the same time to a low value. In DMF, it decreases also at a later step. This happens when the solution is saturated with anhydrous Al 2(SO 4) 3 (0.18 g l −1). Comparison of cut-off curves after polarization to 0.4 and 3 V (SCE) shows that the curves are similar, but the potential decreases more slowly in DMF/H 2SO 4 than in H 2O/H 2SO 4. This proves that the space charge of Al with a Al 2(SO 4) 3 layer in anhydrous DMF has a much higher half-time of potential decay than that of Al with an oxide layer in water. This can be explained on the basis that the oxide layer in water has incorporated water molecules and perhaps H + or OH −, whereas the anhydrous Al 2(SO 4) 3 layer is free from solvent molecules, and the ionic conductivity of Al 2(SO 4) 3 itself is very low. The high space charge in DMF/H 2SO 4 could also be shown by potentiostatic measurements using an interrupter. The different behaviour of Al, when passivated by salt layers on the one hand or by oxide layers on the other, is discussed.