ELECTROCHEMICAL STUDY OF ALUMINUM CORROSION IN ACID CHLORIDE SOLUTIONS

In this work, oxidation and free corrosion of hydroxide-etched high-purity aluminium in deaerated HCl media at concentrations in the range 0.6-2.4 M using the potentiodynamic and galvanostatic transient techniques has been studied. Pitting potentials were determined from cyclic polarization curves. The stationary E(corr) values lay 200-100 m V more negative than the corresponding pitting potentials for the concentration range studied. The evolution of the electrode surface was examined by optical microscopy. For low current densities, the anodic galvanostatic curves show an initial region of constant potential with very low charge and a subsequent slow rise in potential up to quasistationary conditions. When the current density increases, the time corresponding to such an initial region decreases, showing the potential increase sooner and finally, for current densities about 0.3 mA cm-2, an initial sharp maximum is found. The Tafel parameters have been determined in transient conditions from Mansfeld's method. The anodic and cathodic Tafel slopes for potentials +/- 70 mV around E(corr), coincident within the experimental error, were 2RT/F. Free corrosion of Al in concentrated and deaerated HCl solutions is general and takes place in the passive mode through the formation and dissolution of a defective oxyhydroxide film, the total thickness being in the order of few monolayers. No pits develop in the latter conditions, in contrast with the electrode behaviour in aerated HCl solutions, in which pitting with intergranular fissuring attack is found. When a very low anodic current density is applied, aluminium dissolves into the medium through the oxyhydroxide formation and dissolution. The potential rise found at slightly higher current densities corresponds to an oxyhydroxide formation is competition with chloride attack. At a certain value of the potential, pits are nucleated, which propagate in the quasistationary conditions of the galvanostatic curve. When the current applied increases, metal attack is more localized. The potential decrease found after the initial maximum, for current densities about 0.3 mA cm-2 and higher, is ascribed to pit nucleation, the subsequent plateau corresponding to the propagation of pits.