STATISTICAL STUDY OF THE PITTING CORROSION OF THE 8090 ALUMINIUM LITHIUM ALLOY

Abstract
The pitting corrosion of the 8090 Aluminium Lithium alloy has been studied in 0.01 M and 0.1 M Na₂SO₄ solutions having various chloride contents. In this media, pits are induced at random potentials ; therefore a statistical study of pit induction times in potentiostatic conditions and pitting potentials in potentiodynamic conditions has been performed. The induction times obey a log normal law. In solutions with high chloride contents, the distributions of pitting potentials are well described by a normal law, but a more and more important divergence from this law is observed in solutions with low chloride concentrations. This phenomenom results from the fact that with applied potential increasing, the mean induction time firstly decreases then tend to a stable value. The pitting potentials distributions have been calculated from the experimental pit induction times distributions. The calculated distributions are close to the experimental ones, showing that the scattering of the pitting potentials results from the scattering of the pit induction times. The potentiodynamic and potentiostatic tests lead to a same chloride content threshold under which no pit appears. In each sulphate solution, increasing the chloride content leads to a decreasing of the scattering of the pitting potentials. The mean pitting potential value decreases towards a stable pitting potential value obtained particularly in NaCl solutions without Na₂SO₄.