Aluminium corrosion studies. IV. Pitting corrosion

Abstract

Measurements are reported for the variation of the open-circuit potential, E_r, of aluminium in oxygen-saturated sodium salt solutions. The value of E_r was independent of SO₄^2- and NO₃^- concentrations and similar to the value obtained for water (0.04 (s.h.e.)). E_r was a function of chloride concentration given by

                    E_r = -0.475-0.060log[Cl^-] V (s.h.e.)

at 25°C. There was a less well defined relationship between E_r and NO₂^-, I^- and Br^-, and a complex relationship with F^-.

    The potentiodynamic characteristics are reported for aluminium in 1-0.01 mol l^-1 Cl^- oxygen-saturated solutions. Functional relationships were found for E₀, E_p, E_s and E₀' with chloride activity at 5, 25, 50 and 75°C. Hysteresis effects are reported.    

The experimental results are interpreted in terms of a thermodynamic equilibrium condition between the surface oxide and soluble aluminium chloride. As the system oscillates across the equilibrium conditions the surface will passivate or pit. A critical bulk solution chloride concentration is necessary to maintain the growth of the pit; the experimental value was 1.6 mol l^-1 Cl^- and the corresponding open-circuit potential was E_crit = -0.48 V (s.h.e.). The pitting potential, E_v, was interpreted as an overpotential, η_p, given by η = E_p,- E_p-E_crit.