Studies on the kinetics of aluminium metal dissolution in aggressive acid media containing inorganic anions

Abstract EN

The kinetics of chloride ion pitting and inorganic anions inhibition of aluminium metal corrosion in 1.0M solutions of perchloric acid were studied at 30°C in the ranges 0 < CP < 0.8 mol L *; 0 < N03 5 0.18 mol L_l and -0 < ST < 1.0 mol L-1, where ST represents the sum of SO4" and HSO4 species all at an ionic strength I = 2.0, NaC104.

The ^reactions were found to be governed by multistep equilibria in which CF, NO 3, HS04, S042" and dissolved oxygen 02 (g) compete for the metal surface.

The rate of corrosion was found to be zero-order overall with an observed rate constant dependence on CF of the form: kK.K [Cl']2 (k - k ) --2—2- obs 0 KjtOjl + K^Cri + KjK^Cr]2 where 1^ is the equilibria constant for 02(g) adsorption and K2 & K3 are successive adsorption constants for CF ions.

The addition of nitrate or sulfate ions resulted in the inhibition of aluminium corrosion.

The results are discussed in tenrts of multistep competitive equilibria in which the nitrate or the sulfur containing species (SO 4~, HSO4) screen the metal surface from chloride attack.

In all cases a mechanism involving three competitive equilibria viz.

molecular oxygen adsorption, Cl“ adsorption and inhibitor adsorption followed by a rate determining dissolution step is proposed for the reactions.