Passivity and passivity breakdown on a beta-FeAl intermetallic compound in sulphate and chloride containing solutions

The passivity and passivity breakdown of a beta-FeAl-based iron aluminide containing 24 wt% Al has been studied in berate buffer solution (pH 8.4) containing sulphates, chlorides and mixtures of both using potentiodynamic techniques complemented with SEM analysis. The voltammetric response in the berate solution shows the peak characteristic of iron oxidation/reduction processes. Sulphate addition to the buffer solution modifies the voltammetric profile and makes the passive film more stable. It is suggested that a mixed iron (II)-(III) oxide film is responsible for passivation of FeAl aluminide in sulphate-berate solution in contrast to a rich iron (III) oxide film forming in absence of sulphate. The addition of aluminium to iron enables the formation of a passivating layer. While iron suffers from pitting, there was no sign of pitting on the aluminide in these conditions. The addition of NaCl to the berate solution at concentrations higher than 6 x 10(-4)M produces nucleation of pits on FeAl aluminide. The pitting corrosion potential increases linearly with log C-Cl- with a slope of 0.40 V/decade. The pitting potentials are shifted in a noble direction with the addition of 0.5 M Na2SO4, indicating that the latter behaves as pitting inhibitor.