Oxide film thickening on the surface of iron-chromium alloys in relation to anion type and concentration

The variation with time of the open circuit potentials of four FeCr alloys of increasing Cr content is followed in strongly aerated solutions of Cl⁻, Br⁻, I⁻, NO₃⁻, SO₄⁻⁻, WO₄⁻⁻, CrO₄⁻⁻, and H₂PO₄⁻ of varying molarities, till the attainment of steady state values.

Difference in behaviour is noted between Alloy I (7% Cr) and the other three Alloys (⩾ 13% Cr). With Alloy I, film thickening is noted only in very dilute salt solutions, whilst film destruction and dissolution operates in more concentrated media. With the three other alloys film formation describes the behaviour. Film thickening in all solutions follows a direct logarithmic law, with rates depending on the nature of anion in solution, and alloy composition. The action of the various anions is specific. In solution of the same anion the thickness of the oxide film increases with the Cr content of the alloy. Phosphate solutions produce significantly thicker passivating films.

The behaviour of the FeCr alloys in CrO₄²⁻ solutions is unexpected. Whilst passivation is expected, the changes in potential denote film destruction.

The steady-state potentials measured in solutions of equimolar concentrations of the various anions are not the same. These potentials tend towards more negative values with the increase of anion-content of the solution. Under otherwise the same conditions, the steady-state potentials tend towards nobler values with the increase of the Cr-content of the alloys.