Electrochemical behaviour of Alloy 600 tubing in thiosulphate solution

Electrochemical and surface properties of passive films formed on Alloy 600 in a thiosulphate solution were studied. Oxide films formed at various passive potentials contained a bilayer oxide, whose composition changed as a function of the applied potential resulting in a change in the impedance behaviour. Destabilisation of the oxide film at potentials within the passive region was observed, which was due to the breakdown of the oxide film and coincided with the loss of Cr within the passive film. In contrast, a higher degree of corrosion protection was obtained when the Cr content within the oxide film was elevated.

Research highlights
► Passive film on Alloy 600 in 0.1M Na2S2O3 contained a bilayer oxide film.
► Film contained a Cr-rich inner layer and a mixed Ni and Fe outer layer.
► Composition of the outer layer highly dependent upon the applied potential.
► Corrosion resistance dictated by inner layer and highest in the passive region.
► Breakdown of the passive film coincided with depletion of Cr at the outer layer.