Semiconductivity conversion of Alloy 800 in sulphate, thiosulphate, and chloride solutions

Highlights: • Semiconductivity conversion was found in sulphate, thiosulphate and chloride solutions. • Semiconductivity depended on the concentration ratio of different ions. • A mechanism of semiconductivity conversion was proposed. - Abstract: Passive films formed on Alloy 800 in sulphate, thiosulphate, and chloride solutions were investigated using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS), and Mott–Schottky analysis. Experimental results revealed that the p-type or n-type semiconductivity of the passive film depended significantly on the solution chemistries. In a chloride–sulphate system, competitive adsorption of these two ions was believed to determine the type of semiconductivity. However, in a thiosulphate–chloride solution, the type of semiconductivity was attributed to the electrochemical reduction of the sulphur in thiosulphate and the subsequent adsorption of reduced sulphur species in the passive film