Effects of thiosulphates and sulphite ions on steel corrosion

The paper deals with the general corrosion of carbon steel in geothermal plants for conveying and re-injecting condensates. The results of electrochemical and weight loss tests are discussed in function of sulphite and thiosulphate concentrations typical of Italian geothermal power plants. Thiosulphates can inhibit the reactions between oxygen and sulphites and increase the corrosion rate, but they control the acidification resulting from the reactions of sulphites with oxygen. In anaerobic solutions, both species modify mainly the cathodic process. In the absence of oxygen, the corrosion process leads to the formation of mackinawite scales, while iron sulphide forms due to the reduction of thiosulphate and sulphite.