Corrosion of high-alloy stainless steels in the wet phosphoric acid

At present type AISI 316 stainless steel is most widely used stainless steel in wet phosphoric acid environment. However, the acid manufacturing process involves more and more impurities such as chlorides and free fluorides which leads to increasing corrosion resistance requirements for materials. The high-alloy stainless can be utilised in concentrated part of the wet phosphoric acid process due to their good localised and uniform corrosion resistance. For example, Sanicro 28 stainless steel has been developed to resolve the corrosion problems in wet phosphoric acid. For this report the applicability of the high-alloy stainless steels for use in wet concentrated phosphoric acid environment is investigated. A literature survey of the corrosion resistance of the high-alloy stainless steels in wet phosphoric acid containing chloride and fluoride impurities is made and experiments involving both weight loss and electrochemical measurements are carried out. The high-alloy stainless steels are entirely corrosion resistant in wet phosphoric acid at a temperature of 70 °C. However, increase in temperature accelerates the corrosion of steels considerably and at 110 °C all of them suffer from active dissolution having corrosion rates higher than 100 my m/a.