In-situ detection of early corrosion of ferritic Cr-Mo steel in aqueous solutions of different anions using laser Raman spectroscopy

Ferritic Cr-Mo steels are widely used as structural materials in chemical, petroleum processing and power generation industries, due to their favorable creep, toughness and corrosion resistance properties at elevated temperature. Particularly, the modified 9Cr-1Mo steel is one of the important structural materials extensively used for crude heater tubes in refineries and steam generator tubes in nuclear and thermal power plants due to its resistance to stress corrosion cracking and creep at elevated temperatures, together with other properties like low thermal expansion coefficient and high thermal conductivity. Some of the key findings are: In acidic solution containing sulphate anions, uniform compact corrosion deposits are formed, whereas in chloride solution, non-compact loosely bound corrosion products formed at discrete sites via pitting. A crevice type corrosion attack was observed in fresh water. Fe₃O₄ and γ ̶ FeOOH phases were the initially formed corrosion product in all media, except in alkaline solution. α ̶ FeOOH is formed at longer exposure of 120 h. In alkaline solution, the specimen was un-reactive and remained passive during the entire exposure of 120 h. The ex-situ XPS analysis carried out after 120 h of exposure in alkaline medium confirmed higher the concentration of oxidized state of Cr and Fe in the passive film than the native film which shows the surface film thickness is more as compared to native film