A critical review on development, performance and selection of stainless steels and nickel alloys for the wet phosphoric acid process

The wet phosphoric acid (WPA) process equipment is exposed to corrosive and wear conditions with aggressiveness determined by the phosphate rock chemical composition and process parameters employed. Stainless steels and nickel alloys are attractive materials considered for WPA plant construction since they are mechanically and chemically robust to withstand the aggressive environment encountered. This review first describes the WPA process emphasizing on the aggressiveness of the environment to plant construction materials. Then, principles behind design and fabrication of stainless steels and nickel alloys with properties tailored to withstand this aggressive environment are described. To withstand corrosion conditions encountered during the WPA process alloys should contain sufficient chromium (Cr), with some molybdenum (Mo) and nickel (Ni) contents and should possess a homogeneous microstructure. Wear resistance in a specific section of the plant is ensured by adjusting the carbon content and carbide formers contents to obtain adequate carbides for protection from abrasion. Reported data on deterioration rates and failure mechanisms of commonly used alloys is first presented before discussing selection and performance of these alloys in different sections of the WPA plant. Future challenges and opportunities in selection of suitable materials and development of novel materials are finally outlined.