An Evaluation of Nickel‐rich Alloys In Wet Process Phosphoric Acid

Abstract

AbstractThe corrosion behavior of materials in the production of wet process phosphoric acid is a function of acid concentration, temperature and purity. The effects vary depending on the type of process and raw materials used to manufacture phosphoric acid.Laboratory corrosion studies are presented for several nickel‐rich alloys in pure C. P. grade and wet process phosphoric acid manufactured by the sulfuric acid digestion of phosphate rock.The low concentration (30 010 P20,) acids contain contaminants which make the acid more corrosive than the 56O10 P, O, phosphoric acid. At moderate sorption temperatures (< 140 °C) several nickel‐rich alloys demonstrated excellent corrosion resistance.Hot wall corrosion tests were employed to simulate heat transfer conditions in superphosphoric acid processes. The hot wall tests demonstrated excellent correlation between the metal skin temperature and the resulting corrosion rate of the specimen. Excessive corrosion rates were experienced with the nickel‐rich alloys at metal skin temperatures >145°C with the crude phosphoric acids. Electrochemical studies were used to demonstrate that the crude acid can become unstable at elevated temperatures and result in excessively high cor‐rosion attack of the nickel‐rich alloys. Hastelloy alloy G and Haynes alloy No. 625 demonstrated best overall corrosion resistance in the wet process acids, especially at higher temperatures.