“Wet process” phosphoric acid (P2O5) is made by reacting phosphate rock with sulfuric acid. As produced, it contains many impurities, and has a P2O5 concentration of only about 30%, because of the large amount of rinse water needed to separate it from the other main reaction product, calcium sulfate. Typical impurities include unreacted sulfuric acid, various metallic ions, fluoride ions, and chloride ions. The fluoride ions tend to form complexes with the metallic ions, and are therefore less of a problem than the chloride ions, which strongly influence electrochemical reactions between “wet process” phosphoric acid and metallic materials. Particulate matter (for example, silica particles) can also be present in “wet process” acid.
The main use of metallic materials is in the concentration process, where the “wet process” acid is taken through a series of evaporation steps, involving metallic tubing. Typically, the P2O5 concentration is raised to 54% during this process. The concentration effect upon the corrosivity of the acid is somewhat offset by the fact that the impurity levels drop as the concentration increases.
The following chart, comparing HASTELLOY® G-30® alloy with competitive stainless steels, is based on tests in three concentrations (36, 48, and 54%) of “wet process” phosphoric acid (supplied by a producer in Florida, USA) at 121°C (250°F).