More than 95% of the world demand for phosphoric acid is met by the wet-process, which involves the reaction of the phosphate rock with concentrated sulfuric acid. Pure phosphoric acid is only mildly corrosive to metals. However, the presence of impurities in the phosphate ores like chlorides, fluorides and silicates and the free sulfuric acid lead to complex corrosive conditions. The level of impurity depends on the origin of the phosphates and their processing, e.g. washing the ores with sea water further increases the level of chlorides. The corrosive attack is further aggravated by erosion resulting from the presence of phosphate rock particles and gypsum crystals, turbulence and deposit formation. Equipment for production and handling of phosphoric acid typically consists of rubber lined steel, AISI 316L type stainless steel and special steels like 904L and 28. For the most severe conditions nickel based alloys are used.
Even though material selection has been continuously optimized, sufficient advantage is not being taken from the new, cost-effective 6%-molybdenum alloy 31 (UNS N08031) yet. This high Cr and Mo containing austenitic steel was designed to fill the gap in cost and performance between stainless steels and nickel alloys and significantly improves service life and reduces failure by localized corrosion when its lower alloyed counterparts are at the end of the usefulness limits.
This work reports on the success achieved with alloy 31 in critical parts of the phosphoric acid process. Laboratory results are presented together with field experience and examples of application.
Keywords: alloy 31 (UNS N08031), phosphoric acid, 6% Mo alloys, localized corrosion