Corrosion Resistant Alloys for Phosphoric Acid Production and Storage

Abstract

This paper addresses the topic of material selection for several parts of the phosphoric acid production chain, focusing essentially on corrosion resistant alloys and life cycle cost. During the phosphate attack by sulfuric acid, the vessel can be susceptible to corrosion due to the presence of an aggressive slurry which consists of a mixture of phosphoric acid, sulfuric acid, chlorides and hydrofluoric acid. This study reports the results of corrosion tests performed on various stainless steels in this kind of mixture at 80°C and 110°C, corresponding to the temperature of the di-hydrate and hemi-hydrate processes, respectively. From this study, it is highlighted that the super-duplex stainless steel 2507Cu / UNS S32550 (25% chromium, 7% nickel) can be recommended for phosphoric acid production provided that the temperature does not exceed 80°C. This alloy also offers a good resistance to abrasion and may allow the design engineer to decrease the wall thickness thanks to its high mechanical properties. If the temperature is higher than 110°C, materials containing more alloying elements must be selected, like alloy 31 / UNS N08031 or alloy 28 / UNS N08028. For phosphoric acid transportation and storage, the behavior of various stainless steels is also investigated. The influence of several levels of chloride contamination on the corrosion rates is considered. The results reveal that the duplex steel 2202 / UNS S32202 (22% chromium, 2% nickel) is a cost-efficient alternative to conventional austenitic stainless steels, offering better mechanical properties and corrosion resistance in the presence of chlorides.