Among various chemical products, phosphoric acid plays a great role and is mainly used to produce high-efficiency phosphorus compound fertilizers and industrial phosphates. Stainless steel applications often encounter phosphoric acid corrosion problems in phosphoric acid environments.

However, stainless steel materials have better resistance to corrosion by phosphoric acid. Usually in phosphoric acid without impurities, ordinary chromium-nickel stainless steel can completely solve its corrosion problem. For example, storage tanks made of type 18-8 chromium-nickel austenitic stainless steel can store cold phosphoric acid with a concentration of 85% and can process phosphoric acid with a concentration of less than 5% in process equipment; and contain 2%-3% molybdenum. Stainless steel will show better corrosion resistance. As the molybdenum content in stainless steel increases, its resistance to phosphoric acid will become higher.

However, most of the phosphoric acid produced in the wet phosphoric acid production process contains various impurities, such as iron ions, chloride ions, SO42-, Fe3+, Al3+, Mg2+, etc. Among them, except Fe3+, the corrosion rate of stainless steel will be increased, especially iron ions and chloride ions will be more intense. In order to enhance the corrosion resistance of stainless steel in phosphoric acid containing various impurities, the content of chromium and molybdenum elements in stainless steel can be increased. This is the most effective and the effect of chromium is the most obvious.

Tests have proven that chromium has the greatest influence; the effect of molybdenum is only effective when the chromium content in the steel is relatively low; when the chromium and molybdenum content are high, the role of nickel in stainless steel is not obvious. In phosphoric acid containing impurities such as iron ions and chloride ions, in addition to a large number of 0Cr17Ni12Mo2, 00Cr17Ni14Mo2, 0Cr19Ni13Mo3, 00Cr19Ni13Mo3 containing 2%-4% molybdenum, molybdenum-containing duplex stainless steel containing ≥22% chromium, such as 0Cr26Ni6Mo2Cu3 (CD -4MCu), 00Cr22Ni6Mo2N, 00Cr25Ni7Mo3N, high-molybdenum chromium-nickel austenitic stainless steel 00Cr20Ni25Mo4.5Cu (N) (UB6, 2RK65) are also widely used. High-chromium ferritic stainless steel containing no nickel or only a small amount of nickel, such as 00Cr26Mo1, 00Cr30Mo2, 00Cr29Ni2Mo4, 00Cr29Ni4Mo2, etc.

Because the above stainless steel grades not only have excellent resistance to impurity-containing phosphoric acid, they are also corrosion-resistant materials with a wide range of uses. α+γ duplex stainless steel not only has good corrosion resistance but also has high strength, so it is often used as a corrosion-resistant material in phosphoric acid containing impurities.

Practical applications have shown that 00Cr27Ni31Mo3Cu (Sanicro28) and 00Cr26Ni35Mo3Cu, highly alloyed austenitic stainless steels with a chromium content of up to 27% and molybdenum, are the high-grade grades with the best resistance to phosphoric acid corrosion containing iron ions, chloride ions and other impurities and the best overall performance. Stainless steel can be used to manufacture phosphoric acid concentration heat exchangers.