リン酸系におけるオーステナイト系ステンレス鋼の腐食問題

抄録

Although pure phosphoric acid is generally known to be mild to austenitic stainless steel, it is corrosive to austenitic stainless steel in some cases. This paper shows the corrosion behavior of stainless steel (mainly 316L) in a low concentration (about 1% H₃PO₄) phosphoric acid solution at high temperature (about 200°C), and a high concentration (about 33% P₂O₅) phosphoric acid solution at low temperature (about 70°C).
The polarization tests of stainless steels were carried out mainly in 220°C 1% H₃PO₄ solution with or without dissoluted oxygen. The corrosion rate decreased when Cr or Ni contents in the test specimens increased, but even Hastelloy C-22, Ni-based alloy, did not passivate by itself. Dissoluted oxygen also reduced the corrosion rate of 316L, but no self-passivation occurred.
The corrosion behavior of 316L stainless steel in the solution of a phosphoric acid plant was also tested. This solution contained phosphoric acid (33% P₂O₅), surfuric acid (1.5%), plaster (25%), fluorine ion (2.5%) and so on. It was found that corrosion in the plaster slurry settled from the solution was severer than that in the mixed acid solution without slurry, and the quantity of fluorine ions was higher in the slurry. Thus, it is suggested that CaF₂ in the plaster increases the amount of fluoric ions and accelerates corrosion. It is also suggested that ferric ions prevent 316L from corrosion in this environment.