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

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Although pure phosphoric acid is fenerally 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%H_3PO_4) phosphoric acid solution at high temperature (about 200℃),and a high concentration (about 33%P_2O_5) phosphoric acid solution at low temperature (about 70℃).The polarization tests of stainless steels were carried out mainly in 220℃ 1%H_3PO_4 solution with or without dissoluted oxygen.The corrosion tate 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%P_2O_5),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 CaF_2 in the plaster incrases the amount of fluoric ions and accelerates corrosion.It is also suggested that ferric ions pervent 316L from corrosion in this environment.
社団法人日本材料学会の論文
1993-08-15