The Effect of Alloying Elements on Depassivation pH for Stainless Steels

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  • 各種ステンレス鋼の脱不動態化pHに及ぼす合金元素の影響
  • カクシュ ステンレス コウ ノ ダツ フドウタイカ pH ニ オヨボス ゴウキ

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Abstract

Depassivation pH, pHd, for various stainless steels were measured in deaerated 12% NaCl solution at 25°C. The effect of alloying elements on pHd were analyzed by multiple linear regression. Regression expressions were obtained for 4 steel groups of α (ferritic, 15 steels), α+γ (duplex, 10 steels), γ (austenitic, 18 steels with Cu<0.4%) and γt (including austenitic 8 steels with 0.9-3.5% Cu in addition to the γ group steels) as follows. pHdα=-0.157log Ni-0.460Mo+1.15, pHdα+γ=-3.28log Ni-0.13Mo-10.4P+2.95, pHdγ=-0.248Cr+1.29log Ni-0.219Mo+2.66C+74.9S+4.09, pHdγ<sub>t</sub>=-0.144Cr+1.73log Ni-0.206Mo-0.146Cu+1.17Si-0.177Nb+1.89. The letters in the expressions indicate the contents of alloying elements in wt%. For the ferritic 18 steels including 3 steels with 13-17% Cr in addition to the α group steels with 19-25% Cr, another expression similar to that by Onoyama was obtained but with reduced significance level; pHdα′=-0.098Cr-0.224log Ni-0.487Mo+3.36. Standard partial regression coefficients of alloying elements were compared with those for repassivation potential for metal/metal-crevice, ER, measured in 3% NaCl solution at 80°C in our previous report. It is generally found that an element which lowers pHd makes ER more noble and another one which raises pHd makes ER less noble. But these tendencies are not observed for Ni or N alloyed to α+γ- group steels, where Ni is much beneficial to lower pHd but detrimental to make ER less noble, while N has no effect on pHd but much beneficial to raise ER.

Journal

  • CORROSION ENGINEERING

    CORROSION ENGINEERING 36 (11), 702-709, 1987

    Japan Society of Corrosion Engineering

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