Depassivation pH, pH
d, for various stainless steels were measured in deaerated 12% NaCl solution at 25°C. The effect of alloying elements on pH
d 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 γ
t (including austenitic 8 steels with 0.9-3.5% Cu in addition to the γ group steels) as follows. pH
dα=-0.157log
Ni-0.460
Mo+1.15, pH
dα+γ=-3.28log
Ni-0.13
Mo-10.4
P+2.95, pH
dγ=-0.248
Cr+1.29log
Ni-0.219
Mo+2.66
C+74.9
S+4.09, pH
dγt=-0.144
Cr+1.73log
Ni-0.206
Mo-0.146
Cu+1.17
Si-0.177
Nb+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; pH
dα′=-0.098
Cr-0.224log
Ni-0.487
Mo+3.36. Standard partial regression coefficients of alloying elements were compared with those for repassivation potential for metal/metal-crevice, E
R, measured in 3% NaCl solution at 80°C in our previous report. It is generally found that an element which lowers pH
d makes E
R more noble and another one which raises pH
d makes E
R 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 pH
d but detrimental to make E
R less noble, while N has no effect on pH
d but much beneficial to raise E
R.
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