The effect of valence of sulfur on the passivation of Alloys 600 and 690 at 25 and 95 C

Conference · Thu Dec 31 04:00:00 EST 1998

OSTI ID:350081

Fang, Zhi; Staehle, R W ^[1]

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1. Univ. of Minnesota, Minneapolis, MN (United States)

The effects of the valence of sulfur in sulfur oxyanions on the passivation behavior of Alloys 600 and 690 in various heat treatments was studied at 25 C and 95 C using the potentiodynamic polarization method; the effects of scanning rate of the potential, valence of sulfur oxyanions, pH, alloy composition and alloy heat treatment were investigated. In general, the stability of passive films was found to decrease with the decrease of sulfur valence from +6 to {minus}2. Alloy 690 in the thermally treated condition tended to be more stable that the various heat treatments of Alloy 600. Examination of surfaces after polarization over the potential range showed that intergranular corrosion occurred mainly in solutions of SO{sub 4}{sup 2{minus}}, SO{sub 3}{sup 2{minus}}, S{sub 4}O{sub 6}{sup 2{minus}} and S{sub 2}O{sub 3}{sup 2{minus}} whereas S{sup 2{minus}} produced pitting. The increase of S{sub 2}O{sub 3}{sup 2{minus}} at pH 6 and 95 C accelerates anodic dissolution. The variation of pH from 3.5 to 8 in 10{sup {minus}2} M S{sub 2}O{sub 3}{sup 2{minus}} solutions does not change the anodic current density significantly. The transient instability of protective films was assessed by taking the ratio, R{sub SR}, of the current density from a rapid scan at 20 mV/s to a slow scan at 0.2 m V/s. These ratios were determined as a function of potential for the variables studied; in some cases ratios as high as 200 were observed.

• OSTI ID: 350081
• Report Number(s): CONF-980316--
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CORROSIVE EFFECTS
CRACK PROPAGATION
INCONEL 600
INCONEL 690
INTERGRANULAR CORROSION
PASSIVATION
PITTING CORROSION
STRESS CORROSION
SULFATES
SULFIDES
SULFITES
SULFUR
THIOSULFATES
VALENCE