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Inhibition of Crevice Corrosion of Ni-Cr-Mo Alloys by Sulfate, Nitrate and Molybdate

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Nickel base alloys with high chromium and molybdenum contents are very resistant to chloride-induced localized corrosion, even at relatively high temperatures. These alloys include the Ni-Cr-Mo(W) or 
“C” family of alloys and the novel Ni-Mo-Cr alloy HYBRID-BC1 (UNS N10362). Alloy 22 (UNS N06022) stands out due to its versatility: chromium and molybdenum contents were selected to optimize its corrosion behavior in oxidizing and reducing conditions. Alloy 625 (UNS N06625) has outstanding thermal ageing resistance and a longer in-service experience. The

Product Number: 51323-18936-SG
Author: Eduardo Sáenz González, Martín A. Rodríguez, Edgar C. Hornus
Publication Date: 2023
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Crevice corrosion repassivation potential (ER,CREV) of alloys 625 (UNS N06625), 22 (UNS N06022) and UNS N10362 was determined in NaCl-based solutions with various additions of sulfate, nitrate and molybdate at 30, 60 and 90 °C. Tests were performed by the potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) method, which is a modification of the Tsujikawa-Hisamatsu electrochemical method described in ASTM G192. ER,CREV slightly increased or remained fairly constant for low concentrations of inhibitors and increased drastically for high concentrations of inhibitors. Various inhibitor-to-chloride molar concentration ratios (R) were tested. When R was high enough, crevice corrosion did not occur at any potential. The critical inhibitor-to-chloride molar concentration ratio (RCRIT) was determined in such conditions. RCRIT depended on the particular alloy, inhibitor and temperature. In general, RCRIT increased with temperature for all the inhibitors and alloys. Nitrate was the most efficient inhibitor for alloys N06625 and N06022. However, molybdate was the best inhibitor for alloy N10362. The inhibiting performance of molybdate increased with PRE (Pitting Resistance Equivalent), which is mainly influenced by the molybdenum content of alloys.

Crevice corrosion repassivation potential (ER,CREV) of alloys 625 (UNS N06625), 22 (UNS N06022) and UNS N10362 was determined in NaCl-based solutions with various additions of sulfate, nitrate and molybdate at 30, 60 and 90 °C. Tests were performed by the potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) method, which is a modification of the Tsujikawa-Hisamatsu electrochemical method described in ASTM G192. ER,CREV slightly increased or remained fairly constant for low concentrations of inhibitors and increased drastically for high concentrations of inhibitors. Various inhibitor-to-chloride molar concentration ratios (R) were tested. When R was high enough, crevice corrosion did not occur at any potential. The critical inhibitor-to-chloride molar concentration ratio (RCRIT) was determined in such conditions. RCRIT depended on the particular alloy, inhibitor and temperature. In general, RCRIT increased with temperature for all the inhibitors and alloys. Nitrate was the most efficient inhibitor for alloys N06625 and N06022. However, molybdate was the best inhibitor for alloy N10362. The inhibiting performance of molybdate increased with PRE (Pitting Resistance Equivalent), which is mainly influenced by the molybdenum content of alloys.

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