Crevice corrosion of Alloy 22 (N06022) in hot chloride solutions at open circuit potential was studied in short term tests. Mill Annealed (MA) and thermally aged PCA (Prism Crevice Assembly) specimens were used. Thermally aged specimens were obtained by aging at 760 °C for 10 hours. Precipitation of secondary phases (TCP: Topologically Close Packed phases) at grain boundaries occurred. Electrochemical tests performed included open circuit potential monitoring for up to 48 hours, electrochemical impedance spectroscopy (EIS) and galvanic coupling with an MA Alloy 22 foil. Crevice corrosion propagation started in Alloy 22 after a few hours of immersion in pH 2, NaCl at 90 °C. Steady state open circuit potential was higher for thermally aged specimens than for MA specimens. Galvanic coupling was performed in order to asses the effect of an increasing cathodic area. Galvanic current and potential were recorded with time. A potential maximum was correlated with a current minimum, indicating the onset of crevice corrosion propagation. Crevice corrosion kinetics was limited by oxygen reduction on passive Alloy 22 surface areas. Crevice corrosion attacked area and propagation potential (ECORR) were set to balance oxygen reduction rate. Oxygen reduction on passive Alloy 22 was inhibited. Low crevice corrosion propagation rates can be expected for crevice corrosion provided that oxygen is the only oxidant and the ratio of creviced area / passive area is large.

Subject
Foil, Circuit potentials, Sodium chloride, Open circuits, Corrosion attacks, Corrosion propagation, Crevice corrosion, Oxygen, Immersion, Risk reduction, Galvanic current, Alloys, Corrosion potential
Keywords:
N06022, chloride, crevice corrosion, thermal aging, galvanic coupling
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2009
Association for Materials Protection and Performance (AMPP)
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