Crevice Corrosion of Alloy 22 at the Open Circuit Potential in Hot Chloride Solutions

Crevice corrosion of Alloy 22 in hot chloride solutions at the open circuit potential was studied in short term tests.  Mill Annealed (MA) and thermally aged PCA specimens were used.  Thermally aged specimens were obtained by ageing at 760ºC 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 Alloy 22 MA sheet.  Crevice corrosion propagation started in Alloy 22 after a few hours of immersion in pH 2, 1M NaCl at 90ºC.  Open circuit potential stabilized after an initial drop.  Steady state open circuit potential was higher for thermally aged specimens than for MA specimens.  Impedance diagrams indicated a charge transfer kinetic control.  Galvanic coupling was performed in order to asses the effect of an increasing cathodic area.  Galvanic couple current and potential were recorded in time.  A potential maximum was correlated with a current minimum,indicating the onset of crevice corrosion propagation.