Effect of oxygen on the corrosion behavior of alloy 625 from 25 to 200°C

Electrochemical polarization and impedance experiments were conducted on alloy 625 in ammoniacal solution between 298 and 473 K and at various oxygen partial pressures. Increased oxygen concentrations (as a result of higher temperatures or autoclave oxygen loading) resulted in increased corrosion potentials and corrosion rates. The inversion temperature (onset transpassive corrosion) was found to be similar to 373 K at P-TOTAL>40 bar. A binary barrier layer/ outer layer film configuration was assumed and it was found that the outer layer dominated the corrosion process as temperatures and pressures were increased such that oxygen diffusion control was rate limiting. An oxygen solubility model for supercritical water oxidation (SCWO) conditions was generated using the Redlich-Kwong-Soave equation of state. Results of the model experimental work confirm many of the observations from SCWO reactors which suggest that corrosion reaches a maximum at temperatures just short of the critical point. Exchange current densities for oxygen reduction and effective diffusion layer thicknesses have also been calculated. (c) 2007 The Electrochemical Society.