Kinetics and mechanism of reaction between water vapor, carbon monoxide and a chromia-forming nickel base alloy

The kinetic aspects of the competitive reaction of CO and H2O in the formation of chromia layer on a nickel base alloy at high temperature were studied by thermogravimetry combined to gas phase chromatography. It was shown that competitive adsorption occurred between both molecules and that whatever PCO/PH2OPCO/PH2O ratio, CO mainly reacted in the first instants of oxidation and H2O became the main long-term oxidant species. After an interfacial and diffusion mixed kinetic regime, the long-term oxidation rate was parabolic and did not depend on the water vapor content in the gas phase. The oxidation kinetics was well described by considering that the rate limiting step was the outwards diffusion of interstitial chromium cations in the oxide layer.

► Competition of CO and H2O reactivity with a chromia forming alloys was studied.
► CO reacts only during a transient time lasting not more than 12 h.
► H2O is the most reacting species.
► The growth of chromia is rate limited by diffusion of interstitial chromium cations.