The Influence of Hydrogen, Oxygen, and Ammonia on the Corrosion Behavior of Plain Carbon Steel in High Temperature Water

The corrosion behavior of plain carbon steel in high temperature water has been studied using electrochemical polarization techniques. The influence of oxygen, hydrogen, and ammonia on the corrosion behavior has been studied over the range < 0.1-100 ppm. Potentiostatic anodic polarization data indicate that only oxygen has any effect on the anodic dissolution kinetics of carboh steel at 289 C (552 F). Controlled potential corrosion experiments at highly noble potentials in oxygenated water did not produce any form of localized corrosion. Cathodic Tafel polarization data indicate that the primary cathodic partial process during corrosion in ammonia and hydrogenated solutions is the reduction of hydrogen ions. Reduced steady-state corrosion rates observed in ammoniated solutions are due to the severe reduction in the limiting diffusion rate of hydrogen ions because of a simple pH effect. In oxygenated solutions, over the range 0.1-50 ppm O₂, the major reduction process is the reduction of H⁺. At 50-100 ppm O₂ levels, however, the reduction process changes to O₂ + 2H₂O + 4e→4OH⁻, with the formation of a protective oxide film and a reduction of the steady-state corrosion rate to 0.2 mdd. Linear polarization studies indicated that the steady-state corrosion rate of carbon steel in hydrogenated solutions is relatively independent of H₂ content. The corrosion rate in ammoniated solutions decreased with NH₃ content over the range 1-100 ppm NH₃. Corrosion rates in oxygenated water over the range < 0.08-100 ppm increase through a maximum at approximately 50 ppm then decreases to a low value at 100 ppm. This behavior is interpreted by the formation of a protective oxide film which is dependent on oxygen content. X-ray diffraction studies indicate the presence of a duplex oxide film in high oxygen solutions consisting of γFe₂O₃ and Fe₃O₄. At lower oxygen levels, only αFe₂O₃ was observed and also in ammoniated solutions. In hydrogenated solutions, with the highers corrosion rates, only αFe₂O₃ was observed on the surface.