High-temperature oxidation corrosion diffusion barrier

At ordinary temperatures in neutral or near neutral water, dissolved oxygen is necessary for appreciable corrosion of iron. In air-saturated water, the initial corrosion rate may be high. This rate may diminish over a period of time as the iron oxide film is formed and acts as a barrier to oxygen diffusion. Since the diffusion rate at steady state is proportional to oxygen concentration, it follows that the corrosion rate of iron is also proportional to oxygen concentration. 

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