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The effect of Cr(vi) species in HNO3 (concentrations 15–3m) on the corrosion rate of stainless steel has been investigated. The gaseous phase over the HNO3 was carefully controlled using Ar, NO, NO2 or N2 purging. With Ar purging the corrosion rate increased when the Cr(vi) concentrations were above 0.05m. There was evidence that the reduction of Cr(vi) leads to insoluble Cr reduction products on the surface of the steel. Sparging of nitric acid by NO and/or NO2 caused the reduction of any Cr(vi) species present in the solution, to Cr(iii) in a few minutes. However, with no Cr(vi) present, the effect of continuous purging with NO and/or NO2 was to increase the steel corrosion rate when compared with Ar purged solutions, due to catalysis of nitrate reduction by the NOx. The conversion of Cr(iii) to Cr(vi) by nitric acid was shown to require high concentrations of nitric acid, high temperatures and low partial pressures of NO2 over the liquid phase. |
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