51312-01487-Measurement and prediction of corrosion damage in stainless steels in nitric acid containing oxidizi

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Product Number: 51312-01487-SG

ISBN: 01487 2012 CP

Author: V. Kain

Publication Date: 2012

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The austenitic stainless steels are used in nuclear spent fuel reprocessing and waste management plants and the process fluid is nitric acid of concentrations up to 12 M and at temperatures up to boiling point. However incorporation of oxidizing ions e.g. fission products as well as corrosion products in the nitric acid stream makes the environment highly corrosive to stainless steels. Increasing concentration temperature and concentration of oxidizing ions push the operating potential to the transpassive potential rage and this causes severe intergranular corrosion of even the solution annealed stainless steel. In this study it has been shown that the potential attained either by addition of oxidizing ions or by external application through a potentiostat determines the corrosion behaviour of stainless steels. Potentials were applied to types 304L (Nitric Acid Grade) 304L (commercial purity) and 310L in boiling 6 M nitric acid for a period of 48 h. The corrosion rates measured in such experiments were plotted as a function of applied potential. The form of corrosion was established by microstructural examination (optical and scanning electron microscopes). A clear demarcation was observed between uniform corrosion and intergranular corrosion at a potential of 960-970 mVSCE. Above this potential range the corrosion rate increases exponentially and the form of corrosion was intergranular corrosion. Below the potential range uniform and low rate of corrosion occurred. An equation derived from the curve fitting of this data is proposed to be used for predicting the corrosion rate of the same stainless steel at any given operating potential. It was further shown that the corrosion rates measured after exposures for 48 h to 6 M boiling nitric acid containing oxidizing ions and resulting in different potentials (in the transpassive regime) matched well with the predictions from the derived equation. The influence of microstructure (“step” “dual” and “ditch” structures) and alloy types (types 304L 304L (NAG) and 310L) were also studied and are described in this paper.