Papers by Keyword: Transpassivity

Abstract: The corrosion behaviour of a high-alloy CrMnNi steel was investigated electrochemically in a 0.5 M sulphuric acid solution. The characteristic regions of the active, passive and transpassive state for the material were monitored by means of acoustic emissions. To this end, the steel was potentiostatically polarised and the acoustic emission signal was recorded on the specimen’s surface at a distance of 5 cm from the location of corrosion. Analysis of the acoustic emission signals revealed an increase in signal intensity due to hydrogen bubble formation when the material was cathodically polarised. Furthermore, for anodic polarisation, a continuous decline in the signal-time correlation occurred when the material was polarised in the active/passive state, whereas a sharp point of inflexion and drastic reduction in intensity was recognised when the material was polarised in the passive state. Moreover, a further increase in signal intensity was observed when the transpassive state was reached. The increase at transpassivity was related to the onset of oxygen bubble formation at the electrode.

Abstract: The austenitic stainless steels are used in nuclear spent fuel reprocessing and waste management plants and the process fluid is nitric acid at temperature up to boiling point. However incorporation of oxidizing ions e.g. fission products as well as corrosion products, in nitric acid stream make the environment highly corrosive to stainless steels. Present work aims to investigate role of process parameters and material parameters (composition and microstructure) on corrosion behaviour of stainless steels. The process parameters studied are temperature, acid concentration and oxidizing ions. It has been shown that the potential attained on stainless steel is a function of acid concentration and temperature and is further strongly affected by addition of oxidizing ions. This developed potential determines the corrosion behaviour of stainless steel. Increasing the temperature and concentration of nitric acid and concentration of oxidizing species increased the developed potential. Potentials were applied to types 304 L (nitric acid grade - NAG), 304 L (commercial purity) and 310 L stainless steels 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. A clear demarcation was observed between uniform corrosion and intergranular corrosion at a potential of 960-980 mV_SCE. Above this potential range corrosion rate increased exponentially and the form of corrosion is shown to be intergranular corrosion. Below this potential range, uniform and low rate of corrosion occurred. The influence of microstructure (step, dual and ditch) of type 304 L was also studied and is described in this paper.