Electrokinetic and Electrochemical Corrosion Studies Related to Crud Formation

A potentially important mechanism for the flow-induced deposition of CRUD from pressurised high temperature primary water is the effect of “streaming potentials” that develop across the electrochemical double layer of a metallic surface as a result of fluid flow across a pressure gradient or orifice. Thus, under such conditions, streaming currents develop normal to a surface and may result in preferential oxidation, for example of dissolved ferrous to ferric ions with their subsequent deposition as an oxide. The approach presented in this paper was to consider the electrokinetic problem is to divide it into two by, firstly, considering the magnitude of currents that can be developed under a given set of flow/mass transport conditions and, secondly, to consider the way in which these relatively small currents might give rise to oxide deposition. Electrochemical measurements on 304L samples were carried out over a range of temperatures in hydrogenated, alkaline water. The test conditions were chosen in order to simulate PWR primary water conditions. Furthermore, in order to facilitate the electrochemical studies, ferrous ion in the solution was also enhanced by the presence of a mild steel plate left in the autoclave to “corrode”. By employing the cyclic voltammetry techniques combined with the Randles-Sevcik approach it was possible to calculate the concentration of ferrous ion and its diffusion coefficient. A miniature flow cell was been designed as the purpose to create regions of accelerated flow with consequently formation of electrochemical anodic and cathodic reaction so as to be able to measure the streaming currents. A parametric study was carried out in order to better understand the potential which is associated with the streaming potential as function of the velocity and temperature at fixed pH. The electrodes used to measure the streaming potential were investigated with SEM that revealed preferential oxide formation where accelerated flow conditions. Others electrodes anodically and cathodically polarized were used to measure the build up rate of deposition and investigated by XRD and GDOES that showed that the oxide structure and thickness is strongly dependent on the electrochemical corrosion potential. This confirms that anodic processes, e.g. streaming potential, can strongly enhance the oxide deposition. This work underlines the importance of anodic processes, such as those generated by streaming potential, which can strongly enhance the oxide deposition.