Radiation Induced Corrosion of Stellite-6

This thesis presents a study on the aqueous corrosion of a cobalt-based alloy, Stellite-6. Since aqueous corrosion kinetics are strongly influenced by water chemistry conditions (pH, temperature, and redox agents), a systematic study of Stellite-6 corrosion was carried out. The aim was to develop an ability to predict corrosion behaviour and particularly metal dissolution rates for cobalt alloys under a range of conditions. The work focused on a study of the influence of ionizing radiation on corrosion. This is of particular interest in the nuclear industry where radiation fields are present. Ionizing radiation creates oxidizing radiolysis products and alters the redox potential of a corroding solution. A series of electrochemical measurements and corrosion tests along with post-test surface analyses were performed. In doing these tests we gathered information on the state of the oxide formed during corrosion and the sensitivity of the oxide growth to the corrosion conditions. This is a more sophisticated approach than that used in many corrosion experiments that only examine a few facets of oxide formation. The combination of electrochemical measurements and surface analyses provided a highly detailed picture of oxidation. We found that corrosion proceeds through the formation of different oxides at different corrosion potentials. At higher potentials the result is the formation of a Co(OH)2 oxide layer on top of a CoCr2O4 layer. The rate and extent of oxidation and the rate of metal dissolution were sensitive to all parameters studied and particularly the pH of the corroding solution. Oxide growth is promoted and metal dissolution is suppressed at pH ~ 10 where the solubility of CoII is the lowest. The production of water radiolysis products via gamma irradiation was seen to have a net oxidizing effect and stimulated oxide growth at high pH. At all temperatures studied (25 to 150 °C) irradiation did not result in significant oxide compositional changes. However the extent of metal dissolution was seen to be dependent on a combination of both pH and solution electrolyte concentration, as well as the presence of ionizing radiation