Evaluation of aluminum brass coupons in BWR condensate environment in presence of metal ions

Effect of cobalt and cesium ions in the simulated BWR condensate environment (two phase water at 150 °C) on the oxide formed on the aluminium brass has been studied by exposing active and prepassivated coupons in respective environments. Surface changes in the exposed coupons were evaluated by SEM, EDAX and electrochemical studies. The SEM and EDAX data of the exposed coupons indicated marked difference in the surface morphology with varying water chemistry. Presence of nodular grains were seen on SEM images of the pre-passivated Al brass coupons in the Co based media while more granular oxide formation could be seen in presence of Cs. With the mixture of Co and Cs, oxides with larger particle size were seen in the SEM images. The weight change measurement also indicated that Co affects the outer oxide layer to a higher extent as compared to Cs. EDAX measurements indicated incorporation of Co in the oxide layer for the coupons exposed in the Co based media whereas higher aluminum composition was seen in the oxide layer for the coupons exposed in the Cs based media. Cathodic reduction of the oxide layer in sodium perchlorate medium indicated that the oxide grown in only water based media are primarily Cu₂O with a minor amount of ZnO but there is a significant amount of Co in the oxide layer for the coupons exposed in the Co based medium. Impedance measurement of the coupons indicated similar protective nature of the passive layers formed under various conditions based on the values of charge transfer resistance obtained by fitting the experimental data to the Randles circuit. However, the higher capacitance values for oxides formed in the Co based medium indicated its porous nature. Thus, there is significant sorption of Co in the passive layer of the aluminium brass while there is no evidence of Cs sorption over aluminium brass could be obtained in the present study. (author)