Decomposition products of oxygen scavengers and their effect on corrosion of steam generator materials – I. Diethyl-hydroxylamine and carbohydrazide

Hydrazine used as oxygen scavenger in the secondary circuit of pressurized water reactors is hazardous to the environment and potentially carcinogenic, thus, suitable replacement chemicals for it are actively sought. In the present paper, decomposition products of two potential replacements – carbohydrazide and diethyl-hydroxylamine – are analyzed, and their effect on secondary water chemistry and corrosion of the main steam generator materials – carbon steel 22 K, stainless steel 0X18H10T and Alloy 690 – is studied by in-situ electrochemical techniques complemented by ex-situ analyses of the formed oxides by spectroscopic and microscopic methods. Quantitative interpretation of the electrochemical impedance data with the Mixed-Conduction Model allowed for the estimation of oxidation and corrosion release rates depending on scavenger formulation, alloy type and temperature. Conclusions on the extent of interaction of decomposition products with construction materials are drawn based on the experimental and calculational results. • The decomposition of carbohydrazide decreases pH and increases acidic conductivity in the first hours of testing due to the formation of CO 2 . • Acetaldehyde, acetic acid, dimethylamine and ethylamine were the decomposition products that could be analyzed reliably from the DEHA decomposition. • The effect of scavenger type on the austenitic materials' oxide composition is rather small at the studied concentrations. • Electrochemical impedance spectroscopy was successfully applied to assess the structure of the forming oxides. • The differences between corrosion release rates on different materials in all studied scavenger formulations are not very large.