Applicability of Hydrazine Alternatives to Water Treatment in Power Plants: Effects of Hydrazine Alternatives and Decomposition Products on Water Properties and Corrosion of Carbon Steel

The applicability of carbohydrazide (CHZ) and diethylhydroxilamine (DEHA) as hydrazine alternatives to the water treatment of water-steam circuits of power plants was investigated by assessing their effects on the corrosion of carbon steel and water properties, including the conductivity, conductivity after cation exchange (cation conductivity), dissolved-oxygen concentration (DO), and oxidation-reduction potential (ORP) of high-temperature water under simulated conditions of boiler feed water of power plants. Hydrazine and carbon dioxide were generated by CHZ decomposition, resulting in a decrease in DO and increase in both conductivity and cation conductivity. Various substances including diethylamine, ethylamine, and acetaldehyde were generated by DEHA decomposition, and the amines increased conductivity. Under the oxygen-containing condition, acetate was also formed, and cation conductivity was increased by DEHA injection. Measurements of DO and ORP suggested that high CHZ and DEHA concentrations or temperatures were required for them to exert their oxygen-scavenging effect and decrease the electrode potential of metals like hydrazine. Irrespective of the decomposition products of CHZ or DEHA, neither of these hydrazine alternatives accelerated the corrosion of carbon steel. In this study, various characteristics of CHZ and DEHA were clarified, and results suggest that CHZ is a safe choice, as long as hydrazine generation by CHZ decomposition does not conflict with hydrazine restriction.