Corrosion Characteristics of Alloy 800 in Aqueous Solutions up to 200°C

Abstract

The corrosion behavior of Alloy 800 in sulfate solutions containing hydrogen peroxide in the temperature range of 100-200 • C was studied. Electrochemical techniques were used to study both the cathodic and anodic processes. The reduction of hydrogen peroxide showed an increase in the steady state current density and the H 2 O 2 diffusion coefficient by increasing the temperature from 100 to 200 • C. The electrochemical corrosion of Alloy 800 showed a decrease in corrosion potential and a decrease in transpassive potential in accordance with calculations for CrO 4 −2 /Cr 2 O 3 and HCrO 4 − /Cr 2 O 3 couples. The increase in corrosion current density was compared with a predictive model based on both reaction kinetics and water property changes. The findings were compared with weight loss measurements and surface characterization studies.The next generation of power plants aims to approach more efficient and safer designs. One of the candidate designs is the supercritical water cooled reactor (SCWR). Two major advantages are identified for this new design. The first is the higher thermal efficiency because of higher maximum water temperature. 1 The most recent models have proposed that the efficiency of SCWR will be nearly 45% compared to the current reactors' typical efficiency of 33%. The second is the simpler plant design. It is partly because the supercritical water has an appreciably higher heat capacity compared to liquid water and, as a consequence, lower cooling water flow rates would be sufficient at a fixed production rate. The lower flow rate translates into simpler parts in the cooling system loop. 2 However, there are uncertainties regarding the SCWR development due to corrosion characteristics of the environment. The water is exposed to radiation, and the consequent water radiolysis generates radicals such as O .. , H . , HO . and H 2 O 2 which are considered corrosive. 3,4 Therefore, predicting the water chemistry and its interaction with materials at high temperature and pressure should first be fully understood. The corrosion behavior of materials in such an environment is determined by two factors: 1) the operating temperature: the thermodynamic stability of species changes and the reaction rates acceleration which translates into higher corrosion rate. 2) The concentration of radicals such as O .. , H . , H 2 O 2 : the higher the concentration, the more corrosive the solution.The generation of these radicals and their interactions with various metals have been studied. [4][5][6][7][8][9] The maximum transient concentration of hydrogen peroxide in a typical radiation flux is 10 −4 M. 7 Therefore, the corrosion of metals could be simulated by adding hydrogen peroxide up to 10 −4 M. The high-temperature, high-pressure corrosion of 1018 carbon steel, 10 304 stainless steel, 11 316 stainless steel, 12,13 Alloy 182, 14 Alloy 690, 15 and duplex stainless steel 16 in the presence of hydrogen peroxide have been studied.One of the candidate materials of construction is Alloy 800, which has acceptable high...