Study on corrosion properties of WC-Ni cold spray coatings to mitigate flow-accelerated corrosion of carbon steels in nuclear power plants

Abstract

In secondary system of pressurized water reactors (PWRs), carbon steel undergoes severe corrosion owing to high temperature and pressure, and fast flow velocity. These circumstance, named as flow-accelerated corrosion (FAC), induces corrosion and eventually failure of power piping. Flow velocity brings about destruction of oxide layer by electrochemical corrosion and mechanical shear stress at the oxide/water interfaces. The piping system in the secondary circuit of PWRs mainly consists of carbon steels being vulnerable to FAC because Fe, main element of carbon steel, forms Fe3O4 dissolved to secondary water system by changing to Fe ion. To mitigate corrosion of carbon steel, chemical and mechanical treatments to carbon steel or changing secondary water chemistry has been widely studied. Addition of Cr and/or Mo to carbon steel components is one of the adequate countermeasure to mitigate FAC by anodic passivation. Mechanical treatments are controlling surface roughness because reducing surface roughness can decrease flow turbulence at the oxide/water interface. Increasing pH, dissolved oxygen contents, and addition of advanced corrosion inhibitor to secondary system can decrease corrosion rate of carbon steels. However, maintaining homogeneous chemistry condition of water is hardly possible and changing chemical composition of carbon steel needs not only research of corrosion properties but also other mechanical properties. In contrast, application of coatings to the steel surface can be easily applied to the secondary system. Therefore, thermal spray to steel surface using less corrosive metal is widely used. Among them, cold spray has advantage with lower process temperature than that of other spray coatings which leads to low residual stress and oxidation or decarburization of coating. In this study, tungsten carbide (WC) and nickel (Ni) particles are used as feedstock materials for the coating. WC exhibits high corrosion resistance in acidic aqueous media due to the phase stability. Moreover, the oxidation resistance of WC in O2 containing atmosphere at high temperature is applied to industry which is exposed to severe corrosive environments However, in alkaline solution, the dissolution of WC increases by forming WO4-. Moreover, high melting temperature and poor plastic deformation make cold spray coating difficult. To suppress the corrosion of and increase the deposition efficiency of the coating, Ni, which has better plastic deformation and lower corrosion potential and melting temperature than WC, is used as a binder metal. Ni acts like a sacrificial metal to WC so corrosion and oxidation of Ni occur before those of WC. To evaluate the potential of the coating as an effective corrosion barrier, series of corrosion tests at simulated secondary water chemistry is carried out and quantitative analysis is conducted. Surface and cross-section morphologies of the coatings before and after FAC simulation tests is investigated by scanning electron microscope and X-ray photoelectron microscopy. Electrochemical experiments are carried out for quantitative analysis. Cold spray coatings using WC-10Ni with different Ni contents have rough and porous surface but significant changes of surface is not observed by SEM image. However, lower current density is measured at 25Ni coating than carbon steel, P22, 20Ni and 30Ni coating from potentiodynamic polarization experiments in pH 9.3 ETA solution at room temperature. After 2 weeks FAC experiments, weight loss of coating is increased with increases Ni contents in coating but weight of 20Ni coating is increased. On the other hands, weight loss occurs at 20Ni and 30Ni coating and weight of 25Ni coating is increased after 4 weeks FAC experiments. Although weight change of coatings is smaller than carbon steel after 2 weeks and 4 weeks FAC experiments, weight change of 25Ni coating shows better corrosion performance than P22 comparing with 20Ni and 30Ni coating. As a result, Ni has lower corrosion potential than WC and it induces galvanic corrosion of Ni and prevents WC corrosion or oxidation at early stage of corrosion process. Moreover, oxidation product of Ni, Ni(OH)2 is dissolved into water by Ni ion because of poor thermal stability of Ni(OH)¬2. Therefore, high Ni contents in coating increases weight loss during the early stages by Ni(OH)2 dissolution. However, after 4 weeks of immersion, WC oxidizes and transforms into WO3 increasingly which is influenced by Ni contents. 30Ni coating induces continuous weight loss during FAC simulation because of low WC contents which means weight gain by WO3 formation is smaller than weight loss by Ni(OH)2 dissolution. Therefore, weight loss occurs at 30Ni coating. Contrarily, 20Ni coating do not protect WC dissolution after 2 weeks because the absence of Ni and Ni(OH)2.accelerates WC oxidation and WO3 dissolution to water. To be concluded, WC-10Ni + 25Ni coating specimens exhibit the best corrosion resistive performance, the lower corrosion current density and the small weight change than other coatings. Therefore, WC-10Ni + 25Ni coating is possibly suggested to one of the option to mitigate corrosion of carbon steels.