Flow accelerated corrosion of carbon steel piping in nuclear power plants

Abstract

Flow accelerated corrosion (FAC) is a process whereby the normally protective oxide layer on carbon or low alloy steel dissolved into a stream of flowing water resulting in increasing the corrosion rate. Major influencing factors that affect the FAC are flow velocity, temperature, pH, dissolved oxygen concentration, and steel composition. The experimental study described in this paper was focused on evaluating the FAC behavior of carbon steel according to environment conditions. Feasibility tests for the mitigation method against the FAC were also carried out with controlling the water chemistry and with applying the magnetic field. A high temperature rotating cylinder electrode (HTRCE) and a water chemistry control system was developed to perform the electrochemical test in high temperature water environments. The main design concept of HTRCE is to assure stable operation of working electrode in a severe environment, to insulate electrode housing except working electrode surface against external fluid, and to extract corrosion parameter from the rotating cylinder to outside of the autoclave safely. The electrochemical corrosion potential (ECP) and current density were measured as a function of temperature and rotating speed using polarization monitoring. ECP values dropped at a rate of $-1.51 mV/^\circ C$ above $150^\circ C$, which may be come from the formation of magnetite on the steel surface. With increasing rotation of the RCE, the ECP shifted upward in all temperature ranges. This shift may be attributed to the diffusion enhancement of the oxidizing agents in the rapidly flowing of fluid. From the velocity exponent of the cathodic half-cell current density on the steel surface, it was evident that a mass transfer process first dominated the corrosion reaction at $150^\circ C$, and then an activation process partly controlled the corrosion kinetics with increasing temperature. From the results of corrosion experiment at high temperature water, HTRCE has...