Study was focused on elucidating flow accelerated corrosion (FAC) process, which has been defined as one of the major issues in nuclear power plant, in a mechanistic point of view by rotating cylinder electrode from which it was revealed as an effective device to assess velocity sensitive testing. These include proposing electrochemical methodologies in analyzing FAC, analyzing synergistic effect on corrosion behavior of base and weld metal with turbulent flow, and evaluating FAC behavior simulating high temperature, high pressure system to give light on FAC prediction model.
Activation and mass transfer process regarding the FAC of low alloy steel were studied in deaerated weak alkaline solution. Electrochemical corrosion potential (ECP) and electrochemical corrosion current density was measured with a temperature range from 25℃ to 270℃ and various rotating velocity through polarization test. At room temperature, alkaline corrosion appeared above pH 10.4, where passive film was formed from pH 9.8 by the step oxidation and subsequent precipitation of ferrous hydroxyl compound. At higher temperature, formation of magnetite on the steel surface occurred, caused ECP to drop at the rate of -1.51mV/℃. ECP shifted upward with rotating condition in all temperature range by the mechanisms of diffusion of oxidizing agent, which greatly depend on both temperature and dissolved oxygen. From the comparison between electrochemical corrosion current density and cathodic deposition current density on steel surface revealed that activation process firstly dominated at room temperature, soon mass transfer process control the entire corrosion kinetics with the increase of temperature.
Chemical and geometric effects of weld on FAC of SA106 Gr.C low alloy steel pipe was investigated using rotating cylinder electrode in 3.5wt% sodium chloride solution and simulated feedwater of nuclear power plants. Polarization test and weight loss test were conducted at rotating speed of 2000rpm (3.14m/s) with variation of chemical and geometric parameters. The results showed that the chemical effects were relatively larger than the geometric effects, and the welded parts became local anode and preferentially corroded, which was explained by the differences between microstructural and compositional parameters. On the other hand, under active corrosion conditions, the heat affected zone was severely corroded and microstructural effects were dominant.
