Since the nuclear power plant Surry Unit 2 was accident in 1986, flow accelerated corrosion (FAC) has been extensively studied. However, many accidents caused by FAC have been reported such as an accident at Mihama Unit 3 in 2004 and at domestic plants.
During the FAC, a protective oxide layer on carbon steel dissolves into flowing water leading to a thinning of the oxide layer and accelerating corrosion of base material. As a result, severe failures occur in the piping and equipment of nuclear power plants (NPPs). FAC of the carbon steel piping in ‘NPPs’ has been major issue in nuclear industry.
The purpose of this study is the effect of chromium content in material on FAC of carbon steel. We designed a small-scale FAC test loop and constructed it to test carbon steel in an NPP environment. Rotating cylinder facility designed and fabricated and then performance of the facility was evaluated. The facility is very simple in design and economic in fabrication and can be used in material and chemistry screening test.
The facility is equipped with on line monitoring of pH, conductivity,dissolved oxygen(DO), and temperature. Fluid velocity is controlled with rotating speed of the cylinder with a test specimen. FAC test specimens are SA106 Gr. B carbon steel, SA 508 Gr.3, A534 8620H, SA508 Gr.4N, A336 F22V each has a different chromium content. The SA106 Gr. B is used in the secondary piping of the domestic NPPs. Test solution is the D.I water, flow velocity is 4m/s, temperature is 150℃, the pH value was not controlled and initial pH value of the solution was 6.3. Before testing, the solutions were deaerated with a high-purity (>99.99%) nitrogen gas for an hour to adjust the DO content below 1ppb. The test was performed 190 hours and 380 hours more, the total of 570 hours were performed.
After the end of the test specimens was covered magnetite with black color and pits were observed on specimen surface. The specimens after ultrasonic cleaning and analyzed by the Optical microscope, scanning electron microscope, Energy Dispersive Spectroscopy, also FAC rate and the weight loss was calculated.