Application of Corrosion Test for Austenitic SS 304 in PWR with Electrochemical Quartz Crystal Microbalance (EQCM)

The secondary system of a pressurized water reactor (PWR) is a loop composed of: a) steam generator, b) steam transport piping, c) steam turbine and d) condenser and some optional component. The materials of these components are mainly carbon steel, different grades of stainless steel, high nickel alloys (Inconel-600 and Incoloy-800), Copper alloys. Among these materials, austenitic stainless steel 304 is widely used in tubing for large-surface condenser exposed to seawater as coolant of steam condenser. Stainless steels show much greater resistance to erosion-corrosion than copper alloys used in steam condenser tubing and are also immune to ammonia attack, ammonia induced stress corrosion cracking. Also, the chloride induced stress corrosion cracking has not been reported in stainless steel tubing in power plant condenser. Nevertheless, it is well known that stainless steels are susceptible to pitting and crevice corrosion in the seawater environment including chloride ions. Metallic materials exposed to untreated seawater suffer the well-known phenomenon of fouling consisting of the formation of an unwanted deposit that covers the surfaces in contact with the water. Five types of fouling are usually considered as biological, corrosion, particulate, chemical and precipitation fouling. Pitting corrosion is aggravated by stagnant or low flow conditions in which sludge such as corrosion of copper alloys in the secondary system can form in the tube surface. From the operational maintenance viewpoint, pitting attack has destructive effect of structural materials on operation of NPP. Countermeasures such as copper alloys replacement, water chemistry control and chemical cleaning were implemented to mitigate the pitting. Although the many studies are performed macroscopically to reduce pitting corrosion in the secondary system, little work has been done to understand mechanism of fouling formation and to confirm the quantitative analysis of the fouling between the metal and coolant water. Electrochemical Quartz Crystal Microbalance is a new electrochemical technique developed in recent years able to be applied to study the filming kinetics of corrosion phenomena using piezoelectric effect. The high sensitivity of EQCM is the basis for its applications in thin film studies. In addition, interfacial process can be measured with the exact time scale depending on the frequency counter used. The purpose of this study is to understand the corrosion inhibition mechanisms of pitting corrosion for stainless steel 304 using inhibitor based on phosphoric acid and to contribute the application of corrosion test by EQCM techniques for corrosion research of steam condenser tube exposed of seawater coolant in the secondary system of NPP