Effect of the Amplitude in Ultrasonic Nano-crystalline Surface Modification on the Corrosion Properties of Alloy 600

The application of surface modification technology, including water jet and laser peening, was carried out in the early 2000s in many industries [1,2].The nuclear industry has tried to apply this technology to the parts of the primary and secondary sides because the technology can harden the surface and improve the fatigue strength, surface roughness and wear resistance, and form very high compressive stresses on the surface [3-7]. When SCC (Stress Corrosion Cracking) susceptible materials were used in corrosive environments under high tensile stress fields, generally, SCC could be easily induced [8]. Therefore, nuclear power plants are trying to apply various methods, including the substitution to high corrosion resistant materials, reduction of mechanical stress, weld overlay, weld inlay, weld onlay and water chemistry control [1]. However, the above methods have a difficulty because of high cost. As a result, the peening process is emerging in order to reduce the difficulty of the substitution and inspection costs. Since 2016, the nuclear industry in the USA has applied the peening process to nuclear vessels and steam generator, and tried its application to other parts and further research [1]. Applicable peening process includes shot [9-11], laser [12-16], water jet [17,18] and ultrasonic shot peening [19,20], and UNSM etc. Among the peening processes, it is well known that UNSM is very powerful [21]. In the UNSM process, the material is impacted with a hard rigid pin moving at a ultrasonic frequency, typically 20 kHz. A tungsten carbide (WC) tip is attached to an ultrasonic horn, which strikes the specimen surface up to 20,000 or more times per second with 1,000 to 10,000 shots per square millimeter in a very short time. The impact deforms the surface of the target material and converts its microstructure into nanocrystals [22]. The variables in the UNSM process are static load, amplitude, pitch and tip diameter, etc. Researches about mechanical properties and fatigue were performed by using the UNSM technology [23-26], but there is little to the study about the corrosion resistance. Recently, our research team reported the effect of UNSM on the corrosion resistance of stainless steel and Alloy 600. In the case of the stainless steel [27-29], the UNSM treatment improves the passivation film and pitting corrosion resistance, and reduces the chromium carbide Effect of the Amplitude in Ultrasonic Nano-crystalline Surface Modification on the Corrosion Properties of Alloy 600