Abstract
To suppress stress corrosion-cracking, compressive residual stresses, such as shot peening, laser peening, water jet peening, ultrasonic peening, and ultrasonic nanocrystal surface modification (UNSM) are utilized. However, among the numerous techniques, there is little research about the corrosion effect of detailed conditions, such as static load or amplitude in UNSM. A study on UNSM among various techniques of adding compressive residual stress to Alloy 600 was conducted. The focus of this study was on the effect of the static load in UNSM on the corrosion properties of Alloy 600. Microstructure analysis was conducted using an optical microscope (OM), a scanning electron microscope (SEM), and electron backscattering diffraction (EBSD), while compressive residual stress was measured using a nano-indentation technique. A cyclic polarization test and the AC (Alternating Current)-impedance measurement were both used to analyze the corrosion properties. An increase in static load under critical static load enhanced the grain boundary diffusion, consequently strengthened the passive film, and facilitated the surface diffusion, thereby improving the passivation of Alloy 600. However, higher static loads over the critical value can lead to an increase in the friction between the striking tip and the surface, thereby creating an overlapped wave, which reduces the corrosion properties.
Highlights
Ni alloy is among the fcc crystal structures; it has good strength, ductility, resistance to corrosion, and oxidation resistance at high temperatures [1]
This work was centered on the effect of the static load in ultrasonic nanocrystal surface modification (UNSM) treatment on properties
This work was centered on the effect of the static load in UNSM treatment on the corrosion properties
Summary
Ni alloy is among the fcc (face centered cubic) crystal structures; it has good strength, ductility, resistance to corrosion, and oxidation resistance at high temperatures [1]. There are many techniques to mitigate the stress corrosion cracking of Alloy 600, which include substitution with high corrosion resistant materials, modification of water chemistry, and reduction of residual stress. Among these remedies, the reduction of residual stress, including tensile stress and compressive stress, can be achieved through surface treatment. Regarding the UNSM process, the material is impacted with a hard, rigid pin moving at an ultrasonic frequency, typically 20 kHz. A tungsten carbide (WC) tip is attached to an ultrasonic horn, which strikes the specimen surface at about 20,000 times per second with 1000 to 10,000 shots made per square millimeter within a very short time. The effects of the static load on the corrosion properties were discussed on the basis of the microstructure, hardness, and residual stress
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