Surface characterization and corrosion behavior of Monel 400 after high energy micro-shot peening process

In this study,the surface structure,corrosion behavior of Monel 400 alloy are  comparatively investigated after high energymicro-shot peening process with its solution-annealed counterpart. To the aim of this investigation, microhardness, roughness measurements ,grazing-incidence X-ray diffraction  (GI-XRD),field-emission scanning electron  microscopy (FE-SEM), and transmission electron microscopy (TEM) were employed  to characterize the surface layers of high energy micro-shot peened andsolution-annealed samples. Based on the results,the process decreases the surface grain size from 30±4 µm to 76±5 nm,andincreases the density of dislocation structures on the surface. Moreover,electrochemical corrosion tests were carried out in HClsolution to compare the corrosion behavior of solution-annealed,micro-shot peened surfaces. Based on the results of thecyclic potentiodynamic polarization test,the micro-shot peened sample shows a higher initial corrosion current density at theexpense of a lower passivity current density,as compared with the solution-annealed one. The repassivation potential for themicro-shot peened sample is about +150 mV higher than the solution-annealed sample,which is a proof of the higher resistanceof this sample against localized corrosion. Based on the fitting of the data obtained from the electrochemical impedancespectroscopy,the polarization resistance of the passive film in the micro-shot peened sample is more than twice of this one forthe solution-annealed sample.