**1. Introduction**

Nanocrystalline (NC) metallic materials have a high-strength without sacrificing toughness and ductility [1–5]. In general, a NC metallic layer with a certain thickness has often been utilized to improve the environmental service behavior [6,7]. The rotationally accelerated shot peening (RASP) technique can produce nanograins due to the generation of defects and interfaces (grain boundaries), the increasing of polycrystalline free energy, and the inducing of grain refinement, through the application of high strains at high strain rates [8–11]. The material exhibits unique mechanical and corrosion properties on account of strengthening the metal surface with the high density nanocrystals and interfaces.

Recently, some researchers have studied the corrosion mechanism of the surface nanocrystallization material. For instance, Balusamy et al. [12] studied whether, or not, the surface changes had a direct impact on the electrochemical activity of the metallic materials. The grain boundaries and the matrix structure formed numerous tiny electrochemical cells. Ye et al. [13] proposed that nanocrystalline 309 stainless steel (SS) by a DC magnetron sputtering exhibited different corrosion resistance changes in different solutions. Huang et al. [14] reported that the surface mechanical attrition treatment induced grain refinement and dislocations, had positive effects on the corrosion behavior of the Ti-25Nb-3Mo-3Zr-2Sn alloy, annealing experimental results further indicated that the improved corrosion resistance was mainly due to the grain refinement. Li et al. [15] reported that the effect of the grain size on the corrosion resistance of a NC low-carbon steel fabricated through an ultrasonic shot peening technique in a 0.05 M H2SO4 + 0.05 M Na2SO4 solution. When grain size was less than 35 nm, the corrosion rate increased as grain size decreased. They understood this was attributed to the increased number of the active sites caused by surface nanocrystallized low-carbon steel. Zhang et al. [16] found that in a saturated Ca(OH)2 solution with and without Cl−, the micro-cracks in the surface layer of the supersonic fine-particles (about 3 μm) bombarding the low-carbon steel did not degrade its passivity properties and pitting resistance. The interesting work of Chen et al. [17] proposed that stainless steel having underwent the RASP process showed a good corrosion resistance. However, the corrosion mechanisms of the nano-carbon steel and nano-stainless steel were very different. Normally, we think that surface nanocrystalline may worsen corrosion. In contrast, we found corrosion resistance performance of carbon steel increased after the RASP. The influence of RASP on the corrosion behavior of the carbon steel will be discussed below.

The RASP is a newly developed surface nanocrystallization technology for fabricating gradient structure with the grain size varying from nanometer to micrometer without changing the overall chemical compositions of a carbon steel. SA106B low-carbon steel after the RASP has excellent mechanical and corrosion properties, making them ideal for the secondary loop in a nuclear power plant to reduce the flow-accelerated corrosion. In this work, the surface nanocrystallization SA106B carbon steel was fabricated by RASP. The influence of the RASP on the corrosion behavior of the carbon steel has been investigated.
