# Salt-Fog Corrosion Behavior of GCr15 Steels Treated by Ultrasonic Strengthening Grinding Process
## 1 Introduction  
Gcr15 steels are widely used to manufacture mechanical components, such as bearings [1, 2], ball screws [3], and steel balls [4], mainly due to their high hardness and wear resistance [3]. These mechanical components have played a significant role in many industrial fields, such as automotive, aviation, mining, and marine equipment [3, 5]. Metals are easily corroded when working in a marine environment, which causes economic and material loss for marine equipment [6]. The corrosion of metal components can be attributed to multiple factors, including temperature [7, 8], the PH value of the seawater [9, 10], micro-organisms [11], etc. Consequently, a severe degradation occurs in the mechanical properties of the metals and finally leads to fatigue failure.  
To address this issue, many methods have been proposed for the long-term protection of steel components, such as surface coating [12, 13, 14, 15] and surface chemical heat treatment [16, 17, 18]. In particular, surface coating attracts widespread research interests, and great efforts for developing high-performance protective coating have been studied.  
For example, Al coating developed by the thermal-sprayed method can provide efficient protection for steels due to the existence of passivating film [19; 20]. Nevertheless, a serious pitting problem occurs once the passivating film has been damaged [21; 22]. Moreover, the Al coatings formed by thermal-spraying are easy to peel off under heavy loads [23]. Zn-Al [12; 24], Zr-Ti [13; 25], and NiCr-Cr3C2 [14; 15] coatings have also been studied broadly. An example of corrosion behavior of a Zn-Al composite coating in a salt spray corrosive environment, which exhibits the excellent anti-corrosion properties by absorbing the benefits of both Zn and Al, has been investigated by [PERSON] et al. [12]. However, the good corrosion resistance of Zn-Al coatings is only suitable for low-carbon steel substrates [12]. With the rapid development of the mechanical industry, new techniques to improve the anti-corrosion properties of steel are urgently needed.  
As an alternative, mechanical strengthening has emerged as a promising solution to obtain a good corrosion resistance for metals, mainly due to the chilled layer and compressive residual stress microstructures formed on metal surfaces after surface treatment [26; 27; 28], which results in the improvement of fatigue life. So far, mechanical strengthening techniques, such as shot peening (SP) [29; 30] and rolling finishing [31; 32] have been proposed extensively for corrosion resistance investigation of metals. However, traditional shoot peening typically produces a high roughness on the metal surface, and the protection ability is still far from the practical demands [33]. On the other hand, the hardened layer produced by rolling and the internal material have an obvious delamination phenomenon [34], which easily causes the surface layer to fall off. In contrast, ultrasonic shot peening has the characteristics of low cost, large shape correction range, small footprint, good controllability and repeatability. Surprisingly, the use of ultrasonic shot peening for improving the corrosion resistance of GCr15 steels is poorly investigated.  
In this work, the corrosion behavior of four GCr15 steel samples has been investigated in a salt spray corrosion environment. Samples are surface-treated with ultrasonic shot peening (USP), a dry ultrasonic strengthening grinding process (DUSGP), a wet ultrasonic strengthening grinding process (WUSGP), and polishing, respectively. The capacitance arc and polarization curves of these samples were studied. The corrosion morphology and products were further analyzed to reveal possible corrosion mechanisms.