## 4 Discussion
### The Diffusion Behavior of Nitrogen Atoms During the Nitriding Process  
Ion nitriding involves the process of continuous adsorption and diffusion of nitrogen ions. Nitrogen ions, generated through ionization, are accelerated by an electric field and continuously bombard the surface of the cathode specimen. The metal atoms that are sputtered by these nitrogen ions combine with the nitrogen ions to form nitrides [21], which are subsequently adsorbed on the specimen's surface through scattering. As the surface-substable nitrides gradually decompose, active nitrogen atoms are released and diffuse into the specimen. Research has consistently been devoted to the nitriding of stainless steel in order to thicken the nitrided layer while upholding the corrosion resistance of austenitic stainless steel. According to the Arrhenius equation for the diffusion coefficient, there are two primary methods to improve the diffusion coefficient: lowering the activation energy of diffusion or increasing the nitriding temperature. It is well established that these two approaches can effectively increase the diffusion coefficient, as described by the Arrhenius diffusion coefficient equation. The thickness of the penetration layer in the 316L austenitic stainless steel was measured under various deformation conditions, with the average thicknesses calculated as 28.57 \(\upmu\)m, 32.14 \(\upmu\)m, 35.17 \(\upmu\)m, 37.5 \(\upmu\)m, 44.64 \(\upmu\)m, and 66.67 \(\upmu\)m for 0, 10%, 20%, 30%, 40%, and 50% deformations, respectively. The results indicated that, compared to the nitrided specimens alone, the thicknesses of the penetration layers in the nitrided specimens significantly increased after deformation. Specifically, the depth of the nitrided layer without deformation was the thinnest, at 28.57 \(\upmu\)m, followed by the nitrided sample with 10% deformation at 32.14 \(\upmu\)m. The thicknesses of the nitrided layers with 20% and 30% deformation were similar, while the nitrided layer with 50% deformation exhibited the greatest thickness at 66.67 \(\upmu\)m. As the deformation variable increased, the diffusion coefficient of the nitrogen atoms also increased, leading to a higher diffusion rate and a thicker penetration layer. When the morphology variable ranged from 0 to 50%, the thicknesses of the penetration layers increased from 28.57 \(\upmu\)m to 66.67 \(\upmu\)m. However, in the original austenitic matrix, the grain boundary corrosion of the six groups of specimens with different morphology variables resulted in the penetration layer being corroded to a black color. This observation indicates that iontophoresis negatively affects the corrosion resistance of the penetration layer in 316L austenitic stainless steel [22]. To further investigate the influence of various shape variables on the diffusion behavior of nitrogen atoms throughout the ion nitriding process of 316L stainless steel, the cross-sectional morphology of the infiltration layer was characterized using a SEM and EDS line scanning. Figure 9 presents the EDS analysis results for nitrogen (N) in the cross-section of nitrided 316L stainless steel specimens with different morphological variables, along with the corresponding SEM images of the line scanning area and cross-section. The line scan began in the matrix region and concluded in the infiltration layer region. As presented in Figure 9, the N element concentration is relatively stable at the beginning, and there is a peak value of N element on the exterior of the nitrided layer of the specimens with different morphology variables, and the concentration of the N element in the swept area of the specimen cross-section gradually rises along the direction from inside to outside, forming a N concentration gradient [23].