# Effect of Shot Peening on the Evolution of Scale on T91 Steel Exposed to Steam
## 4 Discussion
### Effect of Shot Peening  
Shot peening refines strain-induced grains, leading to a fine grain with a large fraction of grain boundaries on the peened surface. The fraction of low-angle grain boundaries in the surface area increased from 35.6% in the untreated specimen to 49.9% in the shot peened specimen calculated from Figure 2b,d. The grain size and grain boundaries determine the diffusion rates of elements during the formation of protective oxide layer. The effective  
Figure 6: Oxidation scale thickness as a function of exposure time: (**a**) total layer, (**b**) outer Fe-oxide (Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\)) layer, and (**c**) inner (Fe-Cr-O spinel) layer.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Exposure Time**} & \multicolumn{2}{c}{**Total Scale**} & \multicolumn{2}{c}{**Fe-Cr-O Layer**} & \multicolumn{2}{c}{**Fe-Oxide Layer**} \\  & **Thickness (\(\mu\)m)** & \multicolumn{2}{c}{**Thickness (\(\mu\)m)**} & \multicolumn{2}{c}{**Thickness (\(\mu\)m)**} \\ \cline{2-7}  & **Untreated** & **SP** & **Untreated** & **SP** & **Untreated** & **SP** \\ \hline
100 & 63.0 \(\pm\) 2.3 & 49.0 \(\pm\) 2.5 & 28.0 \(\pm\) 1.0 & 18.4 \(\pm\) 1.8 & 35.8 \(\pm\) 1.3 & 30.6 \(\pm\) 1.2 \\
200 & 72.8 \(\pm\) 1.6 & 64.4 \(\pm\) 0.9 & 30.5 \(\pm\) 0.8 & 28.6 \(\pm\) 1.2 & 42.3 \(\pm\) 1.3 & 35.8 \(\pm\) 1.3 \\
400 & 79.0 \(\pm\) 3.0 & 67.4 \(\pm\) 4.0 & 32.3 \(\pm\) 0.9 & 31.1 \(\pm\) 0.8 & 46.7 \(\pm\) 2.1 & 36.3 \(\pm\) 1.0 \\
600 & 102.1 \(\pm\) 2.9 & 67.9 \(\pm\) 0.9 & 35.0 \(\pm\) 1.2 & 30.6 \(\pm\) 1.0 & 67.1 \(\pm\) 2.3 & 37.3 \(\pm\) 1.6 \\
800 &
\begin{tabular}{c} 57.4 \(\pm\) 1.1 \\ (left over) \\ \end{tabular} & 68.9 \(\pm\) 2.1 & 31.6 \(\pm\) 0.8 & 30.3 \(\pm\) 1.2 & 25.8 \(\pm\) 5.4 & 38.6 \(\pm\) 1.9 \\
1000 &
\begin{tabular}{c} 67.2 \(\pm\) 5.8 \\ (left over) \\ \end{tabular} & 70.1 \(\pm\) 2.4 & 33.8 \(\pm\) 1.3 & 31.1 \(\pm\) 0.9 & 33.4 \(\pm\) 5.6 & 39.0 \(\pm\) 1.8 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Oxidation scale layer thickness of untreated and shot peened T91 specimens after steam exposures of various durations (Fe-oxide refers to Fe\({}_{2}\)O\({}_{3}\) and Fe\({}_{3}\)O\({}_{4}\) combined).  
diffusivity (\(\mathrm{D_{eff}}\)) of preferential nucleating elements is the sum of the lattice diffusivity (\(\mathrm{D_{l}}\)) and the diffusivity of the grain boundary (\(\mathrm{D_{GB}}\)), as proposed by [PERSON] [18],  
\[\mathrm{D_{eff}}=(1-\mathrm{f})\times\mathrm{D_{l}}+\mathrm{f}\times\mathrm{D_{ GB}}, \tag{2}\]  
where \(\mathrm{f}\) is the area proportion of the grain boundary. Assuming the grains are cubic, \(\mathrm{f}=2\delta/\mathrm{d}\) (\(\delta\) is the GB width and \(\mathrm{d}\) the grain size). Considering that \(\mathrm{D_{GB}}\gg\mathrm{D_{l}}\), Equation (2) can be simplified as  
\[\mathrm{D_{eff}}=\mathrm{D_{l}}+2\delta/\mathrm{d}\times\mathrm{D_{GB}}, \tag{3}\]  
Thus, the effective diffusivity of Cr noticeably increases with decreasing grain size induced by shot peening through grain boundary diffusion.  
Figure 7 shows cross-sectional SEM micrographs of untreated and shot peened specimens after 100 and 600 h steam exposure. Figure 8 shows cross-sectional EDS mapping micrographs of shot peened specimens after 600 h steam exposure. There is no Cr-depleted zone apparent at the oxide-metal interface of the untreated specimen during the early stage of oxidation (100 h), which is totally different in the shot peened specimen. There, a Cr-depleted zone forms due to outward diffusion of Cr to form a Cr-rich protective layer by decomposition of precipitate \(\mathrm{M_{23}C_{6}}\). This means, shot peening induced plenty of grain boundaries to provide more pathways for the outward diffusion of Cr.  
Figure 7: Cross-sectional SEM images of oxidation scale: (**a**) untreated 100 h, (**b**) untreated 600 h, (**c**) SP 100 h, and (**d**) SP 600 h.