# Effect of Surface Roughness on Early Stage Oxidation Behavior of Ni-Base Superalloy IN 625
## 4 Discussion  
Both methods of roughness description, i.e., standard contact profilometry and fractal analysis, showed similar trends, specifically, roughness parameters where R\({}_{a}\), R\({}_{z}\), R\({}_{\max}\), D, LSFC, and L\({}_{R}\) at 1 \(\upmu\)m, obtained for ground surfaces, are substantially higher than for polished. In contrast, SRC values obtained for the ground surface are smaller than for the polished surface. All parameters clearly showed that surface preparation by grinding increased its roughness [15]. This in turn means, that the ground material possesses a higher surface to volume ratio than the polished. This observation is confirmed by the plot of relative length shown in Figure 2. Therefore, higher oxidation rate could be expected for ground material. Meanwhile, the results obtained in the present work showed exactly the opposite situation, where the material with the smaller surface to volume ratio showed a higher oxidation rate (Figure 5). The post-exposure analyses revealed the formation of Mn, Ni, and Fe-mixed oxide on top of the chromia scale (see Figure 3a or Figure 4a). This mixed oxide originated most probably during the so-called \"transient stage of oxidation\", i.e., the very early stage of oxidation [22]. Contrary to the observation of polished material, on ground alloy formation of previously the mentioned Mn, Ni, and Fe-mixed oxide is not observed. This means, that the transient stage was much shorter in case of ground alloy. However, on top of the chromia scale developed on ground IN 625, slight enrichment of Ti is visible (Figure 3b). Ti is known to increase the chromia growth rate [23; 24], however, in contrast to the material studied in references [23; 24], in case of presently studied alloy, Ti content is only 0.4 wt %, and therefore, its influence on chromia growth rate is smaller. As described above, surface preparation changes the oxide scale composition and morphology, which in turn changes the oxidation kinetics at least during the early stage of high temperature oxidation of IN 625. The exact mechanism responsible for such change is not known so far. However, it was proposed by [PERSON] et al. [8] that during a different kind of surface preparation different amount/kind of defects are introduced into the near-surface region, namely, by the treatment resulting in higher surface roughness, a higher amount of defects are introduced. These defects are claimed to be easy diffusion paths of the elements forming protective oxide scales, as in the present case chromium. In Figure 4b, the presence of several voids was observed. In contrast, no such voids are observed in the case of the polished sample. The most probable formation of such voids is caused by the accumulation of introduced defects, as observed, for example, by [PERSON] et al. [25]. However, to confirm the proposed mechanism, further investigations, including transmission electron microscopy (TEM) analysis and performing an oxidation tests using markers, are necessary and planned.  
This research was financed within the Marie Curie COFUND scheme and POLONEZ program from the National Science Centre, Poland. POLONEZ grant No. 2015/19/P/ST8/03995. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 665778.  
The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.