# Effect of Surface Roughness on Early Stage Oxidation Behavior of Ni-Base Superalloy IN 625
## 1 Introduction  
The materials used in the hottest parts of stationary gas turbines or jet engines face very aggressive atmospheres at high temperature. Therefore, these materials should fulfill a number of requirements, such as, for example, high creep strength and high oxidation resistance at a wide range of operating temperatures, environments, and loading conditions. Ni-base superalloys exhibit excellent mechanical properties at high temperature, due to their microstructure. They possess also good oxidation resistance due to the addition of elements forming a protective oxide layer, like Al or Cr [1]. IN 625 is one of the Ni-base superalloys which is widely used for application at high temperatures. According to the oxidation map developed by [PERSON] and [PERSON] [2], IN 625 can be classified as a chromia forming alloy, as it forms a slow growing Cr\({}_{2}\)O\({}_{3}\) scale. For better protection against oxidation, Ni-base superalloys can be covered by additional oxidation resistant layers such as McArIY type coatings [3] or \(\beta\)-NiAl [4], which at high temperatures form the most protective alumina oxide scale. However, manufacturing of those coatings generate additional costs.  
Literature analysis shows that the oxidation behavior of high temperature material can be controlled by the surface conditions. [PERSON] and [PERSON] investigated the influence of surface deformation and/or grain size of Ni-Cr alloy [5]. They found that the grit blasted material showed similar oxidation behavior to that with fine grained material. The authors claimed that the grit blasted material has recrystallized and the grain boundaries enhanced Cr diffusion along them. [PERSON] et al. investigated the influence of grain boundary density introduced by the cold working of Alloy 800 [6]. The authors found that the higher grain boundary density leads to smaller mass gain, while with decreasing grain boundaries density, the obtained mass gain increased. [PERSON] et al. investigated LDS-1101_HF and CMSX-4 with different surface conditions [7]. They observed strong a correlation between the surface preparation and type of formed oxide scale, namely, on ground surface, thinAl2O3 scale was observed, while on polished sample, a much thicker NiO outer scale with a subscale of Cr2O3-spinel-Al2O3 was found. [PERSON] et al. investigated the role of surface finishing on oxidation kinetics of IN 713 C [8]. It was found that chromium rich oxide formed on polished surfaces, while alumina oxide formed on ground surfaces.  
The literature study revealed that there are limited works describing the influence of surface condition on the oxidation behavior of IN 625. [PERSON] et al. [9] studied oxidation behavior of IN 625 during the long term oxidation in steam at temperature range 700-800 \({}^{\circ}\)C. The samples of IN 625 were prepared by grinding up to 1200 grit SiC paper. [PERSON] et al. [10] compared the oxidation mechanism and kinetics of IN 617 and IN 625 during exposure at 1200 \({}^{\circ}\)C, with the sample prepared by grinding up to 2500 grit SiC paper. [PERSON] et al. [11] studied the oxidation behavior of IN 625 polished up to mirror finishing at temperature range 500-1250 \({}^{\circ}\)C. [PERSON] et al. [12] studied oxidation behavior of IN 625 upon treatment with oxygen and hydrogen plasma at temperatures up to 1330 \({}^{\circ}\)C. The surface of the alloy was prepared by grinding using fine-sandpaper grade. However, no single work dealing with the influence of the surface preparation on the oxidation behavior of IN 625 has been found.  
Therefore, the aim of the present work is to investigate the influence of surface preparation, namely polishing up to 1 \(\upmu\)m and grinding up to 220 grit SiC paper, on oxidation kinetics and oxide scale morphology formed on IN 625 during early stages (up to 24 h) of exposure at 950 \({}^{\circ}\)C in air. The oxidation behavior is correlated with the surface roughness. A mechanism of oxide formation on polished and ground surfaces is proposed as well.