Effect of surface mechanical treatment on the oxidation behavior of FeAl-model alloy
- 74 Downloads
Fe-based alloys are commonly used in almost every sector of human life. For different reasons, the surfaces of the real parts are prepared using different methods, e.g., mirror-like polishing, grit-blasting, etc. The purpose of the present work is to answer the question how the surface preparation influences the oxidation behavior of Fe-based alloys. To answer this question, a high-purity model alloy, Fe–5 wt%Al, was isothermally oxidized in a thermogravimetrical furnace. The post-exposure analysis included SEM/EDS (WDS) and XRD. The surface roughness was determined by a contact and laser profilometer. The obtained results demonstrate that the mechanical surface preparation influences oxidation kinetics as well as the microstructure of the oxide scale formed on the alloy at both studied temperatures. Namely, polishing and grinding caused local formation of Fe-rich nodules and sub-layer of protective Al2O3. In contrast, grit-blasting leads to the formation of a thick outer Fe-oxide and internal aluminum nitridation. A significant increase in the oxidation rate of the material after grit-blasting was attributed to grain refinement in the near-surface region, resulting in an increase in easy diffusion paths, namely grain boundaries.
Fe-based alloys are most commonly used as construction materials at both low and high temperatures. The biggest problem in Fe-based alloy usage is their relatively weak corrosion resistance against high-temperature corrosion. However, Fe-based alloys, such as an example steels, are the most attractive materials to be used as construction materials at elevated temperatures due to their relatively low cost. It is well known that low-alloyed steels possess very low high-temperature corrosion resistance [1, 2]. On the other hand, high-temperature Fe-based alloys are used in almost all sectors of human life, e.g., heating elements in toasters, cooking plates, the constructive materials in turbocharger exhaust systems, etc. The shapes of each part used in different systems demand different methods of forming the elements, e.g., cold  or hot rolling , grit- or sand-blasting [5, 6, 7], grinding  or even mirror-like polishing . As reported, different surface preparations can significantly change the near-surface microstructure of the material or even result in different level of internal stresses [3, 4, 5, 6, 7, 8, 9].
For materials used at elevated temperatures, their oxidation resistance and equally their lifetime at high temperatures become to be a crucial factor during materials engineering. It was found that different surface preparations result in a different oxidation behavior of a wide range of alloys, e.g., Ni-base superalloys [10, 11, 12, 13, 14, 15], iron , or Fe-base alloys [17, 18, 19, 20, 21]. Surface preparation significantly influences the oxidation behavior of these materials. This influence can be positive or negative, depending on the material, exposure condition, chemical composition, etc.
However, most of these researches were performed using commercially available Ni- or Fe-based alloys. Moreover, most of the works about the effect of surface preparation are performed at low temperature (wet corrosion). In most cases, in commercially available alloys, due to several technologically justified reasons, the chemical composition is rather complicated, i.e., they consist of a number of alloying elements. Therefore, it is impossible to unambiguously find out the factor responsible for different oxidation resistances of materials with different surface preparations.
Considering the above-mentioned facts, in the present work, a high-purity Fe–5 at%Al model alloy was investigated to determine the influence of different surface mechanical treatments on oxidation behavior of the investigated alloy. The chemistry of the investigated alloy was chosen due to its ability of forming either a protective Al2O3 oxide scale or non-protective Fe-oxide scale.
Materials and methods
Roughness parameters describing samples surface calculated based on measurement shown in Fig. 2
The calculated real areas are equal to 4.58 cm2, 5.05 cm2 and 6.45 cm2 for polished, ground and grit-blasted surfaces, respectively.
Air exposure at 800 °C
Air exposure at 900 °C
Exposure at 900 °C in an inert atmosphere
To elucidate the reason for different oxidation behaviors depending on the surface mechanical preparation, an additional test has been performed. A set of samples with surface prepared by three different methods, namely polished, ground, and grit-blasted, were heat treated at 900 °C for 24 h. To limit the influence of reaction during oxidation, a heat treatment has been performed in an inert atmosphere of high-purity argon. After heat treatment, the surface was slightly polished to reach the near-surface region (roughly 20 µm of material from the surface was removed). After polishing, the samples’ surfaces were chemically etched to reveal the grains microstructure. The images of the microstructures after heat treatment show that polishing and grinding did not have an influence on the grain sizes and only primary grains are observed, while on the grit-blasted sample fine secondary grains are visible.
The oxidation kinetics measured at both temperatures (800 and 900 °C) revealed clear difference between the samples with differently treated surfaces. At both temperatures, the highest mass gains were obtained for the grit-blasted alloys. The mass change curve of the ground alloy exposed at 800 °C revealed relatively rapid mass gain during the first 30 min of exposure and afterward a very slow increase in mass change. This observation can be explained by the formation of rapidly growing Fe-oxide islands at the beginning of exposure, and a relatively fast Al2O3 formation which slows down the oxidation rate. The polished material showed a slightly different mass gain curve, namely the mass change curve shows a slightly higher oxidation rate up to 2 h of exposure, and after 2 h the mass change curve becomes parallel to that obtained for the ground surface. The latter means that also polished materials have developed a protective alumina sub-layer; however, the polished material needed a longer time. These observations are confirmed by the SEM images of the cross sections of the samples after exposure. The mass change obtained for grit-blasted alloy was substantially higher as compared to the ground and polished. This means that the alloy mainly formed an Fe-rich oxide during the whole exposure time. The latter was confirmed by the SEM analysis. Moreover, formation of AlN in the form of internal precipitates was observed on the grit-blasted material. Therefore, part of the aluminum was tied up in the internal nitridation zone and its further diffusion toward the oxide scale/alloy interface was hampered. Due to this fact, formation of the protective alumina sub-layer was slowed down and formation of the fast-growing Fe-oxide was enhanced.
A similar trend was observed for the samples exposed at 900 °C. The lowest mass change was observed for the ground alloy, the polished sample exhibited a higher mass change, while the highest mass gain was observed for the grit-blasted alloy. However, one should mention that the mass changes observed for the grit-blasted and ground samples are two times lower compared to exposure at 800 °C, while for the polished material, it is at the same level. It was proposed by Nowak et al.  that a rougher surface preparation results in the introduction of a higher number of defects into the near-surface region. These defects are believed to be an easy diffusion path for elements forming the protective oxide scales (aluminum in the present case), which results in a faster formation of the alumina sub-layer, which is indicated by a lower mass change obtained for the ground as compared to the polished surfaces at both studied temperatures. In parallel, similarly to the observation at 800 °C, no nitridation on polished and ground samples was observed. In contrast, a thick zone of AlN below the outer Fe-rich oxide was observed on the grit-blasted sample. However, the thickness of the outer Fe-rich zone is smaller as compared to that formed on the grit-blasted material after exposure at 800 °C. This observation can be explained by two facts. First, as reported by Mehrer , the diffusion coefficient for Al in Fe–10 at% Al alloy increases by an order of magnitude from 2.86 × 10−15 m2 s−1 at 800 °C to 2.74 × 10−14 m2 s−1 at 900 °C. This results in a thicker AlN zone and the formation of a thin sub-layer of Al-rich oxide at the outer Fe-rich oxide and alloy interface. It is the latter, most probably, which limited the rapid growth of the Fe-rich layer. Formation of Fe-rich, spiky-shaped nodules on Fe–10%Al during oxidation at 900 °C in 1 atm. oxygen was previously reported by Saegussa et al. ; however, due to the nitrogen-free atmosphere, formation of AlN below the nodules was obviously not observed. Moreover, no correlation of nodules formation with surface roughness was stated. Usually, if there are no additional factors, Fe–5 wt%Al alloys normally form an external Al2O3 scale accompanied by local Fe-rich nodules, below which an Al2O3 sub-layer is present (as shown in Fig. 12). It has been reported that nitrogen permeability in γ-Fe at 1000 °C is 1.6 × 10−11 cm2s−1, while permeability for oxygen in γ-Fe at the same temperature is 2.4 × 10−12 cm2s−1 [29, 30].
In the present work, the effect of surface mechanical treatment on oxidation behavior of the model alloy Fe–5 wt% Al was studied. The obtained results showed that mechanical surface preparation increases the real specific surface and influences both the oxidation kinetics and the oxide scale microstructures formed on the studied alloy, at both studied temperatures. Namely, polishing and grinding caused local formation of Fe-rich nodules and a sub-layer of protective Al2O3. In contrast, grit-blasting leads to the formation of thick outer Fe-oxide and internal aluminum nitridation. A significant increase in the oxidation rate of the material after grit-blasting was attributed to grain refinement in the near-surface region resulting in an increase in easy diffusion paths, namely the grain boundaries.
The authors would like to acknowledge Kamil Gancarczyk for performing the XRD analysis.
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 Skłodowska-Curie Grant Agreement No. 665778.
- 2.Rujisomnapa J, Seechompoo P, Suwannachoat P, Suebca S, Wongpanya P (2010) High temperature oxidation behaviour of low carbon steel and austenitic stainless steel. J Metals Mater Min 20(3):31–36Google Scholar
- 6.Multinger M, Ferreira-Barragans S, Frutos E, Jaafar M, Ibanez J, Marin P, Perez-Prado MT, Gonzalez-Doncel G, Asenjo A, Gonzalez-Carrasco JL (2010) Superficial severe plastic deformation of 316 LVM stainless steel through grit-blasting: effects on its microstructure and subsurface mechanical properties. Surf Coat Technol 205(7):1830–1837CrossRefGoogle Scholar
- 10.Giggins CS, Pettit FS (1969) The effect of alloy grain-size and surface deformation on the selective oxidation of chromium in Ni–Cr alloys at temperatures of 900 C and 1100 °C. Trans Metall Soc AIME 245:2509–2514Google Scholar
- 13.Nowak WJ, Wierzba B, Sieniawski J (2018) Surface preparation effect on oxidation kinetics of Ni-base superalloy. J Phys: Conf Ser 936:012002Google Scholar
- 22.ISO 25178 part 2 (2012) Geometrical product specification (GPS)—surface texture: areal—part 2: Terms, definitions and surface texture parameters. International Organization for StandardizationGoogle Scholar
- 23.Reizer R, Pawlus P (2011) 3D surface topography of cylinder liner forecasting during plateau honing process. In: Proceedings 13th international conference metrology and properties of engineering surfaces, Twickenham, April, pp 29–34Google Scholar
- 27.Mehrer H (2007) Diffusion in solids, fundamentals, methods, materials, diffusion controlled processes. Springer, BerlinGoogle Scholar
- 30.Wriedt HA, Gonzalez OD (1961) The solubility of nitrogen in solid iron-nickel alloys near 1000-degrees-C. Trans AIME 221:532Google Scholar
- 31.Young D (2016) High temperature oxidation and corrosion of metals, 2nd edn. Elsevier, OxfordGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.