Elsevier

Applied Surface Science

Volume 305, 30 June 2014, Pages 674-682
Applied Surface Science

Oxidation of Inconel 625 superalloy upon treatment with oxygen or hydrogen plasma at high temperature

https://doi.org/10.1016/j.apsusc.2014.03.160Get rights and content

Highlights

  • Inconel 625 alloy was exposed to plasma at high temperatures up to melting point.

  • Evolution of morphological and structural changes was determined by SEM, AES and XRD.

  • Depth profiling revealed different oxides on the surface depending on temperature.

  • Rich nano-structuring was observed but no nanowires appeared.

  • Hydrogen plasma treatment caused surface segregation of minor elements.

Abstract

Initial stages of Inconel 625 superalloy (Ni60Cr30Mo10Ni4Nb1) oxidation upon short treatment with gaseous plasma at different temperatures up to about 1600 K were studied. Samples were treated for different periods up to a minute by oxygen or hydrogen plasma created with a microwave discharge in the standing-wave mode at a pressure of 40 Pa and a power 500 W. Simultaneous heating of the samples was realized by focusing concentrated solar radiation from a 5 kW solar furnace directly onto the samples. The morphological changes upon treatment were monitored using scanning electron microscopy, compositional depth profiling was performed using Auger electron spectroscopy, while structural changes were determined by X-ray diffraction. The treatment in oxygen plasma caused formation of metal oxide clusters of three dimensional crystallites initially rich in nickel oxide with the increasing chromium oxide content as the temperature was increasing. At about 1100 K iron and niobium oxides prevailed on the surface causing a drop of the material emissivity at 5 μm. Simultaneously the NiCr2O4 compound started growing at the interface between the oxide film and bulk alloy and the compound persisted up to temperatures close to the Inconel melting point. Intensive migration of minority alloying elements such as Fe and Ti was observed at 1600 K forming mixed surface oxides of sub-micrometer dimensions. The treatment in hydrogen plasma with small admixture of water vapor did not cause much modification unless the temperature was close to the melting point. At such conditions aluminum segregated on the surface and formed well-defined Al2O3 crystals.

Introduction

Different grades of stainless steel are nowadays widely used as a constructing material due to good mechanical, thermal and chemical properties. Unlike low-grade steels the stainless steels exhibit also good corrosion resistance what is often attributed to formation of passive chromium oxide on the surface upon heating in oxygen-rich environment. Detailed studies of oxidation mechanisms, however, reveal important knowledge on oxidation of stainless steels [1], [2], [3], [4], [5], [6]. Many researchers report growth of thin iron oxides film as the initial oxidation step [2], [7], [8]. The iron oxide is dominant up to elevated temperatures, say roughly 600 K and further heating causes formation of the chromium oxide film which may or may not replace the original iron oxide on the very surface [2], [7], [8]. Oxidation at higher temperatures sometimes cause precipitation of various minority alloying elements found in bulk alloy at small concentration and formation of different surface crystallites [3]. In any case, the type of surface oxide depends on numerous parameters including the type of stainless steel and the properties of oxidizing medium. Oxidation is often reflected also in modification of surface morphology. The presence of oxide film causes enrichment of morphology. The specific morphological features depend largely also on the medium used to promote oxidation.

Stainless steel is also the material of choice for building high vacuum systems due to hermetical tightness, reasonably low degassing and good machining as well as welding properties. In some cases, however, users prefer other types of construction materials and one of them is Inconel, which belongs to a family of austenitic nickel–chromium-based superalloys. These alloys are typically used in high temperature applications and are characterized by excellent corrosion resistance. Inconel retains strength over a wide temperature range, attractive for high temperature applications where steel would succumb to creep as a result of thermally-induced crystal vacancies. Due to such properties, Inconel alloys have been used in construction of vacuum vessels of fusion devices, such as the COMPASS tokamak [9], [10], [11]. The reactor is equipped also with graphite tiles in order to prevent direct contact of hot plasma with metal surfaces. The tiles are eroded due to chemical sputtering with hydrogen ions and the resultant CHx radicals stick to surfaces forming a film of hydrogenated carbon. The film should be removed occasionally and a promising technique is removal by oxygen plasma. Since it is very difficult to form uniform plasma in huge vacuum chambers any application of this method is questionable due to possible local enhancement of plasma power what may cause local heating of Inconel and thus oxidation. The localized effects in large plasma reactors are often difficult to predict, let alone tailor, so behavior of materials upon treatment with oxygen plasma at elevated temperature is better studied ex situ.

Due to technological importance a variety of studies on oxidation of Inconel of different grades has been performed. Authors used oxygen-containing gases, often at atmospheric pressure and essentially in thermal equilibrium. Xiao et al. found that the NiO and Cr2O3 are the main oxides formed during oxidation at high temperature (at 970–1170 K) of Inconel 600 alloy and only NiO has been formed at low temperature (at 870 K)[12]. The NiO layer is located at external surface and the Cr2O3 layer is located at internal surface. Buscail et al. studied high temperature long-term oxidation of Inconel 601 in the range of temperatures between 1270 and 1420 K and found that TiO2 and Ti2Cr7O17 as well as chromia are mixed in the oxide film [13]. Zhu et al. studied oxidation of Inconel 393 alloy and found a variety of compounds in the surface and subsurface films [14]. The external scale was mainly Cr2O3, while the inter-granular thread-like internal oxides were typically Al2O3, sometimes TiOx, and/or NiAl2O4. Ferguson et al. [15], on the other side, reports formation of a protective NiO film on the surface of Inconel. Kumar et al. performed heating of Inconel 625 alloy at atmospheric pressure in pure oxygen and found large amount of niobium oxide upon prolonged heating at 1500 K [16]. At rather low temperature nickel oxide was dominant, followed by chromium oxide which became dominant in the temperature range from about 900 to 1300 K. The authors used EDX, XPS and AES methods and large differences were observed. The only depth profile presented is at 873 K, while profiles at higher temperature are not presented. Still, the conclusion drawn by Kumar is that upon heating to about 1500 K niobium oxide is dominant by far (70 at.% of metallic component), followed by chromium (17 at.%), titanium (9 at.%) and nickel (4 at.%) [16].

The literature survey thus indicates that different Inconel grades behave differently upon high temperature oxidation in oxygen-rich atmosphere at equilibrium conditions so the behavior should be elaborated for each particular material and oxidation medium. Furthermore, according to our knowledge, most studies on oxidation of Inconel were performed in oxygen or air atmosphere, but there is a lack of experiment performed in oxygen discharge (plasma). We have found only works performed by Rives et al. [17], [18]. He has used microwave Ar–O2 plasma for passivation of Inconel 690. Treatment was performed in the temperature range between 573 and 873 K. Oxidation time was quite long up to 480 min. He has found formation of chromium-rich oxide on the surface of Inconel. The oxide thickness was increasing with increasing treatment time as well as with increasing temperature.

In the present paper we report results on oxidation of Inconel 625 alloy used for construction of vacuum chambers of selected fusion reactors. Unlike other authors, we performed the oxidation in a weakly ionized, highly dissociated oxygen plasma in a temperature range between 600 and 1600 K. Treatment times were relatively short since such a procedure is relevant for treatment of inner walls of vacuum chambers contaminated with carbon-containing impurities.

Section snippets

Experimental

Samples were cut from a piece of tubing made from INCONEL® nickel–chromium alloy 625. The tube was provided from COMPASS fusion reactor. The composition is presented in Table 1. The service temperatures are from cryogenic to 1255 K. The melting point is about 1600 K. Samples were polished with sand paper but no attempt was made to make it mirror-smooth. According to our past experience, the fine-sandpaper grade of polishing is sufficient in the regard that it does not result in the distortion of

Results and discussion

Samples were exposed to oxygen plasma and solar radiation for different times in order to achieve different temperatures. Once a selected temperature was reached, both solar radiation and plasma generator were turned off instantaneously. A pyrometer signal obtained at the longest treatment time (70 s) is plotted versus time in Fig. 1. In order to obtain a temperature reading with a pyrometer, it is necessary to know the surface emissivity at the corresponding wavelength (5 μm in our case). The

Conclusion

Three independent techniques for surface and thin film characterization (SEM, AES, XRD) allowed for elaboration of the initial stages in oxidation of Inconel 625 superalloy at high temperatures. Unlike other authors who performed studies in furnaces filled with oxygen or air at thermal equilibrium conditions we used low-pressure highly reactive oxygen or hydrogen plasma as oxidizing medium in order to simulate conditions typical of discharges used in removal of carbon-containing impurities from

Acknowledgements

The authors acknowledge the financial support from Slovenian Research Agency (grant n. P2-0082) and financial support by the Access to Research Infrastructures Activity in the 7th Framework Program of the EU (SFERA Grant Agreement n. 228296). The co-authors from the IPP Prague acknowledge support of the MSMT project#LM201 11021. The authors thank to B. Pracek for help at the AES measurements and D. Pérarnau for XRD characterization.

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