Loading...
Please wait, while we are loading the content...
Similar Documents
High-Temperature oxidation behaviour of model CoRe-Cr alloys at low oxygen partial pressure
| Content Provider | Semantic Scholar |
|---|---|
| Author | Gorr, Bronislava Burk, Steffen Trindade, Vicente Braz Da Christ, H.-J. |
| Copyright Year | 2013 |
| Abstract | The oxidation behaviour of four model Co-Re-Cr alloys and a commercial Co-based alloy were investigated at 1,000°C and low-oxygen partial pressure of p(O2) = 10 bar, in order to proof the feasibility of a pre-oxidation treatment. Under suitable conditions of the pre-oxidation treatment, the oxidation of the highly reactive alloying element Cr is possible. All alloys studied form a continuous and dense Cr2O3 scale on the metal surface. The transport of chromium to the surface occurs mainly from the Cr-rich σ-phase, which becomes completing dissolved in the surrounding matrix after long exposure times. As a result of the Cr2O3 scale growth depletion of Cr occurs in the near surface region, leading to internal oxidation. Introduction Co-Re-Cr alloys present a new class of high-temperature alloys which are promising for applications at service temperatures beyond 1,200°C. The first investigations of this class of alloys showed rather poor oxidation resistance during exposure to laboratory air [1,2]. The aim of the study at this stage of the Co-Re alloy development is to derive options for an improvement of the oxidation resistance at very high temperatures. The improvement in the oxidation resistance of numerous high-temperature alloys by a pre-oxidation treatment is well known in the literature [3, 4]. In most cases, this is accomplished by the formation of a slow-growing protective oxide layer. The pre-oxidation at low oxygen partial pressure allows selective oxidation of only the highly reactive elements in the alloy. Aluminium and chromium are by far the most important reactive suitable elements for many high-temperature alloys due to the formation of protective Al2O3 and Cr2O3 oxide scales on the alloy surface. The previous study has shown that the formation of Cr2O3 oxide in Co-Re-Cr alloys occurs, if the oxygen partial pressure in the ambient atmosphere is lower than 10 bar [5]. Studying the oxidation behaviour of model Co-Re-Cr alloys at low oxygen partial pressure can contribute to an improvement in the corrosion protection in technical applications by a suitable pre-annealing of the material but should also lead to a more complete understanding of the oxidation mechanisms of this class of materials. An atmosphere containing oxygen at a low partial pressure is intended to be used in the operating environment of the Very High Temperature Reactor (VHTR) [6-9]. Helium is applied as cooling gas and contains small amounts of impurities, typically H2, H2O, CO, CH4, CO2 and N2 [10]. The detailed investigations show that water vapour is the main oxidant and that the low oxygen partial pressure is governed by the H2O/H2 ratio. First approach in explaining the observed corrosion effects is usually based on the comparison of either the calculated equilibrium oxygen partial pressure or the experimentally determined oxygen partial pressure in the flowing gas with the equilibrium oxygen pressure of the oxides of all elements present in the alloy. Figure 1 shows the dependence of the equilibrium oxygen pressure on the temperature of oxide formation. In this figure, only alloying elements of the model Re-alloys studied are taken into account. From the thermodynamic point of view, all considered oxides are stable in the reaction of the alloy with air (p(O2)=0.21 bar) at the temperatures of interest (above 1000°C). The low p(O2) value applied allows only selective oxidation of Cr, but not the oxidation of Co and Re. 250 500 75 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://www.mb.uni-siegen.de/lmw/veroeffentlichungen/2011/materials_and_corrosion_gorr.pdf?m=e |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
