Oxidation Behaviors of Inconel 740H in Air and Dynamic Steam

Materials

The alloy ingots were prepared using vacuum-induction melting (VIM) and electroslag remelting (ESR) technic. Sample coupons (10×15×2 mm) were cut from ingots of the Inconel 740H alloy. Before cutting, the alloy ingot was homogenized at 1,204°C for 16 h and hot worked at temperature above 1,050°C for 30 min, followed by water-quenched. The standard precipitation treatment was proceeded at 800°C for 16 h and then air-cooled, which was in accordance with the standard heat treatment provided by the Special Metal Corporation. The specimens were ground to 2,000-grit silicon carbide papers and then ultrasonically cleaned in acetone before high temperature exposure. Alloy composition is shown in Table 1.

Oxidation tests

The oxidation tests under static air were carried out at 750°C and 850°C up to 1,000 h, respectively. Samples were placed in a resistance-heated furnace after temperature met the requirements. Steam oxidation experiments were carried out in a horizontal resistance heated furnace with a sealed quartz chamber under dynamic pure steam. The water used for producing steam was deionized, with a resistivity of about 18.25 MΩ cm and dissolved oxygen content of about 5–7 mg/L. Before heating, high-purity N₂ (99.9%) was used to discharge the air in furnace, pure steam (preheated to 300°C) was fed into the furnace after the temperature reached 750°C. The steam parameters were of 750°C in temperature, 0.1 MPa in pressure and 100–120 mL/s in flow rate. The steam oxidation test lasted for 1,000 h, and several specimens were taken out to weigh during the test.

At least three parallel specimens were weighed under the same testing time. The oxidation kinetic curves were drawn by the average mass changes. The mass changes of the specimens were measured using an electrobalance with a detection limit of 0.01 mg. Surface and cross-sectional morphologies of the specimens were observed by the SEM. Semiquantitative chemical compositions of the specimens were detected by EDS. X-ray diffraction was also used to identify the phase of oxide scales.

Oxidation kinetics

The oxidation kinetics of Inconel 740H alloy under static air at 750°C and 850°C and under dynamic pure steam at 750°C are shown in Figure 1. Figure 1(a) and (b) shows the weight gains and its squared values with the variation of testing time, respectively. The results indicate that the mass gain of Inconel 740H is in accord with the parabolic law approximately for three different conditions. No significant fluctuations were observed in weight gain curves, which was consistent with the visual observation during the testing process. By fitting the kinetic curves according to the formula Δω² = kt + b (Δω: mass gain, t: time, k: oxidation rate constant), the values for oxidation rate constant k of Inconel 740H under static air at 750°C and 850°C and dynamic pure steam at 750°C are 0.6915×10⁻⁴, 3.61×10⁻³ and 1.7936×10⁻⁴ (mg²/cm⁴/h), respectively.

Figure 1:

Oxidation behavior of Inconel 740H: (a) oxidation kinetics and (b) oxidation rate.

Oxidation products

XRD patterns of Inconel 740H after oxidation under three conditions are shown in Figure 2. After oxidation for 1,000 h, the oxide scale formed under static air at 750°C mainly consists of Cr₂O₃ with minor (Ni)CrMn spinel. However, the amount of spinel and TiO₂ increases after oxidation at 850°C and the intensity of peaks from the matrix is significantly reduced. For the samples oxidized under pure steam at 750°C, Cr₂O₃ is the primary oxidation product. Furthermore, the relative peak intensity of Cr₂O₃ was higher than that in static air at 750°C.

Figure 2:

XRD patterns of Inconel 740H after oxidation at three different conditions.

Scale morphologies

Surface morphologies of Inconel 740H after oxidation for 1,000 h are shown in Figure 3. Figure 3(b), (d) and (f) shows the views of Figure 3(a), (c) and (e) at higher magnifications, respectively.

Figure 3:

Surface morphologies of Inconel 740H after oxidation: (a, b) 750°C, in static air, (c, d) 850°C, in static air and (e, f) 750°C, in pure steam.

After oxidation for 1,000 h at 750°C under static air, a dense oxide layer forms on the alloy surface. A typical morphology of nodules with no cracks and spallation is observed in particular regions (Figure 3(a)). In view of local magnification (Figure 3(b)), oxidation product is composed of fine particles with clear boundaries, and the average size is about 0.5 μm. Based on EDS results, relative contents of Cr and O (atomic percent) on alloy surface are 34.47% and 55.10%, respectively, and small amounts of Ni, Mn and Ti are also detected. From Figure 3(c) and (d), it is evident that the oxide scale obtained at 850°C has a much rougher surface compared with the one at 750°C. The particle average size of oxidation products is larger, which is about 2 μm. More Ti-rich oxides were identified by EDS. In addition, a few projections and holes appear on the top surface. For the samples oxidized in pure steam at 750°C (Figure 3(e) and (f)), the oxides with a smooth surface as well as fine particles are observed. However, the particle edges are obviously more blurred in comparison with those in static air.

Cross-sectional morphologies of Inconel 740H after oxidation for 1,000 h are shown in Figure 4. At 750°C in static air (Figure 4(a)), a single layer of oxide scale forms on alloy surface with a depth of about 1.8 μm. Some inner oxides occur beneath the oxide scale. In certain areas, nodule products are observed, which is consistent with the surface morphologies (Figure 3(a)). When temperature is up to 850°C, the thickness of oxide scale increases to 16 μm. Large amount of holes can be seen within the oxide scale and at the alloy/scale interface. Both the internal oxidation zone and oxides amount are remarkably enlarged. The thickness of oxide scale is about 2.2 μm at 750°C in pure steam. The depth of internal oxidation is larger than that in static air.

Figure 4:

Cross-sectional morphologies of Inconel 740H after oxidation: (a) 750°C, in static air, (b) 850°C, in static air and (c) 750°C, in pure steam.

Element distribution of Inconel 740H after oxidation for 1,000 h in static air at 750°C and 850°C and in dynamic pure steam at 750°C is shown in Figures 5–7, respectively. It can be inferred that the thickness of oxide scale increases with temperature rising. In static air at both 750°C and 850°C, the main components of oxide scale are products rich in Mn and Cr. Few inner oxides rich in Al and Ti are observed at the alloy/scale interface and beneath the oxide scale. More Ti-rich oxides are detected on the scale surface at 850°C, and inner oxides with larger dimension are identified as Al₂O₃ (Figure 6). In pure steam, the oxide scale mainly consists of Cr₂O₃ (Figure 7). The results in static air (Figure 5), by contrast, indicate that less Mn and Ti are observed in oxide scale, but the Cr-depleted zone beneath the oxide scale increases significantly.

Figure 5:

Element mapping of Inconel 740H after oxidation in static air at 750°C for 1,000 h.

Figure 6:

Element mapping of Inconel 740H after oxidation in static air at 850°C for 1,000 h.

Figure 7:

Element mapping of Inconel 740H after oxidation in pure steam at 750°C for 1,000 h.

Inconel 740H is a Ni–Cr–Co-based superalloy. Ni, Cr and Co could react with oxygen simultaneously during high temperature exposure. Due to the high Cr content in the alloy, a continuous Cr₂O₃ layer forms during the initial stage of the exposure by the selective oxidation of Cr. Although some nodule products are observed on the top surface, its composition is similar to the oxides formed in flat areas based on the EDS results. Metallurgical defects, heterogeneous phase, grain boundaries extending to the top surface and α-Cr precipitates may account for the formation of these protruding morphologies [13]. Once the protective Cr₂O₃ scale formed, the outward diffusion of alloy elements is restrained, thus reducing the oxygen activity at the alloy/scale interface. Therefore, the oxidation process is stabilized to a relatively steady state (Figure 1(a)). Al is also an antioxidant element in the Inconel 740H alloy, but it is impossible to form a continuous alumina scale due to its low content. In most cases, Al₂O₃ forms as inner oxides instead and grows along grain boundaries as shown in Figure 4 [14]. Additionally, no obvious aggregation of Si or Si-rich oxides was examined within oxide scales in three different atmospheres. Researches had indicated that trace of SiO₂ forms at the alloy/scale interface might be effective to improve the oxidation resistance [15]. However, there is other evidence indicating that Si accelerates the fracture of oxide film, especially in an environment containing water vapor [16]. The impact of Si on the oxidation behavior of Inconel 740H alloy remains to be further studied.

According to the oxidation kinetics in Figure 1, the value of the oxidation rate of alloy at 750°C is about 1/50 of the 850°C value, which suggests that the diffusion rate of the alloy elements and formation of oxide scale increase at higher temperatures. Therefore, oxide scale grows rapidly at 850°C and the scale is much thicker than that at 750°C. Moreover, increasing the exposure temperature changes the scale structure and composition. Minor amounts of Mn/Cr spinel and isolated TiO₂ are observed at 850°C in the Cr₂O₃ scale, which is ascribed to the accelerated diffusion process of these alloying elements as the temperature rising. In addition, defects such as vacancy could be easily induced during the inter-diffusion process between matrix elements and oxygen, due to the disparity of diffusion rate for different elements. These defects gathered and eventually to form a large number of holes as shown in Figure 4(b).

It had been reported that the critical amount of Cr required to form and maintain a protective oxide is affected by the presence of water vapor [17, 18]. However, the present study indicates that Inconel 740H with 25 wt% Cr content could still form a continuous Cr₂O₃ layer even in pure steam atmosphere. Compared with the morphologies in static air at the same temperature, only smaller surface waviness and more blurred edges of oxide particles were observed as shown in Figure 3(e) and (f). This might be caused by dissolution reaction of water molecules involved with Cr₂O₃. Researches indicated that Cr oxyhydroxide may form under water vapor containing conditions and it is volatile at temperatures as low as 650°C [19, 20]. The presence of these volatile products had already been proven by GC-MS (gas chromatography-mass spectrometry) [21]. But according to the equation 1/2 Cr₂O₃ (s) + H₂O (g) + 3/4 O₂ (g) = CrO₂(OH)₂ (g), the formation of these volatiles requires sufficient oxygen to be present. Most of the published works have been carried out employing air–steam mixtures [22, 23]. In the pure steam, however, holes and cracks left by those volatile products are not found in the present research, but similar morphologies by dissolving are observed on the surface of Cr₂O₃ layer [24]. Additionally, the composition of oxide scale is slightly changed by pure steam. The diffusion rate of Cr is accelerated significantly by the pure steam [25], which caused a quick formation of Cr₂O₃ layer and less Ti and Mn dissolved in the oxide scale. At the same time, a larger Cr-depleted zone may be formed by the rapid chromium consumption (Figures 5–7).