Corrosion of Al0.1CoCrFeNi High Entropy Alloy in a Molten Eutectic Salt

The present work investigates the hot corrosion behavior of Al 0.1 CoCrFeNi high entropy alloy (HEA) and Ni-base Alloy 718. Electrochemical tests were conducted to characterize the corrosion behavior of Al 0.1 CoCrFeNi HEA and Alloy 718 in a molten Na 2 SO 4 -NaCl eutectic mixture at 750 ± 5°C in the presence of a platinum catalyzed SO 2 /air mixture. The morphology of the surface of Al 0.1 CoCrFeNi HEA and Alloy 718 was investigated using scanning electron microscopy (SEM) as well as energy dispersive spectroscopy (EDS). The results indicate that the polarization resistance of the HEA was higher and the corrosion rate lower in comparison with Alloy 718. SEM and EDS analyses reveal the formation of a dual oxide layer on the HEA that provides a better corrosion barrier compared to the single chromia scale observed on Alloy 718.

Compositionally complex high entropy alloys (HEAs) have emerged as attractive candidate materials for advanced engineering applications due to their unique microstructures and mechanical properties. Unlike traditional alloys, HEAs contain five or more elements with equal or near-equal amounts (in atom %). 1,2 This unconventional alloying ( Figure 1) strategy results in HEAs with attractive properties, e.g., wear resistance, hardness and high temperature strength coupled with relatively low density and multiple elements that could participate in passive film formation. The random arrangement of multiple elements in solid solutions results in a particular locally-disordered chemical environment, which is expected to result in unique properties. 3 However, the literature is sparse in regard to the stability of these alloys in corrosive environments, e.g. in molten salts. In gas turbine environments, salts can deposit on the turbine blade forming a thin molten film, which will induce hot corrosion. 4 The corrosive salts will attack the protective oxides, forcing the substrate to undergo further degradation. 5 Therefore, it is important to evaluate the corrosion resistance of these alloys to determine their suitability for service under these conditions. The aim of the present work was to evaluate the corrosion behavior of an Al 0.1 CoCrFeNi HEA in a molten salt environment and benchmark it to a conventional Ni-base superalloy, Alloy 718, that is used in aircraft engines.

Experimental
HEA coupons of Al 0.1 CoCrFeNi were obtained from Sophisticated Alloys Inc. The HEA was vacuum induction melted and cast. The control alloy was Alloy 718 (Haynes International). The composition of these two alloys (in at%) are listed in Table I. Al 0.1 CoCrFeNi HEA and Alloy 718 samples were electrochemically tested at 750 ± 5°C in a Na 2 SO 4 -NaCl eutectic mixture as the electrolyte. The electrochemical cell consisted of three electrodes positioned inside an alumina crucible. Tests were conducted in the presence of a platinum catalyzed SO 2 /air mixture. Pure platinum wire was used for both the reference and counter electrodes. The working electrode was fabricated using coupons ground to a 600-grit surface finish. Electrochemical tests were conducted using a commercially available potentiostat. 6 The testing procedure involved recording the open circuit potential (OCP) for one hour. This was followed by a linear polarization (LP) scan from −20 to +20 mV relative to the OCP at a scan rate of 1 mV/s. Following this, a cyclic potentiodynamic polarization (CPP) scan was conducted from −400 to +400 mV relative to the OCP at a rate of 1 mV/s. After exposure to the molten salt environment, the test specimens were electroplated with nickel to help retain the corrosion product, and then epoxy mounted. The test coupons were metallographically prepared to a 1 μm surface finish and analyzed using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS).

Results and Discussion
The average OCP values recorded at the 60 th minute are −897 ± 39 mV (N = 3, where N represents the number of replicates) and −701 ± 227 mV (N = 4) for the HEA and Alloy 718, respectively. The OCP trends as a function of time indicated that the alloys stabilized within the 60 minute time frame. The polarization resistance, R p ( cm 2 ), can be used as a measure of corrosion resistance, i.e.,   Figure 2. The average R p values for the HEA and Alloy 718 are 143 ± 69 (N = 3) and 91 ± 60 (N = 4) cm 2 , respectively. Therefore, the HEA has a higher corrosion resistance than Alloy 718 in Na 2 SO 4 -NaCl molten salt environments. The open circuit potential from the LPR plot for the HEA was −903 ± 34 mV (N = 3) and that for Alloy 718 was −709 ± 229 mV (N = 4). Corrosion current densities, i corr (μA/cm 2 ), can be used to calculate the corrosion rate (mils per year, mpy) assuming uniform corrosion. Figure 3 shows the CPP plots for Al 0.1 CoCrFeNi HEA and Alloy 718. Commercial software (EC-Lab) was used to calculate corrosion potential values and the corrosion current densities using the Tafel extrapolation method. [7][8][9] Regression lines were fitted to the linear portions of the cathodic and anodic sections of the initial portion of the potentiodynamic plot (−400 mV to 0 mV and 0 mV to +400 mV from OCP) and extrapolated until they intersected. This point of intersection defines the corrosion potential and the corrosion current density which was then used to calculate corrosion rates using Equation 1. The corrosion potentials for the HEA and Alloy 718 are −1116 ± 136 mV (N = 3) for HEA and −998 ± 276 mV (N = 4), respectively. The equation for uniform corrosion rate is displayed below.
where ρ is density of the alloy (g/cm 3 ) and EW is equivalent weight (gram equivalent) of the alloy. The HEA exhibited a lower corrosion rate [31 ± 34 (N = 3) mpy] than Alloy 718 [90 ± 39 (N = 4) mpy]. The lower corrosion rate for the HEA can be attributed to the higher chromium activity relative to Alloy 718. This finding is consistent with results reported in the literature. 10 It is important to note that performing cathodic and anodic scans in succession and/or sweeping the potential over large ranges (± 0.4 V versus OCP) can alter the surface of the test coupon and the corresponding test results. It can be seen in Figure 3 that the corrosion potentials of both alloys are significantly lower than those in Figure 2. This difference is likely due to the large overpotentials used in the cyclic polarization scans. However, Alloy 718 continued to exhibit higher nobility than HEA (Al 0.1 CoCrFeNi). The large potential range used in the present study ensures that all corrosion behavior is captured in the scans (i.e., multiple corrosion potentials, active/passive behavior and localized attack), which would otherwise be missing in a scan over small potential ranges (± 0.25 V versus OCP). Figure 4 shows a backscattered electron image and corresponding EDS maps of the cross-section of an HEA coupon exposed to molten Na 2 SO 4 -NaCl at 750°C. The SEM image and the corresponding elemental maps indicate the formation of two distinct oxide layers, i.e., an outer chromium oxide and an inner aluminum oxide layer. The other constituent elements of the HEA, cobalt, iron and nickel, were not present in these oxide layers. Figure 5 shows a higher magnification backscattered electron image and corresponding EDS maps of the same cross-sectioned HEA coupon as above. At this magnification, the dual oxide layer can be more clearly seen. Additionally, a distinctly different region exists beneath the alumina film. This region is depleted of chromium and enriched in cobalt, nickel and iron. The compositions of selected points are provided in Table II and the significance of these points is discussed below. Figures 6-9 show backscattered electron images and elemental maps of the cross-section of Alloy 718 coupons exposed to molten Na 2 SO 4 -NaCl at 750°C. A porous outer layer containing niobium, chromium, aluminum and oxygen and a denser inner layer of chromium oxide are apparent from these figures. Underneath the chromia layer, a region rich in niobium, aluminum and sulfur was observed. Beneath this, niobium sulfide particles embedded in a metallic matrix consisting primarily of nickel and iron was observed.   The present study has shown that Al 0.1 CoCrFeNi HEA forms an outer chromium oxide layer and an inner aluminum oxide layer when exposed to molten Na 2 SO 4 -NaCl. The presence of a dual oxide film on an HEA immersed in molten Na 2 SO 4 -NaCl salt has been observed on an AlCoCrFeNiTi HEA at 900°C. 5 In addition, the cross-sectional morphology of the Al 0.1 CoCrFeNi alloy used in this study is comparable to that of the corrosion behavior of AlCoCrFeNiTi alloy reported Table II. Composition (in at%) of points listed on backscattered electron image shown in Figure 5. in the literature. 5 The Cr 2 O 3 film is beneficial in defending against hot corrosion attack by modifying the ionic character of the electrolyte, i.e., by the formation of compounds such as NaCrO 2 , Na 2 CrO 4 and Na 2 Cr 2 O 7 . 12 This dissolution of Cr 2 O 3 will result in a positive solubility gradient that can interrupt the corrosion process at elevated temperatures. 12 The presence of an alumina film has been reported to be beneficial in several alloy systems because it can extend the initiation stage of hot corrosion as well as delay the propagation stage. 13 Figure 6. Backscattered electron image of cross-sectioned Alloy 718 sample exposed at 750°C in Na 2 SO 4 -NaCl.

Element
) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 169.234.6.216 Downloaded on 2019-12-13 to IP From Table II, the compositions of several points in Figure 5 are presented. These point analyses indicate that the oxide film has an outer region enriched in chromium and oxygen, indicating chromia, and the inner region is enriched in aluminum and oxygen indicating alumina. Some chromium is present in the alumina film. Beneath the oxide layer, chromium and aluminum are depleted. This depletion zone thickness is in the 5-10 μm range. The chromium and aluminum contents increase going into the substrate until they reach the pre-test composition deep into the substrate. This depletion zone is formed by the outward diffusion of aluminum and chromium. The depletion of chromium underneath the alumina film may be an issue if both the chromia and alumina films are ruptured after long exposures to the molten salts, as there will not be enough chromium at the surface to heal the chromia film. The ability of the HEA to form a dual oxide layer suggests that while the outer chromia layer provides an effective defense against the molten salt, the inner alumina layer could serve as a secondary barrier in protecting the underlying substrate.
Alloy 718 develops a single chromium oxide layer upon exposure to molten Na 2 SO 4 -NaCl at 750°C. The formation of an outer chromia layer for Alloy 718 in similar salts has been observed. 14,15 Sulfides can  form in Alloy 718, 15 leading to a chromium depletion zone beneath the chromia film. A depletion zone and the formation of sulfides was also observed in this study.

Conclusions
The corrosion behavior of Al 0.1 CoCrFeNi HEA and Alloy 718 in a molten Na 2 SO 4 -NaCl eutectic salt mixture at 750 ± 5°C was studied using electrochemical tests as well as scanning electron microscopy coupled with energy dispersive spectroscopy. The Al 0.1 CoCrFeNi HEA is less noble, has a higher polarization resistance and a lower corrosion rate relative to Alloy 718. The HEA forms an outer chromium oxide layer and an inner aluminum oxide layer while Alloy 718 forms only a single chromium oxide layer. The chromia layer is an effective deterrent to hot corrosion while the alumina scale is oxidationresistant. This combination makes the Al 0.1 CoCrFeNi HEA more corrosion resistant than a conventional chromia former in environments typical of turbines. Therefore, this alloy appears to be a promising candidate for aerospace applications in light of the corrosion behavior displayed in this study.