Na₂SO₄ + V₂O₅ Corrosion Behavior of BaNd₂Ti₃O₁₀ for Thermal Barrier Coating Applications

Abstract

BaNd₂Ti₃O₁₀ has been considered as a promising thermal barrier coating (TBC) candidate material, which reveals many advantages over the widely used TBC material of yttria partially stabilized zirconia, such as lower thermal conductivity, better high-temperature capability, and higher resistance to environmental deposits corrosion. In this study, the hot corrosion behavior of BaNd₂Ti₃O₁₀ in Na₂SO₄ + V₂O₅ molten salt at 900 °C and 1000 °C is investigated. Exposed to the salt for 4 h at 900 °C, BaNd₂Ti₃O₁₀ pellets had an obvious reaction with the salt, forming corrosion products of NdVO₄, TiO₂, and BaSO₄, and the reaction layer was ~30 μm, beneath which no salt trace could be found. Prolonging the corrosion time to 20 h, the type of the corrosion products had no change. At an elevated corrosion temperature of 1000 °C for 4 h, the corrosion products were still NdVO₄, TiO₂, and BaSO₄, but NdVO₄ revealed a significant growth, which fully covered the pellet surface. The mechanisms by which the corrosion reaction between BaNd₂Ti₃O₁₀ and the molten salt occurred were discussed.

1. Introduction

Thermal barrier coatings (TBCs) are extensively used onto the hot-components (blades, vanes, and combustion chamber) of turbine engines, which give rise to thermal insulation and protect metallic parts from corrosion, increasing the efficiency and performance of turbines [1,2,3]. A typical TBC system usually consists of a ceramic topcoat and a bond coat, the latter of which is designed to enhance the oxidation and corrosion resistance, as well as to improve the adhesion between the ceramic top coat and the substrate. The ceramic top coat is commonly made of yttria partially stabilized zirconia (YSZ), and is usually fabricated by air plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD) [4,5,6]. YSZ TBCs have revealed many advantages, such as low thermal conductivity, excellent toughness, and good thermal expansion match with the substrate [1,7,8,9].

In marine environments or for the usage of low-quality fuel, TBCs face problems of molten salt attack, which caused by vanadium, sulfur, and phosphorous from fuel impurities or corrosive environments condensing the coating surfaces [10,11,12,13,14,15]. At 600–1000 °C, molten salt corrosion to YSZ TBCs is rather severe. The salt melts at high temperatures and infiltrates into the coating, during which a reaction between molten salt and YSZ occurs, decreasing the stabilizer in the TBC. As a result, the coating would undergo an undesirable phase transformation, causing failure much more quickly than if no molten salt is present [16,17,18,19]. Although the degradation mechanism by which molten salt attacks YSZ TBCs has been clear, measurements to solve the corrosion issue still need large studies, and some researchers have made some attempts [15,20,21,22,23,24,25], such as introducing Al₂O₃, CeO₂, Ta₂O₅, and RE₂O₃ (RE = rare earth) into the ZrO₂-based system as a new stabilizer or co-dopants.

In order to further improve engine efficiency, there are great motivations for increasing the engine-operating temperature. However, YSZ TBCs have an operation temperature limitation (1200 °C), above which they suffer from phase transformation and thermal conductivity increasing. Moreover, a practical requirement for TBCs is that the thermal conductivity should be as low as possible for better thermal insulation. Therefore, alternative TBC materials are largely required with even better phase stability, improved thermo-physical and mechanical properties, and superior high-temperature capability. However, for these novel TBCs operated at higher temperatures, threats resulting from molten salts still exist. Hence, it is necessary to understand the hot corrosion behavior of newly developed TBC materials in molten salts.

Recently, the hot corrosion behavior of some TBC candidate materials exposed to molten salt has been evaluated. Ouyang et al. reported the hot corrosion behavior of Gd₂Zr₂O₇ and Yb₂Zr₂O₇ ceramics in V₂O₅ molten salt, and investigated the associated mechanisms [13,26]. Guo et al. indicated that (Sm_0.5Sc_0.5)₂Zr₂O₇ has a better corrosion resistance to Na₂SO₄ + V₂O₅ molten salt than Sm₂Zr₂O₇ [27]. Cao et al. studied the corrosion products of LaTi₂Al₉O₁₉ ceramic resulting from V₂O₅ salt attack, and discussed their formation mechanisms [14]. Chen et al. investigated the corrosion behavior of plasma sprayed LaMg₂Al₉O₁₉ coatings in V₂O₅ molten salt [28]. BaNd₂Ti₃O₁₀ belongs to BaLn₂Ti₃O₁₀ (Ln: La, Nd, Sm) series, which is a perovskite derivative. It has been reported that BaLn₂Ti₃O₁₀ has low thermal conductivities, good high-temperature capability, and excellent resistance to environmental deposits corrosion, giving rise to be promising TBC candidate materials [29,30]. Additionally, BaLn₂Ti₃O₁₀ TBCs have been successfully produced by Guo et al., which have better thermal cycling performance than YSZ TBCs [31]. However, how BaNd₂Ti₃O₁₀ performs in molten salt is still unclear, and no report on the hot corrosion behavior of BaNd₂Ti₃O₁₀ exposed to molten salt exists in open literature.

In this study, the Na₂SO₄ + V₂O₅ corrosion behavior of BaNd₂Ti₃O₁₀ for possible use as a novel TBC material is investigated at 900 °C for 4 h and 20 h, and 1000 °C for 4 h. The emphasis is analyzing the corrosion products of BaNd₂Ti₃O₁₀ due to molten salt exposure by using dense pellets, based on which the related corrosion mechanisms are discussed.

2. Experimental Procedure

BaCO₃, TiO₂, and Nd₂O₃ powders are used as raw materials to produce BaNd₂Ti₃O₁₀ powder by a solid-state reaction method. The raw materials mixed with an appropriate quantity were ball milled in ionized water at a speed of 400 rpm for 10 h, following by drying at 160 °C for 10 h and calcination at 1500 °C for 24 h. The process was repeated until pure BaNd₂Ti₃O₁₀ powder was fabricated. BaNd₂Ti₃O₁₀ pellets for hot corrosion tests were prepared by cold pressing at ~250 MPa, followed by sintering at 1500 °C for 10 h.

Hot corrosion tests were conducted using Na₂SO₄ + V₂O₅ salt (50 mol% Na₂SO₄ and 50 mol% V₂O₅) in a furnace. Prior to the tests, pellets were grinded by 800# SiC papers, followed by ultrasonic cleaning in ethanol and drying at 120 °C. Na₂SO₄ + V₂O₅ salt was uniformly spread on the pellet surface by using a very fine glass rod. The salt content on the pellet surface was ~10 mg/cm², which was determined by calculating the weight difference between the samples before and after the salt coverage. BaNd₂Ti₃O₁₀ pellets covered with Na₂SO₄ + V₂O₅ salt were isothermally heated in a furnace, followed by cooling down to room temperature with the furnace. The heat treatment conditions were as follows: 900 °C for 4 h and 20 h, and 1000 °C for 4 h.

X-ray diffraction (XRD; Rigaku Diffractometer, CuKa radiation, Tokyo, Japan) was employed to conduct phase composition analysis, and the scanning angle (2θ) range was 10°–80° with a scanning rate of 0.1°/s. The peak identification was carried out using MDI Jade 6.0 software. Surface morphologies and composition analysis were conducted by a scanning electron microscope (SEM; TDCLS4800, Hitachi Ltd., Tokyo, Japan) equipped with energy dispersive spectroscopy (EDS, IE 350), and cross-sectional microstructure was analyzed using a scanning electron microscope (SEM; TDCLSU1510, Hitachi Ltd., Japan). For cross-section observations, the samples were cold mounted in epoxy resin, and then grinded by 150#, 400#, 800#, 1200#, and 2000# SiC papers, following by polishing with diamond pastes (5 μm size).

3. Results and Discussion

Figure 1a shows the surface microstructure of BaNd₂Ti₃O₁₀ bulk. It has inhomogeneous grains, with an average size of 5–10 μm. The cross-sectional microstructure is shown in Figure 1b. The fabricated pellet reveals a dense microstructure, and few pores could be found, the relative density of which is above 95%. According to the surface and cross-section observations, it could be considered that in the following corrosion experiments, the salt penetration might be limited due to the dense microstructure of the pellet, and the corrosion may occur at the surface regions resulting from the reaction between molten salt and BaNd₂Ti₃O₁₀.

Figure 2 shows the XRD patterns of BaNd₂Ti₃O₁₀ pellets after hot corrosion tests in Na₂SO₄ + V₂O₅ salt, and that of the as-fabricated pellet is also presented for comparison. Exposed to 900 °C for 4 h and 20 h, the sample surfaces have similar XRD patterns, consisting of diffraction peaks ascribed to NdVO₄, TiO₂, and BaSO₄. This suggests that the corrosion leads to the formation of these products, the types of which are corrosion time independent at 900 °C. Additionally, the substrate phase (BaNd₂Ti₃O₁₀) is detected, which indicates that the corrosion on the surface is uneven causing some regions are uncorroded, or the reaction layer on the surface is thin. After corrosion at 1000 °C for 4 h, only NdVO₄ and some TiO₂ could be detected by XRD, which might be because that the corrosion product of NdVO₄ largely grows covering the pellet surface. This will be further confirmed by SEM observations in the following sections.

Figure 3 shows typical surface morphologies of BaNd₂Ti₃O₁₀ sample after hot corrosion at 900 °C for 4 h. It is evident that many corrosion products are formed on the sample surface, as shown in Figure 3a. In Figure 3b showing these products at a higher magnification, it could be found that they have three different shapes, i.e., rod shaped (marked as A) particle shaped (marked as B), and plate shaped (marked as C), and their chemical compositions are listed in

Table 1. Chemical compositions of compounds A–C in Figure 3 (in at.%).

Compound	Ba	Nd	Ti	V	S	O
A	-	17.1	-	18.3	-	64.6
B	-	-	33.4	-	-	66.6
C	16.8	-	-	-	17.8	65.4

. Compound A consists of Nd, V, and O, compound B has Ti and O, while compound C is composed of Ba, S, and O. Based on the above XRD results, one could determine that compounds A, B, and C are NdVO₄, TiO₂, and BaSO₄, respectively.

The cross-sectional microstructure of the sample after corrosion at 900 °C for 4 h is shown in Figure 4a. A significantly different layer with a thickness of ~30 μm can be observed on the sample surface, in which some parts are dense and other regions are rather loose. Beneath this reaction layer, the pellet keeps structure integrity, and has no molten salt trace. In an enlarged image as shown in Figure 4b, the dense parts could be identified as the original substrate, which could provide the explanation for the detection of BaNd₂Ti₃O₁₀ phase by XRD, and the loose regions are composed of corrosion products, which are composed of rod shaped NdVO₄ and particle shaped TiO₂. One could find that, at the interface of the reaction layer and the original substrate, there are some pores, which might be attributed to the thermal expansion mismatch between the newly formed layer and the original BaNd₂Ti₃O₁₀ pellet.

Figure 5 shows the SEM images of the BaNd₂Ti₃O₁₀ sample exposed to corrosion at 900 °C for 20 h. This is similar to the case of 4 h corrosion; corrosion products are also evident in this case, as shown in Figure 5a. Enlarging this image, one could clearly observe these differently shaped corrosion products, i.e., rod shaped (D), particle shaped (E) and plate shaped (F), as presented in Figure 5b. EDS was employed to analyze the chemical composition of these products, and the results are also listed in

Table 2. Chemical compositions of compounds D–F in Figure 5 (in at.%).

Compound	Ba	Nd	Ti	V	S	O
D	-	18.5	-	19.3	-	62.2
E	-	-	32.9	-	-	67.1
F	17.1	-	-	-	18.9	64.0

. Compound D contains Nd, V, and O, compound E is composed of Ti and O, while crystal F consists of Ba, S, and O. According to the above XRD results, it could be confirmed that D, E, and F are NdVO₄, TiO₂, and BaSO₄, respectively.

BaNd₂Ti₃O₁₀ sample was subjected to hot corrosion in Na₂SO₄ + V₂O₅ salt at 1000 °C for 4 h, and its surface morphologies are shown in Figure 6. The sample surface is almost completely covered by corrosion products, as shown Figure 6a. In the enlarged image (Figure 6b), the corrosion product with a rod shape could be clearly observed, but other shaped products are rare, only presenting a few particle shaped compounds. EDS analysis of the rod-shaped compound indicates that it is composed of Nd, V, and O, and could be identified as NdVO₄ in combination with the XRD result. The particle shaped corrosion products are TiO₂ based on EDS and XRD analysis. BaSO₄ and BaNd₂Ti₃O₁₀ are not detected by SEM, mainly attributed to the significant growth of NdVO₄ which covers the whole surface of the sample.

Based on the aforementioned observations, it could find that in the presence of Na₂SO₄ + V₂O₅ molten salt, BaNd₂Ti₃O₁₀ undergoes evident corrosion, and the type of corrosion products are temperature and time independent, which consists of NdVO₄, TiO₂, and BaSO₄. However, an elevated temperature causes significant growth of NdVO₄, which can cover the whole surface of the sample. The mechanism for the formation of these corrosion products could be understood in terms of the breakdown of the chemical bonds of BaNd₂Ti₃O₁₀ by molten salt [14,32]. From the viewpoint of crystallography, BaNd₂Ti₃O₁₀ crystal has a lattice consisting of a tri-perovskite [Nd₂Ti₃O₁₀] layer separated by a Ba layer along c-axis [29,30]. In the lattice, the Ba insertion layers are weakly bonded, which are easy to be destroyed by molten salt. As a result, Ba-O bonds in BaNd₂Ti₃O₁₀ lattice is expected to first break due to molten salt attack, giving rise to the formation of Ba contained corrosion products. Then, Nd and Ti are enriched in the crystal due to the consumption of Ba, which provides a greater chance for the salt to destroy Nd-O and Ti-O bonds. Thus, other corrosion products containing Nd and Ti are expected to be generated.

For the mixture of Na₂SO₄ and V₂O₅ salts, a reaction occurs above 900 °C by the following expression [11,18,27,33]:

$$\mathrm{N}{\mathrm{a}}_2\mathrm{S}{\mathrm{O}}_4(\mathrm{l})+{\mathrm{V}}_2{\mathrm{O}}_5(\mathrm{l})\to 2\mathrm{N}\mathrm{a}\mathrm{V}{\mathrm{O}}_3(\mathrm{l})+\mathrm{S}{\mathrm{O}}_3(\mathrm{g})$$

(1)

In this study, the formed products resulting from Equation (1) react with BaNd₂Ti₃O₁₀ pellets. Based on the reaction products analysis by XRD and SEM, the reaction between BaNd₂Ti₃O₁₀ and the molten salt could be expressed by the following equation:

$$\mathrm{BaNd}2\mathrm{Ti}3\mathrm{O}10\left(\mathrm{s}\right)+2\mathrm{NaVO}3\left(\mathrm{l}\right)+\mathrm{SO}3\left(\mathrm{g}\right)\to \mathrm{BaSO}4\left(\mathrm{s}\right)+2\mathrm{NdVO}4\left(\mathrm{s}\right)+3\mathrm{TiO}2\left(\mathrm{s}\right)+\mathrm{Na}2\mathrm{O}\left(\mathrm{s}\right)$$

(2)

It has been reported that for YSZ pellets or coatings, molten salt has a large tendency to penetrate inside, reacting with YSZ to form YVO₄ and m-ZrO₂, which causes a large and deep reaction region, where the phase composition and microstructure are largely destroyed [17,18,19,21]. By comparison, in the presence of Na₂SO₄ + V₂O₅ molten salt, there is only a ~30 μm thickness destroyed region/layer on the BaNd₂Ti₃O₁₀ pellet surface, beneath which no molten salt trace could be found (Figure 4). Although EDS line scans or maps of this layer is not provided here, based on the identified reaction products on the pellet surface and cross-section observations, one could confirm that the destroyed region is composed of reaction products. The formation of these corrosion products has largely consumed the salt; as a result, very limited molten salt can infiltrate the deep regions of the pellets. In other words, the reaction layer could be considered as a sacrificial layer, the formation of which can protect the pellet or coating from severe damage. Hence, BaNd₂Ti₃O₁₀ is resistant to Na₂SO₄ + V₂O₅ salt corrosion to a certain extent; especially, its resistance to molten salt corrosion is better than YSZ.

4. Conclusions

BaNd₂Ti₃O₁₀, a promising TBC candidate material, was exposed to Na₂SO₄ + V₂O₅ molten salt at 900 °C and 1000 °C to evaluate its hot corrosion resistance. After exposure to Na₂SO₄ + V₂O₅ salt for 4 h at 900 °C, there was an obvious reaction between BaNd₂Ti₃O₁₀ and the salt, producing NdVO₄, TiO₂, and BaSO₄ as the corrosion products, which formed a reaction layer with a thickness of ~30 μm, and the regions beneath the reaction layer were not destroyed by the molten salt. At 900 °C for 20 h in the salt, the type of corrosion products of BaNd₂Ti₃O₁₀ pellet had no change. At an elevated corrosion temperature of 1000 °C for 4 h, although the corrosion products were still NdVO₄, TiO₂, and BaSO₄, NdVO₄ crystals grew significantly and fully covered the pellet surface. Based on the research results, BaNd₂Ti₃O₁₀ is found to have a good resistance to Na₂SO₄ + V₂O₅ salt corrosion, and due to the excellent high-temperature capability and thermo-physical properties, it has a potential application for TBCs, especially under the corrosive environments containing vanadium and sulfur.