Hot Corrosion Behavior of BaLa₂Ti₃O₁₀ Thermal Barrier Ceramics in V₂O₅ and Na₂SO₄ + V₂O₅ Molten Salts

Abstract

BaLa₂Ti₃O₁₀ ceramics for thermal barrier coating (TBC) applications were fabricated, and exposed to V₂O₅ and Na₂SO₄ + V₂O₅ molten salts at 900 °C to investigate the hot corrosion behavior. After 4 h corrosion tests, the main reaction products resulting from V₂O₅ salt corrosion were LaVO₄, TiO₂, and Ba₃V₄O₁₃, whereas those due to Na₂SO₄ + V₂O₅ corrosion consisted of LaVO₄, TiO₂, BaSO₄ and some Ba₃V₄O₁₃. The structures of reaction layers on the surfaces depended on the corrosion medium. In V₂O₅ salt, the layer was dense and had a thickness of 8–10 μm. While in Na₂SO₄ + V₂O₅ salt, it had a ~15 μm porous structure and a dense, thin band at the bottom. Beneath the dense layer or the band, no obvious molten salt was found. The mechanisms by which the reaction layer forms were discussed.

1. Introduction

Thermal barrier coatings (TBCs) are extensively used in turbine engines, which can protect engine hot-components against thermal attack and corrosion, giving rise to enhanced engine efficiencies and performances [1,2,3]. Usually, a typical TBC system is composed of a ceramic topcoat and a metallic bond coat [4,5,6]. The top coat is important, which provides thermal insulation to the substrate, and is commonly made of yttria partially stabilized zirconia (YSZ) [1,2,7,8,9]. Up to now, many techniques have been developed to produce YSZ coatings [10,11,12,13].

In a marine environment or engines use low-quality fuel, molten salts have severe damage to YSZ TBCs, especially at a temperature range of 600–1000 °C [14,15,16,17,18]. Molten salts infiltrate into the TBCs, and react with YSZ grains to form YVO₄, leading to the depletion of yttria stabilizer in the TBC. During engine heating-cooling cycles, phase transformation of the TBC occurs, causing the coating to spall much faster than if no molten salt exists. Many researchers have studied the corrosion mechanisms of YSZ TBCs resulting from molten salt [18,19,20,21,22]. Some strategies have been proposed to improve the hot corrosion resistance of YSZ coatings, such as doping CeO₂, Al₂O₃, Ta₂O₅ and RE₂O₃ (RE = rare earth element) into the system [18,21,22,23,24].

Increasing engines operating temperature leads to enhanced power output and efficiency [1,2,3]. However, YSZ TBCs suffer from phase transformation and reduced thermal insulation above 1200 °C, which causes them unlikely to meet the long-term requirements for advanced engines. Moreover, even lower thermal conductivity of TBCs is practically required for better thermal insulation. Therefore, alternative TBC materials to YSZ suitable for high-temperature applications are strongly needed. For the application at higher temperatures, a similar threat to new TBCs posed by molten salts still exists. Thus, there is a strong need to understand the hot corrosion behavior of TBC candidates in molten slats.

Recently, the hot corrosion behavior of some newly developed TBC materials in molten salts has been reported. Ouyang et al. have studied the hot corrosion behavior of Gd₂Zr₂O₇ and Yb₂Zr₂O₇ ceramics in V₂O₅ molten salt at various temperatures [16,25]. Cao et al. have investigated the corrosion products of LaTi₂Al₉O₁₉ ceramic resulted from V₂O₅ salt attack, and proposed their formation mechanisms [17]. Recently, Guo et al. have systematically studied the hot corrosion behavior and mechanisms of some TBC candidates in molten salts, such as Ba₂YbAlO₅, (Gd_0.9Sc_0.1)₂Zr₂O₇, Gd₂O₃-Yb₂O₃ co-doped YSZ and rare earth phosphate [26,27,28,29]. They reported that these novel TBC materials reveal better corrosion resistance than YSZ. Specially, LaPO₄ and NdPO₄ are found to highly resist to molten salt corrosion; exposed to high temperatures, the molten salt reacts with the ceramics to form an RE(P,V)O₄ (RE = Nd, La) solid solution, which leads to limited damage to the original microstructure [29]. BaLa₂Ti₃O₁₀ has been considered as a promising TBC candidate material [30], however, how it behaves in molten salts is not found in the open literature.

In order to understand the corrosion resistance of BaLa₂Ti₃O₁₀, its hot corrosion behavior in V₂O₅ and Na₂SO₄ + V₂O₅ salts for 4 h at 900 °C is investigated. In this study, the emphasis is placed on analyzing the corrosion products resulting from the reactions between BaLa₂Ti₃O₁₀ and the molten salts by using dense pellets, and the associated corrosion mechanisms are also discussed.

2. Experimental Procedures

BaLa₂Ti₃O₁₀ powders were produced by a solid-state reaction method. The raw materials contained BaCO₃, TiO₂ and La₂O₃ powders, which were dissolved in ionized water in an appropriate quantity. Then, the powders were ball mixed using zirconia media at a speed of 400 rpm for 10 h, followed by drying at 160 °C for 10 h. Afterward, the mixed powders were calcined at 1500 °C for 24 h. The bulk samples for hot corrosion tests were fabricated from the powders, which were cold pressed at ∼250 MPa and then sintered at 1500 °C for 10 h.

The corrosion media were V₂O₅ and 50 mol% Na₂SO₄ + 50 mol% V₂O₅ (Na₂SO₄ + V₂O₅) salts. Prior to hot corrosion tests, pellets were ground by 800 grit sandpaper. Then, the samples were ultrasonic cleaned in ethanol and dried at 120 °C. The salts were uniformly spread on surfaces of samples, and its content was determined by weighting the samples before and after the salt coverage using an analytical balance. The salt concentration was ∼10 mg/cm². Then, the salt covered samples were heated at 900 °C for 4 h in a furnace, followed by cooling in the furnace.

Phase structures of the corroded samples was identified by X-ray diffraction (XRD; Rigaku Diffractometer, Tokyo, Japan), with 2θ range of 10°–80° at a scanning rate of 0.1°/s. Surface morphologies were obtained by SEM (TDCLS4800, Hitachi Ltd., Tokyo, Japan), the composition analysis was performed using EDS (IE 350), and cross-sectional images were taken by SEM (TDCLSU1510, Hitachi Ltd.).

3. Results and Discussion

Figure 1 shows the XRD patterns of the as-prepared BaLa₂Ti₃O₁₀ pellet and the samples after hot corrosion. BaLa₂Ti₃O₁₀ bulk basically consists of a monoclinic phase. It is possible to observe that the peaks are sharp, suggesting good crystallization of the sample. After V₂O₅ salt corrosion, LaVO₄ (PDF#50-0367), Ba₃V₄O₁₃ (PDF#36-1466) and TiO₂ (PDF#99-0090) phases are detected by XRD on the sample surface. In the case of Na₂SO₄ + V₂O₅ salt corrosion, the corrosion products are LaVO₄, Ba₃V₄O₁₃, TiO₂ and BaSO₄, and some BaLa₂Ti₃O₁₀ diffraction peaks are also detected, as indicated in Figure 1. The corrosion products resulting from the two type of slats are different, which will be further confirmed by SEM and EDS analysis in the following section.

Figure 2a shows the surface image of the as-fabricated BaLa₂Ti₃O₁₀ pellet. Obvious BaLa₂Ti₃O₁₀ grains are observed. After hot corrosion in V₂O₅ salt, corrosion products are observed on the sample surface, as can be seen in Figure 2b. When observing the corrosion products at a higher magnification, one could find three different shapes, i.e., plate-shaped (marked as A), rod-shaped (marked as B) and particle-shaped (marked as C), as shown in Figure 2c. EDS analysis result listed in

Table 1. Compositions of compounds A–F in Figure 2 (in at.%).

Corrosion Products	Ba	La	Ti	V	S	O
A	15.87	–	–	21.56	–	62.57
B	–	–	31.58	–	–	68.42
C	–	16.38	–	17.63	–	65.99
D	16.45	–	–	–	18.53	65.02
E	–	–	33.16	–	–	66.84
F	–	17.57	–	18.46	–	63.97

indicates that A consists of Ba, V and O elements. In combination with the above XRD result, it is possible to determine that A is Ba₃V₄O₁₃. B is composed of Ti and O, and C contains La, V and O. Further analysis confirms that B and C are TiO₂ and LaVO₄, respectively.

After hot corrosion in Na₂SO₄ + V₂O₅ salt, the pellet surface is also completely covered with corrosion products, as shown in Figure 2d. The plate-shaped compounds (D) have Ba, S and O elements, without any evidence of V, as shown in Table 1, and they could be identified to be BaSO₄. Enlarging the image of the surface, one could find many rod-shaped and particle-shaped compounds, as presented in Figure 2e. These two different shaped compounds are marked as E and F, respectively in Figure 2f. As listed in Table 1, compound E contains Ti and O, and F has La, V and O, which could be confirmed to be TiO₂ and LaVO₄, respectively.

Figure 3a shows the cross-sectional image of the BaLa₂Ti₃O₁₀ sample after hot corrosion in V₂O₅ salt at 900 °C for 4 h. A continuous, dense reaction layer forms on the sample surface. Beneath the layer, the bulk keeps structure integrity, where no molten salt trace could be observed. This indicates that this layer has a positive function on suppressing the molten salt penetration. In the enlarged image (Figure 3b), it is possible to find that the layer is highly adhered to the bulk, with a thickness of 8–10 μm. Note that there are two sub-layers in the reaction layer. The upper sub-layer is light-contrasted and has some cracks, while the lower sub-layer is grey-contrasted and reveals a dense structure. EDS analysis was conducted on regions A–C in Figure 3c, and the results are presented in

Table 2. Compositions of compounds A–G in Figure 3 and Figure 4 (in at.%).

Corrosion Products	Ba	La	Ti	V	S	O
A	8.17	5.12	8.38	15.29	–	63.04
B	8.95	3.68	7.31	18.17	–	61.89
C	6.31	12.58	20.74	–	–	60.37
D	10.13	6.99	7.53	8.13	9.86	57.36
E	9.34	5.48	7.25	16.46	–	61.47
F	7.18	13.54	20.35	–	–	58.93
G	6.05	13.26	18.78	–	–	61.91

. The elements in Regions A and B are identical, including La, Ti, Ba, V and O. In combination with the above XRD result and the surface SEM analysis, it could be confirmed that the two sub-layers are composed of Ba₃V₄O₁₃, LaVO₄ and TiO₂. The different contrast of the two sub-layers might be attributed to the difference in the Ba₃V₄O₁₃ content. In region C, Ba, La, Ti and O elements are detected, with no evidence of V, implying that it has not been attacked by V₂O₅ molten salt.

Figure 4 shows the cross-sectional image of the BaLa₂Ti₃O₁₀ sample after hot corrosion in Na₂SO₄ + V₂O₅ salt at 900 °C for 4 h. Being quite different from the case of V₂O₅ corrosion, the reaction layer resulting from Na₂SO₄ + V₂O₅ corrosion exhibits a porous structure and has a larger thickness (∼15 μm). At the bottom of the layer, a continuous, grey-contrasted thin band could be observed. Beneath this band, no molten salt trace could be observed, and the bulk keeps structure integrity, suggesting that the molten salt infiltration has been arrested. Figure 4b shows the reaction layer at a higher magnification. EDS analysis results of regions D–G are presented in Table 2. Region D contains La, Ti, Ba, S, V and O elements. Combining with the above XRD result and the surface SEM analysis, this region is determined to consist of BaSO₄, LaVO₄ and TiO₂. Region E contains Ba, La, Ti, V and O elements. Regions F and G have a close chemical composition, including Ba, La, Ti and O elements. This suggests that they are BaLa₂Ti₃O₁₀ bulk.

Based on the above observations, it could be find that molten salts penetration in BaLa₂Ti₃O₁₀ thermal barrier ceramics could be arrested, and the inner regions of the samples are free from the attack by molten salts. The rationale behind this is the formation of a dense layer on the sample surface resulting from the reaction between BaLa₂Ti₃O₁₀ and the molten salts. The reaction could be understood in terms of the breakdown of the chemical bonds in the crystal by molten salts attack [17,31]. From the viewpoint of crystallography, BaLa₂Ti₃O₁₀ crystal could be seen as a tri-perovskite [La₂Ti₃O₁₀] layer separated by a Ba layer along c-axis [30]. Since the Ba insertion layers are poorly bonded, they are easier to be attacked by molten salts compared with [La₂Ti₃O₁₀] layers. It is thus possible that Ba-O bonds in BaLa₂Ti₃O₁₀ would first break in the presence of the molten salts, resulting in the formation of Ba contained corrosion products. Due to the consumption of Ba, La and Ti are enriched in the crystal, which provides a great chance for the molten salt to destroy La-O and Ti-O bonds. As a result, corrosion products of LaVO₄ and TiO₂ are formed.

Note that the reaction layer on the sample surfaces has different thickness and structure in the two molten salts, which may be related to the type of the corrosion products. In the case of V₂O₅ molten slat corrosion, the corrosion products are Ba₃V₄O₁₃, LaVO₄ and TiO₂, and the reaction could be expressed as follow:

$$3\mathrm{B}\mathrm{a}\mathrm{L}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_{10}(\mathrm{s})+5{\mathrm{V}}_2{\mathrm{O}}_5(\mathrm{l})\to \mathrm{B}{\mathrm{a}}_3{\mathrm{V}}_4{\mathrm{O}}_{13}(\mathrm{s})+6\mathrm{L}\mathrm{a}\mathrm{V}{\mathrm{O}}_4(\mathrm{s})+9\mathrm{T}\mathrm{i}{\mathrm{O}}_2(\mathrm{s})$$

(1)

These reaction products have high melt temperatures and exist as solid states in this study. During the corrosion test, there are two processes, i.e., the penetration of the molten salts into the sample and its reaction with the sample. A reaction layer is formed on the sample surface when the reaction has a higher rate than that of the molten salts penetrating into the porous structure. The reaction layer resulting from V₂O₅ molten slat attack has a dense structure, which could effectively inhibit further penetration of the molten salt. Thus, it is reasonable to accept that V₂O₅ attacked BaLa₂Ti₃O₁₀ pellet has a thin reaction layer.

When the mixture of Na₂SO₄ and V₂O₅ salts are presented, they react with each other at high temperatures. At 900 °C, the reaction could be represented by the following expression [32]:

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

(2)

Then, the formed products react with BaLa₂Ti₃O₁₀. Note that there may exist another possibility, i.e., V₂O₅ or Na₂SO₄ reacts with BaLa₂Ti₃O₁₀ separately before the reaction Equation (2) occurs. We mixed Na₂SO₄ and BaLa₂Ti₃O₁₀ powders together at a weight ratio of 1:1, and annealed them at 900 °C for 4 h. XRD measurements were performed on the as-mixed powders and the sample after heat treatment, and the results are shown in Figure 5. In the XRD pattern of the as-mixed powders, only BaLa₂Ti₃O₁₀ and Na₂SO₄ phases can be detected. By comparison, one could find that the annealed powders exhibit similar XRD pattern appearance to that of the as-mixed powders, and no peak from BaSO₄ could be detected. This indicates that Na₂SO₄ and BaLa₂Ti₃O₁₀ do not react with each other at 900 °C.

After the formation of NaVO₃ and SO₃, the following reaction could occur:

$$\mathrm{B}\mathrm{a}\mathrm{L}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_{10}(\mathrm{s})+2\mathrm{N}\mathrm{a}\mathrm{V}{\mathrm{O}}_3(\mathrm{l})+\mathrm{S}{\mathrm{O}}_3(\mathrm{g})\to \mathrm{B}\mathrm{a}\mathrm{S}{\mathrm{O}}_4(\mathrm{s})+2\mathrm{L}\mathrm{a}\mathrm{V}{\mathrm{O}}_4(\mathrm{s})+3\mathrm{T}\mathrm{i}{\mathrm{O}}_2(\mathrm{s})+\mathrm{N}{\mathrm{a}}_2\mathrm{O}(\mathrm{s})$$

(3)

It has been reported NaVO3 has better fluidity than V2O5 at liquid state [33,34,35], thus it has a larger tendency to infiltrate to the sample along cracks/pores, causing the formation a thicker reaction layer, as shown in Figure 4b. Note that the reaction layer on BaLa₂Ti₃O₁₀ surface resulting from Na₂SO₄ + V₂O₅ molten slat attack is porous, which could be explained in terms of the participation of SO₃ gas during the reaction process. Some unreacted gas escapes from the bulk, leaving pores and resulting in a decrease in the SO₃ content with the increase of the depth. At a certain depth, no SO₃ exists and only NaVO3 reacts with BaLa₂Ti₃O₁₀. The reaction could be described as follow:

$$3\mathrm{B}\mathrm{a}\mathrm{L}{\mathrm{a}}_2\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_{10}(\mathrm{s})+10\mathrm{N}\mathrm{a}\mathrm{V}{\mathrm{O}}_3(\mathrm{l})\to \mathrm{B}{\mathrm{a}}_3{\mathrm{V}}_4{\mathrm{O}}_{13}(\mathrm{s})+6\mathrm{L}\mathrm{a}\mathrm{V}{\mathrm{O}}_4(\mathrm{s})+9\mathrm{T}\mathrm{i}{\mathrm{O}}_2(\mathrm{s})+5\mathrm{N}{\mathrm{a}}_2\mathrm{O}(\mathrm{s})$$

(4)

This causes the formation of a dense reaction layer, as shown in Figure 4b. The absence of BaSO₄ in this layer provides an evidence for this consideration. In the case of Na₂SO₄ + V₂O₅ slat corrosion, the dense reaction layer is formed at a deeper region from the sample surface. Thus, though BaLa₂Ti₃O₁₀ TBC candidate has good resistance to Na₂SO₄ + V₂O₅ slat corrosion, it does not perform as good as that in V₂O₅ slat.

4. Conclusions

Hot corrosion behavior of BaLa₂Ti₃O₁₀ thermal barrier oxide in V₂O₅ and Na₂SO₄ + V₂O₅ molten salts at 900 °C were investigated. After 4 h corrosion tests, a reaction layer formed on the sample surface, the phase constitution and structure of which depend on the type of the molten salt. In V₂O₅ molten salt, the layer consisted of LaVO₄, TiO₂ and Ba₃V₄O₁₃, with a dense structure and having a thickness of 8–10 μm. While in Na₂SO₄ + V₂O₅ molten salt, it contained LaVO₄, TiO₂ and BaSO₄, mainly exhibiting a porous structure, which could be attributed to the participation of SO₃ gas during the formation process of the reaction layer. At the bottom of the porous layer, there existed a dense, thin band, consisting of Ba₃V₄O₁₃, LaVO₄ and TiO₂. Beneath the dense layer or the band, no molten salt trace existed in the samples, indicating that further infiltration of the molten salts has been arrested. Based on this study, it could be concluded that though BaLa₂Ti₃O₁₀ ceramic as TBC candidate has good resistance in both salts, it shows better resistance to V₂O₅ salt corrosion.