Penetration Of Molten Salt Research Articles

Towards enhanced corrosion resistance of PS-PVD TBCs in marine environments by structural design

Thermal barrier coatings (TBCs) prepared by plasma spray-physical vapor deposition (PS-PVD) applied in marine environments encounter a critical challenge of corrosion-induced failure. In this paper, yttria-stabilized zirconia (YSZ) TBCs with different microstructures were prepared via PS-PVD by varying powder feeding rates to investigate the influence of microstructural modifications on corrosion resistance. The effects of structural gradient, initial deposition mechanism, and pore distribution on corrosion resistance were explored. Additionally, the inhibition mechanisms of capillary effect and chemical reaction on molten salt penetration were confirmed. Finally, a microstructure gradient strategy for PS-PVD TBCs was proposed. The structurally graded TBCs tailored using this strategy exhibited excellent corrosion resistance, with the molten salt corrosion resistance surpassing that of conventionally processed PS-PVD TBC by over 50 %. These outcomes highlight the promising potential of the microstructure gradient strategy for enhancing the corrosion resistance of PS-PVD TBCs.

Molten salt corrosion behavior of laser remelted PS-PVD YSZ thermal barrier coatings

Molten salt from the service environment leaches Y2O3 from yttria-stabilized zirconia (YSZ), threatening the security of thermal barrier coatings (TBCs). In this study, we compared the molten salt corrosion resistances of YSZ coatings prepared via plasma-spraying physical vapor deposition (PS-PVD) before and after laser remelting. At 1000 °C, the molten salt reacted with YSZ particulates in the PS-PVD YSZ coating and infiltrated the gaps between quasi-columnar crystals, resulting in phase transition of the coating and column breakage. A dense glazed layer was formed on the YSZ coating surface after remelting by the low-energy-density laser, which prohibited the penetration of molten salt and improved the hot corrosion resistance of the YSZ coating. By contrast, the failure of the high-energy-density laser-treated YSZ coating was accelerated due to the significant interfacial stress from the thick glazed layer and volume expansion caused by the corrosion products. Our investigation revealed the structural evolution of the PS-PVD YSZ coating under hot corrosion conditions, emphasized the role of laser remelting, and could serve as a guideline for TBCs modification.

Hot corrosion mechanism and thermal cycling performance of air-plasma-sprayed LaYbZr2O7 thermal barrier coatings in the vanadate-containing molten salts

LaYbZr2O7 has been regarded as a promising thermal barrier coatings (TBC) material due to its excellent mechanical and thermal properties. In this study, the LaYbZr2O7 coating was prepared by atmospheric plasma spraying (APS), and its hot corrosion mechanism and thermal cycling performance in vanadate-containing molten salts (V2O5 and Na2SO4 + V2O5) were investigated. Results indicated the hot corrosion reaction of LaYbZr2O7 coatings in molten V2O5 was temperature dependent, attributed to the thermal decomposition of ZrV2O7 above 800 °C. A continuous dense corrosion layer could form on the top surface of LaYbZr2O7 coatings after hot corrosion in molten V2O5 salt, effectively preventing the further penetration of molten salt. However, the dense corrosion layer could not form when exposed to Na2SO4 + V2O5 molten salts at 1000 °C, since the formation of NaVO3 could improve the mobility of molten salts and slow the precipitation reaction rate of corrosion products. Furthermore, thermal cycling tests indicated that the Na2SO4 + V2O5 mixture could be more detrimental to LaYbZr2O7 coatings than single V2O5 salt, and the dense corrosion layer could act as a sacrifice layer to protect the inner coating from corrosion degradation. The hot corrosion mechanism of LaYbZr2O7 was discussed in detail based on the Lewis acid-base and dissolution-precipitation rule.

Hot corrosion behavior of EB-PVD YSZ coatings treated by nanosecond pulsed laser

In this study, yttria-stabilized zirconia (YSZ) fabricated by electron beam physical vapor deposition (EB-PVD) were treated by a nanosecond pulsed laser (here called laser micro glazing, LMG). The hot corrosion behavior of YSZ coatings was systematically investigated in the air at 1100 °C with a mixture of 55 % V2O5 and 45 % Na2SO4 for 20 h. The results show that the LMG process can produce a dense and smooth LMGed layer (~0.25 μm) with extremely narrow cracks (~0.15 μm). The phase composition (tetragonal ZrO2) of LMGed 8YSZ does not change at the rapid cooling rate. Nevertheless, it shows a preferred orientation from (200) to (101). Due to the presence of the LMGed layer, the penetration of molten salt is limited during the hot corrosion test. A sintered layer ~10 μm is formed under the top region of LMGed coatings and becomes a second barrier layer. The volume fraction of m-ZrO2 in LMGed coatings decreases from 65.43 % to 35.64 %, and the t-ZrO2 is closely spaced. The YVO4 is also diminished on the surface and between inter columnar gaps, which demonstrates the better hot corrosion resistance.

Mechanisms of crack healing in dense Yb-Si-O environmental barrier coatings by plasma spray-physical vapor deposition

Due to high thermal stress level and low strain tolerance, through-thickness cracks are generally observed in plasma-sprayed environmental barrier coatings (EBCs) with dense microstructures, providing shortcuts to water vapor volatilization and molten salt penetration during service in combustion environments of gas turbines. In this study, crack-free and dense Yb-Si-O EBCs were successfully prepared by plasma spray-physical vapor deposition (PS-PVD) and annealing treatment. Closure and healing of through-thickness crack were observed via scanning electron microscopy, while phase transformation and sintering behavior were characterized by X-ray diffraction and nanoindentation. Furthermore, crack healing mechanisms of dense PS-PVD EBCs were elucidated utilizing a microstructure-based finite element model with physically-based constitutive relation. This work provides a foundation for the development of high-performance EBCs and thermal/environmental barrier coating (T/EBC) systems.

Microstructural study and hot corrosion behavior of bimodal thermal barrier coatings under laser heat treatment

In this study, nanostructured YSZ powders were deposited on the Hastalloy X Superalloy substrate coated with a metallic bond coat by plasma spraying to produce a nanostructured thermal barrier coating with bimodal microstructure. After that, the coated samples were heat-treated using a Nd:YAG laser. Then, the microstructures of the conventional and nanostructured TBCs before and after the laser glazing process were examined using a scanning electron microscope (SEM). The coating phases were studied by X-ray diffractometry (XRD). The high-temperature corrosion behavior of the nanostructured plasma sprayed coating in the presence of Vanadium pentoxide and Sodium sulfate molten salt was compared with that of the conventional coatings before and after laser treatment at 1050 °C. The hot corrosion results showed that the coatings had a similar degradation mechanism based on which the corrosive molten salt reacted with the stabilizer of YSZ, producing hot corrosion products such as YVO4. It led to an unwanted phase transformation from tetragonal (t) to monoclinic (m) Zirconia and the final degradation of the TBC system. However, reducing molten salt penetration, decreasing surface roughness and, reduction of the specific surface area are three important mechanisms that improved hot corrosion resistance, finally extending the lifetime of the glazed samples. The results also showed that the nanostructured TBC had higher hot corrosion resistance in comparison with other samples.

Microstructure modification of Y2O3 stabilized ZrO2 thermal barrier coatings by laser glazing and the effects on the hot corrosion resistance

Y2O3 stabilized ZrO2 (YSZ) thermal barrier coatings (TBCs) are prone to hot corrosion by molten salts. In this study, the microstructure of atmospheric plasma spraying YSZ TBCs is modified by laser glazing in order to improve the corrosion resistance. By optimizing the laser parameters, a ∼18 µm smooth glazed layer with some vertical cracks was produced on the coating surfaces. The as-sprayed and modified coatings were both exposed to hot corrosion tests at 700 and 1000 °C for 4 h in V2O5 molten salt, and the results revealed that the modified one had improved corrosion resistance. After hot corrosion, the glazed layer kept structural integrity, with little evidence of dissolution. However, the vertical cracks in the glazed layer acted as the paths for molten salt penetration, accelerating the corrosion of the non-modified coating. Further optimization of the glazed layer is needed in the future work.

Hot Corrosion Behavior of YSZ Thermal Barrier Coatings Modified by Laser Remelting and Al Deposition

Thermal barrier coatings that are widely used to protect the turbine blades of gas turbines from failure are subjected to hot corrosion conditions. The present study aims to improve their hot corrosion resistance by modifying the plasma-sprayed yttria-stabilized zirconia topcoat by laser remelting and Al deposition. First, a dense remelted layer with some vertical cracks was obtained by laser remelting. Then, Al film was deposited on the surface of laser-remelted coatings by magnetron sputtering, and the dense α-Al2O3 overlay was in situ synthesized through the reaction of Al and ZrO2 during vacuum heat treatment. Hot corrosion tests of all coatings were conducted in a mixture of 55 wt.% V2O5 and 45 wt.% Na2SO4 corrosive salts at 1000 °C for 30 h. The defects such as pores and microcracks in the as-sprayed coatings provided paths for molten salt penetration, which led to the worst hot corrosion resistance of the as-sprayed coatings. The minimal surface roughness of the dense laser-remelted coatings was the main reason for improving the hot corrosion resistance. However, the resistance of laser-remelted coatings to hot corrosion was significantly improved when they were modified by Al deposition, which is mainly due to the reduced reaction surface between molten salts and yttria stabilizer. The corresponding hot corrosion mechanism was discussed.

Enhanced Molten Salt Resistance by Sidewall Pores Repair during Fs Laser Drilling of a Thermal Barrier-Coated Superalloy.

In this study, a novel laser-modified drilling method was used to manufacture cooling holes through thermal barrier coatings (TBCs). Due to the “cooling processing” properties during low-frequency femtosecond (LF-fs) laser drilling, the exposure of the sidewall pores, and the interlayer clearance, the inherent characteristics of plasma-sprayed coatings induced sidewall defects in the drilled holes. After drilling, a high-frequency fs (HF-fs) laser was used to repair the sidewall pores and interlayer clearance of the drilled ceramic holes. Then, the pores and microcracks were healed by local melting using the laser. Moreover, instead of obtaining laser-induced periodic surface structures (LIPSSs), refined and homogeneous grains were produced by the HF-fs laser repair treatment at high transient pressure and temperature. The results from a high-temperature corrosion test showed that healing of the open pores and microstructural improvement in the ceramic hole walls prevented the out-diffusion of Y2O3 stabilizers and the penetration of molten salt, resulting in less corrosive products and producing corresponding phase-transformation stress. Thus, reducing the stabilizer consumption can moderate corrosion fatigue and prolong the lifetime of a cooling hole and TBCs under service.

Molten Salt Corrosion Resistance of Yttria Stabilized Zirconia Coating with Silicon Carbide Interlayer on High Density Graphite

The high temperature corrosion behaviour of yttria stabilized zirconia (YSZ) coated high density graphite (HDG) with and without SiC interlayer was evaluated in LiCl–KCl molten salt at 600 °C. The YSZ coated HDG samples without interlayer were tested for 50, 250, 1000 and 2000 h and the YSZ coated HDG samples with SiC interlayer were studied for 50, 500, 2000 and 3500 h durations. Characterisation of the corrosion tested samples revealed no delamination or failure of the YSZ coating in both the cases, and insignificant weight gain was observed in all the tested samples in the molten salt. The lamellar morphology of the plasma spray coating was intact even after exposure to molten salt for long duration, implying no corrosion attack of the coating and HDG in the molten salt. SEM cross-section examination confirmed neither coating degradation, nor penetration of molten salt into the HDG substrate. X-ray diffraction and Laser Raman spectroscopic analyses ascertained that no phase change occurred in YSZ after the long exposure in molten salt. Present study confirmed the phase stability and durability of YSZ coating in providing corrosion resistance and protection to HDG for high-temperature molten salt applications.

Hot corrosion and mechanical properties of nanostructured Al2O3/CSZ composite TBCs

The purpose of this research is to improve the quality and efficiency of yttria-stabilised zirconia (YSZ) thermal barrier coating (TBC) through replacing CSZ. Three types of top coats including conventional YSZ, CSZ and nanostructured Al2O3/CSZ composite were applied, by thermal spray technique, on IN738LC base metal and the results were compared to one another. In the present investigation, hot corrosion behaviours of the coatings were tested in a furnace containing a mixed environment of Na2SO4 and V2O5 salts at 1050°C. Nanoindentation tests were used to evaluate mechanical properties of the coatings. The experimental results confirmed that nanostructured Al2O3/CSZ composite coating had much hot corrosion resistance than those of conventional TBCs. The transformation of tetragonal zirconia to monoclinic ZrO2 and formation of YVO4 and CeVO4 crystals as hot corrosion products led to the degradation of the mentioned TBCs. Microstructural analysis indicated that the penetration of molten salt was much less in the nanostructured Al2O3/CSZ composite coating where the infiltration of both oxygen and aggressive molten salt was reduced. Mechanical results showed that the superior mechanical properties of nanostructured Al2O3/CSZ composite coating may be attributed to its structural stability.

Preparation of ultrafine-grain graphite by liquid dispersion technique for inhibiting the liquid fluoride salt infiltration

In this work, the mixtures of coke aggregates and coal tar pitch were used as starting materials to prepare ultrafine-grain graphites (UGG) by a liquid mixing method for the inhibition of molten salts in molten salt reactor (MSR). The average diameters of coke aggregates for UGG-1 and UGG-2 were respectively 10 and 5 μm. The liquid dispersion technique could effectively prevent fine particles of raw materials from being conglomerated. Microstructure, thermophysical and other properties of the obtained graphite were studied. A high-pressure reactor was constructed to evaluate the molten salt penetration in the graphite. Compared to isostatic graphite (IG-110,TOYO TANSO CO., LTD.), UGG-2 possessed excellent properties such as high bending strength of 81.0 ± 2.5 MPa, compressive strength of 133 ± 3 MPa, low porosity of 14.0 ± 0.1% and an average pore diameter of 0.52 ± 0.01 μm. The fluoride salt volume occupation of IG-110 and UGG-1 under 1.5 atm was 14.4% and 3.7%, respectively. While the volume occupations of UGG-2 under 1.5 atm was only 0.17%, and continued to be steady even up to 5 atm with the increase of 0.45%, less than the critical index proposed for MSR of 0.5%. The UGG-2 could effectively inhibit liquid fluoride salt infiltration and was a promising candidate as neutron moderator or reflector materials for MSR.

Resistance of Nanostructured Environmental Barrier Coatings to the Movement of Molten Salts

Corrosion of components in a recovery boiler is a major problem faced by the pulp and paper industry. The superheater tubes become severely corroded due to the presence of sulfidic gases in the boiler and molten salts which are deposited on the surface of the tubes. As a result, the boiler must be decommissioned for expensive maintenance and repairs. Yttria-stabilized zirconia (YSZ) coatings have been shown to provide corrosion resistance when applied on gas turbines operating at high temperatures. Air plasma-sprayed YSZ environmental barrier coatings on Type 309 stainless steel were exposed to three different corrosive environments: Test A—600 °C, salt vapors, flue gases, 168 h; Test B—600 °C, molten salt, air, 168 h; and Test C—600 °C, molten salt, flue gases, 168 h. Two different types of YSZ coatings—conventional YSZ and nanostructured YSZ—were tested to study their resistance to corrosion and molten salt penetration. The performances of both types of coatings were evaluated, and a comparative study was conducted. It was found that the nanostructured YSZ samples protected the stainless steel substrate better than their conventional counterparts. This superior performance was attributed to the presence of semi-molten nano-agglomerates present in the coating microstructure, which acted as collection points for the penetrating molten salts.

Hot corrosion control in plasma sprayed YSZ coating by alumina layer with evaluation of microstrcuture and nanoindentation data (H, E)

Thermal barrier coatings (TBCs), which contain yttria stabilized zirconia (YSZ) are used as thermal insulation coatings on hot section components in gas turbine in order to increase operation temperature for higher efficiency. Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which comes as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of two types of plasma sprayed TBCs was evaluated: a) usual YSZ (yttria stabilized zirconia) and b) layer composite of (YSZ/Al2O3) in which Al2O3 was as a top coat on YSZ layer. Hot corrosion tests were carried out on the surface of coatings in molten salt (Na2SO4 + V2O5) at 1050 °C for 40 h. Spallation was observed in YSZ coating due to formation of monoclinic ZrO2 and YVO4 crystals as hot corrosion products. Alumina coating as outer layer in YSZ/Al2O3 reduced the penetration of molten salt into YSZ and resulted in the further resistance of TBC against hot corrosion. Also nanoindentation tests demonstrated that the hardness (H) and elastic modulus (E) of the usual TBC increased substantially after hot corrosion whereas these properties exhibited the less variations in YSZ/Al2O3.

A comparative study on hot corrosion resistance of three types of thermal barrier coatings: YSZ, YSZ + Al 2O 3 and YSZ/Al 2O 3

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBC s) which comes as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: (a) usual YSZ (yttria-stabilized zirconia), (b) particle composite of (YSZ + Al 2O 3) and (c) layer composite of (YSZ/Al 2O 3) in which Al 2O 3 was as a topcoat on YSZ layer. Hot corrosion tests were carried out on the surface of coatings in molten salt (Na 2SO 4 + V 2O 5) at 1050 °C for 40 h. Spallation was observed in YSZ and (YSZ + Al 2O 3) coatings. The formation of monoclinic ZrO 2 and YVO 4 crystals as hot corrosion products caused the degradation of mentioned TBCs. Although monoclinic ZrO 2 decreased in (YSZ + Al 2O 3) in comparison with usual YSZ, dendritic YVO 4 crystals had main role in degradation of (YSZ + Al 2O 3) coating. Alumina coating as outer layer in YSZ/Al 2O 3 reduced the penetration of molten salt into YSZ and resulted in the further resistance of TBC against hot corrosion, because the hot corrosion products considerably decreased in YSZ/Al 2O 3.

Plasma-sprayed yttria-stabilized zirconia coatings on type 316L stainless steel for pyrochemical reprocessing plant

Type 316L stainless steel (SS) is one of the candidate materials proposed for application in pyrochemical reprocessing plants. In the present work, yttria-stabilized zirconia coatings of 300 μm were applied over type 316L SS with a metallic bond coating of 50 μm by an optimized plasma spray process, and were assessed for the corrosion behaviour in molten LiCl–KCl medium at 873 K for periods of 5 h, 100 h, 250 h and 500 h. The as-coated and tested samples were examined by optical microscopy and SEM for homogeneity, penetration of molten salt through coating and corrosion of type 316L SS substrate. The results indicated that the yttria-stabilized zirconia coatings performed well without significant degradation and corrosion attack. Laser melting of the coated samples using CO 2 laser was attempted to consolidate the coatings. The development of large grains with segmented cracks was noticed after laser melting, though the coating defects have been eliminated.

Evaluation of laser-glazed plasma-sprayed thermal barrier coatings under high temperature exposure to molten salts

Thermal Barrier Coating (TBC) systems are frequently used in gas turbine engines to provide thermal insulation to the hot-section metallic components and also to protect them from oxidation, hot corrosion and erosion. Surface sealing treatments, namely laser-glazing, have been showing a high potential for extending in-service lifetimes of these systems by improving chemical and thermo-mechanical resistance. In this investigation, both as-sprayed and laser-glazed TBCs were exposed to hot corrosion in molten salts. The glazed coatings were obtained by scanning the surface of the plasma-sprayed coatings with either a CO 2 or a Nd:YAG laser. The hot corrosion investigation was accomplished by subjecting the specimens to an isothermal air furnace testing under V 2O 5 and/or Na 2SO 4 at a temperature of 1000 °C for 100 h. Spallation has been observed in coatings in the as-sprayed condition under V 2O 5 or V 2O 5 + Na 2SO 4. Na 2SO 4 itself had no or minimal effect on the degradation of the laser-glazed or as-sprayed condition coatings, respectively. The degradation in V 2O 5 was accomplished by destabilization of YSZ as a consequence of depletion of yttria from the solid solution to form YVO 4 and therefore led to the disruptive transformation of the metastable tetragonal phase to the monoclinic phase. Moreover, the presence of both corrosive salts induced the formation of large high aspect ratio YVO 4 crystals that introduced additional stresses and contributed to the degradation of the coatings. The laser-glazed specimens were not efficient in avoiding the molten salt penetration along the thickness direction due to the presence of cracks on the glazed layer. However due to a reduced specific surface area of the dense glazed layer, the corrosion reaction of the molten salts with the YSZ has been lower than in coatings in the as-sprayed condition.