Hot Corrosion Conditions Research Articles

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.

Low-temperature hot corrosion of arc evaporated Ti1−xAlxN on Ni-Cr-Co based superalloys

Cathodic arc evaporated Ti1−xAlxN coatings were deposited on a Nimonic c-263 superalloy and tested under low temperature hot corrosion conditions. Treated with a MgSO4/Na2SO4 salt-mixture, all samples were annealed at 700 °C in a 2800 ppm(volume) SOx-rich atmosphere for 1, 5, 15 and 30 h. A significantly reduced corrosion severity was exhibited by all Ti1−xAlxN variants over the uncoated NiCrCo alloy. A synergistic fluxing mechanism was found to be the dominant factor for the coating breakdown. Depending on the relative Al-to-Ti content on the Ti1−xAlxN sublattice, differences in the coatings’ corrosion and scaling behavior were observed.

Effect of chromium and silicon additions on the hot corrosion resistance of nickel aluminide coatings

The effects of chromium and silicon modification on the microstructure and hot corrosion resistance of slurry aluminide coatings applied on pure nickel was studied by in-depth microscopic analyses. The simple and the Cr-modified aluminide coatings provided limited protection in Type II hot corrosion conditions but the Si-modified coating exhibited outstanding resistance with no sign of attack after 300 h of isothermal exposure at 700 °C. The synergistic dissolution between NiO and Al2O3/Cr2O3 appears responsible for the onset of the corrosion attack for the simple and Cr-modified coatings while silicon is shown to extend the incubation period.

A microscopy study of nickel-based superalloys performance in type I hot corrosion conditions

ABSTRACT Alloy material selection for sustainable, efficient, and cost-effective use in components is a key requirement for both power generation and aerospace sectors. Superalloys are manufactured using a combination of different elements, selected carefully to balance mechanical performance and environmental resistance to be used in a variety of different service conditions. Therefore, a fundamental understanding of each element is critical to alloy design. In this paper, the interaction of alloy chemistry, particularly chromium as a corrosion-resistant element along with titanium and molybdenum, and their effect on alloys performance for the relevant gas turbine industries were discussed. Based on the findings, the single-crystal alloy is found to be a better corrosion resistant alloy exhibited higher corrosion resistance in comparison to polycrystal alloys and proved that microstructure has a significant impact on alloy performance. This study also established that molybdenum level in chromia former alloys can significantly enhance the corrosion damage.

Structure and durability evaluation of blast furnace slag coatings and thermal barrier coatings (TBCs) under high temperature conditions

A high temperature aggressive environment is a great challenge for the long lifetime of thermal barrier coatings (TBCs). In this study, the hot corrosion and isothermal oxidation behavior of blast furnace slag (BFS) and yttria stabilized zirconia (YSZ) coated TBC systems were systematically investigated under high temperature conditions. This study aims to analyze the evaluation of microstructural properties of thermally sprayed CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings before and after high temperature oxidation and hot corrosion tests. For this purpose, the coatings were isothermally oxidized at 900 °C for 8, 24, 50, and 75 h. Hot corrosion behavior of CoNiCrAlY/BFS and CoNiCrAlY/YSZ coatings deposited on Inconel 718 superalloy substrates were studied at 900 °C in a Na2SO4 and V2O5 molten salt environment. CoNiCrAlY/BFS and CoNiCrAlY/YSZ TBC systems exposed to Na2SO4+ V2O5 molten salt at 900 °C investigate the hot corrosion behavior. The microstructure of coatings, formation of thermally grown oxide (TGO) layer, and degradation of coatings during isothermal oxidation and hot corrosion conditions were investigated. The oxidation and hot corrosion mechanisms were discussed. BFS, could be a potential alternative for the YSZ ceramic top coating in TBCs. The TBC system with BFS can be preferred for oxidation damage due to its low cost. YSZ and BFS TBCs are highly resistant to Na2SO4 and V2O5 molten salt environment.

Evaluation of high temperature corrosion resistance of CrN, AlCrN, and TiAlN arc evaporation PVD coatings deposited on Waspaloy

During the regular operation of a petrochemical plant, the degradation and corrosion of some metallic parts frequently occur. These parts are exposed, from the oil extraction to refining processes, to an environment that includes strong acids or bases, under high pressures, and high temperatures, up to 760 °C. Waspaloy (UNS N07001) is a material currently applied to manufacture components exposed to hot corrosion. This study evaluates the high temperature corrosion behavior of CrN, AlCrN, and TiAlN coatings deposited by Arc Evaporation Physical Vapor Deposition (PVD) process onto Waspaloy base material. The AlCrN and TiAlN PVD coatings showed higher hardness, 28 GPa (TiAlN) and 33 GPa (AlCrN), and elastic modulus than the CrN coating. The AlCrN coating presented the lowest corrosion rate values, 1.35 × 10−5 mg2·cm−4·s−1, while TiAlN and CrN showed higher corrosion rate values, 1.66 × 10−5 and 2.09 × 10−4 mg2·cm−4·s−1 under 700 °C hot corrosion condition. The use of X-ray diffractograms, and Raman spectroscopy, to evaluate the coating after hot corrosion tests based on chemical composition indicated that AlCrN and TiAlN coatings formed Al2O3 and TiO2 on the surface, which was responsible for their higher corrosion resistance.

A systematic study on Na2SO4-induced hot corrosion behavior of plasma-sprayed La2(Zr0.75Ce0.25)2O7 coating

To better understand the hot corrosion behavior of LCZ ceramic coatings against Na2SO4 molten salts, three series of hot corrosion tests were performed from the perspectives of Na2SO4 content, corrosion duration and corrosion temperature. Different corrosion products of La2O2SO4, La2O2S and Ce7O12 were identified depending on the hot corrosion conditions and the probable reaction mechanisms were proposed. The effects of Na2SO4 content, corrosion duration and corrosion temperature on hot corrosion behavior were investigated systematically. Results show that Na2SO4 molten salt is hard to react chemically with LCZ coating. The chemical reaction between LCZ coating and Na2SO4 molten salt needs a long time. When the content of Na2SO4 salts is high, the corrosion reaction could be accelerated. The phase transition and decomposition of corrosion products are responsible for the different hot corrosion behaviors at 1050–1500 °C. Moreover, a model describing the hot corrosion process induced by Na2SO4 molten salt is built, which includes two stages of eroding LCZ coating.

Diffusion Aluminide Coatings for Hot Corrosion and Oxidation Protection of Nickel-Based Superalloys: Effect of Fluoride-Based Activator Salts

The influence of two different fluoride-based activator salts (NH4F and AlF3) was studied for diffusion aluminide coatings obtained via pack cementation on a Ni-based superalloy (René 108DS). The resistance to oxidation and hot corrosion was assessed as a function of the concentration of activator salts used during the synthesis process by means of pack cementation. Two different concentrations were selected for activator salts (respecting the equimolarity of fluoride in the synthesis) and the obtained diffusion coatings were compared in terms of morphology, thickness and composition, as well as in terms of microstructural evolution after high temperature exposure. Isothermal oxidation tests were conducted at 1050 °C in air for 100 h in a tubular furnace. The oxidation kinetics were evaluated by measuring the weight variation with exposure time. The microstructural evolution induced by the high temperature exposure was investigated by SEM microscopy, EDS analysis and X-ray diffraction. Results showed that the coatings obtained with AlF3 activator salt are thicker than those obtained using NH4F as a consequence of different growth mechanism during pack-cementation. Despite this evidence, it was found that the NH4F coatings show a better oxidation resistance, both in terms of total mass gain and of quality of the microstructure of the thermally grown oxide. On the other hand, coatings produced with high concentration of AlF3 exhibited a better resistance in hot corrosion conditions, showing negligible mass variations after 200 h of high temperature exposure to aggressive NaCl and Na2SO4 salts.

Hot corrosion properties of plasma sprayed La2Ce2O7/YSZ vis-à-vis La2Ce2O7/cluster paired zirconia thermal barrier coatings

Bilayered thermal barrier topcoats have lead to a remarkable breakthrough in overcoming the disadvantages of the conventional yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The present work is devoted to studying the hot corrosion properties of two different bilayered ceramic topcoats, viz., (i) lanthanum cerium oxide with the formula La2Ce2O7 (LCO)/YSZ and LCO/cluster paired zirconia (gadolinia, dysprosia and yttria stabilized zirconia -(Gd,Dy,Y)SZ) plasma sprayed TBCs. Plasma spray grade YSZ and (Gd,Dy,Y)SZ are prepared by co-precipitation and solution combustion techniques as reported in the literature. The effect of thickness of the plasma sprayed top LCO layer in bilayered LCO/YSZ and LCO/(Gd,Dy,Y)SZ TBC systems under Type I hot corrosion condition is examined. The LCO/(Gd,Dy,Y)SZ bilayered coating with 100 μm LCO thickness exhibits better hot corrosion resistance than the LCO/YSZ system. The phase transformation of t′-ZrO2 to m-ZrO2 in LCO/(Gd,Dy,Y)SZ (100/150 μm) TBC is reduced markedly (60%) compared to LCO/YSZ (100/150 μm) TBC.

Atypical decomposition processes of Sm2Zr2O7 + 8YSZ dual-phase TBCs during hot corrosion

The accelerated decomposition of an Sm2Zr2O7+8YSZ system in the form of air plasma spraying (APS) thermal barrier coatings of composites, double ceramic layer (DCL), and functional graded system (FGS) types under hot corrosion conditions were analyzed. Hot corrosion tests were performed using liquid sodium sulfate salt deposits and then X-Ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersion spectroscopy (EDS) were performed to identify the reaction products. The obtained data indicated that negligible amounts of sulfates or oxysulfates were formed, but significant decomposition of pyrochlore Sm2Zr2O7 to the fluorite phase was detected. These phenomena were also observed in a much higher temperature range during pure oxidation tests, but the lowest decomposition temperature (920 °C) was observed in the sodium sulfate assisted process. The described observations were confirmed during similar tests and realized using model powder mixtures. Microstructural analysis of thermal barrier coatings (TBC) cross-sections revealed a complex decomposition process that was localized to areas near the Sm2Zr2O7 splats. The decomposition of the observed pyrochlore phase was mainly related to interactions between Sm2Zr2O7 and 8YSZ in accordance with the fluxing mechanism typical for the hot corrosion process of Ni-based superalloys. Additionally, the effect of eutectic Na-Sm-O was determined.

An investigation on oxidation behaviour of high velocity oxy-fuel sprayed Inconel718-Al2O3 composite coatings

The grey cast iron (GCI) components such as exhaust manifolds, turbocharger housings, and furnace parts are subjected to high temperature oxidation. GCI alone cannot retain its mechanical strength at elevated temperatures due to this high temperature oxidation. The composite coatings can effectively be used to counter the high temperature oxidation induced degradation of the GCI components. In the present work, Inconel718-Al2O3 (INAL) based composite coatings were deposited on GCI substrate by using high velocity oxy-fuel (HVOF) spray process. The effects of varying Al2O3 contents (10, 20, and 30 wt%) on the microstructure, porosity, micro-hardness, and oxidation resistance of Inconel718 (IN718) coating were studied. High temperature cyclic oxidation testing was conducted on these coatings and bare GCI substrate in a laboratory tube furnace at 900 °C in air. The kinetics of oxidation were determined by using weight change measurements of the bare GCI and coated specimens at regular intervals for a duration of 50 h. The important phases present in the feedstock powders, deposited coatings, and the oxidised specimens were identified by using X-ray diffraction (XRD). The surface morphology and chemical compositions of the coated and tested specimens were determined by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The microhardness of IN718 coating was found to be improved with an increase in alumina contents and a maximum average hardness of about 801 ± 40 HV0.2 was observed in case of IN718-30 wt% Al2O3 composite coating. The increase in alumina contents resulted in an increase in porosity and proportion of un-melted feedstock powder particles in the IN718 coating. The coating with 30 wt% alumina contents showed the maximum resistance to oxidation with a minimum weight gain of 1.29 mg/cm2 as compared to the other coatings. The improved oxidation of composite coatings can be attributed to the formation of protective phases such as Al2O3, NiCr2O4 and Cr2O3 on its exposed surface during the hot corrosion condition.

Effect of surface treatment and crystal orientation on hot corrosion of a Ni-based single-crystal superalloy

The effect of surface preparation and crystal orientation was investigated for a TMS-138 single crystal superalloy under Type I and Type II hot corrosion conditions. The superalloy was cut along the {100} and {110} planes on which grit blasting, mechanical polishing and electro-polishing surface finishes were conducted. At 900 °C grit blasting or electropolishing distorted the surface and altered the γ-γ’ structure of the alloy provoking higher corrosion rates than on mechanically polished surfaces. The electropolished surface revealed that crystal planes with higher atomic number density have an increased corrosion resistance. The anisotropy effects were not observed with Type II exposures.

Analysis of combined static load and low temperature hot corrosion induced cracking in CMSX-4 at 550°C

A CMSX-4 3-point bend specimen was statically loaded under hot corrosion conditions and SEM, (S)TEM and EDX techniques were used to analyse the cracking generated. Sulphur, chlorine, sodium and oxygen were found at the crack tip, and an influence of loading on the corrosion mechanism’s preference to interact with either the γ or γʹ was observed. The microscopy analysis is in support of the corrosive mechanism being a combined stress and electrochemical corrosion linked with low temperature hot corrosion, where crack propagation occurs as a result of localised corrosion enhanced material degradation. High magnification EDX mapping identified W as segregating to the γʹ at room temperature.

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.

Behaviour of Alloy 617 OCC Under Hot Corrosion Conditions Encountered in Boilers in A-USC Power Plants

Superheater and reheater tubing constitute very critical parts of the boilers operating in advanced ultra-supercritical (A-USC) power plants. They are subjected to severe hot corrosion conditions during operation. The operating temperature and pressure in these power plants are very arduous, and materials with high resistance to hot corrosion and creep are required to produce the said tubing. Increased use of superalloys comes into picture in A-USC power plants, because of the demanding operating conditions prevailing there. Inconel 617 has been a superalloy developed for use in the form of superheater and reheater tubing seen in the power plants operating in A-USC environments. There have been researches aiming at improving its creep resistance, and versions with optimum chemical composition have been developed. Alloy 617 OCC, an optimized version of the superalloy 617, has been produced as a candidate material for superheater and reheater tube manufacturing in the context of development of A-USC power plant technology in India. However, hot corrosion issues are equally important for such applications. In light of the foregoing, studies have been carried out to analyse the performance of Alloy 617 OCC in hot corrosion conditions in simulated A-USC power plant environments. Oxidation tests for the alloy were conducted in the laboratory air environment under identical temperature/time conditions for the sake of comparison. Hot corrosion process was examined using thermo-gravimetry, and the corrosion products were characterized using a variety of material characterization techniques. The study reported the comparative corrosion performance of the superalloy in the two environments.

Hot Corrosion of Alloy 617 OCC in Simulated USC Power Plant Environment

Alloy 617 OCC, a variant of INCONEL 617 with optimised chemical composition, has been produced in India for manufacture of superheater and reheater tubing in boilers operating in advanced ultrasupercritical (A-USC) power plants. The tubing encounters intense hot corrosion conditions during service. The present study deals with hot corrosion behaviour of 617 OCC in A-USC environment. The environment occurring in A-USC plants was simulated in the laboratory by exposing the material coated with a mixture of salts at 700°C in a flowing gas mixture. For use in A-USC boiler technology, the metal loss due to fireside corrosion of the material should be less than 2 mm in 200,000 hours. The loss obtained in the present study was nearly 5 times this value. The corrosion processes were studied using SEM/EDAX, XRD and thermogravimetry. The degradation mechanisms coming into play, disqualifying the material for use in A-USC plants, would be discussed.

Mechanisms of hot corrosion of pure nickel at 700°C: Influence of testing conditions

The hot corrosion behaviour of pure nickel was investigated at 700 °C to highlight the influence of testing conditions (composition of the atmosphere and presence or not of a Na2SO4 deposit) on the mechanisms of hot corrosion. The influence of SO2/SO3 inward transport through the porous NiO growing scales was studied by exposing nickel in synthetic air and in synthetic air + 0.5% SO2/SO3 without Na2SO4. The addition of Na2SO4 decreased the pO2 and the pSO3 at the sample surface, which promoted the formation of Ni-S liquid solution at the metal/scale interface. A sufficient pSO3 was required to stabilize liquid Na2SO4-NiSO4 solutions and simulate Type-II hot corrosion conditions.

Impact of Deposit Recoat Cycle Length on Hot Corrosion of CMSX-4

Hot corrosion causes significant problems for both aerospace and power generation industries, where the combination of high temperature, corrosive gases, and contaminants severely limits component operating lifetimes in gas turbine hot gas streams. Multiple laboratory testing methodologies exist to study this hot corrosion, and these can be affected by a range of variables. This paper investigated the impact of varying deposit recoat cycle length when using the ‘deposit recoat’ testing method. CMSX-4 samples were exposed to simulated type II (pitting) hot corrosion conditions, with the same overall deposit load (averaged across the total exposure run), but different deposit recoat cycles. Post-exposure, samples underwent dimensional metrology analysis to compare metal loss resulting from different deposit recoat cycle lengths. Results for CMSX-4 suggest very small differences in corrosion losses, indicating CMSX-4 hot corrosion datasets obtained from deposit recoat experiments with different deposit recoat cycle lengths can be compared with confidence.

Fatigue behavior of uncoated and MCrAlY-coated DS nickel-based superalloys pre-exposed in hot corrosion condition

The effects of a MCrAlY coating on low-cycle fatigue (LCF) behavior of directionally solidified (DS) nickle-based superalloy DZ125 were investigated. Before the fatigue testings, the specimens were pre-exposed in high-temperature hot corrosion (HTHC) environment generating by a burner rig at 850 °C. The results show that the coating in hot corrosion condition has beneficial effects on the fatigue resistance of superalloy. Under corrosion condition, the MCrAlY-coated specimens tested have higher fatigue lives than the uncoated specimens at the same stress level. The coating failure results from fatigue process and numerous fatigue cracks were nucleated at the specimen surface, only one main crack propagates inward and the secondary cracks away from the fracture surface are perpendicular to the loading orientation.

Low-cycle fatigue behavior of a directionally solidified Ni-based superalloy subjected to gas hot corrosion pre-exposure

The influence of gas high-temperature hot corrosion (HTHC) pre-exposure on low-cycle fatigue (LCF) behavior was characterized for the directionally solidified (DS) Ni-based superalloy DZ125. Fatigue tests were carried out at 850 °C in the pre-exposed and unexposed specimens for 2, 15 and 25 h. Experimental results show that the porous corrosion scale and γ′-depleted layer formed in gas hot corrosion condition alter the crack initiation mechanisms of the superalloy. Fatigue cracks of the pre-exposed specimens originate from multiple surface locations where spalling of the corrosion products occur, while nucleation of unexposed specimen begins in the defects close to the surface. There is a significant reduction in LCF behavior for pre-exposed specimens in comparison with unexposed specimens.