An investigation on oxidation,hot corrosion and mechanical properties of plasma-sprayed conventional and nanostructured YSZ coatings

Conventional and nanostructured YSZ coatings were deposited on IN-738 Ni super alloy by atmospheric plasma spray technique. The oxidation was measured at 1100 °C and hot corrosion resistance of the coating was measured in a mixed salt of V₂O₅ and Na₂SO₄ at 1050 °C using an atmospheric electrical furnace. According to the experimental results, nanostructured coatings showed a better oxidation and hot corrosion resistance than conventional one. The improved oxidation resistance of the nanocoating could be explained by the change of structure to a dense and more packed structure in this coating. The improvement in hot corrosion resistance was not as good as the oxidation but much better than conventional coating. The thermo-mechanical properties of the coating were tested using thermal cycles, nanoindentation, and bond strength tests during which nanostructured YSZ coating again showed a better performance by structural stability.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

Hot corrosion behaviour of plasma sprayed YSZ/LaMgAl11O19 composite coatings in molten sulfate–vanadate salt

Hot corrosion studies of thermal barrier coatings (TBCs) with different YSZ/LaMgAl₁₁O₁₉ (LaMA) composite coating top coats were conducted in 50 wt.% Na₂SO₄ + 50 wt.% V₂O₅ molten salt at 950 °C for 60 h. Results indicate that TBCs with composite coating top coats exhibit superior oxidation and hot corrosion resistances to the TBC with the traditional YSZ top coat, especially for which has a LaMA overlay. The presence of LaMA can effectively restrain the destabilization of YSZ at the expense of its own partial degradation. The hot corrosion mechanism of LaMA coating and the composite coatings have been explored.     

Effect of an Enamel Coating on the Oxidation and Hot Corrosion Behavior of an HVOF-Sprayed Co–Ni–Cr–Al–Y Coating

The oxidation and hot-corrosion behavior of a Co–Ni–Cr–Al–Y coating produced by high-velocity oxygen fuel (HVOF) with and without an enamel coating were investigated in air at 900°C and in molten 75 wt.% NaCl+25 wt.% Na₂SO₄ at 850°C. The results show that the enamel coating possesses excellent hot corrosion resistance in the molten salt, in comparison with the HVOF-sprayed Co–Ni–Cr–Al–Y coating alone. In the hot-corrosion test, breakaway corrosion did not occur on the samples with the enamel coating and the composition of the enamel did not significantly change. The oxidation resistance of the Co–Ni–Cr–Al–Y coating, which had good adhesion, was also improved by the enamel coating.     

Microstructure, spallation and corrosion of plasma sprayed Al2O3–13%TiO2 coatings

The electrochemical corrosion behaviours of the steel substrates coated with three different plasma sprayed Al₂O₃–13%TiO₂ coatings were studied in this paper. The three kinds of Al₂O₃–13%TiO₂ coatings were conventional ME coating, nanostructured NP coating and NS coating. There were micro cracks, laminar splats and straight columnar grains in ME coating. For the two nanostructured coatings, the laminar microstructure and columnar grains were not obvious. The NP coating had the highest hardness and spallation resistance. Electrochemical corrosion behaviour of the three coatings was mainly investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in aqueous Na₂SO₄ solution.     

The structure and high temperature corrosion performance of medium-thickness aluminide coatings on nickel-based superalloy GTD-111

Simple and Si-modified aluminide coatings having medium-thickness (40-60 μm) have been applied on the superalloy GTD-111 by a slurry technique. Hot corrosion and cyclic oxidation performance of the uncoated and the coated superalloy were investigated by exposing samples to a molten film of Na₂SO₄-40 %wt NaVO₃-10%wt NaCl at 780 °C and 1 h cyclic oxidation at 1100 °C in air, respectively. The presence of silicon in the aluminide structure increased the oxidation resistance by a factor of 1.7 times. In addition, a SiO₂-containing scale, which formed on the Si-containing coating surface, was stable during of the hot corrosion testing.     

Stepwise Depletion of Coating Elements as a Result of Hot Corrosion of NiCrAlY Coatings

Present investigation deals with the hot corrosion behaviour of the NiCrAlY coatings deposited by HVOF technique on Superni76 under cyclic conditions at 900  °C in the presence of Na₂SO₄ + 60% V₂O₅ salt. The weight change behaviour of the coatings was followed with time up to 200 cycles and K _p value was calculated for the hot corrosion process. Surface and cross-section of the corroded samples were examined by FESEM/EDS and XRD to follow the progress of corrosion up to 200 cycles. In earlier cycles, the corrosive species oxidised top surface of the coatings. With increasing number of cycles, oxidation of the coatings occurred up to 40-μm depth. A Cr-depleted band was seen below the oxide scale. Further increase in number of cycles led to migration and oxidation of Al to form Al₂O₃ sublayer at coating/scale interface, thereby leading to formation of Al-depleted zone in the coating below the Al₂O₃ sublayer. The corrosion resistance of the NiCrAlY coatings is attributed to the formation of the continuous and dense Al₂O₃ sublayer at the coating/scale interface, which acts as barrier to the migration of Cr to the surface. The appearance of Al₃Y after 100 and 200 cycles also contributes to the increased corrosion resistance of coatings after 100 and 200 cycles.     

Characterization of the corrosion products of electrodeposited Zn, Zn–Co and Zn–Mn alloys coatings

The morphology, composition, phase composition and corrosion products of coatings of pure Zn (obtained from two types of electrolytic bath: an acidic bath (Zn_acid) and a cyanide-free alkaline bath (Zn_alkaline)) and of Zn–Mn and Zn–Co alloys on steel substrates were studied. To achieve this, diverse techniques were used, including polarization curves, atomic force microscopy (AFM), scanning electron microscopy (SEM), glow discharge spectroscopy (GDS), X-ray diffraction (XRD), and the salt spray test. In the salt spray test, the exposure time required for the coatings to exhibit red corrosion (associated with the oxidation of steel) decreased in the following order: Zn–Mn_(432h) > Zn–Co_(429h) > Zn_{alkaline(298h)} > Zn_acid(216h). The shorter exposure times required for corrosion of the pure Zn coatings are related to the coating composition and the crystallographic structure. Analysis of the corrosion products disclosed that Zn₅(OH)₈Cl₂·H₂O was a corrosion product of all of the coatings tested. However, the formation of oxides of manganese (MnO, Mn_0.98O₂, Mn₅O₈) in the Zn–Mn coating, and the formation of the hydroxide Zn₂Co₃(OH)₁₀·2H₂O in the Zn–Co coating, produced more compact and stable passive layers, with lower dissolution rates.     

高温防护涂层的熔盐热腐蚀研究进展

高温热腐蚀是热元件主要失效形式之一,Na₂SO₄和NaCl熔盐会加速高温下的热腐蚀,甚至导致灾难性事故发生。本文就Na₂SO₄和/或NaCl熔盐引起的热腐蚀进行了讨论,其中Na₂SO₄是主要的腐蚀反应物,详细介绍了2种典型的热腐蚀行为和性能特点。重点介绍了几种热腐蚀模型和机理,以及Na₂SO₄、NaCl、Na₂SO₄+NaCl熔盐的反应公式和腐蚀机理。根据目前的研究状况来看,制备防护涂层是缓解热腐蚀的最佳途径,总结了近年来MCrAlY涂层、NiAl涂层、热障涂层和新型涂层的发展情况,并探讨了进一步提高涂层耐腐蚀性能的方法。最后,展望了防护涂层的未来发展方向。     

The effect of processing gas on corrosion performance of electroless Ni-W-P coatings treated by laser

An investigation of the microstructural and corrosion characteristics of electroless Ni-5.5 W-6.5P coatings on steel substrates after laser treatment in argon and air is presented. The microstructural characteristics of the coatings, in terms of crystallisation, grain size, microstrain, porosity as well as surface chemistry, were examined using quantitative X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical tests, using potentiodynamic polarisation in 0.5 M H₂SO₄ solution and electrochemical impedances spectroscopy (EIS) in 3.5% NaCl solution, were undertaken to evaluate the corrosion behaviour of the coatings. The results indicated that the laser-treated coatings consisted of nanocrystalline Ni and Ni₃P phases, along with retained amorphous phase; further, the dimensions of the Ni crystallites were larger than those of Ni₃P. The laser-treated coating in argon revealed the presence of submicron scale porosity, while no porosity was evident in the coating surface treated by laser in air. The uniform corrosion revealed in 0.5 M H₂SO₄ solution is mainly determined by the microstructural characteristics of the coating. Pitting corrosion in 3.5% NaCl solution depended on the amount of porosity on the surface. The laser-treated coating in air exhibited better corrosion resistance in both acidic and chloride environments than that laser-treated in argon.     

The hot corrosion behaviour of HVOF sprayed MCrAlX coatings under Na2SO4 (+NaCl) salt films

The hot corrosion behaviour of two NiCoCrAlYTa and CoCrAlYSi HVOF sprayed coatings and a CoCrAlY VPS coating were investigated under laboratory conditions at 900°C using a synthetic gas atmosphere containing sulphur as an impurity. All the coatings tested showed good protection under Na₂SO₄ salt films. In the presence of NaCl in the Na₂SO₄ salt films, the corrosion rates of low Al containing coatings increased considerably but the NiCoCrAlYTa coating with higher Al content still revealed good performance. It is suggested that NaCl in the salt film causes premature failure of the protective scale and reduces the incubation period of corrosion in the coatings of lower Al content. Furthermore, it seems that the finely dispersed Al rich oxide particles in the sprayed and heat‐treated HVOF coating microstructure do not lead to internal corrosion. The experimental investigations include short‐term corrosion kinetic measurements and SEM analyses.     

Fabrication of conventional and nanostructured NiCrC coatings via HVAF technique

The conventional and nanostructured NiCrC （with chemical composition of 80%NiCr-20%CrC） coatings with high quality were fabricated via high velocity air-fuel（HVAF） spraying technique. The microstructures of these coatings were characterized by means of metallographic microscopy, scanning electron microscopy, transmission electron microscopy and X-ray diffractometry. A Vickers microhardness tester was used to determine the mechanical properties of the as-sprayed coatings. The single-line approximation（SLA） method was employed to calculate the grain size and microstrain of as-sprayed nanostructured coating based on the XRD data. The results show that nanostructured NiCrC coating possesses a more uniform and denser microstructure, much higher microhardness and better fracture toughness than its conventional counterpart. Both TEM observation and calculation results based on XRD profile show that as-sprayed nanostructured NiCrC coating has a homogeneous nanocrystalline microstructure with an average grain size of 40 nm.     

Oxidation and Hot Corrosion Behavior of Plasma-Sprayed MCrAlY–Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Coatings

The oxidation and hot corrosion behavior of two atmospheric plasma-sprayed NiCoCrAlY–Cr₂O₃ and CoNiCrAlY–Cr₂O₃ coatings, which are primarily designed for wear applications at high temperature, were investigated in this study. The two coatings were exposed to air and molten salt (75%Na₂SO₄–25%NaCl) environment at 800 °C under cyclic conditions. Oxidation and hot corrosion kinetic curves were obtained by thermogravimetric technique. X-ray diffraction analysis and scanning electron microscopy with energy-dispersive x-ray spectrometry were employed to characterize the coatings’ microstructure, surface oxides, and composition. The results showed that both coatings provided the necessary oxidation resistance with oxidation rates of about 1.03 × 10^?2 and 1.36 × 10^?2 mg/cm² h, respectively. The excellent oxidation behavior of these two coatings is attributed to formation of protective (Ni,Co)Cr₂O₄ spinel on the surface, while as-deposited Cr₂O₃ in the coatings also acted as a barrier to diffusion of oxidative and corrosive substances. The greater presence of Co in the CoNiCrAlY–Cr₂O₃ coating restrained internal diffusion of sulfur and slowed down the coating’s degradation. Thus, the CoNiCrAlY–Cr₂O₃ coating was found to be more protective than the NiCoCrAlY–Cr₂O₃ coating under hot corrosion condition.     

Cyclic oxidation behavior of some plasma-sprayed coatings in Na2SO4-60%V2O5 environment

Cyclic oxidation behavior of plasma-sprayed NiCrAlY, Ni-20Cr, Ni₃Al, and Stellite-6 coatings was investigated in an aggressive environment of Na₂SO₄-60%V₂O₅ by thermogravimetric techniques for 50 cycles. These coatings were deposited on a Ni-base superalloy, namely Superni 600; 10Fe-15.5Cr-0.5Mn-0.2C-Bal Ni (wt.%). X-ray diffraction, scanning electron microscopy/energy dispersive x-ray (SEM/EDX), and electron probe microanalyzer (EPMA) techniques were used to analyze the oxidation products. The uncoated superalloy suffered accelerated oxidation in the form of intense spallation of its oxide scale. After deposition of the NiCrAlY coating, the superalloy showed a minimum mass gain, whereas after application of the Stellite-6 coating, a maximum mass gain was observed among the coatings studied. All of the coatings were found to be useful in reducing the spallation of the substrate superalloy. Moreover, the coatings were successful in maintaining continuous surface contact with the base superalloy during the cyclic oxidation. The phases revealed for the oxidized coatings were mainly the oxides of chromium and/or aluminum and the spinels containing nickel-chromium/cobalt-chromium/nickel-aluminum mixed oxides, which are reported to be protective against high-temperature oxidation/hot corrosion.     

Effect of enamel coating on oxidation and hot corrosion behaviors of Ti-24Al-14Nb-3V alloy

The effect of enamel coating on the isothermal and cyclic oxidation at 900 °C in air and on the hot corrosion resistance of Ti-24Al-14Nb-3V in both 85% Na₂SO₄+15%K₂SO₄ and 15%NaCl+85% Na₂SO₄ molten mixed salts at 850 °C was investigated. The results indicated that Ti-24Al-14Nb-3V alloy exhibited poor oxidation resistance due to the formation of nonprotective Al₂O₃+TiO₂+AlNbO₄ scales and poor hot corrosion resistance due to the spallation of scales formed in molten Na₂SO₄+K₂SO₄ and NaCl+Na₂SO₄. Enamel coating suppressed the migration of oxygen and corrosive ions into the substrate to improve the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy. However, the dissolution of oxides components of the coating into the molten salts degraded enamel coating and the degradation of the coating involved a process by which Cl⁻ anion penetrated into the substrate through voids in the coating to accelerate corrosion of Ti-24Al-14Nb-3V alloy.     

Performance of high-velocity oxyfuel-sprayed coatings on an Fe-based superalloy in Na2SO4-60%V2O5 environment at 900 °C part II: Hot corrosion behavior of the coatings

NiCrBSi, Cr₃C₂-NiCr, Ni-20Cr, and Stellite-6 coatings were deposited on an Fe-based superalloy by the high-velocity oxyfuel (HVOF) thermal spray process. The hot corrosion behavior of the coatings in an aggressive environment of Na₂SO₄-60%V₂O₅ at 900 °C under cyclic conditions was studied. The thermogravimetric technique was used to establish the kinetics of corrosion. X-ray diffraction, scanning electron microscopy/energy-dispersive x-ray and electron probe microanalysis techniques were used to analyze the corrosion products. Hot corrosion resistances of all the coatings were found to be better than the uncoated superalloy. The Ni-20Cr coating was found to be the most protective, followed by Cr₃C₂-NiCr coatings. The Ni-20Cr coating had reduced the mass gain by 90% of that gained by the uncoated superalloy. The hot corrosion resistance shown by the Cr₃C₂-NiCr coating was slightly better compared with the NiCrBSi coating; however, both of the coatings performed better than the Stellite-6 coating. The Stellite-6 coating was the least effective among the coatings studied, but it was still successful in decreasing the mass gain to about one fourth compared with the uncoated superalloy. The formation of oxides and spinels of nickel, chromium, or cobalt may be contributing to the development of hot corrosion resistance in the coatings. This article focuses on the hot corrosion behavior of HVOF coatings. The characterization of these coatings has been presented in part I included in this issue.     

Hot corrosion behaviour of plasma sprayed YSZ/Al2O3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na₂SO₄ and V₂O₅ salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO₄ phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al₂O₃ coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.     

Microstructural Characterization and Cyclic Hot Corrosion Behaviour of Sputtered Co�CAl Nanostructured Coatings on Superalloy

Nanostructured Co?CAl coatings on Superni-718 superalloy substrate were deposited by DC/RF magnetron sputtering in the present work. The microstructure and cyclic hot-corrosion behavior of nanostructured Co?CAl coatings on Superni-718 superalloy were investigated in molten salt of 40 wt% Na₂SO₄ + 60 wt% V₂O₅ at 900 °C. The results showed that a dense scale formed on the coated samples exposed to corrosive environment during thermal cycling. The spinel phases of CoCr₂O₄, CoAl₂O₄ and NiCr₂O₄ were found in the corroded scale of the coatings, resulting in an effective inhibition of O and S diffusion. The sputtered Co?CAl coatings exhibited high hot corrosion resistance due to the formation of ??-CoAl phases in the coating. The relevant corrosion mechanisms substantiating the role of coatings are discussed.     

High temperature corrosion behavior of a multilayer CrAlN coating prepared by magnetron sputtering method on a K38G alloy

A multilayer CrAlN coating of Cr_0.58Al_0.42N/Cr_0.84Al_0.16N/Cr_0.51Al_0.49N has been fabricated by a reactive magnetron sputtering method. It consists of a bonding layer, a Cr-rich intermediate layer and an Al-rich outer layer. The multilayer structure provides the coating with good protection against different types of high temperature corrosion, i.e., high temperature oxidation and hot corrosion. The outer Al-rich layer gives the coating good oxidation resistance at 1000 and 1100 °C due to the formation of a continuous alumina scale. The parabolic rate constants of the coated samples decrease by about 2 orders of magnitude compared with that of the bare alloy samples. The intermediate Cr-rich layer can form a Cr₂O₃ scale to provide good protection under the hot corrosion conditions in the Na₂SO₄ salt fluxing at 900, 950 and 1000 °C. The incubation period of the hot corrosion extends several times longer when the alloy was coated by the multilayer coating at the three selected temperatures.     

Oxidation and Sulfidation Behavior of AlTiN-Coated Ti–46.7Al–1.9W–0.5Si Intermetallic with CrN and NbN Diffusion Barriers at 850°C

Attempts have been made to improve the high-temperature corrosion behavior of an intermetallic alloy, Ti–46.7Al–1.9W–0.5Si, in an H₂/H₂S/H₂O atmosphere at 850°C using AlTiN coating with and without CrN and NbN diffusion barriers. The oxidation and sulfidation behavior of the uncoated Ti–46.7Al–1.9W–0.5Si alloy followed protective kinetics with a parabolic rate constant of 6×10^–11 g²/cm⁴/s. A multi-layered scale developed: an outer rutile (TiO₂) layer, a continuous layer of -Al₂O₃ beneath the rutile layer, and an inner TiS layer, in which pure W was scattered. Fast outward diffusion of Ti within the substrate resulted in the formation of a zone of high concentration of aluminum (TiAl₃ and TiAl₂) between the scale and substrate.The use of an AlTiN coating greatly increased the oxidation and sulfidation resistance of Ti–46.7Al–1.9W–0.5Si. The use of NbN and CrN diffusion barriers further enhanced its corrosion resistance. The protection of the double-layer coatings persisted even after 240 hr exposure. However the mismatch of thermal expansion coefficients between the coating and substrate led to the development of cracks in some locations within the coatings. A 2.5 m thick AlTiN coating on the Ti–46.7Al–1.9W–0.5Si substrate with an embedded defect was modeled using the general finite element (FE) program ABAQUS. The modeling results showed rapid mode I failure of the coating at a temperature of 774°C. The through-fracture of the nitride film caused the nitride coating to shrink back leading to delamination around the crack in the nitride coating. The cracks formed acted as diffusion paths, for the ingress of oxygen and sulfur species and the outward diffusion of substrate elements, which resulted in the formation of nodular corrosion products with similar morphologies and microstructures to the uncoated alloy in those locations where cracks developed.