Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

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.     

Oxidation Behavior of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel-Coated Interconnects in Wet Air

Corrosion of boilers and heat exchangers is accelerated in the presence of vanadium, sodium, and sulfur from low-grade fuels. Several iron- and nickel-based alloys were immersed in 60 mol% V₂O₅–40Na₂SO₄ salt for 1000 h in order to investigate their degradation behavior at 600 °C in air. Materials performance was analyzed by means of substrate recession rate and metallographic characterization. Their corrosion mechanism is characterized by the formation of a sulfide/oxide layer adjacent to the metal, the dissolution of scale oxides in the molten deposit, and their precipitation near the outer surface of the deposit. High Ni- and Cr-containing alloys show the lowest metal loss rates. Al addition was detrimental due to low-melting eutectic AlVO₄–V₂O₅ formation. Fe–Cr-based alloys showed the highest metal loss rates. In such alloys, high Cr additions (above 20%) did not improve the performance due to the negative synergetic effect by simultaneous dissolution of Fe₂O₃ and Cr₂O₃. The predominant salt composition at the corrosion front varied from vanadate rich to sulfate rich during the exposure. This change in the attacking salt makes it difficult to find a protective material for mixed sulfate–vanadate-induced corrosion.     

High-temperature oxidation and hot corrosion of Cr2AlC

High-temperature oxidation and hot corrosion behaviors of Cr₂AlC were investigated at 800–1300 °C in air. Thermogravimetric–differential scanning calorimetric test revealed that the starting oxidation temperature for Cr₂AlC is about 800 °C, which is 400 °C higher than other ternary transition metal aluminum carbides. Thermogravimetric analyses demonstrated that Cr₂AlC displayed excellent high-temperature oxidation resistance with parabolic rate constants of 1.08 × 10⁻¹² and 2.96 × 10⁻⁹ kg² m⁻⁴ s⁻¹ at 800 and 1300 °C, respectively. Moreover, Cr₂AlC exhibited exceptionally good hot corrosion resistance against molten Na₂SO₄ salt. The mechanism of the excellent high-temperature corrosion resistance for Cr₂AlC can be attributed to the formation of a protective Al₂O₃-rich scale during both the high-temperature oxidation and hot corrosion processes.     

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 Exposure Studies of HVOF-Sprayed Cr<Subscript>3</Subscript>C<Subscript>2</Subscript>-25(NiCr)/(WC-Co) Coating

In this research, development of Cr₃C₂-25(NiCr) + 25%(WC-Co) composite coating was done and investigated. Cr₃C₂-25(NiCr) + 25%(WC-Co) composite powder [designated as HP2 powder] was prepared by mechanical mixing of [75Cr₃C₂-25(NiCr)] and [88WC-12Co] powders in the ratio of 75:25 by weight. The blended powders were used as feedstock to deposit composite coating on ASTM SA213-T22 substrate using High Velocity Oxy-Fuel (HVOF) spray process. High-temperature oxidation/corrosion behavior of the bare and coated boiler steels was investigated at 700 °C for 50 cycles in air, as well as, in Na₂SO₄-82%Fe₂(SO₄)₃ molten salt environment in the laboratory. Erosion-corrosion behavior was investigated in the actual boiler environment at 700 ± 10 °C under cyclic conditions for 1500 h. The weight-change technique was used to establish the kinetics of oxidation/corrosion/erosion-corrosion. X-ray diffraction, field emission-scanning electron microscopy/energy-dispersive spectroscopy (FE-SEM/EDS), and EDS elemental mapping techniques were used to analyze the exposed samples. The uncoated boiler steel suffered from a catastrophic degradation in the form of intense spalling of the scale in all the environments. The oxidation/corrosion/erosion-corrosion resistance of the HVOF-sprayed HP2 coating was found to be better in comparison with standalone Cr₃C₂-25(NiCr) coating. A simultaneous formation of protective phases might have contributed the best properties to the coating.     

Corrosion behavior of thermal-sprayed WC cermet coatings in SO4 2− environment

A series of the electrochemical and long-term corrosion tests was carried out in a 3.5 wt% Na2SO4 solution on thermal-sprayed WC-17Co and WC-10Co-4Cr cermet coatings in order to examine the effect of composition of binder materials on the corrosion behavior. The results reveal that the overall corrosion resistance of the WC-17Co coating is inferior to that of the WC–Co–Cr coatings due to the corrosion of binder materials which induce WC particles to fall off. CoO and WO3 oxide films form on the surface of WC-17Co coating in Na2SO4 solution electrochemical corrosion process, which will protect the coating in the process of corrosion. Cr2O3 oxide film formed on the WC-10Co-4Cr coating surface has a strong hindered role to corrosion. The corrosion mechanism of WC-17Co coating in Na2SO4 solution is entire corrosion of Co matrix, while it is film-hole corrosion mechanism for WC-10Co-4Cr coating.     

Hot Corrosion Behavior of Superalloy IN718 at 550 and 650 °C

This investigation was undertaken to evaluate oxidation and hot corrosion behavior of the Fe-Ni-based superalloy IN718, at 550 and 650 °C, to explore its performance as turbine engine components under marine environment. Uncoated and different salt-coated samples (100 wt.% NaCl, 75 wt.% Na₂SO₄ + 25 wt.% NaCl, and 90 wt.% Na₂SO₄ + 5 wt.% NaCl + 5 wt.% V₂O₅) were exposed in air at 550 and 650 °C under cyclic heating and cooling for 100 h. Weight gain was studied for both uncoated and salt-coated samples. X-ray diffraction, scanning electron microscopy, and electron dispersive spectroscopy were used to characterize the oxidation and corrosion products. A possible mechanism of corrosion, based on the corrosion compounds, is discussed. The variation in weight gain with time showed a parabolic growth of oxides. Coating with NaCl was found to be detrimental both at 550 °C as well as 650 °C. On the other hand, the salt mixture of NaCl and Na₂SO₄ had no effect at 550 °C; however, it was detrimental at higher temperature of 650 °C. Coatings of salt mixture of Na₂SO₄, NaCl, and V₂O₅ caused very slow oxidation at both the temperatures. Increase in thickness of salt coating was observed to enhance the rate of hot corrosion. Among the three types of salt coatings, the coating of NaCl was found to be most damaging both at 550 and 650 °C.     

Initial Stages of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Induced Degradation of β-Ni–36Al at 700 °C: I-Intrinsic Behavior

The early-stage scaling behavior of a β-Ni–36Al alloy undergoing Na₂SO₄-deposit-induced degradation at 700 °C was systematically studied using SEM and TEM. After 20 h of exposure in an O₂–1000 ppm SO₂ ambient, the deposit-coated alloy formed a dense but thin Al₂O₃ scale on most areas of the surface; however, large nodules formed locally. Nodule formation occurred where the scale had lost its protective character, with rapid internal oxidation ensuing. The presence of sulfur both in the environment and in the salt played a key role in nodule formation. Removal of SO₂/SO₃ from the gas mixture, or of the Na₂SO₄ deposit from the surface, prevented nodule formation, while removing the sulfur source after nodule formation prevented further nodule growth. The degradation could be linked to the dissolution of reaction products in the Na₂SO₄ deposit and the formation of a low-temperature eutectic liquid. Further, when an Na₂SO₄–48% MgSO₄ deposit was used, the nodule density increased.     

Corrosion of Cr2AlC in Ar/1%SO2 Gas Between 900 and 1200?��C

Cr₂AlC compounds were synthesized by a powder metallurgical route and corrosion tested at 900, 1000, 1100 and 1200 °C for up to 150 h under an Ar/1% SO₂ gas atmosphere. The compounds were resistant to corrosion because a thin ??-Al₂O₃ barrier layer quickly formed on the surface which suppressed sulfidation. Virtually no sulfur was detected inside the scale except during the initial corrosion stage. The superior corrosion resistance of Cr₂AlC originated from the high affinity of Al for oxygen to form the thermodynamically stable Al₂O₃. Unlike Al, Cr was not active because Cr was strongly bound to carbon as Cr₂C layers in Cr₂AlC. The small amount of Cr₂O₃ that had formed was dissolved in the Al₂O₃ layer. The corrosion of Cr₂AlC resulted in the formation of an ??-Al₂O₃ layer and an underlying Cr₇C₃ layer.     

Oxidation of Nickel and Cobalt-Based Alloys in the Presence of Condensed Sodium Sulphate

The hot corrosion behaviour of a number of nickel and cobalt-based superalloys has been examined by exposing samples to a high temperature oxidizing environment supersaturated with sodium sulphate vapour. This test seems more able to reproduce typical service behaviour than other laboratory tests. Pure cobalt is unaffected by the presence of the condensed sulphate, whereas CoW and low-chromium, CoCrW alloys undergo acidic fluxing. However, the major change produced by the continuous supply of Na₂SO₄, as opposed to the limited amount of salt available in the coating test is in the behaviour of the high chromium alloys, when the protective Cr₂O₃ layers are removed due to the formation of a Na₂CrO₄ species. Thus, the normally resistant Co25Cr7.5W alloy suffers acidic fluxing. Similarly, the Cr₂O₃ layer on the binary Co25Cr alloys is rendered ineffective; considerable ingress of sulphur into the alloy occurs. Aluminium and manganese additions seem to reduce this effect slightly by stabilising the protective oxide layer. Both these alloying additions have a higher affinity for sulphur than chromium, and this could be important. Binary Ni20Cr seems less susceptible to accelerated attack than the Co25Cr alloys, presumably due to its greater ability to maintain a protective Cr₂O₃ layer. However, addition of 3 vol.-% Y₂O₃ virtually prevents any attack by the Na₂SO₄, preventing sulphur penetration into the alloy and promoting the formation of a protective Cr₂O₃ layer. Under non-condensing conditions, all of the alloys tested oxidize in an unaccelerated manner, supporting the view that condensation of sodium sulphate is necessary for hot corrosion.     

Investigations on role of HVOF sprayed Co and Ni based coatings to combat hot corrosion

Abstract

This paper aims to investigate the hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed cobalt based (Stellite-6) and nickel based (Ni–20Cr) coatings deposited on the superalloy Superni-718 (Ni–19Cr–18˙5Fe–5˙13Ta–3˙05Mo–0˙9Ti–0˙5AI–0˙18Mn–0˙18Si–0˙15Cu–0˙04C) in the Na₂SO₄–60%V₂O₅ salt environment at 900°C under cyclic conditions. The X-ray diffractometry, scanning electron microscopy/energy dispersive analysis and electron probe microanalyser techniques were used to study the corrosion products with respect to their morphology, phase composition and element concentration. The thermogravimetric technique was used to establish the kinetics of corrosion. The bare alloy underwent severe hot corrosion attack. The Ni–20Cr coating shows excellent hot corrosion resistance with negligible spallation, whereas Stellite-6 coating reveals less hot corrosion resistance and more spallation. The hot corrosion resistance of Ni–20Cr coating has been attributed to the formation of oxides of chromium, nickel and spinel of nickel chromium. The oxides of silicon, chromium, cobalt and spinels of cobalt–chromium and nickel–chromium have contributed for hot corrosion resistance of Stellite-6 coatings.     

Effects of Platinum on the Hot Corrosion Behavior of Hf-Modified �á�-Ni3Al?+?��-Ni-Based Alloys

The establishment of a protective ??-Al₂O₃ scale is critical for providing high temperature protection from oxidation and hot corrosion, thereby improving lifetimes of advanced gas turbine engine components. Recent work by our group has shown that a wide range of Pt + Hf-modified ?á?-Ni₃Al + ??-Ni alloy compositions form a very adherent and slow-growing Al₂O₃ scale and exhibit excellent oxidation resistance. The main thrust of the present study was to understand the effects of Pt addition on the Type I (900 °C) and Type II (705 °C) hot corrosion (HC) behavior of model Hf-modified ?á? + ?? alloy compositions. The salt used to bring about hot corrosion was Na₂SO₄. It was found that the Type I HC resistance of ?á? + ?? alloys improved with up to about 10 at.% Pt addition, but then decreased significantly with increasing Pt content up to 30 at.% (the maximum level studied); however, under Type II HC conditions the resistance of ?á? + ?? alloys progressively improved with increasing Pt content up to 30 at.%. The effect of pre-oxidation on hot corrosion resistance was also examined, and the results indicated that pre-oxidation generally improved Type II HC resistance for the test duration studied.     

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.     

不同Cr含量的Ni-Cr合金熔覆层的高温氧化和热腐蚀特性

采用激光熔覆技术制备了Cr质量分数为10%、20%和40%的Ni-Cr合金熔覆层,研究了其在900 ℃下的高温氧化特性和600 ℃下Na₂SO₄+25% K₂SO₄混合盐中热腐蚀特性。结果表明,Cr含量对熔覆层的高温特性起着关键作用。提高Cr含量对提升熔覆层抗硫酸盐诱导的热腐蚀能力比提升抗循环高温氧化能力更有效。Cr40涂层抗高温氧化和热腐蚀性能最佳。Cr10的氧化产物以NiO为主,极易脱落,内部氧化严重。虽然Cr40表面可以形成单一的Cr₂O₃层,但热应力和生长应力引起的富Cr氧化物内部开裂,使Cr40的抗循环高温氧化能力仅略好于Cr20。面对热腐蚀时,Cr10表面呈现层状NiO和Ni₃S₂叠层分布的腐蚀产物,内部腐蚀区也生成了Ni的硫化物。Cr20表面Cr₂O₃层被破坏,内部腐蚀严重,生成了CrS。Cr40表面生成了致密的Cr₂O₃保护层,有效地防止了进一步腐蚀。     

The hot corrosion of cobalt-base alloys in a modified Dean's rig - III. CoCrMo alloys

A range of CoCrMo alloys have been exposed at 900°C to salt-bearing atmospheres in a modified Dean's rig. The atmosphere consisted of air containing vapours of, respectively, Na₂SO₄, NaSO₄ + NaO, and Na₂SO₄ + NaCl. Both isothermal and 24-h cyclic exposures were used. In general, the presence of molybdenum in the Cr₂O₃-forming alloys caused accelerated and sometimes catastrophic corrosion. The influence of 2.5 Mo addition to the alloys was observed to be minimal. The presence of 10% Mo in the CoO-forming alloys caused acidic fluxing in the pure Na₂SO₄, while the basic salt caused sulphidation corrosion.     

A new solid‐state mode of hot corrosion at temperatures below 700°C

Preliminary results on a single‐crystal nickel‐based superalloy indicated that hot corrosion can occur at temperatures as low as 550°C, where liquid formation, generally believed to be responsible for Type II hot corrosion, is not predicted. Additional tests were conducted on pure‐nickel samples at 650°C and below to more clearly elucidate the mechanism of this very low‐temperature hot corrosion. Environments in dry air and O₂‐(2.5, 10, 100, and 1000) ppm SO₂ were studied. Based on the results obtained, a solid‐state corrosion mechanism was inferred. The mechanism relies on the formation of a previously unreported compound phase, which was identified using transmission electron microscope analysis that indicated the stoichiometry of Na₂Ni₂SO₅. Furthermore, it was nanocrystalline in structure and metastable. It was deduced that the Na₂Ni₂SO₅ formation was responsible for the rapid nickel transport required for the observed accelerated corrosion process. Moreover, its eventual decomposition resulted in a mixed product of porous NiO with embedded particles of Na₂SO₄. Application of the proposed mechanism to nickel‐based alloys is discussed.     

The hot corrosion of cobalt-base alloys in a modified Dean's rig-II. CoCrAl alloys

A range of CoCrAl alloys have been exposed at 900°C to salt-bearing atmospheres in a modified Dean's rig. The atmosphere consisted of dry air containing vapours of, respectively, Na₂SO₄, Na₂SO₄ + Na₂O, and Na₂SO₄ + NaCl. Both isothermal and 24-h cyclic exposures were used. In general, the presence of small amounts of aluminium in Cr₂O₃-forming alloys had relatively little effect. Chromium contents of 20% and above greatly enhanced the hot corrosion resistance of Al₂O₃-forming alloys. The presence of NaCl was always detrimental, leading to scale fracture and enhancing internal sulphidation.     

Hot corrosion behaviour of a laser beam irradiated nickel base superalloy

The structure and the hot corrosion resistance of cast nickel-base superalloy M 38 irradiated by CO₂ laser beam of 1.3 KW output have been examined by means of optical microscopy, XRD, SEM and EPMA in comparison with un-irradiated specimens. The hot corrosion tests were carried out in a crucible filled with a mixture of 75 Na₂SO₄ + 25 NaCl at 900°C for 15, 35, 55, 80 and 100 hrs, respectively. The results showed that the corrosion resistance of the irradiated alloy is by a factor 2–3 higher than that of the cast alloy. In contrast to significant internal oxidation/sulfidation along the grain boundaries in the cast alloy, under a dense protective oxide scale mainly consisting of Cr₂O₃, there was no sign of internal oxidation/sulfidation in the irradiated alloy. The irradiated surface layer may be regarded as an effective barrier forming part of the superalloy itself.     

Accelerated Hot Corrosion Studies of D-Gun-Sprayed Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>–50% Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating on Boiler Steel and Fe-Based Superalloy

In the current investigation, Cr₂O₃–50% Al₂O₃ coating was deposited on ASTM-SA213-T-22 boiler steel and Fe-based superalloy Superfer 800H by D-gun spray process. The high-temperature corrosion performance of the coated as well as bare alloys was evaluated in Na₂SO₄–60%V₂O₅ molten salt, an aggressive environment at 900 °C under cyclic conditions. The kinetics of the corrosion were analyzed by the change in weight measurements which were taken after each cycle (i.e., 1-h heating in a tube furnace followed by 20-min cooling in ambient air) for a total period of 50 cycles. The X-ray diffraction and scanning electron microscopy/energy-dispersive X-ray analysis techniques were used for the analysis of corrosion products. During investigations, it was found that both the selected bare alloys have suffered intensive spallation in the form of removal of their oxide scales, which may be attributed to the formation of non-protective Fe₂O₃-dominated oxide scales, whereas the coated alloys have shown lesser weight gains along with better adhesiveness of the oxide scales with the substrate till the end of the experiment. The oxides of chromium and aluminum were the main phases revealed in the oxide scales of the coated specimens, which are reported to be protective against the hot corrosion.