Cyclic Hot Corrosion Research Articles

High Temperature Corrosion Study on NiCr Coated Low Alloy and Mild Steel

Cyclic hot corrosion behaviour of bare and D-gun sprayed NiCr coated low alloy and mild steel sample having dissimilar coating thicknesses viz. 0.2-0.25 mm and 0.25-0.30 mm have been studied in molten salt environment mixture of Na2SO4 - 25wt. % NaCl at temperature of 750 ̊C. Corrosion kinetics were analysed and parabolic rate constants (Kp) for both bare and coated sample were calculated and observed that 0.2-0.25 mm thick NiCr coated specimen showed better resistance against high temperature corrosion for both materials. SEM/EDX analysis was carried out to examine scale formed on the bare and coated sample after hot corrosion.

Cyclic hot corrosion behavior and mechanism of GH4738 superalloy in mixed molten salt

The research on the hot corrosion resistance of nickel-based superalloys is critical to determining and enhancing their service life. Therefore, this study investigates the cyclic hot corrosion behavior of GH4738 alloy coated with a mixed salt (75 wt% Na2SO4+25 wt% NaCl) at 750 °C and 800 °C. X-ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and transmission electron microscope (TEM) are used to analyze the phase compositions and film structures of the corrosion products, as well as to explore the corrosion mechanism. The results show that a thicker oxide layer is formed at higher temperatures, indicating poorer hot corrosion resistance at elevated temperatures. The corrosion products form at both temperatures to show a distinct layered structure, the outer layer is mainly composed of Cr2O3 and a small amount of TiO2, while the inner layer consists of Al2O3, CrS, and TiS. In addition, it is found that the corrosion mechanism of O, S, and Cl elements in the corrosion process is synergistic and accompanied by alkaline melting.

Hot corrosion behaviour of HVAF coatings deposited on Fe[sbnd]25Cr[sbnd]20Ni support hanger material

Fe25Cr20Ni cast alloy is used as the material of construction for support hangers in fired heaters of oil refineries. These hangers support heater tubes in the fired heaters and are susceptible to hot corrosion due to Na2SO4 and V2O5 containing corrosive ash, especially when low-grade fuels are burnt. API recommends 50Cr50Ni cast-alloy for constructing the support hangers in highly corrosive atmospheres which, is highly expensive as well as brittle. To overcome this problem, in this work, coatings of 50Cr50Ni and 20Cr80Ni were applied on Fe25Cr20Ni substrates by HVAF process. The coatings were characterized by: Optical Microscopy, SEM-EDS Analysis, XRD Analysis, and Raman Spectroscopy. To study the hot corrosion behaviour of these coated specimens, a cyclic hot-corrosion test was carried out at 700 °C using 20 % Na2SO4–80 % V2O5 salt slurry. Along with the coated specimens, cast specimens of Fe25Cr20Ni and 50Cr50Ni were also subjected to the hot-corrosion test. Thus, Type-II hot corrosion behaviour of all four specimens were analyzed comparatively by weight-change measurements, cross-sectional metallography, SEM-EDS, XRD, and Raman-Spectroscopy. The Fe25Cr20Ni cast specimen displayed the least corrosion resistance. The corrosion resistance of the 50Cr50Ni coated specimen and the 50Cr50Ni cast specimen was found to be intermediate and quite comparable. 20Cr80Ni coated specimen recorded the best corrosion resistance. The corrosion mechanism and beneficial effects of high nickel-containing HVAF coatings against hot corrosion are presented.

The effect of oxide scale and microstructural changes during cyclic hot corrosion on high-temperature tensile properties of Rene-80 superalloy

ABSTRACT The effect of oxide scale and microstructural changes during 10, 20, and 40 hot corrosion cycles on the high-temperature tensile properties of Rene-80 superalloy at 950 °C was investigated. Due to the formation of micro-cracks and micro-voids, compressive stresses produced from Cr2O3 and NiO growth, and tensile stresses stemming from NiMoO4 transformation and Al internal oxidation, the oxide scale spalled. By increasing the hot corrosion cycles, UTS (Ultimate Tensile Strength) and El.% (Elongation) first decreased and then increased due to the propagation of intergranular vertical cracks from the oxide scale to the Rene-80 after 20 cycles. During hot corrosion cycles, YS increased due to a rise in the density of near-surface intergranular cracks close to the Rene-80/oxide scale interface resulting from micro-void linkage and γ′-depleted zone. Due to the high area fraction and the small average size of secondary γ′, UTS and YS were the highest and lowest after ten cycles, respectively.

High-temperature corrosion and oxidation properties of borided CoCrFeNiAl0.5Nb0.5 HEA

This study is focused on a detailed investigation of high-temperature corrosion and oxidation behavior of borided CoCrFeNiAl0.5Nb0.5 HEAs, considering their use in advanced engineering applications. CoCrFeNiAl0.5Nb0.5 HEA was produced by arc melting. XRD and SEM-EDS analysis before boriding determined that the alloy consisted of four different phases with different chemical compositions. Powder-pack boriding of a CoCrFeNiAl0.5Nb0.5 HEA was performed at 1000 °C for 3 h in a boriding media containing 90 % boron carbide and 10 % sodium tetrafluoroborate. As a result of the boriding process, complex boride layers consisting of (CoFe)B2, CrFeB, CoNbB, FeB and NiB phases were obtained on the surface with a thickness of 40 μm and hardness of 3004 HV. it was determined that the microstructure with cauliflower appearance evolved towards a dense and non-porous appearance around 20 μm as the boron diffusing into the HEA microstructure filling the gaps of intermetallic compound in the single structure, HEA and borided HEAs were each subjected to a cyclic hot corrosion test at 900 °C in molten corrosion salts of Na2SO4 and V2O5. After hot corrosion tests, long rod-like structures were observed in both samples due to the excessive corrosion, while borided alloys were more resistant to hot corrosion damage. After oxidation tests, HEA consists of a compact protective alumina scale that provided better oxidation resistance, while non-protective mixed oxides with cracks were dominant in the borided HEA.

Hot corrosion behavior of intermixed layered LTA/YSZ thermal barrier coatings

In present work a unique combination of hybridized intermixed five layered coatings structure is developed by using Arc plasma spray method. A Lanthanum-Titanium-Aluminum Oxide (LaTi2Al9O19) that is LTA powder is used as a ceramic top coat material along with YSZ. A bond coat of nickel dominant NiCrAlY having thickness of 90 µm (µm) is applied on the substrate Inconel 718 post shot blasting. A preceding layer of YSZ with 50% and remaining 50% of 50 µm bond coat is sprayed. After that a layer of YSZ having thickness of 100 µm is applied. Then a layer of 50 µm thickness having 50% YSZ and 50% LTA is applied. LTA layer of thickness 100 µm is applied at the end. Cyclic and isothermal hot corrosion is performed using salts Na2SO4 & NaCl in 3:1 mass proportion at 900 °C. The mass increment by as-sprayed intermixed coatings is less than 0.05 mg/cm2 and it is 0.02 mg/cm2 for annealed coating. An annealed intermixed coating shows negligible mass increment and has survived upto 100 h in case of isothermal hot corrosion. In case of cyclic hot corrosion annealed intermixed coatings shown no mass increment and is survived till 70 h. Mass increment of as-sprayed intermixed coatings is less than 0.05 mg/cm2 but after 50 h severe spallation of coating occurred. The coating surface morphology is evaluated through field emission scanning electron microscopy (FESEM) and x-ray (XRD) diffraction.

Life assessment of thermal sprayed alloy 825 in waste heat incineration environment

In this study, the cyclic hot corrosion behaviour of Ni-based superalloy 825 before and after thermal spray coating of Cr3C2–25%NiCr and NiCrAlY powders was investigated in a superheater section of a simulate waste-heat incinerator. The thermogravimetric (TG) technique is used to analyze the corrosion kinetics of the samples that have undergone hot corrosion tests in Na2SO4–40%K2SO4–10%KCl–10%NaCl environment at 750 °C for 60 cycles. The X-ray diffraction (XRD) technique was also used to analyze the phases of coating and corrosion products. Scanning electron microscopy (SEM) was used to characterize the surface and cross-sectional morphology of the coatings, while the compositions were analyzed using energy dispersive spectroscopy (EDS). After hot corrosion tests, the cross-sections of the coated samples were investigated using EDS point and line analysis techniques to characterize the oxide scales formed and assess the depth of the corrosion attack. It is confirmed that the bare specimen showed maximum weight gain compared to the HVOF coated specimens. Minor spallation has been observed in both the uncoated and specimens coated with the Cr3C2–25%NiCr powder, however, not in the NiCrAlY coated specimen. Based on the weight gain results, the Cr3C2–25%NiCr coating exhibits less weight gain than the NiCrAlY coating. However, some failures were noticed in the leading edges of Cr3C2–25%NiCr coated samples with minor spallation and sputtering. The NiCrAlY powder-coated specimen displayed better corrosion resistance than the specimens coated with Cr3C2–25%NiCr powder due to the development of NiO, Cr2O3, Al2O3, and NiCr2O4 scales on the surface of hot corroded samples. The NiCrAlY coating was found to have 20% reduced weight gain compared to the uncoated alloy.

Microstructure and hot corrosion performance of stainless steel 347 produced by wire arc additive manufacturing

The present work reports the feasibility of wire arc additive manufacturing (WAAM) process to manufacture SS 347 components for high temperature applications. The microstructure of WAAM processed SS 347 (WAAM 347) comprised of austenite and residual delta-ferrite while the ferrite fraction varied between 1.40 and 4.20% along the building direction. Cyclic hot corrosion behaviour of WAAM 347 specimens were investigated under the Na2SO4–60%V2O5 molten salt at 700 °C by measuring the weight change. Higher weight gain was noticed in the samples exposed to hot corrosion because of the deposition of molten salt mixture. X-ray diffraction results revealed the formation of oxides like Fe2O3, FeV2O4, and Ni3V2O8 while the presence of NiCr2O4 and Cr2O3 lowers the corrosion rate at elevated temperatures. The absence of spallation due to the good adherence of oxide-scales highlights that WAAM 347 structures are suitable for elevated temperature applications.

Hot corrosion and thermal shock performance of nanoparticulate YSZ coatings deposited via suspension plasma spraying (SPS)

In this research, the preparation, and some thermal behaviors of nanoparticulate yttria stabilized zirconia (YSZ) coatings were assessed. Aqueous suspensions containing YSZ nanoparticles were produced and then used to deposit thermal barrier coatings through the suspension plasma spraying (SPS) method. Cyclic hot corrosion and thermal shock experiments were performed on the coatings in a Na2SO4–Na2VO3 molten salt at 880 °C and under rapid cycles at 1100 °C, respectively. The coating obtained from spraying of the optimal suspension, which includes YSZ nanoparticles dispersed with α-Terpineol as a surfactant, promisingly has a columnar-shaped structure. The lifetimes of this columnar SPS-YSZ coating during the hot corrosion and thermal shock experiments were measured to be 380 h and 120 cycles, respectively, whereas they were 240 h and 100 cycles for a laminar coating obtained from spraying of nonoptimized YSZ. It is concluded that by optimizing the water-based suspension of YSZ nanoparticles, novel thermal barrier coatings with an optimal microstructure can be achieved, which improves the properties of the coating and increases its lifetime.

Behaviour and Mechanisms of Alkali Metal Sulphate-Induced Cyclic Hot Corrosion in Relation to Gradients and Preoxidised MCrAlY-Type Coatings

In this study, two modified MCrAlY-type coatings were prepared by the same arc ion plating process. ‘Gradient annealing’ and ‘gradient annealing plus preoxidation’ were adopted as post-treatments for the two coatings. A two-layer scale of a mixed oxide layer and a pure alumina layer with the alumina next to the substrate was formed on the latter coating. A cyclic hot corrosion test was carried out on them with a conventional MCrAlY-type coating as a reference. The kinetics and evolution of the microstructure showed them to have greater hot corrosion resistance at 900 °C. The preformed inner alumina scale by preoxidation retarded the occurrence of internal oxidation–sulphidation to some extent. Additionally, the internal sulphidation–oxidation model is elucidated and extended.

Evaluation of oxide-scales of T22 steel weldment to characterize the kinetics of cyclic hot-corrosion in molten salt environment

Welding is the primary joining-process, produced the heat affected zone which is mostly damaged severely because of prolonged period particularly in corrosive environment. Most industrial components are joined by shielded metal arc welding (SMAW) process. The present article, therefore, investigated the performance of corrosion of weld-metal (WM) and heat-affected zones (HAZs) of weldment in ferritic T22 steel exposed in salt (Na2SO4–60 wt% V2O5) environment at 900 °C under thermal-cyclic conditions. During the experiments the weight-gain measurements were applied on these regions of weldment to observe the hot corrosion kinetics. The weight gain per unit area increased with the increased number of testing cycles. The parabolic growth rate was described the corrosion kinetics after few initial cycles. HAZs corroded at faster rates than WM due to more spallation and cracking in surface oxide-scale.

Hot corrosion behaviour of Inconel 738 superalloy in presence of NaCl, Na2SO4, V2O5

Superalloy are widely used for high temperature applications in diverse environment such as boilers, turbines and petrochemical industries for longer hours of operation. The application environment may have aggressive salts such as NaCl, Na2SO4, V2O5 which can catalase the hot corrosion thus affecting the normal working of the appliances. In present work, hot corrosion behavior of Inconel 738 superalloy studied by using cyclic hot corrosion test carried out at 700 °C for 10 cycles. For this study to know how the effect of salts on the alloy, each specimen was coated separately by NaCl, Na2SO4, V2O5 salt. After that, the coated specimens placed in the furnace. The cycle consists of 24Hr of heating at 700 °C in muffle furnace. Then, the specimens removed from the furnace and allowed to cool in the air environment. The weight gain after every cycle is measured and is used to determine the kinetics of hot corrosion. The graph of square of weight gain/Area Vs no of cycles was plotted. X-ray diffraction used to study the different types of oxides and compounds formed on the specimen surface. The morphology and electrochemical analysis of the specimens correlated to study the hot corrosion behavior. The rate constant (parabolic rate constant, KP) was determined from the weight gain data. The results showed that the specimen coated with Na2SO4 has lower corrosion as compare to the specimen coated with NaCl and V2O5 due to the presence of protective oxide such as Cr2O3, TiO2, Fe2O3, NiCr2O4, Ni3Ti formed on the specimen surface.

MWCNTS-YSZ coating deposited by plasma thermal spray on INCONEL 738 low carbon substrate

The surface of Inconel 738 LC samples has been coated with carbon nanotube and yttria stabilized zirconia utilizing the plasma thermal spray method. Various percentages of carbon nanotubes and yttria stabilized zirconia were utilized in the coating (100 percent nano YSZ, 96 percent nano YSZ +4 percent CNTs, 94 percent nano YSZ +6 percent CNTs, and 92 percent nano YSZ +8 percent CNTs). After coating, a cyclic hot corrosion testing has been conducted, followed by SEM and XRD on all samples. SEM cross-section micrographs were utilized to quantify coating thickness. After 50 hrs of corrosion in 67 percent wt. V2O5 +33 percent wt. Na2SO4 at 950 degree centigrade, the changing weight of absence and coated specimens was measured. The cumulative weight change per surface area of coated specimens at temperatures of 950 °C was found to be 0.0095, 0.0075, 0.006, and 0.0057 mg.cm–2, (E,F,G,H) respectively. The best coating to resist hot corrosion was showed by specimens coated with 92% wt. yttria +8%wt. carbon nanotube. There are no oxides in the base specimens, according to X-rays of coated samples. Images of scanning electron microscopy for the surface layer revealed that it has a significant porosity and that some of the layer of coating s had microcracks.

Investigation of calcium–magnesium-alumino-silicate (CMAS) resistance and hot corrosion behavior of YSZ and La2Zr2O7/YSZ thermal barrier coatings (TBCs) produced with CGDS method

Thermal barrier coatings (TBCs) commonly expose to oxidation, hot corrosion, and CaO-MgO-Al2O3-SiO2 (CMAS) attacks during the service condition. All of these attacks cause the spallation or delamination of TBCs from the bond coat. In this study, the effect of these attacks on YSZ and La2Zr2O7/YSZ TBC systems was investigated. YSZ and double layer La2Zr2O7/YSZ topcoats were deposited using electron beam physical technique (EB-PVD) on CoNiCrAlY bond coat produced by cold gas dynamic spray (CGDS) technique. CMAS tests at 1225 °C for 4, 8, 12, 16, 20, 24, 28, and 32 h and 5 h cyclic hot corrosion tests at 1000 °C were carried out on TBCs. Before and after the tests, TBCs were characterized using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray diffraction (XRD) and image analysis software program. At the end of the tests, the formed phases, microstructural changes, and general failure mechanisms were investigated in detail for each test. The general result shows that double-layered TBCs provide significant contributions and durability against high-temperature corrosive attacks of conventional YSZ TBC. In addition, in this study, hot corrosion damage mechanism and CMAS attack significantly damaged the column morphology of the EB-PVD method, reducing the thermal insulation properties and strain tolerance of TBCs. As a result of the tests, spallation and phase changes occurred in the damaged TBC systems.

MWCNTs Addition to Al2O3-SiC Binary Coating Deposited by Plasma Thermal Spray on Low Carbon Steel Substrate

Coating with alumina and silicon carbide on low alloy steel specimens surface was conducted by using plasma thermal spray technique. The coating was of different percentages of alumina and silicon carbide (75% alumina +25% silicon carbide, 50% alumina + 50% silicon carbide) and then carbon nanotube was added to the mixtures of powder with percentage of 1%. Cyclic hot corrosion test was done after coating and XRD and SEM were performed to all samples after corrosion. Coating thickness was measured by SEM cross-section micrographs. The weight change of bare and coated specimens after 50 hours of corrosion in 67% wt. V2O5 + 33% wt. Na2SO4 at 950 °C was measured. The maximum reduction in weight was shown by specimens coated with 50% wt. alumina +50%wt. silica carbide with addition of 1% carbon nanotube . X-Ray of coated samples indicated that no phase change occurred to the coating materials. Scanning electron microscopy imaging of surface layer clarified that surface layer has relatively moderate porosity and some coating layers contain micro-cracks.

The Cyclic Hot Corrosion Behavior of Pt–Al–7%YSZ Coating

In this study, the microstructure and cyclic oxidation of Pt–Al–7%YSZ thermal barrier coating applied on Rene-80 superalloy were investigated after Type I hot corrosion. The field emission scanning electron microscope and X-ray diffraction were used to evaluate the microstructure and phase identification, respectively. Na2SO4 was added to the surface of the coating for hot corrosion tests. The tests were carried out by the repeated cycles of heating at 900 °C for 30 and 60 min and rapid cooling to 400 °C in 10 min. Before the cyclic hot corrosion test, the specimens were coated with the platinum electroplating, high-activity low-temperature aluminizing, and the thermal spray of 7% yttria-stabilized zirconia (YSZ), respectively. The results showed that the cyclic hot corrosion mechanisms were the basic fluxing of primary thermally grown oxide (TGO), the formation of chromia, spinel and nickel oxides (C.S.N oxides) and Ni–Cr perovskites on the oxide scale. The type I cyclic hot corrosion resistance of Pt–Al–7%YSZ coating decreased with the number of thermal cycles due to a decrease in Al, an increase in Cr concentration, and increase in the volume fraction of C.S.N oxides and Ni–Cr perovskites in the oxide scale. As the number of thermal cycles increased, the oxide scale growth stresses and thermal mismatch stresses increased, leading to TBC degradation by the rumpling and ratcheting mechanisms in the Pt–Al/TGO interface. However, in lower thermal cycles, internal hot corrosion, and the formation of Kirkendall pores at the substrate/Pt–Al interface caused TBC degradation.

Microstructures and cyclic hot corrosion behavior of laser deposited Inconel 718 alloy under different heat treatment conditions

In this research, the hot corrosion behavior of laser deposited Inconel 718 alloy under different heat treatment conditions (solution plus aging, direct aging and as-deposited) is investigated in a salt mixture of Na2SO4 (87.5 wt.%) + NaCl (5 wt.%) + NaVO3 (7.5 wt.%) at 650 °C. The weight changes of the Inconel 718 specimens that have experienced different heat treatments are measured during the hot corrosion experiments and the data of the weight changes are used to determine the kinetics of the hot corrosion. Field emission scanning electron microscope (FE-SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) are employed to characterize the corrosion products of the specimens, thus the corrosion mechanisms are analysed. It is shown that the hot corrosion kinetic of the as-deposited Inconel 718 alloy follows a parabolic law. The element mapping results of the corroded surfaces reveal that the corrosion products of as-deposited Inconel 718 include two distinct layers, of which the outermost layer is mainly composed of Ni2CrO4, Cr2O3 and Fe3O4, while the inner layer contains Ni3S2. The laser deposited Inconel 718 alloys with different heat treatments behave differently in the hot corrosion. The hot corrosion performance of the solution treated specimens is better than that of the directly aged specimens, while the latter has better hot corrosion resistance than the as-deposited specimens. According to the hot corrosion test results and theoretical analyses, the hot corrosion behavior of as-deposited Inconel 718 alloy with different heat treatments in the salt mixture solution is closely related to basic fluxing mechanism, sulphidation and oxidation as well.

Corrosion behavior of the resistance sintered TiAl based intermetallics induced by two different molten salt mixture

The cyclic hot corrosion behaviors of TiAl based intermetallics were studied in two different molten salts consisting of % wt. Na2SO4-25NaCl and Na2SO4-25K2SO4 at 800 and 900 °C for 180 h, respectively. The aim of the study is to elucidate the effect of alloying elements on hot corrosion phenomena in the corrosive molten salt mixture. Corrosion kinetics revealed the Mo-added samples exhibit poor corrosion resistance in both corrosive salt mixture. It has been found that the NaCl-containing salt mixture clearly influence the extent of degradation due to the self-sustaining reaction mechanism.

Çevrimli ve İzotermal Sıcak Korozyonun Nikel Esaslı Süper Alaşım Malzemeye Etkisi

Nickel-based superalloys have a wide range of usage area such as gas turbines, nuclear reactors, generators. They are especially preferred in areas where mechanical strength and corrosion resistance are important. Hot corrosion is a highly corrosive type of damage in high temperature applications. In hot corrosion, molten salts easily react with reactive components, causing premature damage to the workpieces. In this study, Inconel 718 was subjected to hot corrosion tests as a nickel-based superalloy. 50% V2O5-50% Na2SO4 mixtures were used as corrosion salt. Cyclic and isothermal hot corrosion tests were carried out at 900 ° C for 18 hours. Corrosion products and microstructural changes formed in Inconel 718 were characterized by scanning electron microscopy (SEM), SEM-Energy Dispersive X-Ray (EDX), SEM-mapping and X-Ray Diffraction (XRD) analysis after hot corrosion tests. At the end of the isothermal and cyclic hot corrosion tests, Inconel 718, which has been subjected to cyclic hot corrosion, has been found to be more affected by the damage. Similar results were observed in terms of the corrosion products and the damage mechanism.

Hot Corrosion Behaviour of Electroless Deposited Nano-structured Cerium Oxide Coatings on Superalloy

Cerium oxide film was developed on Superni-75, via electroless method using cerium acetate and triethanolamine. Hot corrosion studies were conducted on bare substrate and the cerium oxide-coated superalloy after exposure to a molten salt environment of Na2SO4–60%V2O5 at 900 °C under cyclic conditions. The weight change measurements were taken to calculate the cyclic hot corrosion kinetics of the coatings at 900 °C. The structural features of the samples were characterized by SEM/EDS and XRD. It was observed that the corrosion rate of cerium oxide-coated superalloy is lower than that of the uncoated superalloy due to the formation of dense and protective oxide scale over the surface of the coatings. In spite of the combined effect of thermal cycling and an aggressive environment, the weight variation per unit area was found to decrease as the cerium concentration increased. Cerium oxide film was able to impede the movement of ions, thereby reducing the short-circuit diffusion of ions. Also, there was an improvement in resistance to spallation in the coated specimens with increase in cerium concentration.