Hot Corrosion Kinetics Research Articles

High temperature corrosion performance of Inconel 625 hard overlays deposited on stainless steel substrate

AbstractIn elevated conditions, austenitic stainless steels suffer intense damage by corrosion because of the severe oxide scale development. Giving stainless steel a corrosion‐resistant surface coating can prevent excessive damage. In this case, welding‐based overlaying is a potential method to restore the damaged parts as it has a higher productivity rate. This investigation articulates the corrosion resistance behavior of the multiple overlapped Inconel 625 (IN625) weld overlays on the AISI 316L substrate plate. The corrosion resistance at elevated temperatures was investigated with kinetic curves, further revealing the corrosion products from the sample surface. Characterization techniques like scanning electron microscopy with energy‐dispersive spectroscopy, X‐ray diffraction, and Raman spectroscopy were employed to analyze the corrosion products. Hot corrosion kinetics in the molten salt environment (Na2SO4 + V2O5) revealed that the IN625 overlay samples gained a weight of 40.15 mg/cm2, and the substrate had a weight gain of 65.42 mg/cm2. From the hot oxidation kinetics, it is evident that the mass gained by the substrate is more than twice that of the Inconel overlay sample. Furthermore, the formation of oxides and spinel phases rich in nickel (NiO, NiCr2O4) highly influenced the corrosion kinetics of the multiple overlapped IN625 overlay sample. The AISI 316L substrate sample revealed the existence of critical oxide phases such as Fe2O3 and Cr2O3, which were lesser and did not influence the corrosion resistance at 900°C. In the present investigation, the multiple overlapped IN625 weld overlay offered excellent corrosion resistance at high temperature compared to the AISI 316L.

Preparation and hot corrosion properties of the AlCrFeCoNi0.5Si high-entropy alloy coating deposited by the air plasma spraying

Normal AlCrFeCoNi and Si-modified AlCrFeCoNi0.5Si high-entropy alloy (HEA) coatings were deposited on a Ni-based superalloy by air plasma spraying technique. Hot corrosion properties of the normal and Si-modified AlCrFeCoNi0.5Si HEA-coated samples were evaluated at 900 °C under the corrosive salt environment containing 45% Na2SO4 and 55% V2O5. The hot corrosion kinetics of both normal and Si-modified coatings were accordingly investigated using mass gain measurements. X-ray diffraction, field emission scanning electron microscopy, and energy dispersive x-ray analysis were used for the microstructural characterizations. The results indicated that the Si-modified AlCrFeCoNi0.5Si HEA coating has a greater resistance to hot corrosion compared with the normal AlCrFeCoNi HEA coating. The results obtained are primarily related to the formation of Si-rich precipitates, which inhibit the outward diffusion and, therefore, reduce the formation of the corrosion products on the AlCrFeCoNi0.5Si HEA coating surface.

Hot oxidation and corrosion behaviour of boiler steel fabricated by wire arc additive manufacturing

Boiler Steels undergo severe degradation in corrosion resistance due to oxide scale formation at elevated temperatures. In this study, the comparative hot oxidation and hot corrosion resistance of wire arc additive manufactured SS 308L (WAAM 308L) was examined in hot air and Na2SO4–60% V2O5 molten salt environments at 700 °C. The corrosion resistance at elevated temperature was analysed using thermo-kinetic curves, corrosion products, and morphology of the oxides. The hot oxidation kinetics revealed that WAAM processed SS 308L specimens has excellent resistance and the weight gain reached 3.10 mg/cm2 with thinner oxide scale formation. Hot corrosion kinetics of WAAM processed SS 308L specimens highlighted the higher weight gain (37.0 mg/cm2) in molten salt environment and is attributed to the acceleration of oxide scale formation by the salts at elevated temperatures. Also, the development of Ni3V2O8 and Fe2O3 along with the depletion of Cr2O3 significantly influenced the corrosion resistance at elevated temperatures. The findings of this study reveal the potential of WAAM to produce customized parts for high-temperature applications.

Effect of Ce addition on hot corrosion behaviours of a cast γ′-strengthened Co–Al–W–Mo–Ta–B alloy at 800 ​°C

Hot corrosion behaviours of a novel Co–9Al-4.5W-4.5Mo–2Ta-0.02B alloy doped with 0.01, 0.05, 0.1 and 0.2 ​at% Ce exposed at 800 ​°C in a solution of 75%Na2SO4/25%NaCl were investigated. The alloys comprised a coherent γ-CoSS/γ′-Co3(Al, W) microstructure (0.01Ce and 0.05Ce alloys) and κ-Co3(W, Mo) precipitates (0.1Ce and 0.2Ce alloys) at grain boundaries. Hot corrosion kinetics curves demonstrated the parabolic time dependency profile with two stages: the first parabolic stage is within the beginning ∼50 ​h corrosion and follows by the second parabolic stage. With an increasing nominal Ce content the weight gain of the alloy significantly decreased from approximately 70.1 ​mg ​cm−2 (0.01Ce) to 40.8 ​mg ​cm−2 (0.2Ce) when exposed for 100 ​h. A two-layer corrosion scale formed, and the scale was composed of an outer layer of Co3O4 oxide with spinel compounds of CoAl2O4, CoWO4and CoSO4, and an inner γ/needle-like Co3W/sulphide layer adhered to the substrate. Heavy spallation of the corrosion scale occurred in the 0.01Ce∼0.1Ce alloys, however, spallation was slight in the 0.2Ce alloy. The excellent corrosion resistance of the 0.2Ce alloy could be attributed mainly to the formation of continuous Al2O3 lines in the corrosion scale, as well as the prolongation of the incubation period of the corrosion product spallation.

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.

Development of CNT mixed HVOF coating for water tube boiler material to improve the degradations

In most of the thermal power plants, one of the vital components is boiler tube. In thermal power plants the down fall in profits due to hot corrosion, oxidation and erosion. In order to improve properties of the above said, the researchers working on the wide area of combating hot corrosion, erosion and oxidation of boiler tubes by using thermal spray coatings (HVOF) with the combination of carbon nanotubes (CNTs). The corrosion occurs, when the material exposed to open atmosphere it will get corroded. The oxidation occurs due to the material is exposed to high temperatures. Erosion occurs due the presence of unburnt particles striking at high velocity. There by the application of HVOF (High-Velocity Oxy-Fuel) coating playing a vital role in approaching using HVOF parameters such as speed, high temperature, oxygen with fuel flow and fuel ratio, powder flow rate. In the present studies the resistance improvement of corrosion, oxidation and erosion of the boiler tube material SA213 T11. The advantages of HVOF coatings with CNTs percentages like CNT 3%, 5%, 7%. are due to its low porosity, high dense coatings, more adhesive in nature and bond strength also more. The kinetics of hot corrosion, oxidation and erosion will be analysed by measuring weight gain after every cycle under thermogravimetric studies (1 h heating and 20 min cooling). The results will be achieved by using visual examination, and advanced microscopy like XRD and SEM/EDS analysis.

Sodium sulphate and V2O5-induced hot corrosion kinetics and oxides characteristics of the weldments in SA213 T11 steels

Fuels used in boilers/gas-turbines, pulp & paper industry, diesel-engines and other machinery, contains vanadium or sulphates and form compounds of low melting phase during combustion which are more corrosive for high temperature materials. This research-article examines the corrosion kinetics and oxide of weldments in steels (SA213- T11) in molten-salt (Na2SO4-60% V2O5) environment under thermal cyclic conditions-at 900 °C. The thermogravimetry technique was used to elucidate the kinetics of corrosion. The results reveal the TIG weldment indicated lower weight gain during some initial cycles than those of SMAW weldment but subsequently increased due to the voids-formation in inner scale of HAZ, and non-availability of Cr in oxide of TIG weldment as reported in the given environment leads to high corrosion rate.

Oxidation and hot corrosion behaviors of Nb−Si based ultrahigh temperature alloys at 900 °C

Abstract Oxidation and hot corrosion behaviors at 900 °C of Nb−Si based ultrahigh temperature alloys were investigated. Both oxidation and hot corrosion kinetics curves of the alloy involve an initial parabolic stage and a later rapid linear stage. In the initial oxidation stage (1−50 h), a thin and continuous scale is formed on the alloy surface, while severe pest degradation phenomenon is observed in the linear oxidation stage. Compared with oxidation of the alloy in static air, a linear hot corrosion stage happens earlier and catastrophic scale disintegration occurs after hot corrosion for 20−100 h, demonstrating that molten salts (Na2SO4 and NaCl) could significantly accelerate the oxidation process of the alloy. STEM results indicate that the corroded scale consists mainly of TiO2, Nb2O5, TiNb2O7, amorphous silicate and NaNbO3.

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.

Hot corrosion behavior of nanostructured and conventional HVOF Cr3C2NiCrBSi coatings on superalloy

In the present work, optimized Cr3C2NiCrBSi nanostructured and conventional powder coatings are sprayed on to Inconel 718 material by high-velocity oxygen fuel coating process for enhancing the high temperature corrosion resistance. The cyclic hot-corrosion study is carried out on uncoated, conventional coated and nanostructured coated superalloy in the salt environment of Na2SO4 and 60% V2O5 at 900 °C. The weight change measurements are carried out after each cycle to investigate the hot corrosion kinetics of uncoated and coated superalloy using thermo gravimetric technique. The corrosion products of coated specimens are analysed using Scanning electron microscopy and Elemental mapping. Cr3C2NiCrBSi nanostructured coating provides dense coating compared to conventional coating. Compared to uncoated sample, Cr3C2NiCrBSi coating is found to be highly effective in corrosive resistance. The hot corrosion resistance provided by Cr3C2NiCrBSi coatings may be associated with the development of oxides of chromium, nickel, silicon and spinels of chromium and nickel.

Comparative study of hot corrosion behavior of thermal sprayed alumina and titanium oxide reinforced alumina coatings on boiler steel

The current work evaluates the effect of hot corrosion on ASTM-SA213-T-22 Steel with a coating of 100Al2O3 and 20TiO2-Al2O3 at 900 °C in the molten salt environment of (Na2SO4-60% V2O5). The coatings were sprayed by plasma spray technique. The bare and coated samples were placed inside the muffle furnace at 900 °C for 50 cycles. Each cycle consisted of heating for 1 h and 20 min of cooling at ambient temperature. The kinetics of hot corrosion was analyzed by measuring mass gain after each cycle. The results were achieved by using visual examination, mass change measurement, XRD and SEM/EDS analysis. The result revealed that bare steel was more affected with corrosion and gained more mass due to the development of iron oxide (Fe2O3) than the coated sample. Fe2O3 and Cr2O3 were found as major phase in oxide scale of T-22 uncoated steel specimen. The 100Al2O3 and 20TiO2-Al2O3 coated specimen indicated the reduction in mass gain by 25.41% and 67.02% respectively as compared to uncoated specimen. Al2O3 coating with the combination of TiO2 showed better adhesion properties. The presence of TiO2 enhanced the durability and strength of Al2O3 coating to withstand high temperatures.

Microstructure and Hot Corrosion of GH2036 Alloy Treated by Laser Shock Peening

The effects of laser shock peening (LSP) on the microstructure, residual stress, microhardness and hot corrosion of GH2036 alloy at high temperature (700°C) were investigated by transmission electron microscopy, x-ray diffraction, Vickers hardness testing and scanning electron microscopy, respectively. The results show that many crystal defects and precipitate phases were induced by LSP. The maximum surface residual compressive stress and micro-hardness of the LSP-treated sample with 9 J of pulse energy were 520 MPa and 275 HV, respectively. The hot corrosion kinetics of GH2036 showed that the sample treated with 9 J of pulse energy recorded the lowest mass loss (3.85 mg/cm2) compared to the untreated sample (11.35 mg/cm2). Higher crystal defects provided the diffusion channels of elements (Fe, Cr, etc.), which facilitated the formation of a denser and more homogeneous oxide layer compared to untreated samples. In addition, the spallation of the oxide layer was obviously alleviated after LSP.

Effect of synthetic biomass ash on high temperature corrosion behavior of super austenitic stainless steel 904L

Cyclic hot corrosion behavior of the super austenitic stainless steel 904L was investigated in the temperature range 500 °C–650 °C, in synthetic biomass ash environment of alkali salts of NaCl, KCl, Na2SO4 and K2SO4, up to 100 h. Sample surface was mechanically polished and spray coated with alkali chlorides and sulfates with two different ratios of chlorine and sulfur, Cl:S = 40:60 with salt mixture SM1 and Cl:S = 60:40 with SM2, to 3–4 mg cm−2. The hot corrosion kinetics was studied by weight change of the salt coated samples and the corroded samples were characterized by SEM/EDS, XRD and EPMA. The hot corrosion rate of the samples coated with SM1 and SM2 at 500 °C was comparable and lowest. However, at higher temperatures from 550 °C–650 °C the rate of corrosion from SM2 was significantly higher than that from the SM1. The higher content of sulfur in SM1 reduced the severity of corrosion resulting from chlorine. There was significant difference in surface morphology of the samples corroded with SM1 and SM2 salt mixtures. The higher rate of hot corrosion from SM2, with higher ratio of Cl:S (60:40) was found to be associated with porosity developed in the protective oxide layer by volatile chlorides and consequent active oxidation of the material.

Studies on High-Temperature Corrosion resistance of low carbon Steel in Actual boiler environment

In coal based power generated plants, hot corrosion and erosion are identified as serious problems, resulting in deterioration of boiler tube. In this present study, the High-Temperature Corrosion behavior of bare SA 210 Gr. A1 steel and Friction stir processed Cr3C2-25(NiCr) inserted Gr. A1 steel samples were compared under cyclic condition of 1500 hours in actual both environments. The kinetics of hot corrosion was measured on the base of weight-change after each cycle. The tool rotation speed for three processed samples were recorded as 1600rpm, 2000rpm, 2400 rpm respectively with transverse speed of 30 mm/min processed by three number of passes. Microstructure evolution of base and FSPed was done by electron back scatter diffraction (EBSD). X-ray diffraction (XRD)and scanning electron microscopy (SEM) along with energy dispersive spectroscopy (EDS) were performed on the exposed sample to analyze the corrosion products. The base steel sample suffered extreme erosion-corrosion in actual boiler environment, where as processed samples exhibits better hot corrosion resistance (64%, 66%, 73%) respectively. The processed samples also showed 4 fold refined grains, as confirmed by (EBSD).

Hot corrosion behaviour of HVOF sprayed Ni3Ti and Ni3Ti + (Cr3C2 + 20NiCr) coatings in presence of Na2SO4-40%V2O5 at 650 °C

Ni3Ti and Ni3Ti + (Cr3C2 + 20NiCr) coatings were deposited on gas turbine based ASTM B265 titanium (Ti-15) and AISI 420 stainless steel (MDN-420) substrate materials using HVOF technique. Thermocyclic hot corrosion tests were carried out at 650 °C in molten salt environment of Na2SO4 − 40%V2O5 for about 50 cycles. Thermogravimetric analysis was carried out to study the hot corrosion kinetics of uncoated and coated titanium and stainless steel substrates. The weight gain per unit area showed that the coated substrate materials displayed better resistance to hot corrosion when compared with that of uncoated substrate materials. The surface morphology of uncoated and coated substrate materials were analysed using scanning electron microscopy and elemental analysis. The formation of different types of oxides and compounds were analysed using x-ray diffraction. The uncoated substrates surface showed microspalling at several regions while coated substrates surface were composed of protective oxide layers. The presence of ternary NiCr2O4 protective oxides on the surface of Ni3Ti + (Cr3C2 + 20NiCr) coated substrates leads to reduction in the diffusion of corrosive species inside the coating.

Correlations between the kinetics and the mechanisms of hot corrosion of pure nickel at 700 °C

This study investigates the kinetics of hot corrosion of pure nickel exposed to different testing conditions at 700 °C. Whereas nickel underwent pure oxidation when exposed to synthetic air only, the addition of 0.5% SO2/SO3 induced oxidation-sulphidation and considerably increased the corrosion kinetics. In the presence of sodium sulphate (Na2SO4), Ni-S liquid solutions systematically formed at the metal/scale interface. This was attributed to the shielding effect of Na2SO4 implying negative pO2 and pSO3 gradients from the gas/salt interface towards the scale/salt interface. The influence of thermal cycling as well as the testing method (i.e. half-embedding and salt-recoating) was also elucidated.

Hot corrosion performance and electrochemical study of CoNiCrAlY/YSZ/YSZ-La2O3 multilayer thermal barrier coatings in the presence of molten salt

One of the effective methods to prevent materials degradation at high temperatures is thermal barrier coating (TBC). In this research, a multilayer (CoNiCrAlY/YSZ/YSZ-10 wt%La2O3) TBC system was deposited on the IN738 superalloy using the high-velocity oxy-fuel (HVOF) and atmospheric plasma spray (APS) techniques. Hot corrosion behaviors were studied in the presence of Na2SO4-20 wt% V2O5 molten salt at 880 °C for 400 h. Hot corrosion kinetics were determined by measuring the thermal grown oxide (TGO) thickness of the specimens at regular intervals. Afterward, the electrochemical behavior of the coated and hot corroded samples was investigated by electrochemical polarization method. Phase identification, TGO, surface morphologies and elemental distribution of the coated samples were also studied during hot corrosion by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The results showed that the multilayer YSZ/YSZ-10 wt%La2O3 ceramic top coat led to a reduction in the penetration of corrosive components into the coating; thereby, delaying the formation of destructive YVO4 phases and postponing the monoclinic-ZrO2 phase transformation.

SMA, GTA and P-GMA dissimilar weld joints of 304LN stainless steel to HSLA steel; Part-2: hot corrosion kinetics

A study of hot corrosion kinetics of dissimilar weld joints of 304LN austenitic stainless steel and HSLA steel, which are welded by commonly used arc welding processessuch as P-GMAW (pulsed current gas metal arc welding), GTAW (tungsten inert gas welding), and SMAW (shielded metal arc welding), has been conducted. The joints were then exposed to a molten salt environment of K2SO4 + NaCl (50%) at 923 K (650 °C) under cyclic conditions, that is, 1 h of heating and subsequently 20 min of cooling. A gravimetric analysis was conducted and it was observed that salt-coated samples follow a parabolic rate law. SEM/EDAX, XRD, and line mapping were used to systematically characterize the resulting oxide scale in the dissimilar weld joints. The influence of the molten salt environment on the hot corrosion behavior of dissimilar welded specimens is also discussed. From the hot corrosion studies, it was concluded that the longer the duration of exposure to molten salt, the higher the weight gain by the weldment.

The corrosion behaviours of plasma-sprayed Fe-based amorphous coatings

ABSTRACTFe-based amorphous coatings are increasingly recognised as promising candidates for the protection of coal-fired boilers against corrosion. The present study prepared Fe-based amorphous coatings on a T91 substrate by plasma spraying technology. The corrosion behaviour of the coating in hot Na2SO4 + K2SO4 salts at 700°C was investigated, and measurements of the mean mass gain were performed after each cycle to establish the hot corrosion kinetics of the coatings using the thermogravimetric technique. The coated specimens, especially specimens with 380-μm-thick coatings, exhibited lower mean gain rates at all operating cycles as compared to the uncoated T91 samples. The highest hot corrosion resistance was a result of the amorphous composite microstructure and high Cr and Ni elemental contents, which contributed to the formation of the protective oxides of chromium and nickel such as Cr2O3, NiO and NiCr2O4.