Hot Corrosion Studies Research Articles

High-Temperature Oxidation and Hot Corrosion Studies on NiCrAlY Coatings Deposited by Flame-Spray Technique

The NiCrAlY coatings deposited by flame-spray technique on the superalloy substrate were oxidized in the presence of air and Na2SO4 + V2O5 salt at 900 °C for 100 cycles. The kinetics of oxidation showed that the coatings deposited by flame-spray technique possess better oxidation resistance compared with coatings deposited by high-velocity oxy fuel (HVOF)-sprayed technique. The oxidized coatings were further characterized by XRD, FESEM/EDS, and x-ray mapping techniques. The mechanisms of the oxidation and hot corrosion were substantiated by analyzing the results obtained from the various characterization techniques.

Effect of CeO2 on Cyclic Hot-Corrosion Behavior of Detonation-Gun Sprayed Cr3C2-NiCr Coatings on Ni-Based Superalloy

The hot-corrosion behavior of detonation-gun sprayed Cr3C2-NiCr coatings with and without 0.4 wt.% CeO2 additive on Ni-based superalloy inconel-718 is comparatively discussed in the present study. Hot-corrosion studies were carried out at 900 °C for 100 cycles in Na2SO4-60%V2O5 molten salt environment under cyclic heating and cooling conditions on bare and coated superalloys. The thermo-gravimetric technique was used to establish kinetics of hot-corrosion. XRD, FESEM/EDAX, and EDX mapping techniques were used to analyze the corrosion products of bare and coated samples. The results indicate that Cr3C2-NiCr-CeO2-coated superalloy showed better hot-corrosion resistance as compared to bare and Cr3C2-NiCr-coated superalloys. The addition of CeO2 has improved micro-hardness, porosity, and surface roughness values of Cr3C2-NiCr-CeO2 coating. The overall weight gain and parabolic rate constant of Cr3C2-NiCr-CeO2-coated superalloy were found to be lowest in the present study signifying that the addition of CeO2 in Cr3C2-NiCr powder has contributed to the development of adherent and dense oxide scale on the coating at elevated temperature.

Hot Corrosion Behaviour of HVOF Sprayed (Cr3C2–35 % NiCr) + 5 % Si Coatings in the Presence of Na2SO4–60 % V2O5 at 700 °C

The hot corrosion behavior of high velocity oxyfuel sprayed (Cr3C2–35 % NiCr) + 5 % Si coatings on three substrate alloys of SA213-T22, MDN-310 and Superfer 800 H was investigated. The as-sprayed coatings are characterized with respect to microstructure and mechanical properties. Thermocyclic hot corrosion studies were performed in a molten salt environment of Na2SO4–60 % V2O5 for 50 cycles at 700 °C. Thermogravimetric technique was used to establish the kinetics of hot corrosion of uncoated and coated steels. It was observed that the coated steel showed better hot corrosion resistance than the uncoated steels. The coated steels follow the parabolic rate law of oxidation and the parabolic rate constant values are appreciably lower in comparison to the uncoated steels. The coated steels showed slow oxidation kinetics which indicated that the reaction rate is diffusion limited. The oxides of Cr2O3 and SiO2 formed on the outermost surface of the coatings are found to be passive in the molten NaVO3 melt.

Evaluation of hot corrosion behavior of plasma sprayed scandia and yttria co-stabilized nanostructured thermal barrier coatings in the presence of molten sulfate and vanadate salt

In this study, the substrates of Inconel 738-LC superalloy were first sprayed with an NiCrAlY bondcoat and then with a nanostructured scandia (3.6mol%) and yttria (0.4mol%) co-stabilized zirconia (4SYSZ) topcoat by air plasma spraying (APS). Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45wt% Na2SO4+55wt% V2O5 molten salt at 910°C for 300h. The results show that the failure of this TBC occurred in bondcoating layer due to the formation of mixed oxide (NiO, NiAl2O4 and NiCr2O4) in this layer.

Chlorine based hot corrosion study of cold sprayed NiCrAlY coating

In this work, the NiCrAlY coating was deposited by cold spray deposition technique on Fe based Superfer 800H superalloy. The cold sprayed coating was found to be dense and hard and adhered strongly to the substrate superalloy. In the present investigation, the cyclic hot corrosion behaviour of the coated and bare specimens under the Na2SO4–10%NaCl environment at 900°C is described. It is found that unlike bare superalloy, uniform, dense and adhered scale layer formed on top of the coated specimen. The various characterisation techniques X-ray diffraction, SEM/EDAX and elemental X-ray mapping were used to analyse the corrosion products. It was found that the coating deposited by this coating technique successfully provided protection to the substrate against the hot corrosion degradation by reducing the weight gain of the superalloy.

Hot Corrosion Behaviour of Detonation-Gun Sprayed Cr3C2–NiCr Coating on Inconel-718 in Molten Salt Environment at 900 °C

Detonation gun sprayed Cr3C2–NiCr coating was deposited on nickel-based superalloy Inconel-718. Hot corrosion studies were conducted on uncoated as well as Cr3C2–NiCr coated superalloy in 75 wt% Na2SO4 + 25 wt% NaCl salt environment at 900 °C for 50 cycles. The thermo-gravimetric technique was used to establish kinetics of corrosion. X-ray diffraction and scanning electron microscopy/energy-dispersive X-ray analysis techniques were used to analyze the corrosion products. The uncoated superalloy suffered intense spalling, while the Cr3C2–NiCr coating provided good protection against hot corrosion in the given salt environment.

Hot Corrosion Studies in Coal Fired Boiler Environment

Hot corrosion behaviour of the bare and D-gun coated superfer 800H exposed to low temperature super-heater zone of the coal fired boiler of Guru Nanak Dev Thermal Power Plant, Bathinda, Punjab, India. The specimens were hanged in the platen super-heater of coal fired boiler where the gas temperature was around 900 °C ±10 °C. Hot corrosion experiments were performed for 10 cycles, each cycle consisting of 100 hours exposure followed by 1 hour cooling at ambient temperature. Weight change measurements were done at the end of each cycle. The weight change data used for predicting hot corrosion behaviour of the coated alloys after the total exposure of 1000 hours. The different phases and their distribution in the hot corroded specimens were analysed with the help of FE-SEM/EDS and X-ray mapping.

Hot Corrosion Studies of HVOF-Sprayed Coating on T-91 Boiler Tube Steel at Different Operating Temperatures

The aim of the present work is to investigate the usefulness of high velocity oxy fuel-sprayed 75% Cr3C2-25% (Ni-20Cr) coating to control hot corrosion of T-91 boiler tube steel at different operating temperatures viz 550, 700, and 850 °C. The deposited coatings on the substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 2%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of uncoated and coated samples. The corrosion products of the coating on the substrate are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for the corrosion mechanisms. It is found that the coated specimens have shown minimum weight gain at all the operating temperatures when compared with uncoated T-91 samples. Hence, coating is effective in decreasing the corrosion rate in the given molten salt environment. Oxides and spinels of nickel-chromium may be the reason for successful resistance against hot corrosion.

A Study of Hot Corrosion of Fe-Cr Alloys Induced by a Thin Film of Fused (Na,K)2SO4 by Electrochemical Impedance Spectroscopy

AbstractTo understand the hot corrosion mechanism of materials it is of significance to establish electrochemical measurements under a thin film of fused salts. In this paper, the hot corrosion behavior of Fe-Cr alloys containing 5, 10 and 25wt% Cr, respectively, in the presence of a fused film of 0.9Na2SO4-0.1K2SO4 (mole fraction) at 1173 K in air has been investigated by electrochemical impedance spectroscopy using a two-electrode system. The Nyquist plots for the corrosion of both Fe-5Cr and Fe-10Cr are composed of a very small semicircle at high frequencies and a line at low frequencies indicating that the corrosion is controlled by the diffusion of oxidants in the fused salt film. The alloys were severely corroded, forming a thick porous corrosion layer containing large amounts of non-protective precipitated Fe oxide particles. Conversely, the Nyquist plots for the corrosion of Fe-25Cr are composed of double capacitive loops, with significantly larger impedance values, which are associated with the formation of a protective Cr2O3 scale. Based on the experimental diffusion impedance results and the theoretic solubility and diffusion coefficient of oxygen in molten salts, respectively, a theoretical value of the diffusion flux of molecular O2 has been calculated.

Evaluation of hot corrosion behavior of CSZ, CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: usual CSZ, layer composite of CSZ/Micro Al2O3 and layer composite of CSZ/Nano Al2O3 in which Al2O3 was as a topcoat on CSZ layer. Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45wt% Na2SO4+55wt% V2O5 molten salt at 1050°C for 40h. The graded microstructure of the coatings was examined using scanning electron microscope (SEM) and X-ray diffractometer (XRD) before and after hot corrosion test. The results showed that no damage and hot corrosion products was found on the surface of CSZ/Nano Al2O3 coating and monoclinic ZrO2 fraction was lower in CSZ/Micro Al2O3 coating in comparison with usual CSZ. reaction of molten salts with stabilizers of zirconia (Y2O3 and CeO2) that accompanied by formation of monoclinic zirconia, irregular shape crystals of YVO4, CeVO4 and semi-cubic crystals of CeO2 as hot corrosion products, caused the degradation of CSZ coating in usual CSZ and CSZ/Micro Al2O3 coating.

Comparison of hot corrosion behaviors of plasma-sprayed nanostructured and conventional YSZ thermal barrier coatings exposure to molten vanadium pentoxide and sodium sulfate

Abstract The purpose of the current study was evaluation and comparison of hot corrosion behaviors of plasma-sprayed conventional and nanostructured yttria stabilized zirconia (YSZ) thermal barrier coatings (TBCs). Hot corrosion studies were performed on the surface of coatings in the presence of a molten mixture of V 2 O 5 +Na 2 SO 4 at 1000 °C for 30 h. Results indicated that the hot corrosion mechanisms of conventional and nanostructured YSZ coatings were similar. The reaction between corrosive salt and Y 2 O 3 produced YVO 4 , leaching Y 2 O 3 from YSZ and causing the detrimental phase transformation of zirconia from tetragonal to monoclinic. The nanostructured coating, as compared to its conventional counterpart, in spite of a further reaction with the corrosive salt, showed a higher degradation resistance during the hot corrosion test due to increased compliance capabilities resulting from the presence of an extra source of porosity associated with the nano-zones.

Hot Corrosion Studies on As-received and HVOF Sprayed Al2O3+CoCrAlTaY on Ti-31 Alloy in Salt Environment

The present investigation is on the High Velocity Oxy-Fuel coating of Ti-31 alloy using fused, blended powder, Al2O3+CoCrAlTaY. Hot corrosion experiments were done on the coated and uncoated Ti-31 under a salt environment of 50% Na2SO4 and 50%V2O5 at 800°C. Thermogravimetric cycles of 1 hour heating and 20minute cooling was followed for hot corrosion study. After each cycle weight measurement was carried out. After hot corrosion, the samples were characterized using XRD and SEM/EDS. It was observed that the coated sample is more resistant to hot corrosion than uncoated sample. XRD and SEM analysis indicated that the surface is rich in oxides. Gravimetric analysis indicated that the sample weight gain was following a parabolic relationship with time. A damage mechanism is discussed for the hot corrosion damage of the samples. Presence of high level of chromium in HVOF sprayed coating imparts improved hot corrosion resistance at 800°C, in a molten salt environment of Na2SO4 + V2O5.

Investigations on the Performance of Gta Welded Dissimilar Aisi 304 and Aisi 310s Subjected to Cyclic Hot Corrosion

This research work aims at investigating the performance of the gas tungsten arc welded dissimilar AISI 304 and AISI 310S when subjected to cyclic hot corrosion in air oxidation and in the molten salt environment of K2SO4 + 60% NaCl environment at 600°C. The weldments are characterized for its mechanical and metallurgical properties. Tensile strength of the weldment is found to be 529 Mpa. Further the hot corrosion studies are performed on the various zones of the weldment and the corrosion products are characterized by the combined techniques of optical microscopy, XRD and SEM/EDAX analysis.

Effect of Filler Metal’s Choice on the Mechanical and Corrosion Properties of Gas Tungsten Arc Welded Aisi 304l

In this research work, an attempt has been made to study the weldability of AISI 304L using filler materials ENiCu-7 and ENiCrMo-3 by Gas Tungsten Arc Welding (GTAW) process. Investigations are carried out on the metallurgical and mechanical properties of AISI 304L weldments. Further, cyclic hot corrosion studies are performed on the various zones of the weldment in air oxidation for 50 cycles at 600 °C. Thermogravimetric data is collected at the end of every cycle and the weldments are characterized using XRD analysis.

Effect of Filler Metal’s Choice on the Mechanical and Corrosion Properties of Gas Tungsten Arc Welded AISI 304l

In this research work, an attempt has been made to study the weldability of AISI 304L using filler materials ENiCu-7 and ENiCrMo-3 by Gas Tungsten Arc Welding (GTAW) process. Investigations are carried out on the metallurgical and mechanical properties of AISI 304L weldments. Further, cyclic hot corrosion studies are performed on the various zones of the weldment in air oxidation for 50 cycles at 600 C. Thermogravimetric data is collected at the end of every cycle and the weldments are characterized using XRD analysis.

Hot Corrosion Studies of GTA Welded Monel 400 Exposed to Thin Film of Molten Na<sub>2</sub>so<sub>4</sub> - 60% V<sub>2</sub>O<sub>5</sub> at 600°C

In this research work, an attempt has been made to investigate the gas tungsten arc welded Monel 400 plates using ENiCu-7 for its mechanical and corrosion properties. Microstructure examination reveals the coarse grain formation of Ni-Cu phase in the heat affected zone (HAZ) and well defined dendrites are observed in the weld region. Tensile tests showed that fracture occurred at the weld interface and the ultimate tensile strength of the Monel weldment is found to be 515 MPa. Furthermore hot corrosion studies were performed on the different zones (Weld, HAZ and Base metal) of Monel 400 exposed to the thin film of molten salt environment of Na2SO4 - 60% V2O5 at 600°C. It was found that corrosion was extremely severe on the fusion zone after exposed to 50 cycles. The corrosion products were characterized using the combined techniques of optical microscopy, XRD, SEM/EDX analysis

Studies on Thin Film Oxide Formation on GTA Welded AISI 304 & AISI 310S Exposed at 600°C

In this research work, an attempt was made to study the thin film oxide formation on the dissimilar gas tungsten arc welded austenitic stainless steels of grades AISI 304 and AISI 310S using E309L filler wire. These combinations are widely used in power plant and marine applications. An assessment was carried out to estimate the metallurgical, mechanical properties of the weldment. Tensile tests revealed that the fracture occurred at the weld zone. In addition, Vickers Micro-hardness tests were performed across the length of the weldment. Furthermore, hot corrosion studies were performed on different regions (heat affected zones, weld zone and parent metal zones) of the weldment subjected to air oxidation at 600°C. It was revealed from thermogravimetric analysis that there was weight loss in the heat affected zone of the AISI 310S side after 50 cycles. The thin film oxide layer formation on various zones of the weldment was examined using the combined techniques of optical microscopy, XRD and SEM/EDAX analysis.

High temperature oxidation and corrosion behaviour of Ni/Ni–Co–Al composite coatings

In the present study, Ni/Ni–Co–Al composite coatings were developed by a potentially simple, scalable, non-vacuum technique namely electrodeposition. These coatings were characterized for their microhardness, oxidation and hot corrosion behaviour. An increase in Co content in the matrix from 8wt% to 70wt% led to an increase in the Al particle incorporation from 12wt% to 21wt%. A change in the surface morphology of the coatings with variation in Co content was seen. The oxidation behaviour of the coatings was studied at temperatures in the range of 400°C to 1000°C. The influence of vacuum treatment on the high temperature behaviour of the coatings was also investigated. The intermetallic aluminide phase formation was observed in the temperature range of 600–800°C and a homogenized structure was seen at 1000°C. The oxidation rate in terms of weight gain was marginally lower for vacuum pretreated NiAl coating annealed at 1000°C. A significant increase in the oxidation rate was exhibited by Ni–70Co–Al coating beyond 800°C showing its poor oxidation behaviour. The characterization studies revealed the formation of stable alumina in the case of NiAl while, metastable alumina was observed in Ni–Co–Al coatings. The hot corrosion studies showed that Co rich Ni–Co–Al exhibited better resistance compared to Ni rich coatings. An optimum cobalt content of 30wt% was desirable for high temperature oxidation and corrosion resistance.

Evaluation of hot corrosion behavior of plasma sprayed ceria and yttria stabilized zirconia thermal barrier coatings in the presence of Na2SO4+V2O5 molten salt

In this study, substrates of Inconel 738 LC superalloy coupons were first sprayed with a NiCoCrAlY bondcoat and then with a ceria and yttria stabilized zirconia (CYSZ; ZrO2−25wt%CeO2−2.5wt%Y2O3) topcoat by air plasma spraying (APS). Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45wt%Na2SO4+55wt%V2O5 molten salt at 1000°C for 30h. The results showed that the coating defects, such as pores and microcracks play important roles as effective paths for the salt penetration in hot corrosion. Based on the results, the reaction between molten salt and stabilizers of zirconia (Y2O3 and CeO2), the formation of YVO4, CeVO4 and CeO2 crystals, the detrimental phase transformation of zirconia from tetragonal to monoclinic due to the depletion of stabilizers and finally, the creation of stresses were recognized to be in the degradation mechanism of CYSZ ceramic coatings in the presence of molten sulfate–vanadate salt.

The effects of post-treatment on the hot corrosion behavior of the HVOF-sprayed Cr3C2–NiCr coating

In the present paper hot corrosion behavior of high velocity oxy-fuel (HVOF) sprayed Cr3C2–NiCr coating on ASME-SA213-T91 boiler steel has been investigated. In addition, influence of post treatment of the coating by two different methods, i.e. sealing and heat treatment, on the hot corrosion behavior has also been investigated. Hot corrosion studies were conducted on bare steel, as-deposited Cr3C2–NiCr coating and on post treated Cr3C2–NiCr coating on T91 steel after exposure to a molten salt (Na2SO4–60% V2O5) environment at 900°C under cyclic conditions. Each cycle consisted of 1h heating in the silicon carbide tube furnace followed by 20min cooling in air. The mass gain measurements were performed after each cycle to establish the kinetics of corrosion using thermogravimetric technique. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDAX) and X-ray mapping techniques were used to analyze the corrosion products.Coated steel showed better resistance to hot corrosion as compared to the uncoated counterpart. As-deposited Cr3C2–NiCr coating experienced two stages of hot corrosion, i.e. initiation stage and propagation stage. Once the coating entered the propagation stage, the protective scale rendered ineffective and substrate became vulnerable to hot corrosion. Post treated coating resulted to be more effective than as-deposited coating in enhancing the corrosion resistance of T91 steel.