Hot Corrosion Kinetics Research Articles

High temperature cyclic oxidation and hot corrosion behaviours of superalloys at 900°C

Oxidation and hot corrosion are serious problems in aircraft, marine, industrial, and land-base gas turbines. It is because of the usage of wide range of fuels coupled with increased operating temperatures, which leads to the degradation of turbine engines. To obviate these problems, superalloys, viz. Superni 75, Superni 718 and Superfer 800H superalloys (Midhani grade), are the prominent materials for the high temperature applications. It is very essential to investigate the degradation mechanism of superalloys due to oxidation and hot corrosion and substantiate the role of alloying elements for the formation of protective oxide films over the surface of the superalloys. Therefore, the present work investigates the oxidation and hot corrosion behaviour of superalloys exposed to air and molten salt (Na2SO4–60% V2O5) environment, respectively, at 900°C under cyclic conditions. The weight change measurements made on the superalloys during the experiments are used to determine the kinetics of oxidation and hot corrosion. X-ray diffraction (XRD), X-ray mapping and field emission scanning electron microscope (FESEM, FEI, Quanta 200F company) with EDAX Genesis software attachment, made in Czech Republic are used to characterize the corroded products of the superalloys. It is observed that the formation of scale rich in Cr2O3, NiO and spinel NiCr2O4 has contributed for the better oxidation and hot corrosion resistance of Superni 75; whereas relatively lesser hot corrosion resistance of Superfer 800H is due to the formation of non-protective oxides of iron and sulphides of iron and nickel. The parabolic rate constants calculated for the superalloys show that the corrosion rate is minimum in air as compared to molten salt environment.

Hot corrosion behaviour of plasma sprayed YSZ/Al 2O 3 dispersed NiCrAlY coatings on Inconel-718 superalloy

Oxide dispersed NiCrAlY bond coatings have been developed for enhancing thermal life cycles of thermal barrier coatings (TBCs). However, the role of dispersed oxides on high temperature corrosion, in particular hot corrosion, has not been sufficiently studied. Therefore, the present study aims to improve the understanding of the effect of YSZ dispersion on the hot corrosion behaviour of NiCrAlY bond coat. For this, NiCrAlY, NiCrAlY + 25 wt.% YSZ, NiCrAlY + 50 wt.% YSZ and NiCrAlY + 75 wt.% YSZ were deposited onto Inconel-718 using the air plasma spraying (APS) process. Hot corrosion studies were conducted at 800 °C on these coatings after covering them with a 1:1 weight ratio of Na 2SO 4 and V 2O 5 salt film. Hot corrosion kinetics were determined by measuring the weight gain of the specimens at regular intervals for a duration of 51 h. X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques were used to determine the nature of phases formed, examine the surface attack and to carry out microanalysis of the hot corroded coatings respectively. The results show that YSZ dispersion causes enhanced hot corrosion of the NiCrAlY coating. Leaching of yttria leads not only to the formation of the YVO 4 phase but also the destabilization of the YSZ by hot corrosion. For the sake of comparison, the hot corrosion behaviour of a NiCrAlY + 25 wt.% Al 2O 3 coating was also examined. The study shows that the alumina dispersed NiCrAlY bond coat offers better hot corrosion resistance than the YSZ dispersed NiCrAlY bond coat, although it is also inferior compared to the plain NiCrAlY bond coat.

Evaluation of cyclic hot corrosion behaviour of detonation gun sprayed Cr 3C 2–25%NiCr coatings on nickel- and iron-based superalloys

In the present investigation, Cr 3C 2–NiCr cermet coatings were deposited on two Ni-based superalloys, namely superni 75, superni 718 and one Fe-based superalloy superfer 800H by detonation-gun thermal spray process. The cyclic hot-corrosion studies were conducted on uncoated as well as D-gun coated superalloys in the presence of mixture of 75 wt.% Na 2SO 4 + 25 wt.% K 2SO 4 film at 900 °C for 100 cycles. Thermogravimetric technique was used to establish the kinetics of hot corrosion of uncoated and coated superalloys. X-ray diffraction, FE-SEM/EDAX and X-ray mapping techniques were used to analyze the corrosion products for rendering an insight into the corrosion mechanisms. It was observed that Cr 3C 2–NiCr-coated superalloys showed better hot-corrosion resistance than the uncoated superalloys in the presence of 75 wt.% Na 2SO 4 + 25 wt.% K 2SO 4 film as a result of the formation of continuous and protective oxides of chromium, nickel and their spinel, as evident from the XRD analysis.

Thermodynamic and kinetic consideration on the corrosion of Fe, Ni and Cr beneath a molten KCl–ZnCl 2 mixture

Thermogravimetric (TG) experiments have been carried out to study the kinetics of hot corrosion of Fe, Cr and Ni, covered by a molten KCl–ZnCl 2 mixture of a composition close to the eutectic (50 mol% KCl–50 mol% ZnCl 2). Furthermore binary and ternary phase diagrams were calculated in order to describe the corrosion process. The tests were conducted at a temperature of 320 °C in an atmosphere consisting of argon and oxygen. For iron different stages are observed in a TG curve. They can be attributed to the different reaction steps of iron chloride formation (incubation phase), oxide precipitation (linear stage) and scale formation (parabolic or logarithmic stage). Based on these observations a model, described by Spiegel [A. Spiegel, Molten Salt Forum 7 (2003) 253], is confirmed. For Cr and Ni these stages are not observed. At 8 vol% O 2 only slight oxidation of Cr and Ni was observed accompanied by evaporation of the salt deposit. At 16 vol% O 2 the rate of oxidation increases and the experiments yield a curve that is either parabolic or logarithmic for both Ni and Cr. As a result it is shown that the solubility of iron chloride in the KCl–ZnCl 2 melt is higher than the solubility of nickel chloride and chromium (III) chloride in the KCl–ZnCl 2 melt. This enables a higher diffusibility of iron chloride to the upper region of the melt where a higher oxygen partial pressure ( p(O 2)) is present leading to a higher oxidation rate of iron.

The preparation and hot corrosion resistance of gradient NiCoCrAlYSiB coatings

An Al and Cr gradient NiCoCrAlYSiB coating was prepared by a two-step deposition: (i) deposition of a NiCoCrAlYSiB layer onto Ni-based superalloy surface by arc ion plating (AIP); and (ii) co-deposition of NiCoCrAlYSiB and Cr–Al by AIP and magnetron sputtering (MS), respectively. The results of EPMA show that Al and Cr have a gradient distribution in the surface layer of the gradient coatings. After annealing, the NiCoCrAlYSiB coatings consist of γ/γ′ and β-NiAl, while the gradient coatings consist of σ-(Cr, Co), AlCr 2, and Al 8Cr 5, besides γ/γ′ and β-NiAl. The phases of σ-(Cr, Co), AlCr 2, and Al 8Cr 5 can act as reservoir phases of Cr and Al to improve the hot corrosion resistance and lifetime of the coatings. In order to investigate the effect of Al and Cr on the hot corrosion of coatings, NiCoCrAlYSiB coatings and the gradient coatings were coated with 0.5 mg (Na 2SO 4+K 2SO 4 20 wt.% salt mixture)/cm 2 and oxidized at 900 °C for 20 h. XRD and EDS results show that there are only the alumina formed on the NiCoCrAlYSiB coatings, so the basic fluxing of Al 2O 3 occurred, which caused an accelerated hot corrosion after 10 h. For the gradient coatings, there formed Al 2O 3 and Cr 2O 3 at the primary stage of hot corrosion. Cr 2O 3 is easier to react with Na 2SO 4 than Al 2O 3, which reduces the oxygen ion activity of Na 2SO 4, so Al 2O 3 was protected. The formation of the continual and protective layer of Al 2O 3 and Cr 2O 3 leads to the hot corrosion kinetics of gradient coatings similar to oxidation kinetics.

Hot Corrosion of Na2 SO 4-Coated Ti3AlC2 in Air at 700-1000°C

Hot corrosion of layered machinable ceramic coated with in air at 700-1000°C has been investigated by means of thermogravimetric analysis, X-ray diffraction, Raman spectroscopy, and scanning electron microscopy/energy-dispersive spectroscopy. The hot corrosion of was slight at low temperatures of 700 and 800°C, while was severely attacked by fused sodium sulfate at 900 and 1000°C. No protective scales were observed on and sulfur-rich layers were present at the substrate/scale interface. The linear hot corrosion kinetics at 800 and 900°C demonstrated that electrochemical reactions of with sodium sulfate at the substrate/scale interface dominated, while the parabolic kinetics at 1000°C implied that the rate-limiting step involved in the hot corrosion was the diffusion of hot corrosion medium through the scale formed. The hot corrosion behaviors were explained by a mechanism of electrochemistry coupled with basic dissolution-precipitation. © 2004 The Electrochemical Society. All rights reserved.

Hot-Corrosion Resistance of Ni–Cr–Al–Y and Ni–18% Si Alloys in Sulfate Eutectic and Sulfate Plus Vanadate Melts at 973 K

The hot-corrosion resistance of Ni–Cr–Al–Y and Ni–17.8 wt.% Si was examined in sulfate and sulfate plus vanadate melts at 973 K. Two salt-deposit compositions were considered: (a) sodium sulfate+50 mole% magnesium-sulfate eutectic and (b) sodium sulfate plus 20 mole% sodium meta-vanadate. Both types of deposit were molten at the test temperature. Cyclic hot-corrosion tests were conducted in a gas mixture consisting of oxygen, sulfur dioxide, and 0.0240 vol.% sulfur trioxide. The hot-corrosion kinetics were evaluated using weight change and the corrosion mechanism deduced from post-test metallography. The results indicate that the nickel–silicon alloy had much better hot corrosion resistance than Ni–Cr–Al–Y under all test conditions considered. The sample preparation process is outlined, the test procedure summarized, and the experimental results are presented and discussed.

The corrosion of Nb-modified Ti3Al in a combustion gas with and without Na2SO4−NaCl deposits at 600–800°C

The corrosion behavior of a Nb-modified Ti3Al intermetallic compound containing 11 at.% Nb in a simulated combustion gas with and without deposits of a Na2SO4−NaCl mixture was examined at 600–800°C for times up to four days. In the absence of salt deposits the corrosion rates were rather low and increased only slightly with temperature, producing very thin scales of mixed oxides of Ti, Al, and Nb without sulfides. The presence of the salt deposits produced higher weight gains during an initial stage of one to two days at 600 and 700°C, after which the reaction stopped. A more important and longlasting effect was observed instead at 800°C, when the kinetics of hot corrosion became nearly linear. The scales formed by hot corrosion were complex mixtures of various corrosion products at all temperatures and showed a porous outer region containing a mixture of unreacted salts with oxides (mainly TiO2), an intermediate region of a mixture of variable composition of oxides of the three metals, and a TiO2-rich layer beneath it. At 800°C the scales tended to form a thin, discontinuous Al2O3-rich layer in the middle and contained an additional innermost region presenting a large concentration of sulfur, very likely as Nb and Ti sulfides. The high rate of hot corrosion at 800°C is attributed to the appearance of sulfides in the inner region of the scale and to a more efficient scale fluxing.

Transport Considerations in the Hot Corrosion of Ni by Molten Alkali Carbonates

The hot corrosion of Ni induced by molten alkali carbonates has been studied to elucidate the mechanism of attack and in particular to determine whether the Rapp‐Goto theory of fluxing is applicable to this system. Two principal rate‐determining processes were considered in some detail: transport of oxidant from the gas phase through the corrosion products, including the molten salt phase, to the site of active oxidation, and dissolution/reprecipitation of oxide within the corrosion product layer owing to a solubility gradient. To help evaluate these processes, we measured the solubility of in various carbonate salts and we experimentally characterized the scales with respect to average porosity and tortuosity. The kinetics of hot corrosion were measured by TGA under conditions expected to affect oxidant transport and/or fluxing. Experimental rates of oxidation were then compared with calculated maximum rates according to the two rate‐determining processes to evaluate the feasibility of rate control by one or the other process. It was concluded that fluxing was very likely to be rate controlling in the early stages of hot corrosion, but later, when the actual hot corrosion rate had increased considerably, oxidant transport was controlling. The distribution of salt within the pores of the scale changes with time, and this is a key issue in determining the rate of reaction.

Kinetics of the Low Temperature Hot Corrosion of Co‐Cr‐Al Alloys

Hot corrosion rates have been measured on many Co‐Cr, Co‐Al, and Co‐Cr‐Al alloys coated with 2.5 mg/cm2 of and exposed in at 750°C. The hot corrosion resistance of the binary alloys increases with the chromium and aluminum content of the alloys. However, the additions of aluminum to Co‐Cr and of chromium to Co‐Al alloys have deleterious effects. These results can be explained on the basis of a mechanism similar to one previously proposed in the literature (1, 2). Additional experiments have been conducted as a function of temperature and the content of oxygen to extend the proposed mechanism.

Investigation of influence of a mechanical treatment ridge on the hermetization process of high-vacuum seals: K I Bespalov and G G Stratinevskiy,Prace PIE, 11 (1), 1970, 3–9 ( in Russian)

Fuel used in diesel engines for naval propulsion contains high levels of impurities, principally vanadium and sulfates, that can react to form aggressive compounds, responsible for hot corrosion phenomena, leading to severe degradation of the engines' components, especially exhaust valves. A protective coating is thus required to increase the service life of the valves in the highly corrosive environment.In this work five different thermal-sprayed metal-ceramic coatings are selected as potential candidates and their hot corrosion resistance is investigated and discussed in terms of microstructure evolution and hot corrosion kinetics. Four coatings were deposited by either HVOF or APS processes starting from commercial powders (Cr3C2–CoNiCrAlY, Cr3C2–self fusing alloy, Cr3C2–NiCrAlY, Cr3C2–NiCr), optimizing deposition parameters by DoE technique. The last composite coating was purposely designed including silica nanoparticles and ceramic fillers (mullite) in a NiCr matrix, and was deposited by HVOF.Microstructural characteristics of each coating in the as-sprayed conditions were evaluated by measuring porosity and microhardness. Hot corrosion tests were performed on samples covered with a mixture of Na2SO4 and V2O5 and placed in a furnace at 700 °C in air for different times (5, 25, 50, 75 and 100 h). Finally, EDS and XRD analyses were carried out on the corroded samples.

Theory of the field effect in amorphous covalent semiconductor films: D F Barbe,J Vac Sci Technol, 8 (1), Jan/Feb 1971, 102–107

Fuel used in diesel engines for naval propulsion contains high levels of impurities, principally vanadium and sulfates, that can react to form aggressive compounds, responsible for hot corrosion phenomena, leading to severe degradation of the engines' components, especially exhaust valves. A protective coating is thus required to increase the service life of the valves in the highly corrosive environment.In this work five different thermal-sprayed metal-ceramic coatings are selected as potential candidates and their hot corrosion resistance is investigated and discussed in terms of microstructure evolution and hot corrosion kinetics. Four coatings were deposited by either HVOF or APS processes starting from commercial powders (Cr3C2–CoNiCrAlY, Cr3C2–self fusing alloy, Cr3C2–NiCrAlY, Cr3C2–NiCr), optimizing deposition parameters by DoE technique. The last composite coating was purposely designed including silica nanoparticles and ceramic fillers (mullite) in a NiCr matrix, and was deposited by HVOF.Microstructural characteristics of each coating in the as-sprayed conditions were evaluated by measuring porosity and microhardness. Hot corrosion tests were performed on samples covered with a mixture of Na2SO4 and V2O5 and placed in a furnace at 700 °C in air for different times (5, 25, 50, 75 and 100 h). Finally, EDS and XRD analyses were carried out on the corroded samples.

Thermally controlled closed system: Y Cany,Le Vide, 26 (151), Jan–Feb 1971, 22–24 ( in French)

Fuel used in diesel engines for naval propulsion contains high levels of impurities, principally vanadium and sulfates, that can react to form aggressive compounds, responsible for hot corrosion phenomena, leading to severe degradation of the engines' components, especially exhaust valves. A protective coating is thus required to increase the service life of the valves in the highly corrosive environment.In this work five different thermal-sprayed metal-ceramic coatings are selected as potential candidates and their hot corrosion resistance is investigated and discussed in terms of microstructure evolution and hot corrosion kinetics. Four coatings were deposited by either HVOF or APS processes starting from commercial powders (Cr3C2–CoNiCrAlY, Cr3C2–self fusing alloy, Cr3C2–NiCrAlY, Cr3C2–NiCr), optimizing deposition parameters by DoE technique. The last composite coating was purposely designed including silica nanoparticles and ceramic fillers (mullite) in a NiCr matrix, and was deposited by HVOF.Microstructural characteristics of each coating in the as-sprayed conditions were evaluated by measuring porosity and microhardness. Hot corrosion tests were performed on samples covered with a mixture of Na2SO4 and V2O5 and placed in a furnace at 700 °C in air for different times (5, 25, 50, 75 and 100 h). Finally, EDS and XRD analyses were carried out on the corroded samples.

Sputtering multilayered conductor films: D G Muth,J Vac Sci Technol, 8 (1), Jan/Feb 1971, 99–102

Fuel used in diesel engines for naval propulsion contains high levels of impurities, principally vanadium and sulfates, that can react to form aggressive compounds, responsible for hot corrosion phenomena, leading to severe degradation of the engines' components, especially exhaust valves. A protective coating is thus required to increase the service life of the valves in the highly corrosive environment.In this work five different thermal-sprayed metal-ceramic coatings are selected as potential candidates and their hot corrosion resistance is investigated and discussed in terms of microstructure evolution and hot corrosion kinetics. Four coatings were deposited by either HVOF or APS processes starting from commercial powders (Cr3C2–CoNiCrAlY, Cr3C2–self fusing alloy, Cr3C2–NiCrAlY, Cr3C2–NiCr), optimizing deposition parameters by DoE technique. The last composite coating was purposely designed including silica nanoparticles and ceramic fillers (mullite) in a NiCr matrix, and was deposited by HVOF.Microstructural characteristics of each coating in the as-sprayed conditions were evaluated by measuring porosity and microhardness. Hot corrosion tests were performed on samples covered with a mixture of Na2SO4 and V2O5 and placed in a furnace at 700 °C in air for different times (5, 25, 50, 75 and 100 h). Finally, EDS and XRD analyses were carried out on the corroded samples.