Effect of impurities on the oxidation mechanism of nickel at 800°C

Abstract

The effect of impurities on the oxidation mechanism of nickel was studied on commercial nickel grades compared to a pure nickel. On the basis of oxidation kinetics, SEM and STEM microstructural and analytical investigations allowed us to identify the oxidation mechanism for both types of nickel at 800°C. The morphology of the oxide scale notably differs according to the purity of the nickel. For oxidised commercial grades, a duplex structure was observed with an outer columnar layer and an inner layer made of equiaxed grains. The inner NiO/outer NiO interface is planar without any segregation, while the NiO/Ni interface is convoluted with large cavities. Mn, Ti and sometimes also silicon impurities were detected at this latter interface. Below the NiO/Ni interface, in the underlying nickel, large internal oxidation was observed. The observed microstructure was quite different for the pure nickel. A single porous NiO layer, composed of equiaxed grains, was observed. The NiO/Ni interface was facetted and no porosity was detected. The presence and localisation of impurities, as well as morphological changes through the scale in the nickel grades, were taken into account to explain the modification of oxidation kinetics with substrate purity.     

Investigation of the microstructure and oxidation behavior of Cr-modified aluminide coatings on γ-TiAL alloys

The microstructure and oxidation behavior of aluminide coatings are investigated. The layers are examined by the optical microscopy, scanning electron microscopy (SEM) equipped with EDS, and the X-ray diffraction method. The isothermal oxidation behaviors of samples are investigated at 950°C for 200 h. The results show that TiAl₃ is formed on the substrate. In addition, the aluminide coating improves the oxidation resistance of γ-TiAl alloys by forming a protective alumina scale. Moreover, during oxidation treatment, the interdiffusion of the TiAl₃ layer with γ-TiAl substrate results in the depletion of aluminum in the TiAl₃ layer and the growth of the TiAl₂ layer. After the oxidation treatment, the coating layer preserves a microstructure with phases including TiAl₃, TiAl₂, and Al₂O₃.     

Formation and cyclic oxidation resistance of Hf–Co-modified aluminide coatings on nickel base superalloys

The codeposition of Co, Al and Hf on nickel base superalloys by pack cementation in a single-step process was investigated in this work. Thermochemical analyses were applied to search for suitable conditions including pack composition and deposition temperature. Co, Al, Hf, NH₄Cl, NH₄I and Al₂O₃ made up the pack powder mixture. According to a series of thermochemical calculations, the pack powder mixture of 20Co–10Al–2Hf–4NH₄Cl–4NH₄I–60Al₂O₃ (wt.%) was adopted. Further experimental results demonstrated that the codeposition of Co, Al and Hf could be achieved practically. The coating consisted of a diffusion zone and an outer layer. The outer layer was mainly composed of Al_0.9Ni_1.1 where a part of Ni was replaced by Co or Hf. The trace element Hf was enriched in the interface between the outer layer and the diffusion zone. The Co–Al–Hf coating exhibited excellent cyclic oxidation resistance due to improvement in adhesion between the oxide scale and the coating.     

Formation of aluminide coatings on low alloy steels at 650°C by pack cementation process

Abstract

The pack aluminising process is normally conducted on alloy steels at temperatures higher than 900°C at which mechanical properties of steels would degrade. This study aims to investigate the feasibility of pack aluminising a commercial 9Cr – 1Mo alloy steel at 650°C in an attempt to increase its high temperature oxidation and corrosion resistance without adversely affecting its mechanical properties and consequently to increase its long-term structural operating temperatures to up to 700°C. It was demonstrated that this could be achieved using packs containing AlCl₃ as an activator and elemental Al as a depositing source. The coatings formed under these conditions consisted of an outer Fe₁₄ Al₈₆ layer and an inner FeAl₃ layer with an abrupt interface between the coating and substrate, suggesting that the coating is formed via a mechanism of the inward Al reaction – diffusion. The pack Al content was varied from 1 to 6 wt-% to investigate its effects on the coating formation process. It was found that the pack Al content in this range affected only the coating thickness and therefore the growth rate of the coating, but not the surface Al concentration. A post-aluminising heat treatment study was also undertaken for an aluminised specimen at 650°C under an argon atmosphere to investigate the kinetics of converting the brittle Fe₁₄ Al₈₆ and FeAl₃ phase layers to a more ductile FeAl phase layer. It was observed that this was a slow process requiring 1132 h for an initial coating layer thickness of 33μm. The coating after the conversion consisted of a uniform top FeAl layer with all other alloying elements in the solid solution and a diffusion zone underneath.     

Specific heats of commercial titanium alloys at 50–1100°C

Specific-heat measurements are reported for nine titanium alloys with and + structures; the temperature of the transition has been revised by DTA. A generalized approximating formula for the specific heat has been derived for the temperature range in which the structural state is stable, which contains a dimensionless parameter that includes the temperature of the transformation.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 38, No. 4, pp. 593–598, April, 1980.     

Limits of the oxidation resistance of several heat-resistant steels under isothermal and cyclic oxidation as well as under creep in air at 650°C

In order to guarantee the oxidation resistance of Cr-steels the Cr content in the alloy must be above a critical limit. Recently developed 9Cr steels are close to that limit and as ongoing oxidation leads to Cr subsurface zone depletion the question arises as to how the oxidation behaviour is affected by the decrease in Cr concentration with oxidation time. Four ferritic heat-resistant commercial steels containing 9–12% Cr and the austenitics AISI 304 and Alloy 800 were investigated at 650°C in air to determine their oxidation behaviour and the course of Cr-depletion in the metal subsurface zone for times up to 3000 hours. In addition to isothermal tests, thermal cycling tests and creep tests were also performed. Surprisingly large differences in oxidation behaviour were found between the two 9Cr steels. Furthermore, of the two steels designated as 12Cr steel, one was even worse than the 9Cr steels while the other one was best. Thermal cycling improved the oxidation behaviour of the steels which was worse under isothermal conditions by almost two orders of magnitude. The oxidation behaviour as a function of time very much reflected the amount of Cr in the metal subsurface zone. The breakaway effects observed could be correlated with a drop in the Cr content in the subsurface zone below a critical value which had been determined by model calculations. The tendency observed under isothermal conditions is enhanced by superimposed creep deformation. It is concluded from the results that growth stresses in the oxide scales combined with the actual Cr-concentration in the metal subsurface zone play a major role in oxidation resistance.     

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.     

Rapid oxidation of liquid tin and its alloys at 600 to 800°C

The behavior of liquid tin and its alloys in oxygen at temperature range 600 to 800°C were investigated. Rapid and nearly linear reaction kinetics were observed for pure tin at temperature higher than 700°C. Marker experiments, which determine the mode of mass transport through the scale, and wetting phenomena between the oxide and melts were studied to delineate the reaction mechanism of oxide growth. Moreover, the rates of oxidation of tin were markedly changed by alloying it with small amount of foreign elements. Significantly increased oxidation rates for binary tin alloys containing Mg, Ba, La or Ca were observed. TEM studies indicated that additional growth stresses were introduced into the SnO₂ scales by these additions.     

The isothermal and cyclic oxidation behaviour of Ti-48Al-2Cr at 700°C

The effect of the microstructure on the isothermal and cyclic oxidation behaviour of the intermetallic Ti–48A1–2Cr (at.%) alloy was investigated at 700°C in air up to 3000 h. Different microstructures, i.e., duplex, near gamma, nearly lamellar, and fully lamellar, obtained by various heat treatments, were used. Results of thermogravimetry showed a good oxidation resistance at 700°C against both isothermal and cyclic oxidation. The growth rate of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material. After equal exposure times, cyclic oxidation induced a higher oxidation rate compared to isothermal oxidation. Oxidation of Ti–48A1–2Cr in air, initially resulted in the formation of α-A1₂O₃, TiO₂ (rutile), Ti₂A1N and TiN, with the latter two near the scale/substrate interface. After longer exposure times, the mixed corrosion scale was overgrown by fast growing TiO₂. The oxide scales, formed under isothermal as well as under cyclic conditions, were uniform.     

High-temperature fatigue behaviour of a second generation near-γ titanium aluminide sheet material under isothermal and thermomechanical conditions

The high-temperature deformation behaviour of a second generation γ-TiAl sheet material with near-γ microstructure was characterised under tensile, creep, isothermal and thermomechanical fatigue (TMF) loading conditions. Test temperature ranged from 500 to 750 °C in isothermal tests and these temperatures were also used as minimum and maximum temperature of in-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests. Under tensile loading, a ductile-to-brittle transition temperature (DBTT) of about 650 °C was observed. At this temperature the material experiences a temperature dependent change in the fracture morphology. Creep tests carried out in the temperature range from 650 to 800 °C under true constant stress conditions revealed a temperature and stress dependence of the Norton stress exponent n and the apparent activation energy for creep Q_app. With increasing temperature, isothermal fatigue life at constant strain amplitude decreased in vacuum, but increased in air indicating an abnormal (inverse) environmental effect. Under IP loading, fatigue is characterised by cyclic softening due to dynamic recrystallisation. OP loading drastically reduces fatigue life and turned out to be an extremely critical loading situation for γ-TiAl alloys.     

A Mössbauer study of the oxidation of Fe-Ni alloys at 535 and 635°C

Fe⁵⁷ transmission Mössbauer spectroscopy, supported by metallography, SEM and X-ray diffraction analysis, has been employed to study the oxidation of Fe-Ni alloys at 535 and 635° C in 1 atm. of air. With increasing Ni content of the alloy, the composition of the scale changed and the oxidation rate decreased. For an alloy containing 0.9% Ni, the oxide scale produced at 535° C was Fe₃O₄ covered by a thin outer layer of-Fe₂O₃, while at 635° C FeO was additionally present as a major phase. The scale formed on a 10% Ni alloy at both 535 and 635° C was similar to that observed for the 0.9% Ni alloy oxidized at 535° C (i.e. of Fe₃O₄ and-Fe₂O₃), although the-Fe₂O₃ layer tended to be relatively thicker. For a 49% Ni alloy, the scale at both 535 and 635° C comprised an inner layer of Ni _x Fe_3–x O₄ (withx0.5, on average) and an outer layer of-Fe₂O₃, of similar thickness. Finally, on an 83% Ni alloy oxidized at 635° C, the scale consisted of roughly equally thick layers of NiO (next to the metal) and NiFe₂O₄, and a thin outer covering of-Fe₂O₃. The decrease in oxidation rate with increasing Ni content of the alloy is discussed briefly in relation to the changing composition of the scale and diffusion in the alloy.     

Microstructure and oxidation behaviour at 1100°C of HfC containing Cr rich iron based cast alloys

Three alloys containing 0·25–0·50 wt-%C, 26–28 wt-%Cr and 4–6 wt-%Hf were elaborated by foundry. They contained a dendritic matrix and HfC carbides, which are expected to strengthen the alloy at high temperatures. They were exposed in air at 1100°C during 46 h. The aged microstructures displayed coarsened chromium carbides but no significant changes to the fraction or morphology of the HfC carbides. The surface characterisation of the oxidised samples showed that the alloys behaved well despite some localised instances of fast oxidation. The hardness was modified by the microstructure stabilisation achieved during the high temperature exposure. A preliminary test showed that the reinforcement by HfC may indeed lead to interesting creep resistance at 1100°C as had been hoped.     

Effect of vanadic corrosion on creep-rupture properties of Superni-600 at 650–750°C

The effect of vanadic corrosion on creep-rupture properties of a nickel base superalloy Superni-600, at 650–750°C, has been investigated. Sodium metavanadate and sodium metavanadate plus 15 wt% sodium sulphate were used as the corrodent salts. Weight change studies have also been made to understand the mechanism of corrosion. Both sodium metavanadate and the sodium metavanadate/sodium sulphate mixture are found to be aggressive and to reduce the creep-rupture life. The degradation of creep-rupture properties and possible hot corrosion reactions are discussed. Cracking and fluxing of the protective scales, together with easier crack nucleation and growth at grain boundaries in the presence of a liquid deposit accounts for the enhanced creep rates and reduced rupture lives.     

Measurement of the monochromatic emissivity of coatings at 100–400°C

Results of measurement of the normal emissivities of certain coatings by a two-beam method in the 2–25 range are reported.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 27, No. 2, pp. 203–207, August, 1974.     

Hot corrosion behavior of porous nickel-based alloys containing molybdenum in the presence of NaCl at 750 °C

The hot corrosion of porous Ni-23Cr-xMo (0%, 4.5%, 9.0%, 13.5%, mass fraction) alloys tested at 750 °C under cyclic procedure was investigated in order to elucidate the effect of Mo addition on hot corrosion in the presence of NaCl. The hot corrosion experiments were performed at 750 °C in air with 4 mg cm^− 2 NaCl deposit. The performance of the alloys was evaluated by the results of weight change kinetics. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize the corrosion products. The results indicate that NaCl accelerated the oxidation of the alloys by chloridized elements Mo and Cr. Among the porous Ni-23Cr-xMo alloys, Ni-23Cr-9Mo alloy exhibited the best hot corrosion resistance due to the formation of NiO-NiCr₂O₄-Cr₂O₃ oxide scales. Furthermore, these oxide scales were confirmed more effective to protect the alloys after adding of Mo.     

Isothermal embrittlement of Fe–8Mn alloys at 450°C

Abstract

Two Fe–8Mn alloys, one of which is alloy 193, stabilised with 0·17%Ti and 0·18%Al, were austenitised at 900°C, ice brine quenched and their DBTTs determined. In this condition, brittle fractures were predominantly cleavage, and thermodynamic calculations on alloy 193 showed that there were 0·0025 wt-%C and <0·03 ppm N in solid solution. Alloys were tempered for 6 min, 1 h and 10 h at 450°C and their DBTTs again determined; in this case, brittle fractures were mainly intergranular. In alloy 193, DBTT rose from 27 to 125°C in 6 min. Hardness values at 450°C were also monitored and the variation of hardness with time is discussed. It is thought that brittle fracture in alloy 193 is due to segregation of Mn per se to prior austenite grain boundaries, unlike an earlier investigation of a pure Fe–8Mn alloy (K1525), where embrittlement was due to a Mn–N and to a lesser extent a Mn–P interaction at prior austenite grain boundaries. The driving force for Mn segregation to prior austenite grain boundaries is thought to be the initial formation of reverted austenite at such sites.     

Effect of carbon content on the oxidation behaviour of FeCrAlY alloys in the temperature range 1200–1300°C

Abstract

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300°C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.

Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling.