The high-temperature corrosion of alloy 800 in carburizing,oxidizing, and sulfidizing environments

This paper reports the results of corrosion experiments of a commercial iron-nickel-chromium alloy in multiple oxidant gases at 800, 900, 1000, and 1100°C. Two sets of experiments were performed at low oxygen activities; one with high sulfur activities and the other with high sulfur and with high carbon activities. The scale growth and the morphologies are interpreted in terms of the calculated thermodynamic stability of the product phases. The calculated chromium sulfide-chromium oxide equilibrium coexistence lies at systematically lower oxygen potential than the experimentally observed transition. The transition from sulfidation to oxidation occurs over a range of at constant .     

Mechanisms of high-temperature corrosion in helium containing small amounts of impurities. II. Corrosion of the nickel-base alloy inconel 617

Application of the technical nickel-base alloy IN 617 in the primary circuit of the high-temperature gas-cooled reactor is limited essentially by the chemical reactions with the impurities (CO, CO₂, H₂, CH₄, H₂O) in the helium coolant. The interactions of the alloy with the reactive impurities in different helium-base gases were investigated by thermogravimetry in a gas-tight microbalance and by simultaneous measurement of the changes in gas composition by a continuous sensitive mass spectrometric analysis. The results demonstrate that the set of six reaction equations deduced in part 1 can be applied to describe the corrosion of the alloy. The occurrence of the various reactions is determined essentially by temperature. For the case of a standardized helium gas (HHT-He), three temperature regions can be distinguished. Below a critical temperature (about 1105 K), the presence of CO can cause simultaneous oxidation and carburization. Above this temperature, this reaction does not reverse itself. Rather, oxidation by CO₂ and H₂O takes place that shows, after a transient period, the same kinetics observed in undiluted oxygen-containing gases. At temperatures above about 1205 K, decarburization of the alloy accompanied by the production of CO takes place, leading to severe destruction of the carbide microstructure and, therefore, limiting the applicability of the material.     

Oxidation of titanium modified type 310 stainless steel in low oxygen partial pressure atmospheres

A series of modified type 310 stainless steels containing 3 wt.% titanium were oxidized in a low oxygen partial pressure atmosphere ( 3.8× 10 ^–15 atm) at 1255 K (1800°F). The scale morphologies and growth rates were similar to those observed on the same alloys in coal gasification atmospheres. The presence of sulfur (as H ₂ S) in the gasification atmosphere apparently has a minimal effect on the corrosion scale formed. The growth rate and phase transformations of the external titanium-rich layer indicate that both titanium and manganese can readily permeate through the apparently compact Cr ₂ O ₃ layer.     

The high-temperature corrosion behavior of Nb-Al alloys in a H2/H2S/H2O gas mixture

The corrosion behavior of pure Nb and three Nb Al alloys containing 12.5, 25, and 75 at.% Al was studied over the temperature range of 800–1000°C in a H₂/H₂S/H₂O gas mixture. Except for the Nb-12.5Al alloy consisting of a two phase structure of -Nb and Nb₃Al, other alloys studied were single phase. The corrosion kinetics followed the parabolic rate law in all cases, regardless of temperature and alloy composition. The parabolic rate constants increased with increasing temperature, but fluctuated with increasing Al content. The Nb-75Al alloy exhibited the best corrosion resistance among all alloys studied, whose corrosion rates are 1.6–2.2 orders of magnitude lower than those of pure-Nb (depending on temperature). An exclusive NbO₂ layer was formed on pure Nb, while heterophasic scales were observed on Nb-Al alloys whose compositions and amounts strongly depended on Al content and temperature. The scales formed on Nb-12.5Al consisted of mostly NbO₂ and minor amounts of Nb₂O₅, NbS2, and -Al₂O₃, while the scales formed on Nb-25Al consisted of mostly Nb₂O₅ and some -Al₂O₃. The scales formed on Nb-75Al consisted of mostly -Al₂O₃ and Nb₃S₄ atT 900°C, and mostly -Al₂O₃ , Nb₃S₄ and some AlNbO₄ at 1000°C. The formation of -Al₂O₃ and Nb₃S₄ resulted in a significant reduction of the corrosion rates.     

Effect of Al on high-temperature corrosion of Co-15a/oNb alloys in H2-H2O-H2S environments

Co–15 at.% Nb alloys containing up to 15 at.% Al were corroded in gaseous H₂–H₂O–H₂S mixtures over the temperature range of 600–900°C. The corrosion kinetics followed the parabolic rate law at all temperatures. Corrosion resistance improved with increasing Al content except at 900°C. Duplex scales formed on alloys consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. A small amount of Al₂O₃ was found only on Co–15Nb–15Al. Contrary to what formed in Co–Nb binary alloys, neither NbS₂ nor NbO₂ were found in the inner layer of all alloys, but Nb₃S₄ did form. The absence of NbS₂ and NbO₂ is due to the formation of stable Al₂O₃ and Al₂S₃ that effectively blocked the inward diffusion of oxygen and sulfur, respectively, and to the reduction of activity of Nb by Al additions in the alloys. Intercalation of ions in the empty hexagonal channels of Nb₃S₄ is associated with the blockage of the transport of cobalt. An unknown phase (possibly Al_0.5NbS₂) was detected. Alloys corroded at 900°C were abnormally fast and formed a scale containing CoNb₃S₆ and Co. Pt markers were found at the interface between the inner and outer layers.     

Investigations of the degradation of high-temperature alloys in a potentially oxidizing-chloridizing gas mixture

The degradation of high-temperature alloys in argon-5.5% oxygen-0.96% hydrogen chloride-0.86% sulfur dioxide at 900°C under isothermal and thermal cycling conditions has been investigated. All the alloys showed reasonable resistance under isothermal conditions, although the Al₂O₂ ***-forming material, alloy 214, gave the lowest amount of corrosion, consistent with Al₂O₃ being a more effective barrier than Cr₂O₃ to inward penetration of chlorine or sulfur-containing species from the environment. Significant internal corrosion was observed for some alloys. Degradation of all the alloys was much more severe under thermal cycling conditions because of the failure of the protective scales. In all cases, formation of volatile chlorine-containing compounds was observed. Degradation of the alloys resulted from the penetration of chlorine-containing species through the initially formed oxide scale and formation of chlorides or, possibly, oxychlorides at the alloy-scale interface or in the subjacent alloy. The sulfur dioxide did not play any obvious role in the process.     

Pressure welding of dissimilar creep-resisting alloys and simulation of the effect of the geometrical parameters of the specimen on the localisation of plastic deformation in the pressure welding zone

Computer modelling is used to investigate the possibilities of increasing the quality of welded joints in pressure welding of dissimilar creep-resisting materials in the presence of a relief on one of the welded surfaces. It is shown that the quality of welded joints depends on the geometrical parameters of the welded specimen.     

Kinetics of oxide equilibration in relaxation experiments with a partial degree of surface control under a Wagner-type rate law for the surface reaction

The importance of the rate of the surface reaction on the kinetics of equilibration of an oxide with the surrounding atmosphere under isothermal conditions is examined by using a general expression of this reaction rate given by Wagner as a boundary condition to solve the relevant diffusion equation by numerical methods. Solutions are obtained for the ideal case of an oxide having a rather simple defect structure corresponding to that of cobalt oxide at high temperatures, but assuming a variable rate constant for the surface reaction, allowing it to change in a suitable range to obtain a change from a diffusion control to a mixed type of control. The effects of the surface reaction rate are discussed in relationship to the different factors affecting the final result, such as the value of the surface rate constant, the sample thickness, the oxygen pressure range examined, and the direction of the pressure change. The data calculated in this way are then analyzed according to the equation which applies for a simpler linear rate law, and it is shown that use of this procedure yields values of the chemical diffusion coefficient more accurate than the analysis made according to the assumption of apure diffusion control. Finally, the need of a more general type of regression analysis of relaxation data to obtain reliable estimates of the chemical diffusion coefficient in cases of mixed kinetic control in presence of a general type of surface rate law is pointed out.     

The in situ synthesis and wear performance of a metal matrix composite coating reinforced with TiC-TiB2 particulates, formed on Ti-6Al-4V alloy by a low oxygen partial pressure fusing technique

A metal matrix composite coating reinforced with TiC-TiB₂ particulates has been successfully fabricated utilizing the in situ reaction of Al, Ti and B₄C by the low oxygen partial pressure fusing technique to improve the wear resistance of Ti-6Al-4V alloy. The results show that increasing the B₄C content is adverse to forming the coating for the formation of interfacial stress; however, the addition of TiC powder as a diluent can favor the formation of this coating and the addition of small amounts of Y₂O₃ can greatly improve the adhesion of the coating. After a pin-on-disc wear test, the wear mass loss of the coating is only about 1/12 that of the Ti-6Al-4V alloy and the wear mechanism of coating is a mixed type of slight peeling-off, adhesion and abrasion.     

Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in a SO2-O2 gas mixture

Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The cyclic oxidation resistance of these alloys was studied at 900°C in a SO ₂-O ₂ gas mixture to elucidate the role of alloy microstructure and Si content on oxidation properties in bioxidant atmospheres. All the large-grained, conventionally processed alloys exhibited breakaway oxidation during cyclic oxidation due to their poor rehealing characteristics. The rapidly solidified, fine-grained alloys that contained less than 1.5 wt.% Si exhibited very protective oxidation behavior. There was considerable evidence of sulfur penetration through the protective chromia scale. The rapidly solidified alloys that contained 1.5 wt.% Si underwent repeated scale spallation that led to breakaway oxidation behavior. The scale spallation was attributed to the formation of an extensive silica sublayer in the presence of sulfur in the atmosphere.     

Investigation of the corrosion for Mg-xGd-3Y-0.4Zr (x = 6, 8, 10, 12 wt%) alloys in a peak-aged condition

The corrosion behaviour of Mg-6Gd-3Y-0.4Zr (GW63K), Mg-8Gd-3Y-0.4Zr (GW83K), Mg-10Gd-3Y-0.4Zr (GW103K) and Mg-12Gd-3Y-0.4Zr (GW123K) were investigated in peak-aged condition using immersion and salt spraying tests and electrochemical measurements in 5% NaCl solution. The corrosion resistances of Mg-xGd-3Y-0.4Zr alloys were found to decrease with the increase of Gd content from 6% to 10% and then increase from 10% to 12%. The corrosion products of Mg-xGd-3Y-0.4Zr alloys were discovered to be composed of voluminous tiny erect flakes by FE-SEM, and the compactness of the corrosion films decreases with the increase of Gd content. The results of AES revealed that the corrosion film is enriched with Gd and Y. The potentiodynamic polarization curves showed that the overpotentials of cathodic hydrogen evolution on GW63K and GW83K are much higher than on GW103K and GW123K. The results of EIS are in good agreement with the morphologies of the corrosion products and the corrosion rates of the corrosion tests.