The initial stages of high-temperature corrosion of Fe-Cr-Ni and Cr-Ni alloys in a carburizing atmosphere of low oxygen partial pressure

The initial corrosion reactions on a number of Fe-Cr-Ni and Cr-Ni alloys when exposed in a H₂-CH₄ gas mixture ofa _c =0.8 and 10^–30 bar at1098 K have been investigated. All alloys were studied with surfaces in boththe cold-worked and electropolished conditions. The experiments, in whichexposure times up to 400 rain were used, were carried out in a conventionalcorrosion rig and also in a small reaction chamber contained within an X-rayphotoelectron spectrometer. The use of XPS, SEM, and TEM showed thattransient -Cr₂O₃ layers formed on all alloy surfaces during the heat-up period.On the electropolished surfaces, the oxide layers developed nonuniformly withrespect to microstructure and thickness, whereas homogeneous, compact, andfine-grained -Cr₂O₃ layers formed on the cold-worked specimens. Followingan incubation period, M₇C₃ carbides nucleated on the surfaces; they grew byconsuming the less stable -Cr₂O₃ and also through diffusion of metal fromthe alloy substrate. The mode of nucleation of the M₇C₃ and its rate of growthwere strongly dependent on the character of the transient -Cr₂O₃ layers. Afterremoval of -Cr₂O₃, carbon ingressed into the substrate where M₂₃C₆ wasformed internally. These results are broadly in line with the thermodynamicstability predictions and are discussed in terms of simple models.     

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 .     

The effect of oxygen partial pressure on the filiform corrosion of organic coated iron

An in-situ time-lapse optical microscopy study, using a novel dual-compartment cell is used to gain mechanistic understanding of filiform corrosion (FFC) affecting an organic-coated iron surface. The apparatus allows independent control of environments surrounding the filament head and tail regions. When oxygen-containing environments surround the filament head, an anterior cathodic arc is formed, constraining the filament head electrolyte. When oxygen is removed from the vicinity of the head, FFC propagation rates remain unchanged, although the constraining arc is not present. Maximum propagation is observed when oxygen is available for transport through the filament tail to the rear of the head.     

Early stages of copper corrosion behaviour in a Tunisian soil

The early stages of copper corrosion in a Tunisian soil were studied using mass loss, surface analysis (optical microscopy, visible spectroscopy, IRTF and atomic force microscopy) and electrochemical characterizations (polarization curves and cyclic voltammetry). The corrosion rate dependence with immersion time was Δm = at^b. Two behaviours for the material surface were evidenced when varying the soil concentration and temperature. Then, the apparent kinetic constant, the soil reaction order and the apparent activation energy were calculated. Analytical and electrochemical characterizations showed that a rough patina layer was build up as a result of the interaction between soil and copper.     

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.     

Electrochemical corrosion response of a low carbon heat treated steel in a NaCl solution

Dual‐phase (DP) steels are produced from a specific heat treatment procedure and have recently emerged as a potential class of engineering materials for a number of structural and automobile applications. Such steels have high strength‐to‐weight ratio and reasonable formability. The present study aims to investigate the effects of four different and conventional heat treatments (i.e., hot rolling, normalizing, annealing, and intercritical annealing) on the resulting microstructural patterns and on the electrochemical corrosion behavior. Electrochemical impedance spectroscopy (EIS) and Tafel plots were carried out on heat treated steel samples in a 0.5 M NaCl solution at 25 °C with neutral pH. An equivalent circuit analysis was also used to provide quantitative support for the discussions. The normalizing and the annealing heat treatments have provided the highest and the lowest corrosion resistances, respectively. The intercritical annealing and as‐received (hot rolled) low carbon steel samples have shown similar corrosion behavior. Although a deleterious effect on the corrosion resistance has been verified for DP steel due to the residual stress from the martensite formation, it combines good mechanical properties with intermediate electrochemical corrosion resistance.     

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.     

An amino-based protic ionic liquid as a corrosion inhibitor of mild steel in aqueous chloride solutions

Protic ionic liquids (PILs) have shown to be promising substances as corrosion inhibitors (CIs). In line with this, the aim of this study is to study the performance and propose the corrosion inhibition mechanism of N-methyl-2-hydroxyethylamine (M-2HEAOL) and bis-2-hidroxyethylamine (B-HEAOL) oleate, for mild steel, in a neutral chloride solution. Electrochemical characterization was conducted under static and hydrodynamic conditions, and it was revealed that M-2HEAOL and B-HEAOL worked as mixed-type CIs with more interference on the anodic reaction. Inhibition efficiency depended on the concentration reaching 97% of inhibition efficiency in 5 mmol/L concentration. Scanning electron microscopy, optical interferometry, Raman spectroscopy, and Fourier-transform infrared spectroscopy are used to elicit the chemical composition of the surface film and corrosion morphology of steel in the presence of CIs, the adsorption processes of which involved physical and chemical adsorption between metal and different parts of ionic liquids. The results allowed the proposition of a corrosion inhibition mechanism.     

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.     

Evaluation of the corrosion resistance of different galvanized steels treated in a cerium salt solution

Analysis of polarization curves and impedance spectra has shown that the surface chemistry of galvanized steels plays an important role in determining their corrosion behavior. Zn‐Ni samples that were untreated or had been treated in a 10 mM Ce(NO₃)₃ solution had the lowest corrosion rates in 0.5 N NaCl. The efficiency of the surface modification process was the highest for electrogalvanized steel. Surface analysis showed that the protective layers on Zn‐Ni and electrogalvanized steel contained mainly cerium oxides/hydroxides, while the layer on hot‐dipped steel consisted mainly of zinc hydroxide.     

Focussed ion beam sectioning for the 3D characterisation of cracking in oxide scales formed on commercial ZIRLO™ alloys during corrosion in high temperature pressurised water

Using FIB sectioning and reconstruction techniques we have performed a quantitative analysis on the microstructure of cracks and the topography of the metal–oxide interface in oxides formed on ZIRLO™ alloys in high-temperature water. The most significant observation is the continuous production of cracks both before and after the transition in kinetics, not a sudden burst of crack nucleation at transition as assumed in the literature. By concluding that cracks are not generated as a result of the transition and are not the primary cause, we suggest that a process by which cracks within the scale become connected to the oxidising environment through interconnected nanoporosity may be critical in controlling the overall rate of oxidation.     

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.     

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.     

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.     

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.     

Field exposure of FeCrAl model alloys in a waste‐fired boiler at 600°C: The influence of Cr and Si on the corrosion behaviour

The aim of this study was to examine the performance of FeCrAl model alloys in a waste‐fired boiler and investigate the influence of chromium and silicon content on the corrosion behaviour. The investigation was executed by utilising an air‐cooled probe, giving a material temperature of 600°C throughout a 672 hr exposure. The material loss measurements were performed by utilizing an ultrasonic thickness gauge in combination with scanning electron microscopy analysis. It was found that increasing the chromium content significantly reduced the overall material loss of the FeCrAl model alloys but further accelerated the corrosion attack on the windward side. Simultaneously, the increased chromium content caused embrittlement of the material. Minor additions of silicon drastically reduced the material loss of the FeCrAl model alloys, whereas the sample ring with no silicon present was completely deteriorated. The trends observed in this field study correlated well with what has been observed in previous laboratory studies. A state‐of‐the‐art alloy in the present environment, Inconel 625, was simultaneously exposed and showed similar performance to the silicon‐containing FeCrAl model alloys with ≥10 wt% Cr.