Penetration of sulfur through preformed protective oxide scales

Thermodynamic assessment of sulfur penetration through otherwise protective scales such as Cr₂O₃, Al₂O₃ has been carried out for Co-Cr- and Co-Cr-Al-base alloys. Limiting conditions for sulfide formation following gas molecular transport and solution-diffusion transport have been established and the results partially confirmed by experiments carried out on Co-10Cr, Co-25Cr, and Co-10Cr-5Al alloys in sulfurous atmospheres. The results show that molecular transport of sulfurous gas species through the growing oxide scale definitely occurs. It was not possible to confirm or disprove the solutiondiffusion mechanism.     

Effect of presulfidation on the oxidation behavior of Co-based alloys. Part I. Presulfidation at sulfur partial pressures above the dissociation pressure of cobalt sulfide

The effects of presulfidation in H₂-H₂S atmospheres of sulfur activity sufficient to form cobalt and chromium sulfides on the oxidation rates of Co-Cr binary alloys containing 0–25 wt.% Cr and Co-25 wt.% Cr alloys containing 0–2 wt.% C have been investigated. Presulfidation increases the oxidation rate, but the effect is not very dramatic. Carbon additions to the Co-25 wt.% Cr alloy progressively increase the oxidation rate, but not to as great an extent as a simple model based on the reduction of the chromium activity in the alloy. Sulfur released from the preformed sulfides by oxidation diffuses into the alloy precipitating fresh sulfides, there appears to be no outward diffusion of sulfur through the oxide scale. These internal sulfides have a liquid-like morphology in cobalt-base alloys when the oxidation is carried out at 1000°C, as compared to 800°C in corresponding nickel-base alloys. When the sulfide layer produced during the presulfidation is thin, so that oxidation destroys the continuous sulfide layer, the subsequent scale morphologies after oxidation exhibit many features similar to samples subjected to hot corrosion in environments containing sodium sulfate.     

The corrosion of ferritic stainless steels and high-purity Fe-Cr alloys in basaltic lava and simulated magmatic gas

Three ferritic stainless steels, types 410, 430, and 446, containing 12, 17, and 26% Cr, respectively, and two high-purity binary alloys, Fe-19Cr and Fe-24Cr, were subjected to molten tholeiitic basaltic lava with a cover gas simulating magmatic gas at 1150°C for periods up to 400 hr. The oxygen and sulfur partial pressures were 9.8×10^?10 and 7.0×10^?3, respectively. All alloys formed Cr₂O₃ scales. Internal sulfidation occurred in the commercial alloys resulting in the formation of chromium and manganese sulfides. Internal oxidation of silicon also occurred. The extent of internal sulfidation decreased with increasing chromium content. There was a “critical” chromium content between 12 and 17%, above which internal sulfidation did not occur in 96 hr. However, the “critical” chromium level increased with exposure time to nearly 26% for 400 hr. Little internal sulfidation was observed in the high-purity alloys. The different behavior between the commercial and high-purity alloys may be attributed to (i) the formation of more perfect scales on the latter, which inhibited the inward migration of sulfur, and (ii) changes in the sulfur activity gradient across the scale caused by the presence of silicon and manganese.     

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.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

Silicon contamination effects in the oxidation of carbide-containing cobalt-chromium alloys

Austenitic Co-25Cr(wt pct) and two phase γ + M₇ C₃ alloys of composition Co-25Cr-xC were reacted with pure, dry oxygen at 1000°C. All alloys reacted according to relatively fast parabolic kinetics if they were prepared without silicon contamination. However, if the alloys were contaminated with silicon during annealing in silica ampoules, or if 0.05 wt pct silicon was added to Co-25Cr-1.0C, parabolic oxidation kinetics two orders of magnitude lower resulted. In the case of the rapid reactions, the scales consisted of an inner layer of CoCr₂O₄ + CoO overlaid by CoO. The slow reactions corresponded to growth of a thin scale of Cr₂O₃ overlaid by CoCr₂ O₄. In the latter case the selective oxidation of chromium led to chromium carbide dissolution in a subsurface zone of the two-phase alloy, but the rates were the same as for the single-phase alloy. Consideration of gas phase conditions in the silica annealing ampoules showed that p_SiO values were high enough to transfer substantial amounts of silicon to the alloy if p_O2 was low enough. This situation arose in well evacuated ampoules where oxygen was consumed by reaction with alloy chromium, or in titanium gettered capsules. In contrast, annealing the alloys under moderate oxygen pressures led to the growth of a protective oxide film which prevented silicon contamination of the oxide surface. It is concluded that the presence or absence of carbon in Co-25Cr is irrelevant to the oxidation mechanism and that the silicon effect is critical. An approximate diffusion analysis shows that bulk alloy properties are not affected by the silicon, and it is concluded that silicon has its effect at the alloy surface, by promoting Cr₂O₃ nucleation.     

The sulfidation of dilute Co-Cr alloys at low sulfur partial pressures(p_{{\text{S}}_{\text{2}} } = {\text{5x10}}^{{\text{ - 4}}} {\text{Torr)}}

The sulfidation of pure chromium and Co-Cr alloys containing 1, 5, 10, 17, and 25 wt. % Cr in H ₂-1%H₂S at 1000°C has been studied in detail by thermogravimetric methods, metallography, and electron probe microanalysis. In this gas mixture, which has an effective sulfur partial pressure of 5×10^–4 Torr, only CrS is formed, on all the alloys containing greater than 1 wt. % Cr, although there is some evidence that it may contain a little dissolved cobalt. The Co-1 Cr alloy is unattached. The sulfidation rate increases with increasing chromium content, the 25 wt. % Cr alloy corroding 100 times slower than pure chromium. Internal precipitation of CrS also occurs, the depth of the affected zone increasing with alloy chromium content. The rate-controlling mechanism appears to be the diffusion of chromium from the interior of the alloy to the alloy-scale interface, there being virtually no chromium remaining there. There is good qualitative agreement between the measured rate constants and values calculated from the rate of supply of chromium from the interior of the alloy.     

The Corrosion of metals in an oxidizing-chloridizing environment at high temperature

An investigation has been undertaken into the behaviour of metals which form the basis of high-temperature alloys in an argon ?5.5% oxygen ?0.96% hydrogen chloride ?0.86% sulphur dioxide gas mixture at 900°C. The intention has been to ascertain the reaction products, with particular emphasis on the formation of volatile species which can cause considerable degradation of commercial alloys in this environment. From consideration of the thermodynamics of the gas system, the potentials of the reactive species can be determined and correlated with the possible reaction products. In this gas mixture, the oxides of nickel, iron, cobalt, chromium, molybdenum and tungsten are the stable phases with respect to the corresponding metals. Indeed, on exposure of the metals to the environment, the appropriate oxide scales are developed. However, the reactions are complicated by formation of volatile corrosion products, particularly for nickel, cobalt and molybdenum. Although a Cr₂O₃ scale is established on chromium, there is evidence for penetration of chlorine-containing species to the scale/alloy interface. The oxide scale on tungsten is not very protective and thickens rapidly while that on molybdenum is volatile, resulting in rapid consumption of the specimen.     

Effect of water vapour on cyclic oxidation of Fe–Cr alloys

Model Fe–Cr alloys containing 9, 17 or 25 wt% Cr were subjected to repeated 1 h cycles of exposure at 700 °C to flowing gas mixtures of Ar‐20O₂, Ar‐20O₂‐5H₂O and Ar‐5O₂‐20H₂O (all in volume %) for up to 400 cycles. The kinetics and morphological development of these reactions were compared with those found during isothermal exposure to the same gases. Under isothermal conditions, all alloys developed thin protective chromium‐rich scales in dry oxygen. Addition of 5% H₂O induced breakaway for Fe‐9Cr within 48 h, but had little effect on higher chromium alloys. Isothermal chromia scale growth on Fe‐17Cr and Fe‐25Cr was accelerated by the addition of 20% H₂O, but breakaway did not result. Under cyclic conditions in dry oxygen, Fe‐9Cr quickly entered breakaway, oxidising according to fast, linear kinetics, but the higher chromium alloys exhibited protective behaviour. When 5% H₂O was added to the oxygen, the 17% Cr alloy also underwent fast breakaway oxidation, but Fe‐25Cr continued to be protected by a chromia scale. In the 20% H₂O gas, all alloys failed under cyclic conditions, producing thick, iron‐rich oxide scales. The synergistic effects of water vapour and temperature cycling are discussed in terms of alloy chromium depletion and the effects of H₂O(g) on oxide transport properties.     

The influence of aluminum on the transition from oxide to sulfide of aluminum-rich Fe-25Cr alloys with low-oxygen and high sulphur pressures

The simultaneous oxidation and sulfidation of Fe-25Cr, Fe-25Cr-5Al and Fe-25Cr-10Al alloys were studied at 1023, 1123, and 1223 K in H₂-H₂O-H₂S gas mixtures. Fe-25Cr and aluminum-rich alloys with 0–10 wt.% Al show, in H₂H₂O-H₂S gas mixtures at high temperatures, a transition from protective oxide-scale formation to the formation of a sulfide-rich corrosion product. The kinetics boundary, which indicates the transition from oxide formation with slow weight gains to sulfide formation with rapid weight gains, has been found in these three alloys. The critical oxygen partial pressures to stabilize oxide formation at the constant-sulfur partial pressures of aluminum-rich Fe-25Cr alloys were systematically below those of Fe-25Cr alloy. When the oxygen partial pressure is much higher than the critical one, the oxide scale formed on the Fe-25Cr alloy was mainly Cr₂O₃ with a small amount of FeCr₂O₄; on the other hand, the oxide scale formed on the aluminum-rich Fe-25Cr alloys was mainly Fe(Cr,Al)₂O₄ with a small amount of Al₂O₃ and Cr₂O₃. The thermodynamic stability diagrams for (Fe, Cr, Al) -S-O systems were constructed, and the experimental results which show the existence of Fe(Cr, Al)₂O₄ in the simultaneous sulfidation and oxidation of aluminum-rich Fe-25Cr alloys are explained by these diagrams. The reaction kinetics were measured by a stainless-steel spring balance, and the reaction products were characterized by x-ray diffraction, Auger spectroscopy, and scanning electron microscopy. The reaction rate usually decreased with an increase of the oxygen partial pressure at a constant sulfur partial pressure. The existence of aluminum plays an important role to suppress the sulfidation of Fe-25Cr alloys.     

Corrosion behavior of Fe(Ni)CrAIX alloys in an HCl−H2O−H2 gas mixture at 800°C

The high-temperature-corrosion behavior of a series of Fe(Ni)CrAlX-type alloys (where X=Zr and Hf, e.g.) has been studied in a gas mixture of 50% HCl-10% H₂O–H₂ at 800°C. The experimental results obtained indicated that Ni-base alloys had superior corrosion resistance to Fe-base alloys in this gas mixture. While the exposed Ni-base alloys showed weight gains due to the formation of oxides (e.g., Al₂O₃, Cr₂O₃) as well as CrCl₂, the Fe-base alloys exhibited substantial weight losses resulting from the formation and subsequent evaporation of FeCl₂. This study also demonstrated that Fe(Ni)Cr8AlX-type alloys, which contained high aluminum, had better chloridation resistance than Fe(Ni)25CrAlX-type alloys, which had high chromium. The improved performance of Fe(Ni)Cr8AlX-type alloys was due to the presence of a high level of aluminum which promoted formation of protective Al₂O₃. Although the presence of chromium in the alloys promoted the formation of Cr₂O₃, the high level of chromium adversely affected the chloridation resistance of Fe(Ni)25CrAlX-type alloys, due to the development of chloride (CrCl₂) at the interface of the oxide scale and alloy substrate.     

Effect of internal oxidation pretreatments and Si contamination on oxide-scale growth and spalling

Internal oxidation pretreatments carried out in quartz capsule with a Rhines pack were found to have a profound effect on the subsequent oxidation behavior of alloys. Specimens of Co-15 wt.% Cr, Co-25 wt.% Cr, Ni-25 wt.% Cr, and Ni-25 wt.% Cr-1 wt.% Al were tested at 1100°C after pre-oxidation treatments. Even without the development of internal oxide particles, pretreated binary CoCr and NiCr alloys oxidized with significantly lower rates. Selective oxidation of chromium was observed on the non-Cr₂O₃-forming Co-base alloys, whereas on the Cr₂O₃-forming Ni-base alloys, elimination of base-metal oxide, reduction in the Cr₂O₃ growth rate, and better scale adhesion were found. These effects were more apparent with pre-oxidation temperatures greater than 1000°C and with longer pretreatment times. Contaimination of Si from the quartz is believed to be the cause.     

The influence of aluminum additions on the oxidation of Co-Cr alloys at 1000 and 1200°C

The isothermal oxidation of Co-Cr-Al alloys, containing 10–30 % Cr and 1 or 4.5% Al in 1 atm flowing oxygen at 1000 and 1200°C has been studied by thermogravimetric methods, optical metallography, electron probe microanalysis, and scanning electron microscopy. The addition of 1 % Al to Co-10% Cr and Co-15% Cr has little effect on the over-all oxidation rate, although there is increased internal oxidation and the outer-inner scale thickness ratio is decreased. The oxidation rate is controlled largely by Co²⁺ ion diffusion out through the entire scale, with oxygen gas transport across voids, spinel blocking effects and doping in the inner layer probably playing subsidiary roles. With Co-15 %-Cr-1%Al, limited healing by Cr₂O₃ increases progressively with time at the alloy-oxide interface. An addition of 1 % Al to Co-30 %Cr assists the formation of an initially protective Cr₂O₃-rich surface layer by internally oxidizing, thereby allowing more of the chromium to diffuse to the surface and form an external scale. This Cr₂O₃ layer tends to lift and crack open, enabling CoO-rich scales to form on the exposed alloy. Co-15%Cr-4.5% Al produces a protective -Al₂O₃ layer on certain surface regions, sometimes with an overlying Cr₂O₃ layer and internal -Al₂O₃ particles in the underlying alloy. In other regions, rapidly growing CoO-rich nodules develop from the outset, or after early lifting and fracture of the -Al₂O₃ scale. Generally, the presence of 28% Cr and 4.5% Al is sufficient to ensure an external scale of -Al₂O₃, the chromium acting as an oxygen getter. If such scale fractures, healing is very rapid.     

Effect of chromium content on the sulphidation behaviour of CoCr alloys in H2/H2S mixtures (Ps2 = 15 torr)

Pure cobalt and CoCr binary alloys containing up to 25wt% Cr, all sulphidize rapidly according to a parabolic rate law at 800 and 1000°C in H₂/H₂S atmospheres of an effective sulphur partial pressure of 15 torr (H₂-65%H₂S at 1000°C and H₂-85%H₂S at 800°C). There is a maximum in the parabolic rate constant at 10wt%Cr at 1000°C, and at 5wt%Cr at 800°C; the latter, however, is not so pronounced.At 800°C, the scales on the alloys consist of an inner layer of cobalt—chromium sulphide solid solution and an outer layer of CoS_1+x containing no chromium; this latter sulphide decomposes on cooling from the reaction temperature to Co₉S₈ and Co₃O₄. The scales on the 1 and 5wt%Cr alloys also contain an intermediate layer of Co₉S₈. The scales on the Co-17 and 25wt%Cr alloys at 1000°C are similar to those at 800°C. However, on the more dilute alloys, a molten cobalt—chromium sulphide solution forms, which may be covered with a thin crust of solid CoS_{1 + x}.The rate-controlling mechanism appears to be the diffusion of chromium and cobalt ions through the cobalt—chromium sulphide solid solution, where this forms, the outer scale layer having little influence on the overall sulphidation rate.     

Protection of cobalt-based refractory alloys by chromium deposition on surface: Part I: Sub-surface enrichment in chromium by pack-cementation and diffusion

The feasibility of surface chromium enrichment by pack-cementation was assessed for different low chromium-containing cobalt alloys, in order to improve their resistance against high temperature oxidation. A binary Co-10Cr alloy, two ternary Co-10Cr-0.5C and Co-10Cr-1.0C alloys and two TaC-containing Co-10Cr-based alloys were elaborated by foundry for the study. 7.5 h-long and 15 h-long cementations at 1050 °C, followed or not by a 75 h-long heat treatment at 1200 °C were performed on these alloys. Microstructure examinations performed using a Scanning Electron Microscope and concentration profiles using Electron Probe Micro Analysis-Wavelength Dispersion Spectrometry were realized in order to analyze the level of Cr-enrichment of the sub-surface region, with as studied criteria: the nature of the external Cr-enriched zone, the maximal chromium content on surface and the depth of chromium enrichment. The Cr-enrichment of the sub-surface succeeded for the Co-10Cr alloy and for the two tantalum-containing alloys, with the formation of an external metallic zone containing around 30 wt.% Cr. In contrast the chromium carbides-containing alloys were effectively enriched in chromium in surface but in the form of a continuous chromium carbide layer which can induce other problems such as spallation and then possible fast oxidation of the denuded alloy. Finally it appeared that only the carbon-free alloys, and the alloys reinforced by carbides more stable than chromium carbides, are potentially able to be successful enriched in chromium in their sub-surface by pack-cementation.     

The effect of certain reactive elements on the oxidation behaviour of chromia- and aluminaforming alloys

The influence of implanted yttrium and lanthanum on the oxidation behaviour of Co-45Cr and Co-25Cr-1Al chromia formers as well as of β-NiAl and Co-25Cr-9Al alumina formers has been studied in isothermal and thermal cycling conditions in the temperature range 1273–1573 K. It has been found that protective properties of Cr₂O₃-scale and its adherence to the substrate are greatly improved by both reactive element additions. This beneficial effect results from the promotion of fine grained scale formation and consequently the elimination of outward diffusion of chromium. The influence of implanted yttrium (but not lanthanum) on the growth rate and adherence of alumina scale is analogous. In this case, however, both these effects result from elimination by reactive element addition of the inward diffusion of oxygen. It may be then concluded that the influence of implanted reactive elements on the oxidation behaviour of both chromia and alumina formers consists mainly in changing the mechanism of scale growth and not in various interfacial phenomena. It has been found also that the protective properties of alumina scale are considerably improved by elimination of grain boundaries from the substrate, this effect being stronger than that resulting from reactive element addition. Beneficial influence of implanted reactive elements should be expected only when the protective scale layer is formed from the very beginning of the reaction, directly on the initial surface of the material exposed to the oxidizing atmosphere.     

Water Vapour Effects on Fe–Cr Alloy Oxidation

Isothermal oxidation at 700 °C of binary Fe–Cr alloys containing 9, 17 and 25 wt% chromium was measured using continuous thermogravimetric analysis. All alloys developed thin, protective chromia scales in Ar–20O₂ (vol%). Chromia scale growth on the 17 and 25 Cr alloys was faster in Ar–20O₂–5H₂O and Ar–5O₂–20H₂O. In these gases, the Fe–9Cr failed to form a chromia scale and suffered rapid breakaway oxidation, growing iron-rich oxides instead. A low oxygen potential gas, Ar–10H₂–5H₂O caused chromia scaling on Fe–17Cr and Fe–25Cr, but internal oxidation of Fe–9Cr. Application of Wagner’s criterion for sustaining external scale growth is shown to account satisfactorily for these observations.     

Effect of Sulfur Partial Pressures on Oxidation Behavior of Fe–Ni–Cr Alloys

Fe–Ni–Cr alloys containing different contents of Si with and without pre-formed oxide scale at the surface were tested in oxidation environments at 1,050?°C with varied sulfur partial pressures. The oxide-scale growth on Fe–Ni–Cr alloys was accelerated by increasing sulfur partial pressures in the oxidizing-carburizing environments. This accelerated oxidation was characterized by the formation of plate-shaped MnCr₂O₄ spinel crystallites and the nodular clusters at the site of scale spallation. Pre-oxidized Fe–Ni–Cr alloys generally did not suffer from sulfur attack because of excellent protection of pre-formed oxide scale. Scale spallation and sulfur attack were found only on high-Si alloy subjected to the maximum sulfur potential, which was attributed to accelerated oxidation and selective oxidation and sulfidation at the sites where oxide scale spallation had occurred. For bare alloys in absence of pre-formed oxide layers, scale spallation was found to occur at lower level of sulfur potential on low-Si alloy than on high-Si alloy. A higher content of Si is necessary for the formation of protective silica sub-layer, which is believed to be the main cause of the difference in scale spallation observed.     

Oxidation of Binary FeCr Alloys (Fe?C2.25Cr, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr) in O2 and in O2?+?H2O Environment at 600???C

The oxidation behaviour of the binary alloys Fe?C2.25Cr, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr (wt%) in dry and wet O₂ at 600???C is investigated by isothermal exposures of carefully polished samples for up to 168?h. The oxidized samples are investigated gravimetrically and the oxides formed are studied by X-ray diffraction. X-ray photoelectron spectroscopy is used for depth profiling of the thin oxides. The scale surface is imaged by SEM. Cross-sections through the scale are analyzed by SEM/EDX for imaging and for measuring the chemical composition. The oxidation behavior of the four FeCr alloys is intermediate between those of iron and chromium. Fe?C2.25Cr oxidizes in a way similar to iron in both environments, forming a poorly protective scale consisting of FeCr spinel at the bottom, magnetite in the middle and a hematite cap layer. In dry O₂, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr form a thin and protective (Fe,Cr)₂O₃ oxide similar to the chromia film formed on pure chromium. In wet O₂, Fe?C10Cr, Fe?C18Cr and Fe?C25Cr initially form the same kind of protective oxide film as in dry conditions. After an incubation time that depends on alloy chromium content, all three alloys go into breakaway oxidation and form thick, poorly protective scales similar to those formed on Fe?C2.25Cr. Breakaway oxidation in wet O₂ is triggered by the evaporation of CrO₂(OH)₂ from the protective (Fe,Cr)₂O₃ oxide.