Scale-metal Interface Research Articles

Nature of indigenous sulfur segregated to the free metal surface and to the scale-metal interface

Sulfur segregation to the free metal surface of a Ni-20(wt.%)Cr-12(wt.%)Al alloy has been documented by high-temperature scanning Auger electron microscopy. Preferential enrichment of sulfur occurs to the surface of the chromium-rich γ phase in the two-phase y nickel-β (NiAl) alloy. Electron spectroscopy for chemical analysis (ESCA) studies have indicated that for a free metal surface, i.e., metal without an overlying thermally grown oxide scale, the surface segregated sulfur correlated with both S−2 and elemental sulfur. Energy-dispersive X-ray analysis (EDAX) characterization of fractured oxide films thermally grown on the Ni-Cr-Al alloy indicated that both a labile, i.e., relatively volatile, and a refractory sulfur species exist at the scale-metal interface. Elemental sulfur as detected by ESCA has the volatility characteristics found for the labile sulfur species found in the scanning electron microscopy (SEM)-EDAX studies.

Transport of reactants during scale formation on copper in SO2+O2 atmosphere at 973 and 1073 K

The reaction between copper and a mixture of SO2+O2 was studied at 973 and 1073 K. The experimental methods included optical microscopy, X-ray diffraction, scanning electron microscopy (SEM), energy-dispersive X-ray analysis, and radiotracer. It is concluded that the inward transport of oxidants through the scale as well as oxygen and sulfur dioxide liberation in the reactions taking place at phase boundaries suggest that secondary processes occur inside the oxide-sulphate layer. Therefore, Cu2O and CuOCuSO4 appear inside this layer. In the metal-consumption zone, a Cu2O layer forms, which contains small amounts of a sulfide phase near the metal-scale interface.

Some comments on the role of yttrium in protective oxide scale adherence

The commercial utilization of materials for high temperature hardware applications requires that such materials form adherent protective scales for protracted times. “Active” elements, such as yttrium and hafnium, added at low levels have been shown to improve oxide scale adherence markedly on thermal cycling. Several models have been proposed to account for this improvement. Some of these models are discussed in the light of relatively recent experimental results. Of particular relevance, the results of studies which indicate that sulfur segregation to the scale-metal interface strongly influences protective scale adherence will be reviewed.

On the reasons for the effects of dispersions of stable oxides and additions of reactive elements on the adhesion and growth-mechanisms of chromia and alumina scales-the ?sulfur effect?

The “sulfur effect” theory proposes that elements such as sulfur segregate to the scale-metal interface, where they have a deleterious effect on scale-metal adhesion, and that reactive elements and dispersions of their oxides are beneficial because they prevent this segregation. Evidence in favor of this theory has been obtained by showing that the adhesion of the scale formed on chromium in 0.1 atm. oxygen at 950°C. is greatly improved if the chromium is annealed beforehand in hydrogen instead of in vacuo. It is shown that the theory can explain not only all the experimental observations regarding the beneficial effects of reactive elements and their oxides on scale-metal adhesion, but also their effects on scale growth-mechanisms.

A comparison between ion implantation and laser alloying of pure iron for oxidation resistance improvement

Following previous results concerning boron alloying by ion implantation or laser surface melting, we study the role of aluminium to improve the high temperature oxidation of iron. Laser alloying has a greater efficiency than ion implantation. The aluminium containing phases formed at the metal-scale interface are responsible for the observed protection. The behaviour of the laser surface alloy is quite comparable to that of conventional alloys Fe-5Al. These two techniques, ion implantation and laser irradiation, seem to be of importance for future developments of corrosion protection.

A relationship between indigenous impurity elements and protective oxide scale adherence characteristics

A new and a radically different mechanism to account for the beneficial effects that small additions of elements such as yttrium have on the adherence of oxide scales is proposed. It has been long:known and has been reaffirmed here that indigenous impurities such as sulfur, known to be present at tramp levels (<100 ppm) within nickel and nickel-based alloys, can segregate to metal surfaces. However, here it has been disclosed that such sulfur segregation can also markedly affect the adherence of the protective oxide scale. In the absence of elements like yttrium, such segregation effects weaken the bond between the protective scale and the substrate metal. The role of the yttrium is to interact with such indigenous sulfur to form a refractory sulfide. This interaction lessens the amount of sulfur available to segregate to and concentrate at the critical scale-metal interface. Results reported here have focused on sulfur because sulfur has been long known to be a common tramp impurity in nickel and nickel-based alloys. However other elements, e.g., phosphorus, chlorine, etc., could very probably produce similar effects. The results of experiments involving Auger spectroscopy, optical, scanning electron microscopy, scanning electron microprobe, and scanning transmission electron microscopy techniques in conjunction with isothermal and cyclic oxidation testing which have led us to propose this mechanism are presented.

STUDY OF THE "REACTIVE-ELEMENT" EFFECT IN OXIDATION OF Fe-Cr ALLOYS USING TRANSVERSE SECTION ANALYTICAL ELECTRON MICROSCOPY

The role of trace additions of reactive elements like Y, Ce, Th, or Hf to Cr bearing alloys was studied by applying a new developed technique of transverse section analytical electron microscopy. This reactive-element effect improves the high temperature oxidation resistance of alloys by strongly reducing the high temperature oxidation rate and enhancing the adhesion of the oxide scale, however, the mechanisms for this important effect remain largely unknown. It is indicated that the presence of yttrium affects the oxidation of Fe-Cr-Y alloys in at least two ways. The reactive element alters the growth mechanism of the oxide scale as evidenced by the marked influence of the reactive element on the oxide scale microstructure. The present results also suggest that reactive-element intermetallic compounds, which internally oxidize in the metal during oxidation, act as sinks for excess vacancies thus inhibiting vacancy condensation at the scale-metal interface and possibly enhancing scale adhesion.

Observations of nodule growth during the oxidation of pure binary iron-aluminum alloys

Oxidation studies were carried out in oxygen at 800°C, on a series of pure binary iron-based alloys with between 1.9 and 9.8 wt. % aluminum. The results are presented in conjunction with the existing literature and these permit the development of a classification of scale morphologies based on alloy composition. Alloys with less than about 2.4 wt. % aluminum form bulky stratified scales composed of Fe2O3 and Fe3O4 with FeAl2O4 and Al2O3 at the scale-metal interface. Alloys with between 2.4 and 6.9 wt. % form an external Al2O3 scale but this is interspersed with iron oxide nodules that penetrate the alloy substrate. Only alloys with greater than 6.9 wt. % aluminum form completely protective Al2O3 scales. Models based on oxide nucleation are presented for the growth of bulky scales and also the iron oxide nodules.

Sulfidation-oxidation of advanced metallic materials in simulated low-Btu coal-gasifier environments

The corrosion behavior of structural alloys in complex multicomponent gas environments is of considerable interest for their effective utilization in coal conversion schemes. Little understanding of the degradation mechanisms of advanced high-temperature alloys in conditions typical of low-Btu coal gasifiers presently exists. Analysis of scale and subscale characteristics of several alloy types after exposure to the aggressive simulated low-Btu gasifier environments yielded a reaction model for these sulfidation-oxidation conditions. Initial competition between reactive metal oxide and base metal sulfide nuclei is followed by base metal sulfide overgrowth, chromium sulfide formation at the scale-metal interface, dissociation near voids in the subscale, and internal chromium sulfide precipitation. Additions of aluminum, titanium, and an oxide dispersion improve the sulfidation resistance by increasing the number of oxide nucleation sites and their growth kinetics on the surface in the crucial competition stage. Thermogravimetric tests carried out in three mechanistic regimes agreed with these hypotheses.

On the influence of metal lattice diffusion on oxidation of metals and alloys

The influence of metal lattice diffusion on oxidation kinetics is discussed for two simple cases: (i) a pure metal, where vacancies generated at the scalemetal interface diffuse to sinks within the metal; and (ii) a binary alloy of metals A and B, with A forming the more stable oxide. In the first case it is shown that vacancy effects are generally negligible. Analyses suggesting the contrary have failed to replace atom concentration gradients by the more appropriate chemical potential gradients. For the alloy, Wagner's condition for breakdown of A oxide is confirmed. It is shown that growth of A oxide cannot be controlled by diffusion of A in the metal, if B atoms can react at the scale-metal interface; scale-breakdown intervenes.

Influence of metal grain growth on the oxidation behavior of a 25Cr-20Ni steel

Comparative oxidation tests with a fine- and a coarse-grain 25Cr-20Ni steel, and cast steel of similar composition in synthetic air between 900 and 1300°C reveal a strong influence of grain growth in the base metal on the oxidation behavior. Metal grain growth destroys the protective Cr2O3-rich scale, and promotes the formation of a scale rich in iron oxides. The result is a higher scaling rate, and a higher Cr depletion at the metal-scale interface of the fine-grain material which undergoes grain growth during oxidation.

The attack of Co-Cr alloys by Ar-SO2 atmospheres

Cobalt alloys containing up to 25% chromium have been exposed to Ar-10% SO2 atmospheres at temperatures between 600 and 1000° C. The results show that, although an increase in chromium content leads to a reduction in the reaction rate, even to negligible rates in the cases of the higher chromium contents, all of the alloys are eventually subjected to rapid attack at more or less longer times, depending on the chromium content. The mechanism of the reaction appears to involve the formation of a more or less protective oxide layer which is eventually penetrated by sulfur. The sulfur forms chromium sulfides at the metal-scale interface, removing the chromium from solution and causing an expansion that cracks the protective scale, allowing both the ingress of gas and the formation of rapidly growing cobalt compounds. The process occurs rapidly with Co-5% Cr alloys, whereas, only the initial sulfur penetration is observed with Co-25% Cr alloys during the time scale of the investigation. The penetration of sulfur is thought to occur as a molecular gas species permeating through the scale down physical defects.

Study of magnetite film formation at metal-scale interface during cooling of steel products

A thin layer of magnetite is sometimes observed in the scales of hot-rolled sheets and wire rod at the scale-metal interface. The results of this study show that this layer of magnetite is produced, during the cooling of the products, by the wustite decomposition. The time/temperature field in which it appears has been defined. This inner layer is composed of pure magnetite, in epitaxy with the neighboring wustite, and the metal surface is not essential to its nucleation. The proeutectoīd reaction which, during cooling, produces an increase in the oxygen concentration of the wustite in contact with the metal favors the subsequent growth of the magnetite layer.

Transient state in the selective oxidation of binary alloys with quasiparabolic kinetics: Solution for a fixed alloy-scale boundary

Analytical solutions for the concentration profile and the surface concentration of an oxidizable component during the selective oxidation of a binary alloy following pseudoparabolic kinetics are examined, assuming a fixed alloy-scale boundary. A solution similar to that already proposed for rate control by metal transfer across the metal-scale interface can be obtained also assuming diffusion control. In the case of formation of a metal-deficit scale growing mainly by cation diffusion it is shown that oxidation is controlled by the diffusion in the alloy. The rate law is found to be pseudoparabolic, although different from that usually found in practice, leading to negative intercepts in a parabolic plot. Positive intercepts cannot be obtained if the regression of the alloy interface during oxidation is disregarded.

Influence of temperature and the role of chromium on the kinetics of sulfidation of 310 stainless steel

The sulfidation of 310 stainless steel was studied over the temperature range from 910 to 1285° K. By adjusting the ratio of hydrogen to hydrogen sulfide, variations in sulfur potential were obtained. The effect of temperature on sulfidation was determined at three different sulfur potentials: 39 N·m−2, 1.4×10−2 N·m−2, and 1.5×10−4 N·m−2. All sulfide scales contained one or two surface layers in addition to a subscale. The second outer layer (OL-II), furthest from the alloy, contained primarily Fe-Ni-S. The first outer layer0 (OL-I), nearest the subscale, contained Fe-Cr-S. The subscale consisted of sulfide inclusions in the metal matrix. Two different phases were observed in OL-II depending on the temperature and sulfur potential. Below 1065°K OL-II is composed of a mixture of monosulfides of iron and nickel (Fe Ni)1−xS and pentlandite (Fe4.5Ni4.5S8) with the pentlandite phase exsolved as lamellae upon cooling. At temperatures higher than 1065°K only the pentlandite phase was formed, which melted above 1145°K at sulfur potentials greater than 10−2 N·m−2, yielding metal-rich iron-nickel-sulfur. Above 1145°K, and at sulfur potentials less than 10−2 N·m−2, OL-II ceased to exist (this temperature is termed transition temperature). Below the transition temperature, where OL-II exists, OL-I could be represented by the general composition (Fe, Cr)1−xS. This phase on cooling transformed into an array of structures differing in Fe∶Cr ratio. These substructures, however, were not observed in quenched samples. Above the transition temperature OL-I changed to a chromium-rich sulfide composition and was associated with a sudden decrease in reaction rate. Subscale formation was found to be due to the dissociation of OL-I at the scale-metal interface, and the extent of subscale growth was found to depend on the temperature and the sulfur potential, as well as the composition of OL-I. At a given temperature and sulfur potential the weight-gain data obeyed the parabolic rate law after an initial transient period. The parabolic rate constants obtained at the sulfur potential of 39 N·m−2 did not show a break when the logarithm of the rate constant was plotted as a function of the inverse of absolute temperature. Sulfidation carried out at a sulfur potential below 2 × 10−2 N·m−2, however, did show a break at 1145°K. This break was found to be associated with the changes which had occurred in the Fe∶Cr ratio of OL-I. Below the transition temperature the activation energy was found to be approximately 125 kJ · mole−1. Above the transition temperature the rate of sulfidation decreased with temperature but depended on the Fe∶Cr ratio in the ironchromium-sulfide layers of the OL-I. A reaction mechanism consistent with the experimental results has been proposed in which the diffusion of cations through OL-I is the rate-controlling step. Below the transition temperature the diffusion of Fe and Ni through OL-I contributes to the scale formation, whereas above the transition temperature the diffusion of Cr through OL-I controls the scale formation. Existing literature on the Fe-Ni-S system is compared with the present results.

The mechanisms of breakaway oxidation of three mild steels in high-pressure CO2 at 500�C

The mechanisms of breakaway oxidation of a rimming steel, a low-alloy steel (1 Cr-0.5 Mo), and a free-cutting steel (En1A) have been studied in high-pressure CO2 at 500°C. Average compressive stresses up to 170–280 ton in−2. in the scale have been derived from foil elongation and creep data. Carbon contents of up to 6 and 15% of the weight gain have been found in protective and breakaway scale and are larger than previously reported.1 Most of the carbon is deposited near the scale-metal interface, showing that CO2 diffuses through the scale. Oxidation in CO2-tritiated water mixtures gives a maximum tritium content in the metal at 250–500 h, which declines thereafter. Treatment with some sulfur compounds before oxidation, or the presence of sulfur in the metal, reduces the rates of protective and breakaway oxidation in wet CO2 and carbon transfer to the metal, but not to the scale. It is proposed that breakaway is initiated by an effect of hydrogen such as the accumulation of hydrogenous gases at the scale-metal interface under pressures sufficient to rupture the inner scale. Carbon deposition may assist initiation, and is probably the main factor in propagating breakaway oxidation.

The influence of oxide adherence on the hot-salt corrosion of mild steels

The corrosion of pure iron and some mild steels by sodium chloride has been investigated in the temperature range 600 to 900° using hot-stage microscopy, a vibration technique, and quantitatively by means of a vapor exposure test. It has been found that for corrosive attack to occur it is necessary for the sodium chloride to have access to the metal surface, and that such access may be prevented by the presence of an adherent, impervious, scale layer at the metal surface. Such a layer was found to be present on an EN2 steel examined but was not formed on any of the other materials tested. The EN2 steel contained 0.2 pct Ni, and 0.21 pct Si, and both have been shown to become concentrated in the scale at the scale-metal interface. This produced an inner adherent scale which was resistant to sodium chloride penetration and which gave the material an appreciably enhanced corrosion resistance.

The high-temperature oxidation of cobalt-21 wt.% chromium-3 vol.% Y2O3 alloys

Alloys of Co-21 wt. % Cr-3 vol. % Y2O3have been prepared by a mechanical alloying method, and oxidized in oxygen at 100 Torr in the temperature range 900–1200°C. The general effects of the dispersed oxide phase are similar to those reported for nickel-base alloys: the selective oxidation of chromium to form a continuous protective Cr2O3scale is promoted; the rate of growth of Cr2O3is reduced compared to dispersoid-free alloys; the adhesion of the Cr2O3is greatly improved; and the scale-forming reaction is probably at the scale-metal interface in the alloys containing the dispersoid, whereas it is at the scale-oxygen interface in dispersoid-free alloys. This last point has not been positively demonstrated. The improvement in adhesion is of particular significance, since the scales on cobalt-base alloys are prone to spallation, and it has been possible to study the mechanism of adhesion in more detail. It appears that in dispersoid-free material the metal recedes from the scale-metal interface, leaving the scale supported on the tops of metal “peaks” but this does not happen in the alloy containing the dispersoid, either because the growth direction of the scale has been changed, or because of changes in the substrate grain size. In general, the observations support the model proposed in an early study for the oxidation of Ni-20 wt.% Cr alloys containing oxide dispersions.

The high-temperature oxidation of nickel-20 wt. % chromium alloys containing dispersed oxide phases

Alloys of Ni-20 wt. % Cr containing 3 vol. % of a dispersed oxide phase have been prepared by a mechanical alloying method and oxidized in oxygen at 100 Torr in the temperature range of 900 to 1200°C. It appears that the dispersed oxide has four distinct effects on the oxidation: (1) the selective oxidation of chromium to form a continuous protective Cr2O3scale is promoted; (2) the rate of growth of Cr2O3is reduced compared with particle-free alloys; (3) the adhesion of the Cr2O3is greatly improved; and (4)the scale-forming reaction appears to be at the scale-metal interface in alloys containing a dispersion, but at the scale-oxygen interface in alloys without a dispersion. It appears that the nature of the dispersed oxide is not important, since very similar effects can be obtained with ThO2,Y2O3,and CeO2dispersions. It is demonstrated that a logical deduction from this evidence is that the growth of Cr2O3scales on dispersion-free systems must involve short-circuit diffusion of chromium through the scale, and that it seems probable that an effect of the dispersion must be to retard or eliminate this short-circuit process. It is suggested that the oxide particles act as nucleation centers for the oxide, thus reducing the oxide grain size; and it is shown that this simple hypothesis is sufficient to explain a number of the experimental observations.

Some Observations on Dilute Iron Alloys

AbstractSamples of pure iron and of 18 pure binary alloys containing a nominal 0.1 or 0.5 wt.% of substitutional alloying element have been oxidised in air at 830 and 930 °C. The effects of the alloying element on the reactions at the scalemetal interface have been studied metallographically and by X‐ray and electron microanalysis. Measurements have also been made of the ambient temperature adherence of the scales so formed. Similar observations have been made on a range of iron‐carbon alloys.