Influence of ion implantation on the microstructure of oxide scales formed on a 20Cr/25Ni/Nb-stabilized stainless steel in carbon dioxide at 825° C

Specimens of a stainless steel (20%Cr, 25%Ni stabilized with niobium and also containing 0.9% Mn and 0.6% Si) implanted with lanthanum to a dose of 10¹⁷ ion cm^–2 , together with unimplanted specimens, have been oxidized in carbon dioxide at 825° C for times up to 9735 h. Transverse sections through the oxide scales formed on the respective specimens have been studied by analytical electron microscopy. After this exposure the scale on the unimplanted 20/25/Nb stainless steel consists of an outer, large-grained, spinel layer, a middle fine-grained Cr₂O₃ layer and an inner, discontinuous silicon rich, niobium and chromium bearing, amorphous layer. The main effects of the lanthanum implantation are to improve oxidation resistance and maintain scale adherence during thermal cycling. The microstructural changes in the scale formed on the lanthanum implanted 20/25/Nb steel include finer Cr₂O₃ oxide grains and an intermediate region between the outer spinel and inner Cr₂O₃ layers comprised of both oxides. The lanthanum concentrates in this region and appears to act as a marker due to its low diffusivity. Mechanisms of scale development on the 20/25/Nb stainless Red and the influence of lanthanum implantation are discussed.     

Scale formation mechanisms of martensitic steels in high CO2/H2O-containing gases simulating oxyfuel environments

Abstract

In oxyfuel power plants, metallic components will be exposed to service environments containing high amounts of CO₂ and water vapour. Therefore, the oxidation behaviour of a number of martensitic 9–12%Cr steels in a model gas mixture containing 70% CO₂–30% H₂O was studied in the temperature range 550–700°C. The results were compared with the behaviour in air, Ar–CO₂ and Ar–H₂O. It was found that in the CO₂- and/or H₂O-rich gases, the mentioned steels tended to form iron-rich oxide scales with significantly higher growth rates than the Cr-rich surface scales formed during air exposure. The iron-rich scales were formed as a result of a decreased flux of chromium in the bulk alloy toward the surface because of enhanced internal oxidation of chromium in the H₂O-containing gases and carbide formation in the CO₂-rich gases. Additionally, the presence of water vapour in the exposure atmosphere led to buckling of the outer haematite layer, apparently as a result of compressive oxide growth stresses. The Fe-base oxide scales formed in CO₂(–H₂O)-rich gases appeared to be permeable to CO₂ molecules resulting in substantial carburization of the steel.     

Oxide/metal interface on 20Cr–25Ni–Nb stainless steel

Abstract

20Cr–25Ni–Nb stabilised stainless steel is used to contain the fuel in the advanced gas cooled reactor. During operation, this steel must withstand temperatures from 600 to 1073 K in CO₂ gas at 40 atm pressure. It is important that the oxide which forms on this steel is thoroughly characterised and the adherence of the oxide to the metal is understood. A technique of sputter ion plating has been used to remove the oxide from the metal without destroying either metal or oxide. This involves plating the oxide with nickel or molybdenum at a temperature of 600 K, while sputtering the surface with argon ions. On cooling, stresses set up between the oxide and the metal cause the oxide plus sputtered layer to peel off allowing both the metal and oxide sides of the interface to be examined. Results are presented from studies of the metal/oxide interface using scanning Auger microscopy. Analysis of grain centres and grain boundaries indicates that silicon and chromium play an important role in oxide/metal adhesion and, together with conventional analysis of the bulk oxide, assist in determining the oxidation mechanism.

MST/862     

Microstructural stability of ODS steels in cyclic loading

The remarkable microstructural stability of high chromium steels prepared by powder metallurgy and strengthened by dispersion of nanometric yttrium oxides in cyclic loading at high temperatures is reported. Contrary to the continuous cyclic softening and profound changes in the microstructure during fatigue of common high chromium steels, the addition of 0.3 wt% Y₂O₃ stabilizes the microstructure and significantly reduces cyclic softening of investigated steels. The evolution of microstructure as a result of fatigue loading at room temperature, 650 and 750 °C, was examined by means of transmission electron microscopy. Only minor changes in the microstructure were detected. The stability of oxide particles after high‐temperature exposure was confirmed by energy dispersion spectroscopy chemical analysis. The microstructural features are discussed in relation to the cyclic behaviour of the oxide dispersion strengthened steels. The analysis of the hysteresis loop indicates that oxide nanoclusters are intersected and dissolved in slip bands of ODS Eurofer steel. This process contributes to cyclic softening.     

Breakdown of protective scales during the oxidation of thin foils of Fe–20Cr–5Al alloys at high temperatures

Abstract

The oxidation behaviour of alumina-forming Fe–20Cr–5Al and similar alloys containing small concentrations of lanthanum or lanthanum plus molybdenum in air at 1,150°C has been studied, with emphasis on thin (0.05 mm) specimens, where the aluminium reservoir in the substrate is soon depleted to a very low value. Oxidation of these alloys involves establishment and growth of protective alumina scales. However, once the residual aluminium concentration in the alloy drops below a critical level, a layer of chromia is able to develop and grow at the alumina–alloy substrate interface. Eventually, breakaway oxidation occurs and iron-rich oxides form and engulf the specimen.

This paper presents some kinetics of oxidation of these alloys and discusses the growth and breakdown of the protective scales, drawing on the results of detailed examinations of the oxidized specimens using analytical scanning and transmission electron microscopy in cross section. It has been shown that lanthanum increases the time to the onset of breakaway oxidation, probably due to beneficial effects on the mechanical integrity of the scale. Molybdenum additions have been found to decrease significantly the rate at which breakaway oxides are able to penetrate and engulf the alloy substrate. Such additions stabilize the ferrite phase in the substrate at the alloy–scale interface, thereby maintaining a high rate of diffusion of chromium to the interface and facilitating establishment of a healing and partially protective chromium-rich oxide layer at the base of the breakaway oxide scale. In the absence of such additions, depletions of chromium in the substrate adjacent to the alloy/scale interface, arising from oxidation of chromium, enable the austenite phase to be stabilized. The relatively low rate of diffusion of chromium in this phase allows chromium-rich oxide to form as internal precipitates in the alloy rather than as a continuous, healing layer; hence, the breakaway oxide scale is able to penetrate and consume the substrate more rapidly than in the presence of molybdenum additions.     

Healing layer formation in Fe–Cr–Si ferritic steels

Abstract

The addition of small amounts of Si can dramatically improve the oxidation resistance of Fe and Fe–Cr steels. It is found that steels with Si contents above a certain critical value oxidise at a much slower rate and also become virtually immune to breakaway oxidation in high pressure CO₂, The critical Si content for this behaviour is found to vary with the Cr content (wt-%) of the steel, from about 2·5% for mild steel to 0·7% for 9%Cr steel to 0·3% for 11%Cr steel in the temperature range 575–650°C. The lower Si content required for Cr steels than for mild steels is advantageous, because it is small enough not to degrade the other metallurgical properties such as creep strength. The beneficial effect of Si is thought to arise from the formation of a near continuous ‘healing’ layer of amorphous SiO₂ at the oxide/metal interface which acts as a diffusion barrier to further transport of metal ions to the scale. The conditions required for the development of such layers are analysed using standard models of selective oxidation. The synergistic effect of Cr and Si is ascribed to the action of Cr as a secondary getter, in which it reduces the oxygen solubility in the metal and so reduces the Si content required to form a healing layer. Chromium also discourages the SiO₂ from converting to fayalite (Fe₂SiO₄) which is a much poorer diffusion barrier. The conventional theory of selective oxidation and secondary gettering is found to describe reasonably well the compositional limits of healing layer behaviour in these ferritic steels. However, the silica also seems to encourage the formation of a chromia based layer at the base of the overlying oxide and the oxidation rate during healing seems to be limited eventually by this chromia layer rather than the silica layer, as would be expected in the conventional model.

MST/1074     

Yttrium implantation effect on 304L stainless steel high temperature oxidation at 1000°C

Scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDXS) and in situ X-ray diffraction techniques were carried out to observe the oxide scale evolutions of yttrium implanted and unimplanted commercial 304L stainless steels during and after their high temperature oxidation at 1000°C for 100 h. Our results clearly demonstrate that yttrium implantation promotes a faster oxide scale growth and the formation of a more uniform chromia layer due to a higher chromium selective oxidation compared to unimplanted 304L stainless steel. Moreover, the presence of yttrium also leads to the formation of an enriched silicon layer at the metal-oxide interface limiting the growth of iron-based oxides which were not detected (even during cooling) in the case of yttrium implanted samples. These results allow to understand the low weight gain of yttrium implanted 304L stainless steel observed by thermogravimetry and underline the beneficial effect of yttrium implantation on the 304L oxidation resistance at high temperature.     

Abscheidung leitender und nichtleitender Hartstoffschichten auf metallischen und keramischen Werkstoffen mittels Hochfrequenz-Plasma-CVD

Deposition of conductive and nonconductive hard coatings on metallic and ceramic materials by RF-PA-CVD Conductive titanium nitride and nonconductive aluminium oxide layers were deposited on conductive and nonconductive substrates by a RF-PA-CVD process. The influence of substrate material, pressure, plasma power and the components of the gas mixture on the layer properties was investigated. TiN coatings with a homogeneous structure could be deposited by using TiCl₄ as precursor. The properties of the layer are strongly influenced by the substrate material. An increasing pressure causes a faster deposition rate and a higher chlorine content. A lower chlorine content and at the same time a faster deposition rate can be achieved by increasing the r.f. power. Aluminium and aluminium oxide layers could be deposited on steel and Si₃N₄ substrates by using AlCl₃ as precursor in dependence on the CO₂ content in gas mixture. Higher CO₂ content facilitates the deposition of aluminium oxide.     

Corrosion of 20Kh13 steel in lead melts saturated with oxygen

We study the high-temperature interaction (650°C, 500 h) of 20Kh13 chromium steel with melts of stagnant lead saturated with oxygen (C _{O [Pb]} ≈ 6 · 10⁻³ wt.%). First (up to 200 h), separate islands of Me₃O₄ oxides (Me: Fe, Cr, Pb) are formed on the steel surface. In the course of time (for 500 h), these islands completely cover the steel surface as a result of lateral growth. The upper part of the oxide layer is formed by the (Fe_{1 − x} Pb_x) O · Fe₂ O₃ complex oxide growing from the initial “solid-metal—melt” interface toward the liquid-metal medium. The inner part of the oxide layer is spinel [(Fe_{1 − x} Pb_x) O · (Fe_{1 − y} Cr_y)₂O₃] enriched with chromium and formed on the basis of the matrix. Both layers symmetrically grow with respect to the initial “solid-metal—melt” interface. Lead does not penetrate into the steel matrix and is fixed only in the oxide layer. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 5, pp. 36–40, September–October, 2005.     

Plasma oxidation of Cu-Al alloy

Selective oxidation of Cu-1.0 wt % Al alloy was found to be achieved by using microwave plasma at 1073 K in a mixed gas of 10 % 0₂ and 90 % N₂ at a total pressure of 2 torr (∼266.644 Pa). The selective oxidation was confirmed by electron diffraction analysis of aluminium oxide particles, microstructural observations and hardness measurements of the internal oxidation layer. Electrical conductivity and the thermal expansion coefficient of the oxidized specimens were measured. The morphology and distribution of the aluminium oxide particles formed in the oxidized alloy is also discussed.     

Effect of silicon content in steel and oxidation temperature on scale growth and morphology

The effect of high silicon content in steel, 1.6 wt.%Si and 3.2 wt.%Si, and high oxidation temperatures (850–1200 °C) on scale growth rate and morphology were investigated. The steels were oxidized in a 15% humid air with short isothermal oxidation times (15 min). The scale growth rate of the non-alloyed steel follows a parabolic law with time; it is an iron diffusion controlled oxidation. The presence of silicon delays scale growth by forming a silica SiO₂ barrier layer at the scale/metal interface, this effect is more important for the steel containing 3.2 wt.%Si and induces a discontinuous scale. Silicon oxides are concentrated at the scale/metal interface; their morphology depends on the oxidation temperature. For temperatures lower than 950 °C, silica is formed. Between 950 °C and 1150 °C, fayalite (Fe₂SiO₄) grains appear in the wüstite matrix close to the scale/metal interface. For temperatures higher than 1177 °C, a fayalite–wüstite eutectic is formed; this molten phase favours iron diffusion leading to high scale growth. After cooling, a continuous fayalite layer with small wüstite grains is obtained at the scale/steel interface.     

Laser raman spectroscopic studies of the surface oxides formed on iron chromium alloys at elevated temperatures

Binary iron-base alloys containing chromium additions of 3, 9, 12 and 18 % were oxidized in air at elevated temperatures. Laser Raman spectroscopy has been used to determine the chemical compounds of the oxides of these alloys. It is found that the oxides formed on Fe-3Cr alloy at various elevated temperatures consist mainly of iron. However, for Cr additions ≥12 %, the surface oxide formed at 400°C consists of αFe₂O₃, Fe₃O₄ and spinel phases. With increasing oxidation temperature up to 850°C, the oxide scale consists of Cr₂O₃ and spinel phases only.     

Cyclic oxidation of Haynes 230 alloy

The cyclic oxidation of Haynes 230 alloy (Ni-Cr-W-Mo alloy) was investigated in air at three different temperatures, 871, 982 and 1093 °C. Studies indicated that during cyclic oxidation, protective scales formed which were predominantly Cr₂O₃, with Kirkendall voids formed both at the scale/alloy interface and grain boundaries. Intergranular oxides were observed at temperatures above 982 °C while internal oxide particles were found above 1093 °C. Both intergranular and internal oxides were identified as aluminium oxide. A 50 m chromium-depleted zone developed after 70 h exposure at 1093 °C and was accompanied by disastrous scale spalling. The lowest chromium concentration within the depleted zone was 14 wt% which still provided a sufficient supply of chromium for development of a continuous Cr₂O₃ rich scale. Decarburization was observed at the higher temperature of 1093 °C, and a carbide-free zone developed. Also, it was found that Haynes 230 is subject to a sensitization process. At the lower exposure temperature of 871 °C, large amounts of chromium carbide formed preferentially at the grain boundaries. While at the surface region chromium carbide precipitation occurred at the twin boundaries.     

Modifying the properties of AISI 316L steel by glow discharge assisted low-temperature nitriding and oxynitriding

Apart from titanium, its alloys and CoCrMo alloys, austenitic steels are widely used in medical applications. In order to improve the frictional wear resistance of these steels, they are subjected to various surface treatments such that the good corrosion resistance of the steels is preserved.The paper analyzes the structure and phase composition of AISI 316L steel after subjecting it to low-temperature nitriding and oxynitriding under glow discharge conditions. The treatments produced diffusion-type surface layers composed of nitrogen-expanded austenite (known as the phase S, i.e. supersaturated solution of nitrogen in austenite) with a thin surface layer of chromium nitride (CrN) zone (after nitriding) or chromium oxide (Cr₂O₃) zone (after oxynitriding). It has been shown that the treatments substantially increase the hardness and frictional wear resistance of the steel without degrading its good corrosion resistance (examined in the Ringer physiological solution at a temperature of 37 °C).     

Oxidation behaviour of some modern heat-resistant cast steels

The kinetics and morphological development of the oxidation of a selection of modern heat-resistant cast steels have been examined and compared with those of the traditional material, HK40. The materials examined had Cr contents of 24 to 29 weight percent (wt %), Ni contents of 30 to 46 wt %and in several cases minority additions of Nb, W, or both. One steel contained 3.3 wt % Al. Kinetics were measured gravimetrically over periods of 6 to 100 h and found to be parabolic in all cases except for the Al-containing steel, which oxidized in an irregular and irreproducible fashion. All steels formed an external scale of Cr₂O₃ with a Mn-rich spinel layer at the outer surface. Beneath this scale was a layer of alloy depleted in both Cr and Mn. Within the depleted layer inter-dendritic carbides had been destroyed, leaving either oxide near the external alloy surface or voids deeper within the alloy.     

Direct observation and analysis of the oxide scale formed on Y-treated austenitic stainless steels at high temperature

Abstract

We have studied the oxidation behavior of conventional austenitic stainless steels using same small amounts of Y as is added for deoxygenation and desulphurisaton in steel making.

The direct observation and analysis of the oxide scale formed on 19Cr–10Ni–l .5Si steels with and without small amounts of Y at high temperature have been carried out using several types of equipment. The following results were found: (1) Steel with 0.03Y showed good resistance to oxidation at l,000°C.

(2) Oxide scale was composed mainly of Cr oxide, and Si oxide was also detected at the oxide scale–metal interface and in the internal oxides. The Si oxide formed a network cell structure in the inner oxide scale with deeper internal penetrations. The steel with Y formed a uniform oxide scale in every oxide layer.

(3) Small amounts of Y and Si were detected at the grain boundaries of the inner oxide scale, but no Y was detected in the oxide grains.

The beneficial effect of Y addition was more notable in the Si containing austenitic stainless steels, as the existence of Y or Si prevents the diffusion of cations and anions through the oxide grain boundaries. As consequence, the steel treated with Y showed good resistance to oxidation.     

Chromium carbide laser-beam surface-alloying treatment on stainless steel

In order to improve the resistance to wear, oxidation and corrosion of a stainless steel die, chromium carbide surface-alloying treatment was carried out on a 12 % Cr stainless steel using a CO₂ laser. Cr₃C₂ powder slurry was coated on the stainless steel and then a 3 kW CO₂ laser beam was used to irradiate the specimen. The thickness of surface-alloyed layer was about 0.3 mm and the chromium concentration was about 40 % throughout the alloyed-region. Large amounts of Cr₃C₂ and Cr₇C₃ were also distributed in this alloyed layer. From the results of the isothermal oxidation test at 960 °C for 100 h, it was found that the surface-alloying treatment improved the oxidation resistance by about 100 times due to the distribution of chromium carbides and the increase in the chromium concentration. The results of the cyclic oxidation test revealed that the oxidation layer was very stable on the chromium carbide surface-alloyed region, while it scaled off very easily from the substrate region due to porous oxidation products. The microhardness was about 1100 H_v due to the dispersion and precipitation of chromium carbides in addition to the martensitic structure in the surface-alloyed region. The microhardness did not decrease much, despite heating at 960 °C for 100 h. The chromium carbide surface-alloying treatment improved the wear-resistance greatly, and the results of the wear-resistance test were very consistent with those of the microhardness test.     

Characterization of the oxides formed at 1000 °C on the AISI 316L stainless steel—Role of molybdenum

In the present work, in situ X-ray diffraction (XRD) was used to identify the oxides formed on the AISI 316L stainless steel (SS) during isothermal oxidation at 1000 °C, in air. The results were compared with those obtained on the AISI 304 SS in order to better explain the role of molybdenum on the oxidation process of the AISI 316L (containing 2% Mo). A good oxidation behavior is observed on the AISI 316L considering kinetics, structural characteristics and scale adherence. It is shown that molybdenum plays a similar protective role as the one observed with silicon. Moreover, it is possible to add a higher content of molybdenum in the stainless steel compared with silicon (usually 0.5 wt%). This higher protective element content hinders the external iron diffusion and leads to the lower growth rate and the better scale adherence. The oxide scale is then composed of Cr₂O₃ with a small amount of Mn_1.5Cr_1.5O₄ at the external interface. The better scale adherence appears to be also related to a pegging effect at the internal interface.     

Electrochemical and surface analytical study of the anodic oxidation of Fe–18% Cr steel in molten NaOH–Li2CO3 mixtures

The anodic oxidation behaviour of a Fe–18%Cr steel in molten NaOH–Li₂CO₃ (50:50 mol%) at 470 °C was characterised by in-situ and ex-situ methods. In-situ electrochemical impedance spectroscopic (EIS) measurements showed that the steady state ionic conductivity of the oxide decreased with potential in the passive range. This indicates an increasing layer thickness or a decrease in the amount of mobile current carriers in the oxide with increasing potential. The two high-frequency time constants in the impedance spectra are most probably related to the semiconductor properties of the corrosion film and interfacial charge transfer, whereas the low-frequency time constant corresponds to the ionic defect transport through the oxide. The surface composition of the corrosion layers was estimated by X-ray Photoelectron Spectroscopy (XPS). The results show that the films can be regarded as mixed iron oxides containing a certain amount of chromium, sodium and lithium.     

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.