A study of the oxidation mechanisms of some austenitic stainless steels in carbon dioxide at 1123k by means of charged-particle nuclear techniques. I

The oxidation behaviour in carbon dioxide at 1123 K and a pressure of approximately 0.lMPa of a 20% (by mass) Cr, 25% (by mass) Ni, Nb-stabilized steel strengthened with a dispersion of titanium nitride has been studied by using charged-particle nuclear techniques and conventional methods. The nuclear techniques were used to study the growth mechanism, thicknesses and surface composition of the oxide. The scales consisted of an outer spinel layer, a Cr 2 O 3 layer and then a silicon-rich layer at the oxide-metal interface. The growth mechanism of the oxide has been studied by using 18 O as a tracer; substantial solid-state diffusion of oxygen took place but indirect evidence indicates that some cation transport also occurred. The relative amounts of the two contributions are not known but the relative contribution from oxygen transport is greater than in the case of the steel without the titanium nitride dispersion. The oxide scales were poorly adherent and sometimes buckled. On two specimens there was evidence for gaseous transport through the scale. The interpretation of the data from which 18 O concentration profiles are obtained is discussed in detail.

The oxidation behaviour in carbon dioxide at 1123 K and a pressure of approximatey 0.1 MPa of a sample of 20% (by mass) Cr, 25% (by mass) Ni, Nb-stabilized steel (the standard steel) coated with ceria has been investigated by means of chargedparticle nuclear techniques and conventional methods. The nuclear techniques were used to study the growth mechanism, thickness and surface composition of the oxide. The scale consisted of an outer spinel layer and an inner layer of Cr2O3. It is not known if there was a silicon-rich layer at the oxide—metal interface because the oxide was adherent. The growth-mechanism of the oxide was studied with18O as a tracer and was found to grow primarily by oxygen diffusion, although there was indirect evidence for some cation diffusion. The growth-mechanism of the oxide is quite different from that on the standard steel without the ceria, and it is suggested that this is because ceria segregates to dislocations and grain-boundaries in the oxide. In the light of the results for the standard steel it is not clear if cerium reduces the cation diffusionandincreases the oxygen diffusion, or merely reduces the cation diffusion.

The influence of cerium, yttrium and lanthanum ion implantation on the oxidation behaviour of a 20Cr25NiNb stainless steel in carbon dioxide at 900–1050°C

Understanding how the so-called reactive elements improve the protection afforded to alloys by Cr203 and Al203 scales continues to be a major topic of high temperature oxidation research. Considerable progress has stemmed from studies on alloy surfaces modified by the controlled addition of these elements, such as yttrium and cerium, by ion implantation. The influence of yttrium ion implantation upon the oxidation behaviour of the technologically important 20%Cr/25%Ni/0.7%Mn/0.7%Si/Nb stabilised (20/25/Nb) austenitic stainless steel in carbon dioxide at 825°C has been the subject of extensive investigation. The implantation of 1017 yttrium ions cm-2 reduced the overall attack of the 20/25/Nb steel and maintained scale adhesion during extended exposure (6500h). These effects derived from the initial stage (<1h) of scale development.Further mechanistic understanding of the role of yttrium is critically dependent upon ascertaining both its location within the scale and its chemical state. This paper reports the preliminary results from an analytical electron microscopy examination of both back thinned(planar) and transverse sections of scales formed on yttrium implanted 20/25/Nb steel following 775 and 6500h oxidation in carbon dioxide at 825°C respectively.

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.

Electron and X-ray diffraction studies of oxide growth on a 20% Cr/25% Ni/Nb-stabilised austenitic steel in carbon dioxide at 750° and 850° are described. The results are interpreted in terms of a duplex structure comprising an inner rhombohedral Cr2O3-layer and an outer spinel oxide of the M2+(M3+)2O4 type. The influence of minor alloying elements, including rare earth metal additions, on the constitution and distribution of the surface oxide phases formed on the same basic steel is also discussed as a means of improving oxidation resistance.

Yttrium ion implantation significantly improves the oxidation behaviour of the 20Cr–25Ni–Nb stabilised stainless steel in carbon dioxide at 750–900°C by the incorporation of this reactive element within the scale. The exact location and chemical state of the yttrium ion were established by transmission and scanning transmission electron microscopy. The implanted yttrium atoms are incorporated as Y2O3 grains and as ionic segregants along the oxide grain boundaries within the outer spinel layer. of the scale next to the inner Cr2O3 layer. As a consequence of the yttrium segregation, the cation diffusion along the oxide grain boundaries, which is responsible for continuing scale growth, becomes energetically less favourable. Scale growth is inhibited to an extent according to the proportion of grain boundaries so affected. The improved spallation resistance could derive from the yttria grains and yttrium segregants inhibiting scale failure, the prelude to decohesion, by increasing the energy for thro...

IRON and mild steel oxidize in high temperature water, steam and carbon dioxide to form a duplex oxide scale. The two oxide layers are identical in chemical composition (Fe3O4) but differ in their physical structure; the layer nearest the metal surface is compact, porous and composed of micro-crystallites but the outer layer consists of symmetrical crystals (Fig. 1) which seem to have grown from the fluid phase. Surman and Castle1 have suggested that the rather high rate of oxidation of mild steel in carbon dioxide at temperatures between 350° C and 550° C may be accounted for if iron is transported across the oxide through the vapour phase, rather than by solid state diffusion. This model suggests that the inner oxide layer forms in competition with the removal of volatile iron from the metal surface which in turn diffuses across the inner layer and deposits to form the outer layer crystals. This mechanism would place the oxidation of steel by CO2 in the same category as the oxidation of steel by water2 and by steam1, for which similar mechanisms have been proposed. Our original suggestion was couched in terms of movement of the pentacarbonyl and therefore applied only to oxidation in high pressure atmospheres. Similar morphological features may, however, be seen on steel oxidized at atmospheric or reduced pressure and the pentacarbonyl is not stable enough to give chemical transport under these conditions. Also, because the compound is exothermic, chemical transport down the CO/CO2 gradient across the oxide would not give the positive experimental activation energy which has been measured for the corrosion process.

The oxidation and carburisation of a 20% Cr/25% Ni/Nb austenitic steel has been investigated between 600° and 850° in carbon dioxide, carbon monoxide and their mixtures at pressures within the range 0·04–760 cm.Under all conditions the oxide formed markedly inhibits the attack of the steel. In carbon dioxide, the major reaction is M + CO2 = MO + CO with the reaction 2M + CO2 = 2MO + C contributing less than 7% to the total weight gain. All the carbon deposition appears to take place within the first few hours of the oxidation. Detailed studies show that the deposited carbon diffuses into the steel.In carbon monoxide, the reaction is essentially M + CO = MO + C, the carbon passing through the oxide film into the steel. Carbon is also deposited in varying amounts on the surface of the oxide by the reaction 2CO = C + CO2. This carbon is present as a filamentary growth and diffuses only slowly into the steel.In carbon dioxide-carbon monoxide mixtures containing up to 75 vol.-% carbon monoxide, the ste...

The effect of surface implantation of yttrium and cerium upon the oxidation behaviour of stainless steels and aluminized coatings at high temperatures

The high temperature oxidation behaviour of a 20% Cr/25% Ni/Nb stainless steel, during isothermal exposures of up to 525 h duration, in carbon dioxide, at 900 and 950°C and on subsequent furnace cooling, has been studied by weight change measurements, in combination with thin layer activation. For this purpose a defined region of the steel was activated by a deuteron beam to produce the radioisotopes 51Cr, 52Mn, 54Mn, 56Co, 57Co and 58Co, with independent depth distributions to 70 μm. Measurements of the activity levels of the radioisotopes before, during and after oxidation, monitored scale spallation, thus enabling effects on the oxidation kinetics due to scale cracking to be distinguished. The involvement of each radioisotope was ascertained in the types of scales formed and in the spall produced.

The improvement by a CVD silica coating of the oxidation behaviour of a 20% Cr/25% Ni niobium stabilized stainless steel in carbon dioxide

The oxidation and spalling behaviour of 20% Cr/25% Ni/Nb-stabilised steel under thermal cycling conditions

An investigation has been carried out into the effects of thin titanium nitride coatings, produced by several PVD and CVD processes, on the sliding friction and wear behaviour of 321 stainless steel in carbon dioxide under reciprocating sliding conditions at temperatures to 500°C. At 20°C and 300°C there was a very considerable reduction in both the coefficient of friction and the wear rate for the coated surfaces compared with the uncoated steel during the early stages of like-on-like sliding. However, after some time, which depended on the type of coating and the temperature, the friction and wear rate of the coated surfaces increased significantly. In some cases, this was accompanied by gross coating failure, followed by a severe rate of wear, similar to that observed for the uncoated steel. The general mechanism of coating failure involved thinning, leading to cracking and fragmentation, which produced areas of exposed substrate. The wear resistance of the coated surfaces was improved at 500°C. Although thermal softening resulted in deformation of the substrate through the coating, oxidation of the exposed alloy prevented severe damage resulting from metal-metal contact. Possible explanations for this behaviour are discussed in relation to the recorded tribological data and morphological features of the wear scars.

304L stainless steel oxidation in carbon dioxide: An XPS study