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

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

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.

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.

The oxidation behaviour of a 20% (by mass) Cr, 25% (by mass) Ni, niobiumstabilized steel in carbon dioxide at 1123 K and a pressure of approximately 0.1 MPa has been investigated by using charged-particle 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, a layer of Cr 2 O 3 inside this and then a silicon-rich layer at the oxide-metal interface. The scale grew primarily by cation transport but some oxygen diffusion also occurred.

The thermal stability of dislocation cellular structures in three additively manufactured (AM) austenitic stainless steels (SSs), 316L SS, 304L SS, and Al modified 316L SS (316L(Al)), were studied. Minor alloying elements, Mo and Al, were found affecting the stability of the cellular structures in AM austenitic SS, resulting in a stability ranking of AM 316L SS > AM 304L SS > AM 316L(Al) SS. As a result, their abilities towards recrystallization also differed. Owing to the high stacking fault energy (SFE) due to Al addition, AM 316L(Al) SS had the least stable subgrain cellular structure and exhibited the lowest recovery temperature. Although 316L SS possessed slightly higher SFE than 304L SS, the pinning effect due to Mo segregation at the cellular walls in AM 316L SS significantly enhanced its thermal stability. While the low-SFE AM 316L SS and AM 304L SS recovered their cellular structures via the equiaxed cell growth, the dislocation cellular walls in high-SFE AM 316L(Al) SS continuously vanished along a preferred direction. The fast recovery of cellular structures led to recrystallization retardation. The Hall–Petch model was found incapable of correlating cell size to strength because of the continuous weakening of cellular walls during heat treatment.

The oxidation of stainless steels 304 and 304L during hot rolling is studied in this paper. Results show the oxide scale thickness decreases significantly with an increase of reduction, and the oxide scales of both 304 and 304L stainless steels were found more deformable than the steel substrate. Surface roughness shows a complicated transfer during the hot rolling process due to the complexity of oxide scale characteristics. Also, surface roughness decreases with an increase of reduction. The friction coefficient increases with reduction in all cases, and the increase is more significant in the case of the 304 stainless steel than that of 304L stainless steel.

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.

The effect of small quantities of water vapour on the oxidation of low-alloy steels in C02–5% CO at 200 psi and at 500° and 550° has been studied. The steels have possible application in the advanced gas-cooled reactor. Almost all the steels exhibited higher oxidation rates at the higher moisture contents. At intermediate moisture contents, edge attack on the specimens was found to contribute up to 70% of the total weight gain. Nominally similar steels showed marked differences in oxidation resistance and cerium additions to steels were found to improve their oxidation resistance.

The oxidation of austenitic stainless steel (316Ti) under dry oxidation and under pressurized aqueous conditions, both at ∼300°C, was studied by applying XPS, depth profiling and x‐ray diffraction. After dry oxidation a relatively smooth surface was found, depleted of chromium, nickel and molybdenum and leaving behind nearly exclusively iron oxides. In the oxide layer chromium oxides were detected, followed by metallic nickel and molybdenum. The growth of the whole oxide layer follows a logarithmic growth law. Under aqueous conditions a rougher surface was found, with oxide layers being a factor of ≥10 thicker than under dry oxidation conditions. A greater variety of the oxides were found, e.g. chromium and nickel oxides and hydroxides are present in considerable amounts already at the sample surfaces. Again, molybdenum is first detected in the oxide layer. During the first 60 days of steel oxidation in pressurized water, the oxide layer formation follows a parabolic growth law. Copyright © 2002 John Wiley & Sons, Ltd.

Exposure and slow strain rate tensile tests were conducted in a simulated pressurized water reactor (PWR) primary water to investigate the oxidation resistance and stress corrosion cracking (SCC) susceptibility of 308L and 309L stainless steel (SS) cladding layers. A double-layer structure oxide layer grown on 308L SS and 309L SS contained the Cr-enriched nanocrystalline internal layer and the Fe-enriched spinel oxide in the external layer. Ni-enrichment at the matrix/oxide boundary was observed. The internal oxide film on 309L SS was thicker and had a lower Cr content than that on 308L SS. Preferential dissolution of inclusions led to pits on 308L SS and 309L SS surfaces during the exposure tests. More inclusions in 309L would decrease its SCC resistance due to the pits’ ability to act as the SCC initiation site. 308L SS had a lower susceptibility to SCC than 309L SS in PWR primary water. Lower ferrite content and higher strength/hardness reduced the oxidation and SCC resistance of 309L SS cladding. The effect of ferrite on oxidation and SCC of the SS claddings is discussed.

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.

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.

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...

Oxidation of stainless steel 304L in carbon dioxide

Early oxidation of Super304H stainless steel and its scales stability in supercritical water environments