Investigation of steam oxidation behaviour of TP347H FG. Part 2: Exposure at 91 bar

Tube specimens of TP347FG were exposed in a test superheater loop in a biomass plant in Denmark. The specimens were exposed to surface metal temperatures in the range of 455–568°C, steam pressure of 91 bar and exposure duration of 3500 and 8700 hours. The oxide thickness and morphology was investigated using light optical and scanning electron microscopy. The oxide present on the specimens is a duplex oxide with an inner chromium rich oxide and an outer iron rich oxide. The inner oxide consisted of a primary iron chromium nickel oxide in the original alloy grain and a chromium rich oxide, “healing layer”, at the grain boundaries. This gave the appearance of uneven inner oxide and it was clear that the varying subsurface grain size affected inner oxide thickness, especially after longer exposure times. Longer exposure times from 3500 to 8700 hours resulted in increased pit thickness. Comparison of pit thickness revealed that increase of temperature from 455 to 525°C increases the oxidation rate, however a further increase in temperature did not result in thicker inner oxide presumably due to the formation of a better healing layer at grain boundaries. These results are compared with the previous paper where the pressure and temperature was higher. A thicker inner oxide is observed at the lower temperatures and pressures compared with higher temperatures and pressures. Reasons for this behaviour are discussed.     

Microstructural investigation of the oxide formed on TP 347H FG during long‐term steam oxidation

The long‐term oxidation behaviour of TP347H FG in ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal‐fired power plant and characterising the oxide layer with reflective light and electron microscopy. Double layered oxide scales formed during steam oxidation. TEM investigations reveal that the inner oxide layer consists of particles of metallic Ni/Fe and Fe? Cr spinel in the interior of the former alloy grains and a compact layer of Fe? Cr spinel and Cr₂O₃ along the former alloy grain boundaries. The morphology suggests that the inner layer grows by internal oxidation of the interior of the alloy grains. The thickness of the inner oxide layer did not change significantly with oxidation time and temperature for exposure times up to 30 000 h. Faster Cr diffusion within the fine‐grained alloy at higher temperatures is held responsible for this observation. This hypothesis is supported by kinetic data. The oxide thickness at low and high temperatures after 58 000 h exposure was higher than expected.     

Surface nanocrystallization of 310s stainless steel and its effect on oxidation behavior

Two techniques, unbalanced magnetron sputter deposition and high-energy short-pulsed plasma discharge, have been used to produce a nanocrystalline surface on AISI 310S stainless steel specimens. The average grain size after surface modification was estimated as ~100 nm by using atomic force microscopy. Cyclic oxidation was performed at 1000°C with treated and untreated 310S stainless steel specimens. The oxide products formed on the specimens consisted of an outer spinel layer that was rich in chromium, iron, manganese, and nickel, and an inner chromium-rich layer. It was found that the concentrations of iron and manganese in the outer layer of treated specimens were higher, and adherence of the scale was better in the treated specimens. The observed oxidation behavior can be explained by the increase of the creep diffusion rate in the fine oxide scale formed on the nanocrystalline surfaces.     

Oxidation Behaviors of Different Grades of Ferritic Heat Resistant Steels in High-Temperature Steam and Flue Gas Environments

For steam tubes used in thermal power plant, the inner and outer walls were operated in high-temperature steam and flue gas environments respectively. In this study, structure, microstructure and chemical composition of oxide films on inner and outer walls of ex-service low Cr ferritic steel G102 tube and ex-service high Cr ferritic steel T91 tube were analyzed. The oxide film was composed of outer oxide layer, inner oxide layer and internal oxidation zone. The outer oxide layer on the original surface of tube had a porous structure containing Fe oxides formed by diffusion and oxidation of Fe. More specially, the outer oxide layer formed in flue gas environment would mix with coal combustion products during the growth process. The inner oxide layer below the original surface of tube was made of Fe-Cr spinel. The internal oxidation zone was believed to be the precursor stage of inner oxide layer. The formation of internal oxidation zone was due to O diffusing along grain boundaries to form oxide. There were Fe-Cr-Si oxides discontinuously distributed along grain boundaries in the internal oxidation zone of G102, while there were Fe-Cr oxides continuously distributed along grain boundaries in that of T91.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature III: Modelling and Simulation of Void-induced Duplex Oxide Growth

9Cr–1Mo steel forms in CO₂ at 550?°C a duplex oxide layer containing an outer magnetite scale and an inner Fe–Cr rich spinel scale. The inner spinel oxide layer is formed according to a void-induced oxidation mechanism. The kinetics of the total oxide growth is simulated from the proposed oxidation model. It is found that the rate limiting step of the total oxide growth is iron diffusion through high diffusion paths such as oxide grain boundaries in the inner Fe–Cr rich spinel oxide layer. In the proposed oxidation model, a network of nanometric high diffusion paths through the oxide layer allows the very fast supply of CO₂ inside pores formed at the oxide/metal interface. Its existence is demonstrated to be physically realistic and allows explaining several observed physical features evolving in the oxide layer with time.     

氧化时间及铬含量对Fe-Cr钢高温氧化行为的影响

为了测定不同氧化时间以及铬含量对高温条件下钢材表面氧化铁皮组织和厚度的影响,将Fe-5Cr钢与Fe-10Cr钢在1000 ℃空气条件下氧化60~180 min,采用增重法绘制其氧化动力学曲线,并利用光学显微镜、能谱仪和X射线衍射仪对氧化铁皮的断面形貌和物相进行研究。结果表明,两试验钢氧化初期为“气-固”反应,中后期为“气相-氧化铁皮-固相”反应。两试验钢氧化铁皮结构均由外氧化层、内氧化层和内氧化区域组成。当氧化时间为180 min时,Fe-10Cr钢检测到了内氧化物Cr₂O₃。空气中氧元素向内扩散与基体中铬元素发生反应生成内氧化物Cr₂O₃,再与氧化铁皮层中的FeO发生固相反应生成尖晶石结构产物FeCr₂O₄。随着氧化时间的增加,由于内氧化物Cr₂O₃不断受到内氧化层的包裹而转为外氧化,内外氧化的转变使得基体不断被腐蚀,氧化铁皮厚度不断增加。     

Kinetics of oxidation of ferrous alloys by super-heated steam

The kinetics of the oxidation of ferrous alloys in steam (10–60 kPa) at 450–550°C have been studied by measuring both the rate of hydrogen emission and the amount of metal oxidized. Excellent agreement has been found between the amount of metal oxidized calculated from both the total mass of hydrogen produced in the reaction and the thickness of the oxide layer formed; rate constants calculated from the rate of hydrogen emission, the mass of hydrogen produced as the reaction proceeds, and the oxide formed agree within experimental error. The rate of oxidation of a 9%Cr-1%Mo alloy at 501°C was found to be independent of the partial pressure of the steam. For this alloy, the activation energy agreed with literature values obtained at higher temperatures and pressures. The effect of the chromium and silicon content on the oxidation rates is compared. The rate constants are compared with theoretical calculations, assuming that the rate is determined by diffusion of iron in the magnetite lattice. For the 9%Cr-1%Mo alloy, the parabolic rate constant and activation energy are in excellent agreement with values calculated using Wagner's theory. The experimental rate constants are greater for the alloys containing smaller amounts of chromium; diffusion of iron along magnetite grain boundaries may be the dominant mechanism.     

Steam oxidation of X20CrMoV121: Comparison of laboratory exposures and in situ exposure in power plants

X20CrMoV121 is a 12% Cr martensitic steel which has been used in power plants in Europe for many decades. Specimens have been removed from superheater tubes to investigate long‐term exposure with respect to steam oxidation. These tubes have been exposed for various durations up to 135 000 h in power plants in Denmark at steam temperatures varying from 450–565 °C. This paper collates the data, compares oxide morphologies and assesses to what extent parabolic kinetics can be used to describe the oxidation rate. The steam oxidation behaviour has been investigated in the laboratory in an Ar‐46%H₂O mixture at 500, 600 and 700 °C for 336 h. It was observed that the morphology of the oxide layers was strongly influenced by temperature, and some of the same morphologies are also observed for power plant specimens. However, the temperatures at which they occur were different for plant and laboratory specimens, e.g. the presence of Cr rich bands within the oxide occurred at a lower temperature in the plant than in the laboratory. In addition there is a greater variety of oxide morphologies for long‐term plant specimens compared to laboratory specimens.     

An investigation of thin oxide films thermally grown in situ on Fe24Cr and Fe24Cr11Mo by auger electron spectroscopy and X-ray photoelectron spectroscopy

AES depth profiling and XPS have been used for the characterization of thin oxide layers thermally grown in situ in the UHV-analysis chamber on pure iron, chromium and the alloys Fe24Cr and Fe24Cr11Mo at a temperature of 384°C. The apparent oxide film thickness and the film composition were monitored as a function of oxygen exposure. The oxidation rate of the Fe24Cr alloy was found to lie in between that of pure iron and chromium. The films formed have a duplex structure, the outer part being iron oxide, the inner part mostly chromium oxide. Alloying with molybdenum decreases the rate of oxidation by a mechanism involving the formation of a barrier layer rich in molybdenum at the oxide-metal interface. No molybdenum is found in the outer part of the oxide film.     

喷丸对马氏体耐热钢高温蒸汽氧化行为的影响

利用高温蒸汽氧化试验装置对比研究P92、G115钢以及喷丸的G115钢的高温蒸汽氧化行为,结果表明,这3个试样的650℃蒸汽氧化动力学曲线符合ΔW=ktn规律.相同蒸汽氧化时间下G115钢的氧化质量增加远小于P92钢,主要是因为G115钢中富Cr层的形成和富Cu相在氧化层界面的析出.经喷丸处理的G115钢氧化质量增加小...     

The initiation of selective oxidation of a ferritic stainless steel at low temperatures and oxygen pressures

Oxidation of a ferritic stainless steel of type Fe-18Cr-2Mo has been performed in the temperature range 285–495°C and oxygen partial pressure range 10^?9-10^?8 torr. The chemical composition of thin oxide layers formed has been analysed by means of Auger Electron Spectroscopy and interpreted in terms of available chromium, iron and oxygen at the solid/gas interface. The selective oxidation of chromium is considered by different probabilities for the oxidation of available chromium and iron respectively. At oxide thicknesses below 100 Å the supplies of chromium and iron are ruled by diffusion in the steel matrix. The results have been used to predict the chemical compositions of two subsequently growing oxide layers provided the thickness of the first oxide layer is below 50 Å.     

Mechanism of electrolytic pickling of stainless steels in a neutral sodium sulphate solution

Oxide scale that mainly consists of spinel‐type metal oxides is formed on stainless steels when they are heated up during the manufacturing cycle. The removal of the oxide scale and chromium depleted subscale by pickling is one of the most important processes during the production. Electrolytic pickling in neutral sodium sulphate is widely used and provides relatively fast removal of the oxide scale. Despite its vast use, the essence of the electrochemical mechanism of electrolytic pickling is not clear. Stainless steel EN 1.4301 and EN 1.4404 samples were annealed in a laboratory furnace to produce an oxide scale similar to process conditions. The oxide scale was protective at potentials equal to the passive region but dissolved at anodic potentials above 1050 mV vs. SCE. The dissolution was found to proceed by electrochemical reactions of the scale. Three different stages were discernible in the dissolution process. At the beginning chromium and manganese of the outer layer were preferentially dissolved. When the chromium content in the outer layer decreased, the scale was enriched of iron and dissolution was hindered causing the electrode potential to increase. At the third stage the potential first decreased and then steady state was obtained. The oxide thickness was greatly reduced and the chromium content was lowered below the base metal level. At the steady state the remaining oxide was rich in iron and silicon. Silicon was not dissolved by electrochemical reactions and remained at the surface after prolonged polarisation.     

An investigation of the high-temperature corrosion (burning) of an automobile exhaust valve

An automobile exhaust valve fabricated from 21-4N austenitic steel and burnt out during normal operation has been examined using conventional metallographic techniques and electron probe microanalysis. In the burnt-out region a thick porous outer oxide has developed, containing oxides of chromium, manganese and nickel; but there is no sign of a healing Cr₂O₃ layer forming. In contrast, regions remote from the burning form a thin dense, adherent oxide apparently comprising three layers: an inner chromium-rich oxide, an intermediate layer containing chromium and manganese, and an outer layer containing iron. The metal beneath the burnt region is depleted in chromium and manganese, but is enriched in nickel. A thin layer contains internal sulphides; some of the larger particles appear to be essentially FeS containing significant amounts of chromium and it seems likely that many of the finer particles are chromium and manganese sulphides. The porous layer contains phosphorus, sulphur and lead: the lead is always associated with phosphorus, and probably with sulphur, presumably as lead phosphate or a mixture of this with lead sulphate. However, much of the sulphur in the scale is not associated with lead: it seems probable that this is sodium sulphate, the sodium (and much of the sulphur) probably entering with the intake air.There is clear metallurgical evidence of a considerable temperature rise associated with the burning out of the valve.It is concluded that the reaction is similar to the sodium sulphate induced hot corrosion encountered in boilers and gas turbines operating in marine environments; the presence of lead oxide, and the cyclic variation of the oxygen potential of the atmosphere can be expected to accelerate this form of attack. There is no evidence of a carburization/oxidation attack.     

The effect of niobium on the oxidation behaviour of fully Austenitic Fe-15% Cr-15%Ni stainless steels

The influence of niobium content (0.5–2.0 wt.%) on the oxidation behaviour of Fe-15% Cr-15% Ni austenitic stainless steels has been studied by cyclic oxidation tests in air at different temperatures in the range 800–1200°C. The isothermal oxidation rate, the scaling index and the cumulative oxide loss decreased with increasing niobium content. X-ray diffraction and fluorescence analysis of detached oxide scales revealed that the niobium in the alloy matrix facilitated the formation of chromium rich (Fe, Cr)₂O₃ and niobium containing mixed oxide near the metal/metal oxide boundary, which decreased further oxidation. SEM/EDX investigations revealed that the external oxide changed from being predominantly iron rich iron-chromium oxide to being chromium rich iron-chromium oxide.     

The Influence of Oxidizing Medium on Structure,Mechanical Properties,and Breakage Pattern of Oxide Films of Zr—1% Nb Alloy under Conditions Simulating Oxidizing Media of WWER and PWR Reactors

This article investigates into the structure, phase composition, mechanical properties, and breakage pattern of oxide films formed on the surface of Zr—1% Nb alloy after corrosion tests in autoclave in water, steam, and water with lithium. Oxidation in water with lithium promotes formation of oxide films with a thickness of more than 100 μm, and, after oxidation in water or steam, the thickness is 5–15 μm. Upon oxidation in water, oxide films are generated with laminar structure: in the substrate, the grains are extended with the thickness of ~80 nm, near film surface the grains are equiaxial with the diameter of ~30 nm. After holding of the specimens in steam the oxide films are comprised mainly extended grains with the thickness of ~95 nm with a minor amount of equiaxial grains, 10%, with a diameter of ~30 nm, after oxidation in water with lithium the mixture of equiaxial and weakly extended nanograins is observed in the film structure. Upon loading of specimens with the structure of equiaxial and weakly extended grains the oxide films are broken due to generation of transversal cracks propagating to basic metal. The oxide films with laminar structure oxidized in water are broken due to exfoliation along the interface between the layers of extended and equiaxial grains. Breakage of films with the structure of equiaxial and weakly extended grains starts at the stage of elastic deformation at a stress of 300 MPa. The highest breakage stresses of 1150 MPa are characteristic for alloy specimens oxidized in steam. Breakage of films with laminar structure after oxidation in water occurs at average stresses of 798 MPa. The cohesive/adhesive strengths of films oxidized in water and steam are nearly the same. In thick alloys, oxide films oxidized in water with lithium, substrate does not open even at loading of 100 N. According to data of Raman spectroscopy, oxide film is mainly comprised of monoclinic phase of zirconium oxide; however, after oxidation in steam at the metal—film interface, a barrier layer of tetragonal phase of zirconium dioxide has been detected that prevents accelerated oxidation.     

Activation of metal oxidation over the zone of electrodiffusion

It is shown that an oxide layer saturated by chromium oxides is formed on the surface of chromium steel at a higher rate under electrocontact (10⁴–10⁵ A/cm²) vacuum dc annealing (10^–2 Torr, 300°C) than under furnace heating. Such activation of oxidation is due to the formation of an electrodiffusion zone in the surface steel layer. At further stages, grain boundaries emerge to the metal surface that act as oxidant transportation channels from the surrounding medium into the conductor bulk, which results in accelerated oxide formation in the bulk of the surface metal layer. Apart from the uniform oxide layer, individual hematite nanoflakes and nanoleaves with the thickness of 50–40 nm and average diameter of 450 nm are formed on the positive electrode and grow vertically on the steel surface. The average surface density of nanoparticles is 10⁸ 1/cm². Such activation of metal oxidation over the zone of electrodiffusion can provide pronounced properties for accelerated formation of protective surface layers, in addition to its intrinsic functional (sensor, catalytic, semiconductor, adsorption) properties.     

The high temperature oxidation of 11% chromium steel: Part I – Influence of pH2O

The oxidation of 11% Cr steel (X20 11Cr1MoV) in the presence of dry O₂ and O₂ + 10 and 40% H₂O was investigated at 600°C. The exposure time was between 1 and 672 hours. The oxidized samples were investigated by a number of surface analytical techniques including GI‐XRD, SEM/EDX, GDOES and Auger spectroscopy. X20 steel (11Cr1MoV) forms a protective chromium rich α‐(Cr,Fe)₂O₃ oxide in dry O₂ at 600°C. In mixtures of oxygen and 10 or 40% H₂O, at the same temperature, the material is affected by chromium vaporization because of the formation of CrO₂(OH)₂(g). The loss of chromium tends to deplete the oxide in chromium. The formation of a more iron‐rich oxide may result in a loss of the protective properties of the oxide scale. The loss of chromium and the tendency to destabilize the protective oxide increases with the concentration of water vapour. The material suffers breakaway corrosion after 336 hours in an O₂/H₂O (60/40) mixture while the rate of oxidation is only marginally increased in the presence of 10% H₂O. The thick oxide formed in O₂/H₂O (60/40) environment features an inner layer consisting of FeCr spinel and an outer layer which is almost pure hematite.     

Influence of water vapour on high temperature oxidation of steels used in petroleum refinery heaters

The oxidation behaviours of three different steels used in the construction of petroleum refinery heaters were investigated by thermogravimetric analysis (TGA) technique. C‐5, P‐11 and P‐22 steel samples were tested in two different environments: air and CO₂ + 2H₂O + 7.52N₂, a gas composition which simulates the combustion products of natural gas, at 450 and 500 °C. P‐22 steel had the best oxidation resistance at both temperatures in air. In CO₂ + 2H₂O + 7.52N₂ environment, the oxidations of all the steels were accelerated and C‐5 exhibited better oxidation resistance than P‐22 and P‐11. Analyses of oxidation products by optical microscopy, SEM‐EDX and XRD were carried out to correlate TGA results to oxide composition and morphology. The lower oxidation rate of P‐22 in air was explained with reference to the formation of a protective Cr‐containing oxide layer between the steel and the iron oxide scale. The higher oxidation rates of chromium containing steels in CO₂ + 2H₂O + 7.52N₂ environment were attributed to the depletion of protective Cr‐containing oxide scale, which was deduced from the lower Cr content of this layer than that formed in air oxidation, as a result of probably faster oxidation of Cr even inside the steel. Therefore, the oxidation mechanisms of Fe? Cr alloys with intermediate Cr contents at higher temperatures could also be valid for steels with low chromium contents such as P‐22 (2.25%) even at 450 and 500 °C.     

Isothermal oxidation behavior of FeCo–2V intermetallic alloy

Isothermal oxidation behavior and the nature of oxide layer formed during oxidation of FeCo–2V alloy were characterized in the temperature range of 500–600 °C. Oxidation kinetics of the alloy follows a parabolic rate law. SEM and XRD studies indicate the formation of an iron rich outer oxide layer and an inner solute rich layer containing cobalt and vanadium rich oxides. The oxidation mechanism of the FeCo–2V alloy is similar to that of low alloy steels. During the initial stages, preferential oxidation of iron and cobalt occurs at the alloy surface and leads to the formation of a solute rich inner layer. Continued oxidation occurs through oxidation of iron and cobalt at the outer layer and internal oxidation of inner layer. The iron rich oxide layer formed at the surface on oxidation of FeCo alloy is semi-conducting in nature and may not provide the necessary insulating barrier required at the surface to minimize eddy current losses during A.C. applications.     

Effect of Cerium on High-Temperature Oxidation Resistance of 00Cr17NbTi Ferritic Stainless Steel

The influence of cerium addition on the isothermal oxidation behavior of 00Cr17 NbTi ferritic stainless steel was studied at temperature up to 1,000 °C for 100 h in air. The results show that cerium additions can reduce the grain size of this ferritic stainless steel, improve the diffusion of chromium and decrease the critical concentration of chromium to form protective Cr2O3 layer. With the increasing of cerium addition, the oxide particles become smaller and this can increase the rupture strength and spalling resistance of oxide layers. The transport mechanism through the oxide layer is varied from metal transport outward from steel to principally oxygen transport inward with the increase of cerium content,which leads to the lower oxidation rate and the better scale adherence of 00Cr17 NbTi ferritic stainless steel.