Investigation on steam oxidation behaviour of TP347H FG Part 1: Exposure at 256 bar

The stainless steel TP347H FG is a candidate material for the final stage tubing of superheater and reheater sections of ultra supercritical boilers operated at steam temperatures up to 620°C in the mild corrosion environments of coal‐firing. A series of field tests has been conducted with the aforementioned steel in coal‐fired boilers and this paper focuses on the steam oxidation behaviour for specimens tested at various metal temperatures for exposure times of 7700, 23000 and 30000 hours as investigated by light optical and scanning electron microscopy. The oxide present on the specimens is a duplex oxide, where the outer layer consists of two sub‐layers, an iron oxide layer and an iron‐nickel oxide layer; the inner layer is chromium rich chromium‐iron‐nickel oxide. Microstructure examination showed that for all these samples the varying grain size of subsurface metal affected the oxide thickness, where the larger the metal grain size, the thicker the oxidation scale. This gave the appearance of uneven inner oxides with a varying pit thickness. Comparison of the pit thickness measurement and oxide composition reveals that the oxidation rate is fast during the initial oxidation stage, but the subsequent growth of oxide from further exposure is slower due to the formation of a healing layer consisting of chromium rich oxide near original alloy grain boundaries. At a temperature region above 600°C a thin oxide rich in chromium and manganese is sometimes formed. In addition precipitation of secondary carbides in the bulk metal also occurs at this temperature region.     

Investigation of the Evolution of the Oxide Scale Formed on 310 Stainless Steel Oxidized at 600 °C in Oxygen with 40% Water Vapour Using FIB and TEM

Detailed microstructure investigations were performed on oxide scales formed on 310 stainless steel exposed isothermally at 600 °C to O₂ with 40% water vapour for 1–336 h. FIB microscopy was used to study the evolution of the surface morphology and to prepare cross-section TEM thin foils of the oxide scales. The foils were investigated by analytical transmission electron microscopy. The results showed that a thin protective base oxide scale had formed after 1 h. Due to Cr loss from the oxide scale through water vapour induced Cr evaporation, local breakaway oxidation occurs, resulting in the formation of oxide nodules. The development of these nodules depends on whether a new Cr-rich healing layer is formed or not. A model for the evolution of the oxide scale is proposed based on the results regarding the composition and distribution of various phases in the oxide scale and subjacent steel.     

Microstructural characteristics of the oxide scale formed on 304 stainless steel in oxygenated high temperature water

The microstructural characteristics of oxide scale formed on type 304 stainless steel in oxygenated high temperature water have been investigated. From outer to inner layer, the oxide scale consists of faceted spinel particles, irregularly shaped hematite particles and a compact layer of nano-sized spinels. Some outmost spinels formed on top of other particles are depleted in Cr, while the hematite particles tightly embedded into the inner layer contain more Cr in the inner than in the outer part. The inner nano-sized oxide grow inwards directly from the bottom of outer particles. The related oxidation mechanism is discussed.     

441铁素体不锈钢在模拟汽车废气环境中的氧化行为

采用增重法研究了441铁素体不锈钢在模拟汽车废气环境中于900~1050 ℃范围内的氧化行为。运用XRD、SEM以及EDS技术分析了试样表面氧化膜的形貌和结构特征。结果表明,441铁素体不锈钢在汽车废气环境中的上限服役温度在900 ℃左右,该温度下形成的氧化膜为双层结构,外层的Mn-Cr尖晶石层和内部的Cr₂O₃层。当氧化温度升到950 ℃及以上时,氧化膜上出现瘤状氧化物,氧化物瘤沿初期Mn-Cr尖晶石层两侧分布,外部主要是铁的氧化物,内部为铁铬氧化物。氧化温度达到1050 ℃时,氧化物瘤内部会形成较多空隙。氧化物瘤的生长导致发生失稳氧化,并使氧化动力学从抛物线规律转变为直线规律。     

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.     

The Role of Silicon in the Reactive-Elements Effect on the Oxidation of Conventional Austenitic Stainless Steel

In this work we evaluate the influence of silicon on the high-temperature oxidation of austenitic stainless steels and propose a mechanism that explains the Reactive-Element Effect (REE) in terms of a synergistic action between the reactive element and the silica layer that forms in the innermost areas of the scale. To do this we have studied the oxidation at 900°C of austenitic commercial alloys (AISI-304, AISI-316 and AISI-310S) and a laboratory-designed high-silicon stainless steel (AISI-304). Lanthanum was selected as the reactive element which was surface deposited by means of ion interchange. Results obtained in this work allowed us to state that the reactive element would enhance the formation of a silica layer that shows diffusion through the scale. The reactive element also changes the expansion coefficient at the scale-alloy interface, increasing the adherence of the oxide layer to the metal.     

The influence of microstructure on the oxidation of duplex stainless steels in simulated propane combustion products at 1000 °C

A low nickel Type S32101 duplex stainless steel has been oxidised in simulated industrial reheating conditions. The surfaces have been studied using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Observations show that local breakaway regions (LBRs) form on the austenitic regions whereas thinner oxides are observed on the ferritic regions of the substrate. The reason proposed for these differences is the formation of a continuous oxide layer on the ferrite region and a discontinuous layer on the austenitic region during the early stages of oxidation. The chemical composition of these LBRs have been shown to be oxide islands of iron and manganese and oxide craters of chromium rich oxides. The more protective regions consist of chromium and manganese rich oxides. A silica layer formed below the oxide which may be attributable to a slight enrichment of silicon in the ferritic regions or due to faster rates of diffusion in ferrite.     

稀土Ce对310S奥氏体耐热不锈钢高温氧化行为的影响

采用热重分析法对不同稀土Ce含量的310S奥氏体耐热不锈钢高温氧化行为进行了系统研究,通过氧化增量曲线分析了相同温度下试验钢的氧化增量规律,并采用场发射电子探针(EPMA)表征氧化膜断面结构及元素分布,同时采用X射线衍射仪(XRD)分析氧化膜的物相组成。结果表明:在循环氧化初期,试验钢的高温氧化增量曲线遵循抛物线规律。试验钢的氧化膜由外层(Cr,Mn)₃O₄“尖晶石”型氧化物和内层Cr₂O₃氧化物组成。适量的稀土元素Ce能促进氧化物/基体界面处的应力释放,同时减少并延缓氧化膜与基体界面孔洞的形成,因而提高氧化膜的抗剥落性。     

成分和热处理对核级不锈钢堆焊材料氧化行为的影响

利用SEM,XRD和XPS等手段研究了二种核级不锈钢堆焊材料在350 ℃/22 MPa高温高压水中的氧化行为. 结果表明,随着腐蚀周期的增加,不锈钢堆焊材料表面氧化物颗粒数量逐渐增多,氧化物颗粒尺寸逐渐增大,氧化膜厚度逐渐增加;不锈钢堆焊材料表面氧化膜为外层富铁内层富铬的双层结构,氧化膜组成成分主要为Fe-Cr氧化物;Cr元素含量的增加会降低材料表面氧化物颗粒的数量和氧化膜厚度, 而焊后热处理(PWHT)增加了材料表面氧化物颗粒的数量和氧化膜厚度.     

Factors affecting initiation of pitting corrosion in super martensitic stainless steel weldments

Abstract

In the present investigation optical microscopy, in combination with sputtered neutral mass spectrometry (SNMS), has been used to examine the conditions for initiation of pitting corrosion in the heat affected zone (HAZ) of two super martensitic (SM) stainless steel weldments. It is shown that the corrosion resistance depends mainly on the nature of the surface oxide, as determined by the oxygen potential in the shielding gas and the HAZ temperature–time relationship, and less strongly on the underlying microstructure. In the absence of H₂S the initiation is associated with spalling of the iron enriched chromium oxide within a narrow region approximately 6 mm from the fusion line, where the contaminated oxide layer is thinnest. As H₂S is introduced, the region close to the fusion boundary becomes susceptible to localised corrosion because of the more extensive metal oxidation. Thus, the high temperature oxidation and iron oxide scale formation accompanying the welding operation appear to be the main factors affecting pitting corrosion initiation in SM stainless steel weldments.     

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。     

A surface study of the oxidation of type 304L stainless steel at 600 K in air

The oxidation of type 304L stainless steel at 600 K in air was studied using a number of surface-analytical techniques, including Auger electron spectroscopy (AES), scanning electron microscopy with energy-dispersive analysis of X-rays (SEM-EDAX), secondary ion mass spectrometry (SIMS), and X-ray photoelectron spectroscopy (XPS). Spectral analysis showed that a duplex oxide was formed, the outer layer of which formed rapidly and was essentially iron (III) oxide. Beneath this was a mixed iron-chromium oxide. SIMS sputter-profile curves showed region of relatively low iron concentration in the oxide film at the metal-oxide interface. This resulted from the rapid diffusion of iron within the oxide film. The oxide grain boundaries were examined using SEMEDAX. Higher chromium and silicon levels were detected in these regions compared with the corresponding grain centers. AES indicated the presence of silicon as SiO₂.     

Influence of Oxidation Temperature and Cr Content on the Adhesion and Microstructure of Scale on Low Cr Steels

Descalability of steels during hot rolling processes is essential in order to maintain the surface quality of steel products, because the remaining scale can be impressed into the steel, producing surface defects. Therefore, a quantitative evaluation of scale adhesion at high temperature is needed in order to find suitable heating conditions for hot rolling processes that promote the formation of scale that easily detach. Round bar shape specimens of low Cr steels were oxidized in a fuel combustion environment, and then compressed at high temperature to detach the scale. The area fraction of the remaining scale was evaluated by image analysis. Under a thick outer oxide scale, a thin continuous inner oxide layer, which consisted of Cr and Si oxides, strongly adhered to the substrate, resulting in poor descalability. On the other hand, if this inner scale changed into a discontinuous layer with a porous FeO layer beneath, the descalability improved. It was found that scales with greater descalability formed on steels containing less Cr and for oxidation at higher temperatures.     

Hot-Corrosion Studies of Separator Plates of AISI-310 Stainless Steels in Molten-Carbonate Fuel Cells

The corrosion behavior of a stainless steel 310S was investigated in amolten carbonate electrolyte at 650°C. The LiFeO₂ andLiCrO₂ oxides were formed in contact with the eutectic 62 mol%Li₂CO₃–38 mol% K₂CO₃mixture. Despite the low solubility of these oxides, the presence offissures in the LiFeO₂ layer does not provide good protection ofthe metallic material. The electrochemical-impedance spectroscopy (EIS)technique has shown low values of polarization resistance up to 80 hr and anincrease of the electrolyte resistance because of the dissolution of part ofthe scale after immersion. A mechanism for the corrosion of stainless steel310S in molten carbonates is proposed between 0 and 80 hr, taking intoaccount thermodynamic simulations, X-ray diffraction (XRD), scanningelectron microscopy (SEM) characterizations, and ac-impedance (EIS)results. This mechanism confirms a high corrosion rate and nonprotectivescale formation during the first hours of immersion. An external anddiscontinuous iron-rich oxide layer covered by a thin chromium-rich oxidelayer and an intermediate chromium-rich oxide and an internal spinel layerwere observed. Internal oxidation of the substrate also occurred to form(Fe, Cr)₂O₃ oxide. The dissolution in the melt of apart of the scale has been detected by EIS.     

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.     

内壁喷丸处理对TP304H耐热钢锅炉管抗水蒸汽氧化性能的影响

将经表面喷丸处理和未处理的TP304H钢管安装于锅炉再热器的高温段运行7474h后,对内壁氧化膜进行形貌观察、能谱分析和X射线衍射分析。结果表明,喷丸处理可大幅度提高TP304H钢的抗水蒸汽氧化性能,氧化膜的生长速率显著降低且氧化膜的粘附性提高。显微分析表明,氧化膜由双层结构转变为单一高Cr氧化膜层,外层Fe的氧化层消失。分析认为喷丸在管子内壁产生的剧烈形变引起晶粒细化和马氏体相变,在高温下促进了Cr向表面扩散及生成富Cr的氧化层。     

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.     

High Temperature Corrosion of Chromium–Manganese Steels in Sulfur Dioxide

This work was aimed at explaining the corrosion mechanism of commercial Cr–Mn steels at 1073, 1173 and 1273 K in the atmospheres containing oxygen and sulfur. Three steels were selected for the investigations, two single-phase austenitic steels (Cr17Mn17 and Cr13Mn19SiCa) and a two-phase austenitic-ferritic steel Cr15Mn19. On all studied steels triplex scales were formed. The inner very thin, fine-grained part of the scale contained manganese, chromium and iron sulfides and oxides, the intermediate layer was built mainly of the MnCr₂O₄ spinel while MnO was the predominant constituent of the outer scale layer. According to the gravimetric measurements, after an initial incubation period, the oxidation of steel follows a parabolic rate law. Thermodynamic and kinetic aspects of the formation of oxide-sulfide and oxide layers were discussed. Oxidation was accompanied by depletion of the subscale region of the metallic core in manganese, which is the austenite former. Consequently austenite transformed into ferrite.     

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.     

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 improvement of the oxidation behaviour of the 20%Cr–25%NiNb stabilized (20/25/Nb) austentic stainless steel, in carbon dioxide, by a silica coating, formed by a vapour reaction upon the pre-oxidized steel surface, has been examined at 825°C. The exposure periods extended to 5975 h duration. The silica coating effectiveness increased with its thickness over the range examined, 0.5–27 μm. Coatings ? 2 μm thick reduced the extent of oxidation by at least a factor of five by providing a barrier to the diffusion of cations outwards and oxidant inwards responsible for oxide growth on the 20/25/Nb steel. In addition, they completely inhibited oxide spallation because oxidation of the steel constituents occurred primarily within the silica coating whose adherence was maintained throughout the exposure. During the prolonged heat treatment of ? 2 μm thick silica coated steel, the iron and nickel constituents in the precoating oxide film were reduced by solid-solid reaction with manganese and probably also chromium from the underlying steel.