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.     

High temperature oxidation of FeCrAl‐alloys – influence of Al‐concentration on oxide layer characteristics

The superior high temperature oxidation resistance of FeCrAl alloys relies on the formation of a dense and continuous protective aluminium oxide layer on the alloy surface when exposed to high temperatures. Consequently, the aluminium content, i.e. the aluminium concentration at the alloy–oxide layer interface, must exceed a critical level in order to form a protective alumina layer. In the present study the oxidation behaviour of six different FeCrAl alloys with Al concentrations in the range of 1.2–5.0 wt% have been characterised after oxidation at 900 °C for 72 h with respect to oxide layer surface morphology, thickness and composition using scanning electron microscopy, energy dispersive X‐ray spectroscopy and Auger electron spectroscopy. The results show that a minimum of 3.2 wt% Al in the FeCrAl alloy is necessary for the formation of a continuous alumina layer. For Al concentrations in the range of 2.0–3.0 wt% a three‐layered oxide layer is formed, i.e. an oxide layer consisting of an inner alumina‐based layer, an intermediate chromia‐based layer and an outer iron oxide‐based layer. In contrast, the 1.2 wt% Al FeCrAl alloy is not able to form a protective oxide layer inhibiting extensive oxidation.     

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.     

Modification of the oxidation behavior of high-purity austenitic Fe-14Cr-14Ni by the addition of silicon

The oxidation behavior of Fe-14Cr-14Ni (wt.%) and of the same alloy with additions of 1 and 4% silicon was studied in air over the range of 900-1100° C. The presence of silicon completely changed the nature of the oxide scale formed during oxidation. The base alloy (no silicon) formed a thick outer scale of all three iron oxides and an internally oxidized zone of (Fe,Cr,Ni) spinels. The alloy containing 4% silicon formed an outer layer of Cr₂O₃ and an inner layer of either (or possibly both) SiO₂ and Fe₂SiO₄. The formation of the iron oxides was completely suppressed. The oxidation rate of the 4% silicon alloy was about 200 times less than that of the base alloy, whereas the 1% silicon alloy exhibited a rate intermediate to the other two alloys. The actual ratio of the oxidation rates may be less than 200 due to possible weight losses by the oxidation of Cr₂O₃ to the gaseous phase CrO₃. The lower oxidation rate of the 4% silicon alloy was attributed to the suppression of iron-oxide formation and the presence of Cr₂O₃, which is a much more protective scale.     

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.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

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.     

Characterisation of oxide films formed on Co–29Cr–6Mo alloy used in die-casting moulds for aluminium

Oxide films formed at 700 °C on Co–29Cr–6Mo alloy were characterised extensively to improve the corrosion resistance of the alloy to liquid Al, enabling its use in Al die-casting moulds. Film of duplex layer consisting of an outer CoO-rich layer and an inner Cr₂O₃-rich layer was observed in samples subjected to oxidation for 4 h. With an increase in duration of oxidation, CoO was gradually replaced by Cr₂O₃, resulting in a single-layered oxide film dominantly composed of Cr₂O₃. The oxide film evolved with duration of oxidation treatment indicating the possibility of optimising films for Al die-casting moulds.     

Oxidation behaviour of Ag-containing TiAl-based intermetallics

The effect of alloy composition on the oxidation behaviour of γ-TiAl alloys with silver additions was studied during exposure at 800 °C in air. For this purpose, a number of Ti–Al–Ag alloys with systematic variation of the chemical composition was prepared by levitation induction melting. Subsequently, the alloys were isothermally and/or discontinuously oxidised at 800 °C in air. The results showed that suitable silver additions can promote and stabilise formation of an alumina scale on γ-TiAl up to the maximum test times of 12,000 h. An oxidation map was derived which allows defining which type of surface oxide is formed on various TiAl–Ag alloy compositions. This result can be applied in further work for development of oxidation resistant coatings for γ-TiAl or/and Ti-based alloys.     

Scanning Auger electron spectroscopy study of the oxide film formed on dendritic and interdendritic regions of C containing Fe3Al intermetallic

The oxide films formed during early stage of oxidation at 800 °C on dendritic and interdendritic regions of the cast Fe–16Al–1C (wt.%) alloy were studied using scanning Auger electron spectroscopy. Microhardness measurement and elemental depth profiles by Auger spectroscopy reveal that the carbide, Fe₃AlC_0.69, is the major constituent of the interdendritic region, while dendrites are predominantly Fe₃Al phase. Between the two, the interdendritic region is found to be more prone to oxidation than the dendritic region, which was attributed to presence of carbides with low-Al content. In spite of the difference in oxide film thickness exhibited by both the phases, they consist of an inner aluminium oxide layer and an outer iron oxide layer.     

低氧分压下NiTi记忆合金氧化行为的研究

本文研究了NiTi形状记忆合金在H₂-H₂O气氛下400-700 °C的氧化行为。合金的氧化过程遵循立方规律,氧化激活能127.52 kJ/mol。氧化显著降低了试样表面的Ni含量。400 °C氧化的试样,其表面形貌与其他试样不同,并且氧化膜较薄,截面结构无法用SEM分析。500 °C、600 °C、700 °C氧化的试样,表面有两种形貌的氧化物,一种是颗粒状氧化物,另一种是晶须状氧化物。截面分析表明,氧化膜分为两层,上层由TiO₂构成,下层由Ni₃Ti构成,两层界面处有孔洞生成。     

Effect of alternating voltage treatment on the microstructure and corrosion resistance of stannate conversion coating on AZ91D alloy

A new stannate conversion coating (CC) on AZ91D alloy was synthesized by the application of alternative voltage (AV) treatment technique. By using AV technique, the formation process of CC can be controlled. SEM results indicated that a continuous and compact dual-layer CC was formed on alloy surface after AV treatment. TEM results revealed that the inner layer consists of magnesium–aluminum–stannum nano-crystals and amorphous, meanwhile, the outer layer is primarily a mixture of magnesium–stannum hydroxides and oxides with amorphous structure. The electrochemical experimental results revealed that AV treatment significantly improved the corrosion resistance of CC, which attributed to the distinguishing microstructure of AV-CC.     

Ni-Cr-W合金在1100 ℃的氧化膜演变

对新型Ni-Cr-W合金在1100 ℃下不同保温时间下的恒温氧化行为进行了研究。采用扫描电镜（SEM）以及能谱（EDS）对合金热暴露后的表面氧化膜形貌、元素含量以及合金基体的恶化情况进行了分析,表面氧化膜的相组成通过XRD进行确定。结果表明：在氧化初期（<3 h）,合金表面生成的单层氧化膜主要由Cr2O3组成,随着氧化时间的延长（>7 h）,在Cr2O3外逐渐形成了一层具有尖晶石结构的NiCr2O4。一旦外表面被均匀致密的尖晶石膜所覆盖,双层氧化膜NiCr2O4·Cr2O3便能有效的减慢合金基体被进一步氧化。合金亚表层的恶化形式包括晶界的内氧化、空洞以及无碳化物区的形成。合金中高的W含量并没有明显恶化合金的抗氧化性能。     

Surface characterization of oxides grown on the Ti–13Nb–13Zr alloy and their corrosion protection

Oxide films were formed on the biocompatible alloy Ti–13Nb–13Zr in a phosphate buffer at open-circuit potential (E_oc), potentiodynamically up to 8 V, or by micro-arc oxidation (MAO) at 300 V. Their electrochemical properties were assessed in a phosphate buffer saline solution (PBS). EIS and SEM results showed that the E_oc and potentiodynamically formed oxide films were compact and behave as a monolayer, while the MAO oxide was a bilayered film (compact inner and porous outer layers). Open-circuit potential and EIS resistance values indicated that the MAO oxide provides the best corrosion protection for the alloy in PBS.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature I: Mechanism of Void-Induced Duplex Oxide Formation

Under CO₂ exposure at an intermediate temperature, typically 550?°C, 9Cr–1Mo steel forms a duplex oxide scale made of an outer magnetite layer and an almost-as-thick inner Fe–Cr rich spinel oxide layer. It is proposed that the inner Fe–Cr spinel layer grows according to a mechanism involving void formation at the oxide/metal interface. The driving force for pore formation is the outward magnetite growth: iron vacancies are injected at the oxide/metal interface then condense into voids. The fresh metallic surface made available is then oxidized by CO₂, which diffuses fast through the scale. The physical aspects, the integrity and the nature of the scale are shown to be very dependent on the oxygen potential existing in the environment.     

Influences of ion migration and electric field on the layered anodic films on Al–Mg alloys

Anodizing of sputtering-deposited Al–Mg alloys containing 27 and 32 at.% magnesium in sodium hydroxide electrolyte is shown to develop two-layered anodic oxide films. The outer layer contains aluminium and magnesium species, and is enriched in the latter species relative to the alloy, particularly towards the film surface. The inner layer also contains the two alloy species but is depleted in magnesium, due to Mg²⁺ ions migrating to the outer layer faster than Al³⁺ ions. The ratio of the thickness of the outer layer to that of the film increases with increase of magnesium content of the alloy. The presence of aluminium species in the outer layer is attributed to the penetration of the outer layer by oxide of the inner layer with lower ionic resistance. This mechanism of film growth appears to be sustainable to alloy concentrations to 40 at.%Mg, when the inner layer may no longer form. Enrichment of alloying elements can accompany film growth on Al–Mg alloys, as shown by enrichment of tungsten to 2–3 × 10¹⁵ atoms cm⁻² in an Al–26 at.%Mg–1 at.%W alloy.     

The Oxidation of cobalt-tantalum base alloys containing carbon

The oxidation of cobalt-tantalum carbon alloys, containing 10 and 15 wt.% Ta and carbon in the range 0–1 wt%, was carried out in oxygen and air at atmospheric pressure at 900, 1000 and 1100°C. The alloys oxidised according to the parabolic rate law with activation energy of about 38 Kcal/mole. In general, the addition of tantalum decreases the oxidation rates, in comparison with cobalt and with the same mass of chromium added to cobalt. Again, the presence of carbon in the Co-Ta alloys decreases its oxidation rates in comparison with carbon-free alloys. The scales formed on Co-Ta and Co-Ta-C alloys consist mainly of an outer layer of cobalt oxide, CoO, and an inner porous layer of mixture of oxides: cobalt oxide; CoO, tantalum oxide; Ta₂O₅, and solid solution of these two oxides; CoTaO₄ at all temperatures in the range of 900°-1100°C. The binary Co ?10% Ta and Co ?15% Ta show an internal oxidation along the internal phase, increasing of alloy tantalum content increases the density of the internal phase. The presence of carbon in the ternary Co-Ta-C alloys has little effect and there is no apparent preferential penetration along the tantalum carbide network. In contrast to carbide present in Co-Cr-C alloys, where these carbides were preferentially attacked, the outer scale was disrupted, due to the formation of carbon gaseous oxides.     

First approach for thermodynamic modelling of the high temperature oxidation behaviour of ternary γ′-strengthened Co–Al–W superalloys

In the present work, thermodynamic modelling of the high temperature oxidation behaviour of a γ′-strengthened Co-base superalloy is presented. The ternary Co–9Al–9W alloy (values in at%) was isothermally oxidised for 500 h at 800 and 900 °C in air. Results reveal that the calculated oxide layer sequence (Thermo-Calc, TCNI6) is in good agreement with the formed oxide scales on the alloy surface. Furthermore, prediction of the influence of oxygen partial pressure on Al₂O₃ formation is presented. The modelling results indicate pathways for alloy development or possible pre-oxidation surface treatments for improved oxidation resistance of the material.     

Si对625合金激光熔覆层高温氧化特性的影响

利用循环氧化法,研究了不同Si含量（0%,1%,3%,质量分数）的625合金熔覆层在700、800、900 ℃下氧化144 h后的高温氧化行为。用XRD分析了氧化物相。通过SEM/EDS研究了氧化物表面和截面的形貌、元素组成和氧化膜的厚度。结果表明,不同温度下试样的氧化动力学都保持抛物线规律,随着温度的升高,氧化增重逐渐增加。通过观察,在900 ℃时,0% Si含量的625合金熔覆层出现了氧化膜大面积剥落的情况,3% Si含量的合金熔覆层氧化膜保持完整。在700 ℃时,随着Si含量增加,氧化膜表面的氧化颗粒尺寸减小且更加致密,同时促进了Cr₂O₃氧化物的生成。在700 ℃下,0 % Si含量的试样出现了大片的内氧化区域;1% Si含量的试样基体部分出现了2处条状的含Ni,Cr,Mo的氧化物相区;而3% Si含量的试样氧化后由于生成了富Si的内氧化层,这阻止了内氧化的发生。外层Cr₂O₃氧化膜和内层SiO₂的联合作用既阻止了O阴离子的渗入也抑制了Fe等金属离子的扩散,提高了合金熔覆层的抗氧化性。     

TC4钛合金高温氧化行为

研究了TC4钛合金在650、750、850℃的循环氧化和25~1000℃范围内的连续变温氧化的氧化行为。采用电子天平或综合热分析仪、XRD、SEM和EDS分析了合金的氧化动力学、氧化膜的物相、表面形貌、截面结构及元素分析。结果表明:650℃循环氧化和连续变温氧化动力学曲线符合抛物线规律、750℃循环氧化符合抛物线-直线混合规律、850℃循环氧化符合直线规律。氧化膜由薄而致密的Al_2O_3外层和厚而疏松的TiO_2内层组成。随氧化温度升高,氧化膜厚度增加,但出现裂纹或剥落。