Degradation behavior of a commercial 13Cr ferritic stainless steel (SS405) exposed to an ambient air atmosphere for IT-SOFC interconnect applications

The objective of this work was to verify the feasibility of using low Cr ferritic stainless steels to improve IT-SOFC interconnect properties in terms of electrical conductivity and Cr evaporation resistance by taking advantage that low Cr ferritic stainless steels may form conductive and moisture-stable oxide layers of chromite spinels. In this context, surface degradation of a commercial 13Cr ferritic stainless steel (SS405) was studied at 700 °C in ambient air (containing ca. 3% H₂O by volume) for up to nearly 5000 h. The results showed a slow, nearly linear oxidation rate indicating that the oxide scale could not act as a fully protective barrier during a prolonged exposure at 700 °C. Oxidation-induced degradation was mainly due to the effects of Cr depletion in the subscale matrix region and internal oxidation attack. However, analysis of the oxide scale after long-term exposure revealed the presence of an interesting example of all-spinel structure consisting of outer layer of iron-substituted Mn–Cr spinel particles on the top of an inner and continuous Fe–Cr spinel layer. Since SS405 steel contains also Al and Si as minor additions, the distribution of these elements in the spinel oxide scale was studied and found to be rather different. Whereas Si was present exclusively in the inner oxide regions, Al migrated also to the outermost part of the oxide scale. Possible explanations for this unexpected Al surface enrichment are discussed.     

Oxidation Behaviour of Laser Remelted Cr-containing Steels

Laser surface remelting of steels with different Cr contents has been performed by using a CO_2 laser.The results of oxidation tests showed that the effects of laser remelting on the oxidation resistance ofCr-containing steels have close relation to the Cr contents and microstructures of the steels. The re-sistance to high temperature oxidation of 18-8 and HK40 at 1273 K can be obviously improved bylaser remelting, whereas laser remelting showed little effect on the oxidation resistance of Fe-6Crand Cr30 at 1173-1273 K.     

Effect of Nickel Pre-Plated on Microstructure and Oxidation Behavior of Aluminized AISI 316 Stainless Steel

Aluminizing of nickel pre-plated AISI 316 is prepared by a high-activity pack at 1050 °C. The effect of initial nickel layer with different thicknesses on microstructure and oxidation behavior of coating is investigated. After aluminization, the surface microstructure of stainless steel mainly consists of β-(Fe, Ni)Al as matrix with β-FeAl + α -(Fe, Cr) precipitates and an inter-diffusion layer with γ phase. Aluminized coating on specimens with 10 and 20 µm primary thicknesses of the nickel layer includes three layers. The outer zone is made up of β-NiAl thin layer and a β-(Fe, Ni)Al layer. As the nickel layer increases to 50 and 100 µm, aluminide layers consist of outer and inner zone with β-NiAl and Ni₃Al phase, respectively. Oxidation tests at 950 °C show that the oxidation resistance of aluminide coatings improves and oxidation kinetics follows a sub-parabolic rate law by increase in thickness of initial nickel layer.     

Long term steam oxidation of TP 347H FG in power plants

Abstract

The long term oxidation behaviour of TP 347H FG at ultra supercritical steam conditions was assessed by exposing the steel in test superheater loops in a Danish coal-fired power plant. The steamside oxide layer was investigated with scanning electron microscopy and energy dispersive X-ray diffraction in order to reveal the effect of oxidation time and temperature on the microstructure.

A double layered oxide formed during steam oxidation. The morphology of the inner Cr-containing layer was influenced by the oxidation temperature. At temperatures below 585°C, it consisted of regions of Fe–Ni–Cr spinel surrounded by Fe–Cr oxide. At higher temperatures almost the entire inner oxide layer was composed of Fe–Cr oxide. Possible mechanisms for the oxide growth are discussed and it is suggested that faster Cr transport within the alloy at higher temperatures explains the change in morphology.

The thickness of the inner oxide layer appeared to be independent of oxidation time and temperature for exposes less than 30 000 h; however, after 57 554 h, the thickness had increased significantly at the lowest temperatures.     

Oxidation of Alloying Elements during ESR of Stainless Steel

The oxidation of alloying elements duringthe ESR of stainless steel has been studied.The model previously developed by WEI and Mitchellfor the chemical reactions and mass transferprocesses during ESR was applied to the remeltingof the high Cr steel 1Cr18Ni9(Ti).The laboratorydata for the unsteady state A.C.ESR were analyzedand dealt with by the model.When the remeltingprocess reached a steady state,an oxidant(Fe_2O_3powder)or a deoxidant(Ca-Si powder or metallicCa)was added to the slag bath.The resultsshowed that this model is applicable to theremelting of stainless steel rather precisely,and it is expected that the model may offera reliable basis for the control of compositionduring practical ESR of high alloy steel.Also,the oxidation of Cr in the steel mustbe noticed when its content is high;but itis entirely possible to adjust the Cr contentof ingot within a considerable range,usinga special technique by means of the slag-metalreactions during the remelting.     

Surface tension and density data for Fe–Cr–Mo,Fe–Cr–Ni,and Fe–Cr–Mn–Ni steels

The temperature dependence of surface tension and density for Fe–Cr–Mo (AISI 4142), Fe–Cr–Ni (AISI 304), and Fe–Cr–Mn–Ni TRIP/TWIP high-manganese (16 wt% Cr, 7 wt% Mn, and 3–9 wt% Ni) liquid alloys are investigated using the conventional maximum bubble pressure (MBP) and sessile drop (SD) methods. In addition, the surface tension of liquid steel is measured using the oscillating droplet method on electromagnetically levitated (EML) liquid droplets at the German Aerospace Centre (DLR, Cologne). The data of thermophysical properties for Fe–Cr–Mn–Ni is of major importance for modeling of infiltration and gas atomization processes in the prototyping of a “TRIP-Matrix-Composite.” The surface tension of TRIP/TWIP steel increased with an increase in temperature in MBP as well as in SD measurement. The manganese evaporation with the conventional measurement methods is not significantly high within the experiments (?_Mn < 0.5 %). The temperature coefficient of surface tension (dσ/dT) is positive for liquid steel samples, which can be explained by the concentration of surface active elements. A slight influence of nickel on the surface tension of Fe–Cr–Mn–Ni steel was experimentally observed where σ is decreased with increasing nickel content. EML measurement of high-manganese steel, however, is limited to the undercooling state of the liquid steel. The manganese evaporation strongly increased in excess of the liquidus temperature in levitation measurements and a mass loss of droplet of 5 % was observed.     

Microstructural characterization of oxide scales formed on steels P91 and P92

Abstract

In this paper, an in-depth study of steam oxidation of two 9Cr ferritic-martensitic steels (P91 and P92), for advanced power plant, has been carried out. The steels investigated were exposed to a 100% flowing steam environment at 650°C for 1,000 – 3,000 hours. Metallographic analysis showed a multilayered scale was formed on both substrates consisting of an outer Fe-rich phase and inner Fe – Cr spinel, of varying oxide stoichiometry, as well as Cr –Mn-rich bands. The results of scale thickness measurements showed that the oxide formed on the P92 steel was thicker compared to that on the P91 steel. Crystallographic analysis showed that the P91 steel exhibited a martensite to ferrite transformation adjacent to scale – substrate interface which was not observed on the P92 steel. The outer oxide layer on both substrates exhibited a region of equiaxed grains followed by a region of columnar crystal growth. However, on the P91 steel the longer exposure time (3,000 hours) resulted in the outer layer having a region with coarsened equiaxed magnetite grains. The roles of alloying elements (Cr, Mo, W and Mn) were investigated to provide a better understanding of the oxide growth behaviour.     

Dislocation engineering and its effect on the oxidation behaviour

Abstract

Shot-peening of the surface of steel prior to oxidation can have a beneficial effect. Shot-peening can improve the oxidation resistance by introducing a localised plastic deformation in the near surface region resulting in an increase of the dislocation density. These dislocations can act in Cr-containing steels as fast diffusion paths for Cr promoting the formation of protective Cr-oxides. However, the effect of shot-peening has some limitations such as working temperature and microstructure. It has different effects on austenitic steels and ferritic martensitic steels. The effect of shot-peening can become futile due to recovery and recrystallisation of the alloy when subjected to higher temperatures for longer periods. In the present work, the main emphasis is put on the type of dislocation arrangement promoting the positive effect on the oxidation behaviour. Dislocation engineering was applied on shot-peened samples by means of some pre-annealing procedures resulting in a recovery process. During the process, dislocations were assumed to rearrange and form certain combinations nearer to the alloy grain boundaries. These arrays of dislocations can result in different oxidation behaviour. In the present study, 18 wt% Cr and 12 wt% Cr steels were shot-peened and vacuum annealed at 750°C for 1 h, 2 h, 3 h, 5 h and 15 h. Subsequently these steels were oxidised at 750°C. The mass gain in all cases is different for both steels, and in the case of both 12 wt% Cr and 18 wt% Cr steels the best oxidation resistance was achieved for the shot-peened +1 h pre-annealed sample.     

Corrosion Behaviour of Stainless Steel Alloys in Molten (Na,K)NO3 Eutectic Mixture

In the present article it is aimed to study the corrosion (oxidation) behaviour of two types of stainless steel alloys, ferritic (15.03% Cr) and austenitic (20.45% Cr, 8.37% Ni), in molten (Na,K)NO₃ mixture at different temperatures ranging from 400–600 °C. In this investigation the technique of potential-time and current-time under the open-circuit conditions is employed. The variation of potential with time at different temperatures was explained in terms of propagation and thickening of oxide film with a rate which depends on the temperature. It is assumed that the oxide film thickens according to a solid state mechanism under high field strength. The activation energy of the corrosion process was estimated using Arrhenius plots. The calculated values are found to be 21.36 KJ/mol for the ferritic stainless steel alloy and 23.3 KJ/mol for the austenitic one. The low value of activation energy signifies diffusion controlled process occurring in the oxide matrix. The corrosion products formed on the surface of stainless steel alloys are identified by X-ray diffraction analysis. Also the melt is chemically analysed to detect the amount of Fe, Cr and Ni in the melt after the completion of experiment.     

碳钢与不锈钢表面高浓度渗铬法

在氩气气氛下，应用固体渗铬填料法对碳钢、不锈钢作了表面高铬浓度的渗铬处理，探讨了渗铬温度、时间和渗铬组分对渗层表面铬含量、厚度，试样增重和物相组分影响，碳钢、不锈钢的表面渗铬浓度分别达８０－９４ｗｔ％和７０－８２ｗｔ％。     

Direct observation and analysis of the oxide scale formed on Y-treated austenitic stainless steels at high temperature

Abstract

We have studied the oxidation behavior of conventional austenitic stainless steels using same small amounts of Y as is added for deoxygenation and desulphurisaton in steel making.

The direct observation and analysis of the oxide scale formed on 19Cr–10Ni–l .5Si steels with and without small amounts of Y at high temperature have been carried out using several types of equipment. The following results were found: (1) Steel with 0.03Y showed good resistance to oxidation at l,000°C.

(2) Oxide scale was composed mainly of Cr oxide, and Si oxide was also detected at the oxide scale–metal interface and in the internal oxides. The Si oxide formed a network cell structure in the inner oxide scale with deeper internal penetrations. The steel with Y formed a uniform oxide scale in every oxide layer.

(3) Small amounts of Y and Si were detected at the grain boundaries of the inner oxide scale, but no Y was detected in the oxide grains.

The beneficial effect of Y addition was more notable in the Si containing austenitic stainless steels, as the existence of Y or Si prevents the diffusion of cations and anions through the oxide grain boundaries. As consequence, the steel treated with Y showed good resistance to oxidation.     

Thermal stability and oxidation behavior of Al-containing nanocrystalline powders produced by cryomilling

Al-containing nanostructured coatings provide excellent protection from high temperature corrosion. Aluminum oxide scales generally provide better oxidation resistance and yield lower oxidation rates than other oxide scale compositions. In this study, nanocrystalline 316L stainless steel containing 6 wt.% Al was synthesized using cryogenic milling (cryomilling). Complete alloying was obtained after 32 h of milling and the average grain size was found to be 7 nm. High temperature thermal stability and oxidation kinetics of the alloyed powders were examined. The powder demonstrated good grain growth stability at 500 °C, at which point, the powders had been heat treated for 120 h and the average grain size was found to be 11.4 nm. The oxidation kinetics of the powder were studied for 48 h at 500, 800, and 1,000 °C, respectively. For comparison, conventional 316LSS powder was also tested. Nanocrystalline 316LSS-6 wt.% Al showed lower weight gain than the conventional 316LSS powders. During the oxidation of nanocrystalline 316LSS-6 wt. % Al at 500 °C, protective aluminum oxide scale formed at the surface. At 800 °C and 1,000 °C, most of the nanocrystalline 316LSS-6 wt.% Al particles showed completed outer aluminum oxide scale. However, at 800 and 1,000 °C, some particles showed growth of chromium oxide scale underneath the aluminum oxide scale. In those samples, Al depletion was also observed due to a non-homogenous distribution of Al during cryomilling. The activation energy of the oxidation reaction was calculated and was found to be affected by the enhancement of the grain boundary diffusion in nanostructured particles.     

Mechanisms of chromia scale failure during the course of 15–18Cr ferritic stainless steel oxidation in water vapour

Abstract

Breakaway oxidation of 15–18 % Cr ferritic stainless steels occurring in water vapour is described in the temperature range 800–1000°C. The failure of the protective chromia scale leads to iron oxide(s) nodule formation with accelerated kinetics. Characterisation of the (Fe,Cr)₂O₃ initial oxide scale by Raman spectroscopy and photoelectrochemistry shows chemical evolution with oxidation time, with increasing Cr/Fe ratio before haematite suddenly appears at the steel-oxide interface. The mechanisms for such a phenomenon are discussed, first on a thermodynamic point of view, where it is shown that chromium (VI) volatilisation or chromia destabilisation by stresses are not operating. It is rather concluded that mechanical cracking or internal interface decohesion provide conditions for haematite stabilisation. From a kinetic point of view, rapid haematite growth in water vapour compared to chromia is thought to be the result of surface acidity difference of these two oxides.     

Initial oxidation of Fe–Cr alloys: in situ STM and ex situ SEM observation

Abstract

In order to understand the initial oxidation of Fe–Cr alloys a single crystal of Fe–15Cr (100) was oxidized at 440°C under controlled oxygen partial pressure in a UHV system and the surface morphology was observed using in situ STM (basic pressure 1×10^?10 mbar); in addition, polycrystalline Fe&15Cr was oxidized at 400°C in an IR-furnace in atmospheric air and the morphology was observed using ex situ SEM. The chemistry of the surface oxide layers was studied by XPS.

Preparation of the single crystal in the UHV system did not lead to segregation of Cr to the surface during heating. In situ STM investigation showed that oxidation of Fe–Cr commenced by nucleation of Cr oxide on the surface, due to selective oxidation of Cr. When the Cr at the surface and at the interface was completely consumed by nucleation of Cr oxide, Fe oxidized and covered the initial Cr oxide nuclei, resulting in an Fe oxide layer on the surface. Ex situ experiments showed that initial oxidation of the mechanically prepared polycrystalline alloy depended on the defect distribution in the surface. It started with formation of whisker-type Fe oxides along defects and proceeded with spherical-type nucleation and growth of Fe oxide. In both experiments, the final product on the surface was Fe₂O₃.     

Oxidation of Hastelloy-XR Alloy for Corrosion-Resistant Glass-Coating

The oxidation behavior of Hastelloy-XR alloy was investigated to obtain the optimum surface condition for corrosion-resistant glass-coatings. The surface morphology of oxide scales changed significantly with variation of temperatureand oxygen partial pressure (po2 ). The oxidation kinetics was mainly parabolic independent of oxidation conditions.The oxide scales were consisted of inner Cr2O3 and outer spinel layers. The phase component of spinel layers wereMn1.5Cr1.5O4 and (Mn,Ni)(Cr,Fe)2O4 for the oxygen partial pressures po2<10 kPa and po2>10 kPa, respectively.The optimum oxidation condition to obtain an oxide scale for well-adhered glass-coating to the substrate was 1248 Kand po2 =0.01 kPa for the oxidation time of 43 ks.     

HP40Nb钢热浸镀Al-Si高温氧化行为及组织研究

为提高高温抗氧化性能,对HP40Nb钢进行了热浸镀Al-10%(质量分数)Si,并进行不同温度扩散处理,研究了不同扩散处理试样在1000℃条件下的高温氧化行为,通过SEM,EDS和XRD分析了经不同扩散处理后的渗层在高温氧化过程中的组织结构变化.结果表明:经800℃/4h扩散处理,渗层由内层(NiAl+ Cr3 Si)...     

High temperature oxidation behavior of ferritic steel in supercritical water at 550–700°C

Oxidation tests were conducted on ferritic steel T22 exposed to deaerated supercritical water at 550–700°C and 25 MPa. Oxide films formed on T22 had a double-layered structure with the outer layer consisting of iron oxide and the inner layer consisting of spinel oxide. Pores formed on the surface of samples initially but healed at longer exposure time and higher temperature. Cracks occurred along the grain boundaries in the oxide scale at 600–700°C for 200 and 400 h. The oxidation kinetics obeyed a near-parabolic law in all cases. The data of activation energy of T22 indicated that the likely oxidation rate-controlling step may be the outward diffusion of iron along the magnetite bulk.     

The influence of chromium on early high temperature corrosion of ferritic alloys under SO2 atmosphere

Comprehensive insights into the early stages of corrosion mechanisms provide fundamental knowledge to further understand and model long time material behaviour. The present work studies the early stages of combined oxidation and sulphidation of ferritic model alloys for time scales up to 250 h at 650°C to observe the influence of chromium during the corrosion under SO₂. Model alloys were used to focus on the reaction of the intended elements: Fe, Cr, S, and O. Pure iron simultaneously forms magnetite and iron-sulphide in an early stage of corrosion, covered by a pure oxide layer after 100 h. Iron with 13 wt-% Cr shows hematite and mixed Fe–Cr-oxides first, before sulphides nucleate in the inner corrosion zone. With increasing ageing time a magnetite layer is observed below the hematite layer. Quantitative phase fractions of all corrosion products observed were determined from cross section images. Characterisation of the Fe13Cr corrosion scale by FIB revealed a highly porous structure in the inner corrosion zone where Cr-rich (Fe, Cr)-sulphides are present, and caused the scale to spall easily.     

High temperature oxidation of Ni–W coatings electroplated on steel

The high-temperature oxidation of Ni–16 at.% W coating electroplated on the steel substrate was studied at 700 and 800 °C in air. Before oxidation, the coating consisted of supersaturated, nanocrystalline Ni grains. During oxidation, oxygen diffused inward, Ni and the substrate elements such as Fe and Cr diffused outward. The outer NiO layer was not pure but had some dissolved ions of W⁶⁺ and Fe³⁺. Some Fe³⁺ ions were dissolved in the inner (NiO+NiWO₄) mixed layer, below which (W, Fe)-supersaturated, unoxidized Ni grains existed. Below these grains, tiny Ni–W–Fe precipitates, which were formed by the outward diffusion of Fe from the substrate, were surrounded by unoxidized (Fe-enriched, Cr-containing) Ni grains. Detailed oxidation mechanism of Ni–16 at.% W coating is proposed.     

Oxidation behavior of a new Fe–Ni–Cr-based alloy in pure steam at 750 °C

The isothermal oxidation of a new Fe–Ni–Cr-based alloy has been investigated in pure steam at 750 °C for exposure time up to 500 h using secondary electron microscope (SEM)/ X-ray energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Results showed that the alloy was oxidized approximately following a parabolic law with a parabolic rate constant k_p of 2.36 × 10^?13 g²/m⁴/s. As revealed by SEM/EDS and XRD results, a duplex-layered external oxide scale was formed, consisting of a thin outer layer of Ni(Fe, Al)₂O₄ and a thicker inner layer of (Cr, Mn)₂O₃. Underneath the external oxide scale, the internal oxidation of Ti to be TiO₂ occurred particularly along the grain boundaries of the matrix alloy. Internal oxide of Al₂O₃ was also observed but at a deeper depth. Based on the detailed compositional and microstructural characterization of the oxidized zone, the mechanism of the external and internal oxidation in steam is presented.