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.     

Effect of silicon content in steel and oxidation temperature on scale growth and morphology

The effect of high silicon content in steel, 1.6 wt.%Si and 3.2 wt.%Si, and high oxidation temperatures (850–1200 °C) on scale growth rate and morphology were investigated. The steels were oxidized in a 15% humid air with short isothermal oxidation times (15 min). The scale growth rate of the non-alloyed steel follows a parabolic law with time; it is an iron diffusion controlled oxidation. The presence of silicon delays scale growth by forming a silica SiO₂ barrier layer at the scale/metal interface, this effect is more important for the steel containing 3.2 wt.%Si and induces a discontinuous scale. Silicon oxides are concentrated at the scale/metal interface; their morphology depends on the oxidation temperature. For temperatures lower than 950 °C, silica is formed. Between 950 °C and 1150 °C, fayalite (Fe₂SiO₄) grains appear in the wüstite matrix close to the scale/metal interface. For temperatures higher than 1177 °C, a fayalite–wüstite eutectic is formed; this molten phase favours iron diffusion leading to high scale growth. After cooling, a continuous fayalite layer with small wüstite grains is obtained at the scale/steel interface.     

Healing layer formation in Fe–Cr–Si ferritic steels

Abstract

The addition of small amounts of Si can dramatically improve the oxidation resistance of Fe and Fe–Cr steels. It is found that steels with Si contents above a certain critical value oxidise at a much slower rate and also become virtually immune to breakaway oxidation in high pressure CO₂, The critical Si content for this behaviour is found to vary with the Cr content (wt-%) of the steel, from about 2·5% for mild steel to 0·7% for 9%Cr steel to 0·3% for 11%Cr steel in the temperature range 575–650°C. The lower Si content required for Cr steels than for mild steels is advantageous, because it is small enough not to degrade the other metallurgical properties such as creep strength. The beneficial effect of Si is thought to arise from the formation of a near continuous ‘healing’ layer of amorphous SiO₂ at the oxide/metal interface which acts as a diffusion barrier to further transport of metal ions to the scale. The conditions required for the development of such layers are analysed using standard models of selective oxidation. The synergistic effect of Cr and Si is ascribed to the action of Cr as a secondary getter, in which it reduces the oxygen solubility in the metal and so reduces the Si content required to form a healing layer. Chromium also discourages the SiO₂ from converting to fayalite (Fe₂SiO₄) which is a much poorer diffusion barrier. The conventional theory of selective oxidation and secondary gettering is found to describe reasonably well the compositional limits of healing layer behaviour in these ferritic steels. However, the silica also seems to encourage the formation of a chromia based layer at the base of the overlying oxide and the oxidation rate during healing seems to be limited eventually by this chromia layer rather than the silica layer, as would be expected in the conventional model.

MST/1074     

Evolution of oxide scale on a Ni–Mo–Cr alloy at 900 °C

The cyclic oxidation behavior of a Ni–Mo–Cr alloy was studied in air at 900 °C for exposure periods of up to 1000 h. The morphology, microstructure and composition of the oxide scale was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Oxidation kinetics was determined by weight gain measurements. The results show that steady state oxidation was achieved within 1 h of exposure. During transient oxidation, the alloy grain boundaries intersecting the alloy surface became depleted in Ni and enriched in Mo and Cr. The scale initially formed at the surface was NiO which grew outwardly. However, a protective Cr₂O₃ layer developed, rapidly retarding the rate of oxidation. Formation of NiMoO₄ was also observed. The presence of Mo in the alloy facilitated the formation of a Cr₂O₃ layer at an early stage of oxidation. The alloy exhibited considerable oxide spalling during prolonged exposure.     

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.     

Anomalous steam oxidation behavior of a creep resistant martensitic 9 wt. % Cr steel

The efficiency of thermal power plants is currently limited by the long-term creep strength and the steam oxidation resistance of the commercially available ferritic/martensitic steel grades. Higher operating pressures and temperatures are essential to increase efficiency but impose important requirements on the materials, from both the mechanical and chemical stability perspective. It has been shown that in general, a Cr wt. % higher than 9 is required for acceptable oxidation rates at 650 °C, but on the other hand such high Cr content is detrimental to the creep strength. Surprisingly, preliminary studies of an experimental 9 wt. % Cr martensitic steel, exhibited very low oxidation rates under flowing steam at 650 °C for exposure times exceeding 20,000 h. A metallographic investigation at different time intervals has been carried out. Moreover, scanning transmission electron microscopy (STEM) analysis of a ground sample exposed to steam for 10,000 h at 650 °C revealed the formation of a complex tri-layered protective oxide comprising a top and bottom Fe and Cr rich spinel layer with a magnetite intermediate layer on top of a very fine grained zone.     

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.     

Fracture behavior of steam-grown oxide scales formed on 2–12%Cr steels

Abstract

A room temperature three point bent-beam test was conducted for preoxidized coupon specimens of ferritic 2%Cr (T22), 9%Cr (T91), and 12%Cr (T122) steels in order to examine the fracture/spalling behavior of steam-grown oxide scale. Test specimens were reacted with atmospheric 100% steam at 550–750°C for 1000-4800 h. Oxide scale thickness of the tested steels was 15–1150 μm for the 2Cr steel, 30–450 μm for the 9Cr steel, and 25–60 μm for the 12Cr steel. External tensile strain of up to 1.86% was loaded to each preoxidized specimen surface and the fracture/spalling behavior of steam-grown oxide scale was investigated visually and by microscopic observation. For the 2Cr steel, scale greater than 380 μm exfoliated without applying any external strain. For thin-scaled specimens of 15–20 μm thick, a tensile strain of 0.25% and more caused through-scale cracking perpendicular to the scale/metal interface. For these specimens, cracks along the scale/metal interface also resulted, and the oxide scale became separated from the base metal. For the 9Cr steel, scale exfoliation due to cooling was not prominent even for specimens with 450 μm thick scales. External tensile strain of 0.91% and 1.86% caused through-scale cracking to the oxide scale, but the scale/metal interface remained intact and scale exfoliation did not take place. This was the same for the 12Cr steel. Clearly, spalling resistance of the steam-grown oxide scale was significantly higher for the 9Cr and 12Cr steels than for the 2Cr steel.     

Characterization of Rust Layer Formed on Low alloy Steel Exposed in Marine Atmosphere

The iron rust phases formed on low alloy steels containing different quantities of Cr element have been characterized using EPMA,Raman spectroscopy,TEM,optical microscopy etc.The ion selective properties of synthesized rust films with the same phase constituent as the atmospheric corrosion products were investigated using self-made apparatus.The results showed that corrosion loss of steels exposed in marine atmosphere decreased rapidly as the Cr content of the steel was increased.Cr-containing steels were covered by a uniform compacted rust layer composed of fing particles with an average diameter of several nanometers.Inner rust layer of Cr-containing steel (2 mass fraction) was composed of α-CrxFe1-xOOH,with Cr content of about 5 mass fraction,Such rust layer showed cation selective property,and could depress the penetration of cl^- to contact substrate steel directly.     

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.     

Effects of carbon content of carbon steel on its dissolution into a molten aluminum alloy

Hot dip aluminizing of carbon steels with different carbon concentration ranging 0.2–1.1 wt.% was carried out in a molten Al–9.08 wt.% Si–0.98 wt.% Fe alloy at 660 °C. The steel specimens lost weight as a result of dissolution into the melt, and an intermetallic layer was formed on the surface of them. The specimens showed varied dissolution rates depending on carbon concentration. The specimen with the highest carbon content exhibited the slowest dissolution rate. The thickness of the intermetallic layer increased with dipping time following a parabolic relationship. The growth rate of the layer decreased with increase of the carbon content. A diffusion mechanism to control the dissolution of the carbon steel into the molten aluminum alloy was suggested, and the effect of carbon content on the dissolution of the steel substrate into the melt was discussed in connection with the proposed diffusion mechanism and microstructural observations.     

Field studies of steam oxidation behavior of austenitic heat-resistant steel 10Cr18Ni9Cu3NbN

The steam oxidation of austenitic steel 10Cr18Ni9Cu3NbN at about 605 °C after in service for 12,000 h and 34,696 h in a 660 MW USC power plant was investigated. Results show that a double layer structure consisting of a magnetite outer layer and Cr-rich inner layer was observed. After 12,000 h, shot blasting treatment improved the steam oxidation resistance of 10Cr18Ni9Cu3NbN steel compared with previous report. Scale oxide which was beneficial from shot blasting treatment spalled off within a short time before 34,696 h. After initial scale exfoliation, the newly formed scale oxide was more likely to be the oxide in 9–12%Cr steels, which was attributed to the low chromium under the initial oxide.     

氯盐环境中新型合金耐蚀钢筋Cr10Mo1的钝化行为

应用线性极化、电化学阻抗谱与电容电位法等方法对比研究了合金耐蚀钢筋Cr10Mo1和普通碳素钢筋在预含不同浓度(0mol·L~(-1)、0.1mol·L~(-1)、0.3mol·L~(-1)、0.6mol·L~(-1))氯盐的较低碱度(pH=12.5)模拟混凝土孔溶液中的钝化行为,利用XPS方法分析钢筋钝化膜组成结构,分析了氯盐作用下两种钢筋钝化行为变化存在差异的原因,揭示了合金耐蚀钢筋强易钝化机制。结果表明:在各氯盐浓度下,合金耐蚀钢筋均能良好致钝且钝化效果并无较大差距,而普通碳素钢筋随氯盐浓度增大钝化效果显著减弱,当氯盐超过一定浓度则几乎不钝化,甚至发生明显点蚀。Cr氧化物作为合金耐蚀钢筋钝化膜不同于普通碳素钢筋的关键成分,高浓度氯盐作用下仍可保持稳定并维持钝化膜层完整密实,从而使耐蚀钢筋呈现强易致钝特性。     

Influence of manganese and sulphur on overheating of some low-alloy steels

Abstract

An assessment has been made of the overheating behaviour of three low-alloy steels used in the electric power generating industries. The steels, 1Cr–Mo–V, 2·25Cr–1Mo, and 3·5Ni–Cr–Mo–V have been prepared as high-purity versions with low tramp element contents, sulphur contents of 0·001%, and manganese contents of 0·02 and 0·2%. For comparison, commercial steels produced by good practice and containing 0·006–0·011%S and 0·17–0·21%Mn have also been examined (all compositions in wt-%). The upper shelf energies of the high-purity versions of the steels in the fully heat treated condition indicate that these steels do not overheat after treatment at temperatures up to 1400°C, whereas the commercial versions do overheat and, in some cases, show a severe reduction in their impact energy levels. In some cases, the high-purity steels show an unusually low tendency to austenite grain growth after reheating at temperatures up to 1400°C. The results obtained show that new specifications for low–alloy steels could be developed which would give freedom from overheating during forging and greatly improved upper shelf energies after heat treatment.

MST/362     

Improvement of high-temperature oxidation resistance and strength in alumina-forming austenitic stainless steels

A new alumina-forming austenitic stainless steel with greatly improved high-temperature oxidation resistance and strength was developed via alloying 3.0 wt.% Al in the Fe-25Ni-18Cr based alloy. Continuous, stable and exclusive alumina scale was formed in either dry air or air with 10% water vapor mixed environment at 800 °C. The long-term high-temperature oxidation performance is appreciably enhanced which is associated with the high density of the B2-NiAl precipitation phase maintaining the Al₂O₃ surface layer. Moreover, when tested at 750 °C in dry air environment, the new steel showed high yield and fracture tensile strength of 310–335 and 480–500 MPa, respectively.     

The abrasive corrosion behavior of plasma-nitrided alloy steels in chloride environments

Both corrosion and abrasive corrosion behavior of plama-nitrided type 304 and 410 stainless steels and 4140 low alloy steel were investigated in 3% NaCl solution (pH = 6.8) by electrochemical corrosion measurements. Surface morphology and alloying elements after corrosion and abrasion corrosion tests were examined by scanning electron microscopy and energy dispersive analysis of X-rays. The results indicated that the plasma-nitrided SAE 4140 steel containing -(Fe,Cr)_{2 – 3}N and -(Fe,Cr)₄N surface nitrides which produce a thick and dense protective layer exhibited a significant decrease of corrosion currents by inhibition of the anodic dissolution of iron, whereas the plasma-nitrided type 304 and 410 stainless steels containing the segregation of chromium nitride CrN exhibited a extensive pitting corrosion by acceleration of the anodic dissolution of iron. It is concluded that the susceptibility to pitting is consistent with the degree of chromium segregation, and decreases as follows: 304 stainless steel > 410 stainless steel > 4140 steel. Also, the results of abrasive corrosion testing for the plasma-nitrided alloys are strongly related to the subtleties of the nitrided microstructures resulting in a pitting and spalling type of abrasive corrosion of type 304 and 410 stainless steels, and excellent abrasive corrosion resistance for SAE 4140 steel.     

Long term high temperature oxidation resistance of a nanoceria coated 316 SS under dry air conditions

In this work, nanoceria dip coated and uncoated 316 stainless steels were exposed to dry air at 1073–1273 K for times of up to 250 h. From this work, measured activation energies, Q = 256 kJ mol⁻¹ and Q = 240 kJ mol⁻¹ were found for coated and uncoated 316 SS, respectively. In the coated steel, the activation energy for oxidation cannot be attributed to a single mass transport mechanism. In addition, the scale morphologies, as well as the dominant oxide phases were determined by X-ray diffraction means. It was found that in the nanoceria dip coated steels, the scale was fine grained and highly adherent. Oxidation at increasing temperatures in the coated steels favored the development of the spinel ((Mn, Cr)₃O₄ structure and at 1273 K with the presence of Fe₂O₃ was severely hindered. In contrast, in the uncoated steel, a relatively thick scale, predominantly Fe₂O₃ developed and it exhibited severe damage through spallation and detachment from the steel substrate. Also, the resultant grain structure was rather coarse and it consisted of faceted grains with continuous nucleation/growth at grain ledges.     

Chemische Diffusion von Molybdän in hochlegierten austenitischen CrNiMo(N)-Stählen

Interdiffusion of Molybdenum in High-Alloy Austenitic CrNiMo(N)-Steels The Volume diffusion of Mo in austenitic CrNiMo(N)-steels was studied using the sandwich method. Accordingly the interdiffusion coefficients of Mo were found to be independent of its concentration for the given steel compositions with 12 to 19 wt.% Cr, 12 to 16wt.% Ni, 0 to 2 wt.% Mo and 0 to 0.27 wt.% N and within the temperature range from 1283 to 1523 K. The diffusivity of Mo is greater than that of Cr, Ni and Fe in such steels. The diffusion parameters of Mo are decreased by alloying addition of nearly 0.25 wt.% N in Steel; possibly as a result of lattice expansion or increasing vacancy concentration caused by N. Small variations in Cr and Ni initial contents of examined specimens showed negligible effect on the absolute values of interdiffusion coefficients of Mo.     

Structural characterization of fluidized bed nitrided steels

The nitriding behaviour of 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels was investigated nitrided in the fluidized bed processes. The nitriding processes were carried out at a temperature of 575 °C for treatment times of 6, 12 and 18 h. The nitrided samples were fully characterized using metallographic, microhardness and XRD techniques. Test results indicated that thickness of the compound layer on the steel surface changed in the range from 10 to 18 μm depending on steel type and treatment time, and γ′-Fe₄N and ε-Fe₂₋₃N formed in the compound layer. The hardness of the diffusion layer was over 1000 HV. Depending on the chemical composition of steels, the case depth ranged from 155 to 525 μm. Kinetics studies showed that the effective diffusion coefficients are 298×10⁻¹⁴, 525×10⁻¹⁴ and 68.8×10⁻¹⁴ m² s⁻¹, for 34CrAlNi7, 42CrMo4 and 40CrMnMoS86 steels, respectively. The fluidized bed process realizes the highest hardness of the case layer, 1095 HV, with fairly high growth rates, 27 μm/h.     

The corrosion of INCONEL alloy 740 in simulated environments for pulverized coal-fired boiler

The corrosion of a new nickel base superalloy, INCONEL alloy 740, has been studied at 550 and 700 °C on exposure to the synthetic coal ash/flue gas environments by means of XRD, SEM, and EDX. Low temperature hot corrosion of the new alloy occurred at two temperatures. The corrosion started to form the thin Cr₂O₃ scale on the alloy at 550 °C and developed as pitting attack resulted from sulfidation. The frontal attack at 700 °C consisted of two successive stages in which the corrosion mechanism started from the sulfidation and ended up in the fluxing of oxide. The compact and protective Cr₂O₃ scale formed and the internal sulfidation took place during the initial stage. The severe hot corrosion occurred due to the presence of the molten CoSO₄ during the propagation stage. The loose and porous outer layer and the compact inner layer consisted of spinels and oxides, respectively. The sulfides of Cr, Ti, and Nb formed on the front of oxide scale and in Cr-depletion zone. The rapid degradation of corrosion resistance of the alloy can be attributed to the dissolution of both cobalt and cobalt oxide on the surface. The alloy of 25% Cr exhibited better resistance to coal ash/flue gas corrosion as compared to the alloy of 23% Cr in the present case.