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.     

Synthesis of austenitic stainless steel powder alloys by mechanical alloying

Fe–18Cr–xNi (x = 8, 12, 13, 15, and 20 wt%) blended elemental powders were subjected to mechanical alloying in a high-energy SPEX shaker mill. The milled powders were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy and transmission electron microscopy techniques. It was shown that the sequence of phase formation in the Fe–18Cr–8Ni, Fe–18Cr–12Ni and Fe–18Cr–13Ni compositions was ferrite in the early stages of milling and then formation of austenite, which eventually transformed to stress-induced martensite on continued milling. The time for the formation of the austenite phase was shorter for the 12Ni and 13Ni powder blends than for the 8Ni powder. However, in the Fe–18Cr–15Ni and Fe–18Cr–20Ni compositions, the initial phase to form was ferrite and then a fully austenitic structure had formed on milling the powder for 10 h. No martensitic transformation occurred in this case on continued milling. The phase formation and microstructural features were confirmed by X-ray diffraction and transmission electron microscopy and diffraction techniques. A new metastable phase diagram was proposed outlining the stability of the austenite phase in ternary Fe–Cr–Ni alloys.     

TEM investigations of the oxide layers formed on a 316L alloy in simulated PWR environment

Atomic scale observations of the oxide formed on stainless steels, under simulated nuclear reactor conditions, are performed to estimate the oxide layer contribution on stress corrosion cracking (SCC) mechanisms. A duplex oxide composed of a chromium enriched inner layer (Fe_1.5Cr_1.5O₄) and an outer layer composed of magnetite crystallites (Fe₃O₄) is found. The oxide layer structure evolves from amorphous, for oxidation times of 1 min, to nano-crystalline at 2 min and mono-crystalline after 5 h. IFFT images, calculated from Cs-corrected HRTEM images recorded on grains oriented in the 〈111〉 direction, highlight a double network of dislocations with ½ 〈10-1〉 and ½ 〈?110〉 Burgers vectors. This network leads to the decrease in non-relaxed deformation and favors an epitaxial growth between steel and oxide. Both crystal structure transformations and epitaxial relations between metal and oxide have provided relevant information which contributed to progress on SCC modeling.     

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.     

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.     

9Cr低活化马氏体钢在超临界水中的腐蚀行为

利用扫描电子显微镜(SEM)、电子能谱(EDS)以及X射线衍射(XRD)技术研究了9Cr低活化马氏体钢在650℃/25MPa超临界水中的腐蚀行为。结果表明,9Cr低活化马氏体钢腐蚀产物的晶粒随腐蚀时间的延长而长大,晶粒尺寸从200h的5.7μm长大到1000h的10.1μm。表面形成的氧化膜为双层结构,外层为Fe3O4,内层由Fe3O4和FeCr2O4共同组成。由氧化增重结果获得了9Cr低活化马氏体钢在超临界水中腐蚀的氧化动力学表达式,同时其腐蚀机理表现为吸氧腐蚀。     

Oxidation of Fe–Si alloys in CO2–H2O atmospheres

Abstract

Iron and model alloys containing 1, 2, and 3wt% Si were reacted with dry and wet CO₂ gases at 800°C. All oxidised in dry CO₂ according to approximately linear kinetics. Additions of H₂O accelerated the reaction until steady-state parabolic kinetics were achieved. However, the effect of H₂O was small in the steady-state reaction stage of Fe – 3Si. Alloy reaction products were a duplex scale consisting of an outer FeO+Fe₃O₄ layer and an inner FeO+Fe₂SiO₄ layer, plus an internal oxidation zone, in all gases. In Fe – 1Si, amorphous SiO₂ precipitates in the internal oxidation zone grew with rod-like morphologies in all gases. However, internal amorphous SiO₂ precipitates grown in Fe – 2Si and Fe – 3Si formed network patterns. Internal penetration rates were initially rapid in Fe – 1Si, but slowed considerably at longer times. In Fe – 3Si, the internal oxidation zone grew wider in the first 20 h of reaction, and then remained constant in dry gas. In the wet gases this zone subsequently diminished, and disappeared after 50 h reaction.     

Evaluation of temperature effect on the corrosion process of 304 stainless steel in high temperature water with electrochemical noise

The effect of temperature on corrosion process of 304 stainless steel (SS) in high temperature water was investigated by electrochemical noise (EN), scanning electron microscope (SEM), Raman spectrum and X-ray photoelectron spectroscopy (XPS). The experimental results showed that the corrosion process could be divided into two stages (passivity and active dissolution) with the increasing temperature. At 100 °C, the oxide film was a single layer mainly consisting of Cr₂O₃. However, at 250 °C, it became a double layer with an inner layer of Cr–Fe spinel compound and an out precipitated layer. The related growth mechanisms of the oxide film were also discussed.     

Oxidation of NdFeB-type magnets modified with additions of Co, Dy, Zr and V

The oxidation characteristics of NdFeB alloys, modified by additions of Co up to 10 at.% and with minor additions of Dy, Zr and V, have been examined in this article over the temperature range 250–500 °C. The surface oxide consisted of an outer surface layer of polycrystalline haematite, regardless of Co content, and an inner layer of Co-modified magnetite. Underlying this two-layer surface oxide was an extensive internal oxidation zone (IOZ) formed as a result of preferential reaction with oxygen of the magnetic phase, Nd₂(Fe, Co)₁₄B. The IOZ consisted of a bcc solid solution of Fe and Co and contained a fine dispersion of Nd-rich oxide particles. The zone grew by inward oxygen diffusion, principally along high-angle grain boundaries within the Fe-rich matrix. The thickening of the IOZ occurred with parabolic kinetics but at a rate which decreased with increasing alloy Co content by a consistent factor of 2–3 in parabolic rate constant at all test temperatures. The activation energy for the growth of the IOZ lay in the approximate range 91–104 kJ/mol for the various alloys but there did not appear to be a systematic variation with alloy composition.     

Formation of boride layers at the Fe–25% Cr alloy–boron interface

Two boride layers based on the FeB and Fe₂B compounds are formed at the interface between a Fe–25% Cr alloy and boron at 850–950 °C and reaction times up to 12 h. The characteristic feature of both layers is a pronounced texture. Each of two boride layers is compositionally two-phase. The outer layer consists of the (Fe,Cr)B and (Cr,Fe)B phases. The inner layer comprises the (Fe,Cr)₂B and (Cr,Fe)₂B phases. The diffusional layer-growth kinetics are close to parabolic and can alternatively be described by a system of two non-linear differential equations, also producing a fairly good fit to the experimental data. Annealing of a borided Fe–Cr sample in the absence of boriding media results in the disappearance of the (Fe,Cr)B–(Cr,Fe)B layer, with the (Fe,Cr)B phase disappearing first. Microhardness values are 21.0 GPa for the outer layer, 18.0 GPa for the inner layer and 1.35 GPa for the alloy base. The abrasive wear resistance of the (Fe,Cr)B–(Cr,Fe)B layer, found from mass loss measurements, is more than 150 times greater than that of the alloy base.     

Steam-side scale morphologies associated with scale exfoliation from ferritic steel T22

Abstract

Observations of steam-formed scales on type T22 ferritic steel reheater tubes have provided some insights into the scale morphologies associated with exfoliation. Measurements indicated that as the total thickness of the adherent scales increased, the ratio of the thicknesses of the main inner (Fe–Cr spinel) and outer (magnetite) layers increased continuously to a maximum of approximately 2.5. The often-assumed thickness ratio of 1.0 did not persist after a laminated structure of pairs of spinel and magnetite layers was developed in the main inner layer. The development of such multi-laminations appeared to be associated with a decrease in the rate of growth of the main outer, magnetite layer. In this particular set of samples, exfoliation occurred in scales of total thickness greater than approximately 270 μm, and involved separation of a single pair of oxide layers of similar thickness that consisted of the original outer magnetite layer and part of the original spinel layer. The scale lost in subsequent exfoliation events at essentially the same site consisted of a further pair of layers comprising the new magnetite layer re-grown after the initial event, and a further part of the original spinel layer. Exfoliation events did not appear to involve the whole of the spinel layer existing at that time, only the equiaxed region immediately subjacent to the magnetite and a portion of the laminated structure.     

Effects of Si,Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract

Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO₂ and Ar–20CO₂–20H₂O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr₂O₃ layer and an inner SiO₂ layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn₃O₄ and MnCr₂O₄ above alloy grain boundaries and Mn₃O₄, Cr₂O₃ and MnCr₂O₄ on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.     

Cyclic Oxidation and Hot Corrosion Behavior of Plasma-Sprayed CoCrAlY + WC-Co Coating on Turbine Alloys

Components in energy-producing systems suffer a variety of degradation processes such as oxidation and molten salt-induced corrosion as a consequence of complex multi-component gaseous environment. Coatings provide a composition that will grow the protective scale at high temperatures having long-term stability. Plasma spraying was used to deposit CoCrAlY?+?WC-Co composite coatings on turbine alloys of Hastelloy X and AISI 321. The thermocyclic oxidation behavior of coated alloys was investigated in static air and in molten salt (Na₂SO₄-60%V₂O₅) environment at 700 °C. The thermogravimetric technique was used to approximate the kinetics of oxidation in 50 cycles, each cycle consisting of heating and cooling. X-ray diffraction and SEM/EDAX techniques are used to characterize the oxide scale formed. Coated alloys showed a lower corrosion rate as compared to uncoated alloys. The coatings subjected to oxidation and hot corrosion showed slow scale growth kinetics. Preferential oxidation of Co, Cr, W and its spinel blocks the transport of oxygen and corrosive species into the coating by providing a barrier, thereby making the oxidation rate to reach steady state. As compared to the substrate alloys, coatings show better hot corrosion resistance.     

Sulphidation of Fe-Nb,Co-Nb and Ni-Nb Alloys in H_2-H_2S at 700℃

The corrosion of alloys based on Fe,Co and Ni containing 15 and 30 wt-% Nb was studied at 700℃in H_2-H_2S mixtures providing 10~(-8) atm S_2.The Fe and Co-based alloys formed duplex scales withan outer layer of pure base-metal sulphide and an inner layer containing both metals.The Nb addi-tions were not able to produce exclusive Nb sulphidation but yielded a significant decrease of thecorrosion rates for both types of alloys with respect to the pure metals even though they were stillmuch higher than that of pure Nb.The Ni-base alloys corroded rapidly forming a single complexscale layer,mainly due to the appearance of a liquid Ni-S solution.     

Microstructure and corrosion behavior of cast Fe–B alloys dipped into liquid zinc bath

The corrosion behavior of cast Fe–B alloys in a liquid zinc bath was examined. As a result, the as-cast Fe–3.5B alloy displays the best corrosion resistance in all tested specimens owing to its netlike structure of eutectic Fe₂B against the liquid zinc. XRD and EDS results indicate that the main phases in the corrosion layer are determined to be δ, ζ and η from the matrix to the zinc zone, respectively. The corrosion process is controlled by the diffusion of zinc atoms and the dissolution mechanism. According to the observations, only three steps occur in the whole dissolution process: the preferential dissolution of α-Fe, the formation of intermetallic compounds, and the spalling of Fe₂B caused by cracks that mainly result from the internal stress produced by Fe/Zn phase transformation. The continuous netlike Fe₂B phase delays the further reaction of Fe/Zn and hinders the diffusion of zinc atoms between the substrate and liquid zinc effectively, improving corrosion resistance of Fe–B alloy in comparison with 1Cr18Ni9Ti stainless steel.     

Microstructure and electrochemical behavior of laser treated Fe---Cr and Fe---Cr---Si---N surface alloyed layers on carbon steel

Laser surface alloyed Fe---Cr and Fe---Cr---Si---N layers on carbon steel were produced by laser irradiation involving preplaced Fe/Cr and Fe/Cr/Si₃N₄ powders, respectively. The effects of Cr content (13–25 wt.%) and microstructure on the corrosion behavior of the laser treated Fe---Cr and Fe---Cr---Si---N alloys in deaerated neutral 3.5 wt.% NaCl solution were studied. Potentiodynamic polarization tests and electrochemical impedance spectroscopy measurements were used to evaluate the corrosion resistance of the surface layers. The results showed that the Fe---Cr---Si---N treated layer with a fine duplex phase microstructure had a higher pitting potential than the Fe---Cr treated layers. The passive film resistances increased and the passive current density decreased with increasing Cr content in the Fe---Cr layer.     

Uniform corrosion behavior of FeCrAl alloys in borated and lithiated high temperature water

The uniform corrosion behavior of model FeCrAl alloys as well as commercial Zircaloy-4 as reference material were investigated in 360℃borated and lithiated water.The results reveal that FeCrAl alloys exhibited better corrosion resistance than Zircaloy-4.It is found that the oxide films formed on the FeCrAl alloys are composed of outer Fe3O4 layer,inner layer consisting of compact spinel layer and porous spinel layer,and transition layer containing Al-Cr-rich oxides and matrix enriched with Fe.The spinel oxides in the inner layer are FeFe2-x-y-zCrxAlyMozO4.The corrosion mechanism of FeCrAl alloys in high temperature water is discussed.     

High-temperature oxidation of Fe-Cr alloys in air

The scales formed on the Fe-Cr (19, 29.5, and 46.8 at % Cr) and Fe₇₀Cr₁₅B₁₅ alloys in the initial stages of oxidation in air at 1470 K have been analyzed using depth profiling. The results demonstrate that the Cr/(Fe + Cr) ratio in the scales on the Fe-Cr alloys is close to 30 at %, independent of the bulk chromium concentration. The top scale layers on the Fe-Cr alloys consist predominantly of M₂O₃ (M = Cr, Fe) solid solutions and CrO₂, the intermediate layers are composed of (Fe, Cr)₃O₄ and FeO, and the internal oxidation and nitridation layers consist of chromium oxides and nitrides. The top scale layers on the oxidized Fe₇₀Cr₁₅B₁₅ alloy contain, in addition, boron oxides, and the internal nitridation product is BN instead of chromium nitrides.     

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.     

Corrosion of armco iron and model Fe−Cr-Al alloys in oxygen-containing lead melts

We studied the influence of oxygen-containing (6·10⁻³ wt.% O) lead on the corrosion of Armco iron and Fe−16Cr, Fe−16Cr−1Al alloys at a temperature of 650°C under stationary conditions. The front of corrosion propagates according to a linear law and this process is periodically repeated. In each period, an oxide film based on Fe₃O₄ magnetite is formed on the surface of the metal and lead penetrates into the suboxide zone. This leads to the exfoliation the external oxide film and then the process is repeated. Under the indicated testing conditions, alloying with chromium and aluminum intensifies the process of corrosion in iron. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 33, No. 2, pp. 84–88, March–April, 1997.