The high temperature oxidation of 11% chromium steel: Part I – Influence of pH2O

The oxidation of 11% Cr steel (X20 11Cr1MoV) in the presence of dry O₂ and O₂ + 10 and 40% H₂O was investigated at 600°C. The exposure time was between 1 and 672 hours. The oxidized samples were investigated by a number of surface analytical techniques including GI‐XRD, SEM/EDX, GDOES and Auger spectroscopy. X20 steel (11Cr1MoV) forms a protective chromium rich α‐(Cr,Fe)₂O₃ oxide in dry O₂ at 600°C. In mixtures of oxygen and 10 or 40% H₂O, at the same temperature, the material is affected by chromium vaporization because of the formation of CrO₂(OH)₂(g). The loss of chromium tends to deplete the oxide in chromium. The formation of a more iron‐rich oxide may result in a loss of the protective properties of the oxide scale. The loss of chromium and the tendency to destabilize the protective oxide increases with the concentration of water vapour. The material suffers breakaway corrosion after 336 hours in an O₂/H₂O (60/40) mixture while the rate of oxidation is only marginally increased in the presence of 10% H₂O. The thick oxide formed in O₂/H₂O (60/40) environment features an inner layer consisting of FeCr spinel and an outer layer which is almost pure hematite.     

Affordable electrolytic ferrotitanium alloys with marine engineering potentials

Fe₂O₃ and TiO₂ powders were compounded in different proportions at elevated temperatures. Porous thin pellets were made from the compounded oxides and then electro-reduced to the respective ferrotitanium alloys and/or intermetallic compounds in solid state in molten CaCl₂. Typical electrolysis conditions were 800–1000 °C, 2.8–3.2 V and 4–15 h. X-ray diffraction, scanning electron and optical microscopy, and potentiodynamic polarisation were used to characterise the oxide precursors and/or the products. The results showed that the obtained Fe–Ti alloys achieved the designated elemental compositions. When the Fe content in the oxide precursor was less than 50 wt.%, the products were mainly mixed Ti and Fe–Ti alloys. At higher Fe contents, the products changed to a mixture of Fe₂Ti and Fe. Between 8 and 15 wt.% Fe, the products sintered most severely. The Fe-rich Fe–Ti alloys had better corrosion resistance than a common ship hull steel (E36) in simulated sea water, i.e. the aqueous solution of 3 wt.% NaCl. The Ti-rich Fe–Ti alloys (8 wt.% Fe) had good corrosion resistance to the 1.0 mol/L HCl solution. The addition of Nb in the alloys improved the corrosion resistance, but the addition of Al caused the opposite effect.     

Corrosion of atomized Fe40Al based intermetallics in molten Na2SO4

The hot corrosion resistance of sprayed and atomized Fe‐40at.%Al, Fe40Al+0.1B and Fe40Al+0.1B+10Al₂O₃ intermetallic materials have been evaluated in molten Na₂SO₄ at 900 and 1000°C using polarization curves and polarization resistance measurements. The results are supported by electron microscopy and microchemical studies. The tests lasted 5 days. At 900°C the Fe40Al material had the lowest corrosion rate (0.03 mA/cm²), and the Fe40Al+0.1B+10Al₂O₃ exhibited the highest. At 1000°C, the Fe40Al+0.1B material, was the material that had the best corrosion resistance with less than 0.02 mA/cm² in the first 50 hours, whereas the Fe40Al presented the worst corrosion resistance with 0.20 mA/cm². The results are discussed in terms of the establishment of an Al₂O₃ layer that gives corrosion resistance to the materials and promotes an Al depletion in the FeAl matrix which allows the sulfides formation.     

Influence of Pre-oxidation on the Corrosion and Mechanical Strength of Fe–25Cr and Fe–25Cr–20Ni Alloys at 973 K

The influence of pre-oxidation on the corrosion and mechanical strength of Fe–25Cr and Fe–25Cr–20Ni alloys was investigated in N₂–0.1SO₂ at 973 K with and without mechanical loadings. About 0.1μm-thick Cr₂O₃ scales formed on the Fe–25Cr alloy by pre-oxidation in Ar. However, spinel oxides of (Fe,Cr)₃O₄ remained on the Cr₂O₃ and voids formed at the oxide/metal interface on Fe–25Cr–20Ni after pre-oxidation in Ar. The preformed oxides are very effective in preventing corrosion of the alloy surfaces. The preformed oxides are also beneficial to increase the strength of the alloys in corrosive environments. The effects of pre-oxidation on Fe–25Cr are stronger than those of Fe–25Cr–20Ni due to the different characteristics of the preformed oxides.     

Oxide growth stress measurements and relaxation mechanisms for alumina scales grown on FeCrAlY

Early‐stage tensile stress evolution in α‐Al₂O₃ scales during oxidation of FeCrAlY at 1000, 1050, 1100, and 1200 °C was monitored in situ by use of synchrotron radiation. Tensile stress development as a function of oxidation temperature indicated a dynamic interplay between stress generation and relaxation. An analysis of the time dependence of the data indicated that the observed relaxation of the initial tensile stress in the oxide scales at 1100 and 1200 °C is dominated by creep in the α‐Al₂O₃. A thin layer of a (Fe,Cr,Al) oxide was observed at the oxide‐gas interface, consistent with a mechanism whereby the conversion of (Fe,Cr,Al)₂O₃ to α‐Al₂O₃ produces an initial tensile stress in the alumina scale.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

Corrosion behavior of AZ91D magnesium alloy in sodium sulfate solution

The corrosion behavior of as‐cast AZ91D magnesium alloy in 0.1M sodium sulfate solution at the corrosion potential (E_corr) was investigated by using electrochemical impedance spectroscopy (EIS), environmental scanning electron microscopy (ESEM), energy dispersive X‐ray spectroscopy (EDS) and X‐ray diffraction (XRD). The results showed that the corrosion of AZ91D started at both the primary α‐Mg and the eutectic α‐Mg. The surface first was covered by a film (MgO, Mg(OH)₂) which became thicker with time. Due to the dissolution of the eutectic α‐Mg, the concentration of aluminum increased, MgAl₂(SO₄)₄ · 2H₂O precipitated at the primary α‐Mg and progressively spread to the eutectic α‐Mg areas. The surface film changed from two‐layer to three‐layer structure with the increase of immersion time.     

Corrosion and tribological behaviors of chromium oxide coatings prepared by the glow-discharge plasma technique

In order to improve the corrosion and tribological properties of steel, chromium oxide coatings were prepared by a new combined process, namely, chromizing and plasma oxidizing treatments using double glow plasma technology under various oxygen flow rates. The composition and microstructure of the coatings were analyzed respectively by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results indicated that the oxygen flow rates had a great effect on the surface structure of the prepared Cr₂O₃ coatings, and dense and smooth Cr₂O₃ coatings were prepared at the oxygen flow rate of 10 sccm. The Cr₂O₃ coatings exhibited the better corrosion resistance which was in good agreement with the results obtained by the microstructure studied. Further mechanical properties test showed that the Cr₂O₃ coatings with high hardness and elastic modulus adhered well to the steel substrates and displayed excellent wear resistance and low coefficient of friction under dry sliding wear test conditions. The wear mechanism was mostly dominated by the “soft abrasion”.     

Das Korrosionsverhalten von sauerstoffionenimplantiertem Magnesium in schwach saurem Elektrolyten

The corrosion behaviour of oxygen implanted magnesium in weekly acid solution Under atmospheric conditions magnesium forms oxide layers which consist mainly of magnesium hydroxide. The protective action of Mg(OH)₂ is minor in sulphate‐, carbonate‐ or chloride containing solutions [2, 18]. Additionally, the crystallographic misfit between magnesium and magnesium hydroxide leads to crack formations inside the hydroxide layer and the metallic surface is then exposed. By using the ion implantation of oxygen (1*10¹⁸ O⁺/cm²) a MgO‐layer can be formed and buried within the bulk metal. Pre‐tests of MgO‐layers produced by ion implantation suggest that it is successful as corrosion protection [17]. Crystallographic stresses between metal‐ and oxide phase will be reduced, depending on the gradient of oxygen concentration. The investigation in weekly acid sodium solution shows, that the oxygen ion implantation leads to a significant improvement in the corrosion behaviour of magnesium.     

Characterization of plasma electrolytic oxide formed on AZ91 Mg alloy in KMnO4 electrolyte

The aim of this work is to investigate microstructure, corrosion resistance characteristics and nanohardness of the oxide layer on AZ91 Mg alloy by applying different voltage with KMnO₄ contained solution. There are lots of closed pores that are filled with another oxide compound compared with the typical surface morphology with pore coated until 350 V of coating voltage. The thickness of oxide layer increases with increasing coating voltage. The oxide layer formed on AZ91 Mg alloy in electrolyte with potassium permanganate consists of MgO and Mn₂O₃. Corrosion potential of the oxide layer on AZ91 Mg alloy obtained at different plasma electrolytic oxidation(PEO) reaction stages increases with increasing coating voltage. The corrosion resistance of AZ91 Mg alloy depends on the existence of the manganese oxide in the oxide layer. The inner barrier layer composed of the MgO and Mn₂O₃ may serve as diffusion barrier to enhance the corrosion resistance and may partially explain the excellent anti-corrosion performance in corrosion test. Nanohardness values increase with increasing coating voltage. The increase in the nanohardness may be due to the effect of manganese oxide in the oxide layer on AZ91 Mg alloy coated from electrolyte containing KMnO₄.     

The Corrosion Behavior of Sulfidation-Resistant Fe–Mo–Al Alloys in H2/H2S Atmospheres at 900°C

The corrosion behavior of Fe–22Mo–10Al (a/o, atom %),Fe–20.5Mo–15.7Al, and Fe–10Mo–19Al was examined inflowing H₂/H₂S gases of 4 Pa sulfur partial pressureat 900°C. Al₂O₃ was stable on all the alloys inthe atmospheres investigated. Fe–22Mo–10Al andFe–20.5Mo–15.7Al reacted slowly, following the parabolic ratelaw. Multilayered reaction products were formed on these alloys and it isuncertain which layer(s) provided the protection. Fe–10Mo–19Alreacted even more slowly, exhibiting two-stage parabolic kinetics. Duringthe early stage of this alloy's reaction, a preferential reaction zone,consisting of an oxide mixture, possibly Al₂O₃+FeAl₂O₄,and nonreacting Fe₃Mo₂, provided the protection. Duringthe later reaction stage, the formation of a continuous, externalAl₂O₃ layer further decreased the alloy reaction rate.     

The high temperature oxidation of 11% chromium steel: Part II – Influence of flow rate

The oxidation of type X20 CrMoV 11 1 steel at 600°C in the presence of dry O₂ and O₂ + 10 or 40% H₂O was investigated. The flow rate was varied between 0.25 to 10.0 cm/s. Exposure time was 168 hours. The oxidized samples were investigated gravimetrically and by a number of surface analytical techniques including grazing angle SEM/EDX, GDOES and XRD. Oxidation is strongly influenced by pH₂O and flow rate. In O₂ + H₂O environment at 600°C, the protective Cr‐rich α‐(Cr,Fe)₂O₃ oxide loses chromium by vaporization of CrO₂(OH)₂. When chromium loss is limited (e. g., in 10% H₂O and in 40% H₂O at low flow rates) the supply of chromium from the alloy compensates for chromium vaporization and the oxide retains its protective properties, resulting in slow oxidation. In 40/60 H₂O/O₂ and high flow rates chromium evaporation becomes so rapid that the protective properties of the oxide are lost and a thick duplex (Fe₂O₃/Fe₂CrO₄) scale develops.     

Improving the Physicochemical Properties of Fe–25Cr Ferritic Steel for SOFC Interconnects via Y-Implantation and Y2O3-Deposition

The present study investigated the role of the reactive-element effect (REE) in improving the corrosion resistance, chromium vaporization rate, and electrical conductivity of the Fe–25Cr ferritic steel modified either by means of yttrium implantation or chemical deposition of yttrium oxide from metaloorganic compound vapors. The corrosion kinetics of the Fe–25Cr steel, both pure and modified, were determined under isothermal conditions in air and an Ar–H₂–H₂O gas mixture at 1,073 K. A significant improvement in corrosion resistance was observed after surface modification. XRD and SEM–EDS investigations showed that the protective Cr₂O₃ layer formed the main part of the scale. Measurements of Cr vaporization rate in the air–H₂O gas mixture revealed that both surface modifications of the steel significantly suppressed the formation of volatile chromium compounds to a large degree. The yttrium-implanted steels oxidized both in air and the Ar–H₂–H₂O mixture were characterized by the lowest area specific resistance and thereby did not exceed the acceptable ASR level (0.1 Ω cm²) for interconnect materials in the temperature range of 973–1,073 K, unlike pure steel and the steel coated with Y₂O₃.     

S-Induced Destabilization of Aluminum Oxide at the Fe(poly)–S–Al2O3 Interface

Auger measurements reveal that, under UHV conditions, interfacial sulfurinduces the destabilization of an aluminum oxide overlayer at theFe–Al₂O₃ interface at temperatures above400 K. One monolayer deposition of Al onto Fe/S results in the insertion ofAl at the Fe–S interface. Exposure of Fe–Al–S to oxygenat 300 K gives rise to the complete oxidation of the aluminum adlayer asevidenced by the disappearance of the Al⁰ Auger signal and thestoichiometric formation of the aluminum oxide. When the resultingFe–S–Al₂O₃ is annealed progressively tohigher temperatures between 400 and 900 K, analysis of the Auger spectrashows a dramatic decline in the Al/O Auger intensity ratio. This declineis accompanied by the appearance of a small signal due to Al⁰,which maintains a constant intensity as the total Al signal (due mainly toAl³⁺) decreases. The appearance of the Al⁰ Augersignal accompanied by the attenuation of the Al³⁺ signalsignifies the chemical conversion of Al³⁺ into Al⁰,probably followed by diffusion of Al into the bulk. The possibility ofalumina dewetting and island formation, however, cannot be ruled out onthe basis of the present data. In the absence of interfacial sulfur, the alumina–Fe interface is stable to 900 K.     

Zinc treatment effects on corrosion behavior of 304 stainless steel in high temperature, hydrogenated water

Trace levels of soluble zinc(II) ions (30 ppb) maintained in mildly alkaline, hydrogenated water at 260 °C were found to lower the corrosion rate of austenitic stainless steel (UNS S30400) by about a factor of five, relative to a non-zinc baseline test [S.E. Ziemniak, M. Hanson, Corros. Sci. 44 (2002) 2209] after 10,000 h. Characterizations of the corrosion oxide layer via grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy in combination with argon ion milling and target factor analysis, revealed that miscibility gaps in two spinel binaries—Fe(Fe_1−mCr_m)₂O₄ and (Fe_1−nZn_n)Fe₂O₄—play a significant role in determining the composition and structure of the corrosion layer(s). Although compositions of the inner and outer corrosion oxide layers represent solvus phases in the Fe₃O₄-FeCr₂O₄ binary, zinc(II) ion incorporation into both phases leads to further phase separation in the outer (ferrite) layer. Recrystallization of the low zinc content ferrite solvus phase is seen to produce an extremely fine grain size (∼20 nm), which is comparable in size to grains in the inner layer and which is known to impart resistance to corrosion. Zinc(II) ion incorporation into the inner layer creates additional corrosion oxide film stabilization by further reducing the unit cell dimension via the substitution reaction

0.2Zn²⁺(aq)+Fe(Fe_0.35Cr_0.65)₂O₄(s)?0.2Fe²⁺(aq)+(Zn_0.2Fe_0.8)(Fe_0.35Cr_0.65)₂O₄(s)     

KCl‐induced corrosion of Ni‐based alloys containing 35–45 wt% Cr

Ni–(35–45)Cr–4Nb alloys containing different fractions of α‐Cr were exposed to potassium chloride (KCl)‐induced corrosion. The corrosion exposures were carried out for 168 hr at 600°C in a 15% (vol/vol) H₂O (g) + 5% (vol/vol) O₂ (g) + N₂ (g; balance) atmosphere using KCl‐free (reference) and predeposited KCl samples. To mimic the KCl deposition in real boilers, 24 hr exposures where KCl vapor condensed continuously onto samples were also performed. The corrosion attack of the studied materials increased significantly when KCl was present compared to the KCl‐free samples. For the KCl exposures, the corrosion attack drastically increased when a significant α‐Cr fraction was present. α‐Cr was either selectively attacked or dissolved through solid‐state diffusion and a layered build‐up of the outer external scale of K₂CrO₄ and chromia could be observed. For the in situ condensed KCl exposure, severe corrosion was observed already within the 24 hr exposure, indicating a higher corrosion rate compared with when KCl was predeposited.     

Corrosion of nickel with small alloy additions of Si,Fe, and Mn in SO2+O2/SO3 at 700°C

The corrosion of nickel with alloy additions of Si, Fe, and/or Mn up to 4 wt% has been studied in SO ₂+O₂/SO₃ at 700°C. All alloy additions greatly improve the corrosion resistance of nickel in oxygen-rich atmospheres (O₂ with about 4% SO₂); the best improvements are achieved with Si, Fe+Si, and Fe+Mn+Si additions. High-purity nickel corrodes rapidly under these conditions; the scale then consists of NiO+Ni₃S₂, and the sulfide forms a three-dimensional network along the grain boundaries of the NiO grains and serves as the diffusion path for rapid outward migration of nickel. From studies of the microstructure and distribution of the alloying elements in the protective scales, it is proposed that the alloying additions exert their beneficial effects by accumulating/segregating at the grain boundaries of NiO (e.g., as silicates) and thereby influence the wetting characteristics and disrupt the sulfide network.     

High-temperature corrosion of Cr2O3-forming alloys in CO-CO2-N2 atmospheres

The corrosion of Fe–28Cr, Ni–28Cr, Co–28Cr, and pure chromium in a number of gas atmospheres made up of CO–CO₂(–N₂) was studied at 900°C. In addition, chromium was reacted with H₂–H₂O–N₂, and Fe–28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H₂–H₂O–N₂ produced a single-phase of Cr₂O₃. In a CO–CO₂ mixture, a sublayer consisting of Cr₂O₃ and Cr₇C₃ was formed underneath an external Cr₂O₃ layer. Adding nitrogen to the CO–CO₂ mixture resulted in the formation of an additional single-phase layer of Cr₂N next to the metal substrate. Oxidizing the binary alloys in CO–CO₂–N₂ resulted in a single Cr₂O₃ scale on Fe–28Cr and Ni–28Cr, while oxide precipitation occurred below the outer-oxide scale on Co–28Cr, which is ascribed to the slow alloy interdiffusion and possibily high oxygen solubility of Co–Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe–28Cr, but varied between the different materials, decreasing in the order chromium >Fe–28Cr>Ni(Co)–28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr₂O₃ is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affects the permeability of the oxide.     

Oxidation behavior of Fe40Al-xWC composite coatings obtained by high-velocity oxygen fuel thermal spray

The Fe40Al-xWC (x=0, 10, 12, 15) coatings with dense structure were successfully deposited by high-velocity oxygen fuel (HVOF) spraying of a mixture of Fe, Al and WC powders. The objective of the present work is to provide insight into the oxidation behavior of the as-deposited coatings at 650 °C under 0.1 MPa flowing pure O₂. The present results show differences in the oxidation behavior of Fe40Al coating and Fe40Al-xWC composite coatings. The irregular Fe₂O₃ layer is seen on the top surface of the composite coatings. Fe40Al coating and Fe40Al-15WC composite coating both suffer a catastrophic corrosion due to the formation of a porous structure during 24 h of oxidation. However, Fe40Al-10WC and Fe40Al-12WC composite coatings show a good oxidation resistance behavior due to their dense structure.     

Accelerated Corrosion of Fe–xCr–10Al Alloys Containing 0–20 at.% Cr Induced by Sulfur and Chlorine in a Reducing Atmosphere at 600 °C

The corrosion behavior of five Fe–xCr–Al alloys with a constant Al content of 10 at.% and Cr contents ranging from 0 at.% to 20 at.% was examined at 600 °C in a H₂–HCl–H₂S–CO₂ gas mixture providing 3.7 × 10⁻²² atm O₂, 2.4 × 10⁻¹⁴ atm Cl₂ and 3.9 × 10⁻⁹ atm S₂. All the alloys formed duplex scales containing an outermost layer of iron oxide plus an inner layer composed of mixtures of the oxides of all the alloy components. Besides, a region of internal attack of Al or Al + Cr, whose depth decreased with increasing Cr content, formed in all the alloys. The simultaneous presence of chlorine and sulfur in the gas mixture significantly accelerated the corrosion of all the alloys with respect to their oxidation in a simpler H₂–CO₂ mixture providing the same oxygen pressure, by forming thick and cracked scales. The effect was particularly large for the high-Cr alloys due to their inability to form external protective alumina scales in the present gas mixture.