Thermodynamic model for estimating the composition of passive films and Flade potential on Fe-Cr alloys in aqueous solutions

A thermodynamic model of a passive film is developed, in which the formation of the film on the surface of an Fe-Cr alloy in an aqueous solution is considered to be a result of the stable solid-phase chemical and adsorption equilibrium at the alloy-inner passive film layer interface. In the calculations, the Cr₂O₃ content in the passive film is determined by both the Gibbs energy change (ΔG < 0) in the chemical oxidation of the alloy components by the water oxygen and the change in the surface Gibbs energy (ΔG _S > 0) of the alloy. The ΔG _S change results in the negative adsorption of chromium atoms, which shifts the 3Fe + Cr₂O₃ ↔ 3FeO + 2Cr equilibrium toward the FeO formation in the passive film. Calculations showed that the enrichment of the passive film in chromium oxide should sharply increase in a chromium content range of 10–20% in the alloy, which agrees with the known data of X-ray photoelectron spectroscopy of the passive films. A formula is derived for estimating the Flade potential of Fe-Cr alloys, which relates the Flade potentials of individual Fe and Cr components to the FeO and Cr₂O₃ contents in the passive film.     

Anodic oxidation of InAlAs

The anodic oxidation of InAlAs is investigated by transmission electron microscopy, Rutherford backscattering spectroscopy and medium energy ion scattering in order to elucidate the mechanism of oxide growth. For this purpose, anodizing was carried out at 5 mA cm⁻² in 0.1 M sodium tungstate and 0.1 M ammonium pentaborate electrolytes at 293 K, which results in relatively efficient film growth over an initial voltage increment of about 70 V. In this period, an amorphous oxide develops with a formation ratio of 2.0 ± 0.2 nm V⁻¹. The film consists mainly of an outer layer of In₂O₃ and an inner layer of units of In₂O₃, Al₂O₃ and As₂O₃, the former representing about 14% of the film. There is suggestion of a fine, intermediate layer containing units of In₂O₃ and Al₂O₃ only. The layering correlates with the bond energies of the cation and oxygen species in the oxide and hence their relative migration rates. Further, for films formed in tungstate electrolyte, tungsten species are incorporated into the outer 40% of the film. Bubbles of oxygen gas are present in the film, probably developed within the In₂O₃ layer. At higher voltages, the film undergoes breakdown, with resulting major changes in the film morphology.     

On the Microstructure of the Initial Oxide Grown by Controlled Annealing and Oxidation on a NiCoCrAlY Bond Coating

Different pre-annealing and pre-oxidation treatments were conducted on a dual phase γ+β Ni–21Co–18Cr–22Al–0.2Y (at.%) bond coating for 1 hr at 1373 K (i) with or without a native oxide upon heating, (ii) in two different atmospheres upon heating, and (iii) under various oxygen partial pressures (pO₂) in the range of 0.1–10⁵ Pa during oxidation. The chemical composition, structure, morphology and phase constitution of the resulting oxide layers were investigated using a range of analytical techniques. It is found that the exclusive formation of a continuous α-Al₂O₃ layer without the simultaneous formation of NiAl₂O₄ spinel was promoted for oxidation at low pO₂. The formation of metastable θ-Al₂O₃ was suppressed for a low fraction of the β phase, coupled with a high fraction of segregated Y at the initial bond coat surface. Initial Y segregation and incorporation of Y₂O₃ and Y₃Al₅O₁₂ within the developing oxide layer was promoted in the absence of a native oxide and for heating in an inert atmosphere. The development of protrusions (i.e. pegs) at the oxide/coating interface, as a result of the incorporation of internal Y₂O₃ precipitates by the inward growing oxide layer, was most pronounced upon heating in an inert atmosphere, followed by oxidation at an intermediate pO₂.     

Improved Cr2O3 adhesion by Ce ion implantation in the presence of interfacial sulfur segregation

As-polished and preoxidized Ni–20Cr alloys were Ce-implanted with a dosage of 1 × 10¹⁷ ions/cm², then subsequently oxidized at 1050 °C in air. The oxide adhesion and the extent of sulfur segregation at the oxide–alloy interface were determined, respectively, using tensile pull testing and scanning Auger microscopy with an in situ scratch device. The critical load for oxide failure was the lowest on the unimplanted Ni–20Cr, and was slightly higher on those with implantation made into a preformed oxide. Oxides that formed directly on Ce-implanted Ni–20Cr never failed under the pull test, which showed the strongest scale adhesion; however, similar amounts of interfacial sulfur, which segregated from the alloy during oxidation, were found at all interfaces. Ce additions were also found to reduce the oxidation rate and affect the extent of voids at the scale–alloy interface. It is suggested that the change in the oxide growth mechanism reduces the number of interfacial voids and, unlike Al₂O₃, these factors are more important for Cr₂O₃ scale adhesion than sulfur segregation to the scale–alloy interface.     

Effect of Platinum on the Growth Rate of the Oxide Scale Formed on Cast Nickel Aluminide Intermetallic Alloys

Thermo-Gravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM) data of the initial stages of oxidation of Ni₅₀Al₅₀ and Ni₄₀Pt₁₀Al₅₀ alloys of low and high sulfur content at 900°C and 1100°C are reported. The results show that the addition of Pt promotes the growth of the transient θ-Al₂O₃ oxide scale. This effect is particularly sensitive in the initial stages of oxidation at 1100°C where Pt considerably increases the total mass gain. It is attenuated in the presence of a high sulfur content in the alloy, indicating a competitive effect of Pt and S on the segregation of Al. The slower θ-to-α transition observed in the presence of Pt leads to an extended lifetime of the θ phase layer, which is proposed to be beneficial to the relaxation of the stresses created by the growth of α-Al₂O₃.     

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.     

Corrosion behaviour of 16%Cr-4%Al and 16%Cr ODS ferritic steels under different metallurgical conditions in a supercritical water environment

The corrosion behaviour of 16%Cr and 16%Cr-4%Al ODS ferritic steels in different heat treatment conditions has been investigated in a supercritical water environment. The exposed coupons were analyzed using scanning electron microscopy (SEM), electron probe micro analysis (EPMA), Auger and X-ray diffraction analysis (XRD). It was found that the formation of oxides depends on the chemical composition and not on the metallurgical condition. The Al-free alloys formed a monolayer oxide film composed of (Cr, Fe)₂O₃. The Al-containing alloys formed an oxide film composed of an outer layer of hematite and magnetite and an inner layer of Al₂O₃. The oxidation mechanisms are discussed.     

Behavior of a barrier layer of corrosion films on zirconium alloys

Capacitances of oxide films obtained on a EZ-1 alloy during the corrosion tests in water-vapor environment at 300, 350, and 400°C are measured. In order to estimate the thickness of a barrier layer, the barrier potential at a constant anodic current (dielectric puncture potential) is measured. It is shown that the barrier layer cannot be treated as a homogeneous environment. At the sites of intermetallide inclusions in the oxide film, the thickness of the dielectrics is locally decreased. In the first approximation, the heterogeneity of the oxide film can be taken into account by inserting two parallel RC subcircuits in the equivalent scheme. One subcircuit (C ₁, R ₁) describes the electrophysical properties of the capacitance whose insulator thickness corresponds to the total thickness of the oxide film. The other subcircuit (C ₂, R ₂) describes the electrophysical properties of the nonporous part of the oxide film between the intermetallide particles and the outer surface. Then, the results of measurements can be written as follows: C _exp = θC ₂ + (1 − θ)C ₁, where θ is the surface part of the oxide film whose dielectric properties are changed due to the inclusion of intermetallide particles. Assuming that the mean spatial size of intermetallide particles falls in the range of 200–400 nm, one can estimate the mean concentration of the particles on the metal surface in agreement with the metallographically determined concentration of the second-phase particles (approximately 10⁶–10⁷ cm⁻²). The obtained results indicate the substantial heterogeneity of the barrier layer structure, which may cause local corrosion and premature failure of zirconium items.     

Effect of pre-oxidation on the hot corrosion of CoNiCrAlYRe alloy

The effect of pre-oxidation on the resistance to hot corrosion was examined by corroding the CoNiCrAlYRe alloy at 900 °C in molten Na₂SO₄. Preoxidized specimens featured strong adhesion of oxide scale with uniform multi-layered structure. The time of pre-oxidation was crucial for controlling Al content sufficient for subsequent hot corrosion. However, direct corrosion yielded a defective and non-protective oxide scale, which allowed detrimental penetration of sulfur into substrate. Sulfur migrating along phase boundary was trapped by yttrium to diminish slightly sulphidation. Thus, two advantages of proper pre-oxidation treatment were presented, as keeping repairing for Al₂O₃ scale and inhibiting sulfur penetration.     

Oxidation Behavior of a Sputtered Ni–5Cr–5Al Nanocrystalline Coating

A NiO-forming Ni–5Cr–5Al (at.%) alloy has been developed anddeposited as a sputtered nanocrystalline coating. The oxide formation andoxidation behavior of this coating have been studied at 1000°C inair. The oxidation rate markedly decreased with time and the oxidationkinetics obeyed the fourth power law. Complex oxide scales, consisting ofNiO, NiAl₂O₄ and -Al₂O₃,were formed during 200 hr oxidation. The outer oxide layer consisted of NiOand NiAl₂O₄ and an inner oxide layer of-Al₂O₃. The sputtered Ni–5Cr–5Alnanocrystalline coating showed good oxidation resistance due to theformation of an -Al₂O₃ inner layer andexcellent adhesion of the complex oxide scales.     

A study of the corrosion properties of plasma nitrocarburized and oxidized AISI 1020 steels

Plasma nitrocarburized AISI 1020 steels were oxidized for 15, 30 and 60 min to evaluate their corrosion and microstructural properties. After plasma nitrocarburizing for 3 h at 570°C in a gas mixture comprising 85 vol.% N₂, 12vol.% H₂ and 3 vol.% CH₄, the compound layer composed of ɛ-Fe_2–3(N,C) and γ’-Fe₄(N,C) phases and the diffusion layer above the matrix were observed. The top oxide layer, consisting mainly of magnetite (Fe₂O₄) and hematite (Fe₂O₃) phases, forms after post-oxidation treatment at 500°C. However, the oxide layer was severely degraded by spallation as a result of increases in post-oxidizing time. The difference in corrosion resistance should be attributed to the thickness of the top oxide layer, which was governed by post-oxidizing time.     

Transition from oxidation to sulfidation of Fe-Cr alloys in gases with low oxygen and high sulfur pressures

Fe-Cr alloys with 17–30% Cr show in H₂-H₂O-H₂S mixtures at 1273 and 1073 K a transition from protective oxide scale formation to rapid sulfidation. The critical oxygen pressure to stabilize the oxide formation increases with increasing sulfur pressure of the gas and decreasing Cr content of the alloy. Cr₂O₃ with traces of Fe₂O₃ is formed under these conditions. Below the critical oxygen pressure, a primarily formed Cr₂O₃ film becomes overgrown by (Fe, Cr)S. The kinetic boundary of oxidation-sulfidation, which lies in the stability field (Fe, Cr)S + spinel Fe_1+xCr_2–xO₄ of the Fe-Cr-O-S phase diagram, is explained with the help of the Fe-Cr-O-S phase diagram and the assumption that Fe diffuses faster through the (Cr, Fe)₂O₃ solid solution than does Cr.     

Esca-studies of the composition profile of low temperature oxide formed on chromium steels—I. Oxydation in dry oxygen

ESCA (Electron Spectroscopy for Chemical Analysis) has been used to analyse the composition of oxide films formed on chromium steels with 6–20%Cr at room temperature in pure oxygen at 0·2 atm. during different times from 2 min to 46 h. Ion etching was used to clean the surface before oxidation and to investigate the composition profile by successive stripping of the oxide layer.It is demonstrated that the total chromium content in the oxide is proportional to the concentration in the metal, but that it varies with depth inside the film. At the oxide-gas boundary the chromium concentration is less than in the metal, while chromium is enriched at the metal phase boundary. It is suggested that the oxide consists of (Fe,Cr)₂O₃ in the outermost zone and FeCr₂O₄ at the inside, with the reservation that the method is not sensitive to deviations from stoichiometry.The thickness of the layer has been determined as 25 Å from the intensity ratio of the deconvoluted ESCA peaks of the metallic and oxidized states. The thickness and the composition profile of the film do not vary noticeably with the oxidation time.     

Growth and characterization of Al2O3 thin films for the buffer insulator in Pt/SrBi2Nb2O9/Al2O3/Si ferroelectric gate oxide structure

Pt/SrBi₂Nb₂O₉ (SBN)/Al₂O₃/Si (MFIS) ferroelectric gate oxide structures were prepared with the rf (radio frequency) magnetron sputtering method for the application of non-destructive read-out ferroelectric RAM (NDRO-FRAM) devices. An Al₂O₃ intermediate layer between the perovskite SrBi₂Nb₂O₉ film and Si substrate prevents the serious inter-diffusion of the SrBi₂Nb₂O₉ (SBN) into the Si substrate. The coercive field that decisively affects the memory window was increased by inserting the Al₂O₃ insulator between the SBN and Si, and thus the memory window also increased with the increase in the electric field to the SBN. The memory windows of the metal-ferroelectric-insulator-semiconductors (MFIS) structures were in the range of 0.7–3.4 V when the gate voltage varied from 3 to 9 V. The memory windows of the MFIS structures were found to be dependent on the thickness and stoichiometry of the buffer layer. We obtained the maximum memory window in an MFIS with an insulator of 11.4 nm in thickness deposited in the deposition condition of a 15∶5 flow ratio (Ar:O₂) during sputtering.     

The Effect of Water Vapor on the Initial Stages of Oxidation of the FeCrAl Alloy Kanthal AF at 900 °C

The effect of water vapor on the initial stages of oxidation of the FeCrAl alloy Kanthal AF is reported. Polished samples were exposed isothermally at 900 °C for 1, 24, 72 and 168 h in a well-controlled environment consisting of dry O₂ or O₂ + 40% H₂O. The samples were investigated using a combination of gravimetry and several surface-analytical techniques, including XRD, SEM, EDX, FIB, AES and TEM. The presence of water vapor significantly accelerates oxidation during the first 72 h. A two-layered oxide forms in both the dry and wet environments. The bottom layer consists of inward-growing α-Al₂O₃ while the outer layer initially consists of outward-growing γ-Al₂O₃. A straight and narrow Cr-enriched band is present at the top of the lower (α-Al₂O₃) oxide, corresponding to the original sample surface. In dry O₂, the top (γ-Al₂O₃) layer is converted into a mixture of γ-Al_2−x (Mg,Fe) _x O_3−(x/2), MgAl₂O₄ and α-Al₂O₃. This transformation does not occur in O₂ + H₂O. The initial acceleration of oxidation by H₂O is attributed to the stabilization of the outward-growing γ-alumina layer by the hydroxylation of the γ-Al₂O₃ surface. A schematic mechanism of the early stages of oxidation of FeCrAl alloys is presented, emphasizing the influence of water vapor.     

Photoelectrochemical study of the growth of the passive film formed on Fe-20Cr-15Ni in a pH 8.5 buffer solution

The structural change of the passive film on Fe-20Cr-15Ni at its growth stage was examined in-situ using a photoelectrochemical technique. The photocurrent spectra showed that the passive film on Fe-20Cr-15Ni for 0.5 h ∼ 25 h in a pH 8.5 buffer solution was composed of (Cr, Ni)-substituted γ-Fe₂O₃ mixed with NiO. Photocurrent spectral analysis suggested that a crystalline film of (Cr, Ni)-substituted γ-Fe₂O₃ formed before 30 min, with its thickness exceeded that of the space charge layer after 1 h of passivation. NiO particles appeared to have gradually precipitated from (Cr, Ni)-substituted γ-Fe₂O₃ film in the early stage of passivation of 1 h.     

Thermal stability behaviors of Cr(N,O)/CrN double-layered coatings by TGA/DTA analysis

In this study, Cr(N,O)/CrN double-layered coatings were synthesized using the cathodic arc deposition (CAD) process. CrN film was first deposited onto a substrate as an interlayer to ensure better adhesion, and Cr(N,O) film was subsequently deposited on top of the CrN layer as the surface layer. Variation in the Cr(N,O) coating composition was achieved through changing the O₂/N₂ flow ratio during the last stage of processing. Phase structure, chemical composition, and morphology of the resulting coatings were analyzed and observed using the X-ray diffractometer, Auger electron spectrometer and SEM. In addition, oxidation behavior of the coatings was investigated using TGA/DTA methods. The tests were carried out by increasing temperature up to 1000 °C in ambient air. With the introduction of oxygen gas during the CAD process, a superficial layer was produced in the Cr(N,O) constituent containing CrN and Cr₂O₃ phases. The formation of the oxide phase attributed to the reaction of chromium and oxygen was more favorable than that of chromium and nitrogen. The results also showed that Cr(N,O)/CrN double-layered coatings exhibited superior oxidation resistance at elevated temperature than that of CrN single-layer coated specimen (870 °C vs. 750 °C).     

Über die Oxidschichtbildung einer Titanlegierung während einer Li2O-haltigen Schutzgasbehandlung bei hohen Temperaturen

Oxide layer formation on a titanium alloy during a high temperature treatment in protective gas containing Li₂O The adhesion of an oxide scale can be improved considerably by the incorporation of Li₂O in the lattice of the growing scale. The oxide layer on Ti alloys generally consists of three zones, an external TiO₂, an intermediate Ti₂O₃ and an internal TiO zone. When, however, the oxidizing gas contains Li₂O, only two layers can be identified, viz. an internal TiO₂ layer (1/8 of the total oxide thickness) and an external Li₂TiO₃ layer. The latter is responsible for the good adhesion; it is obtained primarily in combustion gases obtained by near stoichiometric combustion of natural gas. In gases obtained by sub-stoichiometric combustion TiO₂ is formed perferentially and the adhesion of the oxide layer is low     

Chemical and Morphological Evolution of a (NiPt)Al Bondcoat

Experimental results are reported on the chemical and microstructural evolutions of a (NiPt)Al coating, deposited on a single-crystal, nickel-base superalloy, during isothermal-oxidation tests at 1100°C, up to 50 hr. Analytical STEM studies were carried out, in conjunction with Auger experiments, in order to follow the various changes that occur in the bondcoat (β into γ′ phase transformation, TCP and α-Cr precipitation, S-diffusion pathways) and at the Al₂O₃/bondcoat interface. Efforts were concentrated on the effect of interfacial sulfur segregation (at both intact interfaces and void surfaces) as a function of the oxidation time and its dependence on phase transformations in the external layer of the bondcoat. An important result concerning the co-segregation of S and Cr at the alumina/bondcoat interface was realized in this system due to Cr diffusion from the substrate into the bondcoat. This S segregation adversely affected Al₂O₃-bondcoat adhesion where extensive interfacial voids formed at the interface.     

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.