Inner Oxide Layer Research Articles

Oxidation and sulfidation of implanted and unimplanted AISI 446 steel

AISI 446 steel exhibited parabolic rate kinetics from the beginning during isothermal oxidation in oxygen at 850°C. On the other hand, a pronounced transient oxidation with faster kinetics was observed in Ce- and Xe-implanted AISI 446 steels. The implantation, however, did not affect the steady-state parabolic rate constant, 3.77±0.18×10−5 mg2/cm4 min. The initial response of implanted steels to oxidation with pronounced transient oxidation was attributed to the physical defects of implantation. The oxide grains formed on AISI 446 early in the process of oxidation were rich in Fe and Cr, and after long exposure the spinel MnCr2O4 became the major constituent of the scale. Ce-implantation did not have any effect on the corrosion behavior of AISI 446 in H2/H2O/H2S/Ar at 850°C. The scale had three zones: an outer layer with FeS, (FeCr)S, and spinel oxide; an intermediate layer of (FeCr)S; and an inner layer of Cr-rich oxide and (FeCr)S below the original metal surface.

The stability of hydrous oxide films on gold

The stability of hydrous oxide films produced on gold by anodizing at 2.2 V (RHE) in acid solution was investigated as a function of holding potential in both acid and base. In contrast to platinum, whose behaviour will be described in more detail later, the oxide film on gold was more stable, i.e. more resistant to reduction, in solutions of high pH. This was attributed to a persistent anionic character in the case of hydrous gold oxide. Other factors relevant to the reactivity of such films, e.g. the presence of an inner layer of compact oxide and the heterogeneous character of the hydrous oxide deposit, were also considered briefly. A remarkable coincidence was noted between the minor voltammetric peaks observed earlier for clean polycrystalline gold in base and the much larger peaks observed here for hydrous oxide reduction in the same system: the peak maximum potential values for these two significantly different conditions were virtually the same.

High-temperature oxidation of a 20/25 stainless steel in high-pressure carbon dioxide

Kinetics of high-temperature oxidation, and of oxide spallation, are reported for a 20Cr-25Ni Nb-stabilized stainless steel, exposed in carbon dioxide at 40 bar pressure in the temperature range 1115–1240 K. Gross weight-gain kinetics were parabolic with respect to time, with an activation energy of 370±16 kJ/mol. Oxidation rates are shown to be consistent with diffusion control in the inner chromia layer of a two-layer oxide. Intergranular oxidation of silicon was observed beneath the oxide scale. Oxide spallation occurred at all temperatures and increased linearly with gross weight gain. At 1240 K 50% of the oxide which formed spalled at the first weighing. Thereafter, the fraction spalling decreased to about 10%. The difference is attributed to a change in the oxide composition from an M2O3 structure to M3O4, which requires greater compressive stresses to induce spallation during cooling.

ESCA studies of Ni‐25 At% Fe alloy. 2—dissolution and passivation

AbstractThe surface compositions of a Ni‐25 at% Fe single crystal (100) and (110) were analysed by ESCA with respect to the electrochemical behaviour in 0.05 M H2SO4 (anodic dissolution and passivation). When the alloy surface is exposed to the electrolyte solution, or anodically polarized in the active region (anodic dissolution), iron is selectively dissolved and the surface is enriched with nickel. A stationary surface composition is attained which does not depend on the polarization potential or dissolution current. Once the stable surface composition is attained, the anodic dissolution of nickel and iron takes place with the same ratio as in the alloy. The extent of the surface enrichment with nickel, calculated from a layer by layer model for XPS signal intensities is found to be almost one monolayer of nickel at the alloy surface, each following plane in depth having the same composition as the bulk (Ni3Fe). The passive film is enriched with nickel. It consists or an inner layer of nickel oxide (NiO) and an outer layer of nickel and iron hydroxides. The thickness of the passive film is 8–10 Å. The alloy/oxide interface is also enriched with Ni in its metallic state.

Mechanism of crack decoration of oxides by electrodeposition

The cathodic behaviour of a duplex oxide on 9%Cr steel has been potentiostatically investigated in acidic plating baths based on copper sulphate or nickel sulphamate. Metallography and electron probe microanalysis of decorated specimens has shown that decoration from the copper plating bath involves cathodic dissolution of the outer layer oxide (magnetite). Thus the more stable inner layer of oxide is exposed at cracks and provides an effective site for copper deposition. It is proposed that decoration by nickel occurs by a similar mechanism, but is less readily achieved because magnetite is less readily dissolved in the sulphamate bath.

Diffusion and Partitioning of Elements in Oxide Scales on Alloys

FORMATION of duplex scales on oxidized ferrous alloys and segregation of the alloying elements, are well known. For example, Pfeil1 showed that most of the transition elements, with the exception of Mn, were concentrated within the inner layer of oxide formed on alloy steels in air at 1,000° C and these findings have since been substantiated and extended2–4. It seems that under oxidizing conditions, the alloying elements V, Cr, Mo, Ni, W and Si are concentrated in the oxide layer adjacent to the metal, while Fe and Mn are distributed throughout the double layer. Typical partitioning behaviour is shown in a scanning electron micrograph of a section through a Fe–Cr alloy oxidized at 600° C in CO2-based gas (Fig. 1). The Cr concentration determined from using electron probe analysis is superimposed; it shows that Cr is virtually absent (< 0.2 wt. %) from the outer oxide layer. X-ray diffraction studies established that both layers possess a spinel structure.

The influence of oxide stress on the breakaway oxidation of zircaloy-2

The stresses in oxide films on Zircaloy-2 and zirconium crystal bar were measured after oxidation in the temperature range 500–700 °C. The stresses in films on Zircaloy-2 reached a maximum at an oxide thickness approximately equal to that at which breakaway oxidation occurs. The stresses in films on zirconium crystal bar remained low, and breakaway did not occur under the conditions of the experiment. The results show that breakaway oxidation in Zircaloy-2 is probably due to the mechanical failure of a protective inner layer of oxide under the compressive stresses which arise during growth.

Metal-oxide interface morphology for a range of Fe-Cr alloys

Fe-Cr alloys containing 16, 19, 28 and 38%Cr all develop voids at the metal-oxide interfaceduring oxidation at 950°C for 6h, and the geometry of the interfaces are generally similar. However, there are significant differences in detail: the area of contact between metal and oxide decreases as the Cr content of the alloy increases, and a discontinuous inner layer of oxide has been observed for the two alloys of lower Cr content. It is suggested that the first variation is associated with oxidation rate and diffusion rate changes for these alloys, and that the second phenomenon is due to localized oxygen penetration through the main oxide layer.