Duplex oxide layers

Fig. 24. Potentiodynamic polarization curve of Cu in 0.1 M KOH with anodic and cathodic current peaks and the related reactions of oxide formation or reduction dissolution of cations and the indication of the stability ranges of the CU2O and duplex oxide layer, z ph at CII indicates oscillating photocurrent due to a chopped light beam [86],...

This duplex oxide layer is constituted of a Fe-Cr spinel oxide layer in the case of Fe-Cr steels and a Fe-Cr-Ni spinel oxide layer in the case of Fe-Cr-Ni steels which is in contact with the steel. Above this layer, a porous magnetite layer is observed which is in contact with the liquid alloy. Both layers have similar thicknesses and the interface between them corresponds to the original interface steel/Pb-(Bi). 

Hard plating is noted for its excellent hardness, wear resistance, and low coefficient of friction. Decorative plating retains its brilliance because air exposure immediately forms a thin, invisible protective oxide film. The chromium is not appHed directiy to the surface of the base metal but rather over a nickel (see Nickel and nickel alloys) plate, which in turn is laid over a copper (qv) plate. Because the chromium plate is not free of cracks, pores, and similar imperfections, the intermediate nickel layer must provide the basic protection. Indeed, optimum performance is obtained when a controlled but high density (40—80 microcrack intersections per linear millimeter) of microcracks is achieved in the chromium lea ding to reduced local galvanic current density at the imperfections and increased cathode polarization. A duplex nickel layer containing small amounts of sulfur is generally used. In addition to... 

For Fe in steam, water vapour or COj below 570°C, a two-layered Fej04 layer is observed, the inner layer growing by Oj diffusion inwards. Similarly, Potter and Mann reported the formation of a duplex Fej04 layer during the oxidation of mild steel in steam between 300°C and 550°C. 

In the above considerations, the O/S interface was taken to be a clear-cut boundary between the oxide and the electrolyte. In reality, however, the outer part of the oxide is likely to be hydrated and penetrated by the electrolyte. Hence, the true O/S interface is likely to be withdrawn from the surface to a sufficient depth such that some oxide is left without any electric field imposed across it. This is especially true of thick porous oxide layers, but it can occur with compact layers as well. For example, Hurlen and Haug35 found a duplex film in acetate solution (pH 7-10), composed of a dry barrier-type part and a thicker hydrated part consisting of A1203 H20. Although the hydrated part becomes thinner with decreasing pH and seems to practically vanish at low pH, even a thickness of less than a nanometer is sufficient for the surface oxide to stay outside the electrochemical double layer. 

In conclusion, one can say that most anodic oxide films are of a duplex, or even triplex, character, with only the inner portion being composed of a pure anhydrous oxide. In the duplex films, the outer layer contains anions and often a degree of hydration. There could exist a third thin oxide layer at the surface, again with somewhat different properties, which may have a role in the kinetics of oxide growth. 

Photocurrent spectra of the CU2O / CuO,Cu(OH)2 duplex film suggest a smaller band gap of the CuO layer with respect to the value of Cu20 [97], However, the investigation of a simple anodic Cu(II) oxide layer was not possible. All trials to prepare this layer without a Q12O contribution were not successful. 

Initially, from to the duplex scale grows at a constant rate until time h, when the activity of the metal at the scale surface has fallen to the value given by identity in Equation (7.33b). After this point, only oxide is stable and can form on the scale surface. After time h, the duplex scale is covered with an outer layer of oxide only, which slows down the reaction rate. Consequently, the metal activity at the duplex-layer-oxide-layer interface rises, reflecting the lower rate of transfer of iron ions... 

Sometimes, depending on the system and conditions, the duplex oxide-sulphide layer will appear as a lamellar arrangement, as in the case of iron. " With nickel, however, the sulphide forms in a more massive, less organized arrangement. In either case, the sulphide probably forms as a continuous network to account for the high ionic conductivity of the duplex layer. 

According to Bowsher (1987), the reaction between CsOH vapor and stainless steel surfaces is characterized by the formation of both water-soluble and insoluble cesium-based reaction products it proceeds via a fast physisorption process which is followed by a slower diffusion of cesium into the metal oxide. The cesium is predominantly associated with the inner chromite layer of the duplex oxide which covers the stainless steel surface the results of these studies imply that cesium is present as isolated cations in the chromia layer. Beard et al. (1989) reported that the water-soluble fraction of deposited CsOH is progressively converted upon heating to the insoluble form. At temperatures above 1150 K a significant vaporization of cesium took place, identified tentatively as cesium chromate vapor (Cs2Cr04) however, a substantial fraction was retained in the oxide layer on the metal surface and was not vaporized until the steel began to melt. 

Simms and Little" have examined the early stages of scale growth on 2.25%Cr-l%Mo steel at 600°C in dry flowing oxygen. Between 1 and 22 h they found that a thick oxide layer spreads laterally over a thin oxide layer. After 50 h, no thin areas of oxide layer were left. Whiskers gradually developed on the outer surface and these were well defined after 100 h. Fracture sections of the oxide revealed that the thin scales were duplex whereas the thicker scale was triplex, with a middle layer of Fej04 which spread laterally with time. The authors concluded that the first phases to develop were fine equiaxed a-FejOj overlying a doped spinel. Later, nucleation and... 

During exposure in CO2, mild steels produce protective duplex scales (Fig. 3.15(a)), the interface between the two layers being the original metal surface. The zone where the inner oxide layer grows is observed in Fig. 3.15(b) through the presence of chromium, which does not diffuse during the oxidation process. 

Corrosion studies for COz-cooled reactors and for a S-CO2 Brayton cycle have shown that 9-12Cr steels, in contact with S-CO2 at temperatures above 4(X)°C, suffer from two simultaneous corrosion phenomena oxidation and carburization. They form a duplex oxide scale made of an outer magnetite/hematite layer and an ahnost-as-thick inner Fe-Cr-rich spinel oxide layer, as shown in Fig. 3.16 for exposure at 400°C and in Fig. 3.17 at550°C. 

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