Surfaces passive oxide films

Summary. Scanning tunneling microscopy (STM) provides new possibilities to explore the link between the structure and the properties of thin oxide overlayers (passive films) formed electrochemically on well-defined metal surfaces. Passive oxide films protect many metals and alloys against corrosion. A better understanding of the growth mechanisms, the stability, and the degradation of passive films requires precise structural data. Recently, new results on the atomic structure of passive films have been obtained by STM. The important questions of crystallinity, epitaxy and the nature of defects have been addressed. Data on the structure of passive films on Ni, Cr, Fe, Al, and Fe-Cr alloys are reviewed with enq>hasis on atomically resolved structures. Ihe perspectives of future developments are discussed. 

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

Formation of the first layer (a monolayer) of passivating oxide film on a denuded metal surface occurs very simply by the loss of protons from the adsorbed intermediate oxidation products, such intermediates being common to both dissolution and passivation processes . Thus for example, the first oxidative step in the anodic oxidation of nickel is the formation of the unstable adsorbed intermediate NiOH by... 

This work has been carried out by Marcus and his co-workersand deals with the influence of sulphur on the passivation of Ni-Fe alloys. For sulphur-containing Ni-Fe alloys, sulphur segregates on the surface during anodic dissolution. Above a critical sulphur content a non-protective thin sulphide film is formed on the surface instead of the passive oxide film. 

Thus inhibitive anions can retard the dissolution of both the T-FejO, and the magnetite layers of the passivating oxide layer on iron. This has the dual effect of preventing breakdown of an existing oxide film and also of facilitating the formation of a passivating oxide film on an active iron surface, as discussed in the previous section. 

Passivation is a necessary and natural initial corrosion process that occurs on all hot waterside surfaces. It is the conversion of a reactive metal surface into a lower energy state that does not readily further react or corrode, and it involves the development of a passive oxide film on a clean surface. 

Almost all metallic materials in practical environments perform their service in the state of spontaneous passivation, in which hydrated oxygen moleciiles or hydrogen ions act as oxidants to passivate the surfaces. Stainless steel is a good and widely known example of corrosion resistant metals it is spontaneously passivated and remains in the passive state with a thin passive oxide film even in fairly corrosive environments. 

An interesting example of the kinetic effect in semiconductor photocorrosion is photopassivation and photoactivation of silicon (Izidinov et al., 1962). Silicon is an electronegative element, so it should be dissolved spontaneously and intensively in water with hydrogen evolution. But in most of aqueous solutions the surface of silicon is covered with a nonporous passivating oxide film, which protects it from corrosion. The anodic polarization curve of silicon (dashed line in Fig. 20) is of the form characteristic of electrodes liable to passivation as the potential increases, the anodic current first grows (the... 

Fig. 18. Schematic diagram for a binary alloy with a passivating oxide film in contact to electrolyte with the reactions of (1) oxide formation, (2) electron transfer, and (3) corrosion, including (4) oxidation of lower-valent cations and the indication of ionic and atomic fractions X as variables for the composition of the layer and the metals surface.

Investigation of the effect of particulate properties during CMP of W showed a significant increase in the polish rate in the presence of ferric nitrate compared to the polish rate in de-ionized water, at all alumina bulk density values (shown in figure 4). Kaufmann et al. , attributed the increase in the polish rate in the presence of ferric nitrate to the "softness" of the passivating oxide film compared to W. Potentiodynamic experiments and open circuit potential measured as a function of time indicate passivation of W surface. However, the hardness values of tungsten films exposed for 5 min to 0.1 M ferric nitrate, even at the lowest load (300p,N), were the same as those of as-deposited W films within experimental error. Since a 10 nm indentation depth was observed at the lowest load, it is possible that the thickness of the oxide film is smaller and its effect does not manifest itself on the hardness measurement. 

Many models have been proposed for the siUcon anisotropic etching in alkaline solutions. They can be classified into two large groups those that attribute the relatively slower etch rate of (111) planes to the presence of a passive oxide film on the surface, and those that consider the etch rate difference among different orientations being governed by the reaction kinetics. 

Oxidants. Oxidizing substances added or present in the water restore high electrode potential and prevent the reductive dissolution of the passive oxide film. Oxidants that are specifically adsorbed, such as chromate and nitrite, are especially efficient. Both may form binuclear surface complexes. 

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