Oxide films penetration

Probably the most important powder property governing the formation of atomic bonds is the surface condition of the particles, especially with respect to the presence of oxide films. If heavy oxide layers are present, they must be penetrated by projections on the particles. This results in only local rather than widespread bonding. A ductile metal such as iron which has a heavy oxide layer may not form as strong or as many bonds as a less ductile metal. 

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... 

Strazzi has reviewed methods of sealing oxide films and Omata etal. find that adhesion of paint films to anodised layers depends on penetration... 

The high reactivity of titanium metal is disguised in normal use by its unusually corrosion-resistant protective oxide film (Chapter 16), but it must always be borne in mind by users of Ti process or laboratory equipment. In particular, titanium should not be used with pressurized oxygen pressures of oxygen of as little as 0.4 MPa have been reported to lead to ignition of Ti at ambient temperature if the oxide film is penetrated. 

The intensities of the integrated signals may be evaluated on the basis of well-characterized standards. Consequently ISS provides qualitative and quantitative information on the composition of the surface. Noble gas ions that penetrate the first layers of the surface are backscattered as neutrals, and thus may not pass the energy analyzer. As a consequence, only ions backscattered at the first atomic layer are detected and the method is sampling the outmost atomic layer. A soft sputter process by noble gas ions yields an ISS depth profile with atomic depth resolution. Therefore ISS has been applied to the study of very thin oxide films, as e.g. of passivated Fe/Cr alloys. This method may be applied in addition to XPS due to its high depth resolution. 

The effect of the addition of chlorides to the electrolyte which sometimes hinders the passivity of metal is explained in the following manner chloride ions can penetrate through some oxide films and make them more porous, i. e.,... 

It must be emphasized that when high purity magnesium is converted into turnings, chips or powder, special precautions have to be taken in order to avoid contamination [2]. It should also be noted that magnesium surfaces, exposed to air, rapidly become covered with an oxide film. Methods for activating magnesium (see below) usually depend on the removal or penetration of this surface film. 

The fluxes used when brazing are alkali halide and borate mixtures and compounds, and they have two main functions first, to dissolve the oxide film on the component surface or at least to degrade its adhesion by penetration of naturally occurring flaws and electrolytic action at the oxide-substrate interface, and secondly to prevent formation on the liquid surface of oxide skins which would restrict braze flow. Fluxes can be contained in a bath held at the brazing temperature in which the, usually aluminium, component is placed or else applied as a paste to surfaces of the component or braze. 

It was mentioned above that a chromium anode continues to dissolve in the passive state forming chromate ions in this case the invisible oxide film may be suflSciently porous to allow ions to penetrate it alternatively, the oxide film may become oxidized to C1O3 which dissolves to form chromate, but is immediately regenerated by oxidation of the anode. 

The etch rate of thermally grown oxide may be altered by postoxidation treatments. Ion implantation at a certain level, depending on the nature of damage and distribution, has been found to increase the etch rate. Also, an electric field, by applying an anodic potential onto a thermally grown oxide film, is able to inject hydroxyls into the oxide. The etch rate of the hydroxyl penetrated region, depending on the hydroxyl concentration, is much faster than the unaffected thermal oxide... 

Corrosion. Aluminum is a not a noble metal and is attacked by both alkali and acidic solutions. Because of the presence of a surface A1203 film, the metal is protected against corrosion [Diggle et al.136, Borgmann et al.137]. This oxide film, however, is easily penetrated, for instance, by the presence of chlorine ions which remain in the resist after a chlorine based plasma etch. Also, the presence of Cu in the aluminum weakens the corrosion resistance of the alloy by the presence of an unfavorable electrochemical couple (A1/Cu2+). 

Exposed to an unlimited supply of gas phase particles characterized by the applied pressures and temperature, the surface will adapt on time scales set by the kinetic limitations. Already these time scales could be sufficiently long to render corresponding metastable states interesting for applications. In fact, the classic example is a slow thickening of oxide films due to limitations in the diffusion of oxygen atoms from the surface to the oxide-metal interface or in the diffusion of metal atoms from the interface to the surface [37,38]. Directly at the surface a similar bottleneck can be the penetration of oxygen, which... 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

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