Thin oxide film formation, metal chromium

Corrosion may be defined as the chemical reaction of a solid with its environment fl]. Such reactions may or may not be detrimental to the solid. Iron metal, for example, will react with a moist-air atmosphere to produce an orange rust this reaction will continue indefinitely with eventual destruction of the solid. On the other hand, exposure of chromium metal (or even a chromium-containing alloy) to a similar environment will result in formation of a very thin oxide film (invisible to the eye) that protects the active metal from further deterioration. Both are corrosion reactions, but only one is destructive. 

The initial reaction results in the formation of a continuous film of oxide that is firmly attached to the metal surface. The rate of growth of the film is controlled by the slow diffusion of the Cu ions. However, no corrosion could occur without the transport of electrons, as the mechanism depends on electron transport. The electronic conductivity of the film is therefore of major importance. The reason why both aluminium and chromium appear to be corrosion-resistant lies in the fact that, although oxide films form very rapidly in air, the films are insulators and prevent reaction from continuing. As the thin films are also transparent, the metals do not lose their shiny appearance. 

For metals such as chromium and alloys such as stainless steel, the plot of potential versus corrosion rate above the range is shown in Figure 20.67. Figure 20.68 shows a sudden sharp drop in corrosion above some critical potential. Despite a high level of anode polarization above V, the corrosion rate drops precipitously due to the formation of a thin, protective oxide film as a barrier to the anodic dissolution reaction. Resistance to corrosion above is termed passivity. The drop in corrosion rate above can be as much as 10 to 10 times below the maximum rate in the active state. With increasing corrosion potential, the low corrosion rate remains constant until at a relatively high potential the passive film break down, and the normal increase in corrosion rate resumes in a transpassive region. 

Conversion coating processes produce a thin film of predominantly chromium oxide on metal surfaces. The colour of this film depends on the substrate metal, and may vary in colour, from pale-yellow to gold to dark-brown or black. Today, the most commonly used CCC process for aluminium, zinc and cadmium (Biestek and Weber 1976) is an acid treatment (pH 1—2), based on a two-part solution containing a source of hexavalent chromium ion, e.g. chromate, dichromate or chromic acid. The solution for treating aluminium alloys, generally contains fluoride ion, which assists in the dissolution of the original oxide film, and an accelerator, e.g. ferricyanide, to facilitate the formation of the chromium oxide (Biestek and Weber 1976). 

Another problem in the construction of these devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the electronic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of the device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a barrier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the A1203 layer, aluminium metal can reduce Si02 at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a thin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of chromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

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