Chromium underlayer

Magnetic film 200-500A Chromium underlayer 1000-2000A... 

A critical problem with adhesion layers arises from grain boundary diffusion. Deposited films tend to be polycrystalline and granular. The electrochemistry of the adhesion film is frequently much less desirable than the electrochemistry of the primary film. Moreover, minute contamination of the primary metal film surface by adhesion components can dramatically degrade the electron transfer properties (e.g., electrochemical reversibility, as evidenced by cyclic voltammetric peak potential separation) of the film [58], Thus it is essential that the adhesion layer is not exposed to solution. While the rate of diffusion of adhesion metals through the bulk of the primary layer is quite slow, grain boundary diffusion along the surfaces of grains is much faster. In many cases, the adhesion layer can seriously compromise the performance of the electrode. This is particularly a problem for chromium underlayers. Recently a codeposited Ti/W adhesion layer has been recommended as an alternative to chromium, with reportedly better adhesion and fewer interferences than Cr. A procedure was also described to recondition these electrodes to minimize interference by adhesion layer metals [58]. 

Nickel is also widely used as an electrodeposited underlay to chromium on chromium-plated articles, reinforcing the protection against corrosion provided by the thin chromium surface layer. Additionally the production of articles of complex shape to close dimensional tolerances in nickel by electroforming —a high-speed electrodeposition process —has attracted considerable interest. Electrodeposition of nickel and the properties of electro-deposited coatings containing nickel are dealt with in greater detail in Section 14.7. 

The two metallizations most commonly used to fabricate transducers on AW devices are gold-on-chromium and aluminum. Au is often chosen for chemical detection applications because of its inertness and resistance to corrosion a layer 100-200 nm diick is necessary to provide adequate electrical conductivity. Unfortunately, the inertness of Au also prevents its adhesion to quartz and other oxides utilized for AW device substrates. Therefore, an underlayer of Cr (2-10 nm thick) is utilized to promote the adhesion of Au to the substrate the electropositive (reactive) nature of Cr allows it to form strcxig bonds with oxide surfaces, while alloying between the Cr and Au chemically binds the two metal layers... 

Nickel coatings are usually prepared by electroplating. The metal is plated either directly on steel or sometimes over an intermediate coating of copper. The copper underlayer is used to facilitate buffing of the surface on which nickel is plated, because copper is softer than steel, and also to reduce the required thickness of nickel (which costs more than copper) for obtaining a coating of minimum porosity. The automotive industry uses nickel as an underlayer for microcracked chromium to protect steel [9]. 

On the scale of potentials, zinc is more electronegative than aluminium, while cadmium has a potential very close to that of aluminium. Galvanised or cadmium-coated steel fasteners can, therefore, be used to join and assemble stmctures made from aluminium alloys. It should just be remembered that when these coatings become too worn to protect the steel and the aluminium, the previous scenario applies in which there is contact between the aluminium alloy and bare steel. Chromium-plated steel does not lead to galvanic corrosion with aluminium, as long as chromium covers the nickel underlayer however, the contact between aluminium and the nickel underlayer would lead to galvanic corrosion of aluminium. 

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