The ability of chromates or bichromates to inhibit the corrosion of ferrous and non-ferrous metals, through the formation of a thin passivation layer on the metal surface, has been well-known for a long time.
With regard to the electrochemical interpretation of this phenomenon, it is known that the electrode potential of a passivated layer is shifted toward increasing valuesof the electromotive forces and this leads, by a logical consequence, to increased corrosion resistance, both in an aqueous environment and in moderately corrosive atmospheres.
Conversion coatings, and, in particular, chromating , can be obtained on zinc, cadmium, aluminium, copper, brass, bronze, silver, and magnesium by simple immersion, brushing, or spraying with a suitable solution containing hexavalent chromium.
The resulting conversion layers are very thin (0.5 microns thick, or even less) and not only exert fair protection against corrosion, but they also establish a relationship that is frequently necessary to ensure adhesion to the metal of organic finishing layers, such as varnishes or paints. The first chromating process ever carried out was the M.B.V. process ( = Modifizierte BAUER-VOGEL Verfharen, or modified BAUER-VOGEL process) and it can be applied to pure aluminium and to a few special alloys.
This type of chromating, like all similar treatments on aluminium, goes under the name of “chemical oxidation of aluminium”, because the procedure is based on the same principles as the oxidation and, that is, on the high reactivity of aluminium and on the compactness of its reaction products.
Other applications of chromium layers can be found in the decorative field. A few methods that make use of a chromating film to bond rubber to aluminium have also been developed; however, in these cases, also subsidiary organic binders are needed.
An electroless chromating bath consists in all cases in a solution that includes two fundamental components:
- Hexavalent chromium compounds, in the form of chromic acid, chromates, and bichromates, or any combinations of these components;
- Organic and inorganic compounds that are known activators or catalysts;
Usually, chromating takes place by simple immersion, but, sometimes, electrochemical processes can also be conveniently used, which include the anodic treatment of the piece in the same type of solutions described.
The pH factors, concentration of activators, must be constantly monitored for consistent results. The film is formed by reaction between the hexavalent chromium and the surface of the metal; the latter reduces the chromium to the trivalent form, which precipitates as a gel complex (chromium chromate) when the pH on the metal surface has reached a suitable value following the reaction.
The film produced is amorphous; in the wet state it shows no diffraction image when X-rayed and its colour depends on the initial pH of the solution, the duration of the immersion, the composition of the bath, and, especially, the concentration of the activating radicals. The use of highly acidic solutions on electroplated zinc and cadmium , on brass, copper and bronze, leads to the formation of very thin films with a moderate protective value. With higher PH, iridescent films are obtained, which are thicker and have a higher protective value.
On zinc and cadmium, olive or khaki coloured films give greater protection than transparent films: this is mainly due to the higher thickness and the different composition of the chromating bath. The darkening of the colour usually corresponds to the increased thickness of the chemical conversion layer, also on aluminium.
As soon as it is formed, the film has a gelatinous, soft appearance; after drying, either air-dried slowly at room temperature, or by heating at 50-65 °C, it becomes anhydrous and flexible and reveals a crystalline structure when X-rayed.
It is not yet possible to determine whether this is an actual crystallization process, or if in the hydrated film there is already a microcrystalline structure that cannot be detectable by X-ray.
The film may also contain, but not necessarily, the cations of the base metal, especially if the chromating bath is not very acidic and if the metal chromate is not very soluble.
When the film is fresh, its components, or at least part of them, are quite soluble.
Hexavalent chromium can be bleached quite easily by treatment with alkaline solutions, whether cold or hot, or even, more simply, but also more slowly, with hot or cold water. Trivalent chromium, instead, cannot be dissolved. To the ability of this layer to slowly transfer its hexavalent chromium and inhibitor content, even when exposed to atmospheric moisture, is in large part due the protective value of the layer itself, following logically to an increase of the local potential.
Not all of the protective value of the conversion layer is, however, due to the soluble chromate it contains, because even if this is removed by initial bleaching or rendered insoluble after drying at high temperature, a significant protective value remains on the resulting film.