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Chromating applications

It would be wrong to say that conversion treatments such as chromating aren’t used in race engines, but they certainly aren’t used as widely as on race transmissions. One of its main uses in engine parts is the passivation of components with metallic platings such as zinc or cadmium. Chromating on top of these metallic platings prevents early oxidation of the plating – zinc itself, for example, is highly reactive.

Plating works by using zinc, say, as a sacrificial coating, the oxygen in the air preferentially reacting with the zinc rather than the aluminium or steel substrate. The chromate layer is highly tenacious, and can significantly increase the time it takes for the substrate to become corroded.

Chromate treatments are also used as an ‘undercoat’ for other processes on transmission casings. Magnesium, for example, is very often chromated as a minimum to prevent rapid corrosion. Although chromate coatings in isolation aren’t particularly good at preventing corrosion of magnesium, they are better than nothing and also better than phosphate treatments, both in preventing corrosion and acting as an undercoat.

Chromate treatments do a good job of ‘tidying’ magnesium castings – a new coating of a ‘black’ chromate can make an old casting look much ‘fresher’. This process was reasonably popular when bespoke race engines still widely used magnesium, and some historic F3000 and Formula One engines still sport chromate-treated magnesium cam covers.

The ‘black’ chromating treatment can be anything from a dark brown to black depending on the substrate. The two main chemical solutions used are based on sodium chromate with additions of manganese or ammonium sulphates. Neither process removes any material, so they are suitable for components with tight tolerances on machined bores, and are therefore excellent for new components as well as restoration work.

A more aggressive chromate treatment is based on a more acidic solution, again using sodium chromate but with additions of nitric acid. This is a very much faster process and results in a golden-yellow colour on magnesium. However, it can remove small amounts of material (up to 0.025 mm/0.001 in) so it’s not suitable for bores and other features with tight tolerances on finished size.

Magnesium still remains a popular material for transmission casings in racing, and chromate coatings therefore remain widespread, although they are often combined with other treatments such as painting or resin coatings, since chromate-treated magnesium offers increased adhesion in these applications. This is a real synergy as, if properly applied, the combined coating offers a much longer period of time before significant substrate corrosion is seen than if either the chromate or the paint/resin coating was used in isolation. However, some magnesium materials – including some high-strength wrought magnesium alloys – do not respond to chromate coatings.

Despite the FIA having seen fit to ban magnesium for engine components in Formula One, there are still tens of thousands of new race engines with all manner of magnesium components. Since many race engines are based on production units, there seems little point in replacing the highly engineered, lightweight magnesium components that have routinely been used for well over 25 years – production motorcycles in particular use magnesium cam covers, clutch covers, generator covers and so on. The car manufacturers have not been oblivious to this, and also use the material where it offers an advantage, and very often these components are chromate-treated and painted. 

Written by Wayne Ward

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