The term ‘chromating’ is a contraction of Chromate Conversion Coating, which itself is a little misleading. They are not typical coatings in the sense we would normally define as a separate layer which is clearly distinguishable from the surface below. Chromate conversion processes chemically alter the surface to which they are applied, and the conversion coating becomes an integral part of the surface. The chromated surface is formed by a chemical reaction between the substrate and the chromating chemicals.
The main materials which are directly subjected to chromate conversion processes are aluminium and magnesium alloys. Magnesium is a very reactive material, and the chromating treatment on these materials is not generally held to give any useful degree of protection against corrosion; it is generally used as a pre-treatment for a further process such as painting. Chromate processes are also used as a post-plating treatment on steels. When you hear people talk about ‘yellow zinc plating’, this is actually a zinc-plated component that has subsequently been chromated.
On aluminium alloys, which aren’t as reactive as magnesium, chromate treatments can offer a degree of protection against corrosion. There are various different processes, but a number of these also go under different trade names. Alodine, Iridite and Alocrom are some common examples, and some of the processes marketed under these names are actually equivalent and thus interchangeable. The chromate processes are often used in conjunction with other surface treatment procedures, with the chromate process forming a base layer on the material for subsequent processes such as painting and powder coating. In a racing powertrain, such duplex processes are rarely required, but the chromate conversion processes are often used alone to provide a degree of corrosion protection for vulnerable parts.
Although chromate conversion processes are generally undertaken commercially, the process is one which can be done relatively easily in any workshop. The chromating chemicals are based on chromic acid. There are a number of ways to carry out the process, with a simple dipping treatment being one of the most common, although there are application methods which suit in-situ processing or surface repair, including brushing on the reagents or using special fibre-tipped ‘pens’ that dispense a pre-mixed chromating chemical.
The application of chromate treatments to plated steel parts is to prevent corrosion of the plating. In this instance it is known as a chromate passivation treatment. As we might guess from the word ‘passivation’, the aim is to make the surface of the plating passive, or less reactive, protecting the surface from atmospheric corrosion.
Zinc is a sacrificial coating, and as we can often see from zinc roofing sheet - which is actually corrugated steel which has been coated with zinc - it soon loses its lustrous sheen as the zinc is attacked. The principle of sacrificial corrosion prevention is that the substrate is not attacked while it is in electrical contact with a more reactive element. Once the sacrificial material has been ‘used up’, the substrate will corrode. So, in preventing the rapid oxidation of the zinc plating, we extend the life of the plating and the substrate.
Written by Wayne Ward