Coating systems in which chromium-6 is used have very good properties. As with many regular coating systems, coatings containing chromium-6 consist of an adhesive primer, a base (chromium-6 containing) primer and a topcoat. Chromium-6 compounds ensure that the coating is self-healing. As soon as mechanical damage occurs and the barium of both the topcoat and primer is broken and water enters, chromate ions come to the rescue and prevent the base material to be protected from dissolving and/or being converted to its oxidised form.
The blank starting material of steel (Fe) or aluminium (Al) attracts chromate ions. A very thin layer is formed, sometimes only one molecule thick. This causes passivation to occur and no electrons can be transferred from the base material to the environment. Corrosion processes are thus stopped. The interesting thing about Chromium-6 is that it works again and again. Each time damage occurs, chromate ions come to the rescue, until all chromates are dissolved.
Application of Cr(VI) based coating systems
Applying the coating works as follows: First, a very thin pre-treatment layer is applied to the metal that is to be protected. This layer is only a few micrometers thick. This is applied to allow the subsequent layers to adhere to the metal.
On top of this adhesive primer, the layer containing the anti-corrosion functionality is applied: the primer, which is usually about 25 micrometres thick. And on top of that the so-called topcoat is applied, which is at least 30 micrometers or more. The primer contains a chromium-6 salt. There are many different possibilities and each manufacturer makes its own choice for a specific Chromium-6 salt. Examples of salts that are added to the primer are strontium chromate or barium chromate.
In which applications do you find chromium-6 salts?
Chromium-6 salts are used in many different areas of industry. Not only for coatings, but also for the preservation of wood. During the welding process of alloyed steels, conversions can take place which result in the release of chromium-6 compounds into the atmosphere.
Industrial coatings containing chromium-6 salts have been applied to, for example:
- Aluminium aircraft components
- The exterior of transport pipelines, storage tanks
- Offshore constructions
- Hulls or other parts of vessels
- Public means of transport such as trains and buses
- Civil infrastructure such as bridges and dams
Is chromium-6 (still) added to coatings?
Chromium-6 salts are a very good protection against corrosion as soon as coating damage occurs. It is therefore a good insurance policy for long-term protection against corrosive environmental conditions.
There are still industrial coatings available on the market in which chromium-6 salts have been added. Our estimate is that approximately 40% of the industrial coatings applied worldwide still contain chromium-6 compounds. This is precisely because of the excellent corrosion resistant properties of chromium-6.
There is a lot of research going on to find alternative components that can be added to primers to approach these excellent self-healing properties. There are two relevant developments underway at the moment. A coating in which magnesium is added is being developed. In this, magnesium combines with oxygen.
A second direction is the addition of lithium to the primer. This is a promising development, in which the addition of lithium leads to approximately the same effect as chromium-6 salts.
NOTE: In many cases, chromium-6 salts are not mentioned separately on a material safety data sheet (MSDS). This does not, therefore, provide a basis for assessing whether Chromium-6 has been added to the coating product. It is suspected that chromium-6 is added to coating products on a much larger scale than is currently known. Only an analysis (or several) will give a good indication of whether or not chromium-6 salts have been added.