Knowledge Bank

Etch Primer and Chromate Conversion

What is an Etch primer ?
Etch Primer is a paint that is designed to physically bond itself to the substrate to which it is applied. This is achieved by combining an acid with the paint so that the acid microscopically etches the surface of the substrate, thus forming a physical and chemical bond between the two. In most cases a colour will be added to the Etch Primer so that it can be seen.

2 Pack Etch Primers
As the name suggests 2 Pack Etch Primers (or 2 Part Etch Primers)are supplied in two parts - the Primer and an Activated thinner. It is the activated thinner that contains the acid and thus causes the reaction in the primer.

You can apply the Etch Primer to the substrate, only the thinnest coat is required and a second coat is not necesary. Etch Primers only start to work after they are dry. It is therefore beneficial to only use the thinnest coat possible. Once applied the Etch Primer needs to be left to dry, both to allow the etching process to work and to allow the evaporation of the solvents contained within the paint. All Etch Primers need to be left for a minimum of 24 hours at a temperature of 18 - 20 degrees Celsius to dry properly. Whilst this is the minimum it is advisable to allow significantly longer before applying topcoats. More paint finishes are ruined by the application of paint over Etch Primer that is still drying than almost anything else.

Chromates - what are they used for and why?
Chromates (salts containing hexavalent chromium in the chemistry Metal-CrO3) have the unusual property of affording corrosion protection even when scratched or damaged. They do this by going into solution in the corrosive environment, migrating to the exposed bare metal surface, and forming complexes to inhibit further corrosion. In addition, chromates often form good bonding layers, improving the bond between a paint system and a surface, which also helps to improve corrosion resistance. Consequently chromates are widely used as corrosion inhibitors and passivates in the aircraft and defense industries, and are used wherever corrosion is a serious concern, especially where damage to the surface coating is quite likely. Chromates are also used for chromic acid anodizing (CAA) and etching. CAA is used to create a good bonding surface (because of its roughness and porosity) and as an anodized for components with liquid entrapment areas where acids can be trapped and sulfuric or other acids cause etching. Examples include:

  • Chromate conversion coatings for metal surfaces.
  • Aluminum alloys are typically chromate converted to resist chloride attack in marine environments.
  • Sacrificial corrosion resistant coatings on steels (such as Cd, Zn, Al) are frequently chromate converted to supplement the corrosion resistance of the sacrificial coating. This type of chromate treatment is often termed a "sealer".
  • Phosphate passivation coatings on steels are typically chromate sealed.
  • Anodized layers on Al alloys are usually chromate passivated.
  • Mg alloys used in aircraft gearboxes.
  • Primers and paints.
  • Al alloy surfaces are commonly chromated for corrosion resistance and to enhance paint adhesion.
  • Chromate conversion is the most common treatment prior to painting aircraft.
  • Aircraft primers usually contain chromates so that if scratched they will continue to protect the surface. Corrosion is the biggest problem with aircraft frames and skins.

Chromate conversion is a good passivator and adhesion promoter, It needs a good primer and topcoat on it. More popular in Aerospace industry for Non Ferrous metals. Etch Primer is also a passivator and adhesion promoter by acid etching of the metal. It also functions as a primer by having anticorrosive pigments in it. More popular in General Industry for both Ferrous and Non Ferrous Metals.