conversion coating on fasteners

Conversion coatings form a basis for durable coating systems

Conversion coating is a chemical substance which reacts with metal substrates converting the surfaces into coating layers. In general, these coatings are suitable for ferrous and non-ferrous metals, yet the solutions differ from each other. There are two main reasons to use conversion coatings;

  • Protecting the metal from oxidation;
  • Guaranteeing adhesion of later coating systems on the substrate.

To ease your quest, we divide conversion coatings into steel conversion coating and chromate coating. In this article you will find general information on the types of conversion coatings as well as suppliers and services in the UK.

2 types of steel conversion coatings

There are two different types of conversion coatings suitable for steel which is the most common type of ferrous metal.

1. Black oxide for looks and protection

A steel screw with black oxide conversion coating

A screw with black oxide conversion coating finished with a layer of matt paint.

Black oxide, also known as metal blackening, is a very thin layer conversion coating which prevents steel from rusting and creates a surface where other coatings can adhere well. The protective layer is formed by the chemical substance, reacting with the iron in the steel. The ingredients that cause the reaction are oxidising alkaline salts, which form magnetite on the surface. Magnetite is also know as the black oxide of iron. 

Black oxidising is possible either in room temperature or as hot black oxidising. It is good to know that the hot dipping results in a more durable finish and is also approved by the military and automotive industries unlike cold black oxidising.

After converting the surface into a layer of black oxide, you should apply a finish, to protect the oxide layer. The finish can be a wax, oil, paint or lacquer. In the UK, you can let black oxide conversion coating applied in the following companies:

Metal Treatments (Portsmouth) Ltd.Unit 12, Limberline Spur, Hilsea, Portsmouth, Hampshire, PO3 5HJ
Alderney platingBroom Road, Parkstone, Poole, Dorset BH12 4NP
Chingford Technical Coatings LtdDenmark Street, Maidenhead, Berkshire, SL6 7BN

In case you would like more information on black oxide conversion coatings, or additional company details, do not hesitate to contact us. Our coating experts are happy to help you define whether black oxide is suitable for your products, and where you can get the best service for your needs.

2. Phosphating increases rust inhibition

Another common steel conversion coating is called phosphating. It can be applied for three reasons namely; corrosion protection, adhesion of further coatings or lubrication of components. When it comes to resisting oxidation, phosphating is a more effective rust retardant than black oxide.

For further information on this type of conversion coating, please move to our article dedicated to phosphate coating.

Chromate conversion coating for non-ferrous metals

chromate conversion coating on drills

Chromate conversion coating often has a slightly yellow colour.

The chromate coatings are applicable only on non-ferrous metal surfaces such as aluminium, zinc, magnesium, tin, silver, copper and cadmium alloys. Steel is a ferrous metal, which is why chromate coatings are not suitable for applications on bare steel; however, chromating galvanized or phosphated steel is rather common. 

Generally, chromate coatings contain chromic acid, also referred to as hexavalent acid, which creates unique protective and decorative properties on non-ferrous metals. However, due to the high toxicity of hexavalent acid, the use and application of chormate conversion coatings is strictly controlled and regulated. There are nonetheless, alternative substances for hexavalent acid such as trivalent chromium compounds.

Chromating is suitable for treating general hardware and tools; it is applicable as:

  • Primer;
  • Corrosion inhibitor;
  • Decorative finish;
  • Electrical conductivity increasing coating.

Chromate coatings and applicators

In the UK the most common chromate coating brands are Iridite for aluminium and Alocrom  (a.k.a. Alodine) for aluminium and magnesium chromating.

For example the following companies apply chromate coatings in the UK.

Surface TechnologyLyons Park, Coventry, CV5 9PF (Locations also in Leeds, Aberdeen, East Kilbride and Slough)
Metro Aluminium Treatment SpecialistEskdale Road, Uxbridge, Middlesex UB8 2RT
Metal Finishing LtdWhittle Road, Wimborne, Dorset BH21 7RL

If you are not sure whether one of these companies is suitable for chromating your products, or you would like to request a quote; do not hesitate to contact us. We are happy to answer all your requests regarding conversion coatings.