Phosphate coating, the most common pretreatment for metal
Pretreatment is one of the key components of a successful coating system: especially when coating metal. Each type of metal and types of topcoats require different kind of surface preparation and pretreatment to function optimally. The most common pretreatment for metal is phosphate coating which in a chemical process converts the metal surface into a thin layer of coating. The chemical reaction takes place between the metal (most often steel) and a phosphating agent which contains phosphoric acid. The chemical substance creates a thin coating layer (0,002 – 0,006 microns) on to the metal.
Phosphate coatings are almost exclusively used as pretreatment for preparing steel and iron substrates for further coating. The pretreatment has three main purposes:
- Corrosion inhibitor – phosphating coating offers electric isolation and separates anodic and cathodic parts from each other, which prevents under-film corrosion
- Lubricant base layer – phosphate coating has also a lubricating function especially in machine parts and components
- Adhesion primer – most phosphating services focus on using phosphate coating as a primer that promotes adhesion of the further coating layers
In this article we will introduce the three types of phosphate coating, their uses and suitability for different projects, as well as providing an overview of phosphating services in the UK.
3 types of phosphate coating and their uses
Phosphate coatings prepare steel and iron surfaces for other coating layers such as powder coatings. However, there are three types of phosphating agents: manganese, zinc and iron, which all have preferred substrates, coatings and end uses. Therefore, those are the factors you should base the choice of phosphate coating on. Here below we will take a look at the three types and outline their uses and when you should choose for each of them.
1. Manganese phosphating coating – The toughest
Manganese phosphating coating is the hardest, most rust retardant and abrasion resistant phosphate coating. They form a crystalline absorptive surface and are ideal for substrates that need to be resistant to abrasion such as bearings, bushings, thrust washers and fasteners. Manganese phosphate coating agent also lubricates the treated surface, making it a suitable phosphate coating for the automotive industry: sliding and moving engine and transmission parts benefit from the lubricating effect of manganese.
Benefits: corrosion resistance, friction reduction and uniform, smooth and thin film
Topcoats: Manganese phosphating coating does not require a topcoat, it is a corrosion resistant anti friction layer itself. The coating is often finished with a wax or oil to enhance the properties.
Application methods: immersion
2. Zinc phosphating – the most common
Zinc phosphate coating is the most common of all phosphating pretreatment methods for steel and iron. It is not as corrosion resistant or lubricating as manganese, but those properties can be enhanced by different topcoats. Zn phosphating coating agent creates a much lighter coating than manganese and is therefore, beneficial for projects where the metal’s weight must not be affected.
Zinc phosphate coating is also the most common solution for priming metal for powder coatings after the substrate has been sandblasted. It is slightly more expensive than iron phosphating, but it also provides better quality for longer. Zinc phosphating is the best possible primer for powders, next to galvanising. Zinc phosphated metals are also suitable for liquid paints.
Benefits: corrosion resistance (in combination with other coatings), seamless, smooth and thin film.
Topcoats: layers of oils, anti corrosion wax or lubricants (such as fluoropolymers) different kind of powder coating
Application methods: immersion and spray
3. Iron phosphate coating – the most economical
Steel, zinc and aluminium are all suitable substrates for iron phosphate coating. This is the most flexible option when it comes to the type of substrate. It is also the most economical: the iron phosphating price is clearly lower than the costs of zinc and manganese phosphating services.
Iron phosphating, like Zn, is a common primer for powder coating. However, it is by far not as durable even though corrosion resistance and adhesion are close to each other. Therefore, iron phosphate coating is only recommended for projects that do not require extremely high quality and must be realised within a low budget. In other words, it is not suitable for most offshore and marine projects that are continuously exposed to the elements.
Benefits: the cheapest when it comes to phosphate coating price, degreases normally oily surfaces, operates at low temperature, low maintenance costs
Topcoats: powder coatings, liquid paints
Application methods: immersion and spray
Phosphating services in the UK – Iron, Zinc and Manganese
As phosphate coating is applied as a chemical process, it is usually better to rely on professional phosphating services. Fortunately, there are plenty of them in the UK. Here below you can find a brief overview of some of the phosphating services in the country.
|TYPE(S) OF TREATMENT||PHOSPHATE COATING COMPANY||ADDRESS|
|Zinc and manganese||Ashton & Moore Limited||Smith Street, Hockley, Birmingham B119 3EX|
|Zinc and manganese||Surface Technology||Lyons Park, Coventry, CV 9PF|
|Iron||Rodman Brothers||Tariff Road, Tottenham, London N17 0DY|
|Iron, manganese and zinc||Stovrit Special Coatings||26-28 Baltic Road, Felling, Tyne&Wear, NE10 0SB|
|Zinc||Nexus Surface Treatments||Spurryhillock Industrial Estate, Broomhill Road, Stonehaven, AB39 2NH|
|Zinc||Sherardize UK Ltd.||Bridge Street, Wigan, Greater Manchester, WN3 4EY|
If you are in need of any further assistance regarding phosphate coatings, do not hesitate to contact us! We are experts in finding the best coating solution for any project!