Phosphate coating: the phosphating process for metals

Phosphate coating: corrosion protection for iron and steel

Phosphate coating is a chemical process used for iron, cast iron and metal alloys. It can be carried out in three different ways:

• zinc phosphating;
• manganese phosphating;
• black phosphating.

These three processes all have the common goal of making the treated surface more resistant to wear and corrosion, as well as favouring greater adhesion for later painting. A classic application of phosphating is found in the automotive industry, as corrosion protection for car bodies. Let’s learn how the three types of phosphating are applied.

Zinc phosphating

Developed following the UNI ISO 9717 standard, this treatment gives the surface a colour ranging from light to dark grey based on the type of base material. The coating, mainly composed of zinc and iron phosphates, has a thickness ranging from 5 to 10 microns depending on the specific need. The final phase generally consists in oiling, which aims to reinforce the corrosion protection properties.

Manganese phosphating

This chemical process involves the formation of a coating consisting of manganese and iron phosphates. The thickness varies from 3 to 10 microns. The purpose of this treatment is to develop corrosion protection and low friction. Materials subject to manganese phosphating take on a colour ranging from light to dark grey. The finish of the treatment consists in immersion in an oily compound (oil coating), which reduces corrosive oxidation.

Black phosphating

Like the previous method, this treatment maintains the characteristics, but is used specifically to completely and uniformly blacken the metal surface.

However, there is a technologically more advanced and more efficient solution for dealing with base materials.

Teflon PFA and PTFE coatings are used in Impreglon’s TempCoat® and Halar® surface treatment systems, which make metals smooth, very non-stick and corrosion resistant.