The objective of phosphating is to form a phosphate layer on the metal surface. The process begins with pickling the component to remove the natural oxide layer from the metal. This step leads to cations going into solution, with the evolution of hydrogen.
The component is afterwards immersed in a phosphate solution to precipitate barely soluble phosphates. Depending on the composition of the solution, an iron, manganese or zinc phosphate layer now forms. The process can be repeated if necessary. Phosphating is sub-categorised into layer-forming and non-layer-forming phosphating.
Non-layer-forming phosphating is designated as such because no metal cations pass from the phosphate solution into the treated metal. Instead, all cations which form the layer originate from the material itself. Although providing great dimensional stability, this process provides less corrosion protection compared with layer-forming phosphating.
In this variant of phosphating, cations from the base metal may also be involved in the layer formation, but the majority of the positive metal ions originate from the phosphate solution. For fitted components, it is necessary to take account of the additional layer, which can range in thickness between hundreds of nanometres to a few micrometres.
Metals and types
Another distinguishing feature of phosphating is the metal that contributes to forming the layer structure – the most common are iron, zinc or manganese. Depending on the desired properties, two or three of these metals can be applied at the same time.
Iron phosphating is used to provide corrosion protection and to form the basis for the painting of components such as metal sheets. It is regarded as the simplest and cheapest option, because surface activation and phosphating take place in the same work step. Iron phosphating requires temperatures from 25 °C to 65 °C and a pH value between 4 and 6. Iron phosphate layers weigh between 0.2 and 0.8 g/m2. Iron phosphating is suitable for workpieces made from iron, aluminium, or zinc.
For zinc phosphating, the metal surface must be pre-treated in order that a fine-crystalline zinc phosphate layer can be achieved. For this purpose, the process begins with a titanium salt bath. Nickel is added to the subsequent bath at 35 °C to 80 °C and pH values range from 2.2 to 3.2. This process causes pores to form on the metal surface, which absorb oils particularly well and thus increase corrosion protection. It also results in improved paint adhesion on blank and galvanised steels. The zinc phosphate layer has a matt appearance, and a light grey to medium grey colour. It weighs 1.5 to 30 g/m2.
Manganese phosphating takes place at temperatures from 90 °C to 95 °C and at a pH value between 2.2 and 2.4. Manganese phosphate layers weigh between 10 and 25 g/m2. They provide friction-reducing properties, good corrosion protection, and high absorption capacity for oil – ideal for use in transmissions or slide bearings. Since it produces a dark grey to black colour combined with a silky-matt look, manganese phosphating is also used for the visual enhancement of small arms.
INFO: Coating thickness
The composition of the phosphate solution determines the thickness of the conversion layer.While phosphating with manganese or zinc can produce layers having a thickness of up to 2 μm, iron phosphating layers are usually only a few hundred nanometres thick.
Other metals and types
Trication phosphating is a phosphating process that involves zinc, nickel, and manganese. Other additives frequently used for phosphating are sodium nitrate, sodium nitrite, sodium fluoride, and calcium. Less common is the use of titanium, zirconium, and copper sulphate. Nickel is largely avoided because it is dangerous to health.
Features and advantages
- Adhesion promoter: the phosphate layer is firmly anchored in the base metal
- High level of corrosion protection: Pores and capillaries ensure a high capacity for absorbing oils and paints that provide corrosion protection
- High lubricity: Reduction of friction and wear
- Appearance can be adjusted via the process: fine crystalline to coarse crystalline structure, light grey to black
- Insulation: Phosphate layers offer high insulation resistance
- Rust resistance: damaged layers are nearly insusceptible to rust
Areas of application
While commonly applied to steel, phosphating is often used for aluminium as well as galvanised and cadmium-plated steels. It is primarily used as a preparation for coating. Since the phosphate layer adheres very well to metallic surfaces, and its pores or capillaries form the perfect basis for oils, greases, and paints, it is used as an adhesion promoter.
Phosphating as corrosion protection
The phosphate layer itself already offers corrosion protection; this can be further improved, e.g. by waxing or oiling. In addition, the layer is rust-resistant and can largely prevent underlying corrosion. Zinc phosphating delivers greater resistance to corrosion than iron phosphating.
Improved anti-friction properties because of phosphate layers
Phosphating improves the anti-friction properties of workpieces. Zinc phosphate layers in particular are used for the cold forming of steel. They react with alkaline soaps to form zinc soaps, which withstand higher temperatures and form an ideal lubricant system.
Manganese phosphate layers are also frequently used to prevent workpieces from seizing up because of insufficient lubricant. They can reduce wear on the friction surfaces.
Phosphate layers are ideal for insulation, as they offer high electrical resistance at a small thickness. They are used, for example, in electrical sheets for magnetic cores.
Comparison: Phosphating vs. burnishing
Phosphate layers offer greater corrosion protection than burnished layers, because their irregular contour provides better adhesion for anti-corrosion oils and greases than the amorphous surfaces created by burnishing.
Burnishing is particularly well-suited for precision parts, due to its great dimensional stability because the layer is formed within the component surface. Conversely, layer-forming phosphating requires the thickness of the layer created on the surface to be taken into account.
Special expertise at Härtha
The success of phosphating hinges on the exact coordination of temperatures and reaction times, as well as of chemicals and their concentration. Thanks to our long years of practical experience in bath management and advanced systems, we can guarantee the exact regulation of these parameters. We ensure that your base material is not damaged, and we adjust the thickness of the phosphating layer, the crystal sizes, and the surface adhesion to the specific purpose of your workpieces. To benefit from maximum quality, rely on an expert partner.
Maximum workpiece size:
1,800 x 320 x 500 mm
Customer specifications for phosphating
We are delighted that you have decided to have your workpieces phosphated at Härtha. As a first step, please provide us with the following information:
- Material designation
- If applicable, information on any previous heat treatments
- If applicable, information on the desired layer thickness (in µm)
Refer to our location overview to find a location near you where we offer phosphating and many additional processes.