Zinc and manganese phosphating

1. Characteristics and properties

Phosphate layers reduce friction between friction partners and serve as electrical insulation as well as a primer for paints, lacquers and oils. The colour of this chemically produced layer varies from a light grey coating (zinc phosphate) to a matt dark grey, almost black coating (manganese phosphate). The layer is firmly intergrown with the base material and interspersed with numerous capillaries and cavities that make the layer extremely absorbent. The ductility of the phosphate layers is not particularly good. When the workpiece is bent by 180°C, the layers tear open, but without being detached from the base material. The fine crystalline salt layer produced from heavy metal phosphates consists of 95-98% secondary and tertiary water-insoluble zinc or manganese phosphates and 2-5% iron phosphates.

The speed and type of the layer build-up is exclusively controlled by the exposure time, the composition of the process bath and the temperature. The layer thickness should be about 3-8µm for subsequent painting. For workpieces that are oiled and greased, a layer thickness of at least 10µm is recommended.

Without post-treatment, phosphate layers only have a limited corrosion protection period. This is based on the fact that, even in the ideal case of the layer structure, at least 0.5% of the total surface of the workpiece remain unprotected by the capillaries and cavities in spite of the phosphate layer produced. Within a very short time, flash rust would develop, which would then develop into red rust and infiltrate the layer. Compared to water, phosphate layers are largely insoluble. However, they are completely dissolved by acid and alkaline solutions within a very short time. The phosphating of metals is standardised in DIN EN ISO 9717.

2. Benefits and areas of application

The primary task of a phosphate layer produced on steel with embedded zinc or manganese phosphates is corrosion protection in connection with subsequent painting, or subsequent oiling or greasing of the workpieces. In addition, workpieces are phosphated to facilitate chipless forming by drawing or extrusion. Phosphated workpieces are used in mechanical engineering and in many other branches of industry. Examples of applications can be given: Drive parts from lathes, springs and gear wheels (friction reduction) and machine covers that are painted (corrosion protection).

3. Basic material

The base materials are particularly suitable for unalloyed and low-alloy steels up to medium-alloy steels. Hardened steels (>1000N/mm²) can also be machined at Metoba; due to the risk of hydrogen embrittlement, post heat treatment (usually 190°C, 3h) is required.

4. Required delivery condition

The goods provided must be largely metallically bright. Residual oils or residual impurities must be removable with conventional aqueous alkaline degreasers. Oils containing silicone, alkali/alkaline earth and high-pressure additives must be avoided at all costs.