Phosphating

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The phosphating is a widely used method in the surface treatment in which, by chemical reactions of the metallic surfaces with the aqueous phosphate - solutions a so-called conversion layer is formed by firmly adhering metal phosphates. Phosphating is mostly used for steel , but can also be used for galvanized or cadmium-plated steels and aluminum . The main areas of application are corrosion protection, adhesion promotion, friction and wear reduction and electrical insulation. Phosphating is also available under various brand names such as bonders, Atramentizing or Parkerizing .

execution

During the phosphating process , the base material is first attacked by pickling , in which metal cations dissolve with the evolution of hydrogen. Then the layer is formed by precipitation of poorly soluble phosphates. Depending on the type of solution, these can be iron , zinc or manganese phosphate layers . In the case of non- layer -forming phosphating , the metal cations involved in layer formation come from the base material, the metal cations from the phosphate solution are not involved in the layer structure. In the case of layer-forming phosphating , the layer is built up by metal cations from the phosphate solution; metal cations from the base material can also be involved. The layer thicknesses range from a few hundred nanometers for iron phosphating to two micrometers and more for zinc and manganese phosphating. Phosphating technology is the key to the longevity of metals Before coating metal structures with a paint and varnish composition, the surface is subjected to a special treatment with a mixture of phosphate salts. High-quality phosphating ensures the durability of the outer coating and protects the substrate from corrosion. Thanks to the phosphate film, the adhesion of the paint to the base material is increased and the corrosion of the sub-film due to paint damage is significantly slowed down.

Applications

The great importance of phosphating stems from various properties of the phosphate layer. The phosphate layer adheres very well to the substrate and, thanks to the microporous or microcapillary layer structure, allows subsequent coatings to be well anchored . That is why phosphate layers are very often used as a substrate for coatings. In addition, it makes it difficult to rust underneath damaged areas of the coating. Phosphate layers alone offer useful temporary corrosion protection , which is often sufficient for storage before a subsequent processing step. Zinc phosphate layers are superior to iron phosphate layers in terms of protection. The corrosion protection of the phosphating can be improved by oiling or waxing. The phosphate layers have good sliding properties. This is used in the cold forming of steel. Zinc phosphate layers are used here because they can be easily sheared off. The latter can react with the alkali soaps, which are often used as lubricants , to form zinc soaps and thus form a firmly adhering lubricant system that can withstand higher temperatures. Manganese phosphate layers are used to reduce wear on highly stressed sliding partners. They should keep wear and tear when running in low and prevent seizure in the event of a brief lack of lubricant. The high electrical resistance of phosphate layers is used in electrical steel sheets for magnetic cores to isolate them from one another with a thin separating layer.

For the attachment of gypsum-based materials in interior work, e.g. B. for plasterboard , phosphated screws are used, which do not react chemically with gypsum ( calcium sulfate ). Untreated or galvanized screws can corrode and cause rust stains.

environment

From an ecological point of view, phosphating is viewed critically, as z. B. incidental process sludge and waste water must be disposed of as hazardous waste. Phosphate-free conversion layers for automotive paintwork are under development.

literature

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