Wednesday, November 26, 2008


What is Phosphating:

There are a multitude of separate processes designed to produce thin, crystalline, adherent films of phosphates of iron the surface of steel articles; these often have an admixture of zinc and manganese phosphates. These film absorb oil readily and the oiled film confers a considerable measure of corrosion protection quite cheaply on iron and steel goods where appearance is not important. Phosphate coating are also an excellent pre-treatment for paint or stove enamel on steel, ensuring good adhesion and discouraging lateral corrosion under the paint film from points of damage. Basically this phosphating process consists of immersing the steel articles into a hot solution containing ferrous phosphate with some free phosphoric acid, and with additions of zinc and/or manganese phosphates. Phosphoric acid is tribasic, i.e. it has three replaceble hydrogen ions. The acid phosphates, in which only one or two of the hydrogen atoms are replaced, are fairly soluble in water, but the full phosphate, in which all three hydrogens are replaced by iron, is insoluble. As the phosphoric acid in the solution attacks the steel, the local acidity is reduced and a crystalline film of iron phosphate precipitate onto it.

The simple process described above is rather slow, taking up to an hour to provide a suitable coating; addition of zinc or manganese phosphates accelerates the process and manganese phosphate ensures a thicker coating. But the action can be speeded up more fully by the addition of accelerators which are slightly oxidizing chemicals such as nitrates or nitrites, or complex organic nitro compounds. The range of possible formulations and types of coating is very wide, and their chemistry complex, so that the different processes are mostly serviced by specialist companies under proprietary names, such as Coslettizing (the original, now obsolete, process). Bonderising, Granodising, Parkrising, Walterisation, etc. These companies sell the mixed salts or solutions with full instructions for their use, and often provide a supervision and chemical control service. In some cases they supply plant as well. In these circumstances the user is relieved of much of the technical difficulty and control work, and he can integrate the process into his metal manufacturing operations.

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