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Sheradised Coating

The Sherardizing process is a method of applying a zinc coating to the surface of ferrous components.

The process has been developed into a reliable and accurate coating which, when correctly specified and applied, gives a tough resilient defence against corrosion of the base material. Sherardizing is a diffusion process carried out by heating suitable pre-cleaned ferrous components in zinc powder. The process forms a zinc iron alloy with the surface of the components being treated. The resultant coating has a matt grey appearance, uniform in its thickness and has an iron content of 8-10%.

Coatings are normally commercially applied within the range of 8 to 60 microns. Thicker coatings can be applied if required. 

Sherardizing has been proven to be the most uniform of the range of zinc coatings, the thickness of coating will be uniform to close tolerance over all accessible areas of the individual treated component, this unique factor also applies to the whole production batch contained in a process container. This feature allows repeated batches of components to be treated to an agreed standard. 

Hardness and Adhesion
Due to the iron content of a sherardized coating, it is the hardest and most wear resistant of the zinc finishes having mechanical properties at the alloyed layer of 600 to 700 kg/mm sq with a 10gm load and 300 to 350 kg/mm sq for the surface layer. The excellent alloyed adhesion characteristics are such that the coating will not lift or peel and can only be removed by chemical (acid) or mechanical means (shot blasting). 

Painting of Sherardized Coatings
Paints can be applied to sherardized surfaces without the need for etch primers, normal painting practices may be followed. Powder coatings can be applied, it has been found that to pre-heat prior to powder coating gives excellent results and prevents micro holes due to air entrapment. 

Standards & Specifications
British Standard 4921 covers the sherardizing process. The standard has two classes of thickness. 
Class 1 - 30 microns thickness
Class 2 - 15 microns thickness 

The standard specifies the requirements for appearance, clearances for threaded components and defines the methods to be used to determine that coated components conform to the standard.