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Galvanising difficult steels

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INDUSTRIAL Galvanisers often gets inquiries about galvanising special steels. While these steels can present problems for galvanising, it is sometimes possible to accommodate these problems in the design of the components or through modifications to the hot dip galvanising process.

There are three factors that affect the ability of steel to be galvanised: the chemical composition of the steel, the strength rating of the steel (the yield strength in mPa) and the steel's section thickness.

High silicon steels will produce thick galvanised coatings which may be brittle because the steel reacts very rapidly with the zinc. The effects of high silicon content can be minimised by keeping immersion time in the zinc as short as possible. High manganese steels will produce brownish coloured coatings that may be brittle and easily damaged in handling compared to galvanised coatings on conventional steels. High carbon steels can be successfully galvanised as long as their yield strength is within an acceptable range.

High sulphur steels are used for high speed machined components and should not be galvanised as they can be severely eroded in the galvanising process, rendering threaded items unserviceable.

High phosphorous steel are rarely encountered in galvanising operations but are unsuitable for galvanising. They react rapidly with the zinc to form thick, dark coatings that are easily damaged and may delaminate from the surface.

Stainless steels can be galvanised but are susceptible to liquid metal embrittlement and can fracture under load after immersion in molten zinc. They are only galvanised incidentally if they are attached to mild steel assemblies.

High strength steels (around 1000MPa yield strength and over) are susceptible to hydrogen embrittlement and pickling should be avoided.

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