Base metals, combinations and surface contamination

A fabrication consisting of a variety of materials with different surface conditions should be avoided, as this could affect the uniformity and appearance of the coating. Where differing materials are used, grit blasting the entire assembly can minimise any differences which may arise due to differing effects of pre-treatment. Preferably, the fabrication should be of similar steel type throughout.

Steel fabrications that have been subject to heavy cold work (e.g. bent through a tight radius) may be susceptible to strain age embrittlement and should be stress relieved prior to galvanizing.

When the steel to be galvanized is a high-strength steel with crack initiator and a high level of residual stress, consideration should be given to reducing potential for steel cracking to occur. Such a combination is very rare but when it does exist, advice should be sought from Galvanizers Association or reference made to Publication No 40/05, ‘Galvanizing structural steelwork – An approach to the management of liquid metal assisted cracking’, published by the British Constructional Steelwork Association (BCSA) and ‘Controlling liquid metal assisted cracking during galvanizing of constructional steelwork’, published by the European General Galvanizers Association (EGGA).

Surface contamination

Clean steel surfaces are an essential requirement for good hot dip galvanizing. Contamination in the form of grease, tar, paint and weld slag cannot be removed by chemical cleaning and may result in black bare spots after hot dip galvanizing. Specifiers should ensure that the fabricator takes responsibility for articles being delivered free from contamination.

Steel sections, which have been cut or drilled using suds type oil can give similar problems to burnt-on anti-spatter sprays. The cutting fluid, which has been burnt or baked onto the steel, should be removed prior to sending the steelwork for galvanizing.

Contamination is sometimes difficult to detect on the steel surface and may only show up after galvanizing. The article may then have to be regalvanized, at additional cost.