PLGN 5.05 Galvanising

What is Galvanising?

For structural steel, hot-dip galvanising is normally used. This process involves acid-descaling the steel followed by immersion into molten zinc. On withdrawal a layer of zinc remains on the surface of the steel. British standard BS 729 covers hot-dip galvanised coatings on iron and steel; BS 2982 deals with continuous hot-dip zinc-coated and iron-zinc alloy-coated steel for strip sheet or plate; and BS 3083 covers hop-drip zinc coated corrugated steel sheet. Zinc coatings of 85um minimum average coating weight are considered to be the optimum for most applications. However, thicker layers are sometimes required and in these instances the steel should be blast cleaned prior to dipping. Galvanisers often add traces of other metals to the zinc bath to assist in fluxing and the removal of dross as well as to increase brightness. Typical additives are lead and aluminium. To avoid white rust formation on stored and stacked sheet some galvanisers apply a chromate treatment. This is also known as passivating.

How to Paint it.

Adhesion of paint coatings is normally poor as new galvanising does not provide a good surface for painting. Alkyd resin-based primers especially those formulated on resins containing a high acid value react with the zinc layer on weathering sometimes resulting in loss of adhesion and wholesale flaking. Chromate treatment also causes adhesion problems of paint coatings. Both the size and brightness of the galvanised spangle provide an indication of subsequent coating adhesion. Large bright spangles inevitably provide the poorest adhesion. To enhance the adhesion of surface coatings the use of an acid-etch of mordant solution or an acid containing etching primer can be specified. T-wash developed by British Railways is the best known mordant solution and it is recommended for new galvanised steel work where it blackens the surface on reacting with zinc. When this occurs the excess should be washed off with fresh water. If blackening does not occur in isolated areas it indicates that the surface is not free from grease. Localised degreasing and re-treatment will be needed. Etching Primers can be applied direct but they are not resistant to condensation or moisture and can be difficult to use on-site.

Recommendation.

If the clients are satisfied with the appearance of Galvanising uncoated, then provided the zinc coating is as described (85um minimum average coating weight) we would suggest that this would be the most appropriate and cost effective action to take at this point in time. Should your clients want it coated at a later date the preparatory work would be less onerous than had it been painted from new. A typical specified Dulux System on this Substrate would require re-painting every 8 years whereas the uncoated galvanising will normally last around eight years with the minimum zinc specified. Greater depth of Zinc will obviously increase the period before rust spotting etc will necessitate painting.