Staining and discolouration of galvanised steel
Sound galvanised steel with many years of corrosion-free life ahead can sometimes be rust stained or discoloured. This may give an incorrect impression that the coating has failed and is, in some cases, visually unacceptable. This leaflet sets out and illustrates the main caoses of staining and discolouration and indicates how the problems can be avoided or the effects remedied.
The staining and discolouration of galvanised coatings by rust may occur as a result of one or more factors:
- Direct contact of galvanised parts with unprotected or inadequately protected steel (e.g. galvanised steel sections fastened with unprotected, electroplated or painted steel bolts)
- Deposits of iron dust and swarf from other operations or sources onto the galvanised surface
- Water draining from unprotected or poorly protected steelwork, e.g. from damaged areas on painted steelwork
- Seepage from pickle residues in welds. During pickling , hydrochloric acid may penetrate into the weld area via pin holes or if the welding in intermittent. Residual salts can sometimes pick up water and cause ‘weeping’ from the weld areas. This effect is normally limited to a small area, ceases after a short time and is not detrimental to the coating.
- Rusting of areas welded after galvanising and subsequently left unprotected of inadequately protected.
- Staining of galvanising can occur when water runs of other materials notably metals such as copper, certain hardwoods, e.g. oak, and indeed whenever water can dissolve materials from one surface and redeposit on the galvanised steel.
All parts of the structure should receive comparable corrosion protection where possible. Nuts and bolts and other fasteners should be hot dip galvanised to BS 7371 : Part 6: 1998. The thinner zinc coatings frequently supplied on steel mesh, sheet, wire and tube will not last as long as those of products hot dip galvanised to ISO 1461, the standard which covers all structures galvanised after fabrication.
Welds should be continuous and slag-free wherever possible to minimise the retention of pickle residues.
Design structures to avoid run-off water from other materials on to galvanised steel. In particular avoid run-of from inadequately protected steel and from copper.
Where welding after galvanising is necessary, welded areas should be thoroughly cleaned and the zinc coating resorted either with an appropriate thickness of zinc rich paint or with a proprietary repair compound, in accordance with section 6.3 of ISO 1461.