Ideally, the design and build phase of a structure should only allow for minimum on-site rectification works. By minimising or eliminating field work, the risk of early corrosion of steelwork is reduced, especially in medium to high corrosivity zones.
When severe damage to the galvanized coating has occurred during welding or as the result of rough handling in transport or erection, protection must be restored.
Small areas of the base steel exposed through mechanical damage to galvanized coatings, are protected from corrosion cathodically by the surrounding coating and may not need repair, depending on the nature of the product and the environment to which it is exposed. Small exposed areas (typically 3-5mm in size) normally have little effect on the life of the coating as discussed under ‘Cathodic protection’ on this page.
The coating repair methods detailed below are in accordance with AS/NZS 4680 section 8 - Repair after Galvanizing.
Zinc rich paints The application of an organic zinc rich paint is the most rapid and convenient method of repair. The paint should conform to AS/NZS 3750.9 'Paints for steel structures - Organic zinc-rich primer' applied in two coats by brush to provide a total dry film thickness (DFT) of a minimum of 30μm more than the local coating thickness requirements in AS/NZS 4680 (generally two coats of 50μm, totally 100μm DFT) and should contain not less than 85% zinc in the dried paint film.
Zinc metal spraying In certain circumstances, by prior agreement, zinc metal spraying may be used as a method of coating repair. The damaged area must be grit blasted to Class 3 followed by zinc metal spraying to a coating thickness equivalent to that of the undamaged coating, and seal coated using an aluminium vinyl paint.
For further details on repair of hot dip galvanizing please see the FAQ page and the GAA Advisory Note Repair of Field Welds and Other Damage available for free download here.