Field repair of areas welded after galvanizing
Ideally, the design and build phase of a structure should allow for only the minimum of 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. However, there are times when field welding is a necessary activity and this section provides general information on the repair of hot dip galvanized steel after on-site welding has occurred.
Weld area damage
When severe damage to the galvanized coating has occurred during welding, protection of the steelwork must be restored. The level and extent of the restoration must be more robust and the repair work completed prior to the article being put in service if the steel will be exposed to severe corrosive conditions in service.
The width of the weld damaged zone will depend on heat input during welding, being greater with a slow process such as oxyacetylene welding than with high speed arc welding. In the manual metal arc welding and oxyacetylene welding of galvanized steel, the weld metal itself will corrode in most atmospheres and the application of a protective coating is essential. Suitable materials for coating the weld metal and adjacent damaged areas of the coating are zinc rich paints, and in some circumstances, zinc metal spraying.
In the case of weld repairs, additional surface preparation of the damaged area is usually required to remove any welding slag followed by additional abrasive cleaning of the damaged area.
Appropriate coating repair methods in accordance with AS/NZS 4680 are required to achieve at least 100 µm coating thickness in the repaired area. The Standard provides four options.
- Organic zinc rich epoxy paint complying with AS/NZS 3750.9. This is normally applied to the repair areas in two coats.
- Inorganic zinc silicate paint complying with AS/NZS 3750.15 and can be normally applied as a single coat.
- Zinc metal spray to ISO 2063.
- Zinc alloy solder stick.
The repair of field damage is often of a lower standard than shop coating due to imperfect surface preparation and difficulty accessing some damaged areas. For this reason, we recommend the coating thickness of the repair in the field to be thicker than repairs carried out in the galvanizing plant or in the fabricator’s manufacturing facility.
For field repair of areas damaged after hot dip galvanizing, the GAA recommend the use of organic zinc rich epoxy paint. Further information on the surface preparation and appropriate finishes for different service requirements are detailed below.
It is important to observe normal good painting practice with respect to weather and application conditions. Apply all paint strictly in accordance with paint manufacturers’ recommendations.
Repaired areas of hot dip galvanized steel are normally considered to be most ‘at risk’ of early corrosion. Repaired areas should therefore receive an earlier maintenance inspection than the remainder of the structure.
Surface preparation for all conditions
- Degrease and remove all surface contaminants
- Remove all welding scale, slag, rough edges and corrosion products
- Clean to:
a. AS 1627.2 St 31 (recommended minimum for low to moderate corrosivity zones), or
b. AS 1627.4 Sa 2½ with a surface profile2 (recommended minimum for coastal, marine corrosivity zones).
Zinc rich paint repair
C1 – C3 (low to moderate) atmospheric corrosivity zones
Apply 2 coats of Galvanite® epoxy zinc rich primer or equivalent to 125 µm – 150 µm DFT.
- C4 – C5 (coastal, marine) atmospheric corrosivity zones
Apply 2 coats of 2 pack epoxy zinc to AS 3750.9 to 150 µm minimum DFT followed by 2 pack epoxy enamel to 150 µm DFT.
Decorative finish – all environments
- Apply appropriate repair from the options above, stippling edges of the painted area to achieve optimum appearance of the repair.
- Where the unrepaired galvanizing has a ‘shiny silver’ appearance and if a very close initial colour match is essential, apply 1 coat of a ‘silver’ paint with an aluminium pigment3 over the zinc rich primer for appearance only. Otherwise a more uniform metallic colour match will be achieved over time4.
1 Suitable power tools include power wire brush, needle gun, disc sander, angle grinder and/or chipping hammer.
2 The Bristle Blaster can achieve a Sa 2½ level of cleanliness along with a profile for improved coating adhesion.
3 An aluminium (oxide) platelet or leafing aluminium pigment, which orientate parallel to the substrate and reflect light, giving the coating a shiny appearance.
4 Over time, the galvanized coating weathers to a dull grey and the zinc rich paint will weather similarly. The ‘silver’ paint will stay shiny and the repaired area will stand out beside the weathered dull grey coating.