WELDING GALVANIZED OR THERMAL SPRAYED STEEL
There is a degree of confusion around the ability to weld already Galvanized steel or Zinc Thermal Metal Sprayed steel. It becomes an issue with large or awkwardly shaped frames/items when they are too big to fit in the galvanizing bath or spray workshop, and they have to be joined together after coating. It is also an issue when welding new coated steel to existing coated steel.
The Galvanizing Association of New Zealand (GANZ) in their extensively titled publication After-Fabrication Hot Dip Galvanizing. A Practical Reference for Designers, Specifiers, Engineers, Consultants, Manufacturers and Users, indicate that, with the right adjustments to the welding techniques, the presence of the zinc coating has no effect on weld properties, although more weld spatter may be experienced. For most welding techniques this generally amounts to a slower welding speed among other things. This works because the Zinc boils off before the steel gets close to melting point. For other than Zinc coatings consult applicator for advice.
When welding already Galvanized steel or Zinc Thermal Sprayed steel either
- remove coating locally and carry out a standard weld, or
- leave coating and adjust welding technique to those recommended in the GANZ After-Fabrication Hot Dip Galvanizing document.
Of course this would mean some repair to the coatings will be required, the coating Standards provide some guidance here.
AS/NZS 4680 Hot-dip galvanized (zinc) coatings on fabricated ferrous articles
AS/NZS 2312:2002 Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings
AS 1627.4 Metal finishing - Preparation and pre-treatment of surfaces - Abrasive blast cleaning
Repair weld damaged galvanizing to AS/NZS 4680. 8.4 Site repair, by; removing welding slag, power tool cleaning, grinding off all burrs and sharp arrises, and the finish for:
- Small repairs:- Colour matched zinc rich paint.
- Large repairs:- With approval, colour matched zinc rich paint or other agreed option.
Repair weld damaged thermal metal spray to AS/NZS 2312:2002, 5.2.7, Repair of metal sprayed coatings, if damaged:
- less than 10mm wide, remove welding slag, power tool clean, grind off all burrs and sharp arrises. Finish with colour matched zinc rich paint.
- wider than 10mm, remove welding slag, abrasive blast clean to AS 1627.4, Class 2.5, grind off all burrs and sharp arrises. Finish with metal thermal spray to match original. Sealer coat must be applied within 4 hours of spraying.
To clarify these issues Masterspec has updated it's structural steel sections, other metal work sections and the sections 6781 HOT DIP GALVANIZING or 6782 METAL SPRAY CORROSION PROTECTION (6781 & 6782 available in, Standard, Structural & Civil, Services, Landscape and Interiors).
Galvanizing Association of New Zealand - After-Fabrication Hot Dip Galvanizing. A Practical Reference for Designers, Specifiers, Engineers, Consultants, Manufacturers and Users is available on