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Weld Quality

The low boiling temperature of the zinc of the coating causes it to volatilize in the heat produced by an arc or by an oxyacetylene flame. The zinc in the gaseous state may become entrapped in the molten weld metal as it solidifies. If this occurs there will be porosity in the weld metal and if sufficient zinc is available it will cause large voids in the surface of the deposit.

The presence of the zinc in stressed welds can cause cracking and it may also cause delayed cracking due to stress corrosion. To eliminate this, the weld joint must be designed to allow the zinc vapor to completely escape from the joint. Fixturing, backing straps, etc., should be arranged to allow for the zinc to completely escape. Other ways to avoid zinc entrapment in weld metal is to use sufficient heat input when making the weld.

It is also important to secure complete and full penetration of the joint. The ultimate precaution would be to remove the zinc from the area to be welded.

When welding on galvanized steel or any coated steel, particularly those with coatings that produce noxious fumes, positive ventilation must be provided. Positive ventilation involves the use of a suction hose at the weld area. When using the gas metal arc process or the flux-cored arc process, the suction type gun nozzles should be used. Welding on zinc or other coated steels should never be done in confined areas.

For corrosion resistance of the weld it is sometimes advisable to use a corrosion-resistant weld metal. This can be done by using a bronze deposit such as a copper-zinc alloy, or a stainless steel electrode. In any case, when arc or oxyacetylene welding is used the area adjacent to the weld will lose the protective zinc coating which must be repaired.

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