Properties of welds containing porosity

General effects

Porosity will occur in certain joint designs in galvanized steel, depending on coating thickness, due to volatilisation of the zinc coating and entrapment of gas in the weld.

The type of joint affects pore formation since gases cannot readily escape from tee joints and lap joints or from butt joints in thick materials. In the case of butt joints, a vee edge preparation or provision of a gap between square edges facilitates the escape of gases, minimising porosity.

Pore formation is also influenced by the thickness of the galvanized coating relative to the steel base. Close attention to welding conditions will reduce the extent of porosity but complete elimination is not always possible, and it is important to consider the effect of porosity on static strength, fatigue strength and cracking of the weld joint. 

Effect of porosity on fatigue strength

When joints are subject to fatigue loading, welds in galvanized steel should be made oversize to reduce the influence of any porosity in the weld metal.

When a fillet weld in galvanized steel is large enough relative to plate thickness to fail by fatigue from the toe of the weld in the same manner as in uncoated steel, the presence of porosity in the weld does not reduce the fatigue strength of the joint. 

Where the dimensions of a weld are just large enough to cause fatigue failure from the toe in a sound weld, a weld containing porosity at the root may fail preferentially through the throat of the weld. 

Cracking

Intergranular cracking of fillet welds containing porosity, sometimes referred to as zinc penetrator cracking, does not significantly affect the strength of non-critical joints. For more critical stressed applications, however, it is advisable to carry out procedural tests on material and samples.