The MIG welds in question, after dilution by parent metal, would have a silicon content of 0.3% or more which would account for the grey appearance of the zinc coated welds. The excess could also be caused by the high silicon content because the rate of reaction between zinc and iron depends on silicon content as well as a number of other factors.
In low silicon steels the alloy layer grows rapidly during the first two minutes in the galvanizing bath but the rate then falls. In high silicon steels or weld metal the alloy layer may continue to grow in thickness linearly with time so that the coating thickness will depend on the time that the work is in the bath. Therefore, the zinc coating on the weld may be thicker than that on the parent metal.
Also on thick sections that cool slowly when withdrawn from the bath, the alloy layer continues to grow and may diffuse to the surface in patches. Hot dip galvanizing of welded components is normally carried out without any problems but in cases where appearance is critical, there should be co-operation between the customer, fabricator and galvanizer with the following check list in mind:
- Composition of parent metal.
- Selection of consumables - low silicon rutile electrodes are available but MIG welding wire having less than 0.2%Si is rare.
- Edge preparation - square edge butt joints increase dilution of weld metal which may lower silicon content.
- Surface roughness of ground welds - the smoother the surface, the lower the reaction rate between zinc and iron because of smaller surface area.
- Temperature of galvanizing bath - high temperatures increase reaction rate.
- Time of immersion - should not be prolonged.
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