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Steel wire being "eaten away" during hot dip galvanizing

2007

Hello, I was wondering if anyone could help me with the following fascinating phenomenon:

One of our customers purchases our welded mesh, which is 3.0mm hard drawn, batch coppered wire, manufactured from a standard low carbon (0.08%C), silicon killed steel.

The mesh is supplied to the customer in panel form and he cuts it down to various panel sizes and welds it in to a angled steel frame. The frame is then sent to an external galvanizer for zinc coating.

It appears that occasionally, wires have literally been eaten away when they are submerged in the zinc bath. He says that they have followed the mesh through the galvanizing operation and can see nothing apparently wrong with the wire after the mesh has been cleaned, but straight after immersion in the molten zinc bath certain wires appear to generate a lot of dross and when they are pulled out of the bath, they have been eaten away. This happens both on welds and between welds.

The fault is restricted to certain wires only. Wires next to these appear untouched. However, the chemical composition of "eaten" and "non-eaten" wires looks the same.

Stefaan Watté
Mesh & Wire manufacturer - Zwevegem, Belgium

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2007

Sir:
Check to see if the molten zinc is above 0.3% tin (Sn).
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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2007

Sir:
The flash copper coating on the wire is likely enough to cause the problem. Check to see if the copper (Cu) in the molten zinc is above 0.05%. The silicon and phosphorus should be similar in the wire, in the angles, and in the welding wire (or rod). It is especially bad if the silicon is in the "Sandlin Curve" (e.g. 0.05% to about 0.22% silicon).
Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA

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As Thomas has suggested, this sounds like a case of reactive steel.

Steel grades are not precise levels of elements in the steel, they are a range. At the extremes of the range of allowable levels of say Si in the grade of steel, you could be in the critical area where the steel reacts much more with the Zn. You mentioned extra dross forming, and this corroborates this possibility.

The wires in mesh will not be exactly the same. Each wire can come from a different spool in the mesh making process. While each might be within the spec for the steel grade, the chemistry will not be the same. So adjacent wires will be different.

Coming from a steelmaking background, I can also add that the steel reported analysis is not always representative of the steel. Its very easy to selectively sample steel for analysis to ensure that that batch (or "heat") of steel is in spec. Analysis of the sample shows that the sample is within the spec, but the sample was not representative.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK

2007