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Hot dip galvanizing before or after fabrication?





August 31, 2010

Im interested in knowing what is better to do as in working with steel already hot dip galvanized or is it better to work the steel and then hot dip galvanize it?

John Manuelson
employee - Shanghai



simultaneous replies

Sir:
A few years ago I did a study on cooling tower mounts that were failing due to very excessive white rusting. The failing cooling tower mounts were made with pregalvanized sheet and then formed into the mounts. The failings were due to high aluminum in the pregalvanized sheet (e.g. 0.17% aluminum as opposed to 0.002% aluminum in the post fabricated mounts (aluminum in the zinc). In addition the pregalvanized sheet had bare edges from being sheared. Also the pregalvanized sheet had much thinner zinc than the post galvanized mounts. Your question could only be properly answered if the product(s) are identified.
Regards,

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



It is almost always better to do the welding before galvanizing.

Doing any welding, drilling etc after galvanizing destroys the coating in that place and so means you have no corrosion protection on the welds. And the welds might be the weakest points.

So highest risk of failure....

geoff_crowley
Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK
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