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Does Excess Galvanising Coating Thickness Affect the Steel Structure?

We have a fabricated structural steel like beams, angles, falts, pipes etc etc & this has to be used in refinery booster stations. When galvanised the coating thickness is about 24 mils.Can it be accepted as per ASTM A123?

John Abraham
- Kuwait
March 18, 2010

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March 22, 2010

Sir:

Your 24 mils of zinc (thousands of an inch) equates to 610 microns. IF the zinc coating does NOT chip off, then you have very, very long term corrosion protection indeed.

This thick of coating is from a VERY UNECONOMICAL galvanizing plant. At over $1/pound for zinc this is extremely costly AND unnecessary.

Since reactive steels build a zinc coating of about 1 mil for every minute in the molten zinc, this means the steel product is in the zinc kettle about 24 minutes to "cook out." I strongly suspect bad flux or bad pickling or both.

Thirty years ago a did a transmission powerline project in which the zinc coatings varied between 8 and 16 mils. I even used the now defunct "hammer test" to determine chipping tendency. In the end I "passed" the galvanizing because I knew that very long term corrosion protection would be provided. Had the galvanizer coated with only 4 mils of zinc then the galvanizer would have saved $4 million in zinc (this was when zinc was cheap). WHAT A WASTE OF MONEY.

If you are the galvanizer, you need to do something to correct this problem.

Regards,

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

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March 23, 2010

John:

I responded to your question last night but not sufficiently in an economic way relating to the galvanizing. If you are also the galvanizer, I hope these numbers are useful to you.

From previous experience for 3/8 inch thick average steel, the mills of zinc on the product times 0.7 gives the approximate %GZU. (e.g. %GZU = [zinc consumed/steel galvanized]times 100%) Thus assuming an average steel thickness of 3/8 inch, your %GZU would be about 16% (0.7 times 23 = 16%). For 3/8 inch thick steel it is quite easy to attain a %GZU of 5%. Thus if you galvanize 20,000 metric tonnes/year, by going from %GZU = 16% down to 5% the zinc savings is about $6,000,000/year or about $24,000/day. This would be zinc that would not even be purchased! If the steel thickness average is thinner, then the zinc savings will be greater.

In addition, zinc clean-up, drips, bolt holes, etc. will be much, much better (e.g. much better galvanizing quality). Further, ash and dross are also likely to be very much reduced.

Hot dip galvanizing is like a three legged stool. Leg 1 is sales, Leg 2 is production, and Leg 3 is technology. Presently you are sitting on a two legged stool.

Regards,

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