Conductivity - Galvanized / Galvannealed

A. Galvanneal is merely galvanized steel which has been annealed. The annealing furnace is downstream, from the zinc bath. Galvanneal is easier to paint. The surface is a smooth, dull grey. Usually, it is lightly oiled or even dry.

Galvanize sheets have a bright zinc finish (like garbage cans). The surface is not smooth. Zinc crystals may show through the paint. The surface is normally treated with a chromate solution which is for rust protection. This solution prevents good paint adhesion.

A. The galvanneal process converts most of the zinc and some of the iron at the substrate surface to an intermetallic iron-zinc compound. The electrical conductivity for the intermetallic probably is not as great as that for the zinc or iron metal. Thus, the surface conductivity may not be as high for the galvanneal.

A. Hi Eileen. We appended your question to a thread where Terry & Larry have already explained what galvannealed means.

Pre-galvanized means that the item in question is made by bending, punching, drilling, and/or otherwise working sheet steel that was galvanized before the fabrication work began, rather than galvanizing the item after the other fabrication steps are completed.

Pregalvanized material is used for economy (because it is far less labor intensive to galvanize a continuous sheet than individual fabricated items, and techniques like wiping can be used to limit the thickness). If pre-galvanizing is integrated correctly into the design, the product might be just as good as a post-galvanized item. But if the design is not so good, there will be rusting at the edges and around any holes because there will be no galvanizing on edges that have been created after the galvanizing process. The sacrificial power of the galvanizing offers some level of protection to those bare edges, but it's not as good as having them covered with zinc :-)

A. Hi Vijay. The good part is that your situation is commonplace, so it's pretty easy to get explanations, facts, and opinions even if you are finding nothing in the codes. Some engineering forums which discuss this include:
https://www.eng-tips.com/viewthread.cfm?qid=302126
https://www.eng-tips.com/viewthread.cfm?qid=216646

A good paper at www.dairyland.com is:
"Over-Voltage Protection of Insulated Joints in Pipelines"
Ed. note April 2021: I guess Dairyland wasn't as happy with their paper as I was -- sorry, it's been removed :-)

Another good paper discussing these issues and explaining alternatives such as star washers, spikies, etc., is
"Lined Piping Systems, Static Electricity and Earthing"
That paper also expounds on what to do when you can't find specific guidance in the codes.

There are countless good discussions of your issue if you google the term "electrical bonding across a flanged joint". A simple thing I learned in my fifteen minutes of study is that if you are relying on the bolts to carry current, make sure you have star washers on the head end as well as the nut end. Best of luck.

Regards,