View Full Version : Galvalume finish on metal

Stephen Tashiro
03-02-2010, 12:45 AM
I've read that it is bad to have steel directly in contact with aluminum. So I find it interesting that the often-used "Galvalume" finish consists of aluminum and zinc over steel. The recommended flashing material for metal roofing with Galvalume finish is aluminium, according to the website: http://www.steelroofing.com/trade_prepaint_job.htm

Dave Johnson29
03-02-2010, 11:52 AM
I find it interesting that the often-used "Galvalume" finish consists of aluminum and zinc over steel.


I think you will find that the Steel is Zinc coated then the Zinc is Aluminum coated. It's all tied in with the Chemical Affinity Series or the Periodic Table of Elements.

Much the same way that Chromium plated items are referred to as "triple-plated" Steel and Chromium have very little chemical attraction to each other so the Chromium would not stick to Steel.

However Steel and Copper have great attraction (affinity) so they plate the Steel with Copper. Not done yet as Copper and Chromium aren't great friends either, but Copper and Nickel are. The Steel is Copper coated, then Nickel coated then finally Chromium -- BLING !

My guess is Galvalume is Steel plated with Zinc then with Aluminum. The Steel provides the strength, the Zinc the bonding agent and the Aluminum the great weather and corrosion resistance. All held down with Cadmium plated screws. Don't get me started. :)

Larry Frank
03-02-2010, 9:09 PM
Galvalume is not a plated product. Galvalume® is sheet steel with a hot dip-applied alloy coating of about 55% aluminum and 45% zinc.

Dave Johnson29
03-03-2010, 11:45 AM
Galvalume is not a plated product.

OK, Larry, my mistake, when I said "plated" I did not mean electroplated although using chrome plating as an analogy may have suggested that.

I said "coated" in the first par, but slipped in "plated" in the last. I am old and forget stuff. ;)

Larry Frank
03-03-2010, 8:32 PM
I think that they use the coating process to get around the difficulties with plating not only with the technical difficulties but their are environmental issues with plating. I believe that there are other products that also just combine zinc into a coating rather than hot dip galvanizing or electroplating.