PROTECTING GALVANIZED SHEET METAL

[wynn]

We build sound attenuation systems and structures for power generation projects. Currently, we are looking into insulated panels made from 16 gauge, G90 galvanized, commercial quality sheet steel in 60 inch wide strips.
Here is our problem. The panels are too wide to be unsupported. We could really use a cold formed center stiffener. For best structural performance, we would want to stitch weld this stiffener to the inside of the panel. The problem is that the heat from welding could degrade the galvanizing. If that happens, we have just another patched, zinc primed panel and no longer tha advantage of mill-galvanized material.
Does anyone know of a way to weld the stiffener on the inside face of 16 gauge sheet without damaging the galvanizing on the outside face. We are already thinking about thin wire, pulse arc MIG, but even that may still be too hot used just by itself. Have you other suggestions?

 

[KO]

Braze welding galvanized steel with brass rod is an excellent alternative. If the correct joint and fit-up are used then the end product will be extremly strong and it will maintain the corrosion resistance. If it is done correctly, the temperature will not be hot enough to burn off the zinc on either side of your sheet.

 

[evelrod]

My two cents worth---16g galvanized sheet will, as you have discovered, be nearly impossible to weld without damaging the coating. I'm sorry, guys, but brazing won't do it either. Low temp bonding with galvaloy sticks will work but, again, it may still damage the coating and besides will be VERY questionable as to strength of the bond. I am sorry I can't come up with something that will work aside the fact you could build the assembly and THEN have it hot dipped!
Could you possibly use some of the newer chemical bonding techniques?

Rod

 

[MikeMilburn]

Is there a possability that you could form the stiffener as an independant piece and pop rivet that piece to the main panel. You could use an adhesive tape placed between the two pieces to further aid in strength.
I can't think of any method that involves heat that would not damage the zinc coating...Mike

 

[wynn]

Thanks for your advice. I too am thinking welding won't work here. However, any such rivets would be used on a flat surface exposed to weather for a number of years. This is quite demanding for a rivet. Does anyone know of one compatable with galvanized steel with a reliable shear value that is also able to do this?

 

[MikeMilburn]

If you're thinking rivets won't work, what about self drilling plated screws with the gasket/washer under the head. It'll save an extra step and you'll have your weather tite installation...Mike

 

[ENC]

wynn, all of the above responses are very good. I agree that welding, brazing and any hot work will damage the coating. We recently were working with a client who circumvented the issue by selecting aluminum sheet. They discovered the cost was less than the mill galvanized sheet and they were able to weld the pieces together. The stiffner could then be welded onto the aluminum sheet.

If mill galvanized sheet is the only way to go, consider bending an "L" or "U" at the midspan of two sheet and bolt them together. This will provide the stiffness you need and eliminate the welding. It is commonly done for panel assemblies in packaged equipment such as air handling units, cooling towers, fan housings, etc.