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 the 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?

 

[butelja]

How about riveting instead of welding? Works well for the aircraft folks who deal with unweldable materials.

 

[wynn]

Thanks for your advice. I too am thinking welding won't work here, and we should consider rivets. However, these rivets will 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?

 

[gdodd]

Try self piecing rivets inserted into the opposite unexposed surface.

Alternatively try a structural adhesive. There are a number of adhesives about which are strong enough to remove the galvanized coating.

 

[SCRAengr]

If the temperature range is small enough, adhesives as previously suggested may be a solution worth considering. Aging and environmental effects should also be considered. The more data you have for the adhesive supplier, the closer they can match your needs. There are also some VHB Very High Bond tapes available. The same adhesive issues will need to be addressed. If the stiffener was on the visible side, regular inspection could be performed.
Regards,
Jeff

 

[Chr1s]

Aircraft wings have 100's of rivets on them and are a flat surface and obviously exposed to the elements.

 

[wynn]

Fair comment except we are building architectural devices out of 16 ga galvanized sheet steel -- not an aircraft material. Aluminum skins are less of a problem. Soft aluminum rivets make better weather seals than do tougher material. There is no bimetal action aluminum to aluminum. Aircraft skins do not accumulate bird lime, acid rain, or standing water -- either Ph extreme devastates galvanizing. Also, welding won't strip aluminum of its corrosion resistance, so no need to rivet.
Aircraft to us is not apples to apples, but I would guess the truck body guys have a lot to tell me -- hence this web site, but thank you anyway.

 

[etch]

have you considered composite panels. That is steel galvanised with a foam filling interior basically a sandwich. good for acoustic dampening and also now Approved fire resisitant.

 

[Andybee]

I have a friend who works in metal fabrication. He has procured a galvanising paint which I use on my car body panels na d galv repair sections which is as good as galvanising the steel in the first place. Dont have a product name for it, but it is used in industry when a finished product is galvanised after fabrication and any areas are missed (i.e. where hooks or clamps covered the surface) The ungalvanised area is heated and the paint is applied as it is cooling. I am assured that this gives as good a protective coating as the galvanised areas. Alternatively it can be brushed or sprayed onto cold metal and works almost as well. I use it when fitting galv repair panels and the zinc has burned away.

Sorry i can't help with a product name - it's industrial stuff and should retail at £70 per litre. But if youre serious...