Thread: Gal Screws v Zinc Screws. When to use what?

Doc

Gal is actually pure zinc coating what you may be thinking is Cad plating which is actually Cadmium plating. You can have hot dipped galvanising or electroplating. Hot dipped is far superior.

You wont get gal screws (hot Dipped) in small gauges except I think in roofing screws. Most screws are just zinc plated and aren't much chop as far as being rust resistant.

If you want rust resistance use stainless steel screws.

zinc plating will corrode more rapidly when in contact with cca timber (specifically the copper), as such, hot-dipped galvanised or stainless steel fixings should be used in this situation.

http://www.ocean.udel.edu/seagrant/p...corrosion.html

this is a discussion about galvanic reaction; any pair of metals joined by water will form a circuit and the metal in the reaction that is closer to the cathodic end on the scale will be corroded.

http://en.wikipedia.org/wiki/Galvanic_corrosion

as an aside, another form of this corrosion is when lead flashing is used upstream from zinc-plated zincalum guttering

r's brynk

Zinc coated coach screws are not worth a pinch of tish in outdoor applications and should therefore use galvanised coach screws / bolts outdoors.

Please confirm.

Doc.

Yep - always use Gal outside and in treated timber anywhere - or stainless steel.

So zinc coated is really only a shiney treatment? Not designed for weather resistance?

Doc.

Yeh that is pretty much the case Akillas. Zinc plated is Usually used for screws used internally for hinges, fittings, etc

The zinc coating acts as a sacrificial anode and is obviously put there to protect the steel beneath it from corrosion.
Generally, zinc plating has a maximum of 5 microns and as such remains a little pourous. Hot dip gal has a minimum of 50 microns and has a much longer lasting coating, so it can be used in harsher environments.
There is another type of zinc coating, or more to the point, another way to apply zinc to steel components. This is mechanical plating, most small fasteners (other than Z/P) up to M6 including roofing screws (class 3 & 4) are done this way as part of the process.
With mechanical plating they can apply 50 microns of zinc and not have it fill up the cross recess (phillips drive) or the threads of the screws or make big blobs on the cutting tips on self drillers.
Where the environment is too harsh for HDG, then stainless steel is used, various grades of stainless are used, 400 grade stainless is the lower of the grades for corrosion resistance but has the better hardness and is slightly magnetic, 304 is a general purpose grade and 316 is the marine grade and the most corrosion resistance.
The biggest advantage to stainless is that it is a homogonous material. Coated fasteners will rust if the coating is damaged, eg scratched off during the installation process or by moving parts wearing off the coating.
28 years in building and industrial supplies, the last 12 in fasteners and a bit remains etched in the brain.

Excellent work. Thanks for the info.
Cheers,
Doc.