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How do the following screw types fare in an outdoor (wet) long-term application?

Galvanized screw:

galvanized

Stainless steel screw:

stainless

"Deck" screw:

deck

9

The stainless steel screw will absolutely be the best screw to resist rust. Stainless steel screws are rust-resistant throughout the entire screw, not just on the surface.

The other screws are only covered with a rust-resistant coating on their surface, which will break down or wear off over time. Galvanization is a process that coats with zinc. Other screws may be zinc coated as well using another process, or they may be coated with something else.

I have some screws on an outdoor fence which have rusted inside the slots in the screw head, because the screw driver tip wore away some of the coating in the slots at time of installation. Phillips-head screws in particular are notorious for getting ground up when the tip "cams out" and jumps out of the screw head.

Stainless steel is undoubtedly the best material for resisting rust and corrosion in screws. Why, then, would anyone use anything other than stainless steel outdoors? Two reasons:

(1) Stainless steel is slightly softer than the hard steel used in deck screws or other similar screws. You can't just "go crazy" with the power tools and slam the screws into place. It is easier to chew up the heads on stainless screws, or to snap the heads right off by over-torquing them (though that is more of a problem with bolts than wood screws). You have to slow down and be a little more careful.

(2) Stainless steel screws are more expensive. Galvanized or other coated screws are just plain cheaper.

Despite those drawbacks, the rust prevention is unmatched by other screws. I live in a rainy wet climate, so I use stainless screws on anything outside that I might need to ever take apart again.

  • You want Torx or Robertson drive; stainless steel screws are soft. I'm paying $60 for 5 pounds of 1-1/4" stainless deck screws. It's to do a tongue-and-groove roof. – Harper Jan 3 '17 at 1:24
  • How long would you estimate that a stainless steel screw can be outdoors, in a sometimes wet environment, before it begins to rust or deteriorate? – Fil Jan 4 '17 at 14:09
  • Stainless steel simply will not rust on its own. (If it touches rusty plain steel, that rust can transfer to the stainless. Not generally a problem for wood screws.) You can expect them to easily last a century or more wIthout deteriorating. – Grunthos Jan 5 '17 at 15:15
  • Agreed on the Torx or Robertson (square) drive. The stainless deck screws that I've bought have had Robertson square drive heads. – Grunthos Jan 5 '17 at 15:18
  • Pretty sure stainless WILL rust on it's own. It's simply more resistant to corrosion than other steels. And, yes, if you mix metals you will cause some metallurgic phenomena where the corrosion is accelrated ... search for "dissimilar metals" and "corrosion" ... as a rock climber, I can tell you that our community says 50 years for a stainless bolt correctly installed in an outdoor setting. Corrossive environments, such as near the sea, can rust stainless in only a few years. – chad Jan 13 at 17:36
2

Steel , galvanized or coated, start failing in a deck at 10 years and all fail by 20 years. SS will last "forever". Because SS can only be cold worked. the shanks are soft ( The heads and threads are cold formed so stronger). So they need a clearance hole in the top board and a pilot hole in the bottom board. SS are not used in sea water because the chloride is very corrosive. Copper alloys are used because they inhibit marine organisms and are corrosion resistant. One nuisance of SS fasteners is there is no standardization of the drive so you could need 4 different drivers.

1

Stainless requires exposure to oxygen to avoid rusting; it's the chromium oxide layer that protects the steel. When completely buried, the chromium oxide layer breaks down and the residual oxygen starts a process called crevice corrosion. Passivization restores a fresh layer of chromium, but passivization cannot be done once fasteners are buried. This is why bronze fasteners are used on wooden boats; the bronze does not need a supply of fresh oxygen. Unfortunately, bronze fasteners are very expensive.

  • Hello, and welcome to Stack Exchange. I've never heard of this before, and a fair amount of Googling turned up no supporting information. Where did this info come from? – Daniel Griscom May 5 '18 at 15:29
  • I was curious too and a search for stainless steel crevice corrosion produces many resources, such as this one: ssina.com/corrosion/crevice-pitting.html – binarymax May 8 '18 at 15:55

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