Seeking a recommendation for screws in an always wet environment

[pedr0]

Hi,

Can you recommend some screws for an application where the top of the screw will constantly be in contact with moisture?

It's to screw a 9mm thick plastic board to some battens. The battens will be on the dry side of the plastic board, but the other side of the board will be covered in a felt that will be kept permanently moist for growing plants.

The screw head does not need to be flush/countersunk, if that helps.

 

[ETAF]

this may help
https://www.woodmagazine.com/materials-guide/fasteners/drive-screws-that-dont-corrode
https://www.albanycountyfasteners.com/blog/fasteners-salt-water-environments/
is the water sea water ?

maybe look for decking screws

 

[pedr0]

Thanks @ETAF.

It's not a salt water application. It'll be for a living wall (basically plants on a vertical wall)- so clean fresh water, albeit with some traces of fertilizer and soil.

I had been looking at roofing applications but they tended to be nails. It did lead me to considering polytop screws though. The polytop would be concealed by the felt - so aesthetics aren't a consideration.

Decking screws sounds like a good starting point.

 

[Tigercubrider]

stainless screws. polytop or not

 

[pedr0]

Does the polytop do anything to protect the screw or prevent water ingress down the hole in the board?

If not, it sounds like stainless screws might be sufficient for my purpose.

Does it matter if it's A2 or A4 stainless?

 

[^woody^]

A2 stainless. Simple.

 

[freddiemercurystwin]

Remember stainless is exactly that, it stains less it is not immune.

 

[EddieM]

personally I'd go for 316 stainless (A4). As it's into plastic there's no galvanic issues.

 

[noseall]

Stainless screws can be a wee bit softer than regular gold/silver fellas so the heads are more vulnerable to rounding out etc.

 

[JohnD]

A4 (or "316") is marine stainless, and more resistant, and will stay bright.

https://www.boatfittings.co.uk/types_of_stainless_steel_a2_vs_a4_or_304_vs_316

A2 will discolour over time, which is not much of a problem. Under ordinary conditions, it will not rust, and you can get a special anti-seize "lube" (it is not a grease) for the threads because stainless steel can cold-weld otherwise, and you will never get it apart. This is not like ordinary steel seizing through rust or corrosion. No penetrating oil or extra-long spanner will work.

Cheap Chinese steel is notoriously poor quality. To save money, they often omit the Nickel content, which can double the strength of steel, and is an important part of a stainless alloy. If you buy anonymous stainless screws from a discount supplier, they may be very poor quality. German steel is, IME, quality controlled (but more expensive)

I use A2 outdoors and in the garden and on minor fixings for the car. I am in a coastal location and ordinary or BZP steel suffers very badly.

Ordinary commercial stainless is not strong enough for critical components such as brakes, wheel nuts, suspension on a car.

If going into aluminium, you can get anti-corrosion lubes due to dissimilar metals. I use nickel grease. Copper grease is unsuitable.

 

[pedr0]

Wow thanks everyone. Really helpful responses. Now I've just got to navigate my way to quality steel. Got any recommendations for makes / suppliers.?

I ll be buying a small quantity

 

[JohnD]

I often buy from Kay's.

 

[Old Salt]

Brass