How to attach treated pine to colourbond in a pool area. Will stainless steel screes be ok?

[WThommo]

Help needed choosing the right screw to attach 22mm thick decking treated pine H3 batons to colourbond fence in a salt water pool area. The Bunnings attendant that helped me suggested stainless steel screws. I have since read there may be a corrosion issue with this type of screw.

[MitchellMc]

Welcome to the Bunnings Workshop community @WThommo. It's brilliant to have you join us, and many thanks for your question about attaching treated Pine to Colorbond steel.

Typically, hot dipped galvanised, coated, or stainless steel screws can be used with treated Pine. I'd recommend using galvanised self-tapping screws as they are both suitable for the treated Pine and the Colorbond fencing.

On a side note, if this is the boundary fence for the pool, it would be best to check with your local council to ensure any modifications you make comply with pool safety barrier regulations.

Please let me know if you have further questions.

Mitchell

[Stuckster]

For anyone reading this in the future...

Fasteners aside, Bluescope highly discourages Colorbond being in contact with treated pine since it causes premature corrosion especially in moist environments.

http://www.bluescopesteel.com.au/howto/avoid-corrosion-caused-by-contact-with-timber

If you have to use treated pine, make sure you fully paint or seal it first to avoid direct contact with the steel.

[MitchellMc]

Interesting read @Stuckster.

I must admit that my understanding was that the primer and painted topcoat isolated the metallic coating from the timber and, hence, the corrosive effect. After discussing this with Bluescope's technical team, I also learned that there are concerns about the chemicals in the treated timber affecting the painted topcoat and penetration through it, which is why direct contact is not recommended. Further, it's advised that not only should treated timber be isolated from the Colorbond but that runoff or splashback from the timber should be inhibited.

We'll keep this information in mind when questions of treated pine in contact with Colorbond steel arise in the future.

Many thanks for the heads-up.

Mitchell

[Stuckster]

This requires further clarification.

In the technical bulletin attached to the article, Bluescope states...

"The presence of soluble salts (e.g. sodium sulphate) derived from the treatment process, may contribute to corrosion by increasing ionic conductivity in moist conditions. Treatments of Copper, Chromium and Arsenic (CCA), now used less commonly in construction, also present a highly corrosive influence
for metallic building components either in direct contact or subject to water runoff from the timber."

I would presume that ACQ treatments (also containing copper) are also incompatible. However newer treatments such as LOSP appear to be ok, which is good news.

Some other useful excerpts...

"In instances where no treatments are present, or where corrosively benign treatments are used, or coupling of materials
occurs in a scenario where humidity or moisture exposure is low, and of short duration, it is not expected that timber will have significant
influence on the performance of steel materials."

"However, if treatments are unknown or exposure conditions are unpredictable, it is recommended that a separation strategy be
employed"

The following options can serve as effective methods to separate
timber products from metallic coated and prepainted steels:
• Inert strip-style impermeable membrane, self-adhesive films and damp proof course materials as referenced in BlueScope
Technical TB 34 – Steel Building Frames
• Pliable building membranes (check with manufacturer) and blanket & foil membranes (insulation)
• Packing of timber windows and door jambs can provide adequate separation (via an air gap) from a metal frame
• Barrier paints as advised by reputable paint suppliers.