@Jon, @Freeman, and @ThreePhaseEEl, thank you for your comments.
So the conclusion was that stainless steel is fine as long as the junction between the galvanised steel and stainless is inside the building envelope which ensures that there is never enough moisture to cause galvanic corrosion.
Galvanised fasteners are NOT allowed to be exposed externally for the building site since I am within 500m of the shoreline, although it is a protected inlet and the site is 50m above sea level. Existing galvanised fasteners on the house which has been in place for 40 years are still rust free, so local conditions are more mild than the actual corrosion zone specified on paper. The H3.2 CCA treated cladding requires stainless fasteners, but assumes that they will be exposed.
In my use case, the fasteners will be covered with decorative battens that are affixed with adhesive sealant then painted, so the fasteners will be protected. They hold the cladding on, but are otherwise not structural.
Fastener Test Results
The stack-up is:
- 0.75mm steel framing
- 30mm PIR insulation
- Moisture barrier
- 12mm of plywood cladding (non-structural)
The total thickness is 42mm and a minimum of three threads must be left exposed on the inside. Allowing for the taper or drill point on most screws required between 65mm and 70mm total screw length. Many of the specialty fasteners were only available up to 60mm.
The steel framing is G550 Zincalume and is 0.75mm thick. The thickness turned out to be the major problem which was too thick for most stainless screws made for wood and too thin for self-drilling screws made for 1.5+mm metal.
Bi-metal Stainless Screws
These screws have a steel self-drilling head welded to the stainless steel screw. These should work great, but unfortunately were not available in the 65mm length that I needed.
Wing-tek screws have a drill point and small metal wings on them. The wings drill a clearance hole in the wood and then break off once they hit the metal. Unfortunately the 0.75mm steel was too thin for this to work consistently and sometimes the wings would go through the steel as well. This happened on 1 out of 3 screws in my testing.
Drill-point screws are designed to drill an interference fit and then the threads are cut into the metal. These screws are used for structural connections.
Since I am attaching plywood cladding to the steel framing, I would ideally use these by pre-drilling the wood and then affixing the screw.
From my testing, these screws did not work since the drill point created a clearance hole and the threads were loose such that the screw could be easily moved side-to-side since it was essentially sticking out because of the foam insulation.
Doing a pull-out test by hand, I was able to move the screw back and forth a few times and then pull the screw out. I expect that wind and thermal movement would cause these screws to back out over time.
Great screws, but they suffered the same problem as the drill-point screws of cutting too big of a hole before threading and also failed the pull-out test, but held better than the drill-point screws.
T17 Wood Screws
These screws have a tapered needle-point tip with a slot for cutting threads. Instead of drill a hole, they basically peel the metal away which effectively increases the thickness of the metal. They hold tightly and are often rated for drilling into thin steel.
Needle Point Screws
Chipboard screws have a needle point and are available in stainless steel. I tried several of these and 2 out of 3 dulled when going into the G550 steel. If I pre-drilled, they would often snap off or bend before being fully seated.
There are thousands of different types out there and I ended up finding some 5mm x 65mm coated steel screws with a T17-type tip from Optimaxx. These have passed a 2000+ hour salt spray test and have a 20-year exterior corrosion protection. I screwed some test samples of the screws into steel framing, removed them, and then placed them in sea water on top of a piece of the H3.2 CCA-treated cladding and allowed this to wet and dry out every day. I have another sample with the cladding, insulation, and screw that is sitting outside in the weather. After 3 months, neither have any corrosion on them.
The cladding screws need to have a durability of at least 15 years, so the inspector did not have any concerns.
Given no suitable alternatives, no corrosion with the tests samples, and the fact that the existing structure on the site with exposed galvanised fasteners in the same H3.2 CCA timber and has not shown any corrosion in 40 years gives me some confidence that this will work.