I decided to revisit the whole issue, and over the last few weeks I've had four firms, other than those who quoted for me in the past, look at the solar panel option again for me. The new firms were Origin, True Value, Unleash, and the people who used to be Solar SA under their new name which I don't recall.
Unleash, Origin, and True Value commented from looking at pictures on my roof at various dates on Nearmaps, while the other group sent someone out to have a look on the ground as well.
Unleash were the only one to offer to sell me a system.
The other three advised me to forget it unless I could remove quite a few of the trees in the back yard on the north-western side of the house.
The guy who actually came out said he doubted I'd get more than 50-60% efficiency even with the Tier 1 panels they used.
Also interesting was the screw and joining issue discussed above in this tread.Two of them said coated screws should be used rather than stainless steel ones, while the other two went for stainless steel.
So, I did some on line research and came up with a technical standard for Helipad construction.
It favoured stainless steel over coated screws but added,
"If stainless steel bolts or screws are threaded into aluminium, a suitable thread sealant shall be applied to the threads to prevent ingress of water and corrosion of the threads."The other option for marine application or other wet environments was to use a transitional material called "Triclad".
http://triclad.com/Possibly that would be a bit over the top in cost for home solar installations.
The more local comment was from
http://aluminium.org.au/FAQRetrieve.aspx?ID=43866"What fasteners can I use to join aluminium? Aluminium blind rivets (commonly called ‘Pop’ rivets), 300 series stainless steel screws and bolts and hot-dipped galvanized steel bolts are commonly used. The actual material and joint design depends on the severity of the environment."Overall I get the impression none of the options guarantee
no electrolytic corrosion will occur whatever you use to join an aluminium frame to a Colourbond roof...
Stainless steel frames anyone?