this for the benefit of anyone reading this who's contemplating building or upgrading in aluminium, because clearly Steve's nailed it for his specific needs. Having said that, even you might want to consider some of these options, Steve, further down the track, when you're tossing up the merits of adding isolation bushes and washers in some instances where it seems necessary:
There's another couple of arrows in the quiver, which involve some form of surface coating. The first of these applies specifically when using stainless fasteners or hinge pins or brackets: cadmium plating the SS helps a lot to prevent or delay the onset of the dreaded chalk dust on the adjacent alu.
It's actually pretty effective, especially in combination with something like Tef-gel or Duralac (still not as good as full isolation, but a LOT less work). It even improves the corrosion resistance of the stainless, particularly in respect of the nastier diseases it is occasionally prone to, such as crevice corrosion or pitting from oxygen deprivation, where the stainless item essentially attacks and consumes itself.
It can do this under certain circumstances because it's capable of simultaneously occupying two different locations on the galvanic scale, meaning it acts as both cathode and anode in an electrochemical cell, requiring only a conductive partner like salt water
or damp salt crystals to form an ad hoc
Sorry to use Steve's thread as a pulpit; he'll know this stuff but I'm thinking of interested parties who might not ...
Cadmium over SS is pretty standard in aircraft practice. It's a two step process, first a flash of nickel (known as a 'nickel strike'), then -- with minimal delay because the nickel will passivate otherwise -- plate over that with cadmium.
If you've got a mate in the aerospace industry (shouldn't be hard in Washington
State?) you might be able to get this done to aircraft standards at mate's rates
(Cadmium is a nasty element, so it's harder to get it done than when I first did a batch, many decades ago - I got a big
boxload done for free, tee hee, back in the days of laissez faire
management - well, as a reciprocal favour, to be fair ... There are hopefully still enough left to break the back of my next project)
Another arrow in the quiver is anodising the aluminium. It's not easy to do to big structures (there is such a thing as 'pad anodising', but hardly practical).
However it's quite practicable and very affordable for small stuff, provided you send it to the plater (anodisers usually also do plating) in decent sized batches. It's worth waiting until any new hardware
has been thoroughly debugged in service
, This is so that no further features (holes, chamfers, edge rounding, rebates etc) will need to be added in future which would destroy the protective layer.
It's also worth well rounding any edges where there's any prospect of wear or handling, as the anodising will last a lot longer. You do need to make sure it's done to a marine
standard (thicker build). If you're building a gold-plater, go for Hard-Kote™ !
A third is one of the newer coatings such as Nyalic, often used on aluminium powerboats and flatbeds for upmarket utes. There are also products intended for 'deck liners' - I don't have any direct experience with anything in this paragraph, so hopefully someone who does might chime in.
A related solution: If you need to thread stainless into aluminium, there's a product worth knowing about
: Stainless Helicoils are available these days with a surface coating called "Primer-Free™", specifically intended for isolation. I assume it's dipped in some sort of thermoplastic. I haven't used any yet because I've only recently found out about them.
This seems like a great solution, because helicoils are a match made in heaven for fastening direct to aluminium, which is too soft and weak to handle bolts being run into and out of holes more than a couple of times, especially once a bit of the dreaded chalk-dust puts in an abrasive appearance.
Even in a once-only assembly scenario, Helicoil-style inserts provide a stronger connection with aluminium, partly because the diameter of the thread into the alu is increased wrt the bolt, and partly because the compliance of the spring-like helicoil acts to distribute the load through the entire depth
of the tapped hole, unlike a solid metal-to-metal interface.
Helicoils or similar inserts also help us by eliminating nuts, and sometimes eliminate the need for a second person to hold a spanner on the nut.
... and/or they help by allowing fastening in blind hole situations (which is great on deck or tankbuilding, where you don't want to drill through for leakage prevention reasons)
They're also, at least theoretically, available in cadmium over stainless.
Installing helicoils is not a trivial exercise if you're not a machinist, though: there are a few pitfalls, but anyone who's keen to know more and doesn't have their own domesticated pet machinist can PM me.
I've probably forgotten a few other options but that's something to be going on with for anyone interested.
I first got interested in the interaction of stainless and aluminium when I went to remove the alu frames from the cabin side ports
on my first (FRP) sailboat.
The stainless fasteners were immovable. The heads twisted off every single
one: stupidly I carried on. What I should have done was leave the heads intact, and make up a drill jig bush which fitted over the heads, so I could more easily drill the fasteners away.
This is done using a drill the same size as the fastener - in practice, you need to use a somewhat smaller drill, so it stays within the fastener as long as possible. If it breaks out one side, it dives off in that direction into the adjacent part, which is no fun. Drilling stainless by hand is NEVER fun.
The interesting discovery was that around every hole, the aluminium frame had created so much white powdery corrosion product * around the hole, at the (invisible) interface with the fibreglass, that it had tried to jack the frames off the cabin side. But being prevented by the fasteners, it had instead crushed the fibreglass significantly.
Luckily one of the fasteners was not up to the strain, and broke, because I was unaware of the problem until then. When I tried to punch the shank of the fastener out, it was immovable, and that's when I started to notice a faint stench of rat.
* which I'm led to believe is hydrated aluminium oxide. Some of the attached product in chronic and contained cases like this tends to be a semi gel-like, semi glass-like, more coherent body, semitranslucent, with a greyish /vaguely greenish hue.
The problem is this: the density of this corrosion product is remarkably low in comparison even with aluminium, which means the volume is considerably greater, over six times as great, and in cases like this, there's nowhere for it to go.