On Mon, 13 Dec 2004 06:16:20 -0800, RyanN wrote:

There are several applications I'm looking at, but all involve
galvanically isolatinng the head of the fastener from the item being
fastened.
Two examples that come to mind are attaching aluminum hatches and
stanchions to the deck. In this application, only the head of the
fastener comes into contact with the aluminum, but there is enough
force that a viscous isolator, such as silicone or tef-gel, probably
won't work in the long term.

I'm talking about adhesive sealants such as west marine silicone sealant,
or something more serious like 5200.

As for sizes, my current applications involve #14 oval head screws or
1/4" machine screws.
As for McMaster-Carr, I had spent a good amount of time looking on
their site before posting. Many types of washers, but I can't identify
the ones I'm looking for. There are cup washers for pan heads, and
finish washers for using oval heads on a flat surface, but nothing like
the washers I'm looking for.

Well, I've used screws to attach hatches, and had no problems with
corrosion. The point is that you have so much bedding compound in the hole
with the screw, that during installation the compound oozes out of the
hole and covers the everything, and the aluminum and stainless
are effectively isolated by the bedding compound. Since the screws on
hatches are normally on the dry side of the hatch, this is more than
enough protection, IMO.

Sometimes standing water covers stanchion bases, so extra caution is
order with them. If I had my preference, I would make sure the
stanchion bases were fully anodized. Then I would trust the annodization
along with the bedding compound to protect the aluminum. Second choice
would be powder-coated aluminum, I guess. Last choice would be aluminum
primed with a corrosion inhibiting primer and painted with a decent marine
paint.

But the bottom line is that all of the aluminum needs to be coated, and
the bedding compound will probably isolate the screw from the aluminum
well enough.

Just my $0.02.

--Mac