brass and stainless

[JeffM]

I'm about to apply a brass strip to the keel, and I'm holding a handful of stainless steel screws. I wonder if this is maybe a mistake, combining dissimilar metals in salt water. Anybody had any experience with this? Any chemists out there?

[ken connors]

shouldnt be a problem unless the boat will live in the water. do you plan to moor or keep it at a dock? if not, there will be very little continuity between the two, as wood is a poor conductor.

[Frank Hagan]

The general rule is that fasteners can be "more noble" than the material they are attaching, but not "less noble". Stainless is just about the highest on the nobility scale that we would usually use.

Kasten Marine has a short article on corrosion that is a regular web page: Click Here! and then a longer article that is a PDF file that is about 200KB you can open in your browser if you Click Here!.

The PDF file has the nobility scale, and based on the general rule of thumb and the scale, you could use silicone bronze screws or stainless. The problem I have with stainless screws would be if you are sealing them into the strip, and planning to have your boat in the water all the time (or for long periods ... over a week, say).

The stainless is susceptible to crevice corrosion if it isn't supplied with a constant supply of oxygen (the alloy forms a film over the surface of the steel in reaction to the oxygen, and this film is what protects the steel from corrosion). Boats have real trouble with any water that isn't regularly oxygenated ... like water trapped in the little space between a fastener and the wood that has shrunk a bit. Or in the bilge.

But if you are trailering your boat, and you aren't going to keep it in the water all the time, the stainless should work without getting galvanic corrosion. You could get crevice corrosion of the screws and failure later on. I think a better choice is the silicone bronze screws available at a chandlery or on-line.

[Frank Hagan]

Here's a quote from the PDF file:

Stainless steels each occupy two locations in the galvanic series of metals. Stainless can be both passive and active.

The same piece of stainless can be both in different parts. Stainless is usually passive when it has enough oxygen available to create a tough oxide on the surface. Stainless usually becomes active when there is insufficient oxygen' date=' as when embedded in sea water soaked wood.

Due to its tendency toward crevice corrosion, stainless steel of any kind should never be used as a fastening material below the waterline, especially in a wooden vessel! Stainless should not be used as a fastening material for any part of the hull structure of a wooden boat, even above the waterline. Stainless should not be used for keel bolts on any kind of boat.

Stainless is subject to pitting corrosion in salt air

[/quote']

[JeffM]

Pretty scary words from Kasten Marine! I plan to bed the screws in keyed epoxy bushings, then the whole strip in bedding compound, and the boat will be trailered, so I think I'll use 'em. I don't mind a little maintenance every decade or so! Frank, thanks very much for the info!

[Frank Hagan]

Here's a quote from the PDF file:

........... Stainless should not be used for keel bolts on any kind of boat.

Stainless is subject to pitting corrosion in salt air

Well Production builders do use Stainless bolts ALL of the TIME>

I will grant that there are better choices available' date=' but I really haven't heard of that many keellosses at sea to be honest.

IIRC, surveyors tend to reccomend dropping the keel and replacing stainless keel bolts every 25-30 yrs though.[/quote']

Yep, and there's a bit of controversy about stainless in that application. I guess it all depends on if you have a generally dry bilge or a wet one; its possible today to have a bilge that is kept free of standing water.