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Mark0
03-04-2018, 06:47 PM
Hi,

So I understand not using stainless under the water line, but what about for above the waterline (trim) ? Is 18-8 ok, or should you still go with bronze.

Cheers,
Mark

Thorne
03-05-2018, 11:29 AM
The general rule is that the heads of SS fasteners be exposed to the air, SB coated / capped / plugged. But for trim it may not matter much...hard to say.

Jay Greer
03-05-2018, 02:13 PM
In spite of what others say and ending repairing other people's mistakes in using stainless steel for woodenboat fastenings, I firmly believe in using bronze.
Jay

Mark0
03-05-2018, 08:15 PM
Fair enough Jay, I'll heed your advice on this. It's just easier to obtain SS than bronze locally, but I'll order in some stock. It will be good for ships stores.

Makr

alkorn
03-06-2018, 10:27 AM
There's a tremendous difference between 18-8 and 304 or 316 in a salt water environment.

In my experience, 18-8 will rust in a salt environment even if above the water line. The rusting is a thin layer and is, I think, mainly cosmetic.

304 and 316 should be fine above the waterline, or below the waterline in areas protected from saltwater. If salt water should enter, 316 will resist corrosion better than 304.

MN Dave
03-06-2018, 11:15 AM
Good quality 304 should be OK above the waterline. It has to be passivated, which is standard practice for any decent manufacturer. While 304 is an 18-8, if they don't say 304, you can bet it's 303.

18-8 is a generic name for almost any stainless steel from 302 to 384. The worst of them is 303, which is a free machining grade that contains sulfur and tends to rust a lot more than the others. An awful lot of screws sold as 18-8 are made of 303 and I never trust any fastener that doesn't specify the exact grade. No one brags about crap, but crap is cheap so they use it and describe it accurately, but uninformatively.

This is a good list and general breakdown of the various grades:
http://www.bosunsupplies.com/stainless-steel-info.html
It does not cover the low carbon grades for welding, but is a good overview.

If you want to get into the weeds:
https://www.cartech.com/en/alloy-techzone/technical-information/alloy-selection/how-to-select-the-right-stainless-steel-or-high-temperature-alloy-for-heading

If you chew up the head with a bare steel tool, it can still rust. I have seen A286 bolts side by side from two different manufacturers on an amtrack (ooh-ra, not choo-choo). One batch rusted like plain steel and the other didn't rust at all. Passivation is important. Neither was magnetic, and both were very hard, so there isn't much chance that the rusty ones weren't A286.

When 303 is cast, the sulfur separates out in little blobs of MnS, which stretch out into stringers when the steel is rolled and drawn into wire. The stringers are great for machining, but have poor corrosion resistance and are ideally shaped to initiate pitting.

Time to pull the weeds?

jpatrick
03-06-2018, 12:38 PM
Thank you for the objective response, MN Dave. I, for one, will be extra careful and thorough in my use of stainless from now on. I've always been aware that 316 is good to use but was vague about 304 and the ubiquitous 18-8. I will check my stock for the culprits and the undecided. Fortunately, I prefer bronze to stainless so I don't fear that I have a ticking bomb in my boat.

I wonder how many shipwright's have found rusting "stainless" fastenings of an unsuitable alloy and have formed a permanent bias against all.

Jeff

Phil_B
03-06-2018, 08:52 PM
It depends on what you are using them for.

If you are attaching trim or other cosmetic stuff (for want of a better term) such as a stainless steel window surround or a deck plate to resist wear, then as MN Dave recommends, carefully research the material and cautiously go for it.

If it is structural or where a failure of the fastening will be serious, you would be better off not taking the risk. There are so many factors (such as heat treatment, rate of cooling and hence precipitation of compounds like the sulphur MN Dave mentioned) that it is a gamble. Professionals (such as the Offshore Oil Industry) have banned stainless for structural applications and rely on the known properties and known corrosion rates of non stainless steel.