Crevice Corrosion

[benz]

I've heard folks say that Stainless Steel will only suffer crevice corrosion in an oxygen-starved environment. Is this true? I had an SS wire-gate carabiner from Wichard that corroded most horribly where the gate met the body. It had hung in salt water for a spell, but was by no means oxygen-starved. Could it be that the two different stainless steels corroded each other galvanically?
Thanks
Ben

[Brion Toss]

Hi there,
I think that "hanging in salt water for a spell" might qualify as oxygen-starved. It essentially defines a crevice corrosion environment. I don't know what alloys were in that carabiner, but it would be rare to find much of a potential difference. On the other hand, some grades of stainless are wonderfully vulnerable to crevice corrosion. And it is possible that you had live current juicing the process up.
Fair leads,
Brion

[Stumble]

Benz,

There could be another problem as well. Stainless has a different galvanic potential in and out of water. So a stainless fitting that is partially immersed is actually galvanically active itself.

It could also be that because the corrosion was localized, it has a point failure, either a bad fitting, it was tapped up right there, contaminate from tooling ect.

[Douglas]

Ahoy Benz , West Marine catalog Adviser on SS, is quite good info.

What you have heard about oxygen starvation in salt water is similar to what I have heard too. Where this takes place most is in the cutless bearing and shaft log, on SS prop shafts, but that is called worm-hole corrosion.

My investigations say the enemy of SS is in stagnant salt water environments, like where SS is surrounded closely by entraped salt water, like in lower swaged fitting terminals, and that entrapment turns the salt water stagnant. Further investigation reports that crevice corrosion is a victim of Chloride Ion attack by the free Ion molecule, found in Sodium Chloride or salt water.

Your caribiner's SS alloy , might be suspect. The SS alloy that Winchard uses is probably quite good, as they have a reputation to protect, but I too have found my Winchard SS eye nuts, corrode way too easily ,,, evidensed by rust stains , and thus pitting too.

As for reducing crevice corrosion for your SS gate closure terminals, I have found marrying SS and Silicon Bronze a successful way to reduce the corrosion on SS . Ie: bronze bolt thru a SS plate,,, a SS clevis pin in a bronze toggle, ect., or using bronze link plates instead of SS ones.

The information you will eventually need won't be found in only one place, drats !

This forum and Brion are excellent places to start, but the pleasure boat industry has such a poor history of desiging SS structural fittings and their fabrication and their installations to avoid crevice and other corrosion on their fittings .

[benz]

Thanks all! Phasing out all SS where something else will do is a constant process for me, but I like to keep informed

[Jim Fulton]

If I knew more metallurgy, I might understand all the ramifications of this:

As for reducing crevice corrosion for your SS gate closure terminals, I have found marrying SS and Silicon Bronze a successful way to reduce the corrosion on SS . Ie: bronze bolt thru a SS plate,,, a SS clevis pin in a bronze toggle, ect., or using bronze link plates instead of SS ones.

I don't know the relative galvanic potential of SS and silicon bronze, but wouldn't this combination hasten corrosion?

Jim Fulton

[Douglas]

Ahoy Jim , Yes , of course there is galvanic diferential between SS and Bronze .

Personally , in above water applications,I would rather have my structural bronze parts turn green than to have the connecting SS part rust, pit , and crevice corrode. The bronze turning green with corrosion is a bit like zincs on the prop shaft, sacraficial .

The slowness of the silicon or aluminum nickle bronze to degrade to a non-structural approval is one of it's great assets .

Like Brion has said many times before,,,, " Always try to build in a reserve of neglect ", when fabricating and assembling structural rigging and other boat hardware . He is "Spot On" with this advice.