Crevice corrosion?

J

jstokes

Member
Joined
Sep 29, 2001
RO Number
6292
Messages
115
I am installing a teak platform on my Albin, The brackets are 316 stainless. The through-hull bolts, about 4 of which will be below the waterline, are also 316 stainless. Are the below waterline fasteners doomed to crevice corrosion, or is there something I can do to prevent this situation while the boat is on the hard? All advice appreciated. Thanks.;
 
I am going through some testing of 316SS marine products and found that most marine products that sales persons are selling as 316SS are not 316SS. How do you know that your brackets are 316SS.
 
For salt water contact use 316L Stainless Steel or use Monel. Crevice corrosion in 316 stainless steel occurs due to the lack of Oxygen in direct contact with the material. If you have questions about the material you plan to use, pay a Metallurgist to test the material in question.
 
316 is less susceptible to crevice corrosion than 304 because of the added Molybdanem. 316L would be needed if it was welded. If the brackets are easy to get at so you can check them or to change the bolts then you could use them.

The reason I asked the question above is that a lot of offshore products are using 304SS that was made from recycled scrap and sold as 316, in other words it is made from junk.

Spelling
 
The quick and easy way to check for the "good"
Stainless steel is the old magnet test.
If it sticks it's not "good". If it doesn't,
it is the "Good stuff - 316".
Many times you are billed for 316 when it is
something else, specially with bolts.
Carry a small magnet around when buying bolts.
H
 
To answer the questions: I buy my 316 s/s fasteners from WM and magnet check them too. Guess I'll just have to pull out the transom hull liner and replace the fasteners at annual haulout, unless other ideas come forward.
 
316 and 317 are the only stainless steel that have molybdenum in it. There are a few ways to detect 316/317, they are Magnet Test, Acid Test also known as the Moly Test, X-ray Test and Spark Test. I will be using the X-Ray and Acid Tests.

No stone unturned.

Necessity is the mother of invention.
 
Crevice corrosion requires small amouts of water in a confined area. Well gooped bolts seated in more goop should keep out the water.
 
Eureka! Thanks Bruce, that's the answer. I will goop the fasteners thoroughly with 4200 and avoid 316 s/s crevice corrosion. Once again, Boatered comes through. Thanks to all. J.
 
L Keith is right and there is a huge difference between stainless that is going to get wet and stainless that is going to stay wet. It's the later that is a problem. Theoretically if you can keep the water flow "fresh" and full of oxygen, it will work, but usually there are areas where the sealant has microscopic areas where it did not fully adhere. In these little pockets, the water will go stagnant and crevice corrosion starts at a microscopic level. Once that happens you have have a mechanism for creating voids that themselves propagate more crevice corrosion. You simply can not get enough sealant around the fastener (including the screw head).

That's why you don't use stainless below the water line.

I learned all this after replacing multiple sets of swim step bolts that consistently failed multiple times and would show up on annual haulouts with fasteners eaten away 40-60% on a swim platform often supporting divers carrying hundreds of pounds of weight. It was a big problem.

Go with bronze fasteners and stop worrying about crevice corrosion.
 
I would think bronze in contact with stainless would cerat another problem
 
Ghost: I wonder if you had another problem because there are a lot of SS bolts used underwater without that rapid rate of failure.

I once puled a bunch of ten year old well gooped SS bolts in contract with SS gudgeons and fiberglass and while they had some dark spots there was little necking so it isn’t something that always happens
 
So now, it seems I must choose between risk of crevice corrosion on well-gooped 316 fasteners and risk of electrolytic activity between bronze fasteners and 316 platform brackets. Help!
 
Well, that's boating! Tef Gel will solve the unlike metals issue, and is excellent corrosion preventing goop if you go stainless. Flip a coin and don't over think it. Ask at a good yard what they'd do if that will settle your mind some. My platform, which we refurbished last winter has bronze brackets attached to the transom with silicon bronze bolts through thick solid fiberglass to a backing plate.
The platform bolts to the brackets using stainless bolts. I have no idea how long those bolts were in there, but they were in pretty good shape.

George
Hatteras 56MY
Boston Whaler 130 Sport
 
Geez, I have never seen any problems with my stainless mounted below the water line after years and years of use. That includes nuts, bolts AND PROP SHAFTS. Even my trim tabs are made from stainless.

Foggy
 
There is a 317L also. But more expensive.
For much harder, there is a 2507.
 
From a magnetic point of view, there is no measurable difference between 304 and 316. The chemcial compositions are almost identical. 200 and 400 series SS are both "magnetic" and have the ability to be thermally heat treated for better mechanical properties. These are not suited for Marine use.

The "spark test" works well. Grind a piece of 304 look at sparks they will fork out at the end of their flight. 316 will spark but each spark will stay intact till the end of its flight.
There are other tests you can do, but these are quite extensive and basically test for the presence of Molyb.
The best method for thwarting corrosion is inspection. Encapsulating fasteners may appear to be a good solution, but failure would be catastrophic. Similar to the "coated" anchor chains. You know it failed when your drifting away...........
 
316 is an Austenite metal or non magnetic. A magnet will stick to 302/304 but with 316 a magnet is only slightly attracted. 316 has a higher Nickel/Crome ratio 14% Nickel 18% Crome 3% Molibdenum.

Speelling
 
You must log in or register to reply here.
Back
Top