Corrosion resistant bolts for attaching appendages to hull

I need a few M12 and M16 bolts. The old ones that were in place have failed. Haven't seen them, but friend who owns the boat says one of them literally broke in two, then they had to drill through the remainder of the bolt. Which is weird, but who knows.

They are for attaching something to the steel hull, i.e. will be in seawater. What material should I look for and where can I buy some? Does anyone knows?

I know that "plain" stainless steel does not fare that well in sea water. So, what does?

Every advice would be appreciated...

 

The designation M12 and M16 is just the thread size, in this case metric 12 mm and 16 mm respectively. What material? Are they hex bolts, cap screws, etc.? Thread count? Most importantly what are they holding up, expected sheer and tension loads and what are they threaded into?

Stainless holds up fairly well in salt water, though some grades are better then others as you've guessed. 303 and 304 are the most commonly used stainless alloys, but they aren't as well suited for salt water as 316 or better yet 316L.

There are other, typically application specific stainless steels to select, such as higher chromium content , formability and/or heat resistance, etc. So, what are you trying to do, specifically?

 

Sorry about that. Yes, they were hex bolts for an allen key, fully threaded (thread all along the length). A bit weird choice in my opinion, but who knows. I generally haven't seen allen key bolts until now on boats. Maybe they're more common in some other parts?

They were holding the propeller skeg and the stern thruster. In the case of the thruster they were threaded into a solid steel plate, 28mm thick.

I'll go look for the 316/316L equivalents in my part of the world (Meditteranean, Adriatic sea).

But generally, yes, you gave me an answer.

 

Concur with PAR.

Although standard 316 or 316S16 will suffice. The "L" in the 316L is low carbon, which is required when welding to prevent carbide precipitation. So if not welded, 316 will do, and is cheaper than 316L.

 

"Allen" key bolts are called socket head cap screws. They come in a few different head configurations, such as button and counter sunk, but all use the Allen socket.

Hex head is the tradisional 6 sided bolt head that you put a wrench on.

 

If you include your location it will help. It wouldn't do you any good if someone gives you and address in Rome if you live in Singapur.

 

Well, I'm in Zagreb currently if anyone's familiar with the surroundings, but I don't expect you to find me an actual shop! Can handle that part for myself. I already got the advice I needed, mostly regarding the material.

So, thanks to all. You're more than a friendly bunch when it comes to a constructive advice

 

Titanium Bolts will never corrode but Stainless Steel which is different from CRES (Corrosion Resistant Steel) will depend on a number of things. I have seen SS corrode in Aircraft and boats. Titanium is very expensive but takes the worry away.

http://www.ssina.com/faq/index.html

 

The stock answer here from a corrosion engineering perspective is to bolt with material as close to the parent material as possible.

I'd use a high tensile steel bolt. Grease the thread and shank with waterprrof grease such as Shell Nautilus, torque the bolts evenly and use a locking mechanism. Paint the exposed material with epoxy. Then protect it all with a sacrificial anode ( should be there anyway).

 

A steel bolt is a good solution. SS also works when installed correctly. If your bolts are corroding ,they are leaking. Stop the leak...keep the bolt dry.