Material selection

[skww]

I am designing a Catamaran, and I am looking for a type of stainless steels that can serve well in sea water without any rusting. I know 316L can do the work, but for long term using, is 316L good enough? I know there are other stainless steels, like 317, 2205 or 904L, which can work better. Any suggestion?

Also, is there a specfic welding method that can reduce the chance of rusting?

Thank you =)

[robherc]

whatever grade of steel you pick, it'll still end up suffering SOME form of damage from the water (rust or brittleness being the top 2). The key suggestings I'd make are:
1.) Paint it, Paint it, and paint it again...a well painted & protected mild steel hull can outlast the best SS hull if the latter isn't given the protaction from the paint.
2.) Sacrificial zinc anodes...get one (or better, get 3)

Other than that, though, I can't give you much guidance as to what exact grade of SS to use...not my specialty.

[marshmat]

Hi skww,

Are you thinking of making the hull out of SS, or are you looking for a material to use for fittings and running gear?

[skww]

Robherc, Thank you for you suggestion. I understand if I want to make the hull out of SS, I can paint it and use sacrificial zinc anodes to prevent rusting. But yes marshmat, it is the fittings and running gear that I am having problem to stop it from rusting. I have attached two pictures in this reply, so you will have a better understanding on the rusting problem I am dealing with. Is the welding method not correct? Or is it the material problems?

[alan white]

From my experience, 303, 304, and 316 are all okay and 316 is what I've used for most applications. Each type is a trade-off. block manufacturers brag about 17 ph which is supposed to have a tensile strength of 250k lbs.
The welding method shouldn't matter IF the welder is good, though tig process is much prettier than mig. I've migged all sorts of custom parts from 316 and while my welding is shameful, I am a good grinder and finisher, and so I've had no corrosion issues to speak of.
Polishing the stainless is as important as the alloy itself. Even poorly buffed out scratches not visible to the naked eye will invite discoloration and possible corrosion. Therefore, learniung how to properly grind, sand and polish stainless is key to long term success.
And are you certain that is stainless in the photos and not chromed brass or bronze?

[skww]

I am not sure it is stainless steel either, but that's what the supplier told me. Maybe it is not.

Do I really have to grind and finish the stainless steel after welding? Because someone told me as long as your base and filler materials are good, they won't get rusted, since they are stainless steel. I am not sure is that true.

[alan white]

Yeah, on second closer look, it is stainless. The rust is a crevice type, appearing around and weeping from skinny spaces. All stainless has the capacity to rust that way, but only an analysis of your specific material will tell you whether it ought to have corroded at that rate.
Lubrication and frequent disassembly will help. The exposed material appears to be stained but not corroded. proper cleaning and polishing should clean that up.
It would make sense to investigate aluminums for your multi. The right alloys are lighter and should develop a protective oxidized coating that stops corrosion from continuing. WEelds in aluminum aren't as strong as the base metal, however, nor is the piece going to polish bright and stay that way.
On the bright side, you can machine aluminum parts in a wood shop with carbide tools.

[FAST FRED]

For above the WL SS is grand , esp if electropolished.

Underwater there is no need for SS when a good grade of bronze will work for decades , perhaps centuries.

FF

[PAR]

If you have as little understanding of the physical properties of stainless as it appears, how can you possible expect to engineer this material into a boat? I mean no disrespect, but frankly these are relatively basic attributes one would have to have a reasonable grasp on before attempting such an endeavor.

[MikeJohns]

Originally Posted by skww

I am not sure it is stainless steel either, but that's what the supplier told me. Maybe it is not.

Do I really have to grind and finish the stainless steel after welding? Because someone told me as long as your base and filler materials are good, they won't get rusted, since they are stainless steel. I am not sure is that true.

If a magnet will stick to it it's not 316 the standard marine grade stainless steel. Even 316 exhibits surface corrosion, usually visible as brown staining. This is particularly with crevices and rough surface finish. Warm sea water and its chloride ions are the culprit.

You can get very good results with a simple stick welder and SS electrodes, stainless is a very easy material to weld .

Hulls have been built from stainless steel but the benefit to cost doesn't stack up and it still needs painting, paint also prevents the problematic fatigue acceleration that metals suffer from when immersed.

Look up "Pickling and Passivation of stainless steel" this is probably the most important to understand and usually the reason most amatuers come unstuck fabricating SS.

Hope this helps
cheers

[robherc]

Hmmm...maybe if you clean it off & just make sure to keep it oild/waxed in the future (I know...makes the upkeep almost as much of a chore as non-stainless)...that should, IMHO, keep it from corroding, surface-staining, or whatever else it's going to do to degrade...unless it gets nicked/bumped/exposed to the real world

[skww]

Actually, I am a mechanical engineer, and I am just a beginner in boat industry. I am mainly trying to solve the rusting problem of the crane and some other fitting parts on the boat, which is built by stainless steel. I think it is mainly the material problem that causes the rusting in previous ships, but I also want to know is welding or finishing process have anything deal with Corrosion.

So for the best result to prevent rusting in sea water, I can simply use 316L stainless steel, and some good stainless steel filler materials. After the welding, I need to make sure to grind, sand and polish the parts properly. Is that right? and then maybe add some colorless lubrication on the surface? And should I do the pickling before or after grinding?

Does anyone know which country or company produces good quality stainless steels?

Please feel free to correct me

[rasorinc]

this company produces bronze, aluminum, and SS. They list out all the grades and standards of the metals produced. http://www.alaskancopper.com/ss.php If you post the state you live in it helps in offering advise. Why buy in the state of Washington when it is avaiable in Maine???

[skww]

but our company has factories in China, so it will be expensive to get the materials from the United State. We brought some 316L from China before, but they are all rusted after only one trip, so we are thinking maybe the stainless steel manufacturers in China are not reliable. We are thinking to get the materials from maybe Japan, Singapore or Taiwan. Any idea?

And does anyone know how to test the stainless steel to see if they are qualified? I know we can do spark test or chemical test, but they are not really accurate and chemical test is expensive.

[robherc]

If you have access to high-powered magnification equipment....or maybe use a metal-detector to test magnetics? (sorry, I'm not a chemical engineer...lol)

I'd recommend looking for a supplier in the EU if it'd be too expensive to buy from the US...I don't much trust Far-eastern suppliers....too many deep-discount items coming from over there that are worth every bit of what you pay for them...