Stainless steel grades suitable for sea water Q&A

A. A Yugo and a Corvette are both automobiles, but there is a world of difference between them. It is the same with "stainless steel". One of several stainless steels that are reasonably tolerant to sea water is Monel. 303Se would be a terrible choice. I would certainly want a book on corrosion before making a bold statement either way. NACE, I think it is "National Association of Corrosion Engineers" web site would be a good place to start a search for information.

A. Dear Friends
First of all monel is a well know nickel-copper alloy not steel.
I think using small stainless steel parts like bolt and nuts beside the mild steel structures in seawater, would decrease the corrosion rate of stainless steel. However stainless steel marine parts with large surface area could not be a correct choice regarding to pitting corrosion of stainless steel in chloride containing environment.

Regards,

A. Time flies when you're having fun :-)
But there are a hundred readers for every poster, so I'm sure numerous readers profited from this letter in the last four years, Mehdi, and many more will profit further from your update. Thanks!

A. Dear Friends,
Also remember if you need a classification approval for your piping systems, nearly no society approves the application of SST parts and piping for seawater systems even for small parts like valve disks.

A. Hey Mehdi,
316 or 316L stainless works fine in marine AND pool environments. It has been used in the marine industry for years and will continue to be used. Be sure to have the stainless passivated or electropolished before being put into service to increase the corrosion resistance.
Alternatively you can use titanium. I have seen heat exchangers constructed out of titanium used in chlorine and salt water pool heating systems.

A. In answer to the first question yes s/s is suitable for salt water use. 316L is probably the most generally used worldwide. Propeller shafts would be a good example; even lower grade 304 seems to cope very well. The composition lends itself to being tough but malleable for this purpose. However climate does have an impact on the corrosion factor. Proximity to other metals also needs to be taken into the equation. Environment should also be taken into account as it provides the ideal conditions for corrosion activity. S/S will "tea stain", that brown coating that we see on s/s when left in adverse conditions. Remember s/s, as I read it, means that it is less prone to stain but not 'stain.....less'. Any welding needs to be passivated and is a very good point of corrosion if not treated. Surface finish of course is also a big factor. The better the finish the better the corrosion barrier. S/S yes we all take it for granted, but there are many 'compositions'. Like steel, each has it own use.

A. Here is an article on Stainless Steel and where the various grades should be used:
Which grade of Stainless Steel should I choose for my fastener?

Thank you to everybody for discussion like this, is very useful for everyone who is interested in metallurgical issues.

A. I found the following article very useful and interesting on the subject, although sounds a bit late to share, I see that people would occasionally come for a visit! .

A. If it's a marine environment and you're locked into stainless, I wouldn't want to use anything less than 316. A Duplex grade would be better still.

But I would at least investigate and consider some type of plastic that isn't going to have the corrosion problems.

A. Using austenitic SS (304/316 etc.) with sea water is a gamble.

A lot of folks insist they use SS316 piping with sea water without problems, but there is a risk. The chromium content in 304/316 reacts with environmental oxygen to form the chrome dioxide coating that makes 304/316 corrosion resistant. This is passivation. There is ca. 0,5V pot. diff. between active and passive 304/316, so if the passivation is not good you are effectively using two physically connected dissimilar metals in an electrolyte, which are the conditions you need for galvanic corrosion to take place. The passive 304/316 will 'attack' the active 304/316 resulting in pitting corrosion. Fresh sea water has sufficient oxygen that the protective layer is constantly replenished, so corrosion resistance can be good.

But if you have anywhere that sea water can get trapped and stagnate, like a threaded connection, then you have a big risk of corrosion. Stagnant seawater has no oxygen, so the oxide coating is not constantly replenished, so the mechanism described above kicks in and you get pitting corrosion at the threaded connection. Most maritime class societies do not permit use of 304/316 with sea water.

A. Hi.
The salt content of your water seems to be 20-25X as dilute as seawater; that's really very little salt. And many stainless steels are fine even in seawater. I don't know what your reactor looks like, but chrome plating it would probably be rather impractical anyway. I should think you could use a good grade of stainless like 316 or 317 and not expect any trouble.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Hi again. You're very welcome, but I'm growing old so please try to help others with their questions here when you have a chance; we have thousands, so if you search the site for your areas of expertise you'll surely find open questions.
Luck & Regards,


Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey