Pitting of 316L in seawater

[bob330]

Hi,

Will 316L tend to pit in stagnant seawater at ambient temperatures if there is no fouling deposits or crevices present on it? If not, would we expect to see issues with fouling present?

Also, would anybody expect to see pitting for tube side water in an HE application with up to 1000PPM chlorides at 120F for 316L. I felt that we would not see problems so long as the water flowed adequately enough to prevent fouling.

bob

 

[EdStainless]

With todays chemistries (2.0% Mo) there is no way that I would suggest using 316L at levels above 200-300ppm Cl.
I know, high flow, very clean, high oxygen and maybe you won't get pitting at 1,000ppm. But if any one of those factors fails then you have a disaster.

1,000ppm Cl would tell me to look at 2205. Considering that there are always low flow regions and crevices in heat exchangers, I sure wouldn't risk it.
And today, if you reduce the wall thickness on the 2205 a bit, you can build the unit for less money than in 316L.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[stanweld]

bob330
Regarding both of your questions; my answers are yes and yes. Expect a short service life in the environments stated using 316L.

 

[bob330]

Thanks for the input guys,

I did find a chart that disagrees with your recommendations here and shows that there is little propensity towards 316 pitting at any concentration of Chlorides at temperatures below 50C. However, this chart probably does not consider any crevice behaviour. This chart is from an SPE 56805 document called Application Limits of SS in the Petroleum Industries from 1999. IS this a NACE document or API document or other? Lets discuss more!!!

THanks,
Bob

 

[StoneCold]

Unless the pressure is very high, I would guess that an admiralty brass would be a good fit.

Regards
Stonecold

 

[gr2vessels]

Bob,
Some trivial lines;-
The standard 316L is subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking above about 60°C. It is considered resistant to potable water with up to about 1000mg/L chlorides at ambient temperatures, reducing to about 500mg/L at 60°C. However, a number of secondary factors have also been identified, including amount of ferrite, surface condition, presence of cold work and heat tint at welds, all this contributing to the initiation of pitting and the general SCC, as possible in your HEX environment at much lower temperatures, say 30-40 deg C. Remember the 316L ventillation ducting at your local swimming pool, some of them completely corroded through, at less than 40 deg C.
I could not find one single comprehensive study to answer in a positive, convincing formula, the limits of any Austenitic S/S operating in a corrosive environment. It will take intensive tests and control of your environment , including items like ph level, presence of oxigen, short process excursions likely to initiate a tiny pitting points, but also extensive design experience and fabrication issues, many of them out of your control.
That's way the practical aproach is to play safe, as per the good advice from the guys of this forum.
PS. I just re-designed and built recently a Ti tubes sea water exchanger (cooler), cheaper than in 316L. It operates around the 75 deg C mark and the Client is happy.
Cheers,
gr2vessels

 

[strider6]

The usual recomendations i've found on the use of 316 is that it's suitable for use in Sea Water if it's not in stagnant conditions. Uhlig book recommend for example a minimum velocity of 1.5 mt/s to avoid problem of pitting.
There are some chart for Pitting Test for 316l in different NaCl concentration and temperature on this page of Sandvik:

http://www2.sandvik.com/sandvik/014.../HandbookWeb/4D2F725116DAD2ABC1256B490043561E

hope this help

reagards

Vitt

http://www.corrosionist.com

 

[EdStainless]

I still go back to my earlier values. If your heat exchanger has a continuious ball cleaner and flows of over 8ft/sec then maybe you can survive with 316. But every time that it is shut down you will need to fresh water flush. Forget or delay this once and you will loose the entire unit.
The best bang for the buck today in tubing is either a duplex SS (the lean grades for low Cl or 2205 for medium) or a superferritic (for high Cl). The S44660 SF grade widely used in seawater and chemical process applications. And unlike Ti it is available.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[bob330]

Thanks Guys,

Stagnancy and fouling are no doubt the big concerns. I did want to clarify someting here that I was not celar about earlier. The tube side service is cooling tower water and the 1000ppm chloride is a maximum potential condition and in the past has only reached these levels for short periods of time. Maybe 175-350 ppm is more typical on average.

I am most concerned about either fouling or shutdown periods with water left in there as EdStainless mentioned. Flowing water should be no problem at 50C according to all the data I have been able to gather.

bob

 

[Metalguy]

I'm presently involved with recommending replacement tubing for some nuke-safety-related heat exchangers. Present tubing is Admiralty, severely pitted all over the ID. ID water has at least 1,000 ppm cl, plus sulfates and other stuff.

I'd love to use 2205 or even Sea-Cure, but the eddy-current guys say they can't meaningfully examine them. I'm trying to decide between 90-10 Cu Ni and Ti, and I think Ti is going to win--*if* our 29"-30" span length is OK for 3/4" tubing.

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill

 

[EdStainless]

Metalguy, Tell your NDT guys to go back to school. There are 100s of millions of feet of duplex and ferritic tubing in service that are regularly ETed. Sure, they will need different equipment and test methods, but isn't that what they get paid to do?
Don't give in on this. Price the options, including the fact that with 90-10 you will need thicker walls and still have some risk of corrosion and Cu transport. Also look at all of the costs with Ti (heavier walls also). These are real costs that need to counted.
I have seen AL-6XN used for Nuk safety equipment. It is easy to test, well known, available to Sec.III req. and expensive.

Bob, I have seen 316L (2.7%Mo) used in seawater. It will work for years, but one bad layup and in an hour you can loose the whole thing.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Metalguy]

Thanks, Ed.

Just in time--meeting set for tomorrow PM to try to decide on the material. I had argued that 2205 or Sea Cure will NOT pit in our service, so the ET doesn't have to be all that good--compared to something like nuke steam generator tubing, etc. Also, Cu isn't a factor here like it would be in FW heaters, etc.

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill