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Corrosion of SS316; Chlorides & Sulphides
4

Corrosion of SS316; Chlorides & Sulphides

Corrosion of SS316; Chlorides & Sulphides

(OP)
Hi mates:
I did a search and want to revisit your Thread338-52646.
There it was clear that, if correctly passivated, SS316 or still 304 is/are correct for piping with water with a chloride content of 200 ppm, occasionally more (as high as sea water), provided that:
- The lines are correctly passivated.
- No crevices.
- Non stagnant conditions.
Additionally, from my own, maybe seamless pipe is preferable, to reduce weldings and treatment.
Now, having a critical pipe that normally has:
- 65ºC (continuously)
- 200 ppm chloride.
- jumps to 1000 ppm for, let's say a week or so yearly.
- occasional presence of sulphides, let's say 100 ppm, once per year.
Is still SS304L a good option?
Is SS316L preferable?
Have a safe day
J. Alvarez

RE: Corrosion of SS316; Chlorides & Sulphides

2
316L would definitely be preferrable to 304L in such service.  But if the flow rate is high and fairly constant, either one will survive.

RE: Corrosion of SS316; Chlorides & Sulphides

Metalguy:  you're not worried about eventual SCC under these conditions?

jalvarez:  we're assuming neutral pH here, right?  More or less?

RE: Corrosion of SS316; Chlorides & Sulphides

(OP)
Thanks, people:
Water is neutral, apologize for the forgetfulness, not acceptable speaking about corrosion!
I assume that SCC is Stress Corrosion Cracking. I suppose we are using seamless pipe, maybe this is something that reduces this SCC. Stress relief?
What about incidental sulfur ions content?
J. Alvarez

RE: Corrosion of SS316; Chlorides & Sulphides

No, I don't think SCC would be a problem--provided there isn't a Cl concentration mech. such as evap.  200 ppm isn't a very high amount, even at that temp.  Yes, it certainly can cause pitting in stagnant conditions, but not if the flow rate is above ~7 fps or so.  The exposure time at 1,000 ppm is short enough so I don't think it would cause a problem either--as long as it's flowing well.

Also, the sulfides shouldn't be a concern either.

RE: Corrosion of SS316; Chlorides & Sulphides

Will there be any joints of any type in the system? If so 304 will not survive, and 316 may be marginal depending on the joint types (mechanical, flnged, welded?).
Is there any Fe or Mn in the water.  As little as 30ppb Mn can be a real problem.
What will be used for biological control?
The sulfur compounds don't bother me as long as you are sure that there is no biological activiy.
What is the chance of any scale formation?  Do you have an LI for the water?

In a good clean, flowing system 316 should get you by.
There is no reason to use seamless pipe.  The ID is usually quite a bit rougher than welded which will not be helpful.  If you are concerned about the longitudinal welds you can use tubing made to A249 with S7 weld decay testing in order to assure that it is well annealed.  Just because it is seamless does not meen that it has been heat treated correctly.  Welded product is usually smoother and made to tighter tolerances.

SCC would be a secondary concern.  I have seen hot water service lines in 316 fail rapidly due to external SCC.  Water drips on the outside of the tube then evaporates leaving concentrated impurities.  Internal SCC failure usually requires higher stress or temp.  Though be warned, any sulfides in teh system will make it much more likely to have SCC.  Some breweries use 316 for hot water, some use 2205 and some use AL-6XN.  It depends on their water.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Corrosion of SS316; Chlorides & Sulphides

EdStainless
  I have a lot of data which clearly indicates that TIG welded tubing has much lower pitting corrosion resistance in the weld than in the parent metal. Therefore seamless would be preferred. The loss of pitting resistance is linearly proportional to sulfur and oxygen content. Normal annealing doesn't relieve this effect. To remove the effect requires about 15 minutes above 2000F. Laser welded tubing doesn't display the same loss of corrosion resistance.

RE: Corrosion of SS316; Chlorides & Sulphides

mcguire;  I beg to differ.
Properly specified and annealed welded tubing/pipe has no lower corrosion resistance than properly manufactured seamless.  Either product that has been poorly manufatured will have poor corrosion resistance.  Data that indicates otherwise was generated using under annealed welded material.
The sulfur levels commonly used to assist TIG welding are well below the level that has any measurable impact on pitting resistance.
Our normal furnace anneal process will reduce residual delta ferrite levels in weld below detectable levels and yields welds that are not measurably different from base metal in corrosion resistance.
The reason that this is not seen in laser welded tubing is that the solidification rates are very fast and there is little/no delta ferrite in the welds.  However if you conduct pitting tests on 316 you will find that there is still significant Mo segrigation and accelerated local pitting of the welds, unless the product was furnace annealed.

Neither 304 or 316 have much pitting resistance.  Conditions which result in fouling due to scale or biofilm will quickly lead to pitting even in reasonably good quality potable water at elevated temperatures. The risks of a system leak must be weighed against the cost of higher corrosion resistance materials.  If a leak will only require some maint. activity, then maybe it is OK.  If a leak will shut down a large prodiction plant then you need to consider more robust solutions.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Corrosion of SS316; Chlorides & Sulphides

EdStainless,

I think you and McGuire are really saying the same thing-that TIG welded 304L/316L SS will have reduced pitting resistance *unless* it receives a hi-temp anneal.  Eliminate the ferrite phase and the pitting resistance returns.  

Nowthen, passivation can most definitely increase the pitting resistance of as-welded 304L, but it will still of course pit under the right conditions.

I think we all agree that Jalvarez must carefully evaluate all of the above conditions as they apply to *his* application--per usual.

RE: Corrosion of SS316; Chlorides & Sulphides

jalvarez,

Can you come back with a little more information on the piping in question?
What size pipe?
How Long?
Any elevation changes?
Flow rate?
And anything else.
Water source?

With the installation given, Cl & S, I'm a little skeptical.    

RE: Corrosion of SS316; Chlorides & Sulphides

Please clarify this statement:
"The loss of pitting resistance is linearly proportional to sulfur and oxygen content."
Content of water or metal?
And this one as well: "As little as 30ppb Mn can be a real problem."
We have a problem with a 316L welded assembly and since I have no info on the effects of these two elements, I would like to know more.
Assembly develops pitting in HAZ.  Assembly is passivated using nitric acid after welding.  It operates at 160-180F and pressure of 40-60psig. Installations are exposed to either untreated well water or city treated.  So far we have not been able to find the causes.  Testing using salt solutions have been inconclusive in that we get very good results.
References would be appreciated.

Griffy

RE: Corrosion of SS316; Chlorides & Sulphides

How constant is the temp?  Does the piping sit wet at room temp. for long periods, especially when first installed?  If this is the case, you may well have a sulfate-reducing bacteria problem.  SRB's will go thru a good passive layer like it's not even there.

RE: Corrosion of SS316; Chlorides & Sulphides

(OP)
OK, unclesyd/Metalguy
The line must be redefined, initially we have:
Pipe size: 8"
Length: 25 ft
Elevation: 6 ft
Flowrate: 26 gpm
Pressure: atmospheric
Temperature: 150ºF
pH neutral
To show more details, you can go to the following sketch:
http://ar.briefcase.yahoo.com/bc/jalvarez813
Sorry for the language, click on the folder "Sketchs", click on the unique file there, "sketch2.jpg".
The flow goes to another drum, where the level is stable, so the level in the pipe (as I draw it) is stable too.
Metalguy, you recommended a velocity of 7 ft/sec. With the values above, velocity are much lower than that. We need to modify the diameter, but also we need to assure a more or less constancy on the level in the pipe. Let's see what we can do.
Water (coming from the city drinking water network) is passed through a softener, activated carbon plus cathion plus anion beds. Then, in the process, this water gains 30 to 150 ppm of chlorides and simultaneously is heated up to 150ºF. I don't see there a bacteriological issue.
Nitrogen flow is obviouly pretty wet.
Have a safe day

RE: Corrosion of SS316; Chlorides & Sulphides

Temp graph would approach square wave with narrow lows, wide peaks and fairly steep transitions.
   _|----|_|----|

Corrosion seems to be on inside.  When leaking assemblies are opened there is more evidence on the inside than out.

Griffy

RE: Corrosion of SS316; Chlorides & Sulphides

Griffengm,

Any smelly black deposits?

RE: Corrosion of SS316; Chlorides & Sulphides

Jalvarez,

With those conditions you may well have pitting problems, even with 316L.  The temp. is the killer in such near-stagnant conditions with your Cl levels.  Use a higher-alloyed pipe.

RE: Corrosion of SS316; Chlorides & Sulphides

The Mn pitting has has been discussed in a number of NACE papers.  The paper numbers are 02441 and 02444.  What happens is that MnO is deposited on the surface of the stainless.  Then when chorinated water is introduced the oxide is converted to permanganate, this releases H ions which lowers the pH.  So basicly you form HCl under the deposites and eat through the wall.

Since you have seen HAZ attack you need to try to decide if it is from sensitization (chrome carbide in the grain boundaries) or from surface oxide formation.  There was a very good paper last year, 04291, concerning the effects of surface oxidation on corrosion resistance.

When it comes to welded pipe and tube, rest assured that if you specify S7 Weld Decay test you will get propperly annealed material.  But this will do nothing for your field fabrication welds.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Corrosion of SS316; Chlorides & Sulphides




Jalvarez,
I am still skeptical on the viability of using 340L or 316 SS pipe in this situation due possibility of the establishment of an interface and potential for stress corrosion and then the cracking due to the Cl levels and temperature. The absence of O2 is somewhat of a problem to me both for the SS and the variety of bugs.  The anaerobic species of the "Bugs of Rust" are always looking for a home. Domestic water systems are notorious for having all varieties of the same bugs plus iron bacteria.

If you get bugs you will get the SC and SCC over time at these Cl levels.

I would look hard at a FRP Piping and maybe for the tank as well.

Your conditions are at the 50/50 level of it will work or want work.  Probably half the people have seen it work and
the other half have seen failures.  I've seen it work, but have seen a lot more failures than success under very similar conditions.

What is the tank material?
Are you going to have to insulate the line?   

RE: Corrosion of SS316; Chlorides & Sulphides

(OP)
Thanks, unclesyd
Tank downstream, showed in the sketch is ebonite protected.
I'm planning to reduce line diameter to increase the water velocity. Water is flowing continuously but previously is heated up to more than 150ºF, let's say 170ºF, and then sent here. The bacteria surviving this should be strong. But if this is still possible, we need to do frequent inspections in the line, at least at the beggining.
Another alternative is using FRP, as you suggest. It must work fine.
Thanks to all, great forum!
J. Alvarez

RE: Corrosion of SS316; Chlorides & Sulphides

The little critters are quite resourceful as they encapsulate themselves with a silica shell and can survive some pretty rough treatment.
Remember it only takes one and time.  

The reason of the question on the insulation was you can have a better chance for an interface with insulation than without. Also with this level of chlorides around and the insulation gets wet you can easily get an external problem, more so if the system cycles.

Good luck with your project.

RE: Corrosion of SS316; Chlorides & Sulphides

Resourceful?  As I'm sure you know, some of them think swimming around in battery acid is great fun!

RE: Corrosion of SS316; Chlorides & Sulphides

Somewhere I think I learned a long time ago that 165F is a magic number for killing bacteria, but there is a residence time at that temperature, and I don't remember what it is.

I had a problem to design a process to do just that (heat and hold a flowing stream of water at 165F for a period of some minutes) at a company I worked for years ago, and before I came up with a workable solution I left their employment.  I have often wondered what I would have done if I had stayed, or what my replacement eventually did.

I do know what I would do now, however, if faced with the same project.  But that is beyond the scope of this thread.

rmw

RE: Corrosion of SS316; Chlorides & Sulphides

griffengm
 The sulfur and oxygen I refer to is that which is in solid solution in the steel, not that in the aqueous environment.
These elements precipitate and combine which chromium as the alloy cools from welding temperatures. ( Laser welds cool so quickly that these elements are quenched in situ and don't precipitate) This makes the regions around inclusions relatively poor in chromium and therefore the locus of corrosion pits. A very long, hot anneal remedies this condition by redistributing chromium uniformly. It is coincidental that delta ferrite is also put into solution by this procedure. Delta ferrite is not related to pitting in austenitic stainless steel although this was previously thought to be the case. Pit initiation is associated with inclusions which have grown in the solid state.
 Welds are the most susceptible to pitting because they have no deformation to help the chromium homogenization as the parent metal does. As EdStainless points out annnealing helps, but a normal minute or so at 1950F is insufficient to eliminate the effect after TIG welding.

RE: Corrosion of SS316; Chlorides & Sulphides

Mcquire,
Thanks for the clarification.
In units we have brought back and opened, we generally find the most activity in the uppermost seams.  Cold water enters through the bottom, hits a diffuser and heated water exits through the top.  We also see other relatively random activity toward the top and little in the lower half.  The temp range I gave earlier is probably misleading in that replacement water is likely about 50F while the heated water is in the 160-180 range.
From the discussion, it looks like bio activity would be a good place to start checking. The fact that we can pinpoint specific installation locales with problems might support this.??
Any suggestions on how to find the "bugs" if they're present?

Griffy

RE: Corrosion of SS316; Chlorides & Sulphides

Griffy
 I have to defer to my colleagues on bugs.
Mike McGuire

RE: Corrosion of SS316; Chlorides & Sulphides

Griffengm,
You can start by scraping any deposits into a sterile pack which is made for such testing.  You must also use a sterile knife etc.  Then get the samples to a lab. that has the ability to cultivate and identify any microbes.  There may also be field test kits available.

If you get positive results, you can then deduce what kind of corrosive conditions the microbes create.  I suspect the two most common types, at least in "clean water" systems, are the well-known sulfate-reducers (SRB) and the iron-oxidizers.  The iron-oxidizers create strong oxidizing conditions and you can then expect the austenite phase of a weld to be attacked.  The Cr-rich delta ferrite phase can resist such conditions much better.  The reverse is generally true for the SRB's, but the reducing conditions they create are so strong that both phases are rapidly dissolved.  A good passive layer will offer some protection against the iron-oxidizers but not the SRBs.

RE: Corrosion of SS316; Chlorides & Sulphides

I can assure you that you will find microbes.  I have found them in systems that were claimed to be sterile.  You need to really look at your corrosion and see if MIC looks like a primary cause or just an secondary issue.
Is there any indication of scale or Mn deposition?
Since most of the corrosion is in welds you should look at weld HAZ sensitization and weld surface cleaning methods.  It might be that you have created a home for the bugs.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Corrosion of SS316; Chlorides & Sulphides

There are MIC testing kits available that will give a definitive answer in very short time. I try to find out where we got the kits.  It is extremely hard to get a good sample, that is if you wash your hands you have them all over your hand.

If you see very small rusticles and upon scraping them off and have even the smallest pit on the surface you have  bugs.  If they have been active any length of time they have built condominiums, deep cavernous interconnected tunnels.  Another good hideout for them is on the wetted part of the gasket surfaces.

The irony of this is that these bacteria are synergistic, that is it is extremely had to get rid of them once established especially in a water system such as yours.  The bacterial mass also has the ability to generate HCL and concentrate the Cl's in the system.  This is a good starting point for chloride induced SCC.

Here one of the test kits.
http://www.bti-labs.com/products.cfm

Here is a firm you send a sample to.
http://www.mic-testing.com

RE: Corrosion of SS316; Chlorides & Sulphides

Would you guys know the level of Chlorides/Sulfates that is allowable in a stream in 316 SS L piping that has the following characteristics:

1) temps up to 180 deg. F
2) pH~ 1-2 due to an aqueous solution with lactic acid
3) pipe velocities up to 12 feet/s

We had a chloride excursion at our plant and we were wondering how much chlorides can we live with before equipment is being compromised/corroded.

~lacticman

RE: Corrosion of SS316; Chlorides & Sulphides

Cl could be a big problem *if* the line flow was low.  You should try to determine if the flow was high at the time, and if your 316L was welded correctly so as to avoid sensitization. Can you remove/internally examine a small piece of the pipe?  Can you perform an ultrasonic exam from the OD?

RE: Corrosion of SS316; Chlorides & Sulphides

We were in a start up an shutdown situation so velocities could have ranged from 0 to ~12ft/s.  We can do thickness testing with some of our reliability equipment and will schedule for the near future.  Until then, what would be a good rule of thumb for Cl and SO4 concentrations for the conditions listed above.  This way we can put into place no run conditions to prevent these concentrations from being present in some of our evaps.  Thanks.

~lm

RE: Corrosion of SS316; Chlorides & Sulphides

lacticman,
You are way outside of the sugged operating range for 316 with regards to chloride pitting.  The high flow is no problem, it is low flow that will hurt you.
General rule of thumb for the pitting limit for 316 is that each 100F increase in temp lowers the chloride limit by 10x.  And each drop of pH by two units drops the chloride limit by 10x.
Now, depending on what you want to use for a normal, ambiant chloride limit you can work from there.  I usually suggest 200ppm max in 316.  That is presuming 120F and pH6.  Or we can assume 500ppm at 70F and pH7.
In either case you end up at less than 1ppm choride at your operating conditions.
Considering the risk of choride stress corrosion cracking at 180F also I would look at other alloys.
Let me know if you want more info.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Corrosion of SS316; Chlorides & Sulphides

Lacticman,
I had typed out a long reply (very slow typist here), but the msg. disappeared when I tried to post it.  This time I'll save it on Notepad first.

My experience generally is in agreement with EdStainless.  I have seen one case where ambient temp. city drinking water  was used in a large new passivated 316L piping system.  The Cl was measured at ~600 ppm, and the lines developed many pinhole leaks before the huge plant was turned over to the owners--just before.  I was hired to find the cause, and quickly please!  At first I thought it was a simple case of Cl pitting-the lines had been stagnant for a few months after being filled.  This entire SS piping system was specified because another one of the owner's large plants had used PVC, and two heat-related failures cost them a few million $$$.  A passivated 316L system sounded like it would end those problems, and too bad about the installed cost (this company was/is VERY rich, but it's not MS<g>).

SO, after getting a few pipe samples removed I examined them in a SEM/EDAX.  Surprise, NO Cl in the pits, which were all in the weld metal. But S was everywhere, including lots of dead SRBs.  Seems that SRBs don't hurt humans like they do metals!

I don't think you need to be too concerned with sulfate levels below a few hundred PPM in your system-it's Cl that is the problem.

RE: Corrosion of SS316; Chlorides & Sulphides

Metalguy,
Please, what is "SEM/EDAX"?

Griffy

RE: Corrosion of SS316; Chlorides & Sulphides

Griffy,
A SEM is a scanning electron microscope--a most valuable instrument in failure analyses.  The EDAX attachment is "energy dispersive X ray" or something like that.  It is capable of identifying and quantifying all but the lightest elements while sampling a very tiny area.  

RE: Corrosion of SS316; Chlorides & Sulphides

(OP)
Just being curious
Maybe Energie-dispersive ROENTGEN-Fluoreszenzanalyse as it can be found in
http://acronym.langenberg.com/
???
Have a safe day
J.Alvarez

RE: Corrosion of SS316; Chlorides & Sulphides

Your 316L tubing will have much better pitting corrosion in the welds if you specify very low sulfur content, e.g. 0.003%, in the steel. From a practical point of view, this restricts the welded tubing to laser welded tubing, but that should be no problem. 316L with 0.015% sulfur is no better than 304l with very low sulfur in resisting pitting.

RE: Corrosion of SS316; Chlorides & Sulphides

I am sorry mcguire, but the sulfur contetnt of the steel has insignificant impact on the corrosion resistance of hte welds in stainless steels.
Most grades of SS have 0.005-0.017 S to improve weld fluidity.  It takes S levels higher than this for any negitive imact on the corrosion resistance.
If you are talking about tube mill welds, the deciding factor is the post-weld annealing procedure.  Under annealed material will have significant retained delta ferrite in the welds and this will lead to accelerated weld corrosion.
The corrosion resistance of field welds (un-annealed) is directly realated to heat input and surface condition.  Welds made with low heat input and good shielding have goo corrosion resitance.  Big wide welds with discolored surfaces will pit very rapidly.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

RE: Corrosion of SS316; Chlorides & Sulphides

Sorry to you, EdStainless, but what you say is quite wrong,and there is vomulinous published data to prove it. Sulfur does decrease the pitting resistance of as-welded stainless. Delta ferrite does not. Annealing, if done for very long times, can cause the sulfide preciptation to reach equilibrium and eliminate the chromium depletion which exists around manganese sulfide inclusions, which is what is the cause of the poorer corrosion resistance of TIG welds. See the publication by Ryan in Nature, 415, 770-774, 2002 and the work by Sunniva Collins, which carefully measures the pitting resistance loss in 316L TIG welds for various sulfur levels.

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