Corrosion of SS316; Chlorides & Sulphides 4

[jalvarez]

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

 

[Metalguy]

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

 

[rmw]

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

 

[mcguire]

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.

 

[griffengm]

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

 

[mcguire]

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

 

[Metalguy]

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.

 

[EdStainless]

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

 

[unclesyd]

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

 

[lacticman]

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

 

[Metalguy]

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?

 

[lacticman]

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

 

[EdStainless]

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

 

[Metalguy]

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.

 

[griffengm]

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

Griffy

 

[Metalguy]

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.

 

[jalvarez]

Just being curious
Maybe Energie-dispersive ROENTGEN-Fluoreszenzanalyse as it can be found in

http://acronym.langenberg.com/

???
Have a safe day
J.Alvarez

 

[mcguire]

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.

 

[EdStainless]

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

 

[mcguire]

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.