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]

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

 

[moltenmetal]

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

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

 

[jalvarez]

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

 

[Metalguy]

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.

 

[EdStainless]

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

 

[mcguire]

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.

 

[EdStainless]

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

 

[Metalguy]

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.

 

[unclesyd]

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.

 

[griffengm]

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

 

[Metalguy]

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.

 

[jalvarez]

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

 

[griffengm]

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

 

[Metalguy]

Griffengm,

Any smelly black deposits?

 

[Metalguy]

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.

 

[EdStainless]

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

 

[unclesyd]

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?

 

[jalvarez]

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

 

[unclesyd]

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.