Chloride criteria in MR0175/ISO 15156-3 for austenitic stainless steels 3

[Vahid.A]

It is known that Austenitic stainless steels (SS304,SS316,…) are prone to SCC in chloride bearing environments. The chloride is normally controlled within a certain limit to prevent stress corrosion cracking. In some cases, such as hydrotest, standards normally suggest 50 ppm chloride limitation.

In case the chloride content cannot be controlled, a higher alloyed stainless steel or another suitable material is selected to prevent SCC. ASME Section I (PG-5.5) forbids using the austenitic stainless steels in the boiler pressure parts that are in contact with water.

On the other hand Stainless steels are also prone to crevice and pitting corrosion and their application is normally limited to prevent this type of corrosion (1000 ppm is a common restriction for SS316 at ambient temp.)

My question : Why in table A.2 of ISO 15156-3, the level of chloride content is limited to such a broad range ? (for instance to 50000 ppm in presence of max. 1000 ppm H2S and pH>=4.5). Does this imply chloride content becomes less significant for SCC risk in sour service environments?

 

[hussaindawki]

Because it relies in absence of oxygen ingress, so no probable cracking may happen at temperature of 60 degrees C max. as I remember.

However, that won’t exempt you from pitting corrosion that would happen through the brine liquid you will have, that’s why, even if the NACE tells you, it won’t crack, that does not mean it won’t get pitted with time, at the end chloride weaken chromium oxide layer even with inclusion minor addition of Mo as SS316.

So differentiate between aerated and deaerated environments

 

[EdStainless]

H2S is chemically reducing, hence no oxygen.
However the absence of oxygen will eliminate the ability for SS to re-passivate and pitting is virtually assured.
Just remember that if this system is above ground and there is any leak you will get CSCC.
If water can leak out then oxygen is getting in by diffusion.
I wouldn't use 316L in over 500ppm Cl under any conditions.

 

[Vahid.A]

Because it relies in absence of oxygen ingress, so no probable cracking may happen at temperature of 60 degrees C max. as I remember.

However, that won’t exempt you from pitting corrosion that would happen through the brine liquid you will have, that’s why, even if the NACE tells you, it won’t crack, that does not mean it won’t get pitted with time, at the end chloride weaken chromium oxide layer even with inclusion minor addition of Mo as SS316.

So differentiate between aerated and deaerated environments

H2S is chemically reducing, hence no oxygen.
However the absence of oxygen will eliminate the ability for SS to re-passivate and pitting is virtually assured.
Just remember that if this system is above ground and there is any leak you will get CSCC.
If water can leak out then oxygen is getting in by diffusion.
I wouldn't use 316L in over 500ppm Cl under any conditions.

Thank you for your helpful insights.

Here’s what I understand from your posts: In a reducing environment created by H₂S, significantly higher chloride concentrations are required for stress corrosion cracking (SCC) to occur. However, pitting or crevice corrosion can still develop at chloride levels above 500 ppm. Therefore, despite falling within ISO/NACE limits, using these stainless steels in such service is not recommended.

Does this match your experience/observations?

Please correct me if I’ve misunderstood.

 

[SJones]

NACE/ISO is for types of stress cracking only - it does not deal with pitting or crevice corrosion. I have seen equipment that has been in non-sour service with 100+k ppm of chloride having no untoward effects, and I’ve seen equipment with very much less chloride having major issues. It’s the risk that you run with a lower grade CRA before you even get into the effects of temperature, pH, fabrication practices, etc, etc. It boils down to: how lucky do you feel?

 

[EdStainless]

And how much testing have you done?

 

[Vahid.A]

And how much testing have you done?

I have experience working with stainless steels in RO desalination plants, where I’ve observed severe pitting in cases where SS316 was exposed to seawater.
In RO desalination, chlorine (not chloride) is removed by dosing SMBS (sodium metabisulfite), a reducing agent. However, excessive SMBS can damage membranes, so its concentration must be carefully controlled within an ORP-monitored range. For high-pressure pipelines, duplex or super duplex materials are typically preferred due to their superior corrosion resistance.

I found it interesting—and somewhat surprising—that you and other experienced engineers mentioned the possibility of pitting and crevice corrosion even in oxygen-free environments. According to literature by Dr. Roger Francis, a well-known authority in this field, the critical crevice temperature (CCT) tends to increase at lower electrochemical potentials. For example, in SS316L exposed to synthetic seawater, the CCT shifts from 10°C to 20°C when the potential changes from +200 mV to -100 mV (vs. SCE). Additionally, standards such as ISO 21457 permit the use of SS316 in deaerated seawater systems.

This reinforces my understanding that each environment has unique characteristics that must be carefully evaluated. Could you recommend any published articles that specifically address pitting or crevice corrosion in oxygen-depleted environments in sour media?

 

[EdStainless]

A couple of cautions.
1. Old articles are not to be relied on for a few reasons. First 316 chemistry is different than it used to be. These days it is common to see the Mo run 2.00%. and for products other than sheet and plate the manufacturing methods have likely changed significantly. And honestly many of the older articles were based on limited experience.
2. 316L will work in seawater if you can keep the temperature under ~38C, keep velocities high and never stop flow, and keep the surfaces free from any scale or biofilm. In other words it can't happen in the real world.
Even in oilfield applications with sour water it is usually considered that if the application is above ground then there is nearly a guarantee of oxygen in the system.
Pitting is insidious because even if 99.995% of the surface is fine that one through wall pit will cause a leak.
But at least pitting takes time.
CSCC can happen very fast. In the lab I can cause it in the time that it took me to type this reply.

 

[Vahid.A]

A couple of cautions.
1. Old articles are not to be relied on for a few reasons. First 316 chemistry is different than it used to be. These days it is common to see the Mo run 2.00%. and for products other than sheet and plate the manufacturing methods have likely changed significantly. And honestly many of the older articles were based on limited experience.
2. 316L will work in seawater if you can keep the temperature under ~38C, keep velocities high and never stop flow, and keep the surfaces free from any scale or biofilm. In other words it can't happen in the real world.
Even in oilfield applications with sour water it is usually considered that if the application is above ground then there is nearly a guarantee of oxygen in the system.
Pitting is insidious because even if 99.995% of the surface is fine that one through wall pit will cause a leak.
But at least pitting takes time.
CSCC can happen very fast. In the lab I can cause it in the time that it took me to type this reply.

Thank you for your valuable insights. It’s truly heartwarming to see experienced engineers in this forum dedicating their time to educate others.

I may have overlooked something in this discussion, so I’d appreciate some clarification. To my understanding, pitting corrosion is among the primary initiation mechanisms for stress corrosion cracking (SCC). Given that, why wouldn’t it progress to chloride-induced SCC (CSCC) in this case? Additionally, how does oxygen influence this process?

Additionally, I’m slightly confused about the role of a reducing environment on the pitting corrosion of stainless steels. On one hand, there appears to be an inverse relationship between potential and critical crevice temperature (and likely critical pitting temperature as well)—meaning more reducing conditions could raise the threshold for pitting. On the other hand, stainless steels require some oxygen to maintain their passive films. This makes me think there might be an optimal oxygen concentration that minimizes pitting corrosion risk.

 

[Vahid.A]

316L will work in seawater if you can keep the temperature under ~38C, keep velocities high and never stop flow, and keep the surfaces free from any scale or biofilm. In other words it can't happen in the real world.

Even if we use SS316 in these conditions, crevice corrosion probably remains a concern. I would expect it to occur even in flowing seawater, depending on the crevice geometry.

 

[EdStainless]

CSCC cannot start unless there is active corrosion.
But once corrosion starts the actual conditions dictate which will be faster, SCC or pitting.