Solid SS 316L vs. CS + clad SS 316L

[Chumpes]

Dear all,
We are designing a sour water stripper with sour water containing cyanides and chlorides. Cyanides are as high as carbon steel cannot be used, and chlorides should remain below 50ppm.

Our customer pretends that the solution "CS + clad SS 316L" is better for this column vs. chloride-SCC than the solution "solid SS 316L".

Do you have any reference that may justify this statement ?

I know some Shell standards that show maximum ppH2S, T and ppH2S tested for clad was higher than what was tested for solid SS. Does it mean the clad is better ?

Thanks

 

[weldstan]

The clad is definitely superior if stress corrosion cracking is probable. It should also be cheaper.

 

[Chumpes]

@weldstan : thanks for your fast answer, i have 3 questions for you :
- are you talking about internal or external SCC ?
- would you believe the application of "CS+clad SS 316L" would represent an upgrade compared to "solid SS 316L" if ever the original solution "solid SS 316L" already failed by internal Cl-SCC ? I won't ! ! !
- do you have any reference that can support your statements ?

in this case, INTERNAL stress corrosion cracking is not anticipated by us because operation of the SWS remains within chloride levels, ppH2S and temperature covered by NACE MR0175, NACE paper#10308 and other industry standard like dep shell (no oxygen...).

Our Customer says : "there is a risk of internal chloride-SCC, thus solid SS 316L must be replaced by CS + clad SS 316L... which looks not appropriate to me, in first place because I don't fear Cl-SCC here, and also because the upgrade is not significative enough.

 

[weldstan]

Either internal or external. In the vast majority of cases that I have been involved in the past, clad was selected either due to potential of SCC but most often due to cost. Almost all of the vessels involved were used in petrochemical facilities.

Could your customer have more experience than you in the service involved?

 

[SJones]

Exactly what issue do you have with the customer's steer? If you don't 'fear' SCC, what problem do you have with a clad vessel for a fair amount less outlay?

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

There are a few reasons to prefer clad in SCC risk applications.
-If you do get cracking it will not go clear through the wall
-If the temperature is above ambient the SS will expand more than the CS putting the SS into compression

Realistically though if they know that SCC will happen they should look at clad with a 6% Mo superaustenitic alloy.

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P.E. Metallurgy, Plymouth Tube

 

[cap1a79]

Referring to API RP 945 clause 5.1, it says....

"Clad plate is preferred over solid stainless steel construction to avoid possible through-wall penetration that results from chloride stress corrosion cracking."

Although API 945 handles with amine unit mostly, I would understand the above sentence relevant to "chloride SCC" can be the background why clad is preferred option over solid from metallurgical aspect.


Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[Chumpes]

thanks for your valuable feedbacks

In the present case, it was decided to go to solid SS 316L after economic and a mechanical considerations compared to CS + clad SS 316L (I know the opposite may appear more logical).

Let's point out that I don't fear Cl-SCC of SS 316L here, but the client does now. I understand from your answers that you believe CS + clad SS 316L is better than solid SS 316L with regard to resistance to internal Cl-SCC, but also that the difference is not very significative.

@edstainless : I agree with you, if existing, the risk of internal chloride-SCC of the shell made of solid SS 316L should not be controlled by application of a clad SS 316L on CS shell but by the selection of a material with acceptable resistance in contact with process fluid.

@SJones : In the first place, we specified CS + clad SS 316L. This was accepted by the client. Then it was decided to move to solid SS 316L for mechanical and economic reasons, this was not commented by the client at the time. We finally delivered a column with top section made of solid SS 316L and client refuses it because he wants CS + clad SS 316L versus internal Cl-SCC. We explain to him that we don't have feedbacks of SCC of austenitic stainless steel in the industry for similar services, and that SS 316L solid or clad will be resistant.

@cap1a79 : thanks for this reference from API RP 945. SS piping in amine systems are solid materials that performs very well with no failure history by internal chloride-SCC in amine units.

 

Blanket statements are impossible when it comes to the decision between clad or solid CRA construction. It comes down to cost; for every vessel there is a breakeven point expressed in nominal thickness.
I can tell you as a welding engineer that I loathe clad construction, and that welding of the back cladding strips has several significant pitfalls. However, either approach should be equally corrosion-resistant if fabricated properly.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

And this is another reason that duplex SS is continuing to gain usage.
They are stronger (than austenitic SS or even the steel backing) allowing thinner material to be used.
(this has been a big deal with tank and silo usage)
The also have reasonable resistance to SCC.
As long as you are >-40F and <600F duplex should be on the list of alloys to consider.

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P.E. Metallurgy, Plymouth Tube

 

EdS,
Very true, but I also loathe fabricating DSS, for reasons you will be aware of.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[CoCoE]

@Chumpes,

What is within chloride level for SS? Is there such threshold? I would say Chloride SCC will still happen even though the chloride levels are even lower than 50 ppm. However NO OXYGEN is the key for me, and probability for chloride SCC is almost nil.

Consequence is very high if a solid stainless steel equipment is cracked, than a cladded equipment (since the crack propagation ends in the clad). Also, cladded equipment is less susceptible for SCC since it has a low thickness. Thickness also contributes to SCC.

Usually SS cladded CS are preferred over Solid SS due to economical reason and External SCC. There might be no cases where someone chose cladded SS for only avoiding internal SCC. So It must be external.

COCO

 

[Chumpes]

@ironic metallurgist : I agree, both solutions should have more or less equivalent corrosion resistance "if fabricated properly"
@CoCoE : yes, the presence of oxygen or oxydant is a (the ?) key parameter for Cl-SCC of autenitic stainless steels (people usually focus on chloride ions alone :/ ). I agree that clad cannot be regarded as a relevant upgrade compared to solid with regard to internal SCC

thank you all, see you soon

 

[cap1a79]

Although the presence of oxygen is the factor to accelerate CLSCC, there is no clear threshold value of oxygen content as well as chloride while it is not likely at below 60 C temperature, referring clause 4.5.1.3, i) of API 571.

Generally, the use of SS 316L (including sold & clad) is suitable in SWS unit, free from CLSCC issue as long as it operates at below 60 C.
(Refer to API 571, clause 5.1.1.10.6, b)



Lee SiHyoung,
WorleyParsons Oman Engineering,

 

[Chumpes]

actually API RP 571 (5.1.1.10.6 a) states : "300 Series SS can be used at temperatures below about 140oF (60oC) where Chloride Stress Corrosion Cracking (CSCC) is not likely."
to me, this does not mean : "300 Series SS cannot be used at temperatures above about 140oF (60oC) because Chloride Stress Corrosion Cracking (CSCC) is likely.

can anybody access the following reference ? i cannot find it but it includes a Survey of industrial failures by Cl-SCC of austenitic SS as a function of chloride content and oxygen:

M. Kowaka, Metal Corrosion Damage and Protection Technology, Allerton Press, 1991

In my system, temperature is roughly 92°C >> 60°C. Did anyone here ever had experienced Cl-SCC of SS 316L in sour water stripper above 60°C ?

 

[cap1a79]

I would apologize that I made confusion...


I think Solid SS 316L will be definitely o.k. for your case and you can forget all the issues for CLSCC.

If oxygen is not present and chloride is below 50 ppm, SS 316L is suitable by table A.2 of NACE MR 0175-3 ( I would regard NACE MR 0175 is applicable for your case as mentioned above.)

Generally, threshold as 60C for clscc is applied for general application including oxygen presence. However, if nace mr 0175 is applied (oxygen controlled in upstream field), there is no limit once chloride is below 50 ppm.

Lee SiHyoung,
WorleyParsons Oman Engineering,

 

You can only forget about C-SCC in austenitic SS when you remove either the chloride ions or the elevated temperature.
Solid or clad construction makes no difference.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."