chloride attack of SS

[electricjer]

I have a corrosion problem that I am trying to make sense of. The corrosion is occurring on the plates of a condenser in a skid packaged evaporation unit. The plates are 316 SS and are showing intergranular corrosion on the vapor side of the vapor/water line on the plates. The unit operates at 140F and is showing significant corrosion after about 6 months. Surface deposits showed 1 to 2% chloride. Would this be considered stress corrosion cracking? The feed to this batch evaporation unit is only 20ppm of chloride, but it would get concentrated during evaporation. Can condensing vapor with chlorides produce intergranular corrosion and does the presence of chlorides in deposits make it the source or just a possible contributing factor?

 

[strider6]

If you have stress corrosion cracking you'll see the typical crack in the materials.
Intergranular corrosion occur in sensitized stainless steel, but sensitization is something related with welding or high process temperature. Did the corrosion occur at or near a weld?
140°F is the temperature at which SCC is considered likely in stainless steel in contact with chlorides and in some cases is the limit for Stainless Steel.
this doc is a guide guide to SCC

http://ecow.engr.wisc.edu/cgi-bin/getbig/msae/433/1hellstrom/lecturenot/

http://www.npl.co.uk-lmm-docs-stress.pdf

a "safe" limit for SS is sometimes taken at 50 ppm (see the NACE /ISO 15156). if you hace 20 ppm then with evaporation can easily reach this value.
in this page there is graph that shows the limit for 316L.

http://www.sandvik.com/sandvik/0140/internet/se01598.nsf/cdatas/98A6DA5240941BAF41256618006F4439

http://www.corrosionist.com/Corrosion_Type_Stress_Corrosion_Cracking.htm

hoep this help

S


Corrosion Prevention & Corrosion Control

 

[metengr]

electricjer;
Are you sure it is intergranular stress corrosion cracking? Has this been confirmed by a metallurgical lab or from some type of failure analysis? I would presume if you are seeing visual indications of cracks or using a surface nondestructive test method, like Liquid Penetrant, to detect surface cracking, there would be families of cracks and transgranular if caused by chloride deposits.

.. and yes, evaporation can result in deposit build-up and concentration of deposits leading to TGSCC (transgranular stress corrosion cracking) in 316 SS.

 

[electricjer]

A metallurgical lab did look did look in to this and their findings were as follows: Plate deposits were oxides of 300SS, no corrosion under deposits, plate conforms to 316SS, a pit are was examined after cross sectioning and revealed intergranular attack. 400x pictures show intergranular attack.

There was no mention of SCC, but that seems like a possibility given the temp and Cl concentration. Seems like the typical concentration of Cl is too low to cause pitting, but I haven't ruled out exposure to increased concentrations yet.

I am really trying to determine is if 316SS will fail again or if it was a one time exposure to something else.

 

[strider6]

if the analysis has been made by a metallurgial laboratory expert in failure then they must understand if the failure is caused by sensitization or SCC. a sensitized stainless steel is one thing a stainelss steel with a crack is another... SCC usually start from a pit on the surface but if you don't see any crack...

sure that there are no weld ?

S

Corrosion Prevention & Corrosion Control

 

[stanweld]

The chloride concentrtions observed and service temperature reported are more than enough to cause pitting corrosion in 316 SS. Intergrannular corrosion may result from inadequately solution treated material. Did the Met Lab examine for carbide precipitation adjacent to grain boundaries?

 

[electricjer]

The pit isn't located at a weld or crevice of any kind. It is located where the vapor is condensing on the plates. The corrosion deposits seem to originate where the vapor enters the condenser and dissipate as you move out.

I am curious to know if the stamping process for the plates is enough to sensitize the steel. No analysis for carbide precipitation was done due to the low service temperatures. Cracks are apparent, so I guess that rules out SCC.

If the plates are cleaned up well, could they be passivated in nitric acid or will the corrosion continue with any exposure to Cl? Looking into titanium, but there is a possibility of fluoride (10ppm)which from what I am gathering could produce similar results.

 

[EdStainless]

At this liquid vapor line you may be seeing localized chloride levels of 5-10%. at 140F 316 will not handle this.
As to the attack, just because it is in the grain boundaries does not make it related to sensitization. It the lab didn't so the SEM work to identify the presence of chrome carbides then it may just be crevice corrosion and the grain boundaries are going first.
If you didn't heat or weld these then you did nothing to impact the sensitivity toward this corrosion. It is possible that the material is sensitized, maybe the mill anneal was not very good.
But I would hope that this is 316L with C below 0.025%, in which case it is almost impossible to sensitize it in the first place.

Yes, you can clean them, but this will have no impact on future corrosion.
Is the fluid level constant in this unit? If so consider cutting a band out of it and welding in a better alloy, just in that area. You will want to keep the welds at least 4" away. I would suggest that you do this with a 6% Mo super-austenitic grade (AL-6XN or 254SMO). It is not uncommon to see this kind of construction.

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Plymouth Tube

 

[davefitz]

there is really no safe level of Cl in the case of a interface between vapor and boiling liquid- the process of boiling leaves a much higher Cl concentration at the steam/water interface. This is the main reason 316SS cannot be used in contact with a liquid surface ( economizer, boiler waterwall) in ASME section I boilers.

 

[EdStainless]

to flesh out Dave's comment. at a wet/dry interface there con be concentration well above the wet values. There are cases of localized Cl at the interface in almost any equipment that has this situation.

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