chloride attack of SS
chloride attack of SS
(OP)
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?





RE: chloride attack of SS
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
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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.
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hoep this help
S
Corrosion Prevention & Corrosion Control
RE: chloride attack of SS
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.
RE: chloride attack of SS
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.
RE: chloride attack of SS
sure that there are no weld ?
S
Corrosion Prevention & Corrosion Control
RE: chloride attack of SS
RE: chloride attack of SS
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.
RE: chloride attack of SS
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
RE: chloride attack of SS
RE: chloride attack of SS
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Plymouth Tube