316 Stainless Steel pitting

Date 02-14-2019 18:29 Edited 02-15-2019 22:15

Could very well be MIC (microbiologically induced corrosion).

By chance, was there a mound of brown jello on the wet side where the pitting was observed? If there are leaks, there will be some rather colorful iridescent deposits on the dry side of the leak(as in the embedded photo). If so, MIC is the most probable cause.

Al

Attachment: MIC.pdf (126k)

By ellat1me Date 02-18-2019 14:55

Good morning
Thank you for your response. A video scope was used, but not very good photos. Is there any testing or evaluation to be used on sample to determine if it is MIC? We also took water sample. Is there anything to look for in the water? Thank you.

By 803056

Date 02-18-2019 14:59 Edited 02-18-2019 15:10

Microbes.

My client had through wall perforations in about 90 days. They used city tap water to test their system and then failed to drain it.

The attached photograph shows the microbe excrement on the water side of the vessel.

Al

Attachment: MIC2.jpg (69k)

By Jim Hughes (***)

Date 02-19-2019 15:48 Edited 02-19-2019 15:55

ellat1me,
do you know what type of water was sitting in the pipe? It typically takes brackish water to set up MIC. Every indication of how you describe it indicates MIC. The corrosion cell looks a lot like what we call in the south, a dirt dobber nest. I will tell you this for sure. If it is MIC don't try to repair the welds. You will have to cut out and replace them. This happened on a BP Chemical project I was on. It was 10" Sch 10. Unfortunately, plant utility water was used for hydro. Although most was drained and blown dry with nitrogen, we had a section that was not drained. It was the sleeper rack portion, and it sat in that sleeper rack for about 3 months. We ended up cutting and putting a lot of pups in.

By ellat1me Date 02-21-2019 22:08

I am uncertain concerning type of water but the chemistry: chlorides were about 250 mg/l sulfate 200 mg/l and PH 8

By Jim Hughes (***)

Date 02-23-2019 13:37

ellat1me,
chlorides in systems is a problem. Typically you should have less than 50ppm. But MIC is not chloride corrosion. Chloride Corrosion has to have the presences of chlorides, tensile stress, oxygen and heat. MIC is just typically brackish water at ambient temps sitting in pipe for a few weeks. A micro graph of Chloride corrosion typically looks like lighting bolts. MIC is microbiological animals eating the material. A chief inspector for BP that I worked for, would say, "you can put any ungodly liquid that will kill a human in seconds in 300 series SS, and it will not hurt it, you just can't put water in it, and especially plant utility water". This was after we had a MIC failure.

By JSefco Date 04-03-2019 14:34

Sounds like carbide precipitation to me. When Austenitic stainless steels are heated and cooled through the range of 800 - 1650°F, chromium in the grain boundaries is depleted when it combines with carbon to form chromium carbides. The steel is then "sensitized" to corrosion because of the lower corrosion resistance of the heat affected zone. This can be minimized by using the lowest possible heat input when welding, however, changing the material from 316 to the low carbon equivalent 316L would probably be more productive.