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Spalling of 316 Stainless Steel in Sulfur Dioxide environment

Spalling of 316 Stainless Steel in Sulfur Dioxide environment

Spalling of 316 Stainless Steel in Sulfur Dioxide environment

(OP)
What would cause 316 stainless steel to spall? This particular piece of equipment condenses sulfur dioxide vapor into liquid, there are no moving parts and no possibility of impacts. There are several isolated spalled areas/delamination and cracks throughout both in the welds and plate.

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

Temperature? Age? Are they all at the HAZ? (or near welds)
Is it 316L? What was the weld filler?
When are you removing coupons for lab study?

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P.E. Metallurgy, consulting work welcomed

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

Was the grinding done by you? If so you may have adversely affected any metallurgical analysis. The 2 areas along the weld appear to have been located in arc strikes - one ground out prior to welding and the other left as was prior to circ seam welding.

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

(OP)
I have absolutely no design information (it's at least as old as 2002) for it other than I saw T316, NOT 316L stamped in one of the plates. The spalling is not limited to the HAZ of welds, some of the areas are in the middle of the plate far away from welds. As for the grinding marks, as far as I know these are from the manufacturing of the vessel. No repairs have ever been attempted because it operates near atmospheric pressure and there are SO MANY CRACKS that a wait and see approach has been adopted. I was really curious as to what could cause the spalling because it is not something I've seen before.







RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

What does the other side of the damage look like? Pitting from the outside could push up the chromium oxide layer on the inside and create this appearance. I see it happen with anodized aluminum as the oxide layer is very thick. I guess it could happen on stainless as well.

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

Temperature? Cl levels? pH?

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P.E. Metallurgy, consulting work welcomed

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

(OP)
There's nothing remarkable about the outside, this vessel sits about 40 feet above the ground on its own structure.

I'm new here, so maybe I don't know how things work. I was just trying to start a dialogue about what could possibly cause this type of damage mechanism. I read through API 571 and nothing quite matched. The one thing I found online at corrosionpedia discussing metal spalling mentioned cavitation as a possible cause. I don't know if it would be relevant here. This is a condenser, yes, but the liquid level during operation I believe would not be as high as some of the spalling/delamination I found.

As far as temperature, I think as high as 300, but it's cyclical. There shouldn't be any chlorine vapor, only sulfur dioxide.

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

SO 2 and water , Be worth it to get a micro exam to look for polythionic acid SCC . Sensitized metal in weld HAZ would be suspect L grade may help resistance but does not prevent polythionic SCC. It can develop quickly, does not need to be constantly wet. I have seen high temperature equipment crack during a turn-around with water condensing on sulfide scales.

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

I am with you Blacksmith, PTA cracking is about all that I can see here.
The spalling may actually be cracking in spots where there was weld splatter or arc strikes (or surface contamination during plate manufacture) which caused local sensitization.
Ray, you need to get a grip on how deep this cracking is. That is the starting point of a risk assessment.
I inspected a SS tank once that had thousands of cracks. It we 30' tall and there was no place that you could put your hand and not be touching multiple cracks. But the deepest were only 2% of the wall deep. The tank next to it had 3 cracks, and they all were through wall. Two very different issues.

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P.E. Metallurgy, consulting work welcomed

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

(OP)
Awesome, that's not a damage mechanism I had heard of before, thank you. Now that you mention it there would be some water in the form of steam in the gas that enters. This is an acid process process pulp mill and this condenser is attached to the vents of their blow tanks, chips are digested with acid and steam.

There are probably about 50 cracks that run perpendicular to the weld into the shell plate and are definitely through wall. We suspected it might be some type of SCC coupled with fatigue cracking from vibrations/thermal but I've only really experienced chloride and caustic SCC and neither of these are in this area. We have been hesitant to recommend weld repairs thinking that it will only make it worse. Sounds like we need to recommend the removal of a coupon and send it to the lab to get a definite answer.

Ultimately I think they're going to replace it but like I said, I saw a new-to-me damage mechanism and was trying to figure how what would cause it.

Here is an example of a through wall crack, internal and external view.



RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

This damage mechanism is covered in greater detail in API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. It includes a general description of the damage mechanism, susceptible materials of construction, critical factors, inspection method selection guidelines, and control factors.
PTA SCC is also covered by NACE Standard RP0170-97, Protection of Austenitic Stainless Steels and Other Austenitic Alloys from Polythionic Acid Stress Corrosion Cracking During Shutdown of Refinery Equipment, which lists several methods for preventing or mitigating PTA SCC and details some of its causes.

Most commonly people use a chemical washdown at shutdown to lower the risk. The use of very low C (<0.020% which is common in good L grade) grades and even stabilized alloys (347 or 316Ti) will help avoid the local sensitization issues.

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P.E. Metallurgy, consulting work welcomed

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

(OP)
Thanks, I must have just skipped over that section because I have never heard of polythionic acid.

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

RayHam475
IMO you are in presence of ssc sulfide stress cracking SSC.

As EdStainless told you cracking in spots were there as weld splatter or arc strikes (or surface contamination during plate manufacture) which caused local sensitization.

Polythionic Acid Stress Corrosion Cracking (PASCC) is a type of intergranular stress corrosion that affects sensitized stainless steels in refineries processing feedstocks containing sulphur. This damage mechanism primarily affects standard and high carbon stainless steels, but low carbon and chemically stabilized stainless steels may also be vulnerable under certain conditions. PTA SCC typically does not occur during normal processing, but rather, after the equipment is shutdown and opened up for inspection (i.e. when moist air contacts the surface of equipment that has been exposed to sour hydrocarbons in service).

A common form of "prevention" is to perform a soda acid wash of the equipment before or right after it is exposed to air and moisture. PTA SCC can also be prevented by removing oxygen from the environment by performing a dry nitrogen purge or removing moisture by performing a dry air purge.

luis

luis

RE: Spalling of 316 Stainless Steel in Sulfur Dioxide environment

SSC is a form of hydrogen stress cracking (HSC) and involves embrittlement of the metal by atomic hydrogen that is produced by acid corrosion on the metal surface. Hydrogen uptake is promoted in the presence of sulfides. The atomic hydrogen can diffuse into the metal, reduce ductility and increase susceptibility to cracking. High strength metallic materials and hard weld zones are prone to SSC. Is it possible to have traces of H2S in your So2 System?

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