Stainless Steel Weld Corrosion

[jtoutlaw]

Hello,

I have read the forums for a while. This is my first post, so please bear with me. Hopefully I won't require too much hand holding.

We ran into an issue recently at the plant I work at. The equipment involved are two 304 SS vessels, two pumps, and an assortment of 2"-6" 304 SS piping. Without getting into too much detail of the chemicals involved, they are all compatible with stainless. The equipment has been in service for about two years. Around six months ago, the first leaks appeared. We fixed the leak, and in the following months, more and more leaks started. We have shut down the equipment and are in the process of replacing all the piping. The existing piping is 304 seamed pipe. We are replacing with 316 seamless pipe.

During investigation, we found that the welds were being corroded away. As I said, all chemicals involved are compatible with stainless. None of the pipe walls or plates in the vessels have any indication of corrosion. However, some of the welds seams in the pipe were so corroded that it looked like someone literally took a grinder to the weld. Most of the weld seams in the pipe has varying levels of corrosion, but some have none at all. The radial welds around flanges and fittings were better. Some had serious corrosion, but most had none at all. The system seemingly randomly decided to corrode certain welds and not touch others. We were able to get inside the tank yesterday. It is in much better shape, but a few of the welds in there appear to be beginning to be eaten away at the edges. There is zero pitting throughout the entire system. The welds just seem to be disappearing.

After talking to some people about it, one person said he had seen something like that before. He seems to remember that the issue came from something to do with grounding. It had been a while, so he was not sure of the details. As far as lightning/fire protection, the tanks are grounded as far as I know. We are going to be testing it soon.

My question is, has anyone here seen anything like this before, or have an idea to what may be the cause? It is a strange phenomenon that currently has everyone stumped. We are replacing piping with seamless, which eliminates those welds. It does not solve the issue though, so it will come back eventually unless we do.

As I said, it's my first time here, so let me know what other info I should include or if my formatting needs to change.

Thanks for the help!

 

[don1980]

This is a well known risk when welding stainless steel. It's carbide precipitation which can occur if you don't use the right welding procedure, and don't use a low carbon stainless steel (like 304L, the L stands for low carbon). On the internet you'll find tons of information about this topic, and guidance for how to prevent it from occurring. The welding heat can cause granular changes in a metal alloy, and that's why it's critical to use skilled welders who are following good weld procedures for that specific alloy.

 

[jtoutlaw]

That was my initial thought to the cause when I started to dig. There are a few reasons why I am not sure that is it. The welds done onsite by our contracted welders had the fewest corrosion instances. Being welded by hand in the field/shop, it would reason that the highest occurrence rate would be there. The vast majority of the of the corrosion is occurring in the weld seam on the seamed pipe. As far as I know, that is typically done by machines. I am assuming the pipe manufacturer welded the seams using proper procedures for the alloy, but everyone knows what happens when you assume things. Also, the vessel was built to API 620, which has requirements for the welder qualifications & procedures. I am assuming (again), that they were following proper procedures. API 620 also has inspection requirements for heat affected zones, although I am no expert and don't know what the specifics of the code say.

Correct me if I am wrong, but I thought carbide precipitation causes corrosion in the base metal, just outside the weld, in the heated zone. The welds themselves are being affected, not the base steel.

 

[mk3223]

I believed that you have done some work to investigate the root cause and /or possible issue regarding the corrosion of the 304SS pipe.
In addition to the problem as described, it can be helpful if some photos can be shared for better understanding and preparing answer. Also, it seems a wide spread issue to the operation unit, you may consider to have a material engineer to perform a failure analysis for the damage and suggest a solution.

 

[MJCronin]

Can you post pictures ?

Have you considered posting in the corrosion forum ?

MJCronin
Sr. Process Engineer

 

[Dhurjati Sen]

Dear jtoutlaw,

Nice to see you are concerned.

Need to carry out metallography and chemical analysis of the welds as well as the piping.

Also, the MTCs need to be re-evaluated.

Simply upgrading 304 with 316, I am sorry to say, shows your organization in poor light.

Regards.

DHURJATI SEN

https://www.nace.org/people/dhurjatisen

 

[jtoutlaw]

Attached are some pictures.

Since we aren't positive what the cause is at the moment, the upgrade to 316 seamless was done to enable production to resume. We plan on taking the unit offline again later this year and inspecting it to determine if the problem is still happening. Since we can't leave the unit offline for an extended period, it was decided this was the route that gave us the optimal chance for remedy with the information we have now. I disagree that is shows the organization in poor light. Everything is at atmospheric pressure, and the contents are non-hazardous. There are minimal safety or environmental concerns (I say minimal because I don't like to use the term zero chance). All we would be out if the 316 doesn't work is the cost of the pipe & labor, which is small compared to keeping the unit offline.

I have not posted in the corrosion forum. If that is the better spot for this, I can post there.

We will look into getting some samples to a material engineer for analysis.

 

[mk3223]

Form the attached photos, IMO, the pipe crack along the pipe seam could be resulted from the pipe manufacturing defect. And, it can be a good fix to replace it with SMLS pipe.
To understand the surface corrosion inside the SS pipe, more info may be needed regarding the fluid property, process condition, etc. regardless where to post.
Good luck.

 

[EdStainless]

A slightly acidic environment I am guessing?
Most welded pipe is grossly under annealed. One outcome of this is that there will be significant amounts of residual delta ferrie in the welds. The welds will be slightly magnetic.
And this ferrite is very susceptible to corrosion.
Tubing that is supplied to some applications have rules that limit ferrite in the welds.
In order to do this you must furnace anneal the product, not annealing inline on the weld mill.

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P.E. Metallurgy, Plymouth Tube

 

[SJones]

It does appear to be a classic case of the interrelated aspects of supplier selection and technical specification by the purchaser, and quality control capabilities of the supplier.

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]

I should also add that in many critical services tubing is used, not pipe.
This is because the tubing specifications have options for much more stringent NDT.
And now you know why.

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P.E. Metallurgy, Plymouth Tube