Corrosion on SS 304 welding area 5

[DixieSolar]

Dear All,

I'm developing a solar water heater which combines the collector and storage tank together, heating portable water directly. The product has been made, and tested. The problem we found after 6 months testing is pitting/crevice corrosion on the top of SS 304 tube (welding area).

My case is:
- SS 304 long tube welded on the bottom and top
- portable water in the SS tube, regular working temp. 35-210F.
- Pressure <90 psi
- Max temp. 600F when the SS tube is empty and baked under the sun.

There is very little corrosion found at the bottom or in the middle of tube. Pitting/crevice corrosion is only found at the top around welding area. The solar hot water was rarely used when testing. I suspect that the pressured steam with chlorine in portable water is causing the corrosion, and the S.S. material structure was damaged at the welding area.

My question is:
-Any coating material can be applied at the top of welding area inside the S.S. 304 tube? but it can not contaminate the portable water under high temp. and pressure.
-Any welding process or procedure can be used without destroy S.S. 304 material structure in this case?

Thank you very much for your help and advice!

Dixie Solar

 

[DesignerGuy16]

Could be the inclusion of iron during welding. Anything from the rod itself picking up the iron from a table, brushes or bead blasting media used on carbon steels as well.

Look into passivating the area, and using brushes and other tools that only touch stainless material/welds. Should decrease any cross-contamination.

James Spisich
Design Engineer, CSWP

 

[stanweld]

Assuming the use of welded tube, is corrosion on the ID of the longitudinal weld or at a circumferential weld? If on the long seam, it may well be due to contamination - noting its proximity to the end of the tube.

What is the tube specification? Were the tubes solution treated? Were the tubes cold finished?

 

[Metalguy]

See the replies in your other post, in the Mechanical Engineering section.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[DixieSolar]

The OD of SS tube is 100mm, and the wallthickness is 0.8mm. The SS tube is rolling welded to form tubular first. There may be some cold finish before or during rolling weld process, not sure.

The corrosions were found around the top of circumferential weld area. Attached is a pic of cutting section on the top of the tube. You can hardly see the corrosion on that area. But it is very serious. Almost every tube has tiny holes leaking at the top when the pressure is on. The rust you see outside of the tube is caused by leaking afterwards.

There is no solution or any treatment when welding the top piece on the tube. Can you suggest any treatment to improve the welding, and prevent the corrosion?

Some people say I have to change the material to 316 SS because of chlorine in portable water. But there is no leak or very little corrosion found at the bottom or in the middle of the tube. This leads me to believe 304 may be still OK. I just need some coating to block the steam at the top weld area, especially the inside triangle area at the top.

Thank you very much "stanweld"

Any input, suggestions are much appreciated.

 

[DixieSolar]

It seems that "safe-end" the tube with a short section a Nickel alloy is the solution!!?? because the top of tube is the dead end, it can not be vented.

Maybe weld an incoloy 800 top piece is the cost effective way to do it?

I found "Incoloy 800H/HT alloy was intended for high temperature structural applications. The nickel content makes the alloys highly resistant to both chloride stress-corrosion cracking and to embrittlement from precipitation of sigma phase." on Internet

Please comment.

 

[metengr]

Look at using duplex stainless steel tubes versus high alloy content. This material is higher strength and is highly resistant to SCC from chlorides. Your maximum design temperature is right on the edge that is permitted for this material.

 

[stanweld]

There is rusting along the long seam and rusting in the HAZ of the circ seam. Isolated rust in the pipe is no doubt due to iron contamination. The rust on the long seam indicates that the tubes were not subject to solution annealing. Specify welded 304L tube to be solution annealed and passivated. For better results specify 316L tubes, solution treated and passivated.

How are you making the circ. seam? I doubt that the gas backpurge is adequate.

 

[Metalguy]

Type 316/316L has only about a 20-30 deg.F advantage over 304/304L for Cl- pitting, and your temps. are way beyond what they can resist.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[DixieSolar]

Attached is the bottom section of the tube after cut. You can see the scattered yellow spots (minor rust I suppose). The yellow spots are not particularly on the weld area or on the longitudinal seam. The bottom and middle section are usually submerged into portable water, not exposed to steam. In fact, there is no leak found at bottom or middle section. So I think it’s OK.

The gas backpurge maybe inadequate during the top circ. welding. But now I more intend to believe that 304 SS probably can not stand the steam corrosion at the top.

 

[Guest102023]

Lots of heat tint, but that does not appear to be the main problem. Some of the material substitutions suggested above appear to be in order (i.e., duplex SS).

 

[stanweld]

For high chloride containing potable waters like those from the Colorado River servicing California, Arizona & Nevada, a duplex stainless steel is recommended.

 

[DixieSolar]

Thanks for all your valuable posting!

One more question:
What kind of welding structure do you suggest to weld the "safe-end, either duplex SS or incoloy 800" to the top of 304 SS tube? any treatment before welding or any special technique to weld those two different SS materials together?

Appreciate all your help.

 

[Metalguy]

I suggest you work with a metallurgical lab. and do some realistic testing to see if a good duplex SS will work. If you really have 600 deg F and Cl-, you have very severe conditions.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[DixieSolar]

600 deg F is only in very rare condition when people drain out the system during summer time, and it usually should be a short period of time. There may be some moisture with minimum Cl in it without any pressure in this condition.

The regular working temp. is 35-210F with pressure about 65 pis.

We tested our product from June-Dec. in Texas. Nobody was using the solar hot water. I believe the pressured steam constantly on the top caused the problem.

 

[EdStainless]

1. Were all of these welds bright and clean when they were made? I am guessing not, they are crappy welds, poor cleaning and poor gas shielding.
2. It is most likely that the high ferrite number of standard 304L lead to welds that have much worse corrosion resistance than the bulk metal.
3. Modify the design to remove the tight crevice from a non-flushed area. You will only trap and concentrate impurities.
4. Moving to a lean duplex will give you slightly better pitting resistance than 316 along with MUCH better CSCC resistance. And a lower price. There are a bunch of such alloys on the market, feel free to mix and match, 2003, 2101, 2102, 2404, and whatever has been added this month.

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Plymouth Tube

 

[DixieSolar]

Hey EdStainless,

Thank you very much for your posting.

The current design is concave top for easy and fast welding process.

Convex design with duplex SS (or other alloys you suggested)at the top seems the solution from all the postings here. The top has to be long enough so the circumferential welding line is under the water to protect 304. This solution will increase the material cost quite bit and the difficulty to weld as the wallthickness is only 0.8mm and it's a very long tube (about 7' long).

Is there any coating material I can use to fill the concave gap inside the tube to cover the weld area completely? The pitting/crevice corrosion all happended at the top weld area. The 304 top itself seems OK.

 

[EdStainless]

flip the cap over an do a butt weld. I see these done with automatic (AMI) equipment in 0.7mm and lighter tubing regularly.
Moving to lean duplex will not change the price much, especially if you go to 0.7mm, the alloys are much stronger.
The welds in a duplex retain a higher percentage of original corrosion resistance than do austenitic alloys.

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Plymouth Tube

 

[DixieSolar]

Hey EdStainless,

Can you let me know the name of manufacturers who are doing automatic butt weld of different SS tubes? We are considering to sub-contract this job out.

Thanks,

Dixie Solar

 

[EdStainless]

You can start by talking to AMI (Arc Machines Inc). They build this equipment and may know of sources for welding services. Automated orbital butt welding is the mainstay of the pharmaceutical and semiconductor industries. A lot of people can do a good job of this.

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Plymouth Tube