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. 600 when the SS tube is empty and baked under the sun.

There is very little corrosion found at the bottom. 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

 

[arunmrao]

If you suspect chlorine presence in water,304 will not be right choice,suggest you change to 316 alloy.

_____________________________________
"The richer we have become materially, the poorer we have become morally and spiritually." Martin Luther King Jr

 

[Metalguy]

Sounds like you have severe conditions with your potable water/chlorides at those temperatures and dryout conditions.

You may need something like an Inconel or Incoloy tube.

"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

 

[unclesyd]

You probably have splash zone type of corrosion. This is where if any water evaporates it will leave residue. In you case I agree with the above posts that Chlorides are probably the culprit. A Chloride residue, especially with a little organic matter normally will not rinse off.
We have this problem on all our vertical SS heat exchangers.
One method to mitigate the problem is to provide a vent/s to keep all areas wet.
Another method is to "safe-end" the tube with a short section a Nickel alloy.
The last would be to change the alloy of the tube.

 

[Metalguy]

This has been double-posted. The correct place for it is in the Corrosion section (the other posting).

"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]

Yes, it is double posted. Because I realized that the corrosion section is a better place for my question.

Incoloy tube is probably too expensive for this application, because 316 already seems expensive....

Please post your suggestions to Corrosion section.

Thanks you all!

 

[Guest102023]

No doubt there are chloride ions present, and the higher the temperature, the greater the pitting tendency. Type 316 would offer improvement in pitting resistance.

It is not clear from your post whether the weld zone ONLY is affected; is seems that it coincides with the splash zone (which can be a problem all by itself).

Make sure the corrosion resistance has not been compromised in the heat-affected zone by 'heat tint'. You will recognize this by discolouration after welding (blue to purple to brown lines parallel to the weld). Any such affected areas must be ground and then pickled to restore full corrosion resistance.

 

[RebelBrill]

If the corrosion problems are a result of sensitiaztion of the weld HAZ, several things can be done to prevent it.

Sensitization is the depletion of chromium in the alloy due to the formation of chromium carbides. This can be reduced by (a) using stabilized alloys of stainless, (b) using low carbon containing alloys such as 304L or 316L, and (c) heat treatment of the weldment.

Much information is on-line concerning this subject. Dr. Damian J. Kotecki is a good source.

 

[flash3780]

You may actually consider using a 316L (the low carbon variant) rather than vanilla 316, but I'm not a metals guru. I can't imagine that swapping to 316 would be more expensive than applying a coating.