Nickel alloy-Heat Treatment Inquiry 1

[Normankhan]

Good Day All

I would like to draw your attention to one problem dealing with pitting corrosion in Nickel tube bundle, mostly in the ends of U-tube, of heat exchangers. We are mainly using nickel tubing as it has a Good heat transfer property which off course delivers a Good evaporation of the mixed liquid in the heat exchangers (AlCl3+Ticl4) both are corrosive in nature and I believe are mainly responsible for pitting corrosion.

Below is the material information:
Grade: Pure Nickel (UNS N-02200) Specification: ASTM B 163 Seamless, Nickel 200, UNS N-02200 Annealed Heat.

The details are attached.

I have one proposal that we if we perform heat treatment for nickel-200 tubes; it might reduce pitting in the end-U shape bended sides. As I beleive that during bending of tubes to give them U-Bend shape we introduce internal stresses. Therefore, I believe that these stresses could be eliminated through heat treatment-stress relieve method, may be pitting could be at least reduced in the U-shape bend ends.

do you guys think it is worth to make this kind of trial.Kindly suggest any advice

Thanks

Norman

 

[metengr]

Norman;
Very detailed problem description, nicely done. Several questions as a follow-up;

1). Is the damage to the tube OD surface confined to a narrow band along the neutral axis of the u-bend as shown in your attached photograph?

2). Is there any damage to the tube OD surface observed in the straight length tube sections?

 

[metman]

In chapter 7, "Effects of Stress", in the book Corrosion and Corrosion Control by Herbert H. Uhlig

Work hardening does not create increased corrosion rate for the reason we would intuitively expect. According to Uhlig, ...the residual energy produced by cold working is less than sufficient to account for an appreciable change in free energy... Then he writes about the effect of cold working on the precipitation of carbides or nitrides, dislocations etc and this being the more probable cause of increased corrosion rate in acid. Whereas in natural waters, cold worked vs. annealed steel corrode at the same rate as noted earlier in Uhlig's discussion.

I have no idea if there is any comparisons to be derived here. I just think it very notable about the insufficient free energy issue. But then you have Nickel not Steel.

I find it curious that the pitting is primarily along the nuetral axis. This would seem to suggest some effect different than residual stress.

 

[arunmrao]

A good point raised by metman regarding free energy. But for a reaction to progress,not only is the free energy important,but the kinetics of the reaction is the one that governs the rate at which corrosion proceeds.

Learn the rules,so you know how to break them properly.
Dalai Lama

_____________________________________

 

[MikeMet]

To answer metenger, you can see damage on the straight section in the picture, at least several tube diameters away from the end of the bend. The damage is also on the neutral plane of the bend, which has less residual stress than the back side of the bend which in the picture appears to have less or no damage.

If the bundle is in the orientation shown in the drawing pitting from stagnant conditions does not make sense.

I'm having a hard time finding a reason for corrosive pitting occurring where we see it in the pictures.

It looks to me like caviation, although I have no reason to explain why it is forming on that particular area of the the tube versus another. I recommend you do some serious SEM and metallography work to characterize the damage as corrosion or other. I have seen caviation on hx tubing, but not in that area. It may be an area where the bubbles are forming, but there is not enough energy or virgorous boiling for them to fully release and they collapse back on the tube. But then again, I may be all wet on this one.

 

[EdStainless]

What is the pH of your solution? Ni is happy at very high pH.
There are other alloys used in similar service.
Does it actually boil? That is what looks like is happening.
Remember that this is a very soft material.
Stress relief of the bends will not help at all.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[metman]

I recommend you do some serious SEM and metallography work to characterize the damage as corrosion or other

From your Problem Description:

The tube bundles tend to foul after few days of continuous operation. Vaporizer however is shut down routinely to change out the Ni tube bundles and to clean shell.

How often do you change the Ni tube bundles? Depending on how often, it appears you have ample opportunity to garner samples for evaluation as Mike suggests.

What is the chemical reaction of the two compounds with each other in the bath around the tubes? Is the bath stable i.e. no reaction or…?

What do mean by foul? How can steam foul the tubes? I realize this is academic because the affected area is external on the tubes but I am missing something here.

 

[btrueblood]

I do agree with MikeMet that the location of the damage makes it appear to be cavitation-related, i.e. the location is about where a recirculation zone exists for a tube in cross-flow. The OP should look into the starting sequence for the HX, and see if perhaps the steam flow can be throttled until the entire tank has reached a temperature near the boiling point, or if the steam temperature can be reduced during starting to limit the temperature gradient on the top rank of tubes. This should reduce the likelihood of reflux of the cold fluid into the heating zone of the tube, and consequent "steam" bubble collapse and "cavitation" damage.

 

[metman]

This should reduce the likelihood of reflux of the cold fluid into the heating zone of the tube, and consequent "steam" bubble collapse and "cavitation" damage.

This steam -- are you referring to the volatized bi-chemical bed around the tubes? You cannot mean the steam inside the tubes.

What does it mean HX? I am ignorant of this term or abbreviation.

I agree that it could be cavitation-errosion or maybe cavitation-corrosion but not quite understanding some of the comments.

 

[metengr]

metman;
HX = heat exchanger

 

[rmw]

And sometimes Hx.

rmw

 

[Normankhan]

Guys
Can you suggest any other alloys(Good Thermal conductivity)other than Nickel-200 for (AlCl3+Ticl4)corrosive media.in other words alloys which can resist some how pitting.
Thanks
Norm