Differential Oxygen Pitting in Admiralty Brass Tubes?

[Metalguy]

I am in the middle of a failure analysis of some admiralty brass heat exchanger tube pitting. The water source is some huge open spray ponds, so anything can and does blow in. It is treated with a few biocides, and the Cl- is ~2,000 ppm.

I expected to find some involvement with MIC, but have just about ruled that out. No evidence of S, C, Fe, Mn, and the pits do not look anything like those of MIC or Cl- induced pits on SS. These pits look just like the sharp, open pits typical of diff. O pitting in steel.

I did find the entire ID surface has some dezincification, in that it is covered with thousands of very small "mounds" of nearly pure Cu (from redeposition). They are very well bonded, and resist strong brushing with a SS brush. I had the tubes sawed open such that I could compare the upper half with the lower--no difference. Used ~30% nitric acid in water to remove the scale, which appeared to be a copper-chloride-oxide with a deep green color-a beautiful color indeed.

Although the HXs do sit stagnant of a max. of 2 days, followed by a 1 hr. pump run, I have to eliminate dirt, etc. as the cause of an O blocking deposit, because the upper half of the ID is identical to the lower.

Any ideas? Has anyone seen diff. O pitting on high-Zn brass before?



"When the eagles are silent, the parrots begin to jabber."
Winston Churchill

 

[metengr]

Metalguy;
We have one Admiralty brass condenser at one of our Power plants and it too has suffered from general dezincification attack, and local corrosion pitting attack. The waterside tube surface is exposed to a combination of silting and moderately polluted fresh water.

The dezinficiation occurred because the tube material is not inhibited Admiralty brass as what was originally specified during Plant construction (50 years ago). We have been periodically keeping the condenser tubes clean and this has seemed to keep things under control.

 

[Metalguy]

Thanks. I'm surprised that yours have lasted that long in even moderately polluted water.

I'm going to push for retubing with alum. bronze.

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill

 

[EdStainless]

Is there a reason that you are sticking with a Cu based alloy? In power plant service the trend is to move to high alloy SS, usually a super-feritic grade for any service that admiralty or 304 cannot handle.

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Plymouth Tube

 

[Metalguy]

Good thinking, but my eddy-current guy is in the way, and he's bigger than me. We are considering changing them to new plate-type HX's, for only one reason--time. Not procurement time, but retubing time. Best estimate is 3 weeks (~2500 3/4" tubes, ~50' long, IIRC). We are just finishing our last long outage (hopefully), and there isn't 3 weeks of available time after this.

Had one vendor come in and suggest 254SMO!!! I wasn't there that day, but our water is nowhere near bad enough for that.

If we go with plate type I'll be taking a hard look at 2205, and the eddy current guy can go pick on someone else.

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill

 

[metengr]

MIC and bi-fouling are on-going concerns and have become more prevalent in fresh waters. Going back to copper-base alloys from conventional stainless steels may still be a viable option for some users.

 

[EdStainless]

If people were looking at NAB or 90/10 I would believe it.

We are supplying superferritic tubing in place of brass in many cases. The 6% moly alloys maybe overkill, but at todays prices they are also a great waste of money. They are more than twice the price of the superferritic alloys.

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Plymouth Tube

 

[rmw]

Get your heat transfer or mechanical engineers involved. If you retube with something as high up the food chain as super ferritics you will get the advantage of being able to go to a (much) thinner tube wall which will overcome or exceed exceed the HT coefficient penalty between what you select and the brass, but someone will have to check the pump/system resistance curves to make sure that the pumps and or the effect on system resistance matches the system requirements.

If you go back with plate types that will all have to be sorted anyway for their proper selection.

rmw