Aluminum Heat Exchanger - water treatment

[ziptron]

Hi All,

I have a boiler that has aluminum heat exchanger plates. I have treated water in my recirculating heating system that matches relatively well with the conductivity and the pH specifications that were set out by the manufacturer of my boiler. The one thing that is off however, is the hardness of the water. The water seems almost double as hard as the maximum allowed by the manufacturer. All seemed fine for a while, however I started looking into this because I noticed that I had some type of erosion-corrosion on one of my heat exchanger plates. I had a perforation that actually penetrated through the heat exchanger plate. Could it be that the hardness of the water is causing this?

Any ideas how water hardness affects corrosion or erosion of aluminum with flowing water?

Any ideas or directions for further research would be appreciated.

Thanks!

 

[bimr]

Water hardness is associated more with scale than with corrosion.

However, the water cations represented by the hardness in the water analysis are assocoated with anions such as bicarbonate, sulfate, and chloride.

You do not provide much detail regarding the water analysis, but one would suspect the chloride ion to be responsible. Chloride is an aggressive anion and known to cause corrosion in aluminum.

Please see the attached link:

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=180&pageid=2144416698

Addition of a corrosion inhibitor to the water would seem to be appropriate.

 

[ziptron]

Unfortunately the company that does the water treatment does not report for me the chloride content of the water, that's mainly why I didn't post it.

I can see how chloride can contribute to corrosion, but I don't have corrosion everywhere within the heat exchanger, it seems to all be in one spot around a bend in the heat exchanger. So my first instinct is to say erosion caused it because its at a bend, but if I look closely I can see etched dendrites in the aluminum.. and so I can't decide if the water chemistry is bad or if the flow is too high... or both..

 

[IRstuff]

Maybe punt the alum HX and replace with copper one. Then, you only have to worry about scale.

TTFN
faq731-376

 

[ziptron]

I would, but its only 6 months old.

 

[bimr]

"The initiation of pitting corrosion is often linked to the presence of local defects at the metal surface such as flaws in the oxide or segregates of alloy elements, and the presence of aggressive anions such as chlorides in the environment. The chlorides are believed to locally disrupt the oxide, preferably at pre-existing weak spots, resulting in microfissures of several nm in diameter."

Perhaps it is a local defect in the fabrication. Do you have a picture?

 

[bimr]

Also, a lab test for chloride is relatively inexpensive. Perhaps, you can get a complete water analysis. Then post it.

 

[ziptron]

Unfortunately before I got to the site, the water had been changed and the previous water that was in the system was not retained so I cannot make a good enough analysis on what was in there. Previous test records only tested for pH, conductivity and hardness, and weren't even testing for chlorides at all, so I have nothing to go on from that either.

I don't think this is a pit that was caused by a manufacturing defect in the heat exchanger plates because I've got a 7 boiler system and it happened to 4 boilers, all in the same place within the heat exchangers. Also, it always happens in the same spot on the different heat exchanger plates, it always happens near a turn/bend in the flow of the water through the heat exchanger plate.

Unfortunately I don't have access to the photos now, but if you were to look at it you'd see significant thinning of the walls on the inside around where the perforation occurred. Its not simply a pit or several pits with one that makes its way all the way through the wall, its actually a 1/4 inch by 1 inch hole on the outside and a significant amount of thinning on the inside around where the hole occurred.

Seeing just the hole, I'd say erosion from flow caused this hole.. but, seeing the bad chemical maintenance that was being done prior to my arrival and the fact that I can see dendrites when I look at it under the microscope on the inside of the thinned aluminium surface, I don't know which one was the real and most significant culprit. Perhaps in a good chemistry environment, the flow would have been fine at the rates that we had.. likewise, perhaps with minimal flow, the off set chemistry of the water would have also been fine. Could it be that both combined into a perfect erosion-corrosion storm?

 

[bimr]

The present water quality should be the same as earlier assuming that the water is from the same source.

Some metals are susceptible to erosion, which enhances the corrosion. Perhaps you should compare the design capacity of the heat exchanger to the water flow rate.