Copper deposition on pump parts in hydronic heating system

[gonefishn]

We have a large capacity closed hydronic heating system charged with 50/50 blend of propylene glycol and demin water. The glycol/water is about 10 years old and was never properly inhibited until about 7 months ago. Prior to inhibiting, the glycol/water was tested and found to have a pH of about 6.0 and a reserve alkalinity of 0. In the lab it was determined that the dissolved iron could be dropped out and the pH readily raised with an inhibitor (molybddate/triazole/phosphate type). We drained the system, inhibited the glycol/water, and returned it to the system (less the sludge that settled out). Afterwards the pH in the system was 8.8 and the reserve alkalinity 6.8. Recently, we removed a pump for maintenance, and in adddition to the internal parts showing significant corrosion (thick black coating presumeably oxidized CS) there was a thin plating of copper covering this black coating (see picture). We believe the corrosion was long term previous damage. The question is what does the copper plating indicate. Is it the dissolved copper that was in the uninhibited glycol/water, dropped out by the inhibitor? Or could it be some sort of new galvanic corrosion that is a result of the inhibitor? I believe the pump is a long ways from any copper in the system, i.e. many feet. Any help appreciated.

 

[EdStainless]

Considering the previous poor condition of the system I wouldn't get too concerned. Yes, you are having corrosion of Cu in the system.
Did you do anything to de-oxygenate the fluid when you refilled?
It will take some time for the system to re-stabilize and for new passive film layers to build up on the metals.
I would suggest running for a while (few weeks?) and then taking a look. You also could have fluid checked for Cu and Fe. If they use ICP they can detect ppb levels and tell you if you still have metal disolving in the system.

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Plymouth Tube

 

[chicopee]

" I believe the pump is a long way from any copper in the system" Any bronze bearing in the pump?

 

[EdStainless]

Well the pump touches the water, and Cu is dissolved in the water, so there you have it. I would guess that Chic is right and that this pump have bronze bushings, maybe even bronze wear rings.

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Plymouth Tube

 

[gonefishn]

Thanks for your comments. I believe there is bronze in the radial bearing and a seal leak could have allowed galvanic corrosion between the CS and bronze. No indication from the mechanics of bearing problems, but then the amount of copper deposited on the CS might not have been enough to cause a problem in the bearings yet. Seals were replaced.

Deposition of existing dissolved copper after addition of inhibitor certainly seams a possibility.

We have samples of the glycol out for analysis. My main concern remains whether the inhibitor has caused a problem. Seems unlikely given it corrected the pH and alkalinity reserve to target range, but will continue to watch.