Expansion tank rusting through

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Old 04-23-18, 04:14 PM
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Expansion tank rusting through

I have a closed loop hydronic heating system for my small greenhouse. Heat source is an electric Bosch water heater. System operates at 12 psi and 80-120. Approximately 5 ft of copper piping in the whole system, the balance is schedule 40 PVC and rubber tubing which runs under the greenhouse benches.
EX-15 expansion tank is mounted mid-way between the heater and the circulation pump, about 18" ahead of the pump. Tank is mounted with air separator on top. System runs from December to March.
Problem is that the tank rusts out every 3-4 years. It develops a perforation on the side about where the diaphragm is attached.
Any suggestions on how to prevent the rusting out? Water additives? I'm on well water.
Note, the expansion tank in my hoe is going on 22 years. Another on a solar system is 15 years old (this one uses an anti-freeze solution).
 
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Old 04-24-18, 06:14 AM
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Maybe the dampness in the greenhouse, or some electrolyses .
Sid
 
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Old 04-24-18, 06:34 AM
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You can buy them made of Stainless steel or fiber.
 
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Old 04-27-18, 01:55 PM
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The PVC and rubber tubing undoubtedly lacks an oxygen barrier, so oxygen continuously diffuses into the system. That will cause corrosion of ferrous materials.
 
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Old 04-28-18, 12:16 PM
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I agree with Mike Speed 30..Replace the PVC and rubber hose with copper and/or oxygen barrier PEX and this problem will go away.
 
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Old 04-29-18, 03:20 PM
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In addition to replacing the PVC and rubber hose with copper, steel, or PEX (with oxygen barrier), make sure that the air elimination device is working satisfactorily - not corroded and that the valve cap, if any, on the Schrader valve is not on tight.
 
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Old 04-30-18, 07:24 AM
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Thanks

Thanks to all who responded to my problem. I conclude that the cause of premature failure is dissolved oxygen from the rubber and/or PVC components. Unfortunately, the rubber tubing is engineered into my system so won't be replaced. It is an established, widely used product in the horticultural industry. There's a total of about 15 ft. of PVC pipe. If it is the copper tubing (minimal), why doesn't the system in my house fail? It has hundreds of feet of copper. The air scoop and Schrader valve are in fine shape.
Moving forward, I have replaced the expansion tank (no SS), flushed the system completely, and added a charge of Fornex F1 Protector boiler water treatment.
 
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Old 04-30-18, 10:14 AM
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Copper pipe is compatible with water containing dissolved oxygen. Unlike steel, copper doesn't rust.
 
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Old 05-01-18, 03:13 PM
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Why can't just run a copper wire between the last copper pipe, and the expansion tank sort of an equipment bonding wire?
Sid
 
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Old 05-01-18, 04:54 PM
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I think that is equivalent to joining non-galvanized steel pipe and copper pipe - which is often done, usually not deliberately for the purpose you propose. With water containing dissolved oxygen, the steel will corrode and the copper won't, even if they are joined together, Actually, most all copper plumbing is ultimately connected to steel pipe or components - whether inside the house or outside.

If you look at the electromotive series table, copper and iron are close. So, neither will protect the other on the basis of cathodic protectlon. Iron will corrode (rust) in the presence of oxygen whether or not it is electrically connected to copper, copper will resist corrosion even if connected to steel.
 
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