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Posted by D Sherman [72.47.153.24] on Sunday, February 12, 2012 at 14:39:31 :
In Reply to: Re: Replaceable zinc plugs posted by Jerry in Idaho [69.59.83.175] on Sunday, February 12, 2012 at 14:07:38 :
I own two old buildings in Wallace, each about 15000 sq ft. One is currently heated with a pump-circulated hot-water system that was installed in the 1970s. I have had to open the system up many times to replace rubber gaskets and a stuck valve, but despite the main lines being steel and the branch lines and radiators being copper, there is no visible rust or corrosion inside the system. There is a certain amount of black "slime" in the pipes, but no red rust or pitting. I run plain water in it because there's no easy way to add boiler treatment chemicals. My other building was last heated by its original coal-fired steam boiler, which was shut down in the 1980s. I've been gradually disassembling it to get the pipes out of the way. All of those old steam pipes have a very thin dusting of powdery red iron oxide inside them, but no scaling or rust per se. This system ran for about 70 years.
The lesson from both is that it's oxygen, not water, that rusts iron. Water acts as a catalyst, and salt makes it an even stronger catalyst, but without oxygen, there can be no rusting. Once you've heated the water to near boiling the first time, all the oxygen goes out of it and it's non-corrosive thereafter. Of course steel that's kept damp but is exposed to air, including the inside of a "drained" engine, will rust quickly.
I don't believe dielectric fittings were necessary in your boiler installation. The plumbers probably used them out of force of habit. They're definitely necessary in domestic (fresh) water plumbing, hot or cold, because that water contains plenty of dissolved oxygen. I've seen many examples of where they weren't used, but should have been. The result is usually a slow leak that goes on for decades before it become a big problem. The metals get eaten up where they're in contact, but the resulting corrosion tends to swell up and plug the leaks more or less, so it takes a long time to get to the point where anyone notices it.
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