User: heaterman posted the following hints
I commonly encounter the following failures in all types, stainless or not.
Chemically induced corrosion from poor water treatment, cleaning or installation practices. In a lot of cases the chemical shipped by the manufacturer is a very basic nitrite based composition that has some oxygen inhibitors in it. It doesn't address all the things that can be, and often are encountered in various locations. The only sure fire way to make certain you have what is needed is to find a good independent lab and send in a clean sample. The second thing is a failure on the part of the installer to chemically clean the boiler BEFORE adding the treatment shipped with it. In many cases a lot of junk is in the system from fabricating and cutting oils, soldering flux (that's a biggy) and general debris that can interact with the suggested water treatment and render it useless.
Electrical or stray voltage corrosion (electrolysis) caused by poor wiring, a bad neutral connection somewhere in the system, improper or no ground. This can take out about any kind of steel you can name, stainless or not.
Under deposit corrosion. This happens when deposits form through use and "seal" an area inside the unit from the water. Corrosion then starts at that point. You will commonly see this occur on the bottom of the boiler where things tend to settle or under an improperly cleaned weld.
Once in a great while I'll see dissimilar metal corrosion but it's pretty rare from what I have seen.. This happens when a steel boiler gets a different metal directly connected to it. Such as a copper fitting screwed directly on the boiler. Personally I'd rather see steel pipe on all boiler at least for a short distance. It acts as a nice sacrificial anode for the whole system. Cheaper to replace a section of pipe than the entire boiler. We do all of our near boiler piping with iron pipe and have never encountered a problem. We also use iron circs on all systems be they open, closed, stainless or mild steel.
So, Clean your boiler before adding the treatment chemical Always install a ground rod and bond it directly to the boiler itself Check you water at LEAST once per season through the services of a good lab. (It's not just about the PH level) Drain and flush an open system every couple years to limit crud buildup. Forget using antifreeze in an open system. When it's exposed to air it fails rather quickly and can turn very acidic in some cases.