jimde said:
I will return to Grainger tomorrow and have them switch my order from galvanized fittings to black iron. The tanks already are equiped with gavanized fittings. One of the tanks was from a laundramat and the galvanized fittings are very corroded to the point that I am having trouble removing them. Even the exposed threads that are not in the tank have turned to rusted powder. Can anyone explain why this happened?
If you have a metal, oxygen, and electrolytic water, you get corrosion. If you find iron rust, then iron was the metal that was corroded. If you find rust on the exposed threads outside of the tank it is necessarily true water, oxygen, and an electrolyte were also present on the outside of the tank.
The corrosion of the iron was not caused by the galvanized fittings. Galvanized surfaces on pipes and fittings are put in place to protect the iron from corrosion by forming a protective layer of zinc compounds that prevent corrosion from attacking the underlying iron.
You typically don't find galvanized materials in boiler systems for two reasons.
First, once the oxygen inside the system is eliminated or absorbed, there is no longer any oxygen inside the system to support corrosion. You have iron, and electrolytic water, but no oxygen, so no corrosion (except by acid, hence the need to keep pH on the basic side). In this case the iron won't corrode, so there is no need to go to the expense of galvanized materials to prevent corrosion.
And on the outside of the system the system components are generally hot, warm, or at least above dew point, so no electrolytic water, and no corrosion except 'surface rust'. Here again, in the case of a boiler system, no need go to the expense of using galvanized materials.
Your laundramat tank, on the other hand, was presumably not part of a closed system like a boiler system, rather it was exposed to a continuously replenished supply of oxygenated water, so they used galvanized fittings to provide a corrosion resistant surface layer of zinc hydroxide/zinc carbonate.
The cut threads, however, weren't so lucky, so they rusted.
It is perfectly harmless to believe that galvanized fittings are not compatible with boiler systems -- even though it's not true -- since black iron should perform just as well and should be less expensive. Furthermore, galvanized is incompatible with propylene glycols based antifreeze compounds, a factor that may or may not need to be taken into account.
However, I wouldn't go out and buy new Chinese or Thai black iron if I had any serviceable galvanized pipe and/or fittings in inventory that would otherwise sit around waiting to be used.
--ewd