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Okay, dielectric unions are suppose to be the right way of connecting copper to steel but as known by all they become leak prone rusty pieces of crap. Had to cut one out just the other day on a boiler water make up valve. Now I believe it is or was in the UPC code book as an alternative say in a wall with out an access panel an 8 inch brass nipple was accepted instead of a dielectric coupling. This was in San Diego some years ago. In servicing 14 schools by myself I use both methods, depending on future accessibility.
I was adding waterlines to a home today for a couple of humidifiers, and I noticed that the house was piped almost* completely in copper, and the electrical was grounded onto it with what looked to be a galvanized steel grounding clamp.
*the main coming into the building was black poly, which seems like it defeats the purpose of grounding to the copper line.
No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.
But I'll also summarize. First note that the problem we SHOULD be talking about is galvanic corrosion-- "electrolysis" is different, resulting from externally imposed DC current. (You're totally right, the HWH is grounded AND the pipe is grounded-- there ain't no external DC current.) Galvanic corrosion is self-induced between the different metals, in contact with each other and also in contact with an electrolyte. Saltwater is a great electrolyte and distilled is not an electrolyte at all. "Tap water" is a weak electrolyte, it will support the corrosion process, but only over a period of months/years.
Under these conditions The "less noble" metal will corrode. That pdf shows a table of metals which are widely used in pipes. The "least noble" metals are: #1- Zinc; #2- Galvanized Steel/Iron; #3- Aluminum; #4- Steel, and #5 Iron.
From the "most noble" end, it's #1- Monel; #2- Copper Alloys other than Brass; #3- Bronzes; #4- Copper; and #5- Tin. (I'm leaving out a bunch of metal in the middle of the list which aren't relevant to HWH connections.)
When you use a Galvanized union, you're protecting both the copper pipe and any steel which is exposed within the tank. So of course, if any galvanic corrosion occurs, the part which looks the most horrible and messed-up is the galvanized union. From either the tank steel or the piping copper, the galvanized union part of the union loses the corrosion contest.
Now compare that with the Brass Nipple scenario: Brass is only slightly less prone to galvanic corrosion than your copper pipes, so there's a tiny bit of protection (for the pipes) when you do this. But steel is vastly less "noble" than both of them, so what gets sacrificed if metal-to-metal contact develops between brass and your steel tank? The tank.
You guys who see the "awesome looking" Brass nipples in competition with "look like heck" Galvanized unions might just not be looking in the right place-- if galvanic corrosion has occurred in among the metals in this connection, it's mostly occurred on the other side of the brass, hidden inside the steel tank: There wasn't any "less noble" union which would try to sacrifice itself before the steel when a metal-to-metal contact developed. (And of course, this is why anode rods are made of aluminum and magnesium and sometimes zinc: just like the zinc-galvanized union, the anode sacrifices itself to protect the steel.)
But the anode rod(s) aren't close enough to the piping to handle the joint between dissimilar pipe metals. So I think it's best to use the galvanized unions. But they do require maintenance inspections, with possible replacement, before they've sacrificed themselves into a low-flow rusted out mess. Kind of like the anode rods themselves, and the tank drain-- checking 'em once every year or two can save big money in the long term.