See this all the time.
Copper/Galvanized junction with a dielectric union. And rust nearby.
Not me. I do not run into galvanized potable water pipes often.
In your pic I see; Galvanized steel potable water pipes and tee fitting, what looks like a stainless nipple and copper pipe. Looks like a fitting thread leak on the tee, thread corrosion, and Galvanic corrosion, which occurs when two dissimilar metals are immersed in a conductive solution and are electrically connected.
It’s possible the ground strap has something to do with it also.
At the least the ground strap is on the correct side of the dielectric union.
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Looks like the appropriate way to join to dissimilar metals. No issue here.
My observation has been, while they’re code correct, they fail.
Again and again.
Observationally alone, I think they don’t work as well as traditionally assumed. There’s some backing for that notion elsewhere. I am cautious that any preconceived notion these days, can be confirmed with an Internet search. I was unable to find a good lab study, or Building Science Corporation article on the subject.
See also:
And some semi-quantitative test results from Precision Plumbing Products clearflow, showing a current flow.
And from a large University:
It looks like the only thing not failing in your photograph is the dielectric union. Are you trying to blame the corrosion on the threaded galvanized pipe on the dielectric union? You are aware this happens without the installation of a dielectric union. Dielectric unions do not cause corrosion on galvanized pipe.
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I report…
Galvanized piping present and likely at or beyond useful life expectancy and replacement should be considered. (Galvanized supply pipes generally have a useful life of 60-75 years.)
Evidence of corrosion and past leakage (currently dry)
Verbally, I tell my client that galvanized fell out of common use about 60 years ago. Around 60 years we often start to see problems and failure by 75 years. Of course, there are exceptions in both directions to that.
If the inspected house is that old or older, then there is a high likelihood that the galvanized pipe is at least that old.
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That’s about when galv dies in our area. Give it a decade or two less on the hot. And, if there are pipe wrench marks from excessive tightening, that’s where it leaks first.
But it rusts FIRST in my experience near the copper junction, despite the dielectric.
They installed a dielectric fitting to physically separate the copper from the steel, and then put a copper ground wire in close proximity to steel piping in multiple locations. Go figure.
I am surprised nobody is talking about how if they had used brass fittings to separate the steel from copper, this joint would likely still be relatively rust free (although the steel piping still tends to corrode from the inside).
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There sure are a lot of theories out there about galvanized pipe. So which one of you two are right? I’ve never heard of pipe that leaks from the outside before the inside, just saying.
Plumbers have a theory that it begins to corrode at its weakest point which would be the deepest portion of the thread joint. This is the part located on the interior of a pipe inserted into a fitting. What do I know 
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Both right 
The galvanized pipe gets thinner and thinner from the inside, clearly.
Usually the outside is fine. Then little rusticles start to appear on the outside… sometimes on a seam from the original forming of the pipe (if seamed), or a wrench mark, or the deep threads. Several times I’ve seen the pipe leak first at a deep wrench mark.
What I see observationally is failures right near a copper pipe junction, even if there is a dielectric union.
Some areas are like this. Just about every single family residence in my area was built with Knob & Tube wires, galvanized pipes, with gravity furnaces, and a vented food storage cabinet with icebox. Coal was often the fuel.
Lon had it right in that the copper ground wire in close proximity to the outside of the steel pipe is the likely cause of the exterior corrosion. My comment is that if they had used a brass fittings instead of a dielectric fitting, the copper will not react with the exterior of a brass fitting.
The other part of the problem is the interior, which is filled with an electrically conductive fluid (water) that puts the steel and copper in galvanic contact. Thus, the steel will corrode from the inside, regardless. My hunch is that brass creates a galvanic neutral zone providing greater separation between the two metals, thus it is somewhat more effective than a dielectric union. The steel will still corrode from the inside, only slower.
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The pipe was already weak due to its previous connection before someone had the mindset to place a dielectric union. In your photo, you have 3 dissimilar metals, likely small charge or current and water…imagine.
Is it possible the galvanic corrosion occurred in the past and the dialectic union was installed later as a corrective measure?
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I suspect that the copper section replaced a failed section of galvanized. They unscrewed the old from the tee and added a new nipple with the union to copper. Then added or replaced the jumper bond.
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Yes, in this case, that’s possible. It’s a building I’m under contract to inspect yearly, but I don’t have prior history. The service records of the building indicate the water heater was installed November 2012.
I suspect the history is old water heater died, they cut back a rotted pipe, installed a new one, ran about 3" of copper from the steel tank to the dielectric junction, and left all the old pipe. The bonding jumper is local code, though plumbers put that on the wrong side of the dielectric all the time (here it is on the proper side).
Galv corrosion near the junctions with copper pipe I’ve seen many times, this is not a one off observation.
The copper grounding conductor is in direct contact with the galvanized pipe. This is evident by the rust streaking down from point of contact. The copper conductor should be pulledaway from the pipe on the tee.
Why would it matter? There’s no moisture on the outside of the pipe, beyond humidity.
Copper is a dissimilar metal. Now you have three, maybe four types at one union.
When I was in the Air Force working on guidance systems, the amount of care and caution used on all connections of dissimilar metals was almost fanatical. There was no water source other than the atmosphere.