DD, Excuse the use of your initials but I just can't get used to referring to a nice guy by your handle. I know the fault is mine but there it is...

Anyway, the answer is maybe. Not knowing all the applicable facts of the extant situation, I can't say with much certainty but it is certainly a possibility.

Dissimilar metal corrosion is a powerful destructive force. For example copper pipe and galvanized pipe in contact carrying water is essentially a battery with a short circuit to take all the current produced. And like a zinc carbon flashlight battery, over time the less noble material is consumed, the zinc cup in the case of the old style D cell.

Back when I took a sabbatical and did marine electronics field service engineering I saw lots of examples of dissimilar metal corrosion such as copper shield of coax crimped into a ring terminal to be attached to an aluminum stanchion with stainless steel nuts, bolts, and screws (not about a mass escape from an insane asylum.) Add a little salt and water (ocean going tuna boat in the $10 million category, the ones with a heliport atop the pilot house) and surprise, surprise, surprise, massive corrosion and poor antenna performance. That's OK they have a spare but what do you ask? Isn't the spare done the same way... YUP!!! Oh well the antenna isn't really important it is just the beacon the chopper uses to find the moving tuna boat in reduced visibility or the emergency radio used to summon help or...

I really hate copper pipe for water. A year or so ago two different friends had problems with leaking copper pipes that developed pinholes for no apparent reason. One run was in a buddy's attic to take water from the kitchen sink to the icemaker. He came home from an extended absence to find the sheet rock of the kitchen ceiling on the floor and 3 rooms of the house flooded, all by just a tiny tiny little fine mist coming out of an itsy bitsy pin hole.

The other friend up the road a half mile had the sheetrock in his shop bathroom wall get soggy. He had a copper water pipe running through a plastic sleeve in the slab to a location behind the stool in the bathroom. In the middle of the in-slab run a pinhole developed, the sleeve filled and overflowed at the lower end inside the wall. I replaced the copper with PEX, end of problem. No, I'm not a plumber but neither is the friend who is a DAV who I help with stuff he can't do when I have time.

To the point... frost free hydrants should not contact dissimilar metals anywhere any time period. PEX is a well proven time tested material. If procured from a source of supply that never stores the stuff in the sunlight it can last just about forever. If stored in the sunlight it is seriously degraded and I don''t know how to easily tell. The best insurance is to buy fresh stock from someone who moves it quickly and does not store it outside. I have seen large wholesalers with thousands of feet of rolls of various size PEX in outdoor fenced in compounds. Big chain link fences with razor wire on top. It doesn't get stolen but it does deteriorate. I would never mention Locke Supply by name.

There are lots of ways for a frost free hydrant to fail. Some are easily prevented. avoiding contact with any metal is one.

Pat