Water heater corrosion

Has anyone ever changed the anode rod in their water heater to prevent
corrosion of the METAL tank? We did this frequently aboard ship to all the
seawater heat exchangers. I figured since it is installed thru a threaded
bung accessible from outside the tank that it is meant to be replaced before
the water heater needs replaced as a unit. I drain water from the
pressure/temp. valve and from the tank drain monthly and do get some
brownish water from the drain.
Shawn

If you are referring to the kind of thing stuck into a domestic hot water tank it is an element, not an anode. The difference being that an anode has a cathode elsewhere, so (electric) current flows through the liquid, whereas in an element all (electric) current flows through the element and only heat is transferred to the liquid. As far as corrosion is concerned this is a significant difference. I don't know the first thing about ship heat exchangers, so this may be a crucial difference or it may just be me being really anal and correcting your terminology.
Tim

Why, yes I have...
Quoting me October 25, 2003:
Swapping out the $30.sacrificial anode in your hot water heater right now can save you 400 bucks (and bother) later.
Here is a picture I took of a water heater that didn't get serviced in time. I popped it open to have a look. Ewwww!
(Snip Reference to Rotten Link)
This heater was installed at the end of 1988. By 2003 the unit was leaking. The bluish linear bit to the left of the stack is a piece of anode that fell off and sank uselessly to the bottom of the tank. I swapped in a new water heater and asked the owner to have the anode inspected and perhaps replaced every five years.
______Wavy special effect, with harp denoting a flashback_____
I replaced the anode in our water heater last year. It didn't really need replacing (after only 8 years in service) but the anode cost only $20 and I figured "what the heck, just get a little crazy and swap it out".
______Wavy special effect, with harp denoting a return_____
Just now, it took me about 2 minutes to turn down the thermostat, shut off the inlet water and depressurize the tank from a valve in a nearby bathroom. In another couple minutes, I had the anode out of the top of the tank using a 1-1/16" socket on a breaker bar. Looks pretty good! (Beware that it takes a hard push to unscrew the anode if it hasn't been serviced lately.)
Re-tape the threads and back in the tank, screwed down tight. Valve in the bathroom shut off, Inlet valve opened back up, Thermostat adjusted back where it was before,
Ten minutes tops and I think we will get seven more trouble-free years.
Beware that newer consumer-level water heaters only have a proprietary anode attached to the bottom of the hot water outlet pipe. Your old water heater with its servicable anode just got more valuable because the propriatery anodes are unobtainium. (In my experience.)
Now, I ain't no plumber and I ain't advertising for no business. If you burn yourself it's your own fool fault. You guys are each about 150 IQ points smarter than I am so this little chore shouldn't tax you none and might save you a bundle.
(Big grin)
--Winston

Good to hear that someone else does it, now I won't feel silly when buying the thing.
As for the P/T valve, I've heard both schools of thought and lean toward the regular exercising of it. My reasoning is that a valve that is not exercised regularly will tend to freeze up. I've dealt with enough frozen valves in my time and besides, a P/T valve can't cost THAT much to replace.
Shawn

OK, but doesn't taping the anode threads insulate it electrically from the tank and defeat its purpose?

Actually this is a topic with two schools of though, and one's got to be wrong. Not testing a safety device seems silly to me.
This is a safety device that only functions under abnormal operating conditions. Under normal conditions it doesn't cycle. They do accrete minerals and bind up even at the top of the tank. The rubber seals do deteriorate and prevent resealing. If they do not reseal correctly, it's almost certainly time for a new one anyway. Cycling doesn't guarantee it will operate correctly, but it does identify a major failure mode.
I test my annually (well try to, it works out about every other year.) I end up replacing them every five or ten years. They are cheap (~$15,) and compared to the anode a snap to swap.
If you need a new anode and haven't done the heater element, consider it too.
Jeff Wisnia wrote:

I wondered about that myself, and a few years ago I screwed together a few fittings from my "Hell Box" using a reasonable amount of teflon tape on the male threads.
Every one I tried had good contact between the metal fittings. I think that the teflon justs extrudes out under pressure where there would normally be metal to metal contact if it weren't there at all. After all, you really only need it to fill the "spiral leak path" at the crest of the male threads to prevent leakage.
Thinking about it, the electrical codes call for all metallic piping in a structure to be bonded to ground in case an electrical failure in a piece of equipment tied to the plumbing fails, or some yutz drives a nail into a wall which spears through a piece of Romex and then kisses a water pipe. The object is to prevent electrifying the entire piping sytem with line voltage.
I'd expect that if taping threaded joints had much of a chance of insulating them it'd be prohibited by code for obvious reasons.
Which brings to mind a subject I hope I haven't pontificated about on this newsgroup already. It's that dielectric unions or dielectric couplings at the inlet and outlet fittings of electric water heaters (and probably most gas ones too) don't do shite to prevent corrosion. The reason (and this isn't understood, hence not believed by most plumbers) is that the insulation in the dielectric couplings is shorted out by the building's code required plumbing ground and the heater's electrical ground, both of which are (or should be at least) at the same potential.
If fact, if they are dielectric unions standing on steel nipples screwed into the tank they are even worse than nothing because the nipples will quickly start to rust on the inside and either clog up with corrosion and/or leak. I proved this the hard way on our own water heater when I slavishly installed a pair of dielectric unions where none had been before the last time I was putting in a new water heater. About six months later one of the steel nipples rusted through, and the other wasn't far behind it. See:
That photo was taken after I scraped the heavy rust clumps off the inside.
You don't have to take my word for that. After I doped it out on my own I Googled up a tech memo from The Rheem water heater company which said the same thing and advised /against/ using dielectric fittings when installing their water heaters.
Just my .02,
Jeff

The whole of statics and dynamics was summed by by our prof as "f=ma and you can't push a rope".
Best regards, Spehro Pefhany

"Unless it happens to be a rope" - this according to the physics professor at Queen's in 1958. Gerry :-)} London, Canada

It may depend on the pH of your water. Hard water has a high pH and is not as corrosive as soft water that is often acidic and very corrosive.
Scp