passivation

The Tank passivation

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  • #11890
    seacliff dweller
    Participant

    live in san francisco and water is relative soft from hetch hetchy. installed rheem 75 gal water heater less than 4 years ago and already water is getting heavily rusted. took anode rod out (r tech resistor type) and found rod still have 75 to 80 percent magnesium remaining but covered with thick layer of calcium carbonate. replaced with new anode, but still some rusty water. checked conductivity of new rod and found it to be around 64 ohms from rod to nut (can the resistence number be verified with rheem?), whereas the old one shows NO conductivity (after filing the rod to expose bare metal)!
    Question: was the old anode defective and failed and caused passivation or a combination of relative soft water and the non-conductivity of the rod to the nut?
    If so, is there a way of grounding the rod to the nut so as to bypass the resistor?

    #11892
    Randy Schuyler
    Keymaster

    Nobody is sure what causes passivation, including the manufacturers. I seem to know as much as anybody, and that’s not much. I’ve seen passivation occur in all types of water and several types of situations.

    But Hetch Hetchy water is so naturally soft that it appears to make it hard for the anodic-cathodic reaction between anode and tank to get going. I’ve seen two things in the San Francisco area: the anode passivates and the tank rusts out, as in your case; or a very slow reaction begins and the tank lasts forever.

    It may help to take out the Rheem resistor anode, which slows down the driving current to the level of aluminum, and put in a regular magnesium one. You can’t bypass it, though.

    Randy Schuyler

    #11895
    seacliff dweller
    Participant

    Thanks Randy.
    But the other question is whether the old anode failed (not even registering a resistance value) sometime during the life of the water heater causing the passivation? I will try to break apart the old anode to see what happened.

    #11897
    seacliff dweller
    Participant

    Just cut through a cross section of the nut exposing the inside. The resistor is right under the nut (encased in high temperature grey color expoxy???) connected through a copper wire and it also shows around 61 ohms of resistance (like the new ones), whereas the nut to anode is still non-conductive.
    In my opinion, the anode rod’s connection between the resistor and rod failed during the life time of the water heater and possibly contributed/accelerated the demise of the unit.
    Curious to see if any one else with Rheem r tech anode experiencing passivation also shows non-conductivity of anode to nut.
    If this is the case, Rheem’s anode design is at fault and should be corrected, since I don’t think anyone else had tested the conductivity of the rod to the nut before.

    #14594
    JasonZ
    Participant

    I have heard this suggested, that the resistor or the connection to it fails, resulting in an open circuit. This open circuit would then not allow electrons to flow to the steel, stopping the passivation. I have a water heater that has these symptoms. The tank is corroded and the anode rod has alot of life left in it. I would love to see some hard documentation of this occurring, can anyone point me to a documented case? Or at least have you had a similar experience?

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