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Water Heaters 101 > When Anodes Passivate

What you'll find on this page: Occasionally, a condition occurs where a sacrificial anode stops working normally. It passivates. This page discusses the theories and realities.

Passivated anode from 13-year-old water heater

Passivation: What It Is

Passivation is a corrosion engineering term that is described as "a loss of chemical reactivity under certain environmental conditions" (Corrosion Engineering, Fontana and Greene). In the case of water heater anodes, it means the anode has sort of gone to sleep. So, what does it mean? Anodes are supposed to corrode in a reaction to protect a water heater. If they don't do that, though, it's unclear what then takes place. Is the tank rusting? Maybe, maybe not.

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Anode shows passivation, especially on lower half

It's impossible to tell if aluminum anodes have passivated just by looking. But magnesium anodes are obvious because they appear "like new" after years in a heater. Anodes that are functioning properly look like absolute hell, with rough and pitted surface up and down the rod. Smooth means lack of activity. The one above came from a 13-year-old tank. It's not uncommon for corrosion to finally begin occurring, breaking through the layer of calcium covering the anode. That is most evident here at the very top near the hex nut but also to a lesser degree further down. But 13 years? That's longer than a lot of water heaters last. So was the anode doing some protecting after all? Or was something else going on? I have not heard a good answer yet to that one. The anode to right is from a 11-year-old tank.

The Theory...

The theories that have been put forth as to why passivation occurs include that it's a result of naturally soft water where an anode has trouble getting started; or improperly formulated anode rods; or improper contact between the tank steel and the anode. I've seen situations in the field that call into question all of these theories.

...The Reality? Complicated

Naturally soft water MAY cause this sometimes. Some of my customers were in San Francisco, whose water is from Yosemite: snowmelt and naturally very soft. I saw commercial heaters more than 30 years old (which is astonishing) with almost no sediment and mostly passivated anodes. And heaters five years old that rusted out with passivated anodes.

But one of the situations I found was a light commercial water heater in a different area where the water hardness was 180ppm, which is middlin'. Naturally soft is 50ppm and below. Ten feet away was an identical heater serving another block of apartments and its anode was functioning normally. Both anodes were original equipment. There was another case where I removed an anode and nothing was left but the core wire. I replaced it and the second anode was passivated when I came back a year later. That should have been a highly corrosive environment.

Anyway, if you pull an anode out and it looks like new, it's passivated. If you try to bend it a little, a calcium coating will flake off showing the magnesium underneath. People have tried scraping that off and reinstalling the anode, but it usually passivates again. Unfortunately, changing the anode may not help either. I've seen two or three in a row in the same tank passivate. One possibility is to change to a powered anode because the device can't passivate.