I just learned water heaters have an "anode" and its important

Labor can't replace material. Its called a "sacrificial" anode for a good reason. See here:

Replace it.

Bob

Here's a silly followup question.

Do gas water heaters have an anode too, or just electric?

Both.

so the OP got a used tank hopefully for free...........

by the time he replaces the anode, replaces the elements and cleans the sludge out of the tank.....

Has he wondered where that sludge came from? Almost certinally from the inside of the tank which is nearly rotted out:(

Now he has a old tank, thats less efficent than a new tank with high efficency foam.

So his tank has cost probably cost nearly as much as a brand new cheap short warranty tank

You can't clean it. It's being eroded away on purpose, to prevent damage somewhere else.

I thought the sludge was mostly dissolved minerals in the water, and that the tank is glass-lined and didn't rot? No?

mm wrote in news: snipped-for-privacy@4ax.com:

is the anode aluminum or ZINC?

The website posted by Grandpa (I think) says Magnesium or Aluminum. Good site!

its rust the glass lining isnt perfect and fails over time which is why tanks leak.

the fact the anode is eroded away is another indicator that tank is old and on its last legs.

I love reusing stuff and saving money but this one is a loser.... sorry.

all metal tanks do.............

My understanding is that periodically replaceing the anode can significantly increase a wateh heaters life.

Bob

Why do they call it an anode?

I know what an anode is. I also know what a cathode is. But that doesn't answer my question....I stick this rod in the middle of my water heater - what makes it an anode, and not a cathode?

Did you read down to the part where it says "sacrificial anode" or just skip it? It says: "Sacrificial anode

In cathodic protection, a metal anode that is more reactive to the corrosive environment of the system to be protected is electrically linked to the protected system, and partially corrodes or dissolves, which protects the metal of the system it is connected to. As an example, an iron or steel ship's hull may be protected by a zinc sacrificial anode, which will dissolve into the seawater and prevent the hull from being corroded. Sacrificial anodes are particularly needed for systems where a static charge is generated by the action of flowing liquids, such as pipelines and watercraft."

Then you don't really "know" what an anode (or cathode) is... :)

It's an anode if it attracts anions, and a cathode of it attracts cations...I'm pretty sure that's the definition you know. (DOH! :) )

Which, of course is sort of a circular definition. What actually makes it one or the other is the relative position on the scale of electrical potential of the material from which it is made with respect to the other material.

--

I read it. It still doesn't explain why it's not a "sacrificial cathode".

I found the answer:

"Galvanic anodes are designed and selected to have a more "active" voltage (technically a more negative electrochemical potential) than the metal of the structure (typically steel). For effective CP, the potential of the steel surface is polarized (pushed) more negative until the surface has a uniform potential. At that stage, the driving force for the corrosion reaction is halted. The galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. The polarization is caused by the current flow from the anode to the cathode. The driving force for the CP current flow is the difference in electrochemical potential between the anode and the cathode."

If you know what an anode is it is totally obvious why the water heater anode isn't a cathode.