Sacrificial Magnesium pole for hot water heater

Atwood uses 7072 aluminum for "lining" its tanks. 7072 is composed of:
99% aluminum
1% zinc

Not really in the same class as airplane aluminums...

Francesca Knowles wrote:
I guess it's true to say that most Atwood tanks don't use an anode...but that's only because the tank IS the anode. It's all aluminum, which as a matter of fact is the most common material for anodes in home water heaters. An Atwood isn't lined with plastic, as some seem to believe. It will degrade over time by electrolysis, just as will any other metal.

Here's what Atwood's manual has to say about cathodic protection, and it's gibberish. "Clad aluminum protects against corrosion"???? What????

Atwood wrote:
•
The tank has a clad aluminum lining that protects against corrosion and does not need to be replaced on a yearly or more frequent basis like anode rods do. A more detailed explanation of cladding is found in the back of this manual.
•

Gotta love that carefully parsed sentence about rods...it only says you don't have to replace the TANK as often as you'd have to replace a rod!

The further explanation one's directed to is more of the same. The tank's aluminum, folks, and electrolysis will eat it unless there's something lower-on-the-scale like magnesium present.

Atwood tanks will last a lifetime. Regardless of what this poster states. I have 34 years as an RV Tech and except for BURST tanks from Freeze damage, I have only replaced maybe 5 tanks in 34 years from internal corrosion. As long as you drain your tank at least twice a year and drain when NOT using the Water heater for months at a time, the Atwood Aluminum tank will last until the RV goes to the Junkyard. And YES, I have had to junk Atwoods when people install Hot Rods and install non plastic drain plugs because the metal in the plug they used has frozen the threads and you cannot remove the plug/Hot Rod to then drain for winterization or normal maintenance. Doug

"Clad aluminum" means the metal alloy is coated with a layer of pure (well, ok, 99.9% pure) aluminum, which is highly resistant to corrosion. MUCH more resistant than any aluminum alloy!
That's why "skin quality" aircraft aluminum sheet is nearly always Alclad.
Those of us who have been through aircraft corrosion control classes (yes, I am a licensed A&P mechanic...er..."technician") are well versed in all types of aluminum corrosion.
The water heater is likely made of 6063 aluminum alloy with a pure aluminum coating. 6063 is:
CHEMICAL COMPOSITION
Element % Present
Manganese (Mn) 0.0 - 0.10
Iron (Fe) 0.0 - 0.35
Magnesium (Mg) 0.45 - 0.90
Silicon (Si) 0.20 - 0.60
Zinc (Zn) 0.0 - 0.10
Titanium (Ti) 0.0 - 0.10
Chromium (Cr) 0.0 - 0.10
Copper (Cu) 0.0 - 0.10
Aluminium (Al) Balance

With magnesium and zinc (a trace of zinc) in the alloy, it is obvious why the clad coating!

I guess it's true to say that most Atwood tanks don't use an anode...but that's only because the tank IS the anode. It's all aluminum, which as a matter of fact is the most common material for anodes in home water heaters. An Atwood isn't lined with plastic, as some seem to believe. It will degrade over time by electrolysis, just as will any other metal.

Here's what Atwood's manual has to say about cathodic protection, and it's gibberish. "Clad aluminum protects against corrosion"???? What????

Atwood wrote:
•
The tank has a clad aluminum lining that protects against corrosion and does not need to be replaced on a yearly or more frequent basis like anode rods do. A more detailed explanation of cladding is found in the back of this manual.
•

Gotta love that carefully parsed sentence about rods...it only says you don't have to replace the TANK as often as you'd have to replace a rod!

The further explanation one's directed to is more of the same. The tank's aluminum, folks, and electrolysis will eat it unless there's something lower-on-the-scale like magnesium present.

Also...in certain water types magnesium may give off an odor or dissolve rapidly. In these situations they also have an aluminum rod.

fpresto wrote:

Lobstah wrote:
The anode is zinc

There are both magnesium and zinc anode rods. Magnesium is more common for water heaters.

Magnesium anodes are designed for fresh water and zinc anodes are for salt water.

Magnesium has more negative electropotential and is more suitable for areas where the electrolyte (soil or water) resistivity is higher. This is usually on-shore pipelines and other buried structures, although it is also used on boats in fresh water and in water heaters.

Zinc is generally used in salt water, where the resistivity is generally lower. Typical uses are for the hulls of ships and boats, offshore pipelines and production platforms, in salt-water-cooled marine engines, on small boat propellers and rudders.

A small correction: Years ago Attwood made one heater that needed an anode.

however with the modern Attwoods, not only do they "Not Recommend" using one as some one correctly posted above.

If they find you used one, they void your warranty.

Thanks everyone for your answers, it has been educational.

In addition to replacing the plastic plug yearly, I would also buy one of these and flush the wh 1-2 times yearly to remove the crud. Putting 1 gal of generic white vinegar per 15 gal of water into the fresh water tank then pump through all the water lines until you smell the vinegar, let sit overnight, will also help before flushing the tank with the rinser. Loosens up and dissolves the calcium/lime in the water.

Water Heater Tank Rinser

Lobstah wrote:
The anode is zinc

There are both magnesium and zinc anode rods. Magnesium is more common for water heaters.