Sacrificial Magnesium pole for hot water heater

[raymondfl]

I have an Atwood hot water heater and I was reading about the chemical reaction in hot water heaters that causes rust. Now I do not understand this process but basically as I understand it I should have a magnesium bar that is slowly eroded over time. It is usually attached to the drain plug and inserts into the water of the hot water heater. But I have an Atwood hot water heater, not a Suburban hot water heater. I do not seem to have this part, and no where in the manual can I find any info on one. Does anyone know if I should have this sacrificial lamb in my hot water heater. If it sounds like I am not sure what I am talking about you are correct. Any help??
Thanks

[Francesca_Knowl]

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...

[dougrainer]

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

[mowermech]

"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!

[Francesca_Knowl]

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.

[bob213]

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

[magik235]

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.

[wa8yxm]

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.

[raymondfl]

Thanks everyone for your answers, it has been educational.

[JC2]

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

[fpresto]

Lobstah wrote:
The anode is zinc

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

[Lobstah]

The anode is zinc, and is actually referred to as "sacrificial zinc" in this application. It is required on SOME heaters, but not on an Atwood. This application is very common on boats that sit in salt water. There is a form of electrolysis that attacks metal surfaces due to a difference in grounds. The zinc is put in place as a sacrificial element so that the electrolytic process attacks the zinc and not the more important pieces of metal like a propeller, or propeller shaft, or engine components. Boats will have several large pieces of zinc attached to the boat in various places.
The same concept applies to motorhomes and campers that are plugged into "shore" power, and may have a difference in grounds.

Back to the question...Atwood does not require a zinc anode, and further, if someone installs one, like they did in MY Atwood, it tends to cause problems. Mine got fused to the internal threads of the heater, and was all but impossible to remove.

Jim

[ScottG]

Atwood WH's are aluminum and need no anode. Suburban units are made like the WH in your house and are steel covered in a layer of glass. The latter requires an anode for long life.

[frankdamp]

Atwood heaters are plastic (coated?). Suburban heaters are steel and do need sacrificial anodes. On mine the drain plug includes a sacrificial zinc element, and I have a replacement waiting to replace the one that's in the tank now. It dates from the original purchase in 2002 and is still only about half gone. I expect a couple more years before I need to replace it as we have very benign city water in Anacortes (one of the top five for quality nationwide) and we always leave home with a full water tank.

Most CGs in the western states have pretty good quality water. It's limestone (calcium carbonate) in the mountains where the reservoirs are that cause hard water problems. Our mountains are all granite or volcanic in origin.

[Tom_Barb]

It's actually a zink anode