Sacrificial Anodes in an Aluminum Hot Tub

A sacrificial anode is a block of a more reactive metal placed in the water so that it corrodes preferentially, leaving the metal you care about alone. Standard practice on boats, in water heaters, and on any submerged metal structure. On an AlumiTub they are fitted to Electric, Hybrid and Wood Fired with filtration systems, and they ship with those tubs.

Worth saying up front: anodes are not an optional extra. Along with balanced water, they are one of the two things actually protecting the aluminum. Aluminum protects itself with a thin oxide layer that re-forms continuously on contact with oxygen, which is what lets it last decades on boat hulls and aircraft skins. That layer is stable across a wide range of conditions and unstable at the extremes, so keeping water within the published ranges is what protects the surface. Anodes are the other half of that: they take the galvanic load that would otherwise fall on the tub. Treat them as maintenance you actually do, not as a backup.

Why magnesium

The anode has to be more reactive than the metal it's protecting, or it does nothing. Aluminum already sits fairly high on the reactive end of the galvanic series, which rules out zinc here and makes aluminum anodes largely pointless in an aluminum tub. They're too close to the same potential.

Magnesium is more reactive than aluminum, which is what makes it work.

The AlumiTubs spec is an R-1-MG rudder or trim tab anode, 1-7/8 inch, installed in the designated brackets, one per wall panel side. Electric, Hybrid and Wood Fired with filtration carry three magnesium anodes. A commonly available equivalent is the Martyr CMR01M at the same size, stocked by Fisheries Supply in the US and Steveston Marine in Canada. It is the same physical anode under different supplier part numbering.

If you have anodes dissolving unusually fast, or anodes that came with an older tub, it's worth confirming what alloy they are. Email hi@alumitubs.com and we'll check what was supplied with your build.

Mounting: metal to metal, or it does nothing

This is the detail that catches people out. An anode only works if it is bolted to bare metal with genuine metal-to-metal contact. The protection travels through that electrical connection, not through the water.

An anode mounted on cedar, or on a painted or otherwise coated surface, looks exactly like protection and provides none. It will sit there for years barely wearing down, which reads as reassuring and is in fact the symptom: an anode that is not being consumed is an anode that is not connected. Use the designated brackets, make sure the mating surfaces are clean bare metal, and check the connection whenever you replace one.

Replacement interval

Every 3 to 4 months, or once 30% or less of the anode remains, whichever comes first. In practice this lines up with a seasonal deep clean and water change, so it is less a separate task than a thing you do while the tub is already empty.

A consumed anode provides no protection at all, and a heavily depleted anode is a common reason people conclude anodes don't work. They stop working when they're gone.

What anodes do not do

Worth being direct about, because anodes get treated as a universal fix and they aren't one.

• They don't compensate for aggressive water. If calcium hardness is very low, the water is attacking chemically, and cathodic protection doesn't address that mechanism. Below roughly 100 ppm the water is under-saturated and chemically aggressive. Fix the water, using the ranges in aluminum hot tub water chemistry: alkalinity 80 to 120 ppm adjusted first, pH 7.4 to 7.8, calcium hardness 150 to 250 ppm, bromine 2 to 3 ppm.
• They don't stop galvanic attack from an incompatible fitting. A copper or brass component in direct contact creates a concentrated local cell at that point. An anode elsewhere in the tub doesn't reach it.
• They don't offset incompatible chemistry. Chlorine and copper algaecides stay incompatible regardless of how many anodes are in the water.
• More is not automatically better. If anodes are being consumed rapidly and the surface is still changing, the answer is diagnosis, not additional anodes. What causes pitting separates the mechanisms.

If you've added anodes and seen no improvement, that's useful diagnostic information rather than a reason to add more. Email us with your water numbers, your system, and photos.

Anodes are one part of a system

Anodes work alongside balanced water rather than instead of it, which is why the two are specified together. Good Clean Living Water Care is a curated collection built specifically around aluminum and around our published ranges, so keeping the water where it needs to be doesn't mean assembling a routine from a pool store shelf.

Those two things, water held in range and anodes correctly mounted and replaced on schedule, are what protect the aluminum. The oxide layer does the rest on its own. How the aluminum is protected. The wider question of whether aluminum corrodes at all is answered in do aluminum hot tubs corrode, and the structure itself carries a 25 year structural warranty.

Common questions

Do aluminum hot tubs need sacrificial anodes?

Yes. Electric, Hybrid and Wood Fired with filtration systems are fitted with three magnesium anodes each, and they ship with those tubs. A sacrificial anode is a block of a more reactive metal placed in the water so that it corrodes preferentially. Alongside water kept within the published ranges, anodes are one of the two things protecting the aluminum, so they need replacing on schedule rather than treating as optional.

Why are magnesium anodes used instead of zinc or aluminum?

The anode has to be more reactive than the metal it protects. Aluminum already sits fairly high on the reactive end of the galvanic series, so zinc and aluminum anodes are too close to the same potential to provide meaningful protection. Magnesium is more reactive than aluminum, which is what makes it work.

Does it matter where an anode is mounted?

Yes, and it is the most common installation mistake. An anode only works if it is bolted to bare metal with genuine metal-to-metal contact, because the protection travels through that electrical connection rather than through the water. An anode mounted on cedar, or on a painted or coated surface, looks like protection and provides none. An anode that is barely wearing down is usually not connected.

How often should hot tub anodes be replaced?

Every 3 to 4 months, or once 30 percent or less of the anode remains, whichever comes first. That lines up with a seasonal deep clean and water change, so it is done while the tub is already empty. A consumed anode provides no protection at all.

What size anode does an AlumiTub use?

An R-1-MG rudder or trim tab magnesium anode, 1-7/8 inch, installed in the designated brackets, one per wall panel side, three in total on Electric, Hybrid and Wood Fired with filtration. The Martyr CMR01M is the same physical anode under different supplier part numbering, stocked by Fisheries Supply in the US and Steveston Marine in Canada.

Will adding more anodes fix surface deposits?

Not if the cause is something anodes do not address. Anodes do not compensate for chemically aggressive soft water, do not reach a concentrated galvanic cell at an incompatible fitting, and do not offset incompatible water treatments. If anodes are being consumed rapidly and the surface is still changing, the answer is diagnosis rather than more anodes.