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 matter less than they once did. Every AlumiTub is now built with a marine grade protective barrier at the aluminum surface, so the tub is not relying on them. They are an added layer of protection rather than the thing standing between you and a problem.

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. 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.

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. 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.

Together with the protective barrier built into every tub, that gives two independent lines of defence plus the water itself. How we protect the aluminum. 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.