What Causes Pitting in an Aluminum Hot Tub?

What gets called pitting in an aluminum hot tub is usually mineral scale or aluminum salts depositing on the surface. It is cosmetic, it scrapes off, and it does not thin the metal. The most common cause is water that is too soft, and the fix is calcium rather than anodes.

Two different things get called pitting, and they have different causes. Worth separating them before doing anything about them.

Surface deposits

Small hard nubs, usually white or gray, often appearing within days of a fill. These are mineral scale or aluminum salts precipitating out of the water onto the surface. They scrape off with a plastic edge and they don't reduce the thickness of the metal.

Dulling

The surface goes from bright to matte. This is oxidation, which is the metal protecting itself. Not damage, and it stabilizes. More on that mechanism in do aluminum hot tubs corrode.

The cause most owners don't expect is water that's too soft

Water with very low calcium hardness is chemically aggressive. It's under saturated, and it pulls minerals from whatever it touches, including the structure of the tub.

Some municipal supplies run as low as 10 ppm calcium. A water softener strips calcium by design, so a tub filled through one is below spec before anything else has happened. Rainwater is the same story, arriving with almost no hardness and low alkalinity. Our target range is 150 to 250 ppm, and it's worth testing on every fill and weekly after filling, rather than assuming. The full set of ranges, and what to do about a soft source, is in aluminum hot tub water chemistry.

Anodes and grounding are the two things people usually try first when deposits appear. Neither addresses this particular mechanism. If the water itself is aggressive, the fix is calcium.

Dissimilar metals

Copper, brass and some stainless components in contact with aluminum in water set up a galvanic cell, and aluminum is the one that gives way.

This is concentrated rather than general. It shows up at the point where two metals meet, which is why deposits clustered around a fitting point to the fitting rather than to the water. An anode elsewhere in the tub doesn't reach a cell at a fitting.

Any aftermarket hardware is worth checking before it goes anywhere near the tub. If you're unsure about a part, ask us first.

Incompatible water treatments

Chlorine in any form, salt chlorine generators, copper based algaecides and silver based mineral cartridges are not compatible with an aluminum interior. Several of these are standard for acrylic spas, which is why they get recommended by pool stores that haven't seen an aluminum tub before.

Bromine at 2 to 3 ppm, hydrogen peroxide and plant derived treatments are what we specify. If you'd rather not work out product by product what qualifies, Good Clean Living Water Care is a curated collection built specifically around aluminum and around our published ranges, so everything in it is compatible by design.

This is the one worth passing on to anyone else who services your water. Property managers, cleaners and pool or spa contractors reach for chlorine tablets by default, because almost every other tub takes them.

Anodes past their interval

A consumed anode provides no protection. On the systems that carry them, which is Electric, Hybrid and Wood Fired with filtration, replace every 3 to 4 months or once 30% or less of the anode remains. What anodes do and do not address is covered in sacrificial anodes.

What this means in practice

Surface changes in aluminum come down to a small number of identifiable causes, and all of them are addressable. That's why we publish specific numbers rather than general advice.

Every AlumiTub built since June 2026 is also built with a marine grade protective barrier at the aluminum surface, so the tub carries that protection itself rather than relying on the guidance alone. We still publish water care ranges and still ship anodes, because balanced water protects the equipment, the cedar and the quality of your soak, not only the structure. The difference is that the tub is now built to handle a mistake.

How we protect the aluminum, and the structure itself carries a 25 year structural warranty.

Restoring a tub that already has deposits

Deposits are generally removable and generally cosmetic. Beyond that, the protective barrier can be applied to tubs already in service.

Email hi@alumitubs.com with photos and your water numbers. In most cases this is straightforward to resolve, and we'll walk you through it. For tubs built before June 2026, we can also apply the protective barrier so it doesn't recur.

Common questions

What causes pitting in an aluminum hot tub?

What people usually call pitting is mineral scale or aluminum salt deposits precipitating onto the surface. They are cosmetic and removable and do not reduce the thickness of the metal. The most common cause is water that is too soft. Water with very low calcium hardness is chemically aggressive and draws minerals from whatever it touches. Other causes are dissimilar metals in contact with the aluminum, incompatible water treatments, and sacrificial anodes left past their replacement interval.

Can soft water damage an aluminum hot tub?

Yes. Water with very low calcium hardness is under saturated and chemically aggressive. Some municipal supplies run as low as 10 ppm calcium, a water softener strips calcium by design, and rainwater arrives with almost none. The target range for an AlumiTub is 150 to 250 ppm, tested on every fill and weekly afterward.

Will adding more sacrificial anodes stop deposits?

Not if the cause is aggressive water. Anodes provide cathodic protection, which does not address water that is chemically under saturated. If calcium hardness is low, raising it is the fix. Anodes also cannot reach a concentrated galvanic cell at a fitting, so deposits clustered around one fitting point to the fitting material rather than the water.

Are deposits in an aluminum hot tub a structural problem?

Generally no. Surface deposits are mineral scale or aluminum salts sitting on the surface rather than metal loss, and dulling from bright to matte is the oxide layer doing its job rather than damage. Both are cosmetic. The structure carries a 25 year structural warranty.

Can deposits in an aluminum hot tub be removed?

Generally yes. Deposits scrape off with a plastic edge, never metal or abrasive. Correcting the water chemistry prevents them returning, and Good Clean Living Water Care is a curated collection built around the ranges an aluminum tub needs. The marine grade protective barrier can also be applied to tubs already in service.