Do Aluminum Hot Tubs Corrode?
Aluminum does not rust the way steel does. It forms a protective oxide layer on contact with oxygen, and that layer is what protects the metal underneath. It's why aluminum is used for boat hulls, aircraft skins and architectural cladding, all of which live outdoors in water and weather for decades.
Aluminum is also a metal, and it reacts under specific, identifiable conditions. Every serious aluminum application engineers around those conditions rather than ignoring them, and so do we. We've been building marine grade aluminum hot tubs on the Sunshine Coast since 2001, over 10,000 of them, and the earliest are still in service. More on how we protect the aluminum.
Here's what actually happens in an aluminum tub, and why.
Three different things get called corrosion
Surface deposits. Small hard white or grey nubs on the wall, often forming within days of a fill. Typically mineral scale or aluminum salts precipitating out of the water. They scrape off with a plastic edge and they don't reduce the thickness of the metal. Cosmetically irritating, structurally not significant.
Dulling and greying. The surface loses its mill shine and goes matte. This is the oxide layer doing its job. It isn't damage, and it stabilises.
Localised marks at fittings. Where two metals meet in water the reaction concentrates in a small area, so anything clustered around a fitting points to the fitting rather than to the water generally.
Water that is too soft
The one most owners don't expect. Water with very low calcium hardness is chemically aggressive. It's under saturated, and it will pull minerals out of whatever it's in contact with. Some municipal supplies run as low as 10 ppm calcium.
We specify 150 to 250 ppm. Below roughly 100 ppm, the water itself is doing the work. If your tub is filled through a softener, you're almost certainly below spec, because removing calcium is exactly what a softener does.
pH outside the range
Aluminum is amphoteric, meaning its protective oxide is least stable at the extremes of the pH scale. We specify 7.4 to 7.8, and the lower half of that range is the safer place to sit.
Dissimilar metals in the water
Copper, brass and some stainless components can form a galvanic couple with aluminum in water, with aluminum as the anode. Aluminum sits near the reactive end of the galvanic series, so it loses that exchange.
This matters most at fittings and fasteners, where two metals are in direct contact and the reaction concentrates in a small area. Drain hardware, chimney components, heater connections and any aftermarket fitting are all worth checking for material compatibility. If you're unsure about a part, ask us before installing it.
Incompatible water treatments
Chlorine, chlorine pucks and floaters, salt chlorine generators, copper based algaecides and silver based mineral cartridges are all incompatible with an aluminum interior. Several are standard in the acrylic spa world, which is why they get recommended by pool stores that haven't seen an aluminum tub before.
Bromine at 2 to 3 ppm is what we specify. Hydrogen peroxide and plant derived treatments are also compatible.
Rather than leave owners to work out which products qualify, we put together Good Clean Living Water Care, a curated collection built specifically around aluminum and around our published ranges. Fill purification, balancers, a monthly plant derived stabilizer, filter care and a periodic deep clean. Everything in it is compatible, so there's no cross checking labels.
Trapped moisture without oxygen
Aluminum's protective layer needs oxygen to form and re-form. Where moisture sits against aluminum in a permanently oxygen starved space, under a gasket or beneath a fitting flange, the layer can't rebuild. This is a well documented mechanism in marine aluminum, and it's why installation detail matters as much as water detail.
It's also why we specify a rigid insulation barrier between the tub base and concrete, gravel or pressure treated wood, rather than setting the tub directly on those surfaces.
How the tub is built to handle it
Every AlumiTub now leaves the shop built with a marine grade protective barrier at the aluminum surface, so water sits against the barrier rather than the bare metal. Sacrificial magnesium anodes ship with every system that specifies them. And the water care ranges above are published precisely so there's no guesswork.
That's three independent things working together, rather than one of them carrying all the weight. How we protect the aluminum.
What that looks like after a decade or two
The most useful evidence isn't ours. These are owners describing tubs they have lived with for years.
"We have happily been using our AlumiTub for nearly 20 years. Thank you for building such a remarkable product."
Heidi & Bruce Reugg
"We have been enjoying our hot tub for the last year. Purchased in April 2016. Even kept it burning through the minus 30 C nights of Alberta winters."
Mitch & Sherry Metrunec, Alberta
"After barging our AlumiTub over to our wildfire-burned lot on Loon Lake 7 years ago, two of us rolled it up the hill and installed it. Maintenance and upkeep have been easy. The AlumiTub is built to last and we expect several generations to enjoy it over the years."
Gail Hunt, Loon Lake
"We've had our tub 4 or 5 years and absolutely love it. On a remote site in Northern Manitoba."
Rob & Kate Buchanon, Northern Manitoba
If you already have deposits
They're generally removable, and generally cosmetic rather than structural. Scrape gently with a plastic edge, never metal or abrasive. Then check calcium hardness, pH, and whether any non aluminum hardware has found its way into the water.
If you own an AlumiTub, 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.