Damtech guide
Aluzinc Water Storage Tanks: Engineering Performance and Longevity
Aluzinc has a strong reputation in South Africa. Roofing suppliers sell it on long life in coastal air, and some tank sellers claim it outlasts galvanised steel many times over. One South African seller advertises a seven-fold advantage without citing a test.
The reputation is real, but it was earned on the outside of buildings, not under water. This guide explains how the coating works, where it clearly outperforms zinc, and where the evidence runs out. If you are choosing an Aluzinc water tank, this is the difference that decides whether it lasts. Damtech supplies both AZ150 Aluzinc and Z275 galvanised tanks, each with a 550 gsm PVC liner, so this guide is about fitting the coating to the site, not selling one over the other.
What Aluzinc is
Aluzinc is a trade name for steel sheet with a 55% aluminium-zinc alloy coating. The same class of product is sold as Zincalume and Galvalume. Engineers often write it as "Al-Zn" or "AZ", followed by the coating mass, such as AZ150.
Like galvanised "Z" grades, the number is the total coating mass on both faces in g/m². AZ150 is not 150 g/m² per side.
One supply check matters before anything else. BlueScope's current Zincalume data sheet describes an aluminium-zinc-magnesium alloy alongside AZ labels. A magnesium-bearing grade is a different coating from classic 55% Al-Zn. Confirm the alloy on the mill certificate.
The microscopic layer structure
Under a microscope, Aluzinc is a two-phase composite (GalvInfoNote 1.4):
- Aluminium-rich dendrites. These tree-like crystals make up most of the coating volume. They act as a barrier, because aluminium forms a tough oxide film.
- A zinc-rich interdendritic network. Zinc fills the spaces between the dendrites as a connected labyrinth. This is the part that does sacrificial (galvanic) work, corroding to protect exposed steel.
- A thin intermetallic layer. This bonds the coating to the steel base. It is hard and brittle.
By simple rule of mixtures, the coating is about 75% aluminium by volume. The often-quoted 80% figure could not be traced to a primary source.
Thicker than galvanised, with far less zinc
Aluminium is much lighter than zinc. So for the same coating mass, Aluzinc is thicker. But it holds much less zinc.
| Coating | Nominal thickness per face (µm) | Zinc per face (g/m²) | Zinc compared with Z275 |
|---|---|---|---|
| Z275 galvanised | 19.3 | 137.5 | 100% |
| AZ150 | 20.0 | 32.6 | about 24% |
| AZ185 | 24.7 | 40.2 | about 29% |
| AZ200 | 26.7 | 43.5 | about 32% |
Derived from published equivalence of AZ150 with about 20 µm per face, and 43.5% zinc by weight (GalvInfoNote 1.4).
AZ150 is about as thick as Z275, slightly thicker in fact, but holds only about a quarter of its zinc. That single fact explains most of what follows.
How Aluzinc protects steel: barrier plus galvanic action
In the open air, Aluzinc's behaviour is excellent. The zinc-rich network corrodes first. Its corrosion products then plug the labyrinth, and the loss curve flattens instead of running in a straight line. The aluminium-rich dendrites carry on as a barrier.
That is why long atmospheric programmes show Aluzinc lasting about two to four times as long as galvanised steel of equal thickness. Townsend's 25-year data found it about three times galvanised in severe marine exposure and still intact.
Exterior life by atmosphere
Applying a factor of three to ISO 9224 zinc rates gives these planning lines for one face at nominal thickness:
| Coating | C3 (years) | C4 (years) | C5 (years) |
|---|---|---|---|
| Z275 (for comparison) | about 18 | about 9 | about 4 |
| AZ150 | 54 | 27 | 14 |
| AZ185 | 67 | 34 | 17 |
| AZ200 | 73 | 36 | 18 |
Derived planning lines. No standard publishes Al-Zn rates by corrosivity category, so these columns are constructed, not measured. Multiply by about 0.7 for the thinner face: AZ150 becomes roughly 38, 19 and 10 years.
The exterior advantage is about 3.1 times for AZ150 over Z275, once Aluzinc's slightly greater thickness is included. Against the lighter Z250 grade it is about 3.4 times. Values above about 30 years are arithmetic rather than experience, because the longest exposure records found run to about 30 years.
Warranties near the sea sit well below this arithmetic. BlueScope's next-generation roofing terms run up to 10 years at 100 to 200 m from severe marine influence, rising with distance. Those terms cover a magnesium-bearing roofing grade, not a tank, but they show how fast coastal exposure shortens what a producer will stand behind. That is a useful reality check for coastal sites such as the Overstrand.
Cut edges and scratches: how much "self-healing" is real?
"Self-healing" is a common sales phrase for Aluzinc. It oversells what has been measured for either coating.
What the evidence does show:
- Chloride helps Aluzinc's edges. In chloride, both phases activate and the steel stays protected. In sulphate solution, protection lasted only for short immersion times (Vu et al., Corrosion Science 2011).
- Marine and industrial air are its strong suit. A five-year study found both coatings giving good cut-edge protection in marine and industrial air, lasting longer on Aluzinc, and little protection in rural and urban air (Palma and Morcillo).
- Drying matters. A producer study found corrosion products adhering at cut edges only when a drying step was included (Matsumoto et al.).
- Thickness limits it. A common rule of thumb is that Aluzinc cannot protect a cut edge on steel thicker than about 1 to 2 mm. The test basis for that rule was not found.
No immersed cut-edge or bolt-hole measurement in fresh water was found for either coating. In high-humidity air near the coast, Aluzinc's edge holds up well over the long term. Inside a fresh-water tank, the indirect evidence suggests its edge protection is weaker. Seal cut edges and bolt holes on any wetted face rather than relying on galvanic action.
Aluzinc under water: the evidence gap
This is the most important section for anyone buying an Aluzinc water tank.
No fresh-water immersion corrosion rate for 55% Al-Zn was found from any source. That is a statement about what the research found, not proof that none exists. But it means nobody can honestly quote an under-water lifespan for bare Aluzinc.
What exists points both ways:
- For Aluzinc: in seawater and salt solutions it generally outperforms galvanised steel. In 0.5 mol/L salt solution, its cathodic protection lasted 260 hours against 153 hours for zinc (study record). Aluminium metal also resists neutral soft water well.
- Against relying on it: the same seawater work reports serious pitting in the fully immersed zone (Springer record). Fresh water lacks the chloride that activates Aluzinc's edge protection. And only about a quarter of the zinc is available to act sacrificially.
Producers are cautious too. BlueScope's TB-3 recommends its polymer-laminated product, on a zinc substrate, for water tanks, and says of the strainer that Zincalume "must not be continually immersed". That sentence concerns one component, and BlueScope sells the alternative, so weigh it with care. Cleveland-Cliffs warrants its Al-Zn sheet only under normal atmospheric exposure and excludes constant fresh-water spray.
The fair summary: Aluzinc in fresh-water immersion is unknown, not shown to be poor. Until coupon tests in real South African waters exist, a bare Aluzinc wall should not be the water barrier.
Positives of Aluzinc water storage tanks
- Outstanding exterior durability. About 3.1 times the planning life of Z275 on a freely exposed, draining outer face.
- Better in coastal air. Chloride activates its edge protection, and its marine cut-edge performance lasts longer than zinc's over the long term.
- Better in seawater and salt solutions overall, with a pitting caveat.
- A clean outer skin on a lined tank. Market practice already pairs it this way: one South African maker combines AZ150 with a multilayer PVC liner. That is consistent with the liner carrying the water duty.
Negatives and limitations
- Alkaline conditions attack the aluminium. U.S. Steel warns of alkali attack on both coatings above pH 9 (U.S. Steel). South African drinking water limits allow pH up to 9.7.
- Concrete plinths need isolation. GalvInfo advises no contact between Aluzinc and concrete, including cured concrete that may get wet (GalvInfo). Use a separator membrane, a sealed joint and drainage at the base. Flush water that has stood on fresh concrete.
- Brittle on tight bends. The intermetallic layer cracks more readily in heavier coatings. U.S. Steel favours the lighter coating for tight bends, which limits the gain from moving up to AZ200 on corrugated profiles.
- Thin-sheet only. It is a coil product. It cannot be applied after fabrication, so structural plate and thick panels go to batch galvanising or paint.
- Closed headspace. Trapped moisture causes dark storage staining. Its effect on coating life inside a tank roof is unmeasured.
- Drinking water. No NSF/ANSI 61 or AS/NZS 4020 listing for a bare 55% Al-Zn surface was found. SANS 241 sets an operational limit of 300 µg/L for aluminium.
- Specifications. DWS 9900 does not name Aluzinc. Its galvanising chapter limits zinc-bath impurities to 1.5%, so a 55% aluminium bath is not "galvanising" under that chapter. No public evidence was found that AWWA D103 recognises Al-Zn, though its text was not read. On a contract invoking DWS 9900, an Aluzinc tank needs approval as an unnamed system.
Where an Aluzinc water tank fits
| Situation | Recommended | Avoid |
|---|---|---|
| Lined light-gauge tank, C2 to C3 site | AZ150 shell with a qualified liner | Quoting exterior life as tank life |
| Lined tank, C4 site | AZ185 or AZ200, checked on the real profile for cracking | Assuming a heavier grade always helps |
| Coastal C5, about 150 to 1,000 m from the sea | AZ200 with an organic coat and sealed edges, or a duplex system | Treating bare Aluzinc as maintenance-free |
| Within about 150 m of the sea | Move the tank if possible, or use a full coated system | Bare sheet of any kind |
| Concrete base | Separator membrane, sealed joint, drainage | Bare Aluzinc on concrete |
| Unlined tank in fresh water | Only after coupon evidence in that water | Lifetime claims from atmospheric data |
What Aluzinc water tanks cost
No sourced South African price data for Aluzinc tanks or sheet was found, so no ZAR figures are given here. The main cost drivers are the same as for any steel tank: coating grade, sheet gauge, diameter and height, liner, roof, fittings, freight and installation. On coastal sites, edge sealing and an organic overcoat add cost but also add life.
When comparing an Aluzinc quote with a galvanised one, compare like with like: the same liner, the same coating mass per face, and the same exclusions. For a side-by-side on lifecycle value, see Aluzinc vs galvanised steel water tanks.
Key takeaways
- Aluzinc is about 75% aluminium by volume and holds only about a quarter of Z275's zinc.
- It is the better coating on the outside of a tank, by about 3.1 times over Z275 on planning lines.
- No fresh-water immersion rate was found. A liner should carry the water duty.
- Keep it off concrete and away from alkaline water.
For the zinc side of the story, read galvanised Z275 steel water tanks in South Africa.
Talk to an engineer before you specify
The right tank depends on your site's distance from the sea, your water source and your base. Damtech supplies lined AZ150 Aluzinc and Z275 galvanised steel reservoirs for farms, lodges, estates and industrial users across South Africa. Send us your location, water analysis if you have one, and storage target, and we will recommend the coating, liner and base to suit.
Request a steel reservoir quotation or view steel water storage tanks.
This article is general technical information, not a design or compliance opinion for a specific project. Atmospheric life figures for Al-Zn are derived planning indications, not measured rates or warranties.