Damtech guide
Galvanised vs Aluzinc Steel Water Tanks: What Really Protects the Water
Ask ten suppliers whether Aluzinc or galvanised steel makes the better water tank and you will hear ten confident answers. Most of them compare the wrong thing.
A steel water tank has two very different faces. The outside lives in the air. The inside lives in water. The famous Aluzinc advantage was measured on the outside. Under water, the evidence looks almost reversed, and for Aluzinc it is largely missing.
This comparison sets galvanised Z275, the grade Damtech uses, against 55% aluminium-zinc (AZ150 to AZ200) for South African tanks. Damtech supplies both. It separates what is measured from what is derived, and it ends with a practical rule for specifying a tank that lasts.
The short answer
- On the outside, Aluzinc is the stronger coating, by about 3.1 times over Z275 on planning lines.
- On the inside, neither bare coating should be relied on as the long-term water barrier. Zinc has a documented protective mechanism only in hard, scale-forming water. Aluzinc has no fresh-water immersion rate in anything found.
- So specify the water barrier first: a qualified liner, or a heavier system such as batch galvanising, duplex or a certified factory coating. Then choose the metallic coating for the outside.
Head-to-head comparison table
Read the table by row and by evidence. Rows for the outside face cannot be transferred to the inside face.
| Criterion | Galvanised Z275 | Aluzinc (55% Al-Zn, AZ150) | Advantage | Evidence strength |
|---|---|---|---|---|
| Coating composition | Near-pure zinc, single layer | About 75% aluminium by volume; Al-rich dendrites with a zinc-rich network | — | Secondary sources |
| Thickness per face, nominal | 19.3 µm | 20.0 µm | About equal | Derived from coating mass |
| Zinc per face | 137.5 g/m² | 32.6 g/m² (about 24% of Z275) | Galvanised | Derived |
| Exterior loss rate, C3 | 0.4–1.1 µm/year | 0.10–0.55 µm/year | Aluzinc | Zinc from ISO 9224; Al-Zn derived |
| Exterior loss rate, C4 | 1.1–2.2 µm/year | 0.28–1.10 µm/year | Aluzinc | As above |
| Exterior loss rate, C5 | 2.2–4.4 µm/year | 0.55–2.20 µm/year | Aluzinc | As above |
| Exterior planning life, C3 / C4 / C5 | about 18 / 9 / 4 years (published Z275 ranges 12–42 / 5–12 / 2–5) | 54 / 27 / 14 years | Aluzinc, about 3.1 times | Derived planning lines |
| Wetted face, hard fresh water | 3.9–7.7 years if the rate were sustained; scale can slow it far more | No rate found | Zinc (only one with a mechanism) | Zinc: trade data. Al-Zn: gap |
| Wetted face, soft fresh water | 1.9–3.9 years if sustained | No rate found | Neither proven | Zinc: trade data. Al-Zn: gap |
| Seawater or salt solution | Lower resistance | Better overall, but pits when submerged | Aluzinc, with caveat | Abstracts |
| Cut edge, coastal air, thin sheet | Better in the short term (under about 10 years) | Longer galvanic action over the long term | Aluzinc long term | Producer and abstracts |
| Cut edge, panels above about 2 mm | Weak but more zinc reserve | Rule of thumb says unprotected | Galvanised | Producer, unverified basis |
| Immersed edge or bolt hole | No data | No data | Unknown | None found |
| Forming and tight bends | Better adhesion | Brittle intermetallic; heavier grades crack more | Galvanised | Secondary and producer |
| Concrete contact | Tolerates cured concrete | Advised against, even cured if it may get wet | Galvanised | Consistent secondary sources |
| Drinking water, bare surface | No certification found | No certification found | Neither | Absence from searches |
| DWS 9900 position | Restrictions apply by wording; no route for sheet | Not named; approval route only | Neither | Standard text read |
| Lifecycle cost-effectiveness | Lowest first cost; short life on wet or coastal faces unless lined or overcoated | Higher exterior life per rand on lined shells; needs base isolation and edge care | Depends on configuration | Qualitative; no sourced ZAR data |
Exterior rates: zinc from ISO 9224 bands reproduced in the GAA/GANZ guide. Aluzinc columns are derived by dividing zinc rates by a life multiple of 2 to 4 (GalvInfoNote 1.4); no standard publishes Al-Zn rates by category. Planning line = µm per face ÷ top of the steady-state band (÷ 3 for Al-Zn).
Why the outside and inside give opposite answers
Outside: Aluzinc's labyrinth plugs itself
In the open air, Aluzinc's zinc-rich network corrodes first. The corrosion products plug the network, and the aluminium-rich dendrites keep acting as a barrier. The loss curve flattens. That is the mechanism behind its well-earned roofing reputation.
Galvanised zinc corrodes by roughly uniform thinning. With about 19.3 µm per face on Z275, it runs out sooner.
Inside: zinc has a film, Aluzinc has a question mark
Under water, zinc depends on a carbonate film that hardness and alkalinity support. In hard, scale-forming water it can nearly stop corrosion. In soft water the film struggles.
Aluzinc's plugging mechanism was observed in air. Nothing found shows it working the same way under continuous immersion. In low-chloride water, the working anode is largely the zinc-rich network, which holds only about a quarter of Z275's zinc at AZ150. That is reasoning from laboratory salts, not a fresh-water measurement.
Producers lean cautious. BlueScope's TB-3 recommends a polymer-laminated product on a zinc substrate for water tanks. Cleveland-Cliffs excludes constant fresh-water spray from its Al-Zn warranty.
Water chemistry: which tank suits your water?
| Your water | Galvanised Z275 | Aluzinc | Recommendation |
|---|---|---|---|
| Hard borehole water, scale-forming | Best case; documented film | No scale benefit shown | Zinc may suit better, but test first |
| Soft municipal, e.g. Cape Town | Worst case below 50 mg/L CaCO₃ | Unknown | Line the tank |
| Rainwater | Below zinc's pH range on the Highveld | Unknown | Line the tank; first-flush diversion |
| Chlorinated municipal | Residual not a proven driver | Not studied | Watch alkalinity, not chlorine |
| Water above pH 9 or standing on new concrete | Tolerant | Alkali attacks aluminium | Galvanised, or isolate Aluzinc |
Cape Town supply reported for January to March 2026 had hardness of 16 to 46 mg/L and alkalinity of 8 to 26 mg/L as CaCO₃ (City of Cape Town). Every supply fell below the line where zinc is prone to severe attack. Highveld rain has been measured at pH 4.65 to 4.80 (Water SA).
Coastal sites: where Aluzinc earns its premium
South Africa's corrosion map puts the first 150 m from the shore in CX and 150 to 1,000 m in mid-C4 to mid-C5 (Janse van Rensburg et al., 2019). For a coastal property or farm, the outside of the tank faces the harshest conditions.
On producer rates for severe marine sites, Z275 lasts about 2 years and AZ150 about 9 years before the face is consumed. For marine sites further back, the figures are about 15 years for Z275 and 37 years for AZ150. Those rates come from a BlueScope technical guide seen only as an extract, so treat them as indicative.
Practical rule for coastal tanks: AZ200 with an organic coat and sealed edges, or batch galvanising with a duplex paint system, both over a liner. Measure the site class rather than assuming it.
El Niño 2026/27: why material choice matters this season
South Africa is heading into a very strong El Niño. The South African Weather Service favours below-normal rainfall over most of the country for October 2026 to January 2027, with more frequent heatwaves. Storage bought in a hurry is exactly where coating choices get rushed.
Three findings from Damtech's El Niño research connect directly to tank specification:
- The Western Cape enters summer after a very dry winter. Provincial dam storage was about 78.5% on 2 November 2026, against a national figure of about 95.6%. That region also has soft water and long coastlines: the two conditions where bare zinc performs worst.
- Boreholes are not a guaranteed fallback. In the 2015/16 drought, 29 of 89 emergency boreholes came up dry. Rainwater harvesting and stored municipal water fill the gap, and both are soft. A tank bought for drought resilience must be lined.
- Municipal outages come from leaks and pumping, not empty dams. National non-revenue water is about 47.3%. DWS sees Gauteng and eThekwini as most exposed to summer outages. If backup storage stands on a concrete slab or plinth, an Aluzinc tank must be isolated from the concrete.
A tank that fails in five years turns an emergency purchase into a repeat cost, often in the next dry cycle. Capital security in a drought year comes from the liner and the base, then the coating. For the wider planning picture, see El Niño 2026/27 and water security on South African farms.
Lifecycle cost-effectiveness
There is no sourced ZAR price comparison between Z275 and Aluzinc tanks, so this section reasons from life rather than price.
- Lined tank, inland site (C2 to C3): both coatings can give long exterior life. Z275 is adequate for many farm sites; Aluzinc adds margin. The liner sets the replacement interval.
- Lined tank, coastal site (C4 to C5): Aluzinc's exterior life advantage is worth paying for, provided the base is isolated and edges are sealed.
- Unlined tank: neither bare sheet coating is a long-life design. Batch galvanising at 55 to 85 µm gives about three to four times Z275's zinc per face and is the better long-life route where metal must touch water.
- Concrete plinth: galvanised is the more forgiving coating; Aluzinc needs a separator membrane.
Damtech's current kit prices for lined Z275 corrugated galvanised tanks are listed on the steel water storage tanks page. AZ150 Aluzinc tanks, with the same PVC liner, are quoted on request.
How to specify your tank
- Get a water analysis: hardness, alkalinity, pH, chloride, sulphate and copper.
- Classify the site: distance from the sea and from heavy industry.
- Choose the water barrier: a qualified liner of stated thickness, or a batch-galvanised or factory-coated wetted surface.
- Choose the exterior coating for the site class.
- State the coating per face and require the mill certificate.
- Seal cut edges and bolt holes, isolate dissimilar metals, and keep copper out of the system.
- Design the base: level and compacted, with a separator where steel meets concrete, and dirt and vegetation kept off the wall base.
For the detail on each coating, read galvanised Z275 steel water tanks and Aluzinc water storage tanks.
Standards referenced
- DWS 9900, Department of Water and Sanitation General Corrosion Protection Specification, second edition, July 2022. It names neither coating and applies by contract.
- ANSI/AWWA D103-19, factory-coated bolted carbon steel tanks. Its clause text was not reviewed for this article.
- ISO 9223, atmospheric corrosivity categories. It does not describe tank interiors.
- ISO 1461 / SANS 121, batch galvanising. It excludes continuously galvanised sheet.
- SANS 241-1:2015, drinking water: zinc 5 mg/L (aesthetic), aluminium 300 µg/L (operational).
Get an engineered tank specification
Damtech specifies and supplies lined Z275 galvanised and Aluzinc steel reservoirs for farms, lodges, estates and industrial sites across South Africa. Tell us your water source, site and storage target, and we will recommend a size, liner, coating approach and base, and quote it.
Request a reservoir specification and quote or use our water storage calculators.
This article is general technical information, not a design or compliance opinion for a specific project. Aluzinc atmospheric rates are derived, not measured, and no figure here is a warranty.