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ZINCALUME® Steel in Rainwater Tanks

ZINCALUME® Steel in Rainwater Tanks

In our last article, we compared Poly Water Tanks with ZINCALUME® Steel Tanks. In this article we look further into the features of ZINCALUME® steel in rainwater tanks.


Until recent decades, a common material for steel roofs, sheds and rainwater tanks the littered the Australian outback were galvanised iron – steel that has been coated in zinc to provide protection against rust and corrosion.

Prior to 1976 many steel roofs were also galvanised. Then BHP Steel (now BlueScope Steel) developed and launched ZINCALUME® steel which possessed a higher level of corrosion resistance. It has since become the number one choice in steel roofing, cladding and construction in Australia.

About ZINCALUME® Steel

It is reported that in roofing application ZINCALUME® steel will last two to four times longer than galvanised steel roofs under the same condition. How is this achieved?

Instead of the traditional zinc coating found in galvanised steel which provides electrochemical protection, research and tests found steel dipped in aluminum (55%), zinc (43.5%) and a bit of silicon (1.5%) protected the steel base in two ways. It provides a tough physical barrier to protect the inner core steel against the weather, and secondly it protects the steel if cut at its edges and scratched. In effect, this provides much better protection against rust and corrosion.

In August 2013, BlueScope Steel released next generation ZINCALUME® steel. This introduces magnesium into the mix, resulting in an even tougher protective coating that’s more resistant to scratches and scuffs encountered during construction.

Compared to the older ZINCALUME® steel (AZ150) which has a roofing warranty of up to 25 years against weathering, next generation ZINCALUME® steel (AM125) carries a warranty of up to 36 years. For further information comparing the two ZINCALUME® steel products, read BlueScope’s Product Release Guide for Next generation ZINCALUME® steel.

ZINCALUME® Steel Water Tanks

While ZINCALUME® steel has demonstrated superiority with roofing and wall panels, when used to store rainwater it face several weaknesses and may not last much longer than a more thickly coated galvanised tank.

Numerous factors affect the life of a steel product including environmental conditions, the usage of a product (e.g., roofing, sheds and water tanks), contact with chemicals and dissimilar metals, method of galvanised coating and the coating thickness. A more thickly zinc coated steel tank of 450 g/m2 would obviously outlast a thinner coating of 150 g/m2.

BlueScope’s guidelines for ZINCALUME® steel warns:

  • The material should be clear of the ground and protected from rain and moisture prior to install.
  • Carbon-black should be avoided, for example, with washers used or black carbon (‘lead’) pencils that might be used for marking sheets.
  • Lead and copper, treated timber, uncoated steel, fertilisers, pesticides and abrasive cleaners can cause corrosion (Important: ZINCALUME® steel is incompatible with lead and copper).
  • Galvanised gutters, roofing and steel cannot be used in conjunction with the steel or corrosion will occur.
  • Avoid immersion in concrete, soil or water, or debris build-up against the product.

All these things to avoid means a lot more care needs to be taken when installing to ensure dissimilar metals do not come into contact, moisture is avoided and kept to a minimum. If your ZINCALUME® steel water tank comes into contact with moisture it will more rapidly corrode. ZINCALUME® steel needs to be able to breath and be surrounded by oxygen (air) in order to remain stable.

This weakness in ZINCALUME® steel means an inner poly lining must be used and carefully installed and maintained. This protects water coming into contact with the steel. It helps stop leaks at welded joints, a weak point in steel tanks. In addition, the inner poly lining also stops zinc and other particles leaching into your rainwater which can cause an unpleasant metallic taste.

Sadly, this inner poly lining can cause water condensation to occur. When the water tank fills with water, the liner is pressed up against the inside steel wall. This causes the moisture vapour in the air to condense and become trapped between the liner and the tank wall. Corrosion to ZINCALUME® steel is inevitable due to the moist conditions and the lack of oxygen (air) that it needs to prevent corrosion.

ZINCALUME® Water Tank Warranty

The higher quality next generation ZINCALUME® steel (AM125) water tanks normally carry a conditional warranty of up to 20 years for domestic use. It is very important to understand the conditions of any warranty, and especially if you purchase a ZINCALUME® that has many weaknesses.

In particular some things to be aware to:

  • If you install the tank yourself, or hire your own installer who isn’t authorised, then this will often void your warranty (since great care needs to be taken to ensure correct materials are used during install and the poly liner isn’t torn or damaged).
  • Inner poly liners are not included in your steel tank warranty, and often covered for much less like a 12 month period.
  • Steel tanks need to be installed on a moisture free and solid foundation (otherwise damage caused from foundation settlement or movement will not be covered).
  • Anodes (positively charged electrodes that carry electrons from a device) must be replaced every 10 years to help protect your tank against corrosive effects of moisture.
  • Damage caused from dissimilar metals (e.g., lead, copper), treated timber, fertilisers, pesticides, leaves and vegetation, and the like will not be covered.

In owning a ZINCALUME® steel tank, there is a higher level of knowledge required to maintain your tank and to ensure you do not accidentally damage it, or the inner poly lining when cleaning, which leads to additional expense and may void your warranty.

For an in-depth comparison of ZINCALUME® steel water tanks versus poly water tanks, read our article Water Tanks Compared: Poly Tanks versus ZINCALUME® Steel Tanks.