Aluminium vs Steel: Breaking down the ways in which aluminium won’t break down

We’ve long known that aluminium’s natural properties make it particularly useful for structures that are exposed to the elements. However, this takes on additional significance when it comes to selecting building materials in New Zealand and other parts of Australasia. In Kiwi-land, in particular, even our innermost region is still only 120kms from the coast and the sea.

Staying away from copper on the coast

So, what is the solution for areas subject to coastal climates? Aluminium alloys – provided they don’t contain copper. And the good news is, most aluminium alloys used in construction today don’t include copper, giving them a long and successful service life in salt air conditions.

Aluminium’s chemistry is quite different to that of iron and iron alloys – including the other common construction alternative, steel. These differences render aluminium less vulnerable to structurally significant corrosion, even when it comes into contact with water and salt air. Another reason the ship-building industry have a love of aluminium!

The science is simple

Luckily you don’t need a chemistry degree to understand the science behind the differences either. Most of us will already know that when metals come into contact with oxygen, or other ambient oxidising agents, they form oxide compounds. For iron, this produces ferric oxide or what we more commonly know as ‘rust’.

Aluminium is similar in that it also undergoes an oxidation reaction and produces aluminium oxide. However, what is less well known is that this process imparts quite different physical properties.

Rust is porous and, when it forms on the surface of a steel structure, it allows air and liquid to penetrate to the uncorroded iron beneath – resulting in further damage. Aluminium oxide also forms on the surface of aluminium structures, but unlike rust, it is impermeable and instead acts as a tight barrier coating that prevents further corrosion of the underlying metal.

The facts

Testing by the American Society for Testing Materials suggests that the average rate of penetration for corrosion of aluminium is as low as 0.00051mm/year. For steel, this rate can be as high as 0.0445mm/year.

These might seem like relatively miniscule numbers, but the impact is significant. Evidence from the shipbuilding industry shows that the aluminium alloys used to construct ocean-going vessels corrode roughly 100 times more slowly than their steel counterparts (and those numbers are certainly significant).

And then there’s anodisation

If you want to get even more technical, aluminium’s natural resistance to corrosion is heightened by the process of anodisation. Basically, an electric current and an electrolytic solution can be used to artificially thicken and harden the aluminium oxide layer, adding extra protection from corrosion and abrasion.

The galvanisation of steel works in a similar way by creating a protective surface layer – albeit of a different metal, usually zinc. During this process, a sacrificial layer of zinc is added, which corrodes to form a layer of zinc oxide. This performs a similar function to that of aluminium oxide in that it protects the underlying metal from oxidation. However, it offers significantly less protection because if the surface is scratched and the steel beneath becomes exposed, it will still corrode.

In case it’s not clear already, one of the key benefits of using aluminium in an outdoor or external setting is that if the surface of the metal is damaged and the oxide layer is removed, a new layer will naturally form, thereby preventing any further corrosion.

Choosing aluminium

When it comes to selecting materials, alloy content is a key factor in determining how resistant aluminium is to corrosion. Modern building materials are most often made from ‘pure’ alloys, which have a high aluminium content and offer the best protection.

It’s clear that these qualities give aluminium a long service life without the need for a finishing process like galvanization or the application of a coat of paint – though anodisation and coating can augment its protective properties.

For evidence, we need look no further than the cladding atop the dome of the church of San Gioacchino, which is still in excellent condition over 120 years after it was installed! However, if you could still do with further convincing, download our ebook for a full comparison of steel versus aluminium.

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