Steel vs. Aluminium – which metal should I specify?

A discussion of the relative merits of Steel vs. Aluminium as a material for exterior landscape products

When it comes to exterior landscape products [and planters, in particular], this question is one of IOTA’s most frequently asked FAQs.

Another very common FAQ is Metal vs. GRP – Which Material Should I Specify? There is some commonality between these two material FAQs, as very often the underlying question is actually about weight / loadings. It is a widely-held belief that Aluminium and/or GRP are “the ideal lightweight materials for planters”.

This view is both erroneous, and irrelevant in practical terms – so it is worth addressing the issue first, before developing a considered argument for Steel vs. Aluminium.

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A. The weight myth

Yes, Aluminium is light – in the sense that it is less dense than Steel.

However, Aluminium in sheet form is also a lot less stiff than Steel, so typically one has to ‘gauge up’ with Aluminium [i.e. to achieve the same strength and stiffness as 2.0mm Steel, one might need to use 3.0mm Aluminium]. This gauging-up tends to negate a lot of the perceived weight gain with Aluminium.

Even so, the Aluminium planter will be a few kilos lighter, on a like-for-like basis – but this weight difference is irrelevant in practical terms. In the vast majority of conventional planting schemes, 75-85% of the weight is down to the planting [drainage materials, soil/compost, water, plants/trees etc.], and only 15-25% of the weight is down to the planter. So, if weight is an issue, then the way to address it is via the design of the planting, not the material specification of the planter.

So there are reasons to like Aluminium – but weight is not a reason, in the vast majority of practical, real-world situations. And to truly gain the weight benefit of Aluminium, one needs to be looking at weight mitigation strategies like using artificial plants – and these situations are absolutely at the margin of our industry.

B. Steel vs. Aluminium in context

To set the terms of reference, we will assume that the decision is between Zintec [Zinc-Plated] Steel or Aluminium, both to be painted with a Polyester Powder Coat [PPC] finish.

The above choice covers the overwhelming majority of metal planters ordered from IOTA each year. So we are not discussing here the relative merits of premium steels [such as Stainless Steel or Corten Steel]; and we are not considering the option of Raw Aluminium [which appeals to very few, and which IOTA is asked to make maybe once every two or three years…!]

To put the issue in context, the essential thing to understand is that Aluminium is more expensive than Steel [generally about 30% more expensive, on a like-for-like basis]. Not only is Aluminium itself more expensive as a material; but, as has already been discussed, thicker gauges typically need to be used. Plus Aluminium is a much more difficult metal to work with, so production is slower, and only the most highly skilled welders can be used for Aluminium. All of these considerations accrue to place Aluminium at a significant cost disadvantage to Steel.

On top of this, a Steel planter, and an Aluminium planter painted with a Polyester Powder Coat finish will look exactly the same.

So the FAQ might be restated:

“If the two planters look the same, do the same job, and will in practical terms weigh the same once planted – then why would I pay 30% more for the Aluminium one…?”

Steel is undoubtedly the default choice, and is the material used in the majority of IOTA’s production. However there are reasons to prefer Aluminium, in certain circumstances.

C. When is aluminium the right choice?

The answer to this question is actually very simple, and it relates to the fundamental corrosion properties of the two materials.

Simply put:

If the Steel planter is scratched through the PPC layer down to the bare metal – then you have to fix it, as it will corrode.
If the Aluminium planter is scratched through the PPC layer down to the bare metal – then you don’t have to fix it, as it will not corrode.

The imperative word ‘have’ is crucial here. You may still want to fix the Aluminium planter, as it won’t look too pretty – but that is purely an aesthetic choice. You don’t have to fix it, as it will come to no harm.

And that is the issue in a nutshell. The cost disadvantage for Aluminium can only be justified in circumstances where its superior anti-corrosion properties are worth the premium. These situations include:

High street shop frontages and hotel facades are typically made from Aluminium, as they are prone to damage. The same argument can be made for planters, and for other landscape products, in such high traffic environments.
The rate of Steel corrosion is linked to the environment; so, in high-corrosion environments [like Coastal], there is an argument to prefer Aluminium.
A reason not often considered - Aluminium can be preferred in highly seasonal businesses, where planting is removed and refreshed in the off-season. This allows any repairs to the planters to be delayed until a time that suits the client’s maintenance calendar – which is an acceptable compromise with Aluminium, but not with Steel.
All other things being equal, Aluminium will outlast Zintec Steel or Mild Steel – so if extreme longevity is a requirement, then there is an argument in favour of Aluminium. However, in these kinds of situations, one would typically also evaluate the relative merits of Stainless Steel.
With Aluminium, as we have seen, maintenance is not mandated. So if the brief requires planters to be ‘planted and forgotten’, then Aluminium has merit. Interestingly, in these kinds of situations, it is common for a colour such as RAL 9006 [White aluminium] to be specified, as any surfaces scratches are that bit harder to see.

So totally legitimate reasons exist to argue in favour of Aluminium, in certain circumstances. However ‘cost is so often King’ in landscape projects, that Steel is undoubtedly the default choice. If IOTA’s experience is informative, then 50% of Aluminium specifications typically get value-engineered out of larger projects; and Steel is the material used in the majority of IOTA’s production.