Aluminium - Powder Coated Planters

High Strength-to-Weight Ratio

Aluminium is less strong and stiff than steel, but it has a materially higher strength-to-weight ratio. This means, for example, that steel would typically be preferred for structural landscape scheme elements [like large-scale retaining walls], and for elements exposed to damage in public realm [like planters]. Aluminium, however, would be preferred for elements which are both more protected, and where its high strength-to-weight ratio confers distinct benefits.
 

High Degree of Corrosion-Resistance

Unlike mild steel or galvanised steel [such as Zintec], in most environments aluminum does not corrode outdoors because it reacts with oxygen to form a tough, microscopic barrier of aluminum oxide. This transparent layer seals the metal, is non-porous; and it self-heals immediately if scratched again, preventing further oxygen or moisture from reaching the raw metal underneath.

Unlike rusting or tea staining on steel [which is aesthetically displeasing], aluminium’s protective barrier of aluminium oxide can also be more easily ignored. For example, if the brief requires landscape products to be ‘installed and forgotten’, then it is common for a light/mid-grey powder colour to be specified, such as RAL 9006 [White aluminium], as any surfaces scratches are that bit harder to see.  

Footnote: Aluminium can be used raw, in theory, as it will certainly last. However, in sheet – rather than cast – form, aluminium is quite soft, and it can very easily be physically damaged / degraded; and this is not only unattractive, but it will also impact longevity.  

Aluminium ‘wins’ where its triple benefits of corrosion resistance, longevity and light weight combine, and justify the cost. There are also other, more marginal, situations where aluminium has something distinct to offer – such as in some specialist finishes. 

Cladding, Frontages and Facades

Cladding, frontages and facades are typically made from aluminium, as they are prone to damage, and aluminium’s light weight is a distinct advantage. When designing such cladding etc. it is wise to ensure that the design allows each panel to be ‘popped out and replaced’ in situ, if seriously damaged.

Weight-Sensitive Terraces, Decks etc.

In certain situations, such as on some terraces and decks, weight may be the overriding specification decision. 

However, in IOTA’s experience, such situations are rare, particularly for planters. 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 planters [which will be lighter in aluminium]. So, if weight is an issue, then there are many ways to mitigate it via the design of the planting, not the material specification of the planter. And, if such mitigation strategies can make the required difference, then steel planters will be a lot cheaper. 

Perforated Screening & Canopies

Aluminium’s high strength-to-weight ratio allows designers to create expansive overhead shade canopies, lightweight pergolas, and laser-cut decorative privacy screens, without overloading foundation points.

Wayfinding & Signage

Aluminium is the global standard for public park maps, heritage trail signs, and directional indicators. It is highly customisable via anodising or powder coating, and is essentially immune to the elements.

Public Seating & Furniture 

Most public seating is specified to be natural timber, or occasionally composite timber. However, where metal is specified, aluminium provides a long-lasting, user-friendly sitting surface, as it conducts heat away faster than steel and does not degrade when scratched.

Specialist Finishes. 

Aluminium is the metal chosen for some specialist surface finishes, where the metal [in brushed form] is partially visible beneath a translucent surface paint layer – contributing to a distinctive ‘shimmer’ effect. 

One such finish, commonly used in architecture, is anodised aluminium, prized for its durability and its sophisticated, sleek metallic aesthetic. Because the finish is a translucent oxide layer grown from the metal itself, rather than a painted coating, it delivers a refined, deep sheen; and the decorative look can be customised through different pre-treatments and finishing processes [for example, to change the colour].

For practical application in landscape schemes, anodised aluminium is limited to where it can be used in off-the-shelf sheet form [such as for cladding]. Anodising a prefabricated landscape product, like a large planter, is highly challenging and generally not recommended, if the product has already been welded or assembled. Because anodising is an electrochemical immersion process, attempting to coat a fully prefabricated structural item introduces major technical risks like chemical entrapment and weld discolouration. Plus the process is also extremely expensive, and subject to tank size restrictions. There is good news, however, as there are specialist powder coat ranges which seek to mimic the effect of anodised aluminium, and these can be both highly effective and cost-effective. One such project example is: Princes Street, Hull 

There are other specialist finishes that can be applied to brushed aluminium, using wet paint application. These can be extremely dramatic – even more so than anodised aluminium – if budgets allow. One such finish was developed by IOTA for the Chelsea Flower Show 2026:
Chelsea Flower Show 2026 – Alzheimer’s Society: Microbes and Minds Garden

Strength and Stiffness

Aluminium is less strong and stiff than steel, and there are other differences between the metals in terms of how they perform in real-world landscape situations.

Of particular note, when impacted [such as a vehicular impact] steel exhibits higher absolute strength and impact resistance, permanently deforming less under severe force. Conversely, aluminium is softer and more malleable; it absorbs impact by deforming or tearing more easily than steel. This risk of tearing is a particular concern with aluminium planters, where vehicular impact damage can breach the metal, and cause spillage of the soil / plants etc. Given this, IOTA would not recommend aluminium for planters in public realm where the risk of vehicular impact is high.

Environmental Limits 

Although highly corrosion-resistant, aluminum can still corrode or suffer surface degradation [often forming a white powdery residue] in specific harsh environments:

• Coastal and Salty Air: Chloride ions found in saltwater and heavy sea spray can break down and penetrate the protective oxide layer.
   
• Extreme pH Levels: The oxide layer is stable between a pH of 4 and 9. Highly acidic or highly alkaline conditions [like contact with wet concrete] can dissolve this protection.

A specific risk with aluminium is galvanic corrosion. If aluminium touches a more ‘noble’ metal [such as copper or stainless steel] in the presence of an electrolyte like water, the aluminium can act as an anode and corrode more rapidly. This also means, for example, that aluminium fastenings should not be used for products fabricated from stainless steel [and vice versa].

Demonstrable Manufacturing Competence

Aluminium is expensive, in part because it is much harder to work with than steel, and it requires highly skilled labour – particularly in the welding department. And this consideration is increasingly relevant, of course, the more complex are the designs that have been conceived by the architect.

Any manufacturer seeking to be specified on a project should be able to demonstrate manufacturing competence in aluminium, commensurate with the complexity of the scheme. Evolving – or non-constant – radiuses are particularly tricky, for example in these projects executed by IOTA in aluminium:

Aberdeen – Corporate Headquarters Roof Terrace
Chelsea Flower Show 2026 – Alzheimer’s Society: Microbes and Minds Garden

Galvanic Corrosion

Galvanic corrosion between stainless steel and aluminium occurs when the two metals touch in the presence of an electrolyte [like moisture or salt], causing the aluminium to act as a sacrificial anode and corrode rapidly.

Click here for A Guide To Galvanic Corrosion. However the most important thing to note is that, when specifying these metals, one must make all efforts NOT to mix the two metals within the same product design. However that is not always possible or practical, in which case the mitigating strategies in the above guide should be followed.