Where a client property is literally ‘on the beach’, then it may be invaded by waves and/or salt spray direct from the ocean, and this location is very clearly ‘coastal’. These kind of landscape briefs are not common, but IOTA has worked on several, and extreme proximity to the ocean does not necessarily preclude the use of metal planters.
More common is where salt spray is blown into the garden on the wind, during heavy weather. Depending on the area of the country, the prevailing winds, and local topography the coastal zone may end a few blocks from the ocean or it may extend many miles inland.
Furthermore, in any area there will be a spectrum of environments; and commercial judgement should be exercised, as making a metal planter fit-for-purpose in a Coastal Location will always cost more. So, for example:
Therefore whether the issue requires to be addressed at all is not a simple ‘yes/no’ decision. Even if it does need to be addressed – and as further explained below – many factors need to be considered, to come up with the most appropriate solutions.
Most people understand the basics of rusting; but there are other forms of corrosion which also occur in a Coastal Location.
Rusting occurs when an unprotected alloy that contains iron, like Mild Steel, is exposed to both oxygen and moisture for an extended period of time. This process is progressive but, importantly, it only occurs during those periods when all three conditions [unprotected Steel + moisture + oxygen] are present simultaneously. So how quickly rust develops is a function of how often, statistically, all three conditions are present over a period of time.
Rusting occurs everywhere. But rusting is much more rapid in a Coastal Location, for a number of reasons:
So being in a Coastal Location does not ‘cause’ rusting per se. But in a Coastal Location the conditions required for rusting occur more frequently, and at the elemental level, being in a coastal location makes the process of rusting more rapid.
In addition to rusting, there are other forms of corrosion to be addressed in a Coastal Location.
There are a great many types of other corrosion, and a full discussion of the science behind each is beyond the scope of this article. However the essential points are that:
Therefore, the essential messages from the science are that:
The good news, however, is that there are many such strategies that can be deployed, as explored below.
When considering how to specify for a Coastal Location, most people will think first of Material Specification [for example: most will understand that “Stainless Steel is better than Mild Steel”].
There is a hierarchy of metals for this particular environment, as follows:
The ‘precious’ sheet metals [Lead, Zinc, Copper, Bronze / Brass] are all either elements, or they are alloys with an exceptionally low Iron content. All can be used, unprotected, in Coastal Locations. Whilst their longevity will be compromised, they will still last ‘a human lifetime’ – which is good enough for most briefs. These metals’ individual and characteristic natural patination will tend to become very striking; and IOTA has worked on high-end, coastal projects, where precious sheet metals have been used to stunning effect. However the extremely high cost of these metals puts them out of consideration for all but the highest of high-end briefs. Indicatively, the precious metals are around 250% to 300% more than Powder Coated Zintec [zinc-plated] Steel.
Other, cheaper sheet metals that can be used, unprotected, in Coastal Locations are Aluminium and 316-grade [aka ‘marine-grade’] Stainless Steel. These will also last ‘a human lifetime’, but they have distinct drawbacks. Aluminium will become very dull very quickly, as its protective Aluminium Oxide coat builds up; and longer term the surface will become pitted. It is ‘a look’, but not to everyone’s taste. Stainless Steel meanwhile is not maintenance-free, as is sometimes thought; and it will require ongoing maintenance to remove ‘tea-staining’ [surface corrosion which is non-structural, but aesthetically displeasing]. Also, whilst Aluminium and Stainless Steel are cheaper than the precious metals, they are both still very expensive [indicatively about 30% and 60% more, respectively, than Polyester Powder Coated Zintec Steel].
Aluminium and all grades of Stainless Steel can also be barrier coated, such as with a Polyester Powder Coat. This is, in many respects, the ideal specification for a Coastal Location – marrying the inherent corrosion resistance of the metal, with the protective and aesthetic attractions of a paint finish. There are also maintenance advantages: as if Powder Coated Zintec Steel or Mild Steel is scratched through to the bare metal, then it must be addressed immediately [as it will corrode]; whereas a similar scratch on Powder Coated Aluminium or Stainless Steel can be either ignored, or left to be fixed at a convenient time [as the metals will not corrode]. This is the ideal specification, but in the real world the 20% to 60% premium may dissuade many clients.
Finally, Zintec Steel or Mild Steel can be used in a Coastal Location. And in IOTA’s experience this offers best value to the majority of cases, where budgets are a material consideration. However these metals cannot be used unprotected; and their longevity in a Coastal Location thus relies completely on the quality of their manufacture. There is no reason why these planters should not last 20 to 30 years [not ‘a human lifetime’, but long enough for most clients], so long as they are well made – and this is the subject of the following section: D. Design and Manufacturing Specification.
As explained above, if good design is about balancing form, function and cost, then it is 80% likely, in IOTA’s experience, that Powder Coated Mild Steel or Zintec Steel will be the finally-approved specification. There is a final ‘wild-card’ option, which is Hot-Dipped Galvanised Steel, which will also be discussed here.
In all of these cases, the Design and Manufacturing Specification is equally as important as the Material Specification. As it is only through intelligent design and precise manufacturing, that these most cost-effective solutions can be considered fit-for-purpose in a Coastal Location.
The key design and manufacturing considerations include:
The most common anti-corrosion treatments create a physical barrier of corrosion-resistant material between the damaging environment and the metal. These barriers are generally:
Both options have pros and cons:
In theory, if the metal is completely and evenly encapsulated, by either barrier method, then both offer excellent defence against corrosion. In the real world, however, these barriers are only as good as the application; and this tends to be easier to specify and control in PPC [HDG, in contrast, is a subcontracted-out industrial process, where planters will be consolidated with many other items in a huge batch – and typically ‘you get what you get’].
In the end, the aesthetic considerations will be given precedence in most cases, and PPC is overwhelmingly the most common barrier choice for planters. As an addendum to this article, we have given complete details on a PPC specification recommended for Coastal Locations.
Even if a Barrier Coating, of the highest quality, is expertly applied, much of the benefit may be lost, if the planter was not designed appropriately for coastal use.
When barrier coatings are used to retard corrosion, great care must be taken to ensure complete coverage, without gaps, cracks, or pinhole defects. Small defects can act as an Achilles' heel, allowing corrosion to penetrate the interior and causing extensive damage even while the outer protective layer remains apparently intact for a period of time. And these gaps, cracks etc. are most often inherent to the design.
Even if a Barrier Coating, of the highest quality, is expertly applied to a planter perfectly designed for a Coastal Location – even then, much of the benefit may still be lost, if the planter wasn’t well made in the first place.
Specific manufacturing quality points to note include:
Finally, the kinds of complex, high-performance PPC systems used in Coastal Locations are only as good as their application. Thus it is important to specify that PPC application must be in-house, and/or from an accredited PPC facility, familiar with these systems and with demonstrable track record in supplying to Coastal Locations.
Finally, we have included in this article a recommended PPC specification for Coastal Locations.
Polyester Powder Coating [PPC] technologies have advanced dramatically in the last 20-30 years, and there are now a huge variety of coatings available, from PPC manufacturers such as Akzo Nobel, IGP and Syntha Pulvin. Each have their own brands, and application systems – but the essential specification requirement is to state that:
The PPC coating and application system used must ensure extreme longevity in C3/C4 environments – defined as:
Two such systems commonly used by IOTA in these circumstances follow a 3-coat system with coating products from Akzo Nobel, which might be used as example specs. – these comprise:
Material: Mild Steel
Pre-treatment: Blast Cleaned SA 2.5
Material: Zintec [Zinc-Plated] Steel
Pre-treatment: Degrease and Abrade
The above are also the PPC systems recommended and used by IOTA for Super-Prime Residential Developments >>
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