Specifying Metal Planters for Coastal Locations

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Specifying Metal Planters for Coastal Locations

A review of the material, design and manufacturing issues to be considered when specifying metal planters for use in a coastal location

A. What is a ‘Coastal Location’?

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:

  • If a property is only affected by salt spray during ‘once in a decade storms’ then it is probably legitimate to ignore the issue.
  • A property may be most definitely coastal, but if the planters are to be located behind a high, weather wall then again it might not be cost-justified to ‘coastal-proof’ the planters.
  • In high-end properties with a dedicated gardener, who will assiduously wash-off the salt spray from each planter after each storm, then again the extra spend is debatable.

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.

B. The Science

Most people understand the basics of rusting; but there are other forms of corrosion which also occur in a Coastal Location.


B.1 Rusting

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:

  • The air at the coast tends to be more humid than it is inland. The invisible suspended moisture in humid air still ‘counts’ towards rusting, and visible water will condense out of humid air onto a cold Steel surface more frequently [for example in the early mornings].
  • Salt spray is hydroscopic – meaning that it will draw water out from the humid coastal air to pool onto the Steel surface.
  • Rusting requires the movement of electrons, and this takes place much quicker in salt water, as it is more conductive than rain water or tap water.

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.

B.2 Other Forms of Corrosion

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:

  • Unlike rusting, these other forms of corrosion also affect all metals, to varying degrees – not just Mild Steel.
  • Most relate to electrochemical processes, which take effect due to the high conductivity of sea water [processes which, unlike rusting, do not need the presence of oxygen].
  • These other forms of corrosion tend to be accelerated by poor design and/or manufacturing – such as the inappropriate use of noble and active metals together [i.e. dissimilar metals, which are far apart on the galvanic series], which leads to galvanic corrosion.


Therefore, the essential messages from the science are that:

  • No metal is immune from the effects of a Coastal Location [i.e. it is not just about rusting Mild Steel].
  • The scientific processes behind corrosion are strong, natural, and inevitable – and corrosion cannot be ‘solved’, but its effects can be mitigated.
  • Mitigating the effects of corrosion in a Coastal Location requires considered, technical strategies, grounded in the science.

The good news, however, is that there are many such strategies that can be deployed, as explored below.


C. Material Specification

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:


C.1 The Precious Metals

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.  

C.2 Aluminium and Stainless Steel – Unprotected

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].  

C.3 Aluminium and Stainless Steel – Protected

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.

C.4 Mild Steel or Zintec [zinc-plated] Steel

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.


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:

D.1. Barrier Coatings

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:

  1. Paint finishes, such as Polyester Powder Coating [PPC]
  2. Plating with a more active metal – such as plating Zinc on Steel, via Hot-Dipped Galvanisation [HDG]  


Both options have pros and cons:

  • A paint finish, such as PPC, offers infinitely more aesthetic choice, and it is cheaper than HDG. However painting relies solely on maintaining the physical barrier, and the design life is directly a function of the paint specification and application quality, and how much damage is suffered over time [and on whether this damage is addressed swiftly].
  • Plating, such as HDG, is very much a ‘Marmite’ aesthetic, and it is more expensive than PPC. However plating creates both a physical barrier, and it also provides an outer layer of Zinc which will ‘sacrifice’ itself over time instead of the Steel corroding; and the design life is directly a function of the plating thickness and application quality, and HDG is more immune to damage than PPC.


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.   

D.2. Intelligent Design

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.

To explain:

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.

For example:

  • Open-seamed planters have absolutely no place in a Coastal Location [as complete barrier coverage simply cannot be achieved], and all seams must be specified as ‘closed and fully welded’. 
  • All holes [for irrigation, drainage, lighting etc.] must be designed-in, and laser-cut into the planters prior to barrier coating. Cutting on-site penetrations into the finished planters will destroy the barrier coat; and on-site remedial coatings will never be as good as the original.
  • Barrier coating must be to all surfaces – external, internal, underneath. And, ideally, the planters will be designed such that all elements are independently coated before final assembly. At IOTA, where planters are for a Coastal Location, all internal stiffeners, cross-braces and planter bases are individually PPC painted, and inspected for complete coverage, prior to careful, manual final assembly. 
  • Where planters are subject to salt spray, they should be thoroughly washed-off as part of a regular maintenance plan, and all areas must be accessible and free to dry out as much as possible. For this reason, IOTA recommends that planters in a Coastal Location are designed to include adjustable feet, even if otherwise they might not be strictly necessary.   

D.3. Manufacturing Standards

And finally…

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:

  1. All cut metal areas must be carefully de-burred, and faired off smooth, prior to barrier coating.
  2. All welds must be complete, finished and faired – again to ensure perfect barrier coating.
  3. All seams must be closed, to avoid crevice corrosion [and, as above, open-seamed planters have absolutely no place in a Coastal Location].

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.


ADDENDUM – A recommended PPC specification for 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:

  • C3  Urban and industrial atmospheres with moderate sulphur dioxide pollution
  • C4  Industrial areas and coastal areas with moderate salinity

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: 

Option A

Material: Mild Steel
Pre-treatment: Blast Cleaned SA 2.5
3-Coat application:

  1. Zinc Primer: Interpon PZ 790
  2. Intermediate/Primer: Interpon BPP 330
  3. Topcoat: Interpon D1036
Option B

Material: Zintec [Zinc-Plated] Steel
Pre-treatment: Degrease and Abrade
3-Coat application:

  1. Primer: Cromadex 903 Etch Primer
  2. Intermediate/Primer: Interpon BPP 600
  3. Topcoat: Interpon D1036

The above are also the PPC systems recommended and used by IOTA for Super-Prime Residential Developments >>

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