High-grade Stainless Steel Planters - Powder Coated

In their raw, unpainted, natural metal finish, high-grade stainless steels have myriad uses in landscape schemes – as discussed here: High-grade stainless steels – natural metal finish. By far the most commonly specified of these steels is 316-grade stainless steel [aka ‘Marine-grade].

However, 316-grade stainless steel is rarely specified, with a painted finish, for landscape schemes – for sound commercial reasons:

• 316-grade stainless steel is very expensive [indicatively 50-60% more than Zintec steel]; and, when painted, all sheet metals will look the same. So where is the value-added, and cost-justification, in painting such an expensive metal? 
   
• In fact, painting 316-grade stainless steel is not straightforward, as the metal's natural chromium oxide surface resists paint adhesion, as does its smooth surface finish. To achieve a high-quality, durable and long-lasting paint finish, the surface must be manually abraded, and specialist coatings used – and all this further adds to cost.  

In plain english:

Painting cheaper alternative metals [like Zintec steel, 1.4003-grade stainless steel, or aluminium] is vastly more cost-effective. If choosing to paint 316-grade stainless steel there will be a substantial premium to pay for a base metal that doesn't need to be painted to deliver unparalleled benefits of corrosion resistance and longevity; and then there will be an additional [say] 15–20% on top to pay for specialised, complex surface profiling and coating applications.

There are three specific conditions which together justify the cost of painting 316-grade stainless steel.

One condition is aesthetic, one contractual, and the third is environmental. All conditions are necessary, but none are sufficient: so, to create the justification, the first condition must combine with at least one of the other two. 

Aesthetic Integration

This is the fundamental condition. As there is absolutely no point in painting 316-grade stainless steel unless it is, at the very least, aesthetically necessary. Painting allows landscape products to be closely coordinated with wider design schemes, brand palettes or building finishes – in this case, without introducing the harsh, industrial glint of raw steel, which might overwhelm the harmony of the overall design.

Contractual Warranties

On some projects, there may be contractual warranties demanding a 25+ year Design Life Performance under any-and-all circumstances. With a painted finish on metal, only 316-grade stainless steel can deliver that assurance. In the real world, when the prime contractor is made aware of the cost of standing behind those warranties, sometimes those warranties ‘disappear’. 

Extreme Corrosive Environments

If a painted finish is mandated, and the environment is extremely corrosive [such as coastal areas heavily affected by sea salt], then there is no choice but to paint onto 316-grade stainless steel. For example, IOTA has recently supplied painted tree planters to the Uig Ferry Terminal on the Isle of Skye, and 316-grade stainless steel was the only credible option.  

Paint Finishing

As already alluded to, painting 316-grade stainless steel to a high standard is a highly specialised area, the details of which are beyond what most architects would wish to specify in detail. Therefore most will rely on the manufacturer’s proven track record in delivering this material option.

However, technical considerations [about which an architect might wish to be generally informed] include:

• Because 316 stainless is smooth and possesses low surface energy, regular primer and paint will flake and peel, and the surface must be mechanically abraded to create a mechanical ‘key’ for adhesion.

   
• Specialised materials must be used [e.g., two-part epoxy primers and polyurethane topcoats] that contain less than 50 ppm of available chloride to prevent reactions with the metal.

   
• It is vital to utilize a high-performance epoxy-polyamide primer formulated for marine or industrial metal, followed by a UV-resistant topcoat.

   
• Zinc-based primers must be avoided, as they can react galvanically with the stainless steel.

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.