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Hard black surface for raised vegetation beds

Q. Hello! I'm a landscape architect and working on a project has put me questioning about the best finishing options, so I happened to find this forum.

We want to design raised vegetation beds near a marine environment.

Usually they're made from cort-ten steel sheets, but we want to use about 10 mm thick galvanized steel sheets and have a black finish. Our customer is worried that when for.ex skateboarders use it for sliding even once, the powder coat wears off immediately and it starts to rust.

What would be the most reasonable finish to use in that case. Is there a chemical treatment that also has a completely black appearance and is better? Does passivation make it last longer? We've come to understanding that manual scratches can't be avoided and at a certain moment it will affect the surface anyway. Any help would be appreciated.

Thank You in advance!

Kadri Tere
- Tallinn, Estonia
September 1, 2017

A. Hi Kadri. It is not a trivial matter to select the ideal finish for a complicated situation involving a marine environment, possible earth contact?, an unusual & unpredictable wear situation, unknown cost constraints, etc.

Getting paint or powder coating to adhere to galvanized sheets can be problematic, and when you want sufficient adhesion that skateboard wheels will not cause it to peel or scratch, it's asking for trouble. Black chromate passivation on zinc electroplating might work, but I don't think the corrosion resistance will be adequate. Whether reliable decorative blackening of hot dip galvanizing is practical has been the subject of several threads here. Salt bath nitriding or black chrome plating would give a dark and very hard surface, but if ground contact is involved, I'm not sure of sufficient corrosion resistance.

If you can afford it, stainless steel with a black PVD finish might be best.

Regards,

Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 2017

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