For a coastal urban environment with constant salt air, the most durable planter box is a 316L stainless steel unit with a high-performance powder coat or marine-grade paint system. In severe chloride exposure, specify duplex 2205 stainless steel. Stainless steel resists pitting and crevice corrosion far better than mild or galvanized steel, and 316L is widely available in sheet, tube, and 316 fasteners, so the whole assembly can be built to a marine specification.
Durability also depends on design. Specify fully welded seams, sloped internal drainage, raised feet, ventilation gaps, and a replaceable internal liner or root barrier. Use 316 stainless fasteners and isolate dissimilar metals to prevent galvanic corrosion. Avoid flat bottoms and trapped water, which cause premature failure even in good materials.
If weight or budget rules out stainless steel, marine-grade aluminum with a powder-coated finish is the best lightweight alternative. Fiberglass-reinforced plastic (FRP) is another strong non-corroding option for coastal streetscapes. HDPE is low-maintenance but usually less rigid and less premium in appearance. Concrete or GRC can work but needs a quality sealer and careful detailing to prevent salt ingress and spalling; timber requires frequent maintenance.
For long service life, wash the planter quarterly with fresh water, inspect coatings and welds, and touch up damage immediately. The bottom line: for the most durable urban planter box in salt air, choose a 316L stainless steel planter engineered with drainage, ventilation, and marine-grade hardware.