Aluminium, Steel or Wood Outdoors
Aluminium does not rust, which is why it is used for permanent outdoor frames. Steel is stronger for the same section but corrodes wherever its coating is broken. Wood is the warmest to look at and the only one needing work every year.
Aluminium
It forms an oxide layer that protects the metal underneath, so it does not rust in the way steel does. Scratches do not start a corrosion path. It is lighter than steel, which makes a large structure assemblable by two people, and it is the material used on the frames shown here.
The trade is stiffness: aluminium is less rigid than steel for the same section, so the sections are made larger to compensate.
Steel
Stronger for a given thickness, and the choice where load matters more than corrosion. But outdoors, powder coating is the only thing between steel and rust, and any chip - from assembly, from a stone, from a screw - starts a spot that spreads under the coating.
Wood
The one that looks best and asks the most. Sealing or staining every year or two is not optional, and a structure that misses two seasons shows it. Wood also moves with humidity, which is why a wooden frame is a poor host for a mechanism like a louvered roof.
What this means for a roof that moves
A louvered roof has moving parts that need their tolerances to stay where they were built. That favours a material which neither swells nor corrodes at the pivots, which is the reason these structures are aluminium and not the other two.
Questions
Does aluminium rust?
No. It forms a protective oxide layer instead, which is why it is used for permanent outdoor frames.
Is steel stronger than aluminium?
For the same section, yes. Outdoors the question is usually corrosion rather than strength.
How much maintenance does a wooden pergola need?
Sealing or staining every year or two. It is the only one of the three that needs annual work.