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Question

The oxidation of iron to rust is a problem structural engineers need to be concerned about, but the oxidation of aluminum to aluminum oxide is not. Why?

Answer 1

Rust is not neccessarily a problem for a structural engineer, unless it is galvanic in nature. What this means is that the system is not stable; the material continues to be corroded until there is nothing left.

In the absence of galvanic effects, steel will take on a nice patina and then be stable. The rust layer prevents additional rust. This is how it works on aluminum, as well.

Galvanic corrosion protection is a problem that I have had to solve for a lot of people who unwittingy create galvanic cells in their design without realizing it (usually by coupling stainless for decorative value with aluminum parts for light weight, and operating this system in the presence of an electrolyte, like salt water).

Answer 2

Don't know if this is true, but I have heard that aluminum only oxidizes to a certain point, while iron will continue to oxidize as long as it is exposed to the correct elements (one of them being oxygen), so I imagine structural girders could conceivably rust all the way through but aluminum beams would not.