Steel is more noble than aluminum, in the case you quoted the steel bench will not rust, the aluminum will. If exposed to any electrolyte the aluminum will corrode faster than steel. Even the methanol in our fuel will cause aluminum to corrode if left sitting in the crankcase without evaporating. The only case I can think of on the top of my head where aluminum outlast steel is when exposed to the air where there is little or no condensation and no electrolyte, then the aluminum will outlast the steel. But that is probably the case with most exposed steel and aluminum parts you are used to. If you want to talk about different alloy's there is steel where the rust does form a protective coating and is used for unpainted structures. And then there is stainless steel. So you see if you want to talk about all of the apples and oranges then steel will outlast aluminum in most cases or with the proper care.