The advantages and disadvantages of aluminum?

Answer 1

Aluminum tanks have a standardized width, and are much cheaper and lower maintenance than steel, as they will not rust. Yes, even stainless steel tanks will rust if any water gets in them, as not much won't in 3000 psi. However, they typically wear out more quickly (20+ years, but indefinite for steel) and they weigh less - you would think this would be a plus, but it's not cause you need more weight to make up for it.

Addition

Steel tanks are actually lighter out of the water (since aluminum is a softer metal which requires there to be more aluminum than steel in a Scuba cylinder) and more negative in the water (so you need less weight).