The other upside is that they don't rust and you don't have to tumble nearly as long if at all during annual viz inspections. You will get aluminum oxide but if filled from a good reputable source you likely can just tap the tank when the valve is off upside down and the tiny amount of ALOxide will come out and then you can finish the viz quicker and easier which can lead to a lower cost or less time if you are a viz inspector yourself like I am.
There is the constant confusion and misinpretation about this rusting on Steel
Firstly Steel and Ali both corrode with similar mechanisms:
Atmospheric:
Galvanic (dissimilar metal)
Stress corrosion
Fatigue corrosion
Wormhole or filiform
There are others but we don't need to consider those in this discussion
Just because steel exhibits its corrosion as a red coloured oxide (rust) and Al as a white powdery oxide doesn't make them any different. Indeed it worries me as a materials specialist that people believe the white powder oxide is less of an issue than rust scale.
Just because you can remove the white powder with ease doesn't mean you have removed the corrosion. Think of it as a fungus. you can remove the fungus but the spores are still there and it will grow again unless treated. All you have done is removed the oxide. Would you just knock off the rust scale and say the tank is okay?
Wormhole is the biggest killer. It starts with a pinprick defect in the paint and metallic coating, thus is harder to see.
All wire brushing does it to burr the surface material over the hole. As does sanding. The hole is still there.
Remember the hole is a loss of thickness of the parent material.
The only true way to examine cylinders to establish corrosion damage would be to carry out an ultrasonic inspection to measure material loss over what area.
Eddy current around the necks of tanks will only find surface defects (the depth of its penetration is so small to be considered surface) Especially given the thickness of the material. Eddy current is only suitable for Aluminum. Steel would be easier to inspect for cracking with magnetic particle but all of these are outside the scope of an LDS, both with the cost of equipment and operator training
My Steel tanks are 5 years old. I live in a climate with high temps, high salinity and high humidity yet all of mine are in near perfect condition. The y get washed after each diver and properly washed after every trip The insides are perfectly clean (never been tumbled) and each year I re paint the outsides (ensuring I don't break through the original primer) The top coat ensures salt water doesn't come into contact with the steel and is just the final protective coating.
Aluminum is prevalent because it's cheap not because its better Steel is lighter for an equivalent volume but is negative - hence you don't want a bunch of steel deco cylinders.
I've been to quite a few dive ops that use all steel or have AL 80 and steel for anything bigger.
Buoyancy characteristics, and cost are the main deciding factors Both materials have their pro's and con's.
This constant line of Al is better than steel because it doesn't rust is disingenuous at best and a concern that people can be certified inspectors without being taught or understanding basic principles of corrosion