The eddy current testers used on scuba tanks only work on aluminum.
The testing equipment limitation is only that.
1. It's a cheap piece of junk that someone made good money from selling a lie.
2. It's operated by Level 1 operators who aren't allowed to alter the settings, even if they knew how too and what they were doing.
Eddy Currents are appropriate for finding surface and near surface defects on any conductive materials that you're capable of inducting Eddy Currents into
Granted Ferro Magnetic materials are slightly more tricky since the magnetic domains cause "interference" but I've successfully written inspection procedures for Ferrous, non ferrous steels, as well as AL and Ti. Surface treatments like Cad or Galvanising (anodic to a lesser extent) can create issues while developing the procedure, but they're not insurmountable given the right probe frequency, coil size and playing with filters and gates
You could also inspect with ultrasonics. The advantage being you could examine the complete cylinder from the outside and set the sensitivity of the technique to ignore acceptable corrosion defects.
However certainly at the neck, with threads it would get more tricky because of reflections masking defects, and dealing with the likelihood of mode conversions (sound waves with different angles to the material change speed), the basis of ultrasonics is measuring the time the pulse take to transmit and be received. Highly doable.
The problem with both is cost. Both of the equipment, and training, meaning that its not worth it on a low cost disposable items like scuba cylinders.
Visual inspection are susceptible to interpretation and people erring on the side of caution (or in a few cases I'm sure, being dishonest) and the training is very limited.
