Most divers never give much thought to what their air cylinder is made of. Even so, it can affect aspects of your dive. Besides, it never hurts to know a little about the equipment you're diving with.
Cylinders are made out of two types of material: steel and aluminum (actually an aluminum alloy). Each have different properties that make it appealing for different types of construction and use.
Steel cylinders are tough, making them resistant to damage. They also tend to support higher capacities, because of their increased strength. The downside to steel is that it can rust if not properly cared for.
Aluminum cylinders are softer than steel, so they're not as tough, although they do just fine for general use. To compensate, the walls of aluminum cylinders are thicker than for steel. For this reason, an aluminum cylinder is larger and heavier than a similar capacity steel cylinder. They also don't handle overfilling nearly as well.
Despite this, aluminum is the dominate choice of material for cylinders in many parts of the world. The upside is that aluminum tanks are far more tolerant of corrosion from salt water. As opposed to steel cylinders, when a layer of aluminum oxide, or "rust", forms, it acts as a barrier to prevent further oxidation. 80 cubic feet aluminum cylinders are probably the most common type encountered in tropical dive destinations.
Unless you're into metalworking, it can be hard to distinguish between steel and aluminum tanks. It's even worse when the cylinders are painted for enriched air diving. Fortunately, manufacturers in North America are required to stamp certain information on tanks they produce, including the type of material used.
You can find this stamp at the top, rounded part of the cylinder. It is a sequence of letters and numbers stamped into the metal, arranged into two rows.
To determine the type of metal, look in the middle of the first row. These days, you will most likely see either "3AA" or "3AL". "3AA" is the markings for chrome-molybdenum steel, which is practically all steel cylinders made today. "3AL" is the designation for the aluminum alloy used in cylinder manufacturing.
The type of cylinder you use only has one major effect on your diving: your buoyancy. Aluminum tanks are more buoyant than steel, and thus you will require more weight when diving with them. It is worth knowing what you usually dive with in order to compensate one way or the other when diving with a different cylinder.
The usual recommendation is to add about 5 lbs / 2 kg to your base weighting you get from a weighting guide to compensate for an aluminum cylinder.
Aluminum has a particularly annoying characteristic. A full steel tank is negatively buoyant. An empty steel tank is also negatively buoyant, but less so. This is why during a proper buoyancy check it is recommended to use a near-empty cylinder, or add weight to compensate. Aluminum cylinders also become more buoyant as air is consumed, but they change from being negatively buoyant to positively buoyant. This means a full aluminum cylinder will sink while an empty will float. This makes it harder to pin down a perfect weighting for the entire length of a dive.
If you're curious, a standard steel cylinder weights about 30 lbs, while an aluminum tank weighs about 35 lbs. If aluminum tanks are heavier, then how can they be more buoyant? For the same reason they're not as strong as steel: aluminum is less dense than steel, and thus has a lower specific weight.
I hope you learned something about cylinders today. Next time you go diving, take a quick second to identify what type of cylinder you're using. Use this information to adjust your weighting, instead of defaulting to being overweighted. Keeping note of the tank type (when different than what you usually use) in your log book could also be useful.