Yes they expire due to rust both inside and outside. However, for best cylinder don't use iron gas cylinder bottles. Buy aluminum. Get a plastic and wood pallet put on top a shade from sun.
Be careful of the plastic LPG gas cylinder sold nowadays. A lot of people patronizing these types because it's less heavy and lighter.
Which one is plastic cylinder again? Do you even know that gases has a very high kinetic energy and is capable of expanding plastics. Why are petrol and gas tankers made of iron instead of plastics?
Gases don't produce fire from the cylinder, it's just combustion of the gas jetting out of it... Just a hint..
Avoid explosure to rain and direct sunlight. LPG gas cylinder bottles rust mostly due to moisture around them. Always try to prevent salt water around gas cylinder bottles. This is the number one enemy to rusting. However, the fact remains, no matter what may be done to prevent rusting, it is bound to happen sooner or later. All these tips only helps delay the rusting. It is inevitable, but you can prevent deep rusting that can cause pinholes leading to leakages. Consider respraying your gas cylinder bottles each time you noticed a rusty part, as this will prevent further rust.
Most times, gas cylinder bottle manufacturers uses galvanize method to coat, as iron, with zinc. Galvanized iron: iron that is coated with zinc to protect it from rust.
Just to add another tip on top of that.... whenever you suspect there's a faulty cylinder (gas leakage) please avoid turning on the lights, switch them of ASAP and take the cylinder outside in open air. Then return to your distributor or supplier for exchange. Do not attempt to use, as this is very dangerous.