Diving cylinders – the danger of internal corrosion

Steel diving cylinders are still being found with dangerous levels of internal corrosion. This unseen weakening of the cylinder's structure can lead to an explosive failure which can inflict serious injury. Divers, those around them and the people who fill cylinders are at risk.

Research has shown that corrosion rates in cylinders containing compressed air or nitrox can be more than 100 times faster than normal. If fresh water is present in the cylinder this can mean the loss of more than 2 mm of the thickness of the cylinder wall within 6 months. This is increased to more than 5 mm in the presence of salt water. Diving cylinder walls are generally between 2 and 5 mm thick. Being in date for test (2.5 years) is no guarantee of safety if water has been allowed to enter the cylinder.

There are often no obvious external signs of internal corrosion and the amount of water needed is very small and might not be noticed. Divers therefore need to make sure that water is not allowed to enter cylinders and should take the following precautions:

• Do not leave empty cylinders with their valves open. Moisture will enter from the outside air
• Before connecting a cylinder to a charging line always momentarily crack open the cylinder valve and gas supply valve to blow out any moisture in the cylinder valve or supply line
• Avoid emptying cylinders underwater – if you do, there is a good chance of water getting inside
• Do not charge cylinders (such as delayed SMB cylinders) that have been emptied underwater, by decanting from your main cylinder – water from the empty cylinder may enter the 'charging cylinder' when they equalise
• Only fill cylinders with air/gas which meets the criteria in current British and EU standards

The potential consequences of this unseen corrosion can be serious. If you consider there is any chance of water having entered your cylinder have it internally inspected before it is filled.