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Why are steel cylinders uncommon in tropical diving destinations?
Steel Cylinders versus Aluminium Cylinders use in tropics CSM. Few dive centres that operate in tropical conditions use steel scuba cylinders. In a hot, humid, climate, it is a significant challenge to produce very dry air fills. Additionally experience shows that some dive centres care more about their bottom line than the quality of breathing air their compressors produce. This is exacerbated when poorly educated operators with little training are maintaining compressors and filling cylinders. Â
It is common knowledge that unprotected steel is susceptible to corrosion – steel scuba cylinders corrode significantly in tropical conditions requiring a significant amount of internal cleaning to remove corrosion from moist air fills and the external surfaces will corrode significantly, if the protective paint coat is damaged. Even a scratch will allow the corrosion process to take hold.
refinishing to protect the surface from event maintenance expensive equipment such as automated rotary sand blasters to remove the corrosion.
Observations from decades using aluminium scuba cylinders in the tropics:-
T6061 aluminium cylinders are incredibly resistant to corrosion and rough handling in comparison to painted steel types.
Most dive centres use unpainted aluminium DOT-3AL3000 cylinders. They survive daily dive school use with apparent impunity for many years.
In comparison with steel cylinders, T6061 aluminium scuba cylinders are virtually indestructible. Provably tougher than steel nails in a marine environment and need virtually no maintenance in comparison with steel cylinders. Maintenance of dive gear in remote locations is a major headache for dive center operators, thus aluminium cylinder are loved by everyone.
What about buoyancy? DOT-3AL3000 Aluminium cylinders weigh approx. 14.5Kg. They’re neutrally buoyant when half full by design. About 1kg negatively buoyant when full, 1Kg positively buoyant when empty. Lightweight 3,000psi steel cylinders have almost the same weight and buoyancy characteristics.
Finally, if aluminium cylinders are internally contaminated, they can be easily cleaned by water jet cleaning from a USD200 high pressure water jet cleaner purchased from Amazon. Unlike steel cylinders that will need to be cleaned using a specialist USD10,000 shot, slag or garnet blasting machine to sort out… only to later fail inspection after sand blasting due to cylinder wall thickness issues discovered by Ultrasonic Thickness ‘UT’ inspection or hydrostatic testing processes.
Advice to dive centers in the tropics:-
Stick to using unpainted T6061 aluminium alloy scuba cylinders.
Leave the use of steel cylinders to the gas guzzling dry suit wearing scuba divers found in cold waters.
Advantages of steel cylinders
The Chrome Molly Alloy Steel used to manufacturer scuba cylinders has typically twice the tensile strength of heat treated T6061 aluminium alloy. Thus steel is the preferred material for making really high pressure 4500psi/310 bar scuba cylinders. These high pressure steel scuba cylinders are famously heavy, but that’s not a problem to those who use them.Â
This is since when diving cold water, the extra weight of high pressure steel cylinders helps to offset the buoyancy of the dry suit.  The 50% extra gas the high pressure steel cylinders hold also makes them an attractive choice. Steel is the preferred metal for making high pressure 300 bar and 450bar air bank cylinders plus all industrial gas cylinders used worldwide. Often influencing the decision to use steel for Industrial cylinders is that steel is much cheaper by weight than aluminium alloy.
Scuba Cylinder Manufacturers
Aluminium Cylinder Manufacturers
Steel Cylinder Manufacturers
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Author:   Stephen E Burton. Chartered Engineer.
