23 Aug 95 09:56, Ralph Buttigieg wrote to Terry Smith:
 RB> If I understand this right, steel tanks have the advantage of weighing
 RB> less for the given capacity and are less bouyant, therefore divers need
 RB> less lead in their weight belts. But what are the disadvantages? Do they
 RB> cost more?
Superior buoyancy characteristics are steel's biggest advantage.  They also 
come in a better variety of sizes than aluminum tanks.  However, there is one 
major disadvantage......they can rust.
Steel tanks, even painted steel tanks have a galvanized coating on the 
outside which will prevent rust.  Unfortunately, it's not possible to 
galvanize the interior of the tank (for health reasons....zinc is a heavy 
metal and does nasty things to your liver).  If moisture enters your tank, 
either by being pumped in by a compressor with saturated filters or by 
draining the tank empty so no pressure remains inside to keep it out, then 
the metal oxidizes (or rusts).  The rust can have two deleterious effects:  
First it weakens the walls of the tank, making catastrophic failure a 
possibility if it goes far enough. This is a fairly rare event.  More 
commonly, the rust particles shear off and pass into your regulator, where 
they will do no good.  There is a sintered filter which is supposed to catch 
rust, but I've rebuilt hundreds of regs in my day, and I can tell you a 
filter can only do so much.
There are those who will correctly point out that aluminum tanks also 
oxidize, but the characteristics of aluminum oxide are completely different.  
Rather than shearing off to wreak havoc in your reg, it tends to be tightly 
binding to the walls of the cylinder, effectively protecting the the walls 
from further oxidation.
The final thoughts I have on the matter concern the electrolysis which may 
occur between the tank and tank valve.  The contact of any dissimilar metals 
in the presence of saltwater may cause electrolysis.  Since we visually 
inspect tanks every year, only a moron would forget to lubricate the neck of 
the valve. By doing this annual lubrication, you effectively negate the 
consequences of electrolysis (which would be for the valve to "freeze" onto 
the tank).
I don't work in the diving industry any more, as I've moved on to better 
paying pastures, but for my money aluminum is the way to go.  Your mileage 
may vary.
Jay
PADI M-9033
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