Cylinders shall be cleaned for inspection so that the inside and outside surfaces and all conditions
can be observed. This shall include removal of scale and caked paint from the exterior and the thor-
ough removal of internal scale.
Remove the valve manifold, tank harness and hardware in accordance with paragraph 4-16.
If the valve manifold is not removed correctly valves on new tanks can be ruined.
Tank valves should not be removed unless the proper tools are available. Tanks
with tapered pipe threads should be locked into a chain vise. A big, wide-jawed
wrench should be used to remove the valve. The wrench should be used across
the face or valve opening on a valve without flats for a wrench. If the wrench is
used on the side of the opening, the face will be crushed and will form an oval hole.
If the wrench is too narrow, it will dig into the valve. A tank is ready for inspection
when all attachments are removed.
Internal Inspection of Steel Tanks. Using the inspection light (item 6, Section X, Appendix E) make a close
and thorough inspection around the perimeter of the tank continuing down to the bottom. An internal visual
examination should show the tank free from excessive corrosion, pitting, or dangerous defects. The tank
should be free of moisture, rust, carbon, slag, pits, scale, and loose material. A perfect tank is sometimes
called a cherry tank; it is smooth, clean, shiny, and dry.
When water is found in a tank during visual inspection it should be cleaned or thoroughly rinsed in
fresh water, and warm air dried. Any rust. even light rust, should be removed by tumbling. Any rust
will cause problems because it will eventually break loose and contaminate the valve. Then it will en-
ter the regulator and damage the valve seats. Most foreign particles are sharp and will quickly dam-
age the valve and regulator.
A rusty, heavily scaled tank should be tumbled and reinspected. If it is apparent that the rust cannot
be removed without excessive tumbling, and it is heavily pitted; the tank should be condemned.
Sometimes the pits cannot be seen until the scale is removed.
Forms of Corrosion.
Uniform Corrosion. During uniform corrosion, the metal surface corrodes evenly. The metal becomes
thinner in all areas and eventually fails. if uniform corrosion were the only type of corrosion found in
SCUBA tanks, it would take a relatively long time for the cylinder walls to corrode all over to the point
where they were dangerously thin.
Pitting Corrosion is one of the more serious forms of corrosion. It is localized and can quickly cause
holes to be formed in the metal. These holes may be fairly large or very small. In most cases. they
are small with the hole diameter being about the same as the depth. Also, pits are usually hidden by
surface corrosion products. They may extend deeply into the metal body so that the sub-surface
damage is often greater than is indicated by surface appearance.
If pitting occurs in diving cylinders, the damage effect can be very severe. Once
the pit has started to form, the rate of corrosion attach may continuously increase
and easily penetrate the cylinder wall in a relatively short time. Sometimes the pit
may cause local thinning in a large section of the cylinder wall, or even act as a
stress riser that could cause catastrophic failure of a pressurized tank.