A visual inspection is a careful examination of the internal
and external surfaces of the cylinder. It is designed to find and
correct any internal and external corrosion and damage before the
cylinder becomes dangerous and must be taken out of service. The
inspection must be done at least every year in order for the cylinder
to be filled by a dive center. If a cylinder receives heavy use or
damage is suspected, a visual inspection should be done more than
once a year.
The "visual" is a very simple test, but it must be conducted by a qualified, and competent, maintenance technician. Visual
inspections are usually performed at dive centers, but they can also be performed by a hydrostatic testing facility.
When a cylinder passes a visual inspection, the inspector will place
a sticker on it indicating the date of the inspection and the name and
contact information for the inspection facility. Usually this label
is placed on the lower portion, or bottom, of the cylinder. Before using
a cylinder, be it your own or a rental, look for an inspection sticker and
verify that the examination has been done within the past year.
How does cylinder corrosion occur
The exterior of a cylinder can mask corroded, potentially dangerous, interior damage. A common question among new divers is, "How does
that damage occur?" The answer, in its simplest form, is "water."
Deterioration can also occur from salt deposits, oil vapors,
electrolysis, carbon particles, gouges, dents, scratches, bulges, and
high temperatures.
Water can enter a cylinder and cause internal corrosion in a number
of ways. Moisture can be introduced from poorly filtered air. The
reason diving air is so dry is due to the number of filtering stages
it goes through to remove any water vapor in the air. If the cylinder is
completely emptied, water can enter through the regulator or the
valve. Careless filling is another common way for water to enter a
carefully maintained cylinder. If water is present in the valve
orifice or in the compressor's filler nozzle, it will be injected into
the cylinder.
Draining air rapidly from the cylinder can also produce moisture -
the cause for the damage in a cylinder. Because gas expands as it cools
and can not hold much water, the cold air leaving the cylinder causes
condensation. The resulting water leaks into the cylinder via the
valve.
Corrosion can occur over the entire metal surface or be confined
to a specific area. A light coating of corrosion in a cylinder is
acceptable as long as it does not deteriorate into pitting. Pitting
is a localized loss of wall thickness that can destroy a cylinder's
structural integrity. The pits can be small or large and the degree
of pitting determines the cylinder's continued service.
According to studies, salt water in a steel cylinder can damage
the cylinder beyond repair in less than three months. Corrosion occurs
when oxygen reacts with iron in the metal and forms iron oxide, better
known as rust, which "eats" the metal. Steel cylinders with internal
linings may have hidden corrosion and potentially hazardous pitting.
Inspectors must be careful to look for discoloration where problems
are starting or bulges where moisture has gotten under the coating and
corrosion lifted the lining. Cleaning may need to be done before an
inspection can be made.
Aluminum cylinders don't rust but deteriorate through aluminum
oxidation. Oxidation causes a thin powdery residue to adhere to the
metal, which in some instances protects the bare metal. Even though
aluminum oxidation is usually not a problem, if saltwater corrosion is
left unattended, severe damage can occur. According to the Compressed
Gas Association (CGA), pitting corrosion is the most common form of
attack on aluminum.
Aside from corrosion, the thickness of the cylinder walls can be
reduced by scratches, dents, and gouges. The allowable depth is
regulated by CGA standards and depends on the length, width, and
location of the damaged area. If the walls of the cylinder have been
weakened to the point that a bulge is evident, the cylinder should be
condemned.
Extreme heat can damage a cylinder. A hydrostatic test is required
if the cylinder is exposed to heat over 180' F (82' C). Any cylinder
should be condemned if subjected to fire damage or elevated
temperatures in excess of 350' F (177' C). Elevated temperatures can
be produced by paint baking, galvanizing, or vinyl coating. If a cylinder
has been repainted, the inspector will probably question the owner to
determine what procedure was used.
Electrolysis is another problem for cylinders.
It is a galvanic action in which two different metals fuse together,
or the weaker of the two metals is dissolved. The phenomenon occurs
in moist environments where dissimilar metals react with one another.
Because cylinder valves are made from brass and cylinders from aluminum or
steel, this problem exists in scuba cylinders - especially in
saltwater environments. In severe cases, the threads of the cylinder or
valve can be damaged beyond repair. This is the main reason cylinders,
especially aluminum cylinders, are inspected once a year and the valve
threads lubricated.
A faulty compressor can introduce contaminants into the cylinder.
Poorly maintained compressor filters allow carbon deposits or oil
vapors into the cylinder. The filter's activated charcoal forms a dry
black powder that covers the inside of the cylinder and, in severe
cases, small particles may even be present. While this is not
detrimental to the cylinder, it can damage the valve or regulator. Oil
vapor in a diver's air can present a serious health hazard. Any
cylinder suspected of oil contamination shouldn't be used until the
problem is corrected.
Any questionable problem areas discovered during a visual
inspection are best handled by a certified DOT (Department of
Transportation) inspector followed by a hydrostatic test. Strict
guidelines concerning corrosion and pitting limits in aluminum
and steel cylinders are published by the Compressed as
Association.
Minor cleaning in aluminum cylinders should only be
performed by the dive center's qualified cylinder inspector. To
eliminate minor oxidation, the inspector rinses the cylinder with
distilled water and dries it with warm air. Oil is removed using
a general purpose household cleaner or baking soda mixed with one
gallon of boiling water. The cylinder is rolled or shaken to spread
the solution over the interior, then rinsed and dried. Solutions
of strong alkali (caustic) or acid are not to be used for
internal cleaning. These chemicals can adhere to the pores of
the metal and produce toxic concentrations at depth. According
to the CGA, internal corrosion cleaning should be done with
aluminum oxide pellets, ceramic chips, or other inert media.
Tumbling removes the rust or oxidation without harming the metal
below. After tumbling, a thorough flushing is required to remove
loose residues and cleaning media. Exterior cleaning can be
accomplished by light sandblasting in steel cylinders and shot
blasting in aluminum.
Performing the inspection
To perform a visual inspection, the technician removes the old visual
inspection sticker and checks the cylinder for a current hydrostatic date
stamped into the metal. The cylinder boot is removed and a complete
inspection of the outside of the cylinder is performed. The technician looks
for corrosion, scratches, cracking, gouges, pitting, or other damage such
as exposure to heat. Blisters in the external coating should be probed
with a sharp object to determine if unseen corrosion is taking place.
Where the coating is broken, the surface should be taken down to base
metal and re coated with a compatible coating material.
The cylinder is emptied slowly to avoid forming moisture inside the
cylinder. The valve is then removed. The technician should use proper tools
and procedures to prevent damage to the valve face. Any resistance is
a good indication of corrosion, cross threading, electrolysis, or damaged
threads. The valves threads are examined for damage and this is a good
time to have the valve serviced. The inspector also examines the valve
stem for an indication of foreign residues (oil, saltwater, etc.) that
might have passed into the cylinder.
The cylinder's neck and threads are inspected with a magnifying
glass for signs of cracking, damaged threads, or galvanic action. If
threads are materially reduced or if a gas tight seal cannot be obtained
by reasonable methods, the cylinder is condemned.
Next, an inspection light is lowered into the cylinder. A string
of incandescent, fluorescent or fiber-optic lights should be used. A
single bulb is not powerful enough and causes too many shadows. The
technician checks for pitting, corrosion, oily residue, moisture,
contamination, debris, or other damage. A dental mirror is used to
examine the area around the top of the cylinder.
After the cylinder has been inspected, provided no problems have
been found, the technician replaces and lubricates the neck 0-ring,
lightly lubricates the threads of the valve, and screws it into the cylinder.
A new decal indicating the month and year of the inspection is applied
and the cylinder refilled. During filling, especially with aluminum
cylinders, the cylinder should be checked around the valve, burst disc,
neck, and crown area below the neck for leaks.
When does your cylinder need a
visual?
These are some of the conditions that indicate
when a visual is needed more
than once a year:
Extended time in storage where corrosion could have built up undetected
Rigorous use, especially if the cylinder is constantly filled in a salt air environment such as a resort or charter boat
Whenever the cylinder is completely drained
Bad odors or water vapor coming from the cylinder valve
Anytime signs of corrosion or contamination are detected
When the integrity of the cylinder is in question, such as from gouges, dents, or heat damage
If the filter on the regulator shows a discoloration or if the regulator malfunctions
Rattling or sloshing sounds coming from the cylinder
Anytime maintenance work is done on the valve or cylinder
