Does a nitrox tank rust more inside?

Does a tank that is used for nitrox tend to rust more inside requiring more maintenance compared to a tank used exclusively for air?

scubadude79:

Does a tank that is used for nitrox tend to rust more inside requiring more maintenance compared to a tank used exclusively for air?

Click to expand...

Well...
For starters, if you cylinder is rusting, there is something WRONG -- moisture in the cylinder is bad, whatever the source (compressor, atmospheric air, water from the ocean, etc).

The most ubiquitous scuba cylinder is the aluminum 80. Aluminum doesn't rust -- that just isn't a chemical reaction it participates in.

Steel cylinders rust if you get water/moisture inside. I would bet that nitrox would allow this to rust faster, but if you have any rust inside at all, you need to fix the underlying problem...

Luckily, I don't have any rust since my tank is new but I was contemplating on having it cleaned for nitrox. I have seen dive shops that offer rust removal from tanks and was wondering if nitrox has any effect of the life expectancy of the tank.

Thanks!

People often assume that the higher partial pressure of Nitrox equals more rust.

If that were the case, lower pressure steel tanks (1800, 2250 psi) would rust less than 2400 psi tanks that in turn would rust less than high pressure steel tanks (3442/3500 psi). That really is not the case and any differences have more to do with the alloys used - with the steel alloys used in special permit tanks possibly being more rust prone than the alloy used in 3AA tanks.

Similarly, increases in the % of O2 does not by itself make a tank more likely to rust. If moisture is not present, rust will not occur and if moisture is present rust will occur even with air.

scubadude79:

Does a tank that is used for nitrox tend to rust more inside requiring more maintenance compared to a tank used exclusively for air?

Click to expand...

S-Dude,

You asked two questions. As the other guys noted, the answer to #2 is is "no" - nitrox tanks do not require more maintenance than tanks used exclusively for air.

And SB is also correct when he stated that aluminum does not "rust" in the same way that steel does. But another word for "rust" is oxidize, or 'oxidation', and aluminum DOES oxidize. Aluminum oxidation can form inside aluminum scuba tanks, and aluminum scuba tanks need to be cleaned before being filled with pure O2 (when partial pressure blending nitrox) just as steel tanks do because oxidation and pure O2 do not mix well.

But the answer to question #1 is "yes, higher O2 content in a gas inside a scuba cylinder can, under the right circumstances, accelerate oxidation or corrosion"...because oxidation is a product of O2 and can vary over time with changes in O2 concentration.

Here is some text from a website (cited below):
"Aluminum oxidation is only a minor problem, because once a thin layer of oxide forms, it protects the underlying metal from further oxidation. Although the white powdery stuff will not be good for the insides of your regulator, the tank itself is rarely harmed and can usually be cleaned.

Steel oxidation—rust—is more serious because the process can continue deep into the metal. Salt water inside the tank, and storage of the tank under pressure in one position, which concentrates the water and rust in one spot, can even turn a tank into a missile. Lying on its side will be worse than standing on end, because the walls of the tank are thinner than the bottom. A tank charged with nitrox will also be at greater risk because of the greater concentration of oxygen in the mixture." from this source -
http://www.scubadiving.com/gear/2006/10/a-consumers-guide-to-scuba-tanks

No matter what tanks you are using, it is wise to have your tanks visually inspected annually, and to pay close attention to where and how your tanks are filled. Even if you do, irregularities may occur during filling that you may not notice, and they can cause problems inside your tanks. Whether steel or aluminum, visually inspecting your tanks regularly makes sense to keep you safe.

Regards,

Doc

One of the things they stressed in our scuba class was to avoid, whenever possible, depleting a tank completely of air. So long as there is pressure in the tank, and so long as the pressure is recharged from a properly maintained compressor system with dryers, no moisture should end up inside. It didn't take me long in class to feel the effects of the dry air from the tank, which, considering our typically low humidity here in Tucson, is saying something.

If a tank runs out of air, there is a chance it may allow some moisture back in through the valve either from the water you're diving in, or from the humidity in the ambient air. If the tank is filled from a compressor that doesn't have a good working dryer system on the intake, it can end up with condensation inside. Ideally, we want to avoid those circumstances.

Doc Intrepid:

And SB is also correct when he stated that aluminum does not "rust" in the same way that steel does. But another word for "rust" is oxidize, or 'oxidation', and aluminum DOES oxidize. Aluminum oxidation can form inside aluminum scuba tanks,....

Click to expand...

An Aluminum tank is probably already oxidized on purpose by the manufacturer. They probably have taken steps to make the oxidation layer as thick as possible to strengthen the tank.

Basically what Doc Intrepid said. Aluminium is a much more useful metal for scuba tanks because it's lighter, cheaper, and when it does oxidise - the equivalent of rusting - the oxidisation process actually protects the metal underneath. You will notice that brand new scuba tanks are bright and shiny but they oxidise very rapidly and go a dull whitish sort of colour - again, the same process makes aluminium window frames so durable.

For recreational blends, the increased O2 content should nave no real effect on your tank. In terms of maintenance, this depends on how your Nitrox is mixed. If your fill station uses the partial pressure method - which is to flow pure O2 into a tank then top up with air - then the tank needs to be O2 clean - which means it is serviced to a higher standard than a regular tank to ensure that there is nothing combustible that could start a fire in contact with high pressure O2. Some gas blending methods don't require this.

Hope that helps!

Safe diving,

C.

Crowley:

If your fill station uses the partial pressure method - which is to flow pure O2 into a tank then top up with air - then the tank needs to be O2 clean - which means it is serviced to a higher standard than a regular tank to ensure that there is nothing combustible that could start a fire in contact with high pressure O2.

Click to expand...

Why do I picture a Mythbusters episode where they test just how big a bang they can get out of a contaminated tank filled with O2?

Remember; we're professionals. We do this for a living.

scubadude79:

Does a tank that is used for nitrox tend to rust more inside requiring more maintenance compared to a tank used exclusively for air?

Click to expand...

With all other variables being equal logic suggests yes the higher O2 content will oxidize the tank quicker. But the "requiring more maintenance" is not necessarily true