How to prepare a Aluminium tank(6L) for pure O2 fill

[Wookie]

Any mechanical abrasion has the potential to break the glaze formed on the inside of the cylinder during it's manufacture by the release agent used in manufacturing. That glaze protects the aluminum itself from from anything corrosive (like salt water). If you have to use tumbling media to remove a contaminant like DC111, you stand a real chance of breaking that glaze. Lighter contaminants like compressor oil or salt crystals which fluoresce like crazy can easily be removed by simple wash and rinse with simple green or blue gold. For anything tougher I use steam. If that doesn't work, they make a nice bell.

 

[Superlyte27]

Gotta agree. I can't ever remember a time that I needed to tumble an AL tank for O2 clean.

 

[elan]

So what would water do in the tank anyways? Even if we break that glaze the aluminum oxyde will form protecting from further oxydation. I'm yet to see any water in my the tanks anyway. The only time I saw one was in my inflation bottle and in that case the glaze did not protect it even from fresh water as I got this sticky hydroxide formed.
Even given you are correct about glaze thing I still see no point of condemning the cylinder for that reason. But what do I know

Any mechanical abrasion has the potential to break the glaze formed on the inside of the cylinder during it's manufacture by the release agent used in manufacturing. That glaze protects the aluminum itself from from anything corrosive (like salt water). If you have to use tumbling media to remove a contaminant like DC111, you stand a real chance of breaking that glaze. Lighter contaminants like compressor oil or salt crystals which fluoresce like crazy can easily be removed by simple wash and rinse with simple green or blue gold. For anything tougher I use steam. If that doesn't work, they make a nice bell.

---------- Post added July 30th, 2013 at 10:03 AM ----------

I have to do it after hydros, simple washing does not remove some crap that hydro facilities leave there.

Gotta agree. I can't ever remember a time that I needed to tumble an AL tank for O2 clean.

 

[Wookie]

The way aluminum oxidizes is to pit, where steel usually scales. A pit will work it's way through a 5/16" ship hull that has lost it's protective layer in a matter of months (weeks if electrolysis is present, days if there is any copper in the mix). A scuba cylinder is somewhat thicker than 5/16", but the point remains. You can pit through an aluminum cylinder is less time than annual VIP's will catch in just the wrong conditions.

It's not the water in the cylinder, it's the ions associated with salt water. Aluminum loves to give up electrons, so if there is a place for them to go (the salts in seawater) they are on the move. This weakens the aluminum, and the presence of a high oxygen atmosphere accelerates the process (lots of free oxygen to keep forming aluminum oxide).

The keys are, don't break the glaze, and don't allow salt water in your aluminum cylinders.

 

[elan]

I guess we do not see those issues as we mostly dive fresh water.

 

[Superlyte27]

So what would water do in the tank anyways? Even if we break that glaze the aluminum oxyde will form protecting from further oxydation. I'm yet to see any water in my the tanks anyway. The only time I saw one was in my inflation bottle and in that case the glaze did not protect it even from fresh water as I got this sticky hydroxide formed.
Even given you are correct about glaze thing I still see no point of condemning the cylinder for that reason. But what do I know



---------- Post added July 30th, 2013 at 10:03 AM ----------

I have to do it after hydros, simple washing does not remove some crap that hydro facilities leave there.

In the event that water stays in the tank, the tank will suffer from some oxidation, but even with a lot of water, the oxidation is limited and minimal.

The only crap that a hydro facility leaves in a tank is water. To hydro a tank, you fill it with water to the top, screw in a valve of sorts, then pressurize the tank. Then you dump the water out. That's it. Oh, and it's clean tap water.

---------- Post added July 30th, 2013 at 10:25 AM ----------

The way aluminum oxidizes is to pit, where steel usually scales. A pit will work it's way through a 5/16" ship hull that has lost it's protective layer in a matter of months (weeks if electrolysis is present, days if there is any copper in the mix). A scuba cylinder is somewhat thicker than 5/16", but the point remains. You can pit through an aluminum cylinder is less time than annual VIP's will catch in just the wrong conditions.

It's not the water in the cylinder, it's the ions associated with salt water. Aluminum loves to give up electrons, so if there is a place for them to go (the salts in seawater) they are on the move. This weakens the aluminum, and the presence of a high oxygen atmosphere accelerates the process (lots of free oxygen to keep forming aluminum oxide).

The keys are, don't break the glaze, and don't allow salt water in your aluminum cylinders.

Wookie, it takes a lot to kill an aluminum tank. I can show you aluminum tanks that will pass VIP and Hydro with salt water in them for several years. However, I can also show you steel tanks with salt water in them that are condemned after just a month.

 

[Wookie]

Wookie, it takes a lot to kill an aluminum tank. I can show you aluminum tanks that will pass VIP and Hydro with salt water in them for several years. However, I can also show you steel tanks with salt water in them that are condemned after just a month.

Interesting, because I acquired a set of steel 120's that had last been filled at hydro, and had an inch of standing water in them for 5 years. I stripped them down for hydro and opened them, and thought they were a lost cause. I had the facility tumble them prior to hydro and the rust and crap came out of them and they were not pitted enough to fail hydro and vis. OTOH, I had 32 aluminum 80's that a crewmember VIP'd one year and left gobs of DC111 in. I whipped about 15, then used a tumbler on the rest. The following year I VIP'd them again, and the places where the glaze was scratched had these large balls of AlO2 growing on them. Yes, I could have tumbled them every year and it would have likely been ages before they corroded enough to fail on the pit gauge, but regardless, I wouldn't trust them with nitrox.

 

[Superlyte27]

Wookie, I can show you pictures of an LP85 we sold last year. A month later when we were filling it, we noticed some orange on the o-rings. We bled it down and did a VIP. We found nearly a quart of water in the bottom of the tank. We dumped it out and the tank looked like a volcano had erupted inside. There were rivers of dried rust as large as 2" high. We cut the tank in half (the tank was obviously trashed) and use it as a display for why you shouldn't bleed your tank down to zero.

I wouldn't have believed it if I hadn't seen it. I've never seen anything like that at all on an AL tank. Bear in mind, I'm a steel tank fan. I believe they are better in every way except cost. Unless you fill them with salt water