How much rust after hydro is acceptable?

[Gilldiver]

Next time you could take a drive over the Lake Havasu and let Joel do them at TDL.

A whip will do it and is faster and cheaper than tumboling in most cases. For a private diver a whip should last a lifetime.

 

[AbyssalPlains]

It looks like it is indeed flash rust. I was able to get rid of most of it using a plastic scrubbing pad duct-taped to a wooden pole. Now the rust is gone except in the areas where I couldn't reach (under the dome of the tank). The remaining rust is a very light, golden sprinkling. Should I just not worry about it?

 

[captain]

It looks like it is indeed flash rust. I was able to get rid of most of it using a plastic scrubbing pad duct-taped to a wooden pole. Now the rust is gone except in the areas where I couldn't reach (under the dome of the tank). The remaining rust is a very light, golden sprinkling. Should I just not worry about it?

No, if the tank is dry and only gets dry air put in it will be fine.

 

[oxyhacker]

Good story. If they were able to tell it was a 2400 psi tank they would have been staring straight at the DOT specification, which of course indicates it is USA-legal. I think at that point I would have grabbed the tanks and left since the guy obviously didn't have a clue, and those are the sort of people who screw up tanks.

As far as rust goes, there really isn't all that much in the regulations (in this case, CGA C-6) about it. If the rust is causing actual pitting, of is bad enough that the inspector cannot examine the inside of the tank for pitting or other damage, then its too much. But a light blush, or even worse, is OK.

Flash rust or water in the tank is always irritating, but pretty common given how many tanks a hydroshop has to do to make a decent pay. Of course it was their fault - tape is more likely to keep residual water from the hydro in, than let atmospheric moisture in. But most industrial customers wouldn't even complain, so you can't really blame the shop for trying to blow you off.

What you need, as others have pointed out, is a whip. Let's you do a light derusting anytime you like, and also useful if you ever get into O2 cleaning. You can buy them, but they are really easy to make. Take some braided steel cable (preferably stainless), like the stuff they sell for garage doors, and hose clamp a bunch of 18" to 24" lengths (doubled over) to ta 30" or so 5/16" or so steel or aluminum rod. Get a 1/2" NPT pvc bushing from the hardware store, screw it into the neck of the tank, chuck the whip into a drill, and flail away. Can be used wet or dry, or even with a phosphoric acid solution for more serious rust.

I would never take a steel tank in for a hydro or visual without doing a quick inspection and cleaning as necessary myself. Too easy to fall into the "hydro trap" where a tank has typical neglible flash rusting and the shop insists on tumbling it, then, insists that it needs a hydro since it was tumbled (which is bull****, but that's another story). So much easier to just prep it yourself.

Here is a taste: When I dropped my Fabers off at the hydro facility explaining they were low pressure tanks, the guy said , "no, they are high pressure tanks; see, here it says 2400 psi. Hmmm, they're made in Italy, so I don't know if these are even approved for use here."

I thought, "Oh this is going to be fun."

 

[AbyssalPlains]

Took the tanks to a local cylinder shop this morning and had them look at it. The guy said "They look real good" and if there was any remaining flash rust, he either couldn't see it or I should not worry about it. ;-))

I'm a happy camper now.

 

[oxyhacker]

Their aren't really any regulations against sandblasting tanks that I can find, except in that HP cylinders have minumum wall thickness requirements, so anything that might remove enough material to exceed it - grinding, shot blasting, or abrasive blasting with high powered industrial blasters - is suspect. But this is not as much a concern on the inside since the size of the neck opening limits how powerful a blaster can be used. Erosion of the neck threads remains a possible problem with any blaster, though, since the air and abrasive have to be able to escape. So while I wouldn't worry about using a blaster occasionally I think a whip or tumbler are more appropriate for regular use. I would worry a lot about someone trying to use a blaster not suitable for the job, by shoving it up the neck hole and blasting away, but more out of fear of damaging the threads than removing too much metal from the tank walls.

This is all assuming a steel tank. I wouldn't use a blaster on a aluminum tank without permission from the manufacturer.

BTW there is a quick-and-dirty method of blasting tanks where you dump some abrasive in the tank, insert the end of an HP hose (or is it the LP hose?), wrap a rag around the neck of the tank and the hose to act as a crude filter, and then open the tank valve and blast away (wearing gloves, protective clothing, and, of course, eye protection!). Many old timers swear by it, for getting the bottom of the tank especially.

Could someone knowledgeable please comment, but aren't the 'types' of blasting allowable very limited, and exceeding them means condemning the tanks? From the comments so far, how much confidence would one have that the facility in question knows how to do things properly, or what questions would you ask to try to determine that?

 

[DA Aquamaster]

As Vance indicates, some shops think (or at least state) a new hydro is needed after tumbling. The theory is that tumbling removes metal and weakens the tank. For a steeel tank you could literally leave it on the tumbler for weeks or even months and not harm the tank.

Like Vance, I prep my tanks before taking them in for a hydro test. That way I know they are clean inside and will pass a VIP (since I just did one). I also use the plastic storage plugs that are used when shipping new cylinders, or tape them if the test facility prefers that. Personally I think it is easier to pop out or screw out a storage plug than it is to remove duct tape, but not all testers agree.

Of course I often have to tumble the tanks again when they come back if they are flash rusted (I am picky and most of my tanks are O2 cleaned, and I prefer to remove any rust particles that could otherwise become entrained in the valve or reg.

However for general use, flash rust is not an issue as long as it does not obscure or prevent a proper VIP, does not (potentially) hide pits or is not of sufficient thickness that it may start to attract moisture that then leads to pitting.

 

[moinaman]

I am a certified tech at my dive shop and I service all of our customer tanks. Steel tanks become flash rusted after hydro, so we tumble them lightly with a week acid and ceramic chips to get rid of the rust. Then we completely rinse them out with water and a second rinse with a rust inhibitor and then dry the tanks on a warm air tank drier. This proper care ensures that our customer tanks will be in great condition for the life of it's use. I hope this helps you.

 

[duckbill]

Why doesn't your hydro shop just use the rust inhibitor and save all the unnecessary trouble?! Besides, I thought a tank had to be rehydroed after tumbling.

 

[halocline]

Besides, I thought a tank had to be rehydroed after tumbling.

Where did you hear that one?