Internal rust & flash rust in steel tanks

[FredT]

Tumbling by nature is great for the side walls, but bottoms get little action.

An easy way to get the bottom:

1. Cut the end off a LP hose, or rig a 3/8" OD plastic air line (like used for air brakes) or 1/4" OD stainlees tube to attach to a regulator first stage.
2. Toss a couple handfulls of tumbling media or small stones into the tank. Even silica based beach sand works if you intend to wash the tank later. The pebbles sorted out of masonry sand before mixing mortar are about right if you can get it. That stuff is a bit smaller than pea gravel.
3. Insert the hose or tube to within about 3" of the bottom of the tank and loosely pack the annular around the hose at the top of the tank with a sock or other open weave material. Air needs to come out, but you want the rocks to stay put.
4. Put the reg on a full clean dry tank and dump it through the one to be cleaned while "wiggling" the hose around to be sure to clean the entire base of the tank. You'll need to hold the sock in place, and a good face shield is a good idea.
5. Inspect and repeat as necessary to see good metal at the bottom of the tank, then retumble for a few minutes with a phosphoric acid mix to "passivate" the surface and kill any microscopic rust cells remaining.


FT

 

[pasley]

pdoege once bubbled...
The shop in question is willing to pass them with the rust, I doubt that pits will stop them....
Peter

Hmmmmmm, and you still trust them?

I do not know the standards for a Visual Inspection, but I have trouble believing that it allows for any rust since by the time a year passes the rust that was in the tank will have eaten it to the point of pits. Not to mention the potential for a flake of rust to get into the 1st stage and clog a hole you might want open.

In case anyone wonders what rust in the bottom of a steel tank can do, here are some pictures of a steel tank with rust and the results.
http://www.diveshop-pr.com/pages.dir/engl.dir/tankexplosion.html

I would look for a new shop to inspect my gear, and tumble it too.

Just my $0.02.

 

[pdoege]

I'll give it a try.

As for the shop, I couldn't care less about their standards. As long as they have a tumbler I can use.

Peter

 

[MikeFerrara]

From memory...

Isolated pits have a depth spec...(0.03 in?)

Line corrosion has a length limit (I think like six inches)

Wide spread pitting can't cover more than 25% of the surface regarless of depth.

When they look like you describe, I don't even fool with them cuz they almost always fail anyway after all that work.

I wouldn't buy a used tank without looking inside unless the purchase was contingent on the tanks passing vis and hydro.

 

[FredT]

Loosely install a valve and lift the tank by it a couple inches Whack the tank soundly with a smooth faced hammer under a pound. You should get a a clean tone. If the tank sounds "dead" you may have some serous problems. Be careful not to dent the tank! You want a good tone, but you aren't trying to bend anything.

Now set the tank valve side down on a clean sheet of paper and tap it lightly a few times.

Next lift the tank up and see how much rust falls out through the neck onto the paper.

Much (>2 tablespoons) indicates serious problems. Many steels will have a half teaspoon or so drop out.

Rust bulks amazingly. Lots of rust doesn't mean lots of metal loss. Inspection is done on the clean tank, not on the dirty or rusted one.

 

[MikeFerrara]

FredT once bubbled...
Rust bulks amazingly. Lots of rust doesn't mean lots of metal loss. Inspection is done on the clean tank, not on the dirty or rusted one.

True but you should see how some people spit fire when you tell them that you still want to get paid for cleaning the tank that you failed. LOL

 

[DA Aquamaster]

A light dusting of flash rust in the bottom of a tank is no big deal but I would not use a tank that has any significant rust in it until the rust can be removed and the tank itself inspected for pits and other defects. If you have a shop that is willing to pass the tank in it's present condition - you need to find a new shop.

 

[scubafanatic]

...thank goodness you're in FL and I'm well away in TX....I should be far enough from ground zero when those tanks detonate.

You sound a little too desperate to 'save' your scuba paperweights....chalk it up to experience and let it go...looks like you now know why you got such a good 'deal' on them.

Just because you can slip the tanks past that excuse for a scuba shop you've found doesn't mean you should...I assume you've explored the legal ramifications if/when there's an 'incident'? (not to mention the ethical issues of exposing innocent bystanders to such hazzards.)

Karl

 

[pdoege]

I re-tumbled the tanks using a "copious amount" of abrasive.

About 1/2 full using simple green cut with water as a surfactant. Then a few hours on a tumbler.

Did the trick.

The tanks passed another shop's vis while I stood over the owner's shoulder to make sure things were done right.

Thanks to Mike F and FredT for the helpful replies.

Peter Doege

 

[quimby]

check out global mfg. for your tumbling info at www.gmcscuba.com/ and check out their library. They have all the info you can stand and its solid