Inside of steel tanks black?

[moose_grunt]

I just purchased a couple of used steel tanks, recently hydroed and vip'ed. I dropped a light into one, and it's black. No rust, just black. I asked the seller, and he said that was a byproduct of acid cleaner/rust inhibitor, nothing to worry about. The price was good enough that I'd still be getting a deal even if they had to be tumbled, so I bought em.

I did take a tank inspection and oxygen cleaning class, but this never came up, nor is it in the materials that I received. Searches here and on TDS indicate that acid cleaning with phosphoric acid leaves a black residue, but do not mention how to remove it, or even if it is necessary.

Do I need to have these tumbled, or am I good to go?

Thanks,
Jim

 

[tbone1004]

if you aren't O2 cleaning them I wouldn't worry about it. If you are concerned, build a tumbler rack for a treadmill, and send them for a ride.

 

[Superlyte27]

I've seen this during PP Blending when the O2 ignites. The tank contains the blast, so the filler never even knows it happened. He fills as normal, and later can't figure out why his O2 content is so goobered. But he didn't pull the valve and look inside the tank.

 

[Diver0001]

did you turn the light on? xD

Superlyte probably had the answer, though.

R..

 

[Superlyte27]

Diver0001, that's funny. LOL

 

[Scared Silly]

The acid cleaner contained phosphoric acid which when it comes into contact with steel turns it black. Phosphoric acid removes rust, scale, and oxides, and other crap. It is benign. A light whipping will clean it up.

 

[wedivebc]

This

From Global MFG
42070... Acidic Tank Cleaner
A blend of phosphoric acid and certain suspending agents, this acidic cleaner will expedite
the removal of scale, oxides, concretions, and other stubborn contaminants from inside STEEL dive tanks. It’s biodegradable
and can be
fl ushed down drain systems without harm. It has FDS, USDA, CDA, CDF approval and also meets many
mil-spec requirements. The acid concentrate, when diluted to 5 - 10% strength for use, will speed the process so that only
1-3 hours will be required for completion. NOTE: Use this cleaner on STEEL TANKS ONLY; acids will attack aluminum
models! See Bulletin #42130 for additional information - available at Dive Shop Supplies - Home - Global Scuba Mfg. of Texas. HAZMAT
shipping required at additional cost.

 

[Storker]

I've seen this during PP Blending when the O2 ignites. The tank contains the blast, so the filler never even knows it happened. He fills as normal, and later can't figure out why his O2 content is so goobered. But he didn't pull the valve and look inside the tank.

I've got a slight problem accepting this. Where does the black color come from? There's no carbon involved, and usually black residue after a pyrotechnical event is soot, i.e. carbon. Is it the steel itself blackening? How? What's formed that gives the black color?

 

[halocline]

I've got a slight problem accepting this.

Me too. No way is it an O2 ignition.

 

[ChemGuy]

More than likely a ferric phosphate from using the phosphoric acid. Normally it's benign but I don't know about scuba applications.
It will provide some additional corrosion protection though.