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Is rust inside a steel tank normal?

Discussion in 'Tanks, Valves & Bands' started by scubadiver888, Mar 10, 2018.

  1. scubadiver888

    scubadiver888 Divemaster

    # of Dives: I just don't log dives
    Location: North America
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    Here is the story...

    I have a high pressure steel tank. I brought it to a shop to get it visually inspected. They inspected it and filled it with Nitrox. Now I have a dozen tanks. It turns out I never used this tank.

    Almost a year later and it is due for another visual inspection. I realize I'll probably never use it. So I decide to sell it. We open it up and look inside. It has a light coating of rust over the inside of the tank. I cannot see any pitting but the entire inside is definitely coated with an iron oxide, i.e. rust.

    My question, is this normal?

    I've been reading a few threads on here about flash rust and making sure you dry out the cylinder before you put the valve back on. Could this happen because the shop didn't dry the cylinder enough? Could it be because it was humid in the dive shop and that was enough to cause the tank to develop rust?

    I have 4 other steel tanks this shop visually inspected. Could there be rust in them as well? Should I be concerned?

    TIA.
     
  2. tomfcrist

    tomfcrist NAUI Instructor

    # of Dives: I'm a Fish!
    Location: Virginia, USA
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    I don’t like rust, but that seems like 1 of 2 things...either they hydros it and didn’t use a good flash rust preventer, or they have somehow put water in your cylinder( either by using a wet fill station, or their air is very high humidity and not filtered properly).

    I’d be pissed.
     
    Johnoly and scubadiver888 like this.
  3. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
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    flash rust is normal if they didn't get it hot enough with the water and blast it out well enough with cold dry air. Not a problem and it should all come out really quick with a whip
     
    Scubis, oldschoolto and scubadiver888 like this.
  4. scubadiver888

    scubadiver888 Divemaster

    # of Dives: I just don't log dives
    Location: North America
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    The tank was hydro tested two years ago. So I don't think it was that.

    They used to do partial pressure blending and tanks had to be O2 clean. Recently they switched to a membrane system. I wonder if the new system is introducing moisture in the tanks. If it is the fill station which is doing it then other tanks I've had them fill with Nitrox will have rust in them too.

    I'm hoping there is a good explanation. I like the owner of the shop. I doubt he'd do this on purpose.
     
  5. BRT

    BRT not a soft touch ScubaBoard Supporter

    15,948
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    Scuba air is really dry but raising the pressure inside the tank raises the dewpoint. If your full tanks have been in temperatures of 20 deg F or below even really dry air will condense some water out. This will cause small rust spots. A general light covering of flash rust is probably from not being really dry after a hydro or being cleaned.
     
  6. scubadiver888

    scubadiver888 Divemaster

    # of Dives: I just don't log dives
    Location: North America
    1,514
    285
    So if I had the tanks visually inspected in the Spring and filled then the tanks were dropped to 20F this could cause water to condense out of the air in the tanks. This condensation could have caused some rust to form in the tanks. Is that correct?

    Does this mean in general, if I have my tanks filled then let leave them in my car at temperatures below 20F they might rust inside?
     
  7. Zef

    Zef Divemaster

    2,815
    2,289
    Not sure about where you are but here in Belgium you can opt for a hydro with oil or with water. The hydro test with water is more expensive because the test facility needs to dry the tank where as with oil they just drain the oil out. For tanks using regular air the residual oil creates a barrier that prevents the flash rust. The little snorkel tube on the inside of the valve prevents any residual oil or any other debris from entering the valve when breathing off the tank.

    For nitrox the hydro must be done with water instead of oil as the tank needs to be O2 clean. If the tank is not fully dried, or the tank is not filled directly after the hydro, or the compressor is filling with air that is not completely dried, then rust can form. This is not uncommon and is one of the reasons for the periodic visual inspections.

    -Z
     
  8. Johnoly

    Johnoly ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
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    Steel tanks don't have to flash rust. A good shop takes the extra time and cost and knows how it prevent it from happening. Cheap isn't always worth it, especially if you just lost the sale of the tank(s) because of it.
     
  9. JamesBon92007

    JamesBon92007 Contributor

    # of Dives: 2,500 - 4,999
    Location: Southern California...too far from the ocean
    3,209
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    What kind of oil do they use? I'm always careful not to introduce anything into my tanks that might not be good to breathe. Oil does not sound good.
     
  10. BRT

    BRT not a soft touch ScubaBoard Supporter

    15,948
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    I was getting little rust spots in my steel tanks on the bottom. Couldn't figure out how because I am careful with my compressor and filtration. Finally I looked up how dry scuba air is supposed to be and ran that through an online calculator and found that at 3500 lb Grade E air will condense at about 18 deg F. This has been awhile so the absolute numbers might not be perfect but I realized that when I went over the pass to dive the ocean the temperature was often cold enough to condense water out of scuba air.
     

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