Rust and oxygen

Discussion in 'Materials' started by crabberman, May 23, 2015.

  1. crabberman
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    crabberman New Member

    I have a couple large steel tanks l circulate seawater thru for holding crab. I plan to have the tanks sandblasted for painting by myself and crew.

    Because its winter and conditions are damp the blasted steel will quickly flash rust.
    Does anyone know how long it takes for active rust to begin depleting oxygen to dangerous levels. One 'expert' claimed new rust could remove high percentage breathable air in a day which I don't believe.

    I don't want to add cost of several thousand dollars for BA sets to the job or put my crew and myself at risk.

    Thanks in advance
  2. Petros
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    Petros Senior Member

    30 years as a licensed engineer and I have never head of such a thing, you have to ventilate if you have combustion, particulates or solvents, and you have to ventilate enclosed spaces anyway (breathing would use up the O2 faster than oxidation).

    Seems to me that speed of oxidation would depend on a lot of factors: temperature, type of metal (alloying agents in the steel), presence of corrosive agents, etc. Have you searched for industry standards for working inside steel tanks? I recall there were equations for determining the rate of oxidation, which would allow you to determine how fast the O2 was being used up, but that was a long time ago when I was in collage. Not likely any safety or health inspector would accept a "home calculation" if indeed there was a hazard, there would be regulations in place address it if indeed it was a hazard.

    Can't you just ventilate the tank while people are working inside? I would think it would not take much air circulation to keep the air fresh. If you are using paints and solvents in an enclosed area you will have to ventilate it anyway, there are government and industry standards for how much ventilation you have to provide (I have had to address this issue many times), expressed as fresh air at a certain rate (cubic feet per min, liters per min, etc.) based on the size of the enclosure.
  3. waikikin
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    waikikin Senior Member

    Couple of things, many marine coating are surface tolerant & the light haze of orange doesn't seem to worry them- check with paint company.
    Also more importantly is the health of your workers in regards to solvents in the paint- ventilate & check spaces with an air monitoring device- commercial boats should have one & good for engine spaces for CO as well. Oxygen depletion through rust does happen but more in line with long closed spaces.
    Essentially this sounds like confined space work & training & procedures must be in place in civilised countries- & is mandated by law- if one of your guys dies or gets crook the consequences are far reaching. This happened pretty close to home for me & can happen so easy with a spill or similar
    Also critical to paint is dew point of steel- again your paint company should give a chart for temp v humidity, you may find that heating(safe) is required. No point painting over water....
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I doubt if any regulations would approve anything less than fresh air-fed respirators for a job like this.
  5. NavalSArtichoke
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    NavalSArtichoke Senior Member

    From the standpoint of safety, you should always check the oxygen levels in any space below deck, which is confined and not regularly ventilated, before someone enters it to work or whatever.

    Marine surveyors who enter confined spaces often carry a portable oxygen sniffer in their work kits for testing the atmosphere of the space before entry. These can be had for as little as US $100, which is cheap insurance against serious injury or death IMO.
  6. CDK
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    CDK retired engineer

    From my own observations:

    A hollow steel construction, built from new material, will show a significant decrease of internal air pressure due to the depletion of oxygen. This doesn't happen overnight, but if a sealed plug is removed after a month the sound of air rushing in is unmistakable.
    The actual rate of oxidation depends on temperature, moisture, PH and surface condition. My guess is that freshly blasted steel in saline damp air will noticeably lower the oxygen content within a few days.

    A fan placed over a manhole prevents health issues as far as oxygen levels are concerned but I would be more worried about paint solvents.
  7. Nick.K
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    Nick.K Senior Member

    The tanks will be difficult to coat in any case without breathing apparatus. Last year I had to clean out and epoxy coat ballast tanks on a small converted supply vessel. I used a 3M carbon respirator and we had an efficient air exchange via a high volume compressor line lead through the breather pipes but even so it was uncomfortable and I wouldn't do it again.
    The owner of the boat decided to work late evening on his own, he nearly passed out when the compressor cut out...we would have found him there the next day. Always have someone outside!
    If you don't have breathing apparatus could you use the the air fed blasting helmet and an air line off the compressor for air exchange?
  8. Nick.K
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    Nick.K Senior Member

    Yes, absorbed through skin etc, explosion risk and may inhibit the coating cure.
  9. philSweet
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    philSweet Senior Member

  10. jehardiman
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    jehardiman Senior Member

    It all depends on surface area to volume and how much circulation happens. What I will say is that for a blasted to clean metal tank, 4 ft wide x 8 ft high x 40 ft long with no ventilation and a single 15"x18" access, a single day will reduce the O2 level below 12%, which will require ventilation under most OSHA codes. And I have opened spaces with effectively 0.0% O2 after many years and they had rust "hairs" all over the inside.
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Sounds like a sealed steel container could eventually collapse under atmospheric pressure as 20% of the internal pressure was lost through oxidation of the metal.
  12. jehardiman
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    jehardiman Senior Member

    Nah, the iron oxide displaces volume on the inside and the reaction is exothermic so pressure remains constant :D

    A rusting tight void does lose pressure though, as CDK said you can hear them suck when opening the sounding tube or manhole cover. But void spaces are rarely so airtight that considerable DP could be built up at the rate of the reaction. Like any chemical equilibrium, there is a stable point where the off-gassing matches the deposition. There is still oxygen in the atmosphere, it is just not free oxygen.
  13. mastcolin
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    mastcolin Senior Member

    I'd gusee you never studied Chemistry to a high level. if you gave me time I could probably rack my brains to work out how much o2 you would need to make oxide. The most likely cause of the loss of o2 is the bugs/molds etc in the air. They use the 02, giving off most cases. They die and give off more co2. Don't go entering bilge spaces without o2 testing!! Bore yourself by studying anaerobic corrosion just to amuse yourself (ie you don't even o2 to start corrosion in some cases)

  14. Jammer Six

    Jammer Six Previous Member

    He is right in one very limited, specific case: the inside of a steel scuba bottle that has been flooded. Such bottles need to be tumbled before being filled, to remove the rust. Between the time they are drained and the time they are tumbled, at one atmosphere, don't put a valve back on, hook up a regulator and breathe from it.

    Since you have to remove the valve to drain the tank, and breathing from the bottle would require reinstalling the valve, plus the fact that breathing from a bottle that doesn't have any more than one atmosphere in it, why the case was even mentioned in the class is beyond me, but there you go.
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