How Much Water is Needed to Rust a Hole in Steel?

It depends on the acidity and the dissolved oxygen present. If there is no oxygen and the pH is above 8.5, practically no corrosion will occur.

http://en.wikipedia.org/wiki/Rust

Horace40,

Where do you get your information? I'm a diver. I haven't been diving for almost 20 years, but back when I went diving the tanks were filled to only 3,000 pisg, and during filling they were placed in a tank of water to keep them cool. Under the rules of sport diving, the tanks were visually inspected once a year, (I designed a light on a wire that was dropped into the tank so I could see if any rust or containments were in the tank) and once every 5 years the tanks needed to be Hydrostaticly tested. If well maintained, a tank would last almost forever... so, why the concern?

I am also a diver, but it hasn't been that long since I was last diving (only about 10 years). There was a move by some of the dive equipment companies to up the PIS in the tanks to 3,500 or maybe 4,000 PSI - most of the US Divers regulators could take the extra PSI (engineered for upgrade?) if you had the fittings changes out. The point was to have smaller tanks (80 cubic feet of air in a smaller tank, like a 50 I think) or more bottom time (full-sized 80 cubic foot tank with about 130 or so cubic feet of air).

My father had steel air tanks for diving, and they had annual visual inspections and hydro's every five years. They didn't go away from steel because of rust or corrosion, they went to aluminum because it is much lighter. Those old steel 80's weighed a ton!

The important thing here, besides the annual visual inspections and the hydro every five years, is that the air stations aren't using your average cheap compressor. They are using very nice equipment (I believe with air drying equipment) and the installations have strict requirements that are stringently inspected (at least if they are part of the Clean Air (?) program).

Also, as mentioned below, firefighters aren't using steel and haven't for quite some time. So with visual inspections, hyostatic testing, and good fill practices greatly reduce any rust/corrosion concerns.

Talking about corrosion in high strengh steel:

1. If you have condensation in the cylinder, it's enough to cause rust. Even at minimum quantity.

2. If does not condense, still have a chance for corrosion if unprotected.

3. Cylinder walls have enough safety margin to withstand corrosion between inspections. That's why the standard for pressure cylinders exist.

4. I don't know about diving cylinders, but in other industrial gases, even air, the moisture is normally kept as low as possible to avoid contamination and corrosion.

5. High strengh steel, due to the alloy elements, tends to suffer from pit corrosion. Once starter, due to the oxygen potential difference between the corroded area and the rest of the cylinder, it is localized and accelerated. As you may know, a localized corrosion is much more dangerous to the most commom brown-covering-all corrosion type, which acts like a protective layer (compared to the other type, of course).

6. And, yes, the corrosion would start in a region that is located below the cylinder in its storage position, where the water accumulates (difference of oxygen potential). I have worked with oxygen cylinders for aircraft, and they have extremely tight requirements for cleanlyness and dryness of the gas and cylinder during inspection. And, during inspection, they are also turned upside down and cleaned with dry ozon, to remove moisture. Then, filled again with O2 super dry.

Im a vol firefighter, the air tanks we use are not steel, their aluminum inner shell, wrapped in carbon fiber, fiberglass and resin, it makes for a lighter/stronger tank. Added bonus is none of the corrosion problems you speak of.

see this page 9 of http://www.scotthealthsafety.com/PDFs/FireServiceCatalog.pdf

i could see corrosion being a problem in industrial or agricultural tanks though where steel is used.

yes there is a relationship. the more water the more pressure on different parts of the tank to rust, so more rust. What you should ask is if there is a way of filling the tanks that could extend the life of the tanks. In that case there is. Contact your local welding gass supplier about having the tanks inspected and filled properly with a dry air or compressed gas.

way too many variables for a simple formula. service temp,temp cycles, partial pressure of water, surface treatment, material,, vibration, stray electrical currents,etc. WAY too complex for 'formula' to predict inspection intervals.

I spent 10 years doing Research into rust generation for a major steel company. I agree with you whole-heartedly. There are way to many variables for a simple all-inclusive answer.

As a sales manager for a company which markets corrosion control products I would say outdoor fuel oil tanks are a good example of the variables that can exist to cause corrosion in steel. The condensation present depends on the outside conditions as the temperature changes throughout the day, heating and cooling the tank. As the moisture condensates on the upper inner walls and finds its way to the bottom of the tank the acidity levels increase and corrosion occurs. A Micro-organism "the bug" that eats fuel oil may also be present as well, increasing the acidity levels to such an extent that fuel oil tanks have been known to fail in eight months to a year. If not caught in time the outside visible corrosion will be holes in the bottom 6" of the tank. Many tanks are now designed with doublewalls or made out of fiberglass to combat this problem.

Even a small amount of moisture and you'll have a problem at the same spot.

Thanks all.

My question is related to air quality for respiratory protection. Divers and firefighters need a supply of pure air to keep them alive. The air is stored in cylinders and carried on their backs.

There are standards for air quality that say the air should be free from toxic substances - and dry enough to prevent condensation in the cylinders - at all times in all working conditions. Unfortunately the standards are not observed or enforced to the degree necessary to ensure the cylinders are filled with pure air.

It is the nature of air to heat up on compression. Any water vapour in the atmosphere when sucked in remains as vapour in the compressed (and unless removed by at molecular level by desiccant dryer techniques) there always remains enough vapour to condense when it cools in the cylinder (slowly in atmosphere or rapidly if immersed in cold water).

Either way the vapour condenses on the walls of the cylinder to form water - that then over time causes rust - but short term can freeze in ice cold weather in the airlines and valves - and as rapid expansion causes air to cool the water can freeze in the expansive valve and demand valve - either stop the air or waste it rapidly.

It is also in the nature of compressed air for the water to evaporate as pressure falls - so that empty cylinders appear dry when inspected.

I just wondered how to calculate the rust that can be made from a known amount of water in the breathing air in steel cylinders.

When you consider 'breathable` air the requirements for drying are more stringent

than otherwise, just for the reasons you mention. There were problems with early

high altitude oxygen systems due to freezing before this as figured out.

As to 'the amount of water required...`, as others have pointed out, rusting is

a chemical reaction mediated by the water. The water itself is unaffected.

other conditions govern the amount/duration of the rusting process.

Hello Pragmatist - and other commentators on the chemical process.

Coming from another direction, would it be true to say that there would be no rust if there was no water (liquid), or would there be rust due to vapour in otherwise dry air. In other words, would steel go rusty in conditions of high humidity (but without any liquid).

If high humidity can cause rust (or any humidity come to that) there must be a relationship between the amount of water vapour and the amount of rust.

Logically, the vapour in a confined space such as a cylinder would react with the steel to cause a finite amount of rust until all the vapour is consumed.

Any ideas please.

InRe. Humidity without liquid water:

The question is probably mute. With high humidity, temp. changes would
produce liquid water at some times, probably centered around small salt
deposts, (they're found practically everywhere).

I'm not a physical chemist, but I would guess that the liquid state of water
would be neccessary, but this could be missleading in this case as the
quantity required could be very small.

I'm open to correction & further explanation here.

if yuur looking for a mass of water required, consider the equation of the chemical reaction taking place. Balance it, and it will give you the ratio or reactants(steel and water) needed.

I am afraid I do not know chemistry enough to work this out with any certainty.

My 'simplistic' approach as a mech/elec engineer is to break all components (steel, water, air) down into single molecules/atoms and then 'mix' them together in various combinations to see what you can end up. I am not sure what causes them to join together in some combinations, and not others.

I know that steel goes rusty, and I know that steel is a combination of iron and carbon, so rust must be the addition of oxygen, giving iron oxide.

The reaction (so it seems) is caused by liquid water. So to my mind the oxygen comes from the electrolytic action of the water, leaving hydrogen behind (what happens to the carbon??). Thus there must be a limit to how much rust can be produced and logically will stop when the liquid water has gone.

Or to return to my original point, the cylinder will not rust if there is no water in it.

The air used to fill the cylinder will have to be dry enough to prevent condensation at full pressure in the lowest ambient temperature of the known working environment.

That is my view. But it would be comforting to know that this can be confirmed by experts.

Any chemists out there?

Even my rudimentary knowledge of chemistry suggests that there is no way the oxidation of Fe is going to reduce Hydrogen, (the stronges reducing agent).

As I said before, the water mediates the reaction, but is unaffected, therefore 'a little drop'll do ya`.

corrosion of steel by water is mainly related to the oxygen content in the water. water can dissolve a certain amount of oxygen at a given temp and pressure.

give a reply to your question is not possible also 'cause you don't even know for how long the water will be in contact with steel..

metal corrosion

S.