Pressure drop as oxygen depletes for corrosion in sealed environment
Pressure drop as oxygen depletes for corrosion in sealed environment
(OP)
Offshore design codes state that the internals of monopile structures need not be painted internally to inhibit corrosion provided that they are sealed. I believe the reason is: any corrosion is limited by the depletion of oxygen. However this suggests that there would be a reduction in air pressure inside the monopile. Does anyone know if this is the case, and if so how much could the pressure drop?
The monopiles are very large diameter, thin walled components partially filled with seawater at installation, sealed at atmospheric pressure. Any significant air pressure reduction above the still water level could result in buckling of the piles under load.
The monopiles are very large diameter, thin walled components partially filled with seawater at installation, sealed at atmospheric pressure. Any significant air pressure reduction above the still water level could result in buckling of the piles under load.





RE: Pressure drop as oxygen depletes for corrosion in sealed environment
In your case, the presence of seawater will continue the corrosion for quite a while. However, I would not expect a pressure drop, as the reaction is
Fe + H2O(liq) + 1/2 O2(g) = FeOOH + 1/2 H2(g)
Check for flammable gas.
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
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Rust never sleeps
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RE: Pressure drop as oxygen depletes for corrosion in sealed environment
When steel contacts water, an electrochemical process starts. On the surface of the metal, iron is oxidized to iron(II):
2Fe ? 2Fe2+ + 4e?
The electrons released travel to the edges of the water droplet, where there is plenty of dissolved oxygen. They reduce the oxygen and water to hydroxide ions:
4e? + O2 + 2H2O ? 4OH?
The hydroxide ions react with the iron(II) ions and more dissolved oxygen to form iron oxide. The hydration is variable, however in its most general form:
2Fe2+ + 4OH? ? 2Fe(OH)2
4Fe(OH)2 + O2 ? 2(Fe2O3.xH2O) + 2H2O
Hence, rust is iron(III) oxide, which is formed by the dehydration of iron(II) hydroxide. Corrosion tends to progress faster in seawater than fresh water due to higher concentration of sodium chloride ions, making the solution more conductive. Rusting is also accelerated in the presence of acids, but inhibited by alkalis, through passivation.
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
Sealed/closed loop systems with fresh water perform quite well. Case in point, the hydronic heating system in my own house. Carbon steel and cast iron, 70 years contact with ordinary hot fresh water, exchanged only occasionally. No corrosion inhibitors or protection. The pipe is in perfect condition. Seawater would be a fair bit riskier though.
If the seawater isn't exchanged with fresh periodically, and the system is truly sealed against further oxygen ingress, general corrosion should eventually shut down. The trouble is, "sealed" is an idealization. A small leak or periodic reintroduction of oxygen into the structure could potentially cause significant damage over time- particularly localized corrosion at the vapour/liquid interface etc. Some form of protection sounds like a very good idea.
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
Cheers CM
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
Months ago however I threw away what rather quickly became a very nasty looking comparative sister jar solution prepared at the same time, that contained the same brand bare nail and simulated seawater, but with no Portland cement mortar coupon. While the appearance of the latter jar and solution, and what bloom etc. could be seen through the glass jar looked like the nail would be much deteriorated (and maybe me when the jar fell off the shelf and broke!) once removed , the latter nail actually looked rather sound structurally though fully rusted once I let it dry on the shelf.
I have also been told of at least a couple instances where for whatever reasons some cementlined pipelines have in essence lain fallow after initial installation and testing? for incredibly long periods (30-40 years!) before they were finally disinfected, flushed etc. and placed in service, and I was also not made aware of ill effects once this was done.
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
Thanks to all for your thoughts - all greatly appreciated.
Does anyone know if pressure drop would occur as oxygen depletes. I am currently assuming 0.2bar drop in pressure internally (20% oxygen in air is consumed), but this is not an insignificant load.
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
1) The bottom is effectively sealed by being driven into soil below sea level - even if the soil is porous there is an effective water trap to prevent air ingress by the open end of the bottom of pile.
The top will be sealed deliberately to reduce internal corrosion by means of plates and stiffening beams. There will be an manway access - also sealed by gasket.
2) Air in the pile above sea level has no function. But it is a significant volume. To replace it by nitrogen say would be quite costly.
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
Neil, You would not need to open the structure to test the annode. The electrical test point can be wired to a convienient location out side.
The 'near surface' zone bothers me. Steel rusts when exposed to oxygen. Under some conditions the rust is a protective layer (think CorTen), and in some it is merely wastage.
My concern is that with fluctuations in temperature, fluid level and such, that the metal near the surface may see changing conditions that will reuslt in a surface that is stable at one time being attacked at another.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
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RE: Pressure drop as oxygen depletes for corrosion in sealed environment
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
RE: Pressure drop as oxygen depletes for corrosion in sealed environment
Annodes, inhibitor, biocide; sounds like you need to talk with water treatment guys. Your case actually has more in common with water storage (like for fire fighting emergencies) than typical structure applications.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
http://www.trent-tube.com/contact/Tech_Assist.cfm