Pressure drop as oxygen depletes for corrosion in sealed environment 3

[NeilAA]

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.

 

[kenvlach]

In Design of Welded structures, Section 7.8, Omer Blodgett explains that corrosion stops in sealed steel structures once available moisture is depleted.

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.

 

[EdStainless]

I would not plan on internal coating for sealed, empty spaces. But for sections that will be partially filled with seawater I would plan on some form of internal protection. You don't need oxygen for corrosion. In fact many forms will progress faster once the oxygen is depleated.

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Rust never sleeps
Neither should your protection

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[NeilAA]

I have the following chemical process obtained from wikipedia which suggests that oxygen and water are required to rust steel. It also suggests that oxyegen from the air, or dissolved oxygen in the seawater will be used to form the Fe203. There is no hydrogen release, but I have also seen some mention of possible hydrogen formation as the oxygen is used up.

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.

 

[moltenmetal]

EdStainless: for the general corrosion of carbon steel, don't you need oxygen? And for most of the other forms, don't you need an oxygen concentration differential to drive the electrochemical cell? Galvanic and MIC would be exceptions, because there's something else driving the cell in those cases. But in truly sealed systems, these too should shut down eventually when the electron sink is exhausted.

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.

 

[corrosionman]

If NONE of the internal surface is lined then the corrosive potential oxygen will be spread out over a large surface area, giving minimum damage at any one location. Howvere if the surface is lined but the painter misses one small area ( its always the bit thats difficult to reach) then all the damage will be concentrated at that point. Possibly this is a worse scenario.
Cheers CM

 

[moltenmetal]

corrosionman is right to distinguish "protection" from simple surface coating. What I was referring to as protection was electrochemical protection of some sort. I presumed EdStainless was referring to same. We weren't talking about a simple coating: perhaps anodes swimming in the stagnant seawater, or a zinc-rich cold galvanizing coating etc.

 

[rconner]

As I have no experience with the inside of steel monopiles this post is certainly not intended to be any sort of recommendation for same, nor is it intended to settle any possible differences of opinion on this thread. I am however looking at a Mason jar on my bookshelf, into which a couple years ago I placed a chipped out sample of a cement mortar pipe lining that I knew was per normal practice very rich in Portland cement. I also placed a bare steel nail in the same jar (with length slightly less than the bottom diameter of the jar), though not touching the mortar sample, and then at the same time filled the jar nearly full with some simulated seawater. I did not tighten the lid of the jar tightly, but enough that I figured it would not allow any significant evaporation of the water. After this exposure, the water is still virtually crystal clear and I am interestingly only seeing a little rust only on the obviously more upset, cold-worked head of the nail! When I took the lid of the jar off recently, I did however see that whatever the thin bonded coating was on the inside of it, it had delaminated and the lid rusted some (perhaps with ultra-pure condensation that collected on this area?).
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.

 

[NeilAA]

Moltenmetal: Your suggestion of an anode inside the monopile is a good one. In fact I have just had some discussions with DNV in Denmark and they also suggested putting one sacrificial anode inside the monopile; this could be monitored periodically to check for depletion. The anode would protect the steel at/near the water level surface, which is likely to corrode quickly. However, monitoring the anode would allow the periodic introduction of fresh oxygen into the sealed system - possibly counterproductive in my opinion.

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.

 

[rconner]

These are probably quite stupid questions as I've not worked in this field, but I was curious does "sealed" mean the bottom (and of course top) of the pile will not be open? -- Also, once the pile has been sunk/driven, what is the engineering function of the air to be left inside the pile?

 

[NeilAA]

rconner: In answer to your questions.

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.

 

[EdStainless]

rconner, your nail didn't rust because the pH was high enough that the surface film was stable and passive.

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

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[rconner]

I wasn't thinking of replacing the air with nitrogen, but did wonder a little if it was the air that was causing the problem if the pipe might eventually be more substantially filled with water (i.e. the air exhausted)and the fill hole then air-tight plugged,with the pile essentially full of water (if the top plate/gasket say is less than say ~30 feet above surface?)

 

[SJones]

You do not really have a 'sealed' system: water will circulate through the soil providing replenishment of any detrimental species; air will permeate the gasket; and, removing the manway periodically won't be helpful. The anode will need to sit in the water and will need replacement (provided that it functions correctly in the first place). If you are worried about internal corrosion, try dosing with a biocide/inhibitor/oxygen scavenger chemical cocktail from the word go and keep it topped up periodically.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/

 

[rconner]

I agree in theory with mention of the mechanisms SJones describes; however, while the interior system condition the original poster initially described may not be entirely "bottletight" I nevertheless believe (as I suspect they may have been thinking) that it in effect might be quite "stagnant". Compared however to such a large water mass/exposed steel structure, I wonder if the amount of oxygen that might permeate a finite length and transverse path length of gasket rubber would be significant? I wonder also how much of a meaningful mechanism/driver for circulation or replenishing in this specific system could there be, unless perhaps several vertical feet of air are left in the sea level range area and the water level inside the pile fluctuates signficantly as a result of changing tides? As I suspect these things might be sunk quite deeply in the floor of the ocean in many cases, and there are in many cases perhaps many feet meters of soil that has perhaps limited permeability rate, would it be perhaps interesting to know exactly (perhaps by means of a float and sealed dipstick arrangement?) exactly how much if any this level normally fluctuates? Is it possible that any significant water (level) movement would be some dampened by limited permeability, or eliminated near completely by the barometric effect of atmospheric pressure on the surface of the ocean if say the pile were more filled with water above seal level? Also, if there were hypothetically meaningful circulation of some types "biocides" from inside the pile to say outside the pile into ocean sediments or water, could this perhaps eventually result in some environmentalists out there buzzing their little rubber boats around these things?

 

[EdStainless]

A corrosion inhibitor sounds like a good idea. There are plenty of them that are environmentally safe, I wouldn't worry about that side of it.
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