Dear all, Any idea of what might 8

[replica]

Dear all,

Any idea of what might be the cause of the localized corrosion of the carbon steel in the seawater as in the attached photographs.
The part in the photo was taken from the flange of the floating reducer that was exposed to seawater for 5 years. The floating reducer was used to transport crude oil. The floater reducer consists of flange, reducer, foam and outer layer. The steel part (reducer and flange) was coated with zinc internally and externally. The corrosion/leak took place at the flange side slightly away from the weldment between the flange and the reducer. It was observed that the zinc coating sacrificed and exposing the steel (flange) to the seawater. The reducer did not corrode as it was covered with foam (to make floater afloat) and outer layer made of polyurethane.

While looking for the leak, is was observed that the flange was covered with barnacles and other marine growth. The hole was discovered underneath these barnacle and marine growth and the corrosion started from the outside as shown in the attachment.

Any suggestion on what may cause this type of localized corrosion? The material is A105 carbon steel used for forged flange.

All comments are highly appreciated.

 

Yes, the cause of the localized corrosion of the carbon steel in seawater is carbon steel in seawater.

Relying on carbon steel to contain crude oil on land is bad enough, but in seawater it is criminally negligent.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[MJCronin]

Bravo, ironic !!!! bravo !!!

Relying on carbon steel to contain crude oil on land is bad enough, but in seawater it is criminally negligent

You don't think that replica might have zero corrosion experience or engineering experience in general,..... do you ????

MJCronin
Sr. Process Engineer

 

[EdStainless]

Makes me wonder how bad the rest of the installation is.
Where was the impressed current system?

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P.E. Metallurgy, consulting work welcomed

 

[TugboatEng]

For a floating pipeline? That's going to be a negative.

 

[mk3223]

It looks that this pipe is lack of the routine pipe inspection and painting maintenance.
The damage of the protective painting on the pipe exterior is the obvious root cause of the pipe leak. I thought that the overlook of the Maintenance could be part of the problem.

 

[replica]

Thank you everyone for the comments...I know that carbon steel will corrode in seawater..lol...But I expect it to be a general type of corrosion. What made it to be localized? Any idea ..pitting, crevice, MIC etc...I am sure it was not galvanic...as the zinc is well sacrificed ..Bear in mind that it was covered with barnacle and other marine growth...Was possible to have barnacle induced crevice corrosion?

 

[replica]

Thank you very much for the valuable comments,

Ironic metallurgist
Yes, the cause of the localized corrosion of the carbon steel in seawater is carbon steel in seawater.

Relying on carbon steel to contain crude oil on land is bad enough, but in seawater it is criminally negligent.

Agree ..but people are using it and the manufacturer is selling it probably because it is cheap ..I do not know whether NACE has any guideline for the floater to be used for this application.

EdStainless
Makes me wonder how bad the rest of the installation is.

Not sure ,,probably waiting for the time to leak.

TugboatEng
For a floating pipeline? That's going to be a negative.

Agree but people are using it. Probably hoping that Zinc coating will do the job.

mk3223
It looks that this pipe is lack of the routine pipe inspection and painting maintenance.
The damage of the protective painting on the pipe exterior is the obvious root cause of the pipe leak. I thought that the overlook of the Maintenance could be part of the problem

Totally agreed, looking at the condition, the amount of marine growth on the flange and the damage at the protective coating, I can say that it was not maintained properly. To my understanding, the slight damage to Zinc coating will not caused the steel to corrode but will cause Zinc to dissolve and protect the bare steel due to large anode to cathode ratio. The steel will corrode when there was no more coating to protect the steel. This is what they do in CP ..correct me if I am wrong...

I agree that using carbon steel for this application is inappropriate. Any guild lines from NACE for the material used for the floating reducer?

 

[TugboatEng]

Impressed current only works on the submerged portions of the structure. It would be entirely useless on a floating pipeline. It is also used for structures coated with some sort of insulating material such as paint. The required current would be very high for a structure with a conductive coating such as zinc.

Carbon steel is an acceptable material if you consider its life expectancy. Experience now tells you to expect failure after 5 years so you can replace the pipeline after 2 years to prevent failure.

 

[MortenA]

actually i would like to hear from e.g. ironic metallurgist why we dont see general corrosion? Is the real reason as others have said poor maintenance of the zink coating? Maybe a fault in the zink coating or damage to the same is why the corrosion occured - and only locally? Lots of pipelines that i know of made from CS in sea water. The main oil export pipeline from the Danish fields in the Nort Sea are made of CS and they have lasted for +40 years now. They have anodes sure and are piggeded also with intelligents pigs for monitoring but they are from CS. I would suspect that a assemply such as this not fully submerged but exposed to both salt water and air would be more at risk though?

 

[TugboatEng]

A partially submerged structure is the most difficult to protect as the anodes are ineffective for protecting any part that is not continuously submerged. There is the mechanical wear of being in the splash zone and the concentration of chlorides as the surface is wetted and dries. Finally, you have more UV exposure to wear away at coatings.

 

[replica]

MortenA,

Thank you for the comment.
True..Most of the pipe line subsea transporting hydrocarbon used carbon steel API 5L X60 ..X45..X65 etc and with proper CP and maintenance such Magnetic flux leak (MFL) test through pigging , the pipe can last for years. The issue is at the splash zone of the riser/pipeline whereby the area is exposed to wet and dry condition, temperature fluctuation etc, as mentioned by Tugboat, causing severe corrosion. We have experience on the failure at the splash zone of the riser due to external localized corrosion that induces fatigue failure.

 

[replica]

Tugboat,

A partially submerged structure is the most difficult to protect as the anodes are ineffective for protecting any part that is not continuously submerged. There is the mechanical wear of being in the splash zone and the concentration of chlorides as the surface is wetted and dries. Finally, you have more UV exposure to wear away at coatings.

Thank you for the comment.
Agree with you. I need to clarify with the operator whether the affected location was fully submerged or not.
I am thinking of the probable cause of the leak is either pitting, crevice or MIC as there are stagnancy at the affected area.

For you information we also have the similar leak (appearance-localized corrosion)at almost the same locations (thin flange area not covered with PE outer layer but coated with zinc )on the other floater, also covered with marine growth.

Another issue is that why the designer did not consider covering the thin section flange with PE Outer layer as well to avoid corrosion. The thin section flange has the same thickness as the reducer for ease of welding between them. The flange was hot forged to make the one piece shape without welding.

 

[LittleInch]

Basically if you're going to use Carbon steel elements like this then their life is 3-5 years and then schedule them for replacement.

Judging from the photos the rusting bit looks like the end of the reducer to the flange section which should have been under the foam layer which I can only guess got damaged as it the bit being dragged up the side of the ship time and again and created a local pool of seawater to corrode away. If it was disbondment of the foam then a visual inspection would not have seen this.

Personally I would go for the expense of a super duplex connection piece to avoid this or just have one spare and as soon as anyone sees a few drops of liquid take a boat out and replace it.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mrfailure]

I would think the localized nature of corrosion is related to marine growth/barnacles on the surface. I would think that would set up a local galvanic cell that results in underdeposit corrosion and subsequent pitting. Thoughts?

 

[replica]

LittleInch
Judging from the photos the rusting bit looks like the end of the reducer to the flange section which should have been under the foam layer which I can only guess got damaged as it the bit being dragged up the side of the ship time and again and created a local pool of seawater to corrode away. If it was disbondment of the foam then a visual inspection would not have seen this.

Thank you for the comment. It is possible, but looking at the location carefully it showed that the location of the failure was exactly at the location that was not covered by the PE outer layer ie slightly towards the flange side not the reducer side. Visual inspection during looking for the leaks showed that the PE outer layer was intact.

mrfailure

I would think the localized nature of corrosion is related to marine growth/barnacles on the surface. I would think that would set up a local galvanic cell that results in underdeposit corrosion and subsequent pitting. Thoughts?

Thank you for the comment, Yes, I am thinking of the same mechanism but unfortunately it was hard to prove as all the barnacles/marine growth was removed from the surface during cleaning while looking for the leak. The only evidence that we have is only a photo that shows the presence of hole under the barnacle/marine growth before the barnacles/marine growth was removed. Herewith I attached the photo of leak location (from other floater that I think has the similar failure mechanism),together with the photo before the barnacles/marine growth was removed during cleaning.

 

[LittleInch]

OK, I thought the angled bit was the start of the reducer, not the flange.

The leak looks like it's in the HAZ which often seems to corrode faster for some reason.

Marine growth, especially animal based can secrete acids of various sorts as waste which can eat into the oxide layer. It won't be galvanic.

But whatever the initial cause is, having carbon steel in such a hostile environment subject to air/sea water/splash/marine attack/damage from being hauled up the side of a boat using chains, hooks etc is just asking for trouble. You can't expect miracles.

SO either work on a 3-4 year replacement program or buy a Super Duplex one. IMHO.

The reason the designer didn't consider it is that it worked to start with and then he was gone. No incentive for the EPC contractor to buy something that was going to last 20 years when one which would last beyond his guarantee period was 20% of the cost. Learn for the next time and specify what you want and need from the start and pay for it up front.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[mrfailure]

LittleInch, thanks for the clarification related to marine growth. I recall working leaks in ferritic stainless steel tubing in brackish water service where pinhole leaks occurred under where barnacles attached. Turned out the formulation was not as resistant to oxidation in the environment. Composition was altered by the manufacturer and associated industry specs to solve that problem.

 

[EdStainless]

Often the welds and HAZ are just enough different in chemistry or structure that they will set up very weak galvanic currents with the base metal. While in most cases this is too weak to directly cause corrosion the current does tend to attract biological activity. I have seen this in CS and in many SS applications that growth (micro and/or macro) is fastest along the welds.

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P.E. Metallurgy, consulting work welcomed