pitting on carbon steel 1

[junksci]

Is pitting in wet gas carbon-steel pipeline typically a result of passivated film breakdown or other mechanisms? E.g., condensate oil/water wet, under-deposit attack, bacteria?

 

[Metalguy]

The oxides that form on carbon steel do not protect it, with the exception of the black oxygen-lean Fe3O4 magnetite above ~400 deg F.

"Oxygen pitting" is probably the most common cause. It occurs in low-O2 areas where the rest of the pipe has higher O2.

 

[junksci]

Hi Metalguy, I guess your comment refers to external corrosion problem. If it is internal corrosion, in many cases no oxygen present, what could be the case for pitting? Thanks. (My search for pitting corrosion has not led to many hits for carbon steel, but plenty of stainless steel cases, which means passivation breakdown problem.) Thanks.

 

[SJones]

Answer: all of those you state under the right conditions. If you are researching, try and get hold of

Kermani, MB, Morshed, A - Carbon Dioxide Corrosion In Oil & Gas Production A Compendium

Corrosion, Vol 59, (8), 2003, pp659-683

Steve Jones
Materials & Corrosion Engineer

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

 

[macmet]

This does not answer your question, but I like this webpage for general information.

In regards to pitting,

http://www.corrosion-doctors.org/Localized/Pitting.htm

 

[Metalguy]

I don't know the specifics of a "wet gas" environment, but the various metal-damaging bacteria will certainly cause pitting-frequently severe.

Low pH usually causes wastage/thinning/general attack rather than pitting.

 

[rorschach]

Is Co2 present? Water+ co2=Carbolic acid. I've got a chunk of casing here at my desk that looks like termites have been eating at it that was due to wet CO2 corrosion.

 

[junksci]

Yes, CO2 is usually present in our studies, the paper Steve recommended is really good, it has covered the review for many aspects, it serves a good introduction on effects of oil-wet, wax, H2S etc. and operating conditions.

Do I assume that if someone likes to do a model on pitting of carbon steel, he has to go through all these factors? It is very complicated. Besides, how statistics will affect the pit distribution?

Why pits have different shapes? Can we tell from the pit shape what kind of corrosion?