Pitting corrosion 3

[fakhfakh]

Dear All,

I've a doubt that pitting corrosion is applicable to carbon steel like SA516Gr70. Pitting corrosion is applicable to stainless steel.
Carbon steel is subjected to surface corrosion
Is this correct ?

 

[Chumpes]

hi
there are a couple of words that are used for an application or another.

personnaly, i only use "pitting" or "localized corrosion" or "crevice corrosion" for the processes of localized corrosion in the shape of a pit (or crevice) on stainless steel and resulting from destabilization of the passive film in electrolytic (usualy aqueous) media. pits usualy have a recognizable round shape on an almost unattacked (passive) metal surface.

I never use 'pitting' for non-passivable materials such as carbon steel, but other people may do.
In my point of view, carbon steel is not passivable and thus pitting corrosion is not applicable to carbon steel.
What is sometimes called "pitting" of carbon steel can probably be seen as generalized corrosion on a limited surface area (deposits, high velocities, ...).

regards

 

[bimr]

Not sure why the question has come up.

I have witnessed pits with a recognizable round shape on an almost unattacked (passive) carbon steel surface on the interior of a storage tank. An ultrasonic test on the exterior of the tank indicated the metal thickness was 1/4-Inch. An internal inspection showed numerous pits.

A google search will show a wide range of articles on the subject.

 

[Chumpes]

@birm : What is the thickness and nature of 'passive' films that may grow on carbon steel ?
passive film (a few nanometers) only grows on passivable materials, i.e. stainless steels and other corrosion resistant alloys.
for carbon steel we should better talk about 'corrosion scales' that are much thicker (>> 100nm)

a fast search on Google with keyword "pitting carbon steel" only provides pictures of pitting of STAINLESS STEEL... can you share one of the numerous articles you found on Google and dedicated to pitting corrosion of carbon steel (with pictures) ? thanks.

 

[EdStainless]

There are many environments where the Fe corrosion products have limited solubility and if velocities are low they will tightly adhere tot he surface and inhibit corrosion, acting like a passive film.
I have seen very localized attack of CS in high CO2 environments for example.
However true pits, which grow in a self catalytic manner are a only found in alloys that for true passive films.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

EdS, you are correct, but if it looks like a pit and smells like a pit it's probably a pit. But I agree such distinctions are important (anyway I wouldn't try to argue with you on corrosion matters).
I've seen damage under tubercles in stagnant water systems that were best described as pitting. And I've recently investigated very much pit-like damage to carbon steel HX tubes, caused by deposit corrosion in a wet chloride environment. Scientifically speaking perhaps not corrosion pits, but colloquially speaking they are.

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

 

[bimr]

@bimr : What is the thickness and nature of 'passive' films that may grow on carbon steel ?

Like I said, I witnessed it, so it is possible.

"In environments where the pH is less than 10, carbon steel does not form a passivating oxide film and the addition of chloride results in uniform attack over the entire surface. However, at pH greater than 10 (alkaline) the oxide is very protecting and the addition of chloride results in pitting corrosion."

Link

 

[metengr]

I've a doubt that pitting corrosion is applicable to carbon steel like SA516Gr70.

Try looking up underdeposit corrosion of carbon steel, oxygen cell pitting in carbon steel boiler tubes in stagnant oxygenated feedwater, and of course, microbiological influenced corrosion (MIC).

Ummm lets see,,,

https://www.sciencedirect.com/science/article/pii/S0029801817300689

https://www.nist.gov/sites/default/...-22-Corrosion-of-1018-Carbon-Steel-Avci-2.pdf

https://inis.iaea.org/collection/NCLCollectionStore/_Public/20/011/20011208.pdf

http://corrosionjournal.org/doi/10.5006/1.3280616?code=NACE-prem-site

 

[TugboatEng]

One specific example of pitting or carbon steel I have seen is in diesel engine cooling systems. Once corrosion starts, the film forming additives (typically sodium nitrite) in the coolant can no longer protect the surface under the oxide layer. Corrosion will accelerate under the corroded point and form a pit. This is often very localized.

 

[fakhfakh]

Thanks for all

 

[Dhurjati Sen]

In cooling water and fire water circuits where CS is often used, pits are a regular menace.

DHURJATI SEN

 

[matschka]

Some years back the company that I was working for got a contract to perform 'daylighting' and UT of pipe that was uncovered an other pipe. I was completely 'blown ' away at the amount of corrosion that we found. I got certified myself, (UTth) only. We hired a local NDE contractor to perform the NDE as we required.I saw pipe that looked like the surface of the 'moon'. I was tasked to make drawings of all the pipe that was corroded and to take it to the client,'refinery owner'. Some pipe was paper thin. I would of bet they would replace it. Nope. They said it was alright. Sometimes we would do an RT of the pipe to determine the extent of the corrosion. This RT technique was a 'stand off' technique. The source was placed some distance away from the pipe and 'shot. The exposed film would show a cross section of the pipe and you could see the wall thickness along with the thinning sections and the 'craters' on the interior. 'Daylighting' is the uncovering of pipe from the lowest racks of piping that have been covered for years with dirt.

 

No pitting in carbon steel? Tell it to my car's fenders...

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