Deep corrosion pits on outside of carbon steel piping
Deep corrosion pits on outside of carbon steel piping
(OP)
We have a lot of unpainted carbon steel pipes on our plant which are outdoors.
I am increasingly seeing an unusual corrosion on the outside of the pipe which penetrates deep into the pipe and can lead to leakage.
The "pit" is faily wide and filled with dense corrosion product - generally dark in colour. There is very little flakey corrosion product. I have attached a document with some photos.
Has anyone seen a similar phenomenon or know what it might be caused by? I am trying to get a piece of pipe with this problem so I can get it analysed.
My thoughts so far are surface contamination (spots of material deposited on the pipe) may set up a corrosion cell. We are located next to tidal estuary - could chlorides have a role in this?
Any opinions and information gratefully recieved.
I am increasingly seeing an unusual corrosion on the outside of the pipe which penetrates deep into the pipe and can lead to leakage.
The "pit" is faily wide and filled with dense corrosion product - generally dark in colour. There is very little flakey corrosion product. I have attached a document with some photos.
Has anyone seen a similar phenomenon or know what it might be caused by? I am trying to get a piece of pipe with this problem so I can get it analysed.
My thoughts so far are surface contamination (spots of material deposited on the pipe) may set up a corrosion cell. We are located next to tidal estuary - could chlorides have a role in this?
Any opinions and information gratefully recieved.





RE: Deep corrosion pits on outside of carbon steel piping
I would agree with your preliminary conclusion that this is some type of underdeposit corrosion that could be aggravated by chlorides. High concentrations of chlorides will locally pit carbon steel. Seen it many times on piping we have at our plants near grade level that get exposed to road salts during the Spring.
RE: Deep corrosion pits on outside of carbon steel piping
RE: Deep corrosion pits on outside of carbon steel piping
The corrosion at locations of damage to paint should also be expected in your location.
RE: Deep corrosion pits on outside of carbon steel piping
Metengr - it's useful to know someone else has seen it. I'm still trying to work out where the deposit comes from.
Stanweld - this is not corrosion under insualtion (CUI) - we have plent of that as well! This problem occurs on uninsulated steel. often unpainted, but sometimes where paint breaks down.
The thing that puzzles me is why the corrosion product is so dense and not like typical flakey rust that comes off in layers. I have a section of pipe that's earmarked for replacement and some will be saved for me so I can get some analysis done.
Another thought is whether bird droppings might trigger this. We have a warm plant and we can get a lot of starlings in certain months.
RE: Deep corrosion pits on outside of carbon steel piping
Actually, I did think about bird or animal droppings because the highly localized corrosion could be MIC from under deposit corrosion. You need to evaluate the corroded sections to confirm your observations.
RE: Deep corrosion pits on outside of carbon steel piping
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Plymouth Tube
RE: Deep corrosion pits on outside of carbon steel piping
RE: Deep corrosion pits on outside of carbon steel piping
What grade of pipe is this? My mind goes to material homogeneity, especially after once seeing a cheap grade of plate where engine block parts were clearly visible in the plate. Could this pipe be in the same class of low grade stuff?
rmw
RE: Deep corrosion pits on outside of carbon steel piping
Regards
StoneCold
RE: Deep corrosion pits on outside of carbon steel piping
Thanks for your input. We burn Heavy Fuel Oil on site for boilers and calciners. We also have two power stations down the river from us one on coal the other also on HFO.
We have speculated that there might be something like this on a tank roof that was close to our stacks. What do you think we might find in the corrosion product if it is initiated by sulphuric acid?
rmw
Our steels are ASTM A106 GrB and A234 WPB for fittings. Not very exotic but not suspect quality. The problem is sparse but appears to occur all around the site.
RE: Deep corrosion pits on outside of carbon steel piping
Fe + H2S ---> FeS
2Fe + 3H2SO4 ---> 3H2 + Fe2(SO4)3 etc.