Deep corrosion pits on outside of carbon steel piping

[DefenderJ]

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.

 

[metengr]

DefenderJ;
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.

 

[dba89]

I would second metengr; First figure shows that elbo material is more suseptable to pitting than the pipe materials and external conditions are promoting the corrosion; probably under deposit.

 

[stanweld]

Assuming that the higher temp lines were insulated, the pipe is ripe for under insulation corrosion when water seaps through the insulation and would be more expected at fitting locations.

The corrosion at locations of damage to paint should also be expected in your location.

 

[DefenderJ]

Thanks everyone for the input.

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.

 

[metengr]

DefenderJ;
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.

 

[EdStainless]

look for other things that could drip onto the piping also. Even before installation. A wide range of common substances can be rather corrosive at these slightly elevated temps.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Compositepro]

This type of corrosion is common on very old steel that gets wet. I never gave any thought as to why. My guess would be that salts on the pipe concentrate into droplets as they dry and rewet over time. A divot would be the last place to dry, so it would be wet for longer periods and have greater salt concentration. This type of corrosion is not usually seen where there is flaking rust so the rust must be coming off in small particles.

 

[rmw]

Well, it is clear to me that the turbine worms have escaped and are now eating the pipe...... sorry.

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

 

[StoneCold]

Well, I will throw my guess out there as well. I think that it might have something to do with ash from furnaces/stacks in the area. Ash settles on the pipe and you get little spots of sulfuric acid on the pipe. What kind of plant is yours, could this be coming from your industrial neighbors??

Regards
StoneCold

 

[DefenderJ]

StoneCold
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.

 

[rconner]

Not going to claim much expertise here, but if you have anything going on involving sulfuric acid, or for that matter hydrogen sulfide, I would think you would most likely find a good bit of sulfur in the deposits. [If you got your nose down in corrosion deposits, or crushed them under your nose, and smelled a rotten egg odor that would of course probably mean some presence of H2S -- and I think sulfur corrosion deposits are often black in color, whereas sulfur is sort of yellow]:

Fe + H2S ---> FeS

2Fe + 3H2SO4 ---> 3H2 + Fe2(SO4)3 etc.