Internal Corrosion of gas pipeline

[09091960]

In a recent cleanning process of a Gas pipe line, we have found considerable amount of black powder from the internal surface of the pipe line. Mineralogy sample shows that

Mackinawite (FeS-0.9)- Dominant
Metacinnabar(HgS) - Trace
Halite(NaCl)- Trace

What is the approach to find these compositions are aiding the internal corrosion of the pipe line or not?

 

[strider6]

it would be necessary to know what is the type of fluid, operating conditions, and material. based on what you find it seems a service with a sour fluid (gas + liquid?) that contains also mercury and water with some chlorides..

S

Corrosion Prevention & Corrosion Control

 

[09091960]

Operating conditions for the gas pipe line

MAOP- 10 MPa
Diameter - 169.3 mm
Wall thickness - 4.2mm
Material - API5LX42
Design factor - 0.72

What methods I'll be using to find the mentioned compersitions won't aid the corrosion process?

 

[SJones]

The black powder IS a corrosion product. Therefore, an H2S related corrosion process is occurring, or has occurred, in the pipeline. The other corrosion monitoring and inspection techniques that are deployed on the pipeline should help to give a fuller picture. For example, if you can't detect any H2S in the entry gas, it may indicate that MIC is occurring somewhere if conditions are conducive to its development.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[strider6]

free water inside the pipe? gas composition?

S

Corrosion Prevention & Corrosion Control

 

[stanweld]

Is the corrosion located at low points in the p[ipeline where water can collect?

 

[brimmer]

Black powder is completely normal in many pipelines, especially gas gathering lines and sour gas lines, and especially if they have never been cleaned with maintenance pigging. The product you mention is a corrosion product as others have mentioned. You will care about the quantity of free water in the pipeline, or low areas where this water can accumulate as others have mentioned. These areas will accumulate constituents like Cl and they will break down the balck FeS film. Remember FeS film is generally a tight adhering film which actually will protect the pipe from corrosion, forms a barrier, but where this film breaks down or is in too much abundance you will get localized pitting. Don't know the quantity of powder you are getting out, but a thin film is normal, you can set up a cleaning frequency based on amount of powder and amount of water coming out. You can also look at corrosion inhibitor either in continuous or batch form when you pig.