I am looking at a wet gas pipeline

[Kiranpatel]

I am looking at a wet gas pipeline with a small quantitiy of C02 in the gas composition (1 to 2 % mol). The pipeline pressure is in the rage of 5 to 10 bars.
I think I remember, from a long time ago, about a rule of thumb which states that, below a certain value of partial pressure of CO2, the reactions between the CO2 and the water can be regarded as insufficient to initiate any significant internal corrosion.
Am I talking rubbish ?
If not, can anybody throw some more light on this rule of thumb (what is the threshold value of the partial pressure)?

 

[rustbuster1]

Even low concentrations (<1 % mol)of CO2 can acidify water if the water has no buffering. Therefore I would suggest you plan to address this as a corroding environment and not hope it won't be.

 

[Kiranpatel]

Your point is noted. However, considering that I am looking at very low pressure pipelines (operating at 5 bars or so) with a CO2 content of 2%, I get a CO2 partial pressure of 0.1 bar. As I mentioned in my original post, I remember of a project where it was stated that below a certain threshold (I cannot remember what value but it was certainly above 1 bar) of CO2 partial pressure, risks of corrosion are considered to be inherently included within the design safety factors.
Maybe that was just b**t that I was fed.

 

[Hush]

API 6A purchasing guidelines provides the following for wet CO2 in wellhead equipment (mind you it doesn't mention temperature):

<7 psia partial pressure - non-corrosive
7 < 30 psia partial - slightly corrosive
>30 psia partial - moderate to highly corrosive

 

[Carburize]

One of the relationships between corrosion rate , partial pressure and temperature which used to be used for these types of environments was published by Dewaard and Milliams and if my notes are correct the equation is:

Log (corrosion rate mm/yr) = 5.8 - {1710/(273+t DegreeC)} + (0.671 log Partial Pressure CO2 bar)

 

[stonerabroad]

carburize, is that corrosion equation for a mild steel pipeline?

 

[Francesb]

CARBURIZE has given the a basic corrosion rate formula which is in DeWaard & Lotz, prediction of CO2 corrosion in carbon steel, Paper 69, NACE Corrosion '93. It's a good first pass, but there are correction factors and modifications that can be made to this. Lots of the Oil and Gas operators have their own modified models. If you have a materials/ corrosion technology group within your organisation it may be useful to contact them as they may have software you can use.