Material Selection basis of corrosion-resistant alloy pure material (corrosion mechanism of natural Gas Field)

According to NACE MR0175, the development and production environment of natural gas field can be divided into acidic environment and sweet environment.

The acidic environment contains H2S and H2S partial pressure is greater than 0.03mpa.

The sweet environment contains CO2 and Cl-, but does not contain H2S or H2S partial pressure is lower than 0.03mpa.

Corrosion mechanism in acidic environment

Corrosion mechanism in acidic environment

H2S                     H+  +  HS-

HS-                      H+  +  S2-

2H+ + Fe            Fe 2+  +  H2 ↑

Therefore, in acidic environment, in addition to uniform corrosion and local corrosion on the surface of pipes, a series of metal hydrogen-related corrosion cracking will also be caused, among which Cl- has a certain accelerating effect on metal corrosion.

Corrosion mechanism of sweet environment

CO2+H2O       →             H2CO3

H2CO3          →             H+  +  HCO3-

HCO3-          →             H+  +  CO32-

Fe + H2CO3       →             FeCO3   ↓   +  H2 ↑

Therefore, in the sweet environment, metal corrosion behavior is closely related to the corrosion product film on the surface, and the corrosion rate depends on the structure, thickness, stability and permeability of the corrosion product film on the surface of the metal, so that the pipe presents uniform corrosion or local corrosion, or the pipe wall thickness gradually becomes thinner or even perforated.



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