In crude pipelines, sour (H2S in separated gas approx. 0.1% and CO2 1%) corrosion is always observed. The formation of both FeS and FeCO3 scale were identified in the deposits. The question is which type of corrosion is prevailing? and whether corrosion inhibitor can prevent further damage? Can biocide be a option to reduce H2S and further sour corrosion?
Are the fluids within the pipeline reducing? [If FeS is actively precipitating it suggests the fluids are reducing]. Are the fluids exposed to atmospheric O2? If so, CO2 is probably your issue? Any exposure to oxygen, meteoric or otherwise, may produce H2SO4.
NACE Corrosion Paper 06121 suggests a CO2/H2S ratio 'rule of thumb'. For CO2/H2S ratio below 500, the corrosion process is mixed with contributions from both acid gases, as in your case. Above 500, CO2 corrosion prevails or dominates and the primary corrosion product is likely to be FeCO3. An appropriately selected corrosion Inhibitor can mitigate further corrosion in either case, and biocide is an option to reduce H2S and further sour corrosion depending on the source of the H2S.
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