CO2 Partial Pressure
Effect of CO2 partial pressure (PCO2) is related to solution pH as well. In the condition where scale-free CO2 corrosion occurs, the increase of PCO2 will increase the corrosion rate. This can be explained by the increment of H2CO3 concentration …show more content…
Sour corrosion also occur as dry corrosion which means that corrosion can still occur in the absence of water. Which therefore means that the school of thought which holds that corrosion can be combated by dehydration was a farce.This can be apparent in the casing annulus as H2S gas produced with water vapor will corrode the outside of the tubing and casing.H2S is known to accelerate the effect of localized corrosion, crevice corrosion and pitting corrosion. On dissociation H2S releases its protons in two step reactions as shown in subsection …show more content…
An ideal working system that will minimize the effect of corrosion should be maintain at a pH range ranging from 9 – 10 this pH range is called the passive zone as alredy discussed in the previous section.
Analogous to the effect of CO2 partial pressure (PCO2), The partial pressure of H2S is also related to the solution pH as well. In the condition where scale-free H2S corrosion occurs, the increase of PH2S will increase the corrosion rate. This can be explained by the increment of H2SO4 concentration as PH2S increase that will lead to accelerated cathode reaction. At high pH, where FeS formation is favorable, the increase in PH2S will increase sulfide ion and sulfide ion concentration, followed by higher super saturation of FeS. High iron sulfide super saturation enhances precipitation and scale formation. Nesic et al. showed that when PH2S is increased, pH at the surface initially increases due to corrosion and release of Fe2+, and then decreases since precipitation of FeS occurs rapidly. They also concluded that at constant pH, as PH2S increases, FeS precipitation can increase faster than the corrosion