| Abstract |
CO2 rich steam condensate from Upper Mahiao was diverted to the spray tower to elevate the pH to around 6.1 to 7.0 from as low as 4.5. The resulting solution that is aerated is still corrosive to carbon steel construction materials with corrosion rates of 0.4 to 1.0 mm/yr with localized pitting attack. Treating the degassed and aerated condensate with NaOH to adjust its pH to 7.5, gives a drop in the corrosion rate to around 0.3 to 0.5 mm/yr measured using on-line electrical resistance type CorrosometerTM probes and weight loss corrosion coupons in short term tests. The drop in corrosion rate, however, was still above the acceptable corrosion rate set at 0.12 mm/yr and localized pitting was still observed. However, on-line probes suggested a decrease in corrosion rate as a function of time, implying that lower rates would be observed for long-term exposures. Mineralogy and morphology of corrosion products collected from the carbon steel materials were evaluated using surface analytical techniques such as XRD, EPMA, SEM with EDX, and reflected light microscopy. Magnetite was found in the corrosion products for both the treated and untreated condensate while goethite was observed only when the condensate was treated with NaOH. Iron, sulfur and oxygen were found in all of the samples analyzed by EPMA and SEM, suggesting the presence of an iron sulfur compound,. Potential-pH diagram showed that the stable products are pyrite, troilite and magnetite in this system. Increase in pH shifted the equilibrium of the solution and dissociation of acid components H2CO3 and H2S into less corrosive species This species change is accompanied by aeration of the fluid in the spray tower and the combined chemistry resulted in the formation of films containing magnetite, goethite and presumably hydrated ferrous sulfate. It is postulated that the formed films have some protective properties but it is recognized that the corrosion rate decrease with pH increase may be a complex function of carbonate and sulfur species as well as oxygen concentration. |