Effect of pH on Carbon Dioxide CO2 corrosion in carbon steel pipe lines(Oil and natural gas tranportation pipes)

Effect of pH on Carbon Dioxide CO2 corrosion in carbon steel pipe lines

One of the important cathodic reactions in the CO2 corrosion process is the reduction of H+ ions, thus pH plays an important role in the cathodic reaction. In 1989, Tebbal and Hackerman showed that there is change in the pH immediately adjacent to the electrode surface in the electrolyte and it has a major effect on the physical properties of precipitates such as iron carbonate and iron sulphide.

The corrosion rate of carbon steel at room temperature changes considerably, with change in solution pH. With change in electrolyte pH, the concentration of dissolved species such as HCO3-, CO32- and H2CO3 changes, which in turn affects the rate of cathodic reaction. This has been proven experimentally and theoretically.In 1996, Nesic et al., found that the anodic dissolution reaction rate of iron in CO2 environments is not strongly dependent on pH and the effect was found to be below than the dissolution of iron in HCl solutions.

In CO2 corrosion systems, when the solution pH is less than 4, the available H+ ions makes H+ reduction the dominating cathodic reaction. Also the corrosion rate is found to be flow sensitive at this low pH. The fact that reduction of H+ ions is much more flow sensitive than H2CO3 reduction, causes the corrosion rate to be more sensitive to flow at low pH values (pH<4) than at high pH values (pH >6). Between pH 4 and pH 6, the corrosion rate decreases due to the depletion of H+ in the electrolyte which is required for one of the cathodic reactions in CO2 corrosion. In addition, another significant cathodic reaction takes place: the direct reduction of H2CO3.

The reduction of H2CO3 can be either under charger transfer control or under chemical reaction control which comes from the chemical step: hydration of CO2 into H2CO3. At this intermediate pH (4<6), the cathodic limiting current (ilim) in CO2 solutions decreases threefold since the limiting current is a combination of chemical reaction and a H+- limiting current. The chemical reaction limiting current does not vary with pH, whereas the limiting current for H+ reduction changes with pH and is proportional to [H+]. At pH=6.5, the bulk solution contains about 30% H2CO3 and 70% HCO3- ions, which leads the cathodic process to occur in several pathways simultaneously.

At room temperature either the pH or the Fe2+ ion concentration has to be high (pH> 6) in the bulk solution to attain a protective iron carbonate film on the metal surface in a short time period. The pH has to exceed a critical value above which the concentration of Fe2+ and CO32-+ exceeds the solubility limit and precipitates as iron carbonate. This critical value depends on temperature, Fe2+ concentration, etc. Above the critical value, the concentration of Fe2+ and CO32- ions exceeds the solubility limit and precipitate as iron carbonate on the metal surface. Nesic et al., found that the precipitation of iron carbonate film initiates at a pH equal to 6, but no

protective dense film formation occurs until the pH reaches 6.8 which is the critical pH for the experimental conditions opted by them. Once this critical pH of 6.8 is attained, supersaturation of Fe2+ and CO32- ions exceeded and precipitation of iron carbonate starts. In general the increase in pH of the bulk solution decreases the solubility of iron carbonate and increases the precipitation rate, thereby increasing the rate of formation of the protective iron carbonate layer.

The increase in concentration of H2CO3 and HCO3- ions with increasing pH increases the overall rate of cathodic process until the iron carbonate layer formed as a result of anodic process reaches its critical thickness. The kinetics of film removal by chemical reaction i.e., dissolution, also depends strongly on pH. The concentration of Fe2+ and CO32- in the bulk solution increases as the dissolution of FeCO3 takes place leading to an increase in pH value.

 See Also:

Effect of temperature on Carbon Dioxide CO2 corrosion on carbon steel pipe lines

Effect of hydrodynamics on Carbon Dioxide CO2 corrosion on carbon steel pipe lines 

Effect of CO2 partial pressure on  CO2 corrosion on carbon steel pipe lines

Effect of Fe2+ concentration on Carbon Dioxide CO2 corrosion on carbon steel pipe lines

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