1 Fe - CO2 Corrosion Rahmad Budi Arman Muhammad Pribadi

2 Internal Corrosion The internal corrosion of vessels, equipment and piping in the offshore / onshore facilities depends on internal process fluids and operating conditions. The process fluid may vary from corrosive liquid or gaseous hydrocarbon, corrosive chemicals, produced water and brine. Based on this, the corrosion occurring under these conditions can be broadly classified as:  Sweet corrosion ( CO2 present )  Sour corrosion (H2S or combination of H2S and CO2 present);  Amine corrosion and cracking.  Chloride corrosion.

3 CO 2 Corrosion Wet CO2 corrosion is generally referred to in the oil and gas industry as ‘sweet corrosion’ when H2S is absent. When CO2 is present in the gas phase, any water in contact with this gas will dissolve CO2 to a concentration proportional to the partial pressure PCO2, of the CO2 in the gas.

4 CO 2 Corrosion In the well fluid, CO2 will be in equilibrium between the three phases, water, oil and gas. The quantities of CO2 present in each of these phases are therefore interrelated, with differences in concentration and activity coefficient. In multiphase effluents, which are generally highly turbulent, even if the CO2 is rarely in perfect equilibrium between the three phases, the deviations can never be very large.

5 CO 2 Corrosion Hence,  The CO2 is at its solubility equilibrium in the water and hydrocarbon phases.  The CO2 content of the water is determined by its fugacity in the gas phase in contact with the water, or by default, the last gas phase.

6 CO 2 Corrosion Based on CO2 concentration proportion, partial pressure and temperature, the CO2 corrosion rate will vary. The type of corrosion that takes place in the topside facilities due to CO2 corrosion is usually in the form of localized attack (Mesa) pitting and preferential weld corrosion.

7 CO 2 Corrosion The overall reaction by which CO2 corrodes steel is as follows:  CO2 + H2O → H2CO3 (Carbonic acid)  Fe + H2CO3 → Fe CO3 + H2

8 CO 2 Corrosion Based on this a monogram, which provides a relationship between corrosion rate of steel, temperature and CO2 partial pressure was developed by deWaard and Milliams of Shell International. This was later modified to take care of various parameters, such as high system pressure, velocity, presence of crude, pH, glycol in the system and corrosion protective film formation at high temperatures.

9 CO 2 Corrosion The empirical relationship developed between PCO2 and the corrosion rate is: Log υ =5.8 – log (ƒ CO2) T Υ =predicted corrosion rate for CS (mm/yr) T =Temperature (°K) ƒ CO2=a x PCO2 a=Fugacity Coefficient

10 CO 2 Corrosion  The use of fugacity, rather than partial pressure, allows for the non-ideality of the gas with increasing pressure and temperature.  Starting with the worst-case corrosion rate prediction by the deWaard-Milliams equation, correction factors can be applied to quantify the influence of environmental parameters and of corrosion product scales formed under various conditions.

11 CO 2 Corrosion From field studies, it has been found that carbonic acid can form protective scale above 60°C.  The correction factor Fscale for the basic corrosion rate equation can be calculated: Log Fscale =2400 – 0.6 Log (ƒ CO2) – 6.7 T With a maximum value of Fscale of 1.

12 CO 2 Corrosion From field studies, it has been found that carbonic acid can form protective scale above 60°C.  The correction factor Fscale for the basic corrosion rate equation can be calculated: Log Fscale =2400 – 0.6 Log (ƒ CO2) – 6.7 T With a maximum value of Fscale of 1.

13 CO 2 Corrosion Similarly, pH level has an influence on the corrosion rate. With increase in pH the corrosion rate decreases and this is particularly significant in cases where bicarbonate is present in produced water. Wet CO2 corrosion may be mitigated by the use of inhibitors. The effect of inhibitors varies depending on the type and operating conditions. Usually inhibitors can provide about % efficiency in reducing the corrosion rate.

14 CO 2 Corrosion Taking into account all the above factors the Corrected Corrosion Rate (CCR) is as follows: CCR=UCR x Fc x Ft x Fi x FPH mm/yr UCR=Uninhibited corrosion rate Ft=temperature factor (scaling) Fc=Water Condensation factor Fi=Inhibitor factor (0.05 for 95% inhibitor efficiency) FPH=pH factor

15 CO 2 Corrosion This approach to carbon steel corrosion due to wet CO2 has been incorporated into software known as ‘The Electronic Corrosion Engineer’ and this program has been used for corrosion rate calculations