Electrochemical Corrosion Behavior of X 52 and X 60 Steels in Carbon Dioxide Containing Saltwater Solution

X52 and X60 high strength low alloy (HSLA) steels are widely used in the construction of petroleum pipelines. This paper discusses the corrosion resistance of X52 and X60 steels in CO 2 containing saltwater at pH 4.4 and 50 °C. A circulating flow loop system inside an autoclave was used for conducting the experimental work. The rotating impeller speed was 2000 rpm. The corrosion rate was monitored using in situ electrochemical methods such as potentiodynamic sweep, linear polarization resistance, and electrochemical impedance spectroscopy (EIS) methods. Results indicated that the corrosion rate of X60 steel is relatively higher than that of X52 steel.


Introduction
Carbon dioxide (CO 2 ) gas is one of the most common gases which occurs with the crude oil.The dissolution of CO 2 gas in water forms corrosive environments which attack the carbon steel pipe walls 1 .CO 2 gas occurs naturally in oil and gas reservoirs and dissolves in water to form carbonic acid (H 2 CO 3 ).
The processes, which simultaneously take place in CO 2 corrosion of mild steel are 2 : Chemical reactions.CO 2 is sparingly soluble in water (Reaction 1) and produces H 2 CO 3 , which is a weak acid (Reaction 2): H 2 CO 3 partially dissociates in two steps to form bicarbonate ( -3 HCO ) ions (Reaction 3) and carbonate ( 2- 3 CO ) ions (Reaction 4): The process of homogenous Reactions 3 and 4 occur at much faster rates than the other simultaneously occurring processes in the system.The processes of Reaction 1 and particularly Reaction 2 have been known to occur as much slower processes (rate controlling), and may lead to local non-equilibrium in the system.
The formation of solid iron carbonate (FeCO 3(s) ) (Reaction 5) is an important chemical reaction usually observed in aqueous CO 2 solutions: The precipitation of FeCO 3(s) (predominantly on the surface of the steel) will occur only once the concentrations of local Fe 2+ and CO 3 2-exceed the solubility limit of FeCO 3(s) .The formation of FeCO 3(s) usually plays an important role in the process of corrosion since the FeCO 3(s) layer increases the mass transfer resistance of the corrosive species and reduces the exposed steel surface area to the corrosive environment.In fact, in many cases, the presence of the FeCO 3(s) layer largely controls the CO 2 corrosion rate.
Electrochemical reactions.There are a number of electrochemical reactions that take place in CO 2 corrosion process.The reduction of H + (Reaction 6) is considered one of the key cathodic processes.2 H e H + + → (6)   Reaction 6 is usually limited by how fast the H + can be transported from the bulk solution to the steel surface through the mass transfer boundary layer and the FeCO 3(s) layer (if it exist).
The adsorption of H 2 CO 3 at the steels surface (Reaction 7) is followed by the reduction of H + , which is referred to as "direct reduction of carbonic acid".Reaction 7 is, in essence, an alternative pathway of Reaction 6.
The direct reduction of water is another possible pathway for hydrogen evolution (Reaction 8).Reaction 8 is very slow compared to the above cathodic reactions and commonly is neglected in estimating effects of practical CO 2 corrosion environments.
For the anodic reaction, there is often only one dominant anodic reaction (at the corrosion potential) is involved in the CO 2 corrosion process.Iron oxidation is the anodic reaction of mild steel in CO 2 corrosion process (Reaction 9).
Transport.Concentration gradients are built up between steel surface and bulk solution as a result of the consumption and evolution of certain species at the steel surface, which leads to molecular diffusion.The rates of the overall reaction by transport will be limited compared to some fast electrochemical reactions such as Reaction 6 since mass transfer of H + proceeds much slower.
Oxygen can contribute to the corrosion process by oxygen reduction reaction (Reaction 10) and by hydrogen evolution from direct reduction of water (Reaction 11), although it is not considered a common corrosive specie in oil and gas pipeline systems 3 .
Reaction 10 is comparatively very slow, always possible, and is important only at CO 2 partial pressure << 0.1 bar and pH > 6 4,5 .
Experimental research conducted by Han et al. 6 indicates that the formation of a FeCO 3 layer only failed to passivate the surface.The surface passivated after the formation of magnetite phase (Fe 3 O 4 ).
The loss in the metal mass caused by general corrosion is generally proportional to temperature and CO 2 partial pressure 7 .Nevertheless, it is well documented that a precipitation layer on the steel surface is promoted at certain conditions, i.e. elevated temperature and pH > 6.This layer is reported to be iron carbonate (FeCO 3 ) 8 and it has a mitigating effect on CO 2 corrosion 9 .
Modelling has been performed by a number of researchers 2,3,[10][11][12] .The state-of-the-art in modelling of internal corrosion of oil and gas carbon steel pipelines was reviewed by Nesic 13 .
The CO 2 corrosion in petroleum facilities is a serious concern since it is difficult to predict, understand and control.The failure of in-service components as a result of erosion-corrosion and CO 2 corrosion has long been responsible for major safety concerns, lost production time, and cost in the maintenance of the steel materials used in petroleum industries.
Approximately 60% of the failures in oilfields are related to CO 2 corrosion.This CO 2 failure is mainly related to low corrosion resistance of carbon steels and to lack of prediction 14 .CO 2 corrosion has been reported as the major cause of failures in some oilfields 15 .
In latest years, the mechanical properties of plain carbon steels have been greatly improved by micro alloying the plain carbon steels with small amounts (max 0.1 wt.(%)) of strong carbide and nitride forming elements such as Nb, Ti and V.The produced steels are known as High Strength Low Alloy (HSLA) steels 16 .
HSLA steels are widely used in oil and gas production and transportation pipelines.HSLA steels have a low price-to-yield strength ratio.The weldability of HSLA steels is considered good due to their low carbon contents.The minimum yield strength value is used to designate the API grade HSLA steels.For example, the designed yield strength of API-X60 steel is 60 ksi (414 MPa) 16 .
Data on electrochemical corrosion behavior of HSLA steels exposed to CO 2 containing solutions are not widely reported or available.Moreover, the use of a flowing system inside an autoclave is not commonly investigated as part of the other testing methods mentioned above.Therefore, it is important to study the behavior of the HSLA steels in CO 2 containing solution environment.The study presented below aims at filling in this gap.In addition to generating new knowledge on corrosion of HSLA steels in CO 2 containing solution environment using flowing system inside an autoclave.The objective of this work was to compare the corrosion behavior of two HSLA steels (X52 and X60) that are commonly used in pipe fabricated for use in petroleum transport pipelines.The steels were exposed to CO 2 containing saltwater solution in a circulating fluid system contained within an autoclave.

Experimental Apparatus
The apparatus used in this experimental work was a high-pressure/high-temperature autoclave.This autoclave is equipped with fluid circulating system as illustrated in Figure 1.The main components of the experiment setup are described below.

Autoclave (heating pressure vessel)
The autoclave is a 5.6 liter pressure vessel acting as a reservoir for the solution surrounded by an electric heater to heat the solution.The autoclave is made from 316 stainless steel internally clad with Hastelloy C276, which has an excellent erosion-corrosion resistance to hot acidic solution.It is equipped with a variable speed motor to rotate the impeller, a pocket to house a thermocouple for measuring the temperature of the solution inside the autoclave, and a CO 2 gas connection.The design pressure is 20.7 MPa at 260 °C.

Electrochemical testing
The experimental set up is equipped with the following electrodes in order to perform electrochemical measurements: Working electrodes.There are two working electrodes inserted on the working electrode shaft as illustrated in Figure 1.The working electrode is a circular ring inserted on the working electrodes shaft.The dimensions of the working electrode are 25 mm outside diameter, 15 mm inside diameter, and 7.5 mm height.The working electrodes are made from the material to be tested, which is X52 and X60 HSLA steels in this study.The working electrodes were machined from actual X52 and X60 HSLA pipes.Counter electrode.The counter electrode is a circular ring placed around the working electrodes as illustrated in Figure 1 in order to ensure a symmetrical current distribution during electrochemical polarization measurements.The counter electrode is made from Hastelloy C276.
Reference electrode.There is only one external Ag/AgCl reference electrode attached to the autoclave.The reference electrode is a simple Ag rod coated by AgCl which was prepared by anodically polarizing the Ag rod in a saturated KCl solution at room temperature.A current density of 10 mA.cm -2 was used for three minutes in order to coat the rod with a durable AgCl layer.In the experiments the reference electrode was continuously wetted by the solution which was extracted from the autoclave and cooled to room temperature, by natural convection.The IR drop resulting from varying distance between the reference electrode and the two working electrodes was measured to be very small due to the high conductivity of the solution.

Flow straightener
The flow straightener is placed just before the working electrodes shaft as shown in Figure 1 in order to stabilize the flow.The flow straightener allows the flow to pass through four separate paths and recombine again before passing over the working electrodes shaft.

Experimental
The specimens were prepared by grinding with 600 grit paper, then washed by ethanol and allowed to dry.The electrolyte was made by dissolving 3 wt.(%) NaCl in distilled water.The selected concentration of NaCl was chosen to simulate to the concentration in actual oil production process.The NaCl was analytical reagents grade.
The NaCl solution was placed in the autoclave and the solution was heated to 50 °C while running the motor at a speed of 2000 rpm and de-aerating the solution with CO 2 for about 3 hours till the pH reached 4.4, then the following electrochemical measurements started.The impedance spectra were normalized to 1 cm 2 .

Results and Discussion
The corrosion rate was monitored using in situ electrochemical methods such potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy (EIS) methods.

Potentiodynamic polarization measurements
The potentiodynamic polarization curves of the X52 and X60 steels are illustrated in Figure 2. The scan range was -250 to 250 mV vs. open circuit potential (E oc ) and the scan rate was 0.167 mV/s.The potentiodynamic polarization curves indicate that E corr of X52 steel (-437 mV) was relatively more positive than that of the E corr of X60 steel (-454 mV).
Figure 2 shows that cathodic curves of X53 and X60 steels showed little change, although the anodic curves were dependent on the steel grades.The anodic current density of X60 steel was slightly higher than that of X52 steel.
The values of anodic (β a ) and cathodic (β c ) Tafel slopes of the X52 and X60 steels were determined as illustrated in Table 1.

Polarization resistance measurements
This method was used to obtain the polarization resistance (R p ) from the slope of the potential/current curve.The measurements were done by polarizing the working electrode 6 mV above and below the open circuit potential at scan rate of 0.1 mV/s.
The corrosion current (i corr ) was calculated by using Equations 1 and 2: where: i corr is the corrosion current density in A.m -2 ; R p is the polarization resistance in Ω.m 2 and B is the proportionality constant in V.decade -1 : 2.3( ) where: The corrosion rate (CR) measured by the electrochemical method was calculated by using Equation (3): where: w is the equivalent weight of Fe, F is Faraday constant, and ρ is the density of Fe.
The corrosion rate of X60 steel was slightly higher than that of X52 steel (Figure 3).The corrosion potential (E corr ) of X60 is relatively more negative than that of X52 steel (Figure 4) which indicates that the corrosion rate of X60 steel is more than that of X52 steel which fully agreed with Figure 3.These results are consistent with data obtained from potentiondynamic polarization technique.
The standard deviation and confidence level (95.0%) of the corrosion rate and corrosion potential were determined using Microsoft Excel as illustrated in Table 2.
In order to quantitatively evaluate the effect of the CO 2 containing saltwater on the resistance of each of the two steel grades to erosion-corrosion, the total amount of metal lost during the whole experiment was determined for X52 and X60 steels, by integrating the areas under the corrosion rate curves shown in Figure 3.The results for the two steels are shown in Figure 5.It can be seen that the metal loss of X60 steel was higher than that of X52 steel.

Electrochemical impedance spectroscopy (EIS) measurements
EIS measurements are shown in Figures 6 (Nyquist) and 7 (Bode).The frequency range is 0.1-10 6 Hz.EIS results show a clear difference between the two materials, and are considered more useful than polarization techniques for comparison of the materials.The corrosion resistance was found, in comparison, to be 196 Ω.cm 2 (X52 steel) and 147 Ω.cm 2 (X60 steel).The EIS measurements thus confirm the findings from potentiodynamic polarization and linear polarization resistance techniques.
Based on the previous findings, the corrosion rate of X60 steel grade is higher than that of X52 steel grade as measured by potentiodynamic polarization, linear    polarization resistance, and electrochemical impedance spectroscopy methods.The corrosion rates of both steel grades decreased significantly and the corrosion potential increased significantly during the first hour of immersion then stabilized with little fluctuation during the rest of the experiments.This significant decrease in corrosion rate and increase in corrosion potential might be attributed to the formation of a protective film.
There were no major differences in the final corrosion rates, measured at the end of the experiment, across the  two working electrodes.They varied between 0.5 and 0.7 mm.y -1 .Moreover, it seems that, passivation happened within the first hour of the experiment for both steels substrates, which was confirmed by the rapid increase in the corrosion potential.

Microstructure
The two steels (X60 and X52) have similar microstructure, as shown in Figure 8 (SEM imagery).Both images show pearlite (dark) with ferrite (bright) at the grain boundaries, and with relatively higher ferrite content for X52 steel.This is in agreement with similar microstructure obtained by others [37][38][39] .The microstructure has an effect on corrosion through the galvanic effect between pearlite and ferrite, which enhances the corrosion of the ferrite 39 because the cementite which is contained in the pearlite is electrochemically more stable than ferrite 40 .The higher corrosion rate of X60 steel can be related to the relatively lower content of ferrite than that of X52 steel.

Conclusion
Two principal conclusions may be drawn: Polished HSLA steel coupons (X60 and X52) showed corrosion rates (typically) of 1.0 to 1.8 mm.y -1 while exposed to CO 2 containing saltwater, although the rate was reduced after one hour.This rate reduction is attributed to formation of a protective film.
Steel of the type X52 showed superior resistance to corrosion as compared X60 steel, under conditions of pH 4.4 and 50 °C.

Figure 1 .
Figure 1.A cross section of the autoclave.

Figure 2 .
Figure 2. The potentiodynamic polarization curves of the X52 and X60 steels.
of X52 and X60 Steels in Carbon Dioxide Containing Saltwater Solution

Figure 5 .
Figure5.The total amount of metal lost for X52 and X60 steels.