Research Article | Open Access
Anodic Dissolution of API X70 Pipeline Steel in Arabian Gulf Seawater after Different Exposure Intervals
The anodic dissolution of API X70 pipeline steel in Arabian Gulf seawater (AGSW) was investigated using open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS), cyclic potentiodynamic polarization (CPP), and current-time measurements. The electrochemical experiments revealed that the X70 pipeline steel suffers both general and pitting corrosion in the AGSW solution. It was found that the general corrosion decreases as a result of decreasing the corrosion current density (), corrosion rate () and absolute currents as well as the increase of polarization resistance of X70 with increasing the exposure time. On the other hand, the pitting corrosion was found to increase with increasing the immersion time. This was confirmed by the increase of current with time and by the SEM images that were obtained on the steel surface after 20 h immersion before applying an amount of 0–.35 V versus Ag/AgCl for 1 h.
API X70 pipeline steel is characterized by its good combination of strength and toughness, good weldability, low crack sensitivity coefficient, and low ductile to brittle transition temperature . Therefore, several studies [1–5] have been conducted on the corrosion and corrosion protection of X70 pipeline steel in different aggressive media. Alizadeh and Bordbar have  reported the influence of microstructure on the protective properties of the corrosion product layer generated on the welded API X70 steel in sodium chloride solutions and found that the corrosion resistance for the steel was increased after heat treatment due to formation of fine and compact corrosion product layer with fewer defects. Bordbar and Alizadeh  also investigated the effects of microstructure alteration on corrosion behavior of welded joint in API X70 pipeline steel. Li et al.  studied the anodic dissolution of the X70 steel in H3PO4 solution with the halide ion perturbation at the interface and reported that the way how the halide ions affect the anodic dissolution is related to the types of the ions and the property of the film formed on the surface of the electrode. The corrosion of welded X70 pipeline steel in near-neutral pH solution was also characterized by Zhang and Cheng  using the microelectrochemical technique and confirmed that the resistance of corrosion product layer of the steel decreases with hydrogen charging and heat-affected zone has the largest dissolution current upon hydrogen charging. Furthermore, Sirong et al.  have invented a new method for preparing bionic multiscale superhydrophobic functional surface on X70 pipeline steel.
It is generally believed that general and pitting corrosion of passivated pipeline steel often occur when this steel is in contact with aggressive ions such as the chloride ions. In this regard, the corrosion of steel in various media such as carbonate/bicarbonate solution [6–9] and acidic solutions [3, 10, 11] has been reported. Some other investigators have focused their studies on the corrosion of steels under disbanded coating [12–14] and hydrogen damage . Although there are a lot of investigations on the corrosion of API X70 pipeline steel, the corrosion of this steel in Arabian Gulf seawater (AGSW) has not yet been reported to the best of our knowledge. The aim of this work was to investigate the corrosion of API X70 pipeline steel after its immersion in the freely aerated AGSW for different exposure periods, namely, 1 h and 20 h, using varied electrochemical techniques such as potential-time, electrochemical impedance spectroscopy, cyclic polarization, and potentiostatic current-time measurements along with scanning electron microscopy investigations. A particular attention was paid to the effect of immersion time on the pitting corrosion of the API X70 pipeline steel.
2. Materials and Methods
2.1. Materials and Chemicals
API X70 pipeline steel with a rectangular shape and dimension of 1 cm length and 1 cm width was employed in this study. The main chemical compositions for this steel were 0.04% C, 1.70% Mn, 0.035% , 0.035% , and 0.55% ; all these elements were in mass percent. Arabian Gulf seawater was brought from the Arabian Gulf at Dammam, Saudi Arabia.
2.2. Electrochemical Corrosion Measurements
The electrochemical measurements were collected using a conventional electrochemical cell with a three-electrode configuration. The API X70 pipeline steel rod, a platinum foil, and an Ag/AgCl electrode (in saturated KCl solution) were the working, counter, and reference electrodes, respectively. Before measurements, the surface of the working electrode was ground successively with metallographic emery paper of increasing fineness up to 1200 grit; it was then cleaned using doubly distilled water, degreased with acetone, washed using doubly distilled water again, and finally dried with dry air.
The electrochemical measurements were carried out using an Autolab Potentiostat (PGSTAT20 computer controlled) operated by the general purpose electrochemical software (GPES) version 4.9. The electrochemical impedance spectroscopy (EIS) tests were performed at open-circuit potential (OCP) over a frequency range of 100 kHz to 100 mHz, with an ac wave of ±5 mV peak-to-peak overlaid on a dc bias potential, and the impedance data were collected using Powersine software at a rate of 10 points per decade change in frequency. The cyclic potentiodynamic polarization curves were obtained between −1.2 and 0.25 potential rang at a scan rate of 1 mV/s versus Ag/AgCl. Potentiostatic current-time experiments were carried out by stepping the potential of the steel samples at −0.350 V versus Ag/AgCl. All experiments were carried out using a fresh electrode surface at room temperature in a cell that contains 200 mL of the test solutions.
2.3. Scanning Electron Microscope (SEM) Investigations
The SEM images were obtained by using a JEOL model JSM-6610LV (Japanese made) scanning electron microscope with an energy dispersive X-ray analyzer attached for acquiring the EDX analysis.
3. Results and Discussion
3.1. Open-Circuit Potential (OCP) Measurements
Figure 1 shows the variation of the potential of API X70 pipeline steel in AGSW against time for circa 20 h. It is seen that the potential of the steel in AGSW increased towards the more negative values from the first moment of electrode immersion, which resulted from the dissolution of an air oxide film which was formed on the steel surface before its immersion. The potential further decreased in the more negative direction due to the continuous dissolution of the steel surface under the aggressiveness action of the anions present in the AGSW. After about 4 h, the steel potential abruptly shifted in the less negative direction indicating that the steel surface developed an oxide film and/or corrosion product layer that could provide certain protection and decreased the corrosive attack of the AGSW on the surface. The potential then continued shifting in the less negative direction with time till the end of the run. This was due to the oxide film and/or corrosion product thickening with time, which provided more protection for the steel surface and led to the increase of potential towards the less negative values.
3.2. Electrochemical Impedance Spectroscopy (EIS) Measurements
Figure 2 shows the EIS Nyquist plots obtained for the API X70 pipeline steel after its immersion for (a) 1 hour and (b) 20 h in Arabian Gulf seawater. The data obtained from Figure 2 were best fitted to the equivalent circuit model depicted in Figure 3. The values of the elements of the equivalent circuit shown in Figure 3 are listed in Table 1, where RS represents the solution resistance, Q the constant phase elements (CPEs), and RP the polarization resistance [16–19].
It is clearly seen from Figure 2 that there is only one semicircle whether the immersion time was 1 h or 20 h. The diameter of the semicircle is higher for the steel that was immersed for 20 h, curve (b), than that obtained after 1 h. This indicates that increasing the immersion time decreases the aggressiveness action of the AGSW towards the steel, which is due to the fact that the increased immersion time allows the surface to develop a corrosion product layer and/or an oxide film that gets thicker with time and provides some protection to the surface against corrosion. This was also confirmed by the data recorded in Table 1, where the RS and RP values are higher for the steel that was immersed for 20 h in AGSW before measurements. The constant phase elements (CPEs, Q) with their n values are around 0.8; CPEs thus represent double layer capacitors because their n values are close to unity. The value of Q decreased with increasing time, which proves that the mass transport from surface is limited, particularly after 20 h of the steel immersion in the AGSW [20–22]. According to Arzola-Peralta et al. , who studied the corrosion of X70 pipeline steel at similar conditions, at high frequencies a transfer process takes place. This emphasizes the fact that increasing the immersion time before measurements decreases the corrosion of the steel, which can result from the formation of an oxide film and/or corrosion products that could partially protect the surface from being attacked by the chloride ions present in solutions under investigations.
The data obtained from Nyquist plots were also confirmed by the Bode impedance and Bode phase angle plots that are shown in Figures 4 and 5, respectively, for the API X70 pipeline steel electrode after its immersion in AGSW for (a) 1 h and (b) 20 h. Figure 4 depicts that the impedance of the interface for the X70 increases with increasing the immersion time from 1 h to 20 h, particularly at lower frequency values. According to Mansfeld et al. , the high impedance values at the low frequency region confirms the passivation of the surface. This indicates that the increase of the immersion time from 1 h to 20 h increases the corrosion resistance of the steel in AGSW through increasing the passivity of its surface. Figure 5 also showed that the increase of the immersion time from 1 h to 20 h before measurements increased the maximum degree of the phase angle, which gives further confirmation on the increased surface passivity with increased exposure intervals.
3.3. Cyclic Potentiodynamic Polarization (CPP) Data
The CPP curves obtained for API X70 pipeline steel after its immersion for (a) 1 h and (b) 20 h in the Arabian Gulf seawater are shown, respectively, in Figure 6. These curves were collected in order to understand the change of current with potential and to report the corrosion parameters for the API X70 steel after varied exposure intervals in AGSW. The values of the corrosion parameters such as cathodic (βc) and anodic (βa) Tafel slope, corrosion potential (ECorr), corrosion current density (jCorr), pitting potential (EPit), pitting current density (jPit), protection potential (EProt), polarization resistance (Rp), and corrosion rate (RCorr) obtained for the API X70 steel electrodes from the CPP curves shown in Figure 6 are listed in Table 2. The values of all these parameters were obtained as reported in the previous studies [16–19].
It has been reported [24–28] that the cathodic reaction of metals and alloys in an open to air near neutral solution is the oxygen reduction. On the other hand, the anodic reaction takes place via the dissolution of iron from Fe(0) to Fe(II). According to Alizadeh and Bordbar , the cathodic and anodic reactions can be represented, respectively, as follows: In the chloride containing carbonate/bicarbonate, which is the case of the natural seawater, AGSW, the FeCO3 can be formed and deposited on the steel surface as follows : Fu and Cheng  have also reported that the formation and the deposition of FeCO3 provide an inhibition for the steel surface from further dissolution.
It is clearly seen from Figure 6 that the X70 pipeline steel shows an active-passive anodic behavior, where the steel forms a passive region in the potential range between −800 and −400 mV for the steel in the AGSW solution; the appearance of such region is due to the formation of an oxide film and/or corrosion products. At these conditions, the corrosion products might be iron carbonate and chloride compounds such as FeCO3, FeCl2, and FeOCl and the oxides can be Fe3O4 and Fe2O3 . The formation of such oxides can be represented as follows :
Increasing the applied potential in the less negative direction leads to increasing the output current due to the dissolution of the formed passive film and the occurrence of pitting corrosion. Reversing the potential in the backward scan resulted in an increase in the output current and the appearance of a hysteresis loop, which indicates the occurrence of pitting corrosion for the API X70 steel in the AGSW test medium. Table 2 also indicated that the jCorr, anodic current, and RCorr recorded higher values, while the (Rp) is lower for the steel in AGSW after 1 h immersion. Increasing the immersion time from 1 h to 20 h decreased the values of jCorr and RCorr and increased the values of (Rp). This also agrees with the impedance data that the corrosion of API X70 steel decreases with increasing the immersion time before measurements.
3.4. Potentiostatic Current-Time and Scanning Electron Microscopy Investigations
The variation of the potentiostatic current versus time at −0.350 mV versus Ag/AgCl after (a) 1 h and (b) 20 h in AGSW is shown in Figure 7. These experiments were carried out to report the anodic dissolution of the API X70 steel at an active anodic potential and to see whether pitting corrosion occurs after varied exposure periods in the test solution. The current of the steel after 1 h immersion in AGSW, curve (a), recorded very high value that increased in the first few seconds before decreasing again till the first 400 s. The increase of current might have resulted from the dissolution of an oxide film which was formed on the surface due to the contact with the solution. On the other hand, the decrease in the current values was either due to the formation of an oxide and/or corrosion products film that decreases the effect of the aggressiveness action of the test solution. The current then shows almost a stable change with increasing time up to the end of the run.
Increasing the immersion time to 20 h before measurements could decrease the initial currents for steel in the AGSW solution to its minimum due to the formation of corrosion products and/or passive film on the electrode surface during its immersion before applying the constant potential. The current then gradually increases with increasing the time of the experiment as a result of the dissolution of the formed passive film and the occurrence of pitting corrosion. This can be also explained by the dissolution of iron as represented by (2) and then the formation of FeCl2 and FeC13 on the surface of the steel according to the following reactions [31, 32]: Due to the applied potential and concentration gradients, the FeCl2 and FeC13 species at the interface diffuse through the porous film and the diffusion boundary layer and are then carried away to the bulk solution leading to a continuous dissolution of the alloy and the occurrence of pitting corrosion as indicated by the continuous increase of the current with time.
This was confirmed by the SEM micrographs obtained for the API X70 pipeline steel surface after performing the current-time experiment shown in Figure 7 (curve (b)). Figure 8 shows the SEM micrographs obtained for the API X70 pipeline steel surface after its immersion for 20 h then applying −0.35 mV for 1 h, which shows an increased magnification for the steel surface. Figure 8(a) depicts that the majority of the steel surface has few pits and some of these pits were propagated and could lead to the continuous increase of the current with time which we have seen on curve (b) of Figure 7. Figures 8(b) and 8(c) show clear images of the propagated pits and prove that the increase of current with time at −0.35 V was due to the occurrence of pitting corrosion. The data obtained by potentiostatic current-time thus confirm the data obtained by EIS and polarization measurements that increasing the immersion time of the API X70 pipeline steel decreases the uniform corrosion and increases the pitting one.
The corrosion of API X70 pipeline steel after 1 h and 20 h immersion in Arabian Gulf seawater (AGSW) has been reported using variety of electrochemical measurements. Potential-time experiments indicated that the potential of the steel shifts to the more negative values in the first few minutes of the immersion and then forms a layer of oxides and/or corrosion products that partially protect the steel surface and shift its potential towards the less negative values with time. Electrochemical impedance spectroscopy data revealed that X70 steel shows one semicircle in AGSW solution and increasing the immersion time to 20 h increases the surface and polarizations resistances. Cyclic polarization technique confirmed that the steel shows lower corrosion current and corrosion rate and higher polarization resistance in AGSW with the increase of the immersion time from 1 h to 20 h, which indicates that the general corrosion decreases and the pitting corrosion increases. Potentiostatic current-time experiments at −0.35 mV versus Ag/AgCl indicated that the X70 steel confirmed the data obtained by EIS and polarization measurements that the current recorded higher values with 1 h exposure period. On the other hand, increasing the time to 20 h decreased the absolute currents of the steel, where the initial currents were very low and increased with increasing the time of the applied potential. All measurements thus were consistent with each other and proved that the increase of the immersion time of the API X70 steel in the AGSW solution before measurements decreases the uniform corrosion through decreasing the absolute current values, while it increases the pitting corrosion by increasing the current with time.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors extend their appreciation to the Deanship of Scientific Research at KSU for funding the work through the research group Project no. RGP-VPP-160.
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Copyright © 2014 El-Sayed M. Sherif and Abdulhakim A. Almajid. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.