Effect of Microalloying with Ti on the Corrosion Behaviour of Low Carbon Steel in a 3.5 wt.% NaCl Solution Saturated with CO 2

A problem is defined to investigate the effect of titanium traces on the corrosion behaviour of low carbon steel. In theory titanium effects surface properties like abrasion resistance in medium carbon steels and corrosion resistance in low as well as medium carbon steels. The present research as indicated by the topic is aimed to experimentally mark the effect of titanium traces on corrosion resistance in the available low carbon steel specimens. The effect of microalloying with titanium (i


Introduction
Low-carbon steels (LCSs) are often used for pipeline applications mainly due to their cost [1][2].However, given the extreme conditions in the gas/oil reservoirs, LCS corrodes easily.The majority of failures reported in oilfield are due to CO2 corrosion (sweet corrosion) and H2S (sour corrosion) [3].CO2 is often used to enhance oil and/or gas recovery [4].CO2 dissolves in the operating fluid forming carbonic acid, which corrodes the LCS substrate.[5].Many researchers have studied the effect of some factors such as CO2 partial pressure, temperature, immersion time, and pH on the corrosion resistance of carbon steel [6][7][8][9], but the effect of microalloying elements on the corrosion behavior of alloy steels in CO2 environments is rarely described [10][11].Some elements such as Ti, Al, Cr, etc., when added to the alloy have a strong tendency to stimulate the formation of a uniform passive layer (i.e.passivity promoters) [12] hence, increasing the corrosion resistance of steel.Liu et al. [13] observed that the addition of Ti had a beneficial effect on the corrosion resistance of the low-carbon steel, promoting the formation of a dense and compact corrosions products layer.Yu et al. [1] reported the effect of Ti alloying on the corrosion behavior of the low-carbon steel in a H2S/CO2 environment.The authors observed that the Ti-bearing steel showed an improvement in corrosion resistance compared to the Ti-free steel, resulting from a more compact corrosion product.However, the corrosion behavior of the low-carbon steel in an environment with the presence of pure H2S or H2S/CO2 is different from that of a pure CO2 environment [1].H2S has a strong influence on the corrosion behaviors of the steel, and also in a minimum amount, it is strongly absorbed on the metal surface so that the surface is rapidly covered by a black layer of iron sulfide (i.e.mackinavite) [14].The corrosion of the steel in the presence of CO2 is more complex.The formation of iron carbonate on the metal surface can influence the corrosion resistance of the steel.Moreover, the combination of chlorides-containing fluid and CO2 increases the corrosion rate by a factor of 16 [5].Therefore, the understanding of this type of corrosion is important for corrosion engineering in the oil and gas field.Consequently, this study reports the effect of Ti on the corrosion behavior of low-carbon steel in saline-saturated CO2 environment.In this study, the impact of Ti was assessed via electrochemical experiment such as electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP).The microstructure and morphological analysis (i.e.SEM and Raman) before and after the corrosion test were also performed in this study.

Materials and preparation
Two low-alloy steels with the chemical composition listed in Table 1, named LCS and LCS+Ti, were investigated in this work.The samples were machined into a cylindrical shape and embedded into an epoxide resin, leaving a surface area of 1 cm 2 .Each sample was ground with sandpapers up to a grit size of 1200, degreased by alcohol, and before corrosion tests.

Electrochemical experiments
The measurements were performed on a Gamry reference 600 potentiostat/galvanostat electrochemical system in a threeelectrode cell consisting of a saturated calomel electrode (SCE) as a reference electrode, a platinum foil as a counter electrode (CE), and the tested LCs used as working electrodes.The electrochemical impedance spectroscopy (EIS) experiments were carried out after 24 h of immersion over the frequency range of 10 kHz to 10 mHz and with an amplitude of 10 mV at open circuit potential (OCP).
The increase in corrosion resistance (efficiency, E%) was calculated from the polarization resistance (Rp= Rf + Rct) obtained from the fitting process with the following equation [6]: where R p Ti and Rp are the polarization resistance values in the presence and absence of the titanium, respectively.The potentiodynamic polarization (PDP) experiments were carried out after 24 hours of exposure to the tested solution at OCP, with a sweep rate from -0.3 V to 0.3 V versus OCP, and a scan rate of 1 mV s -1 .The E% was calculated from the measured corrosion current densities (j corr ) values using the following equation [15]: where j corr  and j corr are the corrosion current densities values in the presence and absence of the titanium, respectively.
Where icorr is the corrosion current density; Ew is the equivalent weight of the metal and d is the metal density.3.27 × 10 −3 is the conversion factor.
Each time before a test, the steel was ground and polished up 1 μm, ultrasonically washed in ethanol, rinsed, and dried.The experiments were performed in a 3.5 wt.% NaCl solution saturated with CO2 at T = 25 °C.

Surface analysis
The metal surface of the LCS samples was studied via SEM and Raman spectroscopy analysis after they were exposed for 24 in the tested solution.The SEM analysis was carried out by using a JEOL scanning electron microscope.The Raman measurements were performed by using an InVia Raman Spectrometer Renishaw equipped with a charge-coupled device (CCD) camera and the confocal microscope.The spectra were collected with a spectral resolution of 1 cm -1 and in the 4000-50 cm -1 spectral range in three different pints of the surface [16][17].

Microstructure
Fig. 1a and b, show the microstructure of LCS and LCS+Ti after etching in Nital solution, respectively.As can be seen, the microstructure of the Ti-bearing steel sample appears much finer than that of the Ti-free steel sample.The result is in agreement with the studies reported in the literature that showed that the Ti element was a good inoculant agent for grain refinement [1,18].During the liquid phase solidification, non-metallic micro inclusion particles are formed [19][20].The grain refinement is due to the precipitation of TiC particles at the grain boundary, which inhibits the grain growth [1,[21][22].

Electrochemical experiments
Fig. 2a and b, show the EIS measurements performed on the LCs in the tested solution after 24 hours of immersion time.The EIS plots were fitted with the electric equivalent circuit (EEC) shown in Fig. 1a (inset) and the fitting parameter were listed in Table 2. Rs represents the resistance of the solution.Rf and Qf are the resistance and the capacitance of the film (i.e.corrosion/passive film), respectively.Rct and Qdl are the charge transfer resistance and the capacitance of the double layer, respectively.Due to the imperfection of the metal surface, the double-layer capacitance (Cdl) was simulated via a constant phase element (CPE) [6,15].The impedance of CPE is described by the following equation: where Q is the CPE constant, n is the exponent, j is the imaginary number and ω is the angular frequency at which Z reaches its maximum value.The parameter n quantifies the imperfections of the surface.For n = 1, CPE is a pure capacitor, and for n = −1, CPE is an inductor [6,15].The data shows that Rp increased in the presence of Ti micro-inclusion in the metallic matrix from 231.41 to 325.74 Ω cm 2 , reflecting an improvement in corrosion resistance of around 30%.Moreover, the steel with Ti has a higher Rf compared to the Ti-free steel, indicating that the trace addition of Ti improved the resistance of the low carbon steel [23].This improvement may be ascribed to the formation of a denser corrosion layer on the metal surface, as observed from the PDP experiments.Fig. 3 and Table 3 show the PDP plots and the corrosion kinetic parameters of the steels with and without the presence of Ti after 24 hours of immersion in the tested solution, respectively.As can be seen from the data, the addition of Ti leads to a decrease of a few orders of magnitude in corrosion current density.Furthermore, the anodic region of the Ti-bearing steel shows a narrow activepassive region (Fig. 3 inset), which is in contrast to the Ti-free steel.This small passive region may be ascribed to the formation of a relatively dense corrosion layer.Then as the potential increases, the current increases, which is likely due to the destruction of the corrosion layer.The cathodic curves of the polarization curves also shifted towards lower current densities in the presence of Ti.
. Fig. 3. PDP plot recorded after 24 hours of immersion at OCP in 3.5 wt.% NaCl saturated with CO2 The corrosion process in the presence of CO2 is controlled by the cathodic (Eq.5-7) and the anodic reactions (Eq.8) [9,24]: 2H + + 2e -→ H 2 (7) Fe → Fe 2+ + 2e - The pH of the tested solution is 4.3.At pH>4, the main cathodic reactions are reactions 5 and 6.The active domain referred to the metal dissolution (Eq.8) [16].The change in the shape observed is likely attributed to the formation of a thin passive protective surface film on the metal surface due to the presence of Ti, in agreement with the literature [12,23,25].The results suggest that the addition of titanium hindered both the rate of the anodic dissolution (Eq.8) and the cathodic reactions (Eq.5-6), by either covering part of the metal surface and/or blocking the active corrosion sites on the steel surface.The efficiency calculated with the Eq. ( 2) shows an improvement of around 40%, which is consistent with the result of EIS.

Surface analysis
In a CO2-containing solution, carbon dioxide dissolves to form H 2 CO 3 , which it dissociates in HCO 3 -and in CO 3 2-, according to the following reactions [26]: H 2 CO 3 ↔ H + + HCO 3 -( 10) If the concentrations of Fe 2+ and CO 3 2-ions exceed the solubility limit, FeCO3 precipitates on the surface [27][28].However, no traces of FeCO3 were found, but the Raman results reported in Fig. 4 and Table 4 show that the corrosion products observed were mainly composed of magnetite (Fe3O4) and α-Fe2O3 or/and γ-Fe2O3 [29][30][31].The presence of these species is likely ascribed to the decomposition of FeCO3, according to the following reaction [9,27]: In turn, FeO transforms into Fe3O4 [9,27].
In the air: No traces of titanium oxide were observed, evidencing that the thickness of the passive TiO2 layer was at a trace level below the detection limit of the instrument.
The SEM images of the steels after PDP experiments are shown in Fig. 5.The Ti-bearing steel showed a smoother surface compared with the Ti-free steel.The improvement may be attributed to the formation of a thin TiO2 passive layer on the Ti-bearing surface resulting from the minor addition of Ti [25].

Conclusions
The effect of microalloying with Ti on the corrosion behavior of low-carbon steels in a saline CO2-containing environment can be summarized in the following points:

•
The microstructure of Ti-bearing steel was finer compared to Ti-free steel.

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The EIS experiments showed that the addition of Ti improved the corrosion resistance of the LCS by around 30%.Moreover, the EIS experiment showed that the value of Rf (i.e., the polarisation resistance of the film) of the steel with Ti was higher than the Ti-free steel, indicating that Ti improved the corrosion resistance of the steel.

•
The PDP results showed that microalloying with Ti, the corrosion current density decreased and a small passive region is observed.

•
The corrosion products were mainly composed of different types of iron oxide.