Influence of alternating voltages on passivation and corrosion properties of X80 pipeline steel in high pH 0.5 mol L − 1 NaHCO 3 +0.25 mol L − 1 Na 2 CO 3 solution

Material and solutions

The material studied was the API X80 pipeline steel with the chemical composition of Fe–0.036C–0.197Si–1.771Mn–0.012P–0.002S–0.223Cr–0.278Ni–0.22Cu–0.021Al–0.019Ti–0.184Mo–0.001V–0.11Nb–0.005N (wt-%). The X80 steels with a dimension of 10 × 10 × 3 mm were used as working electrodes. The specimens were coated with epoxy resin, leaving an exposure surface area of 1 cm². The working surface of the electrodes was ground sequentially up to 2000 grit emery paper, followed by rinsing with double distilled water and finally dried with ethanol.

The test solution was a carbonate/bicarbonate solution with a concentration of 0.25 mol L^{− 1} Na₂CO₃+0.5 mol L^{− 1} NaHCO₃ (pH 9.4), which is a simulation solution of the high pH environment under coating effects formed by a reaction between carbon dioxide in soil and hydroxyl ions produced by CP, and these ions protect the pipeline steel surface from corrosion. The solution was made from analytic grade reagents (Fisher Scientific) and ultrapure water (18 MΩ cm in resistivity).

Electrochemical measurement

The electrochemical measurement of X80 steel in the concentrated carbonate/bicarbonate solution without superimposition of AVs was performed using a three-electrode cell through a Princeton 2273 electrochemical system. The X80 steel electrode was used as the working electrode, a platinum sheet as counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The OCP of X80 steel in the solution with and without cathodic prepolarisation [ − 1.3 V(SCE) for 5 s] was recorded from 0 to 5 h. The potentiodynamic polarisation curve was measured in the potential range from − 1.0 to 1.1 V(SCE) with a scanning rate of 0.5 mV s^{− 1}. Electrochemical impedance spectroscopy measurement was performed after the X80 steel was immersed in the carbonate/bicarbonate solution for 0 and 5 h. The ac disturbance signal of the EIS was 10 mV peak to peak and in a frequency range from 100 kHz to 0.01 Hz. The surface morphology of the X80 steel in the solution before and after immersion was observed using a Nikon S4200 digital camera.

Alternate current experimental procedure

The ac experimental set-up is shown in Fig. 1. The sinusoidal ac voltage signals were supplied through a slide rheostat using a Tektronix AFG3000 signal generator. The X80 steel was used as the working electrode, and two working electrodes, namely, WE_left and WE_right, which were the same in material and exposure area, were connected to the two poles of the ac generator. The ac voltage amplitude between the two WEs was measured using a FLUKE289 electric multimeter. The OCP values of the two WEs (versus SCE) were also measured using the FLUKE289 electric multimeter. To investigate the influence of ac voltage amplitude on the corrosion of X80 steel in the concentrated carbonate/bicarbonate solution, sinusoidal ac voltages of 100, 300, 500 and 700 mV(rms) in amplitude and 50 Hz in frequency were superimposed to the two WEs. The ac voltage was applied for 5 h, and the OCP of the two WEs was recorded. The EIS of the two WEs was then measured using a Princeton 2273 electrochemical system, and the corrosion morphology was observed through a Nikon S4200 digital camera. All tests were performed at ambient temperature (∼22°C) and repeated several times to ensure the accuracy and reproducibility of the results.

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Experimental set-up of application of alternating voltages on X80 pipeline steel

Electrochemical properties without AV application

Figure 2 shows the polarisation diagram of the X80 steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution. The metal displays an active–passive transition when anodically polarised, and two anodic current peaks are observed in the polarisation curve. The current peak at the lower potential [ − 0.67 V(SCE)] is attributed to the reactions below ¹⁸

1

2

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Potentiodynamic polarisation curve of X80 steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution

The iron carbonate is a rust layer on the steel surface, which can slow down the corrosion process by presenting a diffusion barrier for the species involved in the corrosion process and by covering up a portion of the steel surface and preventing the underlying steel from further dissolution.

The second current peak at higher potential [ − 0.29 V(SCE)] is ascribed to the formation of γ-Fe₂O₃/Fe₃O₄ film ¹⁹ 3

4

5

Therefore, a more compact passive film is formed, and passivation of the steel occurs. From Fig. 2, two corrosion potentials are observed, which means that a self-passivation will be achieved if the X80 steel is immersed in the solution, which can be explained by the Evans diagram in Fig. 3. Figure 3 illustrates that the equilibrium potential of the cathodic reaction E _e,c is higher than the passivation potential E _P, thereby resulting in the two corrosion potentials, one at the activated potential region E _corr1 and the other at the passive potential region E _corr2. The cathodic reduction current density at E _p is equal to the passive current density. Thus, self-passivation can be achieved when the steel is immersed in this solution. ²⁰

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Evans diagram of X80 steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution

The above analysis was proved by the OCP measurement (Fig. 4) and EIS testing (Fig. 5). Figure 4 shows that the OCP of the non-prepolarised steel rapidly increases and then stabilises at about − 0.36 V(SCE) in the solution. In this situation, the surface of the steel is covered by a compact passive film. On the contrary, when a − 1.3 V(SCE) cathodic potential was applied to the steel before OCP testing, the OCP stabilises at about − 0.85 V(SCE), indicating that the steel is active. This is because the cathodic prepolarisation creates a fresh surface that promotes the anodic reaction and shifts the OCP negatively.

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Open circuit potential of X80 steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution

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a non-prepolarised; b prepolarised

Figure 5 shows the EIS of the non-prepolarised steel and the prepolarised steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution. Fitting of the spectra was performed using equivalent electrical circuits. Several models of circuits that can fit the experimental data were created, and the best results (best agreement between experiment and fitting) were obtained with the equivalent electrical circuits presented in the figure. The equivalent electrical circuit in Fig. 5a comprises solution resistance R _S, in series with a Q ₁/[R _fQ ₂/(R _ctW)] parallel combination. Q ₁ represents the capacity of the passive oxide film, including the semiconductor space charge layer in parallel with the resistance of the passive film R _f. Q ₂/(R _ctW) represents the contribution of the charge transfer process, in which the Warburg impedance W describes the diffusion behaviour. Electrochemical impedance spectroscopy collected from the prepolarised steel follows the equivalent electrical circuit shown in Fig. 5b. In this equivalent electrical circuit, R _S is the solution resistance, Q ₁ and R _f are the capacity and resistance of the corrosion scales respectively and Q ₂ and R _ct represents the double layer capacitance and the charge transfer resistance respectively. The EIS fitting results of the two circumstances are given in Table 1. The results indicate that a compact passive film is formed on the surface of the non-prepolarised steel and thickens with increasing of the immersion time based on the fact that the R _f value significantly increases from 0 to 5 h. On the contrary, corrosion scales which are loose and poor in protection are formed on the surface of the prepolarised steel. This phenomenon suggests that more corrosion scales are formed with increasing immersion time.

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Table 1

Electrochemical impedance spectroscopy fitting data of non-prepolarised and prepolarised [ − 1.3 V(SCE)] steel immersed in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na2CO₃ solution for 0 and 5 h

	Non-prepolarised		Prepolarised
Immersion time	0 h	5 h	0 h	5 h
Rs/Ω cm− 2	8.6	8.2	4.7	4.6
Q1/S sn cm− 2	4.3 × 10− 3	9.7 × 10− 3	3.9 × 10− 3	3.1 × 10− 3
Rf/Ω cm2	220	38 720	224	267
Q2/S sn cm− 2	1.4 × 10− 4	8.7 × 10− 5	7.6 × 10− 5	8.0 × 10− 5
Rct/Ω cm− 2	335	347	323	496
W/S s5 cm− 2	1.4 × 10− 3	1.1 × 10− 3

Influence of AVs on passivation behaviour

To investigate the influence of AVs on the passivation behaviour of X80 steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution, different amplitudes of AVs were applied to the non-prepolarised steel, and the OCP of the samples were recorded. Figure 6a shows the OCP values of the two WEs under the influence of AVs. The OCP increases when low amplitudes of AV were superimposed to the steel, indicating that the passivation of steel is enhanced by low amplitude AVs, which is confirmed by the corrosion morphology in Fig. 7 and the EIS in Fig. 8a.

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a non-prepolarised; b prepolarised

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Surface morphologies of X80 steel before and after application of alternating voltages

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Electrochemical impedance spectroscopy diagrams of non prepolarised steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution after application of alternating voltages

Figure 7 shows that the surface of the two WEs are bright, and no corrosion occurs when the AV amplitudes are not higher than 0.3 V(rms). Figure 8 shows the EIS Nyquist diagrams of the non-prepolarised steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution after the application of ac voltages for 5 h. The equivalent electrical circuits are also presented in the figure, and the fitting data are given in Table 2 and Fig. 9. The fitting results show that the protectiveness of the passive film increases when 0.1 and 0.3 V(rms) AVs are applied to the system. This phenomenon is consistent with findings of Kwiatkowski and Mansfeld, ²¹ who found that an alternating potential step (AV) passivation can significantly improve the corrosion resistance of stainless steels in acid media. As shown by Song et al., ²² the AV increased the thickness of the passivation film, which consequently resulted in an increase in corrosion protectiveness. Figure 10 illustrates the schematic diagram of X80 steel under different AV interferences. The steel is speculated to always be in the passivation potential region when 0.1 and 0.3 V(rms) oscillations are superimposed to the WEs. The thickness of the passive film increases with the superimposition of the low amplitude AVs based on the fact that the R _f values keep an upward tendency with the increasing AV amplitude when it is no higher than 0.3 V(rms) (Fig. 9).

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Table 2

Electrochemical impedance spectroscopy fitting data of non-prepolarised steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution after superimposition of different amplitudes of ac voltages for 5 h

	0.1 Vrms	0.3 Vrms	0.5 Vrms	0.7 Vrms
ac voltage	WEleft	WEright	WEleft	WEright	WEleft	WEright	WEleft	WEright
R s/Ω cm2	8.7	8.4	7.7	9.4	12.2	8.6	13.4	11.1
Q1/S sn cm− 2	5.0 × 10− 3	6.4 × 10− 3	2.2 × 10− 3	2.4 × 10− 3	1.1 × 10− 4	1.2 × 10− 4	1.2 × 10− 4	1.3 × 10− 4
Rf/Ω cm2	83 510	47 530	130 600	121 400	5060	2130	4463	1734
Q2/S sn cm− 2	8.5 × 10− 5	8.2 × 10− 5	5.4 × 10− 5	8.0 × 10− 5	1.6 × 10− 4	2.6 × 10− 4	1.82 × 10− 4	2.8 × 10− 4
Rct/Ω cm2	695	643	1118	743	3764	943	2838	736
W /S s5 cm− 2	4.9 × 10− 4	6.9 × 10− 4	3.6 × 10− 4	4.4 × 10− 4

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9

a Rf; b Rct

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10

Schematic diagram of X80 steel under different alternate voltages with small and big amplitudes

With the application of low amplitude AVs, the OCP of the WEs shifts positively as shown in Fig. 6, resulting in the enhancement of the passive film. This has the same effect as anodic potentiostatic polarisation, by which the passive film is enhanced through potential augmentation. To investigate the relationship between the polarisation effect induced by AVs and the anodic potentiostatic polarisation effect, − 0.2 and − 0.3 V(SCE) potentiostatic polarisations were applied to the steel for 5 h, and the EIS after the polarisation are shown in Fig. 11. The EIS of the alternating and direct polarisation can be fitted by the same equivalent electrical circuit as shown in Fig. 11. When 0.1 V(rms) AV was applied to the system, the OCP of the left WE shifts positively to about − 0.3 V(SCE), resulting in the same extent of increase in R _f and R _ct as that of the − 0.3 V(SCE) potentiostatic polarisation. Similarly, with the application of 0.3 V(rms) AV, the OCP of the left WE shift positively to about − 0.2 V(SCE), resulting in an increase in R _f and R _ct values similar to the − 0.2 V(SCE) potentiostatic polarisation. Thus, the same effect on passivation between low amplitude AVs and anodic potentiostatic polarisation is confirmed.

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11

Electrochemical impedance spectroscopy diagrams of non-prepolarised steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ after anodic potentiostatic polarisation and application of low amplitude alternating voltages

When the AV amplitude was further increased to 0.5 and 0.7 V(rms), the OCP of the WEs shifts dramatically in the negative direction (Fig. 6a), and the film resistance declined markedly (Fig. 9a), indicating that the passive film is broken down and the steel is corroded. This result is verified by the surface morphology in Fig. 7.As shown in Fig. 7, after the application of 0.5 and 0.7 V(rms) AVs for 5 h, the surface of the WEs is not bright anymore and is covered with grey and brown corrosion products. From Figs. 6a and 7, we can speculate that a more negative OCP shift by the AVs indicates a more serious WE corrosion and that more corrosion products are generated.

It is worthwhile to note that the left and right WEs have several differences. The OCP of the left WE shifts more positive than the right one by low amplitude AVs, resulting in a thicker passive film and a higher resistance against corrosion. On the contrary, the right WE exhibits a more negative OCP value than the left one after the application of high amplitude AVs. Figure 7 shows that the left WE is corroded more severely than the right one by 0.5 and 0.7 V(rms) AVs, and more corrosion products are formed on it. The existence of the corrosion product layer blocks the diffusion of the reaction ions and suppresses the dissolution of the steel. Thus, the R _f and R _ct values of the left WE are higher (Fig. 8). Based on the above experimental facts, we suggest that the polarisation effect of the AVs on the two WEs, which are completely the same, is not identical, and a dc flowing from one to the other is generated. Higher AV amplitudes indicates that more dc is produced, thereby resulting in bigger differences in OCP between the two WEs (Fig. 6a).

Influence of AVs on activation corrosion behaviour

Different amplitudes of AVs were superimposed to the prepolarised samples to investigate the influence of AVs on the activation corrosion behaviour of X80 steel. The OCP of the WEs is recorded and shown in Fig. 6b. As previously discussed, no passive film is formed on the prepolarised steel, and corrosion scale, which is poor in protection, is formed instead. Consequently, the OCP of the steel is much more negative than that of the non-prepolarised steel (Fig. 4). With the application of AVs, the OCP of the steel shifts negatively, and the shift magnitude increases with increasing AV amplitude. Similarly, a dc flowing from one WE to the other is produced and caused the difference in OCP of the two WEs. Higher AV amplitudes indicated that more dc is produced.

After the application of AVs for 5 h, the corrosion morphologies are observed and shown in Fig. 7. Some grey and brown corrosion products are formed on the surface of the steel. To investigate the characteristics of the corrosion products produced by AV polarisation, EIS of the WEs was carried out on the WEs. Figure 12 shows the Nyquist diagrams of the two WEs under the influence of AVs and the equivalent electrical circuit that fits the EIS. The fitting results are shown in Table 3 and Fig. 13. The lower R _f value in comparison with R _ct for the same sample implies that the charge transfer process is the rate controlling step rather than the diffusion of ions through corrosion scales. The increase in the R _f and R _ct values implies that more corrosion scales are formed on the surface of the steel with increasing AV amplitudes.

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Electrochemical impedance spectroscopy diagrams of prepolarised steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution after application of alternating voltages

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Table 3

Electrochemical impedance spectroscopy fitting data of prepolarised [ − 1.3 V(SCE) for 5 s] steel in 0.5 mol L^{− 1} NaHCO3+0.25 mol L^{− 1} Na2CO3 solution after superimposition of different amplitudes of ac voltages for 5 h

	0.1 Vrms		0.3 Vrms		0.5 Vrms		0.7 Vrms
ac voltage	WEleft	WEright	WEleft	WEright	WEleft	WEright	WEleft	WEright
R s/Ω cm2	4.9	4.9	5.1	5.8	6.0	5.7	6.1	6.0
Q1/S sn cm− 2	3.3 × 10− 4	3.5 × 10− 4	2.5 × 10− 4	2.3 × 10− 4	1.3 × 10− 4	1.6 × 10− 4	1.3 × 10− 4	1.8 × 10− 4
Rf/Ω cm2	278	285	310	351	468	579	836	982
Q2/S sn cm− 2	8.7 × 10− 4	9.0 × 10− 4	6.6 × 10− 4	7.5 × 10− 4	3.1 × 10− 4	3.3 × 10− 4	2.8 × 10− 4	2.8 × 10− 4
Rct/Ω cm2	631	652	800	943	1941	2080	2579	2857

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13

Electrochemical impedance spectroscopy fitting data of prepolarised steel in 0.5 mol L^{− 1} NaHCO₃+0.25 mol L^{− 1} Na₂CO₃ solution after application of alternating voltages