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J. Electrochem. Sci. Technol > Volume 11(2); 2020 > Article
Sun and Chen: Potential Difference of Cyclic Polarization Curve of an Aircraft Al Alloy: ΔE (Esec,corr – Ecorr)

Abstract

This paper presents a hypothesis and its experimental validation that ΔE (Esec,corrEcorr) of cyclic polarization curve of an Al-Zn-Mg-Cu alloy decreases firstly and then increases with the increasing of corrosion rate or corroded fraction Fcorr of alloy surface. The minimum value of ΔE is obtained when Fcorr ≈ 50%. In addition, a proportional relationship between ΔE and |50% – Fcorr| was found. This non-monotonic relation between ΔE and extent of localized corrosion indicates that additional attention should be paid on using ΔE to assess localized corrosion behaviour of Al-Zn-Mg-Cu alloys.

1. Introduction

Localized corrosion of Al and Al alloys in chloride-containing environments has been intensively investigated [15]. Various electrochemical methods such as cyclic polarization, EIS (Electrochemical Impedance Spectroscopy), SECM (Scanning Electrochemical Microscopy) and SKP (Scanning Kelvin Probe) were used in the last several decades [612], among which cyclic polarization (or pitting scans) has become one of the most important methods for the characterization of localized corrosion. This method can provide lots of quantitative data such as corrosion potential Ecorr, pitting potential Epit, pitting transition potential Eptp, corrosion current density jcorr and corrosion potential Esec,corr of reverse scan as well as qualitative interpretation of localized corrosion of Al alloy. A typical cyclic polarization curve for 7150-T77 Al alloy is shown in Fig. 1. The potential is first increased in the anodic or noble direction (forward scan). The potential scan direction is reversed at some chosen current or potential (the reverse is at the potential of Erev = −0.2 VSCE in Fig. 1) and progresses in the cathodic or active direction (backward or reverse scan).
Potential differences such as EpitEsec,corr, EpitEcorr and Esec,corrEcorr as criteria have been used for several decades to predict localized corrosion susceptibility of alloys [1324]. For instance, after conducting experiments on AA 1050, AA 6082, AA 5754, AA 5083 and AA 2014 in chloride-containing solutions with and without additional cathodic reactants, M. Trueba and S.P. Trasatti [20] proposed that the trend of EpitEsec,corr is useful to predict the propensity towards pitting in some specific environments.
Another important potential difference is ΔE (Esec,corrEcorr). In the early 1990s, Silverman [18,25] proposed that, for a given experimental procedure, the higher the value of ΔE is, the harder certain metals can repassivate. His work has been introduced into the famous book of Uhlig Corrosion Handbook as a whole chapter which cited by many following researchers [19,22,26,27]. However, the physical meaning of ΔE is still unclear and ΔE has not been theoretically derived so that it is empirical in nature. For example, Esec,corr can change with experimental variables such as point-of-scan reversal and scan rate [14, 23,24,28,29] and the value of ΔE can be greatly influenced by the amount of prior damage done to the passive surface [18,21,23,24]. The farther the polarization scan is generated in the anodic direction, the greater corrosion degree of surface region will be. These results may lead to an erroneous prediction of corrosion behavior especially when the pitting potential is exceeded or some other electrochemical transformation is participated [18,21,23,24]. However, the reason that using ΔE criterion to evaluate corrosion behavior of alloys may lead to an erroneous prediction remains unknown. The scan rate also can alter the scan and cause a misinterpretation of features. Generally the scan rate should be slow enough so that double-layer capacitance remains fully charged and the current collected reflects only the interfacial corrosion process at every potential [28,29].
In this work, we conducted cyclic polarization tests of 7150 Al alloy under four groups of corrosive environments. The suitability and limitations of using ΔE for 7150 Al alloy corrosion evaluation are discussed. The relation between ΔE and corrosion rate/corrosion degree is established as well.

2. Experimental

2.1. Alloy and heat treatment

The investigated material is a rolled 7150 Al alloy plate received from Aluminum Corporation of China. The chemical composition (mass fraction) of the plate is Al-6.5Zn-2.4Mg-2.2Cu-0.15Zr-0.08Fe-0.061Si. The samples, cut into 15×15×3 mm plates, were solution heat treated for 30 min at 480°C, followed by cold water quenching and RRA/T77 ageing process (120°C * 24h + 180°C * 0.5h (water cooling) + 120°C * 24h). The optical microstructure of alloy after heat treatment has been shown elsewhere [30].

2.2. Cyclic polarization curve

For electrochemical characterization, 7150 Al alloy samples were wet ground through successive grades of silicon carbide abrasive papers from P240 to P1500, followed by diamond finishing to 0.1 μm. A CHI 660C electrochemical workstation (Shanghai Chenhua, China) connected to a three-electrode cell was used for the electrochemical measurements. The working electrode was the test material with an immersed area of 0.5 cm2 and platinum plate and saturated calomel (SCE) electrodes were used as the counter and reference electrodes, respectively. Cyclic polarization curves were performed under different solutions and different temperatures. As listed in Table 1, four groups of experimental corrosion environments were selected. Cyclic polarization curves were obtained after 1 h of immersion, at a scanning rate of 1 mV/s. For group A, B and D, the scanning potential ranges from −1.2 VSCE to −0.2 VSCE and for group C −1.2 VSCE to −0.4 VSCE. All the electrochemical tests were performed in a Faraday cage.

2.3. Surface corrosion fraction calculation after cyclic polarization

The corrosion morphologies of 7150 Al alloy after the cyclic polarization tests were characterized by optical microscopy (LEICA NEOPHOT-21). The surface corrosion fraction (Fcorr) of sample after polarization test was calculated by ImageJ2x software, using at least 5 images for each experimental point. Fig. S1 (a) shows the corrosion morphology of alloy after cyclic polarization in 0.1 mol/L HCl + 1 mol/L NaCl at the temperature of 318 K. As can be seen, the corroded surface corresponds to the dark area in Fig. S1 (a). And corrosion fraction Fcorr equals to the ratio of the dark area and the whole area. The corrosion fraction is 64.3% from the analysis of binary image using ImageJ2x software, as shown in Fig. S1 (b).

3. Results

3.1 Cyclic polarization curve

Fig. 2 shows the anodic section of cyclic polarization curves of 7150 Al alloy obtained for group A, B, C and D. For group A and B, pitting potentials Epit occurs at solutions with lower chloride concentrations. Corrosion current density jcorr and current density at reverse potential jrev of group A and B are shown in Fig. 3a. Clearly, both jcorr and jrev increased with higher chloride concentrations. Compared with group C, obvious Epit can be found in group D and the value of Epit shifts to the negative direction with increasing temperature. From jcorr and jrev values of group C and D shown in Fig. 3b, it can be easily concluded that, for both group C and group D, the corrosion rate of 7150 Al alloy is accelerated by higher temperature.

3.2 Corrosion morphology

Representative corrosion morphologies of 7150 Al alloy after cyclic polarization are shown in Fig. 4. The mean corrosion fraction Fcorr of alloy surface after polarization test is obtained from at least five images with different magnifications for each experimental point. In the case of 303 K (group A), the corrosion fract ion of alloy surface after cyclic polarization is 13.5%, 20.9%, 29.1%, 42.1%, 48.5%, 53.5% and 57.6% as x equals to 1, 5, 10, 50, 100, 500 and 1000, respectively; In the case of 353 K (group B), the corrosion fraction is 25.4%, 33.3%, 40.4%, 43.8%, 52.8%, 62.6% and 68.3%, for x equals to 1, 5, 10, 50, 100, 500 and 1000, respectively. In the case of 0.1 mol/L HCl + 1 mol/L NaCl (group C), the mean Fcorr value of Al alloys after cyclic polarization under 298 K, 308 K, 318 K and 333 K is 38.5%, 50.1%, 68.1% and 100%, respectively. In the case of 0.1 mol/L Na2SO4 + 20 mmol/L NaCl (group D), the mean Fcorr of Al alloys under 298 K, 308 K, 318 K and 333 K is 14.2%, 28.5%, 38.6% and 68.4%, respectively. As implied by Fig. 5, the corrosion degree of alloy surface increased with higher chloride concentration and higher temperature.

3.3 Potential difference

As shown in Fig. 5, for group A and B, ΔE (Esec,corrEcorr) decreases firstly and then increases with the increasing of chloride concentration. The turning point of group A and group B is the same as 100 mmol/L chloride. For group C and D, similar trend between ΔE and also does not decrease linearly with increasing temperature. The turning temperature of ΔE values for group C is 308 K, while it is 318 K for group D. Furthermore, ΔE (Esec,corrEcorr) as a function of corrosion current density jcorr for group A, B, C and D are shown in Fig. 6. As can be easily seen, for all groups, ΔE exhibits non-mono tonic trend with jcorr. The critical value of jcorr at turning point varies for different groups.
The link between ΔE (Esec,corrEcorr) and corrosion fraction Fcorr after cyclic polarization test is exhibited in Fig. 7. As expected, ΔE value does not decrease linearly with the propagation of corrosion. For all groups, it has a minimum turning point at which the corrosion faction equals to approximately 50%. Therefore we can conclude that at the first stage of localized corrosion, ΔE decreases with promoted corrosion. On this stage, higher values of ΔE means less susceptible to localized corrosion of the metal. When it moves into next stage where more than half of the surface area corroded, inversely, ΔE increases with further corrosion propagation.

4. Discussion

Wilde and Williams [13] proposed that Esec,corr varies with the amount of pitting propagation that has taken place. Pitting propagation varies not only with the specific electrolyte and temperature, but also with cyclic polarization parameters such as scan rate and reversal potential. Point of reversal is such a factor that can influence the amount of pitting propagation. It is well-known that the farther the polarization scan is generated in the anodic direction, the greater the corrosion degree of the surface region will be. As shown in Fig. 8, Esec,corr descends dramatically as Erev changes from −0.7 VSCE to −0.6 VSCE, followed by a slight fluctuation from −0.6 VSCE to −0.2 VSCE and an obvious upward from −0.2 VSCE to 0 VSCE. The value of ΔE (Esec,corrEcorr) first decreases and then increases as Erev shifts to the anodic direction. This is exactly the expected result on the basis of the non-monotonic relationship between ΔE and corrosion degree of alloy surface. Thus, Fig. 8 is a re-validation of this non-monotonic trend observed above. This non-monotonic trend between ΔE and Fcorr, though empirical in nature, is of great importance for the interpretation of localized corrosion of 7000 series Al alloys using cyclic polarization method. One must very careful when using ΔE as a criterion to assess localized corrosion of 7000 series Al alloy, because it can be less rigorous and or even deceptive.
Furthermore, a linear relationship between ΔE (Esec,corrEcorr) and |50% – Fcorr| was found, as shown in Fig. 9. |50% – Fcorr | is the absolute value of the difference between 50% and Fcorr. The linear regression equation between ΔE (Esec,corrEcorr) and |50% – Fcorr| for the four groups are listed below:
  • Group A: y = 0.2072x − 0.1654 R2 = 0.5978

  • Group B: y = 0.4068x − 0.3265 R2 = 0.8622

  • Group C: y = 0.0779x − 0.0381 R2 = 0.9834

  • Group D: y = 0.3583x − 0.3098 R2 = 0.9576

Though a linear trend between ΔE and |50% – Fcorr| existed (especially for Group C and D, R2 values are quite high), the mechanism of which still is not well understood. It probably due to that the difference in features between forward scan and reverse scan is a result of changes in surface state before and after anodic polarization. For example, in very aggressive environment (Group C, 333 K high temperature and 1h prior immersion duration, see Fig. 2c), the forward scan and the reverse scan are completely over-lapped, thus ΔE value is zero. In this case, anodic polarization altered slightly on alloy surface because the surface has been completely corroded by immersion before anodic polarization, as one can observe a totally black area on alloy surface caused by prior immersion using un-aided eye. The value 50% might be corresponds to the largest degree that can be altered by anodic polarization for these four groups of corrosive environments, especially for Group C and D. We emphasize that 50% is not a universal value, because corrosion is a complex phenomenon that can be influenced by various factors such as ion concentration, temperature and immersion duration, and more importantly, corrosion potential (no matter Ecorr and Esec,corr) is a mixed potential which altered by both anodic and cathodic reactions.

5. Conclusions

ΔE (Esec,corrEcorr) exhibits a non-monotonic trend with corrosion current density jcorr. It decreases firstly and then increases with the increasing of jcorr. For 7150 Al alloy, the link be tween ΔE value and corrosion fraction (Fcorr) after cyclic polarization was determined. At the first corrosion stage where Fcorr is less than one certain critical percentage (~50%), ΔE value decreases with higher corrosion degree. And then, by contrast, ΔE value increases with further corrosion propagation. We believe this non-monotonic relation between ΔE and corrosion degree is applicable at least for 7000 series Al alloys. Empirically, proportional relationship between ΔE and |50% – Fcorr| was found for 7150 Al alloy in the four studied groups of corrosion environments.

Acknowledgement

The authors are happy to acknowledge financial support from National Key Research and Development Program of China (No.2016YFB0300801). The financial support of the China Scholarship Council and Guangdong Young Innovative Talent Project (2018KQNCX227) are also gratefully acknowledged.

Supporting Information

Supporting Information is available at https://doi.org/10.33961/jecst.2019.00507

Fig. 1
Schematic representation of the characteristic parameters of cyclic polarization curves: Ecorr – corrosion potential; Epit – pitting potential; Erev and jrev– potential limit of the forward scan and the corresponding current density; Eptp – pit transition potential; Esec,corr – second corrosion potential. The curve corresponds to experimental cyclic polarization of polished 7150-T77 Al alloy in naturally aerated 0.1 mol/L Na2SO4 + 20 mmol/L NaCl solution with Erev = −0.2 VSCE.
jecst-2019-00507f1.jpg
Fig. 2
Cyclic polarization of 7150 Al alloy for different experimental conditions: (a) group A; (b) group B; (c) group C; (d) group D. The details of different groups are listed in Table 1.
jecst-2019-00507f2.jpg
Fig. 3
Corrosion current density jcorr and current density at reverse potential jrev of different groups: (a) group A and B; (b) group C and D.
jecst-2019-00507f3.jpg
Fig. 4
Representative corrosion morphologies after cyclic polarization: a1, a2, a3 and a4 is for x = 1, 10, 100 and 1000 under the temperature of 303K, respectively (group A); b1, b2, b3 and b4 is for x = 1, 10, 100 and 1000 under the temperature of 353K, respectively (group B). c1, c2, c3 and c4 for 298 K, 308 K, 318 K and 333 K in 0.1 mol/L HCl + 1 mol/L NaCl, respectively (group C); d1, d2, d3 and d4 for 298 K, 308 K, 318 K and 333 K in 0.1 mol/L Na2SO4 + 20 mmol/L NaCl, respectively (group D).
jecst-2019-00507f4.jpg
Fig. 5
Ecorr, Esec,corr and ΔE (Esec,corrEcorr) of 7150 Al alloy for different groups: (a) group A and B; (b) group C and D.
jecst-2019-00507f5.jpg
Fig. 6
ΔE (Esec,corrEcorr) as a function of corrosion current density jcor r for group A, B, C and D, respectively.
jecst-2019-00507f6.jpg
Fig. 7
(a) ΔE (Esec,corrEcorr) as a function of surface corrosion fraction Fcorr after cyclic polarization test for group A, B, C and D, respectively. A minimum turning point can be seen when the corrosion faction equals to approximately 50%.
jecst-2019-00507f7.jpg
Fig. 8
Ecorr, Esec,corr and ΔE (Esec,corrEcorr) of 7150 Al alloy as a function of Erev. Cyclic polarization curves were performed in 3.5 wt% NaCl solution under room temperature, scan rate 1 mV/s.
jecst-2019-00507f8.jpg
Fig. 9
ΔE (Esec,corrEcorr) as a function of |50% – Fcorr| for group A, B, C and D, respectively.
jecst-2019-00507f9.jpg
Table 1
Four groups of experimental condition used for cyclic polarization test
Group Solution Temperature
A 0.1 mol/L Na2SO4 + x# mmol/L NaCl 303 K
B 0.1 mol/L Na2SO4 + x# mmol/L NaCl 353 K
C 0.1 mol/L HCl + 1 mol/L NaCl 298, 308, 318, 333 K
D 0.1 mol/L Na2SO4 + 20 mmol/L NaCl 298, 308, 318, 333 K

# x = 1, 5, 10, 50, 100, 500, 1000

References

[1] R. Arrabal, B. Mingo, A. Pardo, M. Mohedano, E. Matykina and I. Rodríguez, Corros Sci, 2013, 73, 342–355.
crossref
[2] B. Wang, Y. Su, X. Mou, Y. Xiao and J. Liu, Mater Des, 2013, 50, 15–21.
crossref
[3] S. Chen, K. Chen, G. Peng, L. Jia and P. Dong, Mater Des, 2012, 35, 93–98.
crossref
[4] Q. Sun, Q. Han, R. Xu, K. Zhao and J. Li, Corros Sci, 2018, 130, 218–230.
crossref
[5] Q. Sun, R. Xu, Q. Han, K. Zhao, I. McAdams and W. Xu, Appl Mater Today, 2019, 14, 137–142.
crossref
[6] SS. Jamali, SE. Moulton, DE. Tallman, M. Forsyth, J. Weber and GG. Wallace, Corros Sci, 2014, 86, 93–100.
crossref
[7] J. Izquierdo, S. González and RM. Souto, Int J Electrochem Sci, 2012, 7, 11377–11388.

[8] A. Maljusch, C. Senöz, M. Rohwerder and W. Schuhmann, Electrochim Acta, 2012, 82, 339–348.
crossref
[9] M. Rohwerder and F. Turcu, Electrochim Acta, 2007, 53(2), 290–299.
crossref
[10] Q. Sun, X. Liu, Q. Han, J. Li, R. Xu and K. Zhao, Surf Coat Technol, 2018, 337, 552–560.
crossref
[11] Q. Sun, Q. Han, X. Liu, W. Xu and J. Li, Surf Coat Technol, 2017, 328, 469–479.
crossref
[12] Q. Sun, K. Chen, H. Fang, J. Xu, P. Dong, G. Hu and Q. Chen, Int J Electrochem Sci, 2016, 11, 5855–5869.
crossref pdf
[13] B. Wilde and E. Williams, Electrochim Acta, 1971, 16(11), 1971–1985.
crossref
[14] A. Broli and H. Holtan, Corros Sci, 1973, 13(4), 237–246.
crossref
[15] A. Broli and H. Holtan, Corros Sci, 1977, 17(1), 59–69.
crossref
[16] N. Nilsen and E. Bardal, Corros Sci, 1977, 17(8), 635–646.
crossref
[17] M. Yasuda, F. Weinberg and D. Tromans, J Electrochem Soc, 1990, 137(12), 3708–3715.
crossref
[18] R. McGuire and D. Silverman, Corrosion, 1991, 47(11), 894–902.
crossref
[19] M. Dabala, K. Brunelli, E. Napolitani and M. Magrini, Surf Coat Technol, 2003, 172(2), 227–232.
crossref
[20] M. Trueba and SP. Trasatti, Mater Chem Phys, 2010, 121(3), 523–533.
crossref
[21] E. Rosen and D. Silverman, Corrosion, 1992, 48(9), 734–745.
crossref
[22] U. Trdan and J. Grum, Corros Sci, 2012, 59, 324–333.
crossref
[23] D. Silverman, Uhlig’s Corrosion Handbook. Third Edition. 2011. p.1129–1166.
crossref
[24] DC. Silverman, Tutorial on cyclic potentiodynamic polarization technique. NACE International, Houston, TX (United States), 1998.

[25] DC. Silverman, Uhlig’s Corrosion Handbook, 2011, 1129–1166.
crossref
[26] B. Zaid, D. Saidi, A. Benzaid and S. Hadji, Corros Sci, 2008, 50(7), 1841–1847.
crossref
[27] M. Finšgar and I. Milošev, Corros Sci, 2010, 52(7), 2430–2438.
crossref
[28] A. Poursaee, Electrochim Acta, 2010, 55(3), 1200–1206.
crossref
[29] H. Leckie, J Electrochem Soc, 1970, 117(9), 1152–1154.
crossref
[30] Q. Sun, J. Hu, J. Li, K. Chen, P. Dong, X. Liao and Y. Yang, Int J Electrochem Sci, 2017, 12, 5363–5377.
crossref pdf
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