Localized corrosion of Al and Al alloys in chloride-containing environments has been intensively investigated [1–5]. 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 [6–12], 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 E_corr, pitting potential E_pit, pitting transition potential E_ptp, corrosion current density j_corr and corrosion potential E_sec,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 E_rev = −0.2 V_SCE in Fig. 1) and progresses in the cathodic or active direction (backward or reverse scan).

Potential differences such as E_pit – E_sec,corr, E_pit – E_corr and E_sec,corr – E_corr as criteria have been used for several decades to predict localized corrosion susceptibility of alloys [13–24]. 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 E_pit – E_sec,corr is useful to predict the propensity towards pitting in some specific environments.

Another important potential difference is ΔE (E_sec,corr – E_corr). 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, E_sec,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.

Wilde and Williams [13] proposed that E_sec,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, E_sec,corr descends dramatically as E_rev changes from −0.7 V_SCE to −0.6 V_SCE, followed by a slight fluctuation from −0.6 V_SCE to −0.2 V_SCE and an obvious upward from −0.2 V_SCE to 0 V_SCE. The value of ΔE (E_sec,corr – E_corr) first decreases and then increases as E_rev 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 F_corr, 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 (E_sec,corr – E_corr) and |50% – F_corr| was found, as shown in Fig. 9. |50% – F_corr | is the absolute value of the difference between 50% and F_corr. The linear regression equation between ΔE (E_sec,corr – E_corr) and |50% – F_corr| for the four groups are listed below:

Though a linear trend between ΔE and |50% – F_corr| 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 E_corr and E_sec,corr) is a mixed potential which altered by both anodic and cathodic reactions.

ΔE (E_sec,corr – E_corr) exhibits a non-monotonic trend with corrosion current density j_corr. It decreases firstly and then increases with the increasing of j_corr. For 7150 Al alloy, the link be tween ΔE value and corrosion fraction (F_corr) after cyclic polarization was determined. At the first corrosion stage where F_corr 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% – F_corr| was found for 7150 Al alloy in the four studied groups of corrosion environments.