PSI - Issue 37

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Dimitris Georgoulis et al. / Procedia Structural Integrity 37 (2022) 941–947 Georgoulis et al. / Structural Integrity Procedia 00 (2021) 000 – 000

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2.2. Methods Electrochemical measurements were performed on polished small rectangular specimens (10 mm * 20 mm * 3.2 mm) with the use of a typical three electrodes cell system that was composed of a platinum electrode as counter electrode, a silver chloride electrode Ag/AgCl as reference electrode and the small rectangular specimens of the investigated aluminium alloys as working electrodes. A surface area of 0.5 cm 2 of the working electrode was exposed to the electrolyte. The experiments were carried out in naturally aerated 3.5 wt. % NaCl solution at temperature of (25 ± 2) °C. A PalmSens ® potentiostat was used for the Electrochemical Impedance Spectroscopy (EIS) measurements and a PStrace5 ® software was used for the data fitting of the impedance spectra. The voltage perturbation range used in the EIS measurements was 10 mV (rms), and the acquisition rate was 10 points per decade in a frequency range of 0.01 Hz to 10 5 Hz. Each test was performed at least three times to evaluate the reproducibility of the results. 3. Results and discussion The EIS results for the two different investigated tempers (T3 and T8) as a function of immersion time in 3.5 wt. % NaCl solution are shown for the AA2198 in Fig. 1, in the form of Nyquist and Bode phase angle diagrams. Two clear capacitive loops in all immersion times are evident from the Nyquist plots for both aluminium alloys, e.g. Fig.1a and Fig.1b. According to the literature, e.g. Wang et al. (2015) and Moreto et al. (2014), the two capacitive loops in EIS correspond to the two parts of the alloy ’s surface; more precisely the high frequency (HF) capacitive loop (complete semicircle) corresponds to the flat passive film, while the capacitive loop that is present in low frequency values (LF) (one-fourth arc) is associated with the localized corrosion phenomena, such as pitting, intergranular and exfoliation corrosion. The one-fourth arc capacitive loop evolves to a diffusion-controlled process and represents the diffusion actions near the electrode surface. It is obvious from Figs. 1a and 1b that the impedance modulus (radius of the semi-circular Nyquist plot) of AA2198-T3 is higher than the respective of AA2198-T8 during the whole test period, indicating that the alloy in T3 temper is more corrosion resistant (or less susceptible to corrosion attack). A different trend in impedance is noticed for the two different tempers of the alloy; a sudden drop of the impedance modulus is evident after 3 h of immersion in T3 condition and then it is continuously decreasing with increasing immersion time, while no significant decrease of the impedance modulus is evident for up to 3 h of immersion in T8 temper. A respective sudden drop of the impedance modulus is noticed after 6 h of immersion in T8 temper where further increase of the immersion time seems not to decrease the impedance essentially. However, the impedance decreased with increasing immersion time for both tempers. These results suggested the presence of two-time constants up to 6 h of immersion in both alloy tempers (T3 and T8) - distributing in medium frequency (10 1 - 10 3 ) and in low frequency (10 -2 – 10 -1 ) regimes - while a new time constant seems to appear after 12 h of immersion in T3 and after 24 h in T8 condition in medium frequency (10 1 - 10 3 ). Time constants in medium frequencies are related to charge transfer processes coupled to the charging of the double layer while low frequencies time constants are associated with a diffusion-controlled process. According to Campestrini, et al. (2001), when the resistance associated with the diffusion becomes very large the R // CPE element (time constant) may be replaced by a simple CPE (constant phase element) with the exponent “ n ” value of 0.5, which, in an ideal situation, is represented by a Warburg element, as was the case for the present research work. In order to gain a better insight into the corrosion mechanisms, EIS response is modelled using equivalent electrical circuits shown in Fig. 2. The R 1 element in the circuit model represents the solution resistance. The pair R 2 // Q 1 stands for the capacitance of the oxide film in parallel with conductive pathways associated with defective sites created by the intermetallic particles (IMCs), which lead to the breakdown of the oxide film and consequently to unprotected metal surface. A constant phase element (CPE) - that is denoted with Q in the present work - was used instead of an ideal capacitor to take into consideration the heterogeneity of the surface, as already mentioned in the literature, e.g. Schmutz et al. (2006), Blanc et al. (1997). R 3 // Q 2 pair represents the charge transfer resistance coupled to the charging of the double layer, while the W refers to the Warburg impedance that is ascribed to the low frequency corrosion processes that kinetically control the alloys’ dissolut ion, which evolves to a diffusion-controlled process.