PSI - Issue 37

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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(a)

(b)

Fig. 2. Equivalent electrical circuits proposed to fit the data of AA2198-T3 & AA2198-T8 for (a) up to 6 hours and (b) from 12 h of immersion to 3.5% wt . NaCl solution.

Table 1 shows the evolution of the R // Q values with increasing exposure time. It is obvious that the Q 1 increases with increasing immersion time for both tempers. This is attributed to the formation of new active sites on the specimens’ surfaces – e.g. pits as a result of the dissolution / detachment of the IMCs due to the formation of a galvanic cell between them and the matrix - which increase the effective area exposed to the electrolyte. AA2198 in T3 condition exhibited lower Q 1 values for the whole test period, indicating the presence of a thicker surface oxide layer than in T8 condition. The increase in Q 1 was accompanied with a decrease in R 2 for both alloys and the lower Q 1 values in T3 temper accompanied with higher R 2 when compared to T8 temper. The lower R 2 values observed in the T8 temper can be attributed to the higher kinetics of surface attack in the case of this temper, which are related to the higher amount of T 1 precipitates compared to the T3 temper. The higher amounts of T 1 precipitates in the artificially aged alloy (T8) also resulted in increased corroded areas with the removal of passive film. Despite of the fact that the T3 temper showed higher values of the oxide layer’s resistance ( R 2 ) when compared against T8 temper for the whole test period, lower decrease rate with increasing immersion time was noticed for the T8 temper as can be seen in Fig. 3. An initial increase in R 2 of T8 temper indicates an initial reduction in surface activity that could be due to the partial coverage of active sites at the surface. Regarding the evolution of R 3 // Q 2 components - that are associated with the charge transfer reactions taking place at the interface of the matrix at the defective sites of the oxide layer (e.g. near IMCs) and the electrolyte - it is noticed that the R 3 of T8 temper was essentially oscillating with increasing immersion time, which can be attributed to the detachment and nucleation of new IMCs with corrosion evolution. For this temper, the Q 2 remained almost constant from 12 to 24 h of immersion revealing that the electrochemically active surface area remained almost unchanged. However, an increase in Q 2 after 48 h of immersion might be attributed to the formation of new active sites. Conversely, for the T3 temper R 3 decreased continuously. This behaviour might be due to the enrichment in the more noble components of the remaining active IMCs (e.g. Cu) which even though in lower density they lead to enhanced galvanic activity between the matrix and the Cu-enriched IMCs. Analogous behaviour was noticed for the Q 2 in T3 temper where its increase with corrosion evolution indicates that the electrochemically active sites augmented. The lower decrease rate of both resistances ( R 2 and R 3 ) noticed at the T8 temper might be due to the intragranular corrosion attack which is the pre-dominant corrosion-induced degradation mechanism in this temper as was referred in Araujo et al. (2019).

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2198-T3 2198-T8

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Fig. 3. Variation of the oxide film resistance R 2 as a function of immersion time to 3.5% wt . NaCl solution for AA2198-T3 and AA2198-T8.