Thermomechanical treatment and corrosion resistance correlation in the AA2198 Al–Cu–Li alloy

Abstract

The influence of T3, T8 and T851 thermomechanical treatments on the microstructure and corrosion resistance of the AA2198 was investigated. Differential scanning calorimetry and scanning electron microscopy were used for microstructural characterisation, whereas electrochemical methods were employed to analyse the corrosion behaviour of the alloy. The morphology and composition of constituent particles were similar for the T3 and T8 thermomechanical treatments but varied in the T851. There was an inverse relation between T1 phase density and corrosion resistance. The T3 treatment with the highest corrosion resistance was the one with the lowest density of T1 phase. The mechanisms of corrosion varied with the thermomechanical treatments.

Exfoliation and intergranular corrosion resistance of the 2198 Al–Cu–Li alloy with different thermomechanical treatments

“…[ 42 ] For the 2198‐T3, a similar behavior was observed in the first hours of immersion; however, after an initial potential drop (nearly 1 hr of the test), it increased slowly and reached stability after 24 hr of immersion. The slow corrosion kinetics related to the T3 temper, due the lower amount of hardening precipitades, was already reported in previous works [ 50 ] and, consequently to their slower kinetics and electrochemical activities as compared with the T8 and T851 tempers. For these two last tempers, the initial potentials were more negative than for the alloys with T3 temper due to the lower electrochemical activity related to this last temper.…”

“…This type of attack has been related to slip bands by other authors. [ 50,57–60 ] Grain boundaries were preserved for T851, as shown by red arrows in Figure 9a,b and also in Figure 10a,b. Analyses by EDX maps in the areas of heavy corrosion attack in 2198‐T851 (Figure 11) showed copper enrichment at the borders of the attacked areas, indicating that elements more active than Al are dissolved from the alloy.…”

“…[ 43 ] The transgranular attacks in the 2189‐T8 and 2198‐T851 alloys occurred due to selective dissolution of particular grains that were more active due to their higher density of T1 phase, compared with others. Consequently, galvanic coupling occurred between grains with different amounts of T1 phase, [ 8,50,65 ] as observed in the EXCO and IGC tests (Figures 9, 10, 13a,b, and 14).…”

“…Taking into consideration that natural and artificial aging influence precipitation of the T1 phase (Al 2 CuLi) in Al–Cu–Li alloys, this phase was considered to be mainly responsible for localized corrosion in the artificially aged Al–Cu–Li alloys. [ 63 ] A recent work [ 50 ] showed, by differencial scanning calorimetry curves, that the tempers T8 and T851 have a higher density of T1 phase when compared with the T3 temper. This explains the similarities in the electrochemical behaviors of the T8 and T851 alloys, despite the differences in the corrosion kinetics of these two tempers (Figures 1–5).…”

“…However, the choice of temper according to application usually does not take into account the damage that these materials might cause due to limited corrosion resistance if they are used in improper tempers. [ 50 ] The corrosion susceptibility of age‐hardened Al–Cu–Li alloys (such as T8 and T851) when compared with the naturally hardened alloys (T3), is known [ 50 ] due to the heterogeneous microstructure of the age‐hardened alloys when compared with the fairly uniform microstructure related to the T3 alloy. Usually, T3‐tempered aluminum alloys exhibit improved corrosion resistance in comparison to age‐hardened alloys; these last ones with numerous precipitates form a large number of galvanic cells.…”