PSI - Issue 28

Dimitris Georgoulis et al. / Procedia Structural Integrity 28 (2020) 2297–2303 Author name / Structural Integrity Procedia 00 (2019) 000–000

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among the highest tensile properties during artificial ageing (peak-ageing condition) and had an appropriate artificial ageing heat treatment from the T3 temper. Tensile specimens were machined from the longitudinal (L) rolling direction and according to ASTM E8 specification. The geometrical dimensions of the tensile specimens were 12.5 * 3.2 mm in the reduced cross-section and 150 mm in length for both aluminium alloys.

2.2. Methods

2.2.1. Corrosion exposure Tensile specimens were exposed for different hours to the laboratory 3.5 wt. % corrosion environment (hereafter called NaCl solution) according to the ASTM G44 specification. The concentration of the NaCl solution consisted of 3.5 g NaCl for each 96.5 ml water. The solution volume was calculated per exposure area of the specimens and it was constant at 20 ml/cm 2 for all specimens. Further details can be found in the respective specification. The specimens were cleaned with alcohol prior to corrosion exposure according to ASTM G1 specification. Additionally, the specimens were masked with appropriate insulating PVC tape in order to be exposed only at the reduced surface area of approximate 55 mm in length. The experimental procedure was carried out in laboratory environmental conditions and at room temperature. After the corrosion exposure the corroded specimens were immediately cleaned according to ASTM G1 specification and then they were subjected to tensile testing. 2.2.2. Tensile tests Additionally, tensile tests were carried out after the immersion of the tensile specimens to the corrosive environment for different exposure times. Tensile tests were carried out in a servo-hydraulic Instron 100 kN testing machine according to the ASTM E8 specification and the crosshead displacement rate was kept constant and equal to 0.7 mm/min. An external Instron extensometer with 50 mm ± 10 mm maximum travel was attached at the reduced cross section gauge length of the specimens. A data logger was used during all tensile tests and the values of load, displacement and axial strain were recorded and stored in a computer. 3. Results and discussion 3.1. Microstructural characterization Exposure of specimens to the corrosive environment (3.5 wt. % NaCl solution in the present study) results in the deterioration of the surface of the specimens due to the nucleation of corrosion - induced surface pits, as can be seen in Fig.1. The depicted corroded area has dimensions of 12.5 mm x 55 mm being the dimensions of the exposed reduced surface area of the specimens. It is noticed that for the short exposure times the surface deterioration remains limited since pits were not identified after 6 h of exposure. However, the pitting density and size tend to increase with increasing exposure time; corrosion damage in the form of pits was observed after 168 h of exposure while more pits of higher diameter as well as pit coalescence are evident after 720 h exposure.

Fig. 1: Typical photographs of AA2024-T3 pre-corroded tensile specimens exposed to 3.5 wt.% NaCl solution (a) 6 h, (b) 168 h, (c)-(e) 720 h.