*2.1. Welds*

Sheets with a thickness equal to 4 mm were considered. Table 1 shows the chemical composition of the AA 7075 T6 alloy. Sheets of 200 mm × 80 mm were welded by using a custom-made tool designed with a smooth plane shoulder (16 mm diameter) coupled with a frustum cone-shaped pin (maximum and minimum pin diameters were 6 mm and 4 mm, pin height was 3.8 mm). The rotational speed of the tool (S) was 1500 rpm and feed rate (F) was 10 mm/min. The set-up of the welding procedure was reported in a previous work [26]. The mechanical properties of the base materials and the welded joints are reported in Table 2. Good reproducibility was observed in terms of tensile strength of the FSW joints. Owing to non-uniform strain distribution, it was not possible to calculate the yield strength of the welded specimens.

**Table 1.** Chemical composition (% weight) of the alloy 7075-T6.


**Table 2.** Mechanical properties of the base materials and the welded joint.


### *2.2. Metallographic Analysis*

The microstructure was revealed through Keller's etchant after grinding by means of emery papers up to 4000 grit, then polishing with 0.1 μm alumina aqueous suspension and finally observed with Nikon (Eclipse MA100N, Tokyo, Japan) optical microscope and SEM (Zeiss EVO 50, Zeiss, Oberkochen, Germany) Oxford x-act sdd (silicon drift detector, nitrogen free) EDX equipment type model (Oxford Instruments, High Wycombe, UK).

#### *2.3. OCP Monitoring*

OCP was measured on the three di fferent zones of FSW butt welds, i.e., the nugget, the thermomechanical a ffected zone/heat-a ffected zone (TMAZ/HAZ—it was not possible to separate the two zones) and the base metal. The specimens were cold-mounted in epoxy resin with an embedded wire to allow electrochemical measurements. A 20 mm × 20 mm area was grounded with emery paper and then polished with colloidal alumina up to 0.1 μm. After polishing, the specimens were stored at room temperature (23 ◦C) with relative humidity in the range of 25–30% for 48 h before the immersion, to allow the formation of the natural protective film. The specimens were dipped in di fferent testing cells filled with aerated 35 g/<sup>L</sup> NaCl solution (Carlo Erba RPA reagent, Cornaredo, Italy). The OCP was measured by using a saturated calomel electrode (SCE E = +0.240 V vs Standard Hydrogen Electrode, Amel Instrument, Milano, Italy) positioned close to the surface of the specimens to reduce the ohmic drop in the electrolyte during electrochemical tests.

#### *2.4. Four-Point Bending Tests*

Four-point bending (4PBB) tests have been performed according to ASTM G39 on 4 mm × 20 mm × 160 mm prismatic specimens obtained by FSW joints (Figure 1). The specimens were polished by emery papers up to 1000 grit and degreased in acetone in ultrasonic bath for 5 min. After polishing, the specimens were left in still air for 48 h to allow the formation of the protective film.

**Figure 1.** Schematic representation of specimens obtained by means of friction stir welding (FSW) welded sheet (draw not in scale).

The specimens were positioned inside the bending device to attain a uniform tensile strain distribution over the welded surface. Four glass cylinders were used to avoid galvanic coupling between the stainless-steel specimen holder and the aluminum alloy. Four specimens were loaded up to 80% of the tensile strength of FSW joints derived by tensile tests (refer to Table 2). The other four specimens were dipped without loading (unloaded condition). Tests have been performed on four specimens for each condition (loaded and unloaded). All the specimens were exposed in a cell filled with about 30 L water, and 35 g/<sup>L</sup> sodium chloride (Carlo Erba RPS analytical grade reagents, Cornaredo, Italy) for 1500 h. To avoid the contamination of the testing solution by corrosion products of aluminum during long-term exposures, the test solution was refreshed after 150 h and 500 h. The amount of water that was substituted was equal to 10 liters. During the procedure, the specimens were always left well below the waterline. The OCP was measured during the exposure. More details about the experimental set-up are reported in a previous work [27].

At the end of the long-term exposure tests, the specimens were unloaded, washed in distilled water, and then rinsed with acetone in an ultrasonic bath. The surfaces were then observed by means of optical microscopy up to 600 times magnification and SEM equipped with EDX.

#### *2.5. Constant Load Tests*

CL tests were carried out on a specimen of 4 mm × 8 mm × 82 mm gage length, with the weld positioned at the center of each specimen (Figure 1). The size of the specimens is reported in Figure 2. The specimen was settled in a double compartment cell (Figure 3a). All the compartments were maintained at room temperature filled with aerated NaCl 35 g/<sup>L</sup> solution (Carlo Erba RPS analytical grade reagents, Cornaredo, Italy). Water recirculation in the cell was granted during the tests. The OCP of the specimens was continuously monitored using a SCE positioned close to the metal surface by means of a Lugging capillary.

The specimens in the cell were positioned into the CL machine (no loading), then they were dipped in the test solution. After 120 h of OCP monitoring, the specimens were loaded at 190 MPa, corresponding to 54% of the ultimate tensile strength (UTS) of the weld. This value is slightly lower than the elastic limit of the weld obtained by means of tensile tests. The specimens were left at this value for 350 h and the OCP was measured during all the period. Then, the applied load increased up to 279 MPa, corresponding to 80% of the UTS of the weld (homogeneous plastic deformation field). The OCP was measured until 720 h of exposure (one month).

**Figure 2.** CL and SSR specimen: the weld is in the center of the gouge length, the thermomechanical affected zone/heat-affected zone (TMAZ/HAZ) extension is 40 mm for each side.

**Figure 3.** CL test device (**a**) and SSR test device (**b**).

At the end of the test, the specimens were unloaded and observed under the optical microscope and under the SEM to analyze the corrosion morphology. Metallographic cross-section was also taken along the longitudinal direction of the specimen.

#### *2.6. Slow Strain-Rate Tests*

The SSR tests were carried out by using specimens of the same dimensions as the CL tests. The set-up of the tests is shown in Figure 3b. The tests were carried out on a 30 kN testing machine at displacement rates varying from 5 × 10−<sup>7</sup> to 5 × 10−<sup>3</sup> mm/s. A displacement rate of 8.2 × 10−<sup>4</sup> mm/s was used in order to have a strain rate of 10−<sup>6</sup> s<sup>−</sup><sup>1</sup> as average value on all the gage length. During the tests, the load and OCP were measured continuously. At the end of the tests the specimens were washed in distilled water and rinsed in acetone to permit the observation of the fracture surface under the SEM. Then the specimens were longitudinally sectioned for the metallographic observation.
