2.3. Potentiodynamic Polarization Test

Electrochemical measurements consisting of pitting corrosion tests by dynamic polarization were carried out in a conventional electrochemical cell consisting of a 150 cm³ volume cell, open to the air, and a three-electrode system. A cylindrical Ø 8 mm working electrode (tested sample) was embedded in an isolating rubber sleeve so that only the desired surface was exposed to the solution (exposed area of 50.3 mm²). The counter electrode (XM140) was platinum foil with a total surface area of 128 mm² (Radiometer Analytical, Villeurbanne, France) and a saturated calomel electrode (SCE, ESCE0 = +0.248 V vs. SHE at 20 °C) was used as a reference electrode, directly in contact with the working solutions. The potential of the working electrode was controlled by VoltaLab PGP201 potentiostat, and the response current was recorded in VoltaMaster 4 software (Radiometer Analytical). Measurements were performed using a scan rate of 3 mV/s.

The temperature of the corrosion cell was controlled thermostatically using a water bath ICC basic (IKA–Werke, Staufen im Breisgau, Germany) and maintained at 20 °C during all the tests. The electrolytes were prepared with purified water of >18.2 MΩcm resistivity using Millipore Direct-Q water purification kit (Merck, Rahway, NJ, USA) and analytical grade sodium chloride and concentrated (65%, 14 M) nitric acid (Chempur, Piekary Śląskie, Poland). The tests were carried out with three different corrosive solutions: 0.61 M NaCl, 1 M HNO₃, and 4 M HNO₃. Experiments in NaCl were performed additionally as general, all-purpose tests that produce valid comparisons for most metals [23], as it corresponds to the mean salinity of seawater (~3.5 wt.% NaCl) and is commonly used in other studies. The total volume of the electrolyte in each experiment was 70 mL, and it was replaced with a fresh solution after each test. The tested samples were ground and polished using a set of emery papers (800–3000 grade) and 0.05 μm alumina slurry to achieve a mirror surface finish. Prior to the experiment, the samples were cleaned in an ultrasonic bath for 10 min in ethanol, washed with water, wiped, and immersed in the test solution shortly after. The three-electrode system was immersed in stagnant (non-stirred) solutions for 2 h before starting measurements in order to reach a steady-state value for the open-circuit potential (OCP).

Corrosion current density Icorr was obtained by the Tafel extrapolation method (Figure 1) [24], utilizing a well-known Stern–Geary equation [25,26]. As shown in Figure 1, the intersection of the straight lines on anodic and cathodic polarization curves corresponds to Icorr, whereas Ecorr is the corrosion potential. Polarization resistance Rp was estimated as the slope of the polarization curve in a narrow potential region (±10 mV) around Ecorr.

Tafel extrapolation with fundamental parameters used to calculate corrosion behavior (after [27,28]).