*2.2. Experimental Methods*

The specimens for the microstructural examinations were mounted in conductive resin, ground on silicon carbide papers up to 1600 grit, and then progressively polished with 9, 3, and 1 μm diamond paste. The etchant used was 3% ethanol solution of picric acid. The secondary electron (SE) micrographs were acquired with a JEOL JSM 7600 F (Jeol Ltd., Tokyo, Japan) high-resolution field emission scanning electron microscope (SEM), operating at an acceleration voltage of 15 kV. The microstructural examinations were coupled with semi-quantitative analysis of chemical composition of phases by using energy-dispersive spectroscopy (EDS). The basis of the carbide particles' classification and the determination of their quantitative characteristics were elaborated on earlier, and are demonstrated in recent papers [14,22]. In brief, the eutectic carbides (ECs) are vanadiumbased MC-particles, hence, they differ from the secondary M7C3-carbides (SCs) in terms of their chemistry. Therefore, EDS analysis was used to differentiate between these two carbide types. In order to differentiate between the small globular carbides (SGCs) and other particles, a classification according to their size was used, and the particles smaller

than 500 nm were then denoted as SGCs. For the determination of the population densities of ECs, SCs, and SGCs, 25 randomly acquired SEM micrographs, at a magnification of 3000×, were used. Then, the mean values and standard deviations were calculated from the obtained experimental data.

Semi-quantitative chemical composition of phases of sub-micron size (metallic elements only) and the microstructure of both the martensite and the austenite were examined by transmission electron microscopy (TEM). Thin foils for TEM were prepared by sectioning specimens, with a special saw, to obtain pieces with a thickness of 0.1 mm, which were then ground mechanically on silicon carbide paper (1200 grit) to a thickness of approximately 0.02 mm. Final thinning was carried out using an electro-polisher Struers-Tenupol 5. A TEM JEM ARM 200 cF (Jeol Ltd., Tokyo, Japan) equipped with an energy-dispersive spectroscopy (EDS) detector was used for acquisition of micrographs as well as for the determination of phase compositions.

The phases in differently heat-treated specimens were identified from the X-ray diffraction (XRD) profiles by using a Phillips PW 1710 diffractometer. A filtered Coα1,2 characteristic radiation, obtained at the voltage of 40 kV and current of 40 mA, has been used for acquisition of diffraction line profiles, within a range of 37–130◦ of two-theta angles. The analysis was coupled with Rietveld refinement of the X-ray line profiles. The retained austenite amount was determined following the ASTM E975-13 standard [23].

Corrosion resistance studies were carried out by the potentiodynamic polarisation measurements (TAFEL). The TAFEL measurements were completed by using the potentiostatgalvanostat ATLAS 0531 EU (Atlas-Sollich, R˛ebiechowo, Poland) and IA ATLAS SOLLICH (Atlas-Sollich, R˛ebiechowo, Poland). A platinum electrode was used as an auxiliary electrode, while a calomel electrode was used as the reference electrode and tested specimens as the working electrodes. The data were recorded by AtlasCorr (Atlas-Sollich, version 3.19) and AtlasLab (Atlas-Sollich, version 2.24) computer software. After corrosion tests, the tested specimens were examined by using SEM coupled with EDS.

Before testing, all the specimens were ground using metallographic emery papers with a grit size up to 2000, and finally, were polished using 3 μm diamond slurry. Just prior to measurements, the specimens were degreased in ethanol and warm air-dried. The 3.5 (in mass %) NaCl water solution was prepared using distilled water and high-purity reagent grade sodium chloride. Then, the solution was subjected to Ar gas bubbling for 30 min in order to its deaerate. For the corrosion tests, the temperature of the solution was kept constant at 22 ◦C.

The potentiodynamic polarisation measurements were carried out within the range of potentials of −1.5 to 1.5 V, and at a scan rate of 1 mV/s.

The corrosion rate for each specimen was estimated according to the ASTM G 102-89 standard [24]. The calculation was based on the validity of the Faraday's Law, and it followed Equation (1):

$$\mathbf{C}\_{R} = K\_{1} \cdot \frac{i\_{\rm cor}}{\rho} \cdot E\_{W} \tag{1}$$

where *CR* is the corrosion rate (mm/year), *Icor* is the current density (μA/cm2), *ρ* is the specific density of the alloy (g/cm3), *<sup>K</sup>*<sup>1</sup> = 3.27·10−3, (mm × g/μ<sup>A</sup> × cm × year), and *EW* is the equivalent weight of the experimental steel.

Equivalent weight, *EW*, represents the mass of metal, in grams, that will be oxidised by the passage of one Faraday of electric charge. The value of *EW* of the experimental material was calculated upon its known chemical composition, atomic weight of the major elements, *W*, the most common valence of a particular element *n*, and as a reciprocal value of the sum of the electron equivalents of all the major elements, *Q*, following Equations (2) and (3):

$$E\_W = \frac{1}{\sum \frac{n\_i \cdot f\_i}{W\_i}} \tag{2}$$

$$Q = \sum \frac{n\_i \cdot f\_i}{\mathcal{W}\_i} \tag{3}$$

where *fi* is the mass percentage of the *i*th element in the alloy, *Wi* is the atomic weight of the *i*th element in the alloy, and *ni* is the most common valence of the *i*th element of the alloy. All the input data for the calculations are collected in Table 2. Additionally, it should be noted that an assumption of corrosion uniformity was adopted in the present study, in order to simplify the considerations.

**Element** *f* **(Mass %)** *W* **(g)** *n* **(-)** *Q* **= (***n* × *f***)/***W* C 2.1 12.0107 4 0.699376389 Si 1.0 28.0855 4 0.142422246 Mn 0.4 54.9380 2 0.014561857 Cr 6.8 51.9961 3 0.392337118 Mo 1.5 95.9400 6 0.093808630 V 5.4 50.9415 5 0.530019729 Fe 82.8 55.8450 3 4.448025786 *Qtotal* 6.320551755 *EW* 15.821403555

**Table 2.** The mass percentage, *f*, atomic weight, *W*, most common valance, *n*, electron equivalents of the main elements in the Vanadis 6 steel, *Q*, and calculated electron equivalent of the steel, *Qtotal*, as well as its equivalent weight, *Ew*.

In order to analyse the differences in nobility between the carbides and matrix, the Kelvin probe force microscopy (KPFM) has been adopted. This technique enables to distinguish between local potentials of different phases, at the sub-micron level. Hence, it is suitable for analysing fine-grained PM ledeburitic steels. The analyses were performed at The University of Manchester, Department of Materials Corrosion, by using a Multimode 8 instrument (Bruker), in the amplitude modulated mode. The imaging parameters were the following: 50 × 50 μm scans, potential maps obtained from a 50 nm lift height, using a 0.3 Hz scan rate, 512 points per line, 256 lines, and height images obtained using 100 nm peak force amplitude.
