**2. Materials and Methods**

Wrought ZE10 magnesium alloy samples with dimensions of 20 × <sup>20</sup> × 1.6 mm3 were used as a substrate material for deposition of the electroless Ni–P coating. The microstructure of the ZE10 magnesium alloy, shown in Figure 1, was characterized using the light microscopy (LM) and the individual microstructural features were identified using a Zeiss EVO LS-10 scanning electron microscope (SEM, Carl Zeiss Ltd., Cambridge, UK) with an energy-dispersive X-ray spectroscopy (EDS) Oxford Instruments Xmax 80 mm<sup>2</sup> detector (Oxford Instruments plc, Abingdon, UK) and AZtec software analysis (version 2.4). Elemental composition of the coated material shown in Table 1 obtained using the GDOES (glow-discharge optical emission spectroscopy) corresponds to the ASTM standard [18].

To reveal the ZE10 magnesium alloy microstructure, prepared and polished metallographic samples were poured into an acetic picral etchant (consisting of 4.2 g of picric acid, 10 mL of acetic acid, 10 mL of distilled water, and 70 mL of ethanol) for 3 s and then into 2% Nital for1s[19]. The microstructure of the substrate, characterized on the surface parallel to the material processing,

was formed by solid solution grains of α-Mg, Mg7Zn3(RE) (La, Ce)-based particles and undissolved Zr particles (Figure 1b) [1,5]. Via EDS analysis, the presence of Ce, La, Pr, and Nd elements was found in the Mg7Zn3(RE) (La, Ce)-based particles.

**Table 1.** The elemental composition of the wrought ZE10 magnesium alloy (wt %) (GDOES).

**Figure 1.** Surface microstructure of the ZE10 wrought magnesium alloy: (**a**) microstructure (LM); (**b**) intermetallic phase particles in microstructure (SEM).

To achieve a sufficient activity and roughness of the magnesium substrate surface before electroless Ni–P coating deposition, a suitably chosen specific pre-treatment is required. First, the samples were ground using SiC paper no. 1200 and then cleaned in an alkaline degreasing bath with the content of soil-releasing agents. To activate the magnesium substrate surface by partial etching, pickling in an acid pickling bath based in acetic acid was performed. Partial etching of the substrate surface led to an increase in the surface roughness and activity. This process was shown to have a positive influence on adhesion and mechanical properties of deposited Ni–P coating due to the mechanical interlocking of deposited Ni–P coating to magnesium substrate [20].

The electroless nickel bath contained a nickel source, NiSO4·6H2O, a reducing agent, NaH2PO2·H2O, a complexing agent, and the substance-activating NaH2PO2 molecule. Samples were localized in the middle of the nickel bath to ensure uniform coating creation and the elimination of the thermal gradient from the bath surface [9]. The deposition of the electroless Ni–P coating proceeded for 60 min. For the microhardness measurement, a thicker coating was required, so a time of 180 min was taken for the coating preparation.

The distribution and the average content of nickel and phosphorus in deposited Ni–P coatings were determined using the Zeiss EVO LS-10 with an EDS Oxford Instruments Xmax 80 mm<sup>2</sup> detector SEM and the AZtec software (version 2.4). SEM observations were used to characterize Ni–P coating surface morphology and for the evaluation of the mechanism of corrosion degradation of the magnesium substrate with the subsequent violation of Ni–P coating after exposition in 0.1 M NaCl.

The microhardness measurement of the deposited Ni–P coating was carried out using the LECO AMH43 microhardness tester (Saint Joseph, MO, USA). The average value of the microhardness was obtained from 10 valid indentations performed within the coating cross section under the applied load of 25 g for 10 s, according to the ASTM E384 standard [21]. The indents were performed parallel to the coating surface.

Physical properties of the electroless deposited Ni–P coating were evaluated using the REVETEST scratch tester CSM Instruments with the Rockwell diamond indenter with a top angle of 120 and 200 μm radius hemispherical tip [22]. The progressive load type method was applied to the measurement. The substrate surface was polished to the roughness of *R*<sup>a</sup> ≈ 2 μm using a DP-Paste M (diamond paste from Struers, Ballerup, Denmark) during the pre-treatment process before the alkaline degreasing in the addition. The friction force, the friction coefficient, the penetration depth, and the acoustic emission were recorded during the scratch test. Normal force was recorded. The applied normal force was set up in the range from 1 to 20 N. The speed of indenter was 1.58 mm·min<sup>−</sup>1, and the total length of the trace was 3 mm.

The electrochemical corrosion characteristics of the ZE10 magnesium alloy and material with deposited Ni–P coating were analyzed in a 0.1 M solution of NaCl using the Bio-Logic VSP-300 potentiostat/galvanostat (BioLogic, Seyssinet-Pariset, France). Electrochemical polarization tests were performed on three specimens. A standard three-electrode cell was used for the measurements: a Pt gauze was used as a counter-electrode, a saturated calomel electrode (SCE) as a reference electrode, and a prepared sample as a working electrode. The analyzed sample area exposed to the solution during the polarization test was 1 cm2. The stabilization time of the samples exposed to the corrosive environment was 5 min. The polarization range of the measurements was from −50 to +200 mV vs. open circuit potential (OCP). The corrosion potential *E*corr and the corrosion current density *i*corr, were determined applying the Tafel analysis, and the corrosion rate *v*corr was calculated from the *i*corr, according to the literature [23].

For the evaluation of the mechanism of the corrosion degradation of the ZE10 magnesium substrate with subsequent violation of the deposited Ni–P coating, the sample was immersed in the 0.1 M NaCl for 1 h. After this time, the rinsed and dried surface of the samples was analyzed via SEM.
