*3.1. Ni–P Coating Morphology and Chemical Analysis*

Uniform Ni–P coating was successfully deposited on the previously pre-treated surface of the ZE10 magnesium alloy substrate. As shown in Figure 2a, grain boundaries and intermetallic phase particles appeared on the surface of the material after the pre-treated, pickled substrate. The pickled surface also shows a "honeycomb-like microstructure." Revealed intermetallic phase particles/α-Mg solid solution interface and the grain boundaries served as places where the initiation of the electroless deposition of the coating initiated due to the galvanic coupling [24]. The higher roughness of pickled and activated surface increased the adhesion of deposited Ni–P coating to the magnesium substrate.

Usually, zinc immersion is used for the magnesium alloy pre-treatment before Ni–P coating deposition. El Mahallawy et al. [14] studied the electroless Ni–P coating of different magnesium alloys, using zinc immersion as the pre-treatment of magnesium alloys. The zinc immersion was used to remove the residual oxides and hydroxides from the surface of the magnesium alloys, and a thin layer of zinc formed on the Mg surface preventing back oxidation.

Based on the obtained results, a partial re-oxidation of the ZE10 magnesium alloy surface occurred without the use of zinc immersion (Figure 2a). During the following immersion of the activated sample into the electroless nickel bath, the substrate became catalytically active when the surface oxides were dissolved in the nickel bath, and the replacement reaction occurred between the substrate and nickel ions. Even though some contamination of the substrate surface was observed before Ni–P coating deposition, it seems that it did not negatively affect the coating process [9].

The surface morphology of the deposited Ni–P coating with a nodular structure, formed by typical cauliflower-like shapes is shown in Figure 2b. Wang et al. [25] showed that deposited Ni–P coatings are formed by a columnar microstructure. However, the deposited Ni–P amorphous coatings improve the corrosion resistance of magnesium, the inherent columnar microstructure of the coating does not provide the best protection against the corrosion. The high concentration of inter-column defects, such as microvoids and micropores, [26,27], form channels where the corrosion ions and environment can pass through the coating and react with the substrate. The presence of microcavities was not evident between nodular cusps of the deposited coating (Figure 2b), which is in agreement

with observations in [28]. Based on an evaluation of SEM figures (Figures 2b and 3a), no defects and cracks were observed in the deposited Ni–P coating at the ZE10 magnesium substrate/Ni–P coating interface.

The average thickness of the coating prepared for 60 min determined from the cross sections was approximately 10 μm. In the case of a longer deposition time (180 min), prepared with the aim to increase the coating thickness to obtain relevant microhardness values, thickness was 30 μm.

**Figure 2.** SEM microstructure of the pre-treated substrate and deposited Ni–P coating: (**a**) pre-treated surface of the ZE10 magnesium alloy before deposition; (**b**) fine morphology of the deposited Ni–P coating.

Deposited Ni–P coating with an average thickness of about 10 μm was chosen for EDS analysis (Figure 3). Using EDS mapping analysis, it was determined that the distribution of Ni and P in deposited Ni–P coating was homogeneous in the entire cross section, as shown in Figure 3b,c, respectively. The EDS analysis determined that the Ni content in the deposited Ni–P coating was 95.6 wt % and the P content was 4.4 wt %. Based on the literature [9,29], it was determined that the deposited Ni–P coating is low-phosphorus, as in the cases of the AZ31 magnesium alloy presented in [30] and the AZ61 magnesium alloy in [28].

**Figure 3.** EDS analysis of deposited Ni–P coating on the ZE10 magnesium alloy: (**a**) a cross section of the Ni–P-coated sample, (**b**) nickel, (**c**) phosphorus, and (**d**) magnesium.

#### *3.2. Ni–P Coating Microhardness Analysis*

Based on the measured data, it was determined that the average value of the microhardness of the Ni–P coating was 690 ± 30 HV 0.025, measured in the cross section. The microhardness of the plain ZE10 magnesium substrate was 60 ± 4 HV 0.025.

The surface microhardness of the coated samples increased approximately 11-fold compared with the ZE10 magnesium alloy.

It is assumed that the measured hardness of low-phosphorus Ni–P coatings is higher compared to the high-phosphorus coatings [9]. The addition of filler (SiC, Al2O3) into the high-phosphorus Ni–P matrix led to a substantial increase in hardness [9]. The microhardness (690 ± 30 HV 0.025) of the deposited low-phosphorus Ni–P coating on the ZE10 magnesium alloy reached a value higher than that of the Ni–P/SiC composite coating prepared on the AZ91 magnesium alloy presented in [31] and [32]. The microhardness of the Ni–P/SiC composite coating (7.33 wt % P) was 620 HV [31], and that of the electroless Ni–P/SiC nanocomposite coating (10 wt % P) was 600 HV 0.025 [32].

The microhardness of the deposited low-phosphorus Ni–P coating was higher compared with the values obtained for the high-phosphorus Ni–P coatings. The hardness of high-phosphorus Ni–P coatings ranges from 410 to 600 HV [29,33]. As the content of phosphorus in Ni–P coatings increases, the microhardness of the coating decreased due to the microstructural changes (a decrease in crystallinity) [9].
