*3.4. The Electrochemical Corrosion Test in 0.1 M NaCl*

Figure 6 shows the potentiodynamic polarization curves of the ZE10 magnesium alloy and the ZE10 alloy with the deposited Ni–P coating in 0.1 M NaCl obtained at laboratory temperature. The polarization curve of the Ni–P-coated sample is significantly shifted to more electropositive values, which means better corrosion properties of the Ni–P-coated sample compared with the untreated ZE10 magnesium alloy.

Therefore, a deposited Ni–P coating appears to be suitable for the protection of magnesium alloys. Based on the Tafel extrapolation analysis [39], the values of the corrosion potential, *E*corr, and the corrosion current density, *i*corr, for samples with the deposited Ni–P coating and the ZE10 magnesium alloy were determined. The average values of the *E*corr, for the ZE10 magnesium alloy and the Ni–P-coated alloy, were −1701 and −505 mV, respectively, and the values of the *i*corr, for the ZE10 magnesium alloy and the Ni–P-coated material, were 23.7 and 0.4 <sup>μ</sup>A·cm<sup>−</sup>2, respectively (Table 3). It is generally known [25] that the columnar structure of deposited Ni–P coatings contains a network of defects (grain boundaries, pores, and microcavities). These defects are precursors for a micropitting and may cause the corrosion attack of the material under the deposited Ni–P coating. However, no local corrosion attack (pitting) was observed in the anodic area of the polarization curves.

Based on the determined values of *i*corr, the corrosion rate, *v*corr, was calculated for a short-term experiment. As indicated by Table 3, the corrosion rates of the ZE10 alloy and the Ni–P-coated samples in 0.1 M NaCl were 530.00 μmpy and 8.95 μmpy, respectively.

Experimental deposited Ni–P coating was ranked among the low-phosphorus Ni–P coatings, which are characterized by their lower corrosion resistance when compared to the medium- and high-phosphorus coatings, [9]. As indicated in the work of S. Narayanan [12], deposited Ni–P coatings with low phosphorus content (3.34 wt % P) showed an *E*corr of −0.536 V and an *i*corr of 4.22 <sup>μ</sup>A·cm−2, whereas medium- (6.70 wt % P) and high-phosphorus (13.30 wt % P) Ni–P coatings showed *<sup>E</sup>*corr values of −0.434 and −0.411 V, respectively, and *<sup>i</sup>*corr values of 1.17 and 0.60 <sup>μ</sup>A·cm−2, respectively. When comparing the experimentally obtained results and the results presented in [12], it is evident that the value (Table 3) of the corrosion potential, *E*corr, is between *E*corr values of 25-μm-thick low-phosphorus (3.34 wt % P) and medium-phosphorus (6.70 wt % P) Ni–P coatings presented in [12].


**Table 3.** Corrosion parameters of the ZE10 magnesium alloy and the ZE10 alloy with a deposited


**Figure 6.** Characteristic potentiodynamic polarization curves for the ZE10 magnesium alloy and the ZE10 alloy with a deposited Ni–P coating in 0.1 M NaCl obtained at laboratory temperature.

Although the plating rate of high-phosphorus Ni–P coatings in [12] was higher, high-phosphorus Ni–P coatings were deposited under more energy-intensive conditions (90 ± 1 ◦C) [12]. It is evident that the obtained value of the corrosion current density, *i*corr, of high-phosphorus Ni–P coating was worse compared to the presented low-phosphorus Ni–P coatings on the ZE10 magnesium alloy (Table 3), and the electroless deposited coating on the ZE10 alloy exhibits greater corrosion resistance. This can be attributed to the fact that the experimentally prepared electroless deposited Ni–P coating was probably less defective or that the Ni–P coating contained only a small amount of microcavities when compared to the coatings analyzed in [12].

#### *3.5. The Immersion Test in 0.1 M NaCl*

As indicated by Figure 7a,b, after the exposition of the sample with deposited Ni–P coating in 0.1 M NaCl for 1 h, the degradation of the Ni–P coating occurred due to the corrosion of the magnesium substrate under the coating.

Corrosive agents may accumulate between the nodules (grains) creating the Ni–P coating and migrate to the substrate surface. According to the results in [25,40], the inherent columnar porous microstructure of the coating (Figure 7d) does not adequately protect the magnesium substrate against corrosive environments [41]. Furthermore, electroless Ni–P coatings contain a certain amount of microcavities in their volume. These microcavities form nucleation sites for micropitting when the material is exposed to a corrosive environment. Microcavities are created as a result of the hydrogen evolution during the deposition process when small bubbles of hydrogen H2, as a byproduct of the nickel reduction, are adsorbed on the surface of the growing Ni–P coating [40]. Stirring of the nickel baths or the rotation of the deposited objects is a partial solution of this problem. However, the addition of surfactants into the nickel bath was shown to be a more effective solution, while the reduction of these microcavities resulted in an increase in the corrosion resistance of the coated substrate [42,43].

As described above, due to the high concentration of inter-column defects, such as microvoids and micropores [27], the interaction between the corrosive environment containing chloride ions and magnesium substrate under the deposited Ni–P coating resulted in the creation of oxides or chlorides of magnesium. This reaction has an effect on the increase in corrosion product volume when compared to the bulk material and the evolution of hydrogen is an accompanying process. The increase in the volume of the material under the Ni–P coating leads to the formation of cracks and the subsequent destruction of the coating and thus to the acceleration of the corrosion of the magnesium substrate (Figure 7c).

Even though the coating internal defects were not observed in SEM analysis, even the intercolumnar areas can provide a path for the transfer of corrosive medium elements into the coating and react with the substrate. Figure 7d reveals the damaged structure of the Ni–P coating showing the columnar structure. The revealed columnar structure supports the theory that the grain boundaries and microdefects present on the grain boundaries acted as the paths for the corrosive medium transfer to the substrate.

Observed local corrosion attack did not correlate with the results obtained with potentiodynamic tests; however, the electrochemical test was limited to the range of polarization used, and a larger range for the measurement likely can reveal the pitting attack.

**Figure 7.** Corrosion degradation of the Ni–P coating on the ZE10 magnesium alloy: (**a**) surface, (**b**) cross-section, (**c**) cracks in Ni–P coating caused by the increase in volume under deposited coating due to the corrosion, (**d**) cracked Ni–P coating.
