3.1. Microstructural Characterization
Representative SEM images of the two HEAs are presented in
Figure 1a,b, both of which showed a compact microstructure, without cracks. The microstructure of both alloys was dendritic, with a grain refinement tendency in the case of Sample 2. A semi-quantitative analysis was performed in order to reveal the chemical composition of the investigated samples on the micro-areas labeled Area 1 and Area 2.
Table 3 presents the quantified values and estimated uncertainties of the elements found in the alloys’ composition. From the EDS analysis, it can be observed that the dendritic zone, labeled Area 1 in both samples, was rich in Ni, Fe, and Co. Area 2, corresponding to the interdendritic space, was made up of alloy rich in Cr and Mo, with the concentration of elements such as Co, Fe, and Ni being slightly lower. Low concentrations of Zr were identified in Sample 2, with slightly higher concentrations in the interdendritic areas. A comparative analysis of the microstructure of the two alloys highlights that the volume fraction of the interdendritic zones decreased when Zr was added to the CoCrFeMoNi alloy. In this type of alloy, the σ phase frequently appeared alongside the FCC structure. The addition of Mo to the CoCrFeNi system caused the formation of eutectic containing intermetallic phases (σ and µ) in the FCC phase. The eutectic microstructure influences the mechanical properties, determining the increase in hardness and yield strength [
44]. The key elements favoring the appearance of the σ phase are Mo and Cr, whose concentrations are higher in the interdendritic zone [
44]. In the high-entropy alloy CoCrFeNiMo, about 14% Cr- and Mo-rich σ phase was identified [
19]. In our alloys, the addition of 0.48 at.% Zr caused the Mo concentration to decrease from 20 at.% to 17 at.% in the interdendritic zones, while the Cr concentration remained almost unchanged, at approximately 25 at.%. In this way, the tendency to form the sigma phase was diminished.
Table 3 shows the chemical composition determined using the EDS method, where a higher concentration of Mo and Zr in the interdendritic areas can be noted. The X-ray diffraction patterns for both investigated samples are shown in
Figure 2. The alloys presented an FCC solid solution with some weak peaks, in addition to the (111) high-intensity peak. Thus, the lower-intensity peaks were attributed to the secondary σ phase, corresponding to the rich Cr- and Mo-rich areas determined in the EDS investigations.
3.3. Electrochemical Measurements
The samples were immersed in 3.5% NaCl solution, and the open-circuit potential (OCP) was recorded as a function of time up to 24 h. The OCP values changed continuously, fluctuating more rapidly during the first hours of immersion and reaching relatively stationary values only after 24 h. Was observed that the OCP values after 24 h of immersion were negative for both Sample 1 (−342 ± 63 mV vs. SCE) and Sample 2 (−458 ± 43 mV vs. SCE). This negative shift was related to the alteration of the film from the sample surface. The steady-state potentials corresponding to the corrosion potential (Ecorr) were obtained. Then, the linear polarization test in a potential range of 25 mV ± 1 versus Ecorr was performed, and the polarization resistance Rp was obtained. Tafel measurements were started from the cathodic to anodic direction in the range of −1.2 V to +1.2 V vs. SCE in order to obtain the Tafel slopes.
The potentiodynamic polarization curves were obtained during the tests carried out to estimate the corrosion rate (CR) of the samples. The current values are presented on a semi-logarithmic scale in
Figure 5.
The potentiodynamic polarization curves showed an increase in anodic current densities with the addition of zirconium to Sample 2, indicating a decrease in corrosion resistance in the conditions of simulated seawater used in experimental tests. In the anodic range of both curves, small increases of the current were observed, suggesting the acceleration of the oxidation reaction due to local corrosion and repassivation processes.
The value of the cathodic current density decreased with the addition of zirconium. Consequently, this implies that, in the simulated environment used for testing, the zirconium acted as an inhibitor of the cathodic reaction, restricting the hydrogen evolution process.
The corrosion rate was calculated as follows:
where
Icorr is the corrosion current (A),
K is the constant that defines the units of corrosion rate (3272 m/A·cm·year),
Ew is the equivalent weight (g/equivalent),
ρ is the density (g/cm
3), and
A is the sample area (cm
2).
Table 5 shows the electrochemical values obtained with these curves. The calculated CR data show values ranging from 2.44 × 10
−3 mmpy for Sample 1 to 2.80 × 10
−3 mmpy for Sample 2, representing a ~16-fold increase compared to Sample 1.
A more positive corrosion potential value can be observed for Sample 1 than for Sample 2. The corrosion current (icorr) is representative of the degree of oxidation of the alloy. A higher polarization resistance (Rp) denotes that the alloy is more resistant to corrosion; thus, Sample 1 was more resistant to corrosion than Sample 2.
The Tafel slopes (βa and βc) were obtained through an analysis of the curve plotted in an interval of ±250 mV versus the open-circuit potential (OCP). Sample 1 showed a tendency toward passivation because it had a value of βa greater than βc, while Sample 2 presented a corrosion tendency because the anodic slope was lower than the cathodic slope.
The characteristics of the oxide layer formed on an alloy surface can be estimated by means of the impedance technique, also known as electrochemical impedance spectroscopy (EIS). The graphs obtained from the EIS tests are presented as Nyquist plots in
Figure 6 and Bode plots in
Figure 7.
As can be observed in the Nyquist plots (see
Figure 6), the radius of the semicircle for Sample 2 was smaller than that for Sample 1, indicating a low polarization resistance (a low corrosion resistance) for Sample 2.
In the Bode vs. |Z| plot (see
Figure 7), a slight shift toward a higher value of the impedance module at the lowest frequency can be observed for Sample 1, indicating a slightly increased corrosion resistance of this alloy. In the Bode phase plots shown in
Figure 6, a specific performance of the growth of a passive film can be observed for both alloys. This passive layer had a capacitive behavior with a phase angle approaching 90°. In the case of Sample 1, the higher phase angle was constant in a wide frequency band, a phenomenon related to an increase in the effective surface area.
After analyzing the shapes of the impedance diagrams, the experimental results could be fitted to an appropriate physical pattern consisting of an equivalent electrical circuit (EC). This circuit consists of several series or parallel configurations of resistors, capacitors, Warburg elements, etc. and provides the most relevant corrosion parameters of the substrate/electrolyte system. The equivalent circuit is similar to that proposed for Ti−xMo, Al
xCoCrFeNi alloys, TiO
2 nanofibers, and polymer electrolytes [
45,
46,
47].
The equivalent circuit used to fit the experimental impedance data is presented in
Figure 7, and the values of the corresponding elements are shown in
Table 5.
Within the circuit presented in
Figure 8, the ohmic resistance of the simulated seawater is labeled R
1, the resistance of the passive layer is labeled R
2, and the capacitance of the passive layer is represented as Q
2. As a consequence of the heterogeneous thin oxide film built up on the surface of the HEA alloys and the remarkable deviations of the Bode diagrams, it was required to replace the “ideal” capacitance by a constant-phase element (CPE) [
48], the impedance of which is given [
49] by Z = (
jω)
−nY
0, where
j is an imaginary number (
j2 = −1),
ω is the angular frequency (rad·
s−1), Y
0 is the constant of the CPE [S(s·rad
−1)
n], n is the power number denoting the drift from ideal performance, n = α(π/2), and α is the constant-phase angle of the CPE (rad). Therefore, one of the parameters obtained when modeling the process is the ideality coefficient “
n”, in such a way that the answer of the real process is more similar to the ideal as the value of n gets closer to unity and, consequently, the surface is more uniform. Thus, for n = 1, the CPE element is reduced to a capacitor with a capacitance Y
0 and, if n = 0, to a simple resistor [
50].
With the aim of estimating the total impedance of the equivalent circuit, we computed the admittance of the parallel arrangement (
R2Q2) as follows [
51]:
Although a constant-phase element was used to fit the experimental results, the achieved value was considered as the capacity in the following equation:
Multiplying by
Rct, we get
Once the electrolyte ohmic resistance is added, the resulting impedance is
R2 was taken as the corrosion resistance of the analyzed HEAs. The
R2 values calculated by fitting the experimental data with the simulated results of the corresponding equivalent circuit are given in
Table 6.
This decrease in R2 (and, consequently, in corrosion resistance) when adding Zr was due to the fact that the passive film formed on the surface of the HEAs changed its properties as a result of this addition and became thicker (Y0 decreased with Zr addition).
It can be noted that the passive film resistance
R2 decreased with the addition of Zr because of the increase in the number of defective spots in the film. Without Zr, the passive film formed on the surface of the alloy was more compact and protective (see values of Y
0 in the
Table 6).
Doping with oversized atoms, such as zirconium, obstructs the grain coarsening of HEA, creating a supersaturated solid solution. Tekin et al. [
38] observed the in situ formation of ZrO
2 through TEM analysis; in contrast, in our XRD, although we had the peaks of the phases, these were not effective because the volume fraction of ZrO
2 was below the detection limit of XRD. The formation of ZrO
2 explained the high corrosion rate of Sample 2 with zirconium in comparison to Sample 1 without zirconium.
After performing the corrosion tests, the surface was covered with a thin layer of oxide, which was subjected to SEM analyses, and the results are presented in
Figure 9. The surface of both samples was covered with a very thin and transparent layer of oxide due to the chemical reactions during immersion in the corrosive solution. In the case of Sample 2 alloy, the oxide layer was thicker and further blurred the surface microstructure.
On the surface of Sample 1, a dendritic microstructure under the oxide layer and some corrosion pits were visible.