Electrochemical assessment of the barrier ability of galvanic Zn coating primers electrodeposited on low-carbon steel

Low-carbon steel is highly susceptible to corrosion, necessitating protective coatings capable of efficiently shielding its surface from corrosive environments. This brief research focuses on evaluating the protective capacity of electrochemically deposited Zn coatings, both with and without the addition of benzalacetone. Two independent electrochemical methods, Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Scanning (PDS), were employed after exposing the samples to a 0.01 M NaCl model corrosive medium (MCM) for 24 hours. The results demonstrate the discernible barrier performance of the Zn protective films compared to the bare steel substrates.


Introduction
Zn coating remains the most widely employed method for protecting steel-based materials from corrosion [1][2][3][4].Recent research in this field has placed a strong emphasis on this type of coating primers [5][6][7][8].In this context, Giurlani et al. [9] underscore the escalating interest in galvanic Zn layers, highlighting their diverse applications in corrosion protection, decorative purposes, microelectronics, automotive, urban planning, aircraft, and spacecraft sectors, among others.Furthermore, Zn layers can be subjected to additional anodization, acquiring further advantageous properties [10,11].Apart from the application of specific electrical regimes [10,12], the electrolytes used for Zn deposition [13-15] play a crucial role in determining the composition, structure, and resulting properties of the Zn layers.Furthermore, these coating primers can undergo modifications, such as anodization [7, 8, 16] or additional sealing [17,18].The potential utilization of these Zn layers in alternative energy components [16][17][18][19][20][21][22] and potential biomedical applications [23] continues to drive increasing interest in this type of coatings.The formation mechanism and characteristics of galvanic layers are significantly influenced by additives in the electrodeposition electrolyte.Notably, some authors have directed attention to the role of benzalacetone as a brightener for galvanic coatings [5,24].Consequently, the utilization of benzalacetone in Zn electrochemical deposition has demonstrated a 3. of this organic additive for further and more detailed research activities.Another anticipated advantage of introducing the organic compound is its potential corrosion inhibition effect, as elucidated in recent studies [26][27][28][29][30].In alignment with these research trends, this brief study aims to assess the impact of benzalacetone addition on the performance of the resulting Zn galvanic films in diluted NaCl solutions.The assessment employs two independent electrochemical analytical methods, and the results demonstrate the noticeable barrier properties of the formed films in comparison to the bare steel samples.However, no substantial impact of the suggested organic additive on the coating behavior was observed.

Electrochemical film deposition conditions
The electrochemical deposition of Zn films was carried out on low-carbon steel substrates in a thermostatic double-chamber electrolytic cell with a volume of 500 cm 3 .The current density (c.d.) was maintained within the range of 2 A dm⁻² for a duration of 15 minutes at room temperature.Recognizing the significance of electrolyte pH, as mentioned in [18], we maintained its value at pH = 5.Continuous circulation of the solution at 150 rpm was ensured, with Zn-counter electrodes used.Two different electrolyte compositions were employed, as detailed in table 1 below: Table 1.Compositions of the used electrolytes.

Electrochemical film assessment conditions
The electrochemical assessments of the films were conducted on three sets of samples, each comprising two specimens, as follows: Set 1 with coating deposition from Electrolyte 1, Set 2 with coating deposition from Electrolyte 2, and Set 3 consisting of bare reference substrates.
The measurements were taken after 24 hours of exposure to a 0.01 M NaCl solution, using threeelectrode electrochemical cells with a volume of 100 cm 3 .The conventional electrochemical analytical techniques employed were Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Scanning (PDS).For EIS, the spectra were acquired in a three-electrode configuration connected to an Aurolab 30 PG-stat and supported by a Frequency Response Analyzer FRA-2 block.All measurements were referenced to an Ag/AgCl/3M KCl reference electrode positioned 10 mm above the corrosion test areas (CTA) of the specimens, which had an area of 1 cm².The excitation signals were introduced into the electrochemical cells via a cylindrical Pt-mesh mounted around the reference electrode.PDS curves were acquired within a potential range from -50 to +500 mV relative to the reference electrode, with a potential sweep rate of 10 mV s -1 .

EIS Spectra Acquisitions
The spectra obtained after 24 hours of exposure to the 0.01 M NaCl model corrosive medium reveal distinct differences in the surface layer depositions compared to those of the reference samples.These differences are highlighted by the circled data points in figure 1.
The acquired EIS data were assessed through fittings of the real spectra with a model equivalent circuit (MEC).We employed a simple Randle's cell as the MEC, following the principle that the simplest appropriate MEC corresponds to the actual corroding surface/corrosive medium system.This MEC consisted of the Ohmic resistance of the 0.01 M NaCl model corrosive medium (RMCM), a constant phase element (CPEedl) representing the electric double layer, and charge transfer resistance (Rct).It's worth noting that, while the Nyquist plots of the reference samples indicate diffusion processes, these were disregarded in order to ensure comparability with the rest of the sample sets.

Barrier Ability of Zn Films with
Benzalacetone.However, the addition of benzalacetone does not appear to influence the barrier ability of the resulting Zn films.Specifically, the films deposited from Electrolyte 1 (indicated by up triangles in figure 1) remain positioned between the spectra of the Zn coating primers (represented by down triangles in figure 1).This observation strongly suggests that these films possess identical barrier abilities.Similarly, the respective CPEedl values approach 4.10 -4 s n Ω -1 cm -2 for Set 1, compared to about 3.10 -4 s n Ω -1 cm -2 for Set 2 and nearly 6.10 -4 s n Ω -1 cm -2 for the references.This observation indicates that the electric double layer for the reference samples possesses higher capacity due to the larger surface area, a consequence of the greater roughness of the steel substrates.The multiplier n also exhibits lower values for the references, further supporting the notion of easier access for corrosive species from the model corrosive medium to the substrate surfaces.
Lastly, the average Rct values are calculated as follows: 3.06 kΩ cm 2 for Set 1, 2.95 kΩ cm 2 for Set 2, and 1.80 kΩ cm 2 for Set 3. Consequently, both Zn layers possess nearly identical Rct values, approximately 3 kΩ cm 2 , whereas the bare steel exhibits almost half of this value.This suggests that the corrosion process on the bare alloy is nearly twice as intensive compared to the coated specimens.

PDS Data
All the PDS curves exhibit uniform corrosion, as evidenced by the absence of typical sharp rises in the anodic branches (figure 2).This finding suggests a high level of homogeneity between the metallic substrates and the Zn coating primers.However, it is worth noting that both galvanic films exhibit significantly more negative values of corrosion resistance when compared to the bare steel samples, as indicated by the results in table 3. The shift in open circuit potential, induced by the galvanic coatings, results from the more negative standard electrode potential of Zn when compared to that of Fe.Furthermore, the table illustrates that the polarization resistance (Rp) for Set 1 is approximately 3.85 kΩ cm 2 , while for Set 2, it is around 3.65 kΩ cm 2 , and for Set 3 (bare steel), it's merely 1.30 kΩ cm 2 .Consequently, the Zn coating primers exhibit Rp values approximately twice as high as those of the bare steel.

Conclusions
This concise study investigated galvanic Zn coating primers deposited on low-carbon steel, both with and without the addition of benzalacetone in the deposition electrolytes.The analysis of the results has yielded the following insights: • The measurements conducted after 24 hours of exposure to a diluted model corrosive medium have demonstrated that both the Zn coating primers significantly enhance the barrier properties of the steel.In fact, the values of Rct and Rp obtained through EIS and PDS, respectively, are approximately twice as high as those observed for the reference samples.
• Importantly, the addition of benzalacetone does not substantially impact the barrier properties of the investigated Zn coatings.In all cases, the measured parameters for both Zn layers exhibit nearly identical values.
2 times lower corrosion current, as reported by Morón et al. [24], and a reduction in the lattice grain size of the Zn layer, as observed by Li et al. [25].This, in turn, results in the smoothening of the deposited Zn film, highlighting the potential

Table 2 .
Impedance Component Values Obtained from EIS Fitting with MEC.

cm 2 ) CPEedl (s n Ω -1 cm -2 ) 10 -5 n / Rct (kΩ cm 2 )
Comparison of Results.A comparison of the results presented in table 2 reveals that the deposited films in Sets 1 and 2 possess approximately 720 Ω cm² of resistance within the model corrosive medium, compared to 580 Ω cm² obtained from the reference samples.This indicates that the samples undergo smoothening during galvanic coating electrodeposition, resulting in a reduced contact surface area and higher RMCM values.

Table 3 .
Data acquired from the PDS curves analysis.