Thus, a more insight into corrosion behavior can be gained from Fig. 6 (a&b) wherein corrosion current (a measure of corrosion rate) and the corrosion potential ZM447439 presented in the form of bar charts for the Zn coatings obtained by three modes of deposition. Amongst the electrodeposited coatings, pulse reverse electrodeposited Zn exhibits the lowest corrosion rate and therefore effectively protects the underlying mild steel substrate for longer duration.
Fig. 6. Variation of corrosion current (a) and corrosion potential (b) as a function of coating deposition mode.Figure optionsDownload full-size imageDownload high-quality image (211 K)Download as PowerPoint slide
3.2.2. (b) Electrochemical impedance spectroscopy
Fig. 7. Nyquist plot after impedance tests in 3.5% NaCl along with equivalent circuit model (inset).Figure optionsDownload full-size imageDownload high-quality image (162 K)Download as PowerPoint slide
The equivalent circuit in the case of PRC deposited Zn coating is messenger RNA (mRNA) characterized by predominant two time constants (see inset of Fig. 7) and includes an additional circuit elements compared to the DC and PC deposited Zn. It can be noted that, the first time constant (Rct − CPEdl) at lower frequency corresponds to charge transfer at the electrical double layer. The second time constant (Rcoat − CPEcoat) can be attributed to the presence of compact ZnO passive layer present on the surface of the Zn (as discussed later). After fitting the experimental data with the equivalent circuits presented in Fig. 7, the values of Rs, CPE-n, Rct, CPEdl, Rcoat and CPEcoat were obtained and are presented in Table 4. The CPE (CPEdl in PC and DC CPEcoat in PRC) was not found to change much with deposition technique. An increase in Rct in the case of PRC Zn coatings compared to DC and PC Zn is clearly evident.