EIS

A standard cell with three electrodes was used: WE was a CS coupon, with a surface of 32 cm²; RE was a SC; and AE was Pt. The electrodes were connected to the measuring equipment (EGG Mode l 273A potentiostat/galvanostat, with three entries).

The testing solution was 3.5% NaCl. Impedance measurements were carried out with a response frequency recorder, from 10⁵ Hz to 1 mHz, with 5 points per decade.

Polarization curves

In order to plot the polarization curves, PDP measurement, with a E variation of +250 mV vs. E_corr, at a SR of 0.166 mV/s, was performed. This test was carried out only once, at the start of the coating steel immersion, on a separate coating.

CA

CA consists in following I evolution vs. time, when the system is subjected to overpressure. Herein, cathodic delamination test was used, which consists in imposing on the electrode a E lower than its E_corr, and measuring i_corr vs. time. This test allows to evaluate the coating performances over time, under cathodic protection.

In order to simulate real conditions, all steel coatings were scratched until the metal surface appeared (coating surface scratch: 2 cm × 2 mm). Imposed E was 1.2 V/E_corr.

Experimental results

Study of bare metal

Reduction E

The follow-up curves of the free E during the bare metal corrosion, for three studied R_a values, are presented in Fig. 1. Whatever the considered surface R_a, E decreased, since the immersion start, from a noble value, to a stabilized one located from -700 to -750 mV/SCE. The following values resulted from several tests: 1 (120 mesh) = -746 mV/SCE; 2 (600 mesh) = -719 mV/SCE; 3 (1200 mesh) = -708 mV/SCE.

E became progressively less noble (Fig. 1) at the immersion beginning. There was a continuous attack on CS, and the free E stability shows that the systems that went through the corrosion process had the same kinetics during the samples immersion in NaCl ²¹.

Figure 1: Follow-up of the free E of bare metal in NaCl 3.5 % for the three samples surface R_a. 

Polarization curves

In order to determine Ec parameters for corrosion evolution, PDP plots were performed on the bare CS samples, at various surface states. Polarization curves were obtained by scanning E from +250 mV around free E. E scanning was done from the cathodic to the anodic field, at the SR of 0.166 mV/sec. This type of curve allowed determining I_corr and E_corr. Fig. 2 represents polarisation curves obtained for the bare CS electrodes, in aerated conditions, for the three surface states. Measurements were done after free E stabilization. E_corr and j values were obtained by the polarization curves (extrapolation method of Tafel lines). Ec parameters values obtained from these curves are shown in Table 3.

Figure 2: Polarization curves of bare metal at various surface states in a 3.5% NaCl medium. 

Polarization curves (Fig. 2) show that the surface state influenced E and J. It should be noted that E_corr shifted in the negative direction of approxim. 58.9 mV/E for the surface polished at 120 mesh vs. CSE, compared to the surface polished at 1200 mesh. J values were larger for the electrodes polished at 120 and 1200 mesh, than for the one polished at 600 mesh.

EIS

EIS was successfully applied for the study of CS corrosion mechanisms. EIS diagrams, in Nyquist and Bode representation of bare metal, obtained for various studied R_a values, are shown in Fig. 3.

C_res and C_rev results obtained from these curves are shown in Table 4. Whatever the surface state, the impedance diagram was composed of a loop in the low frequencies field that reported corrosion phenomena at CS/electrolyte interface.

In Bode plot, impedance diagrams show the presence of only one time constant at low frequencies. Concerning the impedance phase, diagrams loops are similar for the three considered surface states.

Figure 3: Impedance diagrams in the complex and Bode plan of bare CS in 3.5% NaCl, for the three surface states. 

R_p values recorded after polishing with abrasive paper of grades 120, 600 and 1200 mesh, respectively (Table 4), show that they increased in the order: R_p (1200 mesh) < R_p (600 mesh) < R_p (120 mesh). One also noted an increase in the capacity with a decrease in C_res value. R_p decreased with increased C_rev values. This behaviour was due to the increase in the corroded CS surface.

Study of the coating

E _corr

E_corr curves vs. IT of CS samples, for the studied surface states, are shown in Fig. 4. E_corr shifted towards the negative direction, after two days IT. Then, it shifted towards the noble direction, and, finally, moved towards the negative direction, until the experiment end, at 91 days. E displacement towards the noble direction indicates that the cathodic/anodic reactions ratio increased, and also that O and water penetrated the coating, reaching the CS/coating interface. E shift towards more negative values indicates that the anodic/cathodic reactions ratio became significant. The increase in E_corr towards increasingly positive values, over time, indicates that alkaline conditions were locally developed at the CS/coating interface. E_corr displacement towards increasingly negative values, over time, indicates the formation of corrosion products and the limited coating service life ^22,23.

Figure 4: Follow-up of E_corr during CS immersion in a NaCl 3.5 % medium, at different surface states. 

EIS

Low frequency impedance modulus (1 MHz) is commonly used to evaluate the overall anticorrosion performance of organic coatings ^24-27. Figs. 5, 6, 7 and 8 present the impedance diagrams evolution, during IT of CS in the 3.5 % NaCl solution, for the EC, at different R_a values.

Figure 5: Impedance diagrams of the system in 3.5% NaCl after 2 days IT. 

A collapse of the impedance was observed from 2 to 7 days IT for the three R_a values (10⁹ → 10⁸ Ω). Then, for longer IT, the impedance diagrams remained relatively stable, around average values (1.77.10⁸, 1.44.10⁸ and 1.16.10⁸ Ω, for 120, 600 and 1200 mesh, respectively). Thus, the metal/coating system resisted corrosion. At the IT end (86-91 days), impedance had a visible evolution, but it was still significantly lower (10⁷ Ω). C_rev value was estimated at C = 10^-11 F/cm^-2, which is characteristic of an organic coating ²⁸.

Figure 6: Impedance diagrams of the system in 3.5% NaCl after 35 days IT. 

Impedance diagrams are characterized by two time-constants: high frequency field, which represents the coating properties, and low frequency field, which is attributed to phenomena occurring at depth in the paint pores.

Figure 7: Impedance diagrams of CS/coating in 3.5% NaCl after 77 days IT. 

Figure 8: Impedance diagrams of the metal/coating in 3.5 % NaCl after 91 days of immersion. 

R_p and C_rev values were extracted from Ec impedance diagrams, in order to assess changes in the coating properties with IT. Fig. 9 represents evolution of R_p values extracted from the high frequencies loop, which characterizes the EC effect of the three electrodes on the three surface states. The systems R_p and C_rev values are shown in Tables 5 and 6, respectively.

Figure 9: Evolution of R_p values extracted from high frequencies loop vs. IT. 

According to Fig. 9, for the electrodes polished with 120 and 600 mesh, C_res values decreased in the first IT, then stabilized. This shows that, due to the system adhesion, corrosion protection remained rather significant and constant for a relatively long IT. At the experiment end (70-91 days), one noticed a decrease in C_res that was due to the electrolyte penetration in EC pores, which put CS in direct contact with NaCl.

In the case of the electrode polished with 1200 mesh, C_res values decreased with IT and were weaker than those observed for the other electrodes. This evolution was accompanied by an increase in C_rev values. R_p and C_rev evolution was due to the increase in the corroded MC surface under EC, which led to the formation of an oxides layer.

The increase in C_res during IT can be explained by the following phenomena: corrosion products growth on the material surface (in EC pores) had a barrier effect which slowed down CR; the localised character of corrosion involved a reduction in EC active surface, artificially inducing an increased R_p; and the decreased medium aggressiveness related, e.g., to the increase in pH, involved a deceleration in CR. Whatever the surface state, R_s value in EC pores decreased, due to their enlargement and hydration during IT.

According to this evolution, electrodes polished with 120 and 600 mesh showed a better barrier effect against the electrolyte than the one polished with 1200 mesh.

C_rev values (Table 6) remained relatively stable over IT of the three polished electrodes (120, 600 and 1200 mesh). Stable or slightly increased C_rev values were due to the solution saturation and accumulation in EC and under it ²⁷.

For longer IT, C_rev values of the electrode polished with 1200 mesh tended to increase, which agrees with various researches that employed Ec impedance measurements to study metals coatings degradation. Thus, due to the aggressive medium high polarity, the system dielectric constant and C_rev increased, as written in eq. (1).

()1

Absolute permittivity is the medium property that reveals the flux density generated at a given point, due to a certain electric field intensity at another point in that field. The increased ξ_r constant value involved higher C_rev values ^29,30. R_p values decreased significantly during the first 7 days IT, due to EC non-destructive delamination. Then, a relative R_p stability occurred for longer IT. Finally, a reduction in R_p happened due to EC detachment.

Test of cathodic delamination

To protect steel structures, such as underground pipelines, cathodic protection is used in conjunction with organic coatings. Organic coatings, on the other hand, deteriorate with time. In addition, combining a cathodic protection with an organic coating causes a high alkalinity at the coating/steel interface, resulting in coating cathodic disbondment. As a result, more electric power is consumed over time, to preserve the steel structure. In the end, E demand may surpass the protection system designed capabilities ^29-33. Herein, the protective E imposed on EC tested in different surface states was checked in order to assess its detachment. Fig. 10 shows EC amperometric curves for the three surface states.

The amperometric curve corresponding to EC of the electrode polished with 1200 had lower C_res for CS undercoating cathodic protection, because I increased from -218 to -125 µA. This did not happen with the other electrodes (600 and 120 mesh) of which cathodic I remained practically stable. As such, organic EC are incompatible with E cathodic protection.

Figure 10 CA curves of CS/ EC for the three electrodes. 

It should be noted that EC with various R_a values underwent a cathodic delamination test for an overprotection of approxim. -600 mV/SCE, knowing that E_applied - E_corr = -1200 - (- 600) mV/SCE.

It was seen that the tested systems were not degraded, since neither delamination, nor blisters formation occurred around the scratches. This shows that EC systems adhered well on CS. However, the surface polished with 1200 mesh was almost mirror-like. Therefore, the coating detachment was easier.

Salt spray test

A salt spray test was carried out to complete the results obtained by various Ec techniques. This is an accelerated corrosion test generally defined as a method that enables to evaluate coatings performance, and of which results are obtained with shorter IT than those of natural exposure.

In this work, the solution was 7.69% NaCl, and room temperature was 35 ºC. IT was 15 days. The saline solution was periodically added after every 80 h. Blisters formation and CR of the EC metal plates were regularly evaluated by visual observations. Samples photographs were taken after 15 days IT in salt spray. Fig. 11 shows that the primer formulated with EC 1200 mesh had better anti-corrosion performance. The blisters are spaced apart, and their diameter is much smaller than that of the primer formulated by Zn₃(PO₄)₂ with 600 and 120 mesh, of which blisters are more numerous and close together.

Figure 11 Salt spray test, with an IT of 15 days.