Fabrication of a Conductive Additive for the Anticorrosion Enhancement of Zinc-Rich Epoxy Coatings

Abstract

In the study, a conductive polypyrrole (PPy) is deposited on the lamellar sericite powder (SCP) surfaces by an in situ oxidization growth method and the prepared PPy/SCP conductive additive is successfully applied on the zinc-rich primer (ZRP) coating. The equal mass substitution and the equal volume substitution methods of the conductive additives to zinc dusts are discussed, as well as the optimal replacing ratio to achieve the best corrosion protection effect of the ZRP coatings. The results indicate that the equal volume substitution method is in favor of corrosion resistance of coating film. The salt spray test and the electrochemical impedance spectroscopy (EIS) and polarization curves show that the prepared ZRP coating with a 66% zinc content and replacing ratio of 1:3 possesses the best corrosion-resistant performance and an optimal adhesion strength. The replacement of PPy/SCP particles to zinc dusts using the equal volume substitution method is feasible to achieve the improvement in anticorrosion ability through a synergic function of the cathodic protection effect and barrier effect.

1. Introduction

Sea water with mainly sodium chloride is the largest natural electrolyte liquid, with strong corrosivity. Corrosion of metal in contact with severe environmental media causes the destruction of appearance, color, mechanical properties, etc., of materials, resulting in a reduction in their use value and huge economic loss [1]. Hence, corrosion is considered as the greatest issue threatening marine vessels and infrastructures in high-salinity environments and has aroused the attention of the whole of society [1,2]. Steel, as an important material widely used for offshore platform, is generally easily corroded in a complex and diverse marine environment, which is mainly attributed to electrochemical corrosion. Coating protection is a facile and applied method for corrosion protection of metal substrates [3]. Especially, heavy-duty anticorrosive coatings with the characters of thick film can achieve the purpose of long-term protection of substrates [4,5].

Reactive metal zinc (Zn) is the most frequently used anodic material for cathodic protection of steel, which is widely applied on the anticorrosion field. Zinc-rich epoxy coating as an anticorrosion primer has been used in the protection of steel substrates. The anticorrosion mechanism of the zinc-rich primer (ZRP) film is attributed to the cathodic protection of zinc and the shielding effect of corrosion powder products [5,6]. Generally, epoxy coating films with higher proportion of zinc dusts can establish effective electronic channels with metal and achieve superior cathodic protection performance under the action of corrosive media (mainly oxygen and moisture), which largely improves the anticorrosion property. Generally, higher zinc dust content means a higher galvanic protection, and the zinc content must be above 74% in ZRP coatings to achieve a continuous electrical connection and a reliable percolation path in the coating [5,7,8]. However, a higher content of zinc dust causes poor adhesion, permeability, weak impact resistance and rising economic cost [5,9]. Moreover, excessive addition is not environmentally friendly, and is even harmful to human health [10]. Reducing zinc content is necessary but also causes poor connectivity between particles. Hence, discussion and study on how to save zinc content without decreasing the performances of ZRP coatings is very important.

The addition of conductive additives in ZRP coatings to replace partial zinc can play a role in connecting zinc dusts and strengthening the physical barrier effect, thereby improving the protective performance of the metal substrate. For example, graphene with excellent electrical property, chemical inertness and mechanical strength can decrease the quantity of zinc dusts and also be an ideal additive for the improved corrosion resistance of the coating [2,11,12]. However, its poor dispersion in water or organic solvent trends it towards aggregating together and limits its practical application [11]. Conducting polymers such as polypyrrole (PPy) and polyaniline (PANI) act as a barrier layer, and the corrosion inhibitors (electrochemical protection effects) have been widely studied for the corrosion protection of metal substrates [13,14,15]. Due to its remarkable electrical property and stability, PPy coating has a wide and potential application prospect for metal corrosion protection. The PPy coating coated on metal substrates can play a role in the barrier and shielding effect of metal [16]. In addition, two-dimensional lamellar materials with excellent barrier effects can form a labyrinthine structure to prevent the permeation of water and corrosion media, which are used for realizing an improvement in anticorrosion performance [17,18]. Li et al. [19] developed an anticorrosion epoxy-modified silicone resin (MSR) coating based on graphene oxide (GO) and lamellar mica powder (MP) using a spraying method. Ramezanzadeh et al. [20] studied the effect of the addition of lamellar micaceous iron oxide (MIO) on the corrosion resistance of ZRP coatings, and they illustrated that MIO particles can provide a barrier effect and reduce the oxidation rate of zinc dust, thereby improving the cathodic protection duration. The application of sericite powder (SCP) with lamellar structure and high aspect ratio in ZRP coatings can form a compact barrier network and enhance the impermeability, and thus largely improve the corrosion protection performance of coating film [21,22].

To meet the requirements on environmentally friendly and high corrosion resistance in the harsh marine environment, conductive PPy was synthesized on a lamellar sericite powder (SCP) surface by an in situ oxidization growth method and applied in a ZRP coating. The synthesis of conductive SCP particles based the PPy material can be better dispersed in the ZRP coating to play a role in connecting zinc dusts while producing a shielding effect. The optimal anticorrosion performance was studied by adjusting the replacing method and replacing ratio of the conductive additives to zinc powder. The effect of the content of zinc powder and the replacing ratio of PPy/SCP particles on the anticorrosion property of the coating was investigated. The selection of substitution methods of the conductive additive is important to improve the corrosion resistance of the coatings. The salt spray test, EIS analysis and pull-off strength test show that the prepared ZRP coating with a 66% zinc content (PPy/SCP replacing ratio of 1:3) possesses the optimal anticorrosion performance and adhesion.

2. Experimental Section

2.1. Materials

Zinc dust with a particle size of ~23 µm was obtained from Jiangsu Sanmu Group Co., Ltd. (Wuxi, China). Epoxy resin was purchased from Hunan Xinweiling Metal New Material Technology Co., Ltd. (Yueyang, China). The dispersant (BYK613, BYK), defoamer agent (680, TEGO), polyamide wax as antisagging agent (BYK410, BYK), fumed Silica as antisediment agent (AEROSIL 200, Evonik) and other additives were used for the fabrication of ZRP coatings. Lamellar sericite powder (SCP) (1250 mesh, oil absorption 39 g/100 g, whiteness 75%) was obtained from Chengdu Weisigude New Material Co., Ltd. (Chengdu, China). Pyrrole (Py) and ammonium persulfate (APS) (AR, ≥98%) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). and p-toluenesulfonic acid monohydrate (TSA) was purchased from Energy Chemical.

2.2. Preparation of Conductive PPy/SCP Particles

In situ oxidization-growth Py with the APS as oxidants and the TSA as dopants on the sericite powder (SCP) particle surface was achieved to prepare conductive SCP/PPy particles. A total of 1.5 g pyrrole (Py), 4.18g p-toluenesulfonic acid monohydrate (TSA) and SCP in a certain ratio (Py: SCP of 2:1, 1:2, 1:4 and 1:8) were dispersed into 50 mL deionized water and vigorously stirred for 30 min at 0 °C in an ice bath. Then, 2.6 g ammonium persulfate (APS) was added into 50 mL deionized (DI) water followed by a stirring treatment for 10 min. The prepared oxidant solution was slowly dripped into the above mixture solution and reacted for 12 h at 0 °C in an ice bath. The reacted solution was filtered and rinsed repeatedly with DI water to remove residual APS and water-soluble TSA, and the obtained precipitates were dried at 60 °C for 24 h. Hence, the conductive PPy/SCP particles were obtained.

2.3. Fabrication of ZRP/PPy/SCP Coating

The PPy/SCP particles and zinc dust in a certain ratio were added into epoxy coating with toughener, dispersant, antisettling agent and other additives both dispersed into xylene and n-butanol solvents, and then stirred 20 min. The curing agent in a ratio of 1:10 to EP resin was added into the prepared ZRP/PPy/SCP suspension and stirred for 20 min. The above suspension was sprayed onto steel substrate using a spray gun with a 15 cm distance between the spray gun and the substrate. The coated surface was cured in an oven at 60 °C for 2 h and the ZRP/PPy/SCP coating was successfully obtained. The thickness of general coatings and thick films was controlled at 65 ± 5 µm and ~130 µm, respectively.

To optimize the anticorrosion performance of the prepared composite coatings while reducing the content of zinc dusts, two ways (equal mass and volume substitutions) were selected to explore the replacing ratio of PPy/SCP particle to zinc dusts. The detailed mass fraction of zinc dusts, conductive PPy/SCP particles and other additives were shown in

Table 1. The mass fraction of ZRP/PPy/SCP coating system through equal mass substitution.

Label	1	2	3	4	5
Zinc dust/wt %	80	60	60	60	50
PPy/SCP particle/wt %	0	4	6	8	12
Ferrotitanium powder/wt %	0	16	14	12	18
EP + other additives/wt %	20	20	20	20	20
Replacing ratio of PPy/SCP	0	1:5	1:3	1:2.5	1:2.5

and

Table 2. The mass fraction of ZRP/PPy/SCP coating system through equal volume substitution.

Label	1	2	3	4	5	6
Zinc dust/wt %	80	(60.0)	66.9	66.5	66.4	70
PPy/SCP particle/wt %	0	6.6	8.9	4.5	3.0	3.3
Ferrotitanium/wt %	0	10.2	1.9	6.8	8.5	5.1
EP + other additives/wt %	20	23.2	22.3	22.2	22.1	21.6
Replacing ratio of PPy/SCP	0	1:3	1:1.5	1:3	1:4.5	1:3

. The mass difference between zinc reduction and PPy/SCP addition was supplemented by Ferrotitanium. The replacing ratio of equal mass substitution was directly calculated by Equation (6), while the replacing ratio of equal volume substitution was calculated through Equations (1)–(6).

The mass amount of SCP/PPy was calculated by Equation (1) and R′ was defined as original replacing ratio. M_SCP/PPy, M_original and M_zinc are the mass of conductive PPy/SCP particles, original zinc dusts and current zinc dusts, respectively.

$$M_{SCP/PPy}=(M_{original}-M_{zinc})\times R^{\prime }$$

(1)

The volumes of zinc dust (V_zinc), EP + other additives (V_EP) and SCP/PPy particles (V_SCP/PPy) were calculated by Equation (2), and ρ_i is density of these components.

$$V_i=\frac{M_i}{{\rho }_i}$$

(2)

The volume (V_ferro) and mass (M_ferro) of supplementary ferrotitanium powder were calculated by Equations (3) and (4).

$$V_{ferro}=V_{total}-V_{zinc}-V_{EP}-V_{SCP/PPy}$$

(3)

$$M_{ferro}=V_{ferro}\times {\rho }_{ferro}$$

(4)

After equal volume substitution, the mass fractions of zinc dust, EP + other additives, ferrotitanium powder and SCP/PPy particles and the apparent mass replacing ratio of SCP/PPy particles were calculated by Equations (5) and (6), respectively, where w_PPy/SCP, w_original and w_zinc are the mass fraction of conductive PPy/SCP particles, original zinc dusts (80%) and current zinc dusts, respectively.

$$w_i=\frac{M_i}{M_{zinc}+M_{EP}+M_{ferro}+M_{SCP/PPy}}$$

(5)

$$Replacing ratio \left(R\right)=\frac{{\omega }_{PPy/SCP}}{{\omega }_{original}-{\omega }_{zinc}}$$

(6)

2.4. Characterization

The morphologies of particles and coating samples were observed by a field emission scanning electron microscopy (SEM) (Regulus 8100, HITACHI, Tokyo, Japan). Resistivity tests of powders were conducted using a powder resistivity tester (ST2742B, JR, Suzhou, China). The powder samples were compacted into a mold and conductivity of samples was recorded with the increasing pressure. A chamber salt spray (KK-60, KeCe, Shanghai, China) with 5 wt % NaCl solution was used to evaluate the corrosion performance of coating samples according to ISO 9227:2017. The temperature of chamber and the saturated pressure barrel were set as 35 °C and 47 °C, respectively. The corrosion resistance of the zinc-rich coatings was discussed using an electrochemical workstation (PARSTAT 2273, AMETEK, PA, USA). Adhesion strength of the coating films was performed by an adhesion tester (BGD 500/S, Biuged, Guangzhou, China). The coating film was glued to a circular spindle with a diameter of 20 mm. After 48 h, the spindle was pulled up by the adhesion tester under the applied pressure ranging from 1.0–20 Mpa and a pulling rate of 1.5 Mpa/s described in the standard GB/T 5210. Film thickness was measured by a PosiTector 6000 thickness gauge.

3. Results and Discussion

3.1. PPy/SCP Particle Analysis

The particle morphologies of pure PPy are shown in Figure 1a,b. The synthesized PPy particles with an average particle size of ~400 nm show a microspherical structure, which are closely linked, thereby forming a connected conductive network. Hence, the pure PPy can transmit electrons through intermolecular contact and thus possesses prominent conductivity.

Lamellar SCP particles with a high aspect ratio can provide a good physical barrier effect and also work as a platform for the in situ oxidization growth of Py. Figure 2 presents the surface morphologies of PPy/SCP particles with the Py and SCP particles at the mass ratios of 2:1, 1:2, 1:4 and 1:8. When the mass ratio of Py and SCP is 2:1, the excessive deposition of PPy particles on the lamellar SCP surfaces is shown in Figure 2a,b, and the morphology is similar with the pure PPy (Figure 1). With a low content of SCP, it cannot be achieved that the conductive PPy particles do not disperse well in the ZRP coating. Some micropores are formed on the SCP particle surface due to the aggregation of PPy particles, and the multiple stacks of PPy may cause a negative effect on the preservative effect of the coating, thereby affecting the corrosion resistance and mechanical durability of the anticorrosive coating. With the increase in SCP content, the distribution of PPy on the SCP surface is thinner in Figure 2c,d and the number of micropores increases significantly. When the mass ratios of PPy and SCP are 1:2 and 1:4, it is observed that the dense and compact micro-PPy particles are dispersed on the SCP surfaces in Figure 2c–f. However, the synthesized PPy displays a loose deposition and an incomplete covering on the SCP surfaces with the mass ratio of PPy and SCP particles at 1:8, as shown in Figure 2g,h. The loose dispersion of PPy particles on the SCP material will affect the construction of conductive network.

Figure 3 shows the relationship of conductivity Vs pressure of pure PPy particles and PPy/SCP particles at different mass ratios. The conductivities of PPy/SCP particles at the mass ratios of 2:1 and 1:2 are higher than those of pure PPy particles. This is attributed to the excessive addition of Py and the uniform dispersion of conductive PPy particles on the SCP particles, respectively. However, the addition of Py with higher content causes a noteworthy decrease (Py and SCP of 2:1 Vs 1:2) in conductivity due to the multiple stacks of PPy on SCP particles, which may affect the connectivity between conductive Ppy particles. When the mass ratio of Py and SCP is 1:4 and 1:8, the conductivities of Ppy/SCP particles decrease significantly, which is ascribed to the lower content of deposited PPy particles. The conductivity of PPy/SCP particles (Py: SCP of 1:2) is much higher than that of PPy/SCP particles (Py: SCP of 1:4). The higher conductivity of the coating, the better the cathodic protection effect and anticorrosion property [13]. Hence, The PPy/SCP particles (Py: SCP of 1:2) are utilized for further study.

3.2. ZRP/SCP/PPy Coating Analysis

3.2.1. Equal Mass Substitution

To reduce the content of zinc dust without reducing the corrosion resistance of the zinc-rich coating, the prepared conductive PPy/SCP (1:2) particles and ferrotitanium powders with different mass proportions are used to replace zinc dusts (original zinc content of 80%) with a certain ratio, as shown in Table 1. The replacing ratios of PPy/SCP particles to zinc dust are set as 1:2.5, 1:3 and 1:5 based on 60% zinc content. Keeping the same replacing ratio (1:2.5), the anticorrosion performance of the ZRP coating with the zinc content at 50% was also investigated in Table 1.

The thickness of the coating films was controlled between 65 ± 5 µm and a neutral salt spray test was used for evaluating the corrosion performance of the ZRP coating to discuss the best replacing ratio of PPy/SCP particles to zinc dusts. After 500 h of exposure to the salt spray environment, it is noted that a mass of white corrosion products (nonconductive ZnO or Zn (OH)₂) attributed to the rapid consumption of zinc dusts appear on the coating surface (Figure 4). This phenomenon leads to a decrease in contact area between zinc dusts and a decline in the cathodic protection ability of coatings. Moreover, brown rusts along the scratched area and large area of blisters are formed on these coating surfaces, especially when the zinc content is decreased from 80% to 50%. It indicates that the excessive addition of Ppy/SCP conductive particles cannot contribute to the protection effect of the coating and even produces a serious blistering phenomenon. Higher zinc content (60%) has a better corrosion protection ability at the same replacing ratio of PPy/SCP particles to zinc dusts (1:2.5) compared with the ZRP coating with the zinc content of 50%. The formation and accumulation of corrosion products lead to a decrease in the adhesion strength of the organic ZRP coating. In addition, the accumulation of corrosion products at the scratched area accelerates bubbling, peeling and even aging of the surrounding coating film. As a result, the reduction in zinc dust content and the equal mass substitution of conductive PPy/SCP particles and ferrotitanium powders do not improve the corrosion resistance of coatings compared with the original zinc-rich coating.

The line roughness of ZRP coatings with different content of zinc dusts and PPy/SCP particles before and after 500 h of exposure to salt spray is shown in Figure 5. It can be seen that the average roughness of ZRP coatings has a significant change after a salt spray test of 500 h, which further illustrates the formation of coating defects and even detachment between the coating and substrate. Under the same replacing ratio of PPy/SCP particles to zinc dusts (1:2.5), the lower content of zinc powder (50%) leads to a higher roughness variation and a worse surface defect.

3.2.2. Equal Volume Substitution

In the equal mass substitution of additives (conductive PPy/SCP and ferrotitanium powders) to zinc dust, the prepared ZRP coatings do not achieve an excellent anticorrosion effect on the substrate while decreasing the zinc content. When the same quality of zinc powders is replaced, conductive PPy/SCP and ferrotitanium powders as the replacement materials are nearly twice the volume of zinc dusts because of the difference in density, as shown in Figure 6a. The addition of conductive additives with higher volume concentration in the ZRP coating system causes greater oil adsorption, and therefore the poor adhesion, workability and mechanical performance of the coatings, thereby decreasing the anticorrosion effect while reducing the zinc content.

To avoid the coating defect, an equal volume substitution method was proposed to discuss the best replacing ratio of PPy/SCP particles and to better improve the anticorrosion performance of the coating, as shown in Figure 6b.

The film thickness of the coating films was also controlled between 65 ± 5 µm. Similarly, the ZRP coating with 80% zinc dust was used as original coating. The replacing ratios of PPy/SCP particles to zinc dust were 1:1.5, 1:3 and 1:4.5 at a zinc content of 66%; 1:3 at a zinc content of 60%; and 1:3 at a zinc content of 70%, as shown in Table 2.

Surface Morphologies

The surface morphologies of ZRP coatings with different zinc content and replacing ratios of PPy/SCP particles are shown in Figure 7. The PPy/SCP particles are incorporated into the EP resin and dense conductive networks combined with zinc dusts are formed. By controlling the same replacing ratio of the PPy/SCP particles, the increase in the zinc content causes the decrease in the content of PPy/SCP conductive particles, which further affects the surface roughness. A higher content of conductive additives means greater surface roughness, as shown in Figure 7b–d. By controlling the same zinc content, the increasing replacing ratio of Ppy/SCP particles to zinc dusts means a decrease in the conductive additives, which reduces the surface roughness, as shown in Figure 7c,e,f. These results are consistent with the roughness analysis results in Figure 8.

The line roughness of ZRP coatings with different content of zinc dusts and replacing ratios of PPy/SCP particles before and after 500 h of exposure to salt spray is shown in Figure 8. It can be seen that the average roughness of ZRP coatings has a minor change after a salt spray test of 500 h, and this variation is significantly less than that of the ZRP coatings prepared by equal volume substitution. It further indicates that a few of the coating defects appear on these coating surfaces, which is consistent with the results in Figure 9. In particular for the ZRP coating with a 66% zinc content and a PPy/SCP replacing ratio of 1:3, it has the lowest surface roughness and minimum roughness variation after a salt spray test of 500 h, which illustrates a more stable mechanical property and workability.

Salt Spray Test

The salt spray test is the major method for detecting the corrosion resistance of coating films. After being subjected to 500 h of exposure to the salt spray test (5 wt % NaCl solution), the coating appearance is shown in Figure 9. Minor bubbling phenomena and brown rusts around the cross-cut of film appear on the coating surfaces. The anticorrosion properties of the ZRP coatings with PPy/SCP additives prepared by equal volume substitution are superior to or comparable to the those of the original ZRP coating (80% Zn) in Figure 9a compared with the ZRP coatings prepared by the equal mass substitution. Therefore, the addition of conductive PPy/SCP particles in the ZRP coating system is feasible to improve the corrosion resistance of the coating film while reducing zinc dusts.

Comparatively, different coating protection abilities are also shown in Figure 9 by adjusting the zinc content and the replacing ratio of PPy/SCP particles. The defects of the part marked by the red cycles are enlarged at the top of the figure. Keeping a same PPy/SCP replacing ratio of 1:3, the higher the content of zinc dusts, the better the anticorrosion performance. Furthermore, keeping the same zinc content of 66%, obvious blisters are observed along and in the vicinity of the scribe when the replacing ratios of PPy/SCP particles are 1:1.5 and 1:4.5 (Figure 9d,e). This is attributed to the fact that the addition of a low content of conductive PPy/SCP particles is not conducive to forming a conductive network through the mutual connection of conductive particles. Furthermore, the addition of a high content of PPy/SCP particles causes a reduced adhesion of film to the substrate and failure in the anticorrosion property. When the replacing ratio of PPy/SCP particles to zinc dusts is 1:3, the coating sample is relatively intact, with minor blisters. Hence, the ZRP coating with a 66% zinc content and a PPy/SCP replacing ratio of 1:3 possesses the most superior anticorrosion ability.

The anticorrosion effect of ZRP coating with different zinc content and replacing ratios of conductive PPy/SCP particles is improved by increasing the film thickness to ~130 µm. In Figure 10, the corrosion resistance is improved with increasing film thickness after neutral salt spray test of 500 h. Although the ZRP coating with a PPy/SCP replacing ratio of 1:4.5 has fewer brown rusts at the same 66% zinc content, rust spots appear that are likely to prejudice the protective ability of coating film. Compared with original ZRP coating (80% Zn), the coating at a 66% zinc content with a 1:3 replacing ratio of PPy/SCP particle still exhibits the best corrosion resistance. In addition, the coating sample shows fewer rusts and is intact without any bubbling phenomenon, even after 1000 h of exposure to the salt spray environment. Therefore, the steel substrate pained with the ZRP coating using the equal volume substitution can resist corrosion for a greater longevity.

Adhesion Strength

The pull-off strength test results are listed in

Table 3. The pull-off strengths of ZRP coatings with different zinc content and replacing ratios of conductive PPy/SCP particles.

Sample	60% Zn (1:3)	66% Zn (1:3)	70% Zn (1:3)	66% Zn (1:1.5)	66% Zn (1:4.5)
Pull-off strength /Mpa	8.24	8.38	7.30	6.44	8.27

. The results show that all prepared ZRP coatings show an adhesion strength of higher than 5 Mpa, which meet the requirement of the coating in practical applications. The addition of conductive PPy/SCP particles does not affect the adhesion of coatings to substrates and even largely improves the coating adhesion. High adhesion can further prevent corrosion media from spreading into the coating and can enhance the anticorrosion ability of the coating film.

Corrosion Resistance

The corrosion resistance of the ZRP coatings coated on steel substrates are evaluated by electrochemical impedance spectroscopy (EIS). Figure 11a presents the Nyquist plot of bare steel substrate and ZRP coatings. Generally, the radius of the capacitive loop describes the magnitude of the impedance. The higher the radius, the better corrosion resistance. Comparatively, the ZRP coating with 66% zinc content and 1:3 replacing ratio of SCP/PPy indicates a larger radius than other ZRP coatings, indicating an optimal corrosion performance. Figure 11c reflects the Bode plots of the phase angle Vs log frequency of the ZRP coatings. A higher phase angle in a wide frequency range represents a better anticorrosion property [23]. The 66% Zn (1:3) ZRP coating exhibits a relatively larger phase angle than other ZRP coatings, suggesting a stable corrosion resistance. Figure 11b,d illustrates a Bode plot of impedance after immersion in NaCl solution for 144 h and the relationship between low frequency (|Z|0.01 Hz) and immersion time of the ZRP coating with and without PPy/SCP conductive particles after immersion in 3.5 wt % NaCl aqueous solution for 24 h, 48 h, 108 h, 144 h and 312 h. The impedance modulus at the lowest frequency (|Z|0.01 Hz) can be used for measuring the anticorrosion ability. The impedance modulus at low frequency decreases with increasing immersion time. A larger |Z|0.01 Hz value represents a better corrosion barrier in the ZRP coating system [24]. After 144 h of immersion, the 66%/1:3 ZRP coating has a higher magnitude than other ZRP coatings in Figure 11b. As noticed, the |Z|0.01 Hz value of the 66%/1:3 ZRP coating is still greater than 106 Ω·cm² value, which is much higher than other ZRP coatings even after immersion in NaCl solutions for 312 h, as shown in Figure 11d. Therefore, the ZRP coating with 66% zinc content (replacing ratio of 1:3) possesses an optimal barrier function.

The potentiodynamic polarization curves of bare steel substrate and ZRP coatings with different zinc content were performed in 3.5 wt % NaCl solution, as shown in Figure 12, and the corresponding electrochemical parameters, the corrosion potential (E_corr), corrosion current density (I_corr), corrosion rate (CR), anodic slope of Tafel (β_a), cathodic slope of Tafel (β_c), polarization resistance (R_p) and protection efficiency (PE), are presented in

Table 4. The electrochemical parameters of bare steel substrate and ZRP coatings.

Sample	E Corr (mV)	I Corr (A/cm2)	CR (mpy)	βa (mV)	βc (mV)	Rp (Ω·cm2)	PE (%)
Bare steel	−811	2.4 × 10−1	7.8 × 10−1	76	196	99.2	-
80% Zn	−953	3.2 × 10−4	1.1 × 10−5	106	176	9.0 × 104	99.87
60% Zn (1:3)	−1035	9.4 × 10−3	3.1 × 10−4	195	87	2.8 × 103	96.08
66% Zn (1:3)	−1006	1.9 × 10−4	6.1 × 10−6	108	142	1.4 × 105	99.92
70% Zn (1:3)	−1001	5.5 × 10−4	1.8 × 10−5	62	108	3.1 × 104	99.77

. The protection efficiency (PE, %) is calculated by Equation (7).

$$PE \left(\%\right)=\left[1-\frac{I_{corr}}{I_{corr}^0}\right]\times 100\%$$

(7)

where $I_{corr}$ and $I_{corr}^0$ are the corrosion current density of ZRP coatings and bare steel substrate, respectively. The ZRP coating with 66% zinc content (1:3) shows a lower corrosion potential and corrosion current density, which indicates a better galvanic protection effect and a slow consumption of zinc content, respectively [24,25]. Compared with other ZRP coatings with different zinc content, the R_p value of ZRP coating with 66% zinc content is observed to be maximum. Hence, the ZRP coating with 66% zinc content possesses the lowest corrosion rate (6.1 × 10⁻⁶) and highest protection efficiency (99.92%), and therefore an optimal corrosion protection effect.

3.3. Anticorrosion Mechanism

Figure 13 represents two anticorrosion mechanisms of ZRP coatings with PPy/SCP conductive particles. Compared with the original ZRP coating (80% Zn), the improved corrosion-resistant property of the ZRP coating with conductive PPy/SCP particles is attributed to the effect of electrical conductivity and barrier property. The addition of conductive PPy/SCP particles forms a conductive PPy/SCP/zinc network in favor of the cathodic protection effect of the coating film. In addition, the conductive lamellar particles are superimposed parallel to each other in the coating film and play a “labyrinth” effect, thereby prolonging the permeation path and time of water and corrosion media to the substrate. Hence, the ZRP coatings with PPy/SCP particles have high impermeability and thus extend their service lives. In addition to the barrier and shielding effect of PPy/SCP particles, the epoxy organic coating can also act as a protective barrier to prevent the invasion of corrosive chloride ions. The synergic function of the cathodic protection effect and “labyrinth” effect shows an outstanding improvement in the corrosion protection capability of the ZRP coating.

4. Conclusions

In the study, a conductive PPy/SCP particle was prepared by an in situ oxidization growth method and it was effectively applied on the ZRP coating system using the equal mass/volume substitution methods. The results show that the equal volume substitution of PPy/SCP particles to zinc dusts is more conducive to improving coating adhesion and corrosion resistance compared with the equal mass substitution. The corrosion protection effect increases with increasing film thickness. Different corrosion protection abilities are discussed by adjusting the replacing ratio of the conductive PPy/SCP additives and the zinc content. The prepared ZRP coating with a 66% zinc content and a PPy/SCP replacing ratio of 1:3 possesses the best corrosion protection capability. The replacement of PPy/SCP particles to zinc dusts using the equal volume substitution method in the ZRP coating system can greatly improve the utility ratio of zinc dusts and prolongs the resistance time to the salt spray test, thereby achieving long-term corrosion resistance attributed to the optimal cathodic protection efficiency and good shielding effect.