1 INTRODUCTION

The hot-dip aluminized-galvanized steel (HDAGS) (55%Al–43.4%Zn–1.6%Si, wt %) was a highly corrosion-resistant coated steel developed by Bethlehem Steel in the United States. It was widely used in marine construction, equipment and other fields because of its corrosion and oxidation resistance [1–3]. However, several elements, such as Cl and S, could destabilize it, resulting in the acceleration of corrosion, which limited its application and development [4–6]. Therefore, further protective technologies were required. The corrosion inhibitors were more applicable because it had the advantages of simple operation, less dosage and high efficiency compared with other methods [7, 8].

The research on corrosion inhibitors for HDAGS hadn’t been carried out for now, but the researchers had made certain progress on aluminum and zinc alloys corrosion inhibitors. Ma et al. [9] studied the effect of L-cysteine/ZnO compound inhibitor on the corrosion resistance of aluminum alloy in alkaline environment. The results showed that the compound inhibitor significantly reduced the anode corrosion rate by controlling the diffusion process of Zn2+. Zhao et al. [10] studied the inhibition effect of phthalocyanine sulfonate on 7075 aluminum alloy in acidic environment. Phthalocyanine was verified as a hybrid corrosion inhibitor because it was validated to adsorb to aluminum by molecular dynamics simulations and XPS analysis. Anyiam et al. [11] extruded galvanized steel by starch physically and evaluated its corrosion inhibition effect in 1 M HCl. It was found that the modified starch belonged to a hybrid corrosion inhibitor by electrochemical method, and its adsorption process obeyed the Langmuir adsorption isotherm. Meeusen et al. [12] applied odd random phase electrochemical impedance spectroscopy (ORP-EIS) to study the time-effect. Silica-based and phosphate-based corrosion inhibitors were evaluated for hot-dip galvanized steel in 0.05 M NaCl.

In order to fill the research gap of HDAGS’s corrosion inhibitors, the effect of nine common corrosion inhibitors for HDAGS was evaluated in simulated marine environment by Immersion corrosion test, Tafel polarization, EIS test, XRD, SEM&EDS in this paper. Effective corrosion inhibitors were added to the WPUC to assess their influence on the comprehensive performance of coatings.

2 EXPERIMENTAL

2.1 Synthesis

HDAGS (Baoshan Iron and Steel Co.) was cut into rectangular coupons, whose size were 50 × 25 × 1 mm3. The HDAGS coupons were immersed in ammonium acetate (CH3COONH4) solution with the concentration of 10 wt % at 70°C for 5 min first to remove the oxide and then cleaned in deionized water. 5 wt % medium alkaline degreasing agent was applied to degrease the HDAGS coupons’ surface by ultrasonic cleaning for 5 min. After washing with deionized water, the HDAGS coupons were soaked in absolute ethanol, then dried with cold air for use. The simulated seawater was configured according to the Mocledon formula [13]. The specific components of the simulated seawater were as follows: sodium chloride 26.726 g/L, magnesium chloride 2.26 g/L, magnesium sulfate 3.248 g/L, calcium chloride 1.153 g/L, sodium bicarbonate 0.198 g/L, potassium chloride 0.721 g/L, sodium bromide 0.058 g/L, boric acid 0.058 g/L, sodium silicate 0.0024 g/L, disodium tetrasilicate 0.0015 g/L, phosphoric acid 0.002 g/L, aluminium chloride 0.013 g/L, ammonia 0.002 g/L, lithium nitrate 0.0013 g/L. Corrosion inhibitors were added to the simulated seawater by 1 wt %. The cor-rosion inhibitors were as followed: disodium L‑(+)-tartrate, sodium molybdate, gluconic acid, sodium salt, N-methyldiethanolamine, benzimidazole, triethanolamine, calcium carbonate, 2-mercaptobenzimidazole, and zinc phosphate. The medicines used were purchased from Shanghai Aladdin Co. The specific groups of simulated seawater containing corrosion inhibitors are shown in Table 1.

Table 1.   Groups of simulated seawater containing different inhibitors
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The selected inhibitors were added to the WPUC at a ratio of 1 wt %. Magnetic stirred at 25°C for 6 h. The WPUC with and without inhibitor were roll-coated on HDAGS coupons with an RDS wire rod coater (#3, USA) [14], then dried at 200°C for 37 s. The immersion time of the coated HDAGS coupons in the simulated seawater was extended to 400 h to obtain a complete corrosion process.

2.2 Characterization

The microstructure and composition of HDAGS coupons were observed by a scanning electron microscope (SEM, HITACHI S-4800, Japan) equipped with an energy dispersive spectrometer (EDS) [15]. The composition of the corrosion product was determined by an X-ray diffractometer (XRD, Rigaku, Japan) [16]. The scanning speed was 10 deg/min in the 2θ ranges from 10° to 90°.

The effect of corrosion inhibitors on the HDAGS coupons and WPUC were evaluated by static immersion test. The corrosion area of HDAGS was obtained referring to ISO4628. The corrosion rate of HDAGS after being immersed was calculated by formula (1). The inhibition efficiency was calculated by formulas (2) and (3)

$$V \pm = \frac{{{{m}_{{\text{0}}}} - {{m}_{{\text{1}}}}}}{{St}},$$
(1)
$$\eta = \frac{{{{V}_{0}} - {{V}_{1}}}}{{{{V}_{0}}}},$$
(2)
$$\eta = \frac{{Jcor{{r}_{0}} - Jcor{{r}_{1}}}}{{Jcor{{r}_{0}}}},$$
(3)

where V± was the corrosion rate, g/(m2 h); m0 was the HDAGS coupons’ mass before corrosion, g; m1 was the HDAGS coupons’ mass after corrosion, g; S was the area exposed to the solution, m2; t was corrosion time, h; η was the inhibition efficiency, %; V0 and V1 were the corrosion rates of HDAGS coupons in the simulated seawater without and with corrosion inhibitor, g/(m2 h), respectively; Jcorr0 and Jcorr1 were the self-corrosive current density of HDAGS coupons in the simulated seawater without and with corrosion inhibitor, A/cm2, respectively.

Potentiodynamic polarization curves and AC Impedance spectroscopy (EIS) tests of HDAGS coupons with and without WPUC coated were performed in simulated seawater at 25°C via Solartron 1260 + 1287 electrochemical workstation. The saturated Hg/Hg2Cl2 electrode and graphite electrode were used as the reference electrode and counter electrode [17], respectively. During the potentiodynamic polarization measurements, the potential was swept from –300 to +300 mV (vs. open circuit potential) using a scan rate of 1 mV/s. The EIS data were obtained in the frequency range of ~105–10–3 Hz with an amplitude of 5 mV at open circuit potential.

3 RESULTS AND DISCUSSION

3.1 Static Immersion Test

Table 2 was the corrosion rate of HDAGS in different groups, which was calculated by static immersion corrosion test. The corrosion rate of HDAGS in simulated seawater without inhibitor (Group A) was 6.072 × 10–4 g/(m2 h), and the thickness index was 1.151 × 10–3 mm/yr. Disodium L-(+)-tartrate (Group B) would accelerate the corrosion of HDAGS, while other inhibitors played a certain degree of corrosion inhibition effect on HDAGS in simulated seawater, among which the best inhibitor was zinc phosphate (Group J). The corrosion rate was 3.333 × 10–7 g/(m2 h), the thickness index was 6.319 × 10–7 mm/yr, and the inhibition efficiency was up to 99.94%.

Table 2. Corrosion rate of HDAGS in simulated seawater of different groups
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3.2 Electrochemical Test

Figure 1 showed the potentiodynamic polarization curves of HDAGS coupons in simulated seawater containing different inhibitors at 25°C. Figure 2 presented the electrochemical impedance spectroscopy of HDAGS coupons in simulated seawater of different groups. Figures 3a, 3b were the equivalent circuit model of HDAGS in simulated seawater without and with different inhibitors, where Rs was the solution resistance, CPE was the constant phase angle element, Rf was the resistance of HDAGS, CPEdl, and Rct were the CPE of the double electric layer and the charge transfer resistance, respectively. Table 3 was the fitted data of the potentiodynamic polarization curves and electrochemical impedance spectroscopy.

Fig. 1.
figure 1

Polarization curves of HDAGS coupons of different groups in simulated seawater.

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Fig. 2.
figure 2

EIS curves of HDAGS coupons of different groups in simulated seawater.

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Fig. 3.
figure 3

The equivalent circuit of HDAGS for fitting Nyquist plot.

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Table 3.   Fitted data of the electrochemical test shown in Figs. 1 and 2
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It can be seen that the regular of the corrosion current density and impedance values of HDAGS was consistent with static immersion corrosion rate. The effect of deposited film inhibitors (Groups J and H), adsorbed film inhibitors (Groups I, G, F, and E) and oxidized film inhibitors (Groups D, C, and B) were from superior to inferior for HDAGS. Among adsorbed film inhibitors, O atoms of N-methyldiethanolamine and triethanolamine (Groups E and G) were adsorbed, while N atoms of benzimidazole and 2-mercaptobenzimidazole were adsorbed. It can be differentiated from static immersion test and electrochemical test that N adsorption (Groups F and I) was better than O adsorption (Groups E and G) for HDAGS. The best corrosion inhibitors were still calcium carbonate, 2-mercaptobenzimidazole, and zinc phosphate. Zinc phosphate and calcium carbonate were deposited film inhibitors [18, 19]. Zn2+ and Ca2+ could react with \({\text{CO}}_{3}^{{2 - }}\), \({\text{PO}}_{4}^{{3 - }}\), and OH– in the cathode region of HDAGS and adhere to the coupons’ surface. 2-Mercaptobenzimidazole was an adsorbed film inhibitor [20, 21]. N atoms contained in its benzo heteroring could coordinate with metal ions and firmly adsorbed on HDAGS, thereby isolating the contact between the HDAGS and the corrosive medium.

3.3 Characterization of HDAGS after Immersed for 144 h

Figure 4 and Table 4 were SEM morphologies and EDS element composition of HDAGS after immersed in simulated seawater (Groups A, H, I, and J) for 144 h. The deposition film formed by calcium carbonate and zinc phosphate on the surface of HDAGS were small and entangled, while 2-mercaptobenzimidazole formed large adsorption film on the surface of HDAGS. The Zn2+ and Al3+ on the surface of HDAGS promoted the forming a deposition film containing Ca with a content of 60.87 wt %. The higher content of Zn in Group J was due to the formation of the deposition film. 2-Mercaptobenzimidazole was adsorbed film inhibitor, and the adsorption film had less binding force than the deposition film. However, 1.58 wt % N element could also prove the presence of the adsorption film.

Fig. 4.
figure 4

SEM morphologies of HDAGS after immersed for 144 h.

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Table 4.   EDS analysis of HDAGS after immersed for 144 h
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The HDAGS was analyzed by XRD in Fig. 5, which were immersed in simulated seawater of Groups A, H, I, and J, respectively. There was no peaks other than the matrix peaks of Al, Zn, and Fe in the XRD pattern of the HDAGS immersed in the simulated seawater without corrosion inhibitors. After doping different inhibitors in simulated seawater, the peaks of matrix presented varying degrees of attenuation, and the element peaks of corrosion inhibitors also began to appear. In the simulated seawater doped with calcium carbonate (Group H), the peak of Al decreased significantly, indicating that part of the coated Al element was consumed during the deposition process, and the peak of CaCO3 (PDF 86-2341) appeared, accompanied by heterogeneous peaks. In the simulated seawater doped with 2-mercaptobenzimidazole (Group I), the peak of Fe16N2 (PDF 78-1865) presented. Element N indicated the occurrence of the adsorption process. In the simulated seawater doped with zinc phosphate (Group J), the peak of AlPO4 (PDF 89-4201) showed up, which indicated the deposition process. Overall, the results of XRD were consistent with SEM&EDS.

Fig. 5.
figure 5

XRD of HDAGS after immersed in simulated seawater for 144 h (Groups A, H, I, and J).

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3.4 Performance of Composite WPUC

Figure 6 was the corrosion area of WPUC with and without different inhibitors in simulated seawater. The corrosion area of the WPUC with inhibitors expanded much more slowly than the WPUC without inhibitors. The corrosion spot of WPUC without inhibitors was firstly discovered at 96 h, while that of WPUC with inhibitors showed up at 144 h. The rust class of WPUC without inhibitors was Ri5 according to ISO4628, while that of WPUC containing 1 wt % zinc phosphate was Ri4. It proved that the inhibitors could delay corrosion whether it was added to the corrosive medium directly or into the coating. Calculated from the corrosion area, with the addition of corrosion inhibitor, the service life of the WPUC can be extended by 2–3 times.

Fig. 6.
figure 6

Corrosion area of WPUC with and without inhibitors in simulated seawater.

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Figure 7 presented the curves of the impedance changes of the WPUC with and without inhibitors in simulated seawater at 25°C. After immersed for 12 h, the impedance of all coatings dropped rapidly, which was caused by the dissolution of the coatings in seawater solution [22]. The impedance of WPUC doped with inhibitors were slightly lower than that of WPUC without inhibitors, because the addition of inhibitors changed the resin structure in the coating, resulting in a certain amount of defects. During the immersion process of simulated seawater, the inhibitors formed inhibition film, which worked in tandem with WPUC. In the subsequent immersion test, the impedance of WPUC containing the corrosion inhibitors were always higher than that of WPUC without inhibitors. The impedance of WPUC containing the corrosion inhibitors were consistent with the efficiency of the inhibitors itself.

Fig. 7.
figure 7

Impedance changes of WPUC with and without inhibitors in simulated seawater.

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4 CONCLUSIONS

The influence of nine common corrosion inhibitors on the corrosion behavior of HDAGS in the marine environment was studied in this paper. It was found that the oxidized film inhibitors (Groups B, C, and D) had a poor inhibition effect because the Al–Zn coating was difficult to passivate. The effect of adsorbed film inhibitors varied according to the adsorption element. N adsorption (Groups F and I) was better than O adsorption (Groups E and G). The deposited film inhibitors were the most effective, among which the corrosion current density of HDAGS in simulated seawater containing 1 wt % zinc phosphate was 1.309 × 10–7 A/cm2, while the corrosion rate of HDAGS in simulated seawater was 4.111 × 10–4 A/cm2, and the inhibition efficiency was 99.94%. The performance of WPUC containing inhibitors were evaluated, and the impedance all decreased rapidly at the beginning of immersion. The deposited film inhibitors presented to be the most effective, and the service life of WPUC was extended by 2–3 times.