Influence of Partial Rust Layer on the Passivation and Chloride-Induced Corrosion of Q235b Steel in the Carbonated Simulated Concrete Pore Solution

Abstract

A partial pre-rusted wire beam electrode (WBE) was designed to study the influence of the rust layer on rebar corrosion in the carbonated simulated concrete pore solution (SCPS). The results show that the passive film generated on the pre-rusted steel area is more fragile than that formed on the fine polished steel area in carbonaceous media. Nevertheless, the pitting corrosion resulting from the presence of chloride ions still tends to occur on the fine polished steel surface due to the local acidification process being hindered by the rust layer. The rust layer could play a more important role than the passive film in inhibiting the initiation of chloride-induced corrosion on rebar. The expansion path of the corrosion product would be blocked by the rust layer, leading to the pit propagating in the fine polished region. Furthermore, the growth of pitting corrosion is greatly accelerated due to the catalytic cathodic reaction of the rust layer.

1. Introduction

Reinforced concrete is extensively used as the preferred material for civil and infrastructure construction in the marine environment due to its low cost, high structural durability, and wide application range [1,2,3,4,5]. Normally, due to the high alkalinity of the pore solution, a protective passive film would spontaneously form on the surface of carbon steel. It is believed that the passive film is an ultrathin (<10 nm) oxide or hydroxide layer, which could inhibit the anodic dissolution to a negligible level [6,7,8]. However, the partial or complete loss of passive film, known as depassivation, could lead to pitting or the active corrosion of rebar in concrete [9,10]. There are two main reasons to induce the depassivation of rebar in concrete, which are the concrete carbonation and chloride ions (Cl⁻) ingression. The concrete structures located in the marine environment would suffer the dual effect of concrete carbonation and Cl⁻ ingression [11,12,13,14]. Concrete carbonation leads to the degradation of the passive film, resulting in the passive film being more easily destroyed by Cl⁻. Various researchers have focused on the environmental parameters that control the depassivation of rebar in concrete, namely the chloride ion concentration [15], the pH of concrete pore solution [16], and the content of dissolved oxygen [17].

In most previous studies, the reinforced steels are usually pretreated before testing by sandblasting or polishing to ensure the repeatability of experiments [18,19,20]. The surface condition of these steel samples is different from those in field applications [21,22]. Since rebar is exposed to the atmosphere during transportation and storage for a considerable period before its installation into the concrete structures, rebar would corrode in the atmosphere, and the rust layers might locally cover the steel surface [23,24]. Some researchers have studied the influence of the rust layer on the bonding strength of reinforced concrete. Fu [25] reported that a slight corrosion treatment of steel rebars could increase the roughness of the rebar, thereby enhancing the friction between the rebar and the surrounding concrete. Meanwhile, Batis [26] found that the bond strength between the rebar and concrete showed a significant decrease in the aggravation of rebar corrosion.

Although several experimental studies were conducted in the past few decades to understand the rust layer on the corrosion behaviors of reinforcement steel, the previous researchers could not reach a general conclusion. Some researchers indicate that the rust layer has a negative effect on the corrosion resistance of reinforced steel [27,28,29]. Gonzalez [28] suggested that the rust layer itself might induce the active corrosion of steel in mortar. Sagûës [29] affirmed that the pre-rusted steel is prone to pitting corrosion due to the rust layer undermining the continuity of the passive film. Nevertheless, some researchers suggested that the rust layer on rebar could not obviously affect its corrosion performance, and sometimes, it could even be beneficial [30,31,32]. Li [31] found that the pitting corrosion resistance of the pre-rusted steel was significantly higher than that of the fine polished steel in a simulated concrete pore solution (SCPS). Maslehuddin [32] reported that the corrosion rate of the pre-rusted rebar would be less than that of the as-received rebar.

Conventional electrochemical techniques such as open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and linear polarization resistance (LPR) could only detect the average information of rebar corrosion. The mechanism of pit initiation and propagation may be misinterpreted by these methods. The wire beam electrode (WBE) technique was successfully applied in studies of localized corrosion [33,34]. WBE is composed of many wire electrodes arranged together to simulate a one-piece electrode. Such an assembly allows WBE to follow the development of corrosion processes at all the local zones of the whole electrode surface.

In this work, a partial pre-rusted WBE sensor was used to study the pitting corrosion of rebar in carbonated SCPS containing Cl⁻. The combined tests of WBE with electrochemical impedance spectroscopy (EIS) and galvanic current measurement were carried out, and the initiation and propagation of pitting corrosion were visualized based on the current distribution map. The effect of the rust layer on the passivation and chloride-induced corrosion behaviors of rebar in a simulated concrete pore solution was discussed.

2. Materials and Methods

2.1. The Preparation of the Partial Pre-Rusted WBE and the Corrosion Coupons

The rust layer on the surface of reinforced steel is produced using the wet/dry cyclic corrosion acceleration pre-rusting process [35,36,37,38]. A dilute solution of NaCl (0.5 wt %) was used as a corrosive solution to simulate the coastal atmosphere environment. As shown in Figure 1a, all specimens were suspended horizontally in a customized corrosion device during the pre-rusting process. The instrument consists primarily of a thermotank, water tank, linear actuator, electric control panel, air pump, and two thermometers. The air temperature and solution temperature could be measured. The linear actuator can push all the specimens into and out of the solution to create a wet–dry cyclic corrosion environment. The pre-rusting process contains 12 wet–dry cycles, and each cycle was conducted in two steps: (1) immerse the specimens into the corrosive solution with a temperature of 35 °C for 2 h; (2) lift the sample out of the solution and wait 4 h for them to dry naturally. After the pre-rusting process, the corrosion coupons and the WBE were removed from the water tank and gently washed with distilled water to clean the superficial NaCl contamination. Then, the silicone rubber was removed with a plastic knife. The steel surface was carefully cleaned with a cotton swab stained with alcohol to eliminate the rubber residual.

The steel coupons and the WBE used in the test were made of Q235 carbon steel whose chemical compositions are (mass %): C 0.22, Si 0.3, Mn 0.4, P 0.045, S 0.04, Cr 0.01, Ni 0.01, and Fe for the balance. As shown in Figure 1b, the coupons were machined to dimensions of 10 × 10 × 10 mm. The WBE was fabricated by 100 identical wires with a total working area of 4 cm². The dimensions of each electrode were machined as 2 × 2 × 10 mm. The 100 wires were arranged into a 10 × 10 matrix and embedded in epoxy resin with an interval of 0.5 mm from each other. The working surfaces of the coupons and the WBE were both successively abraded by 240, 400, and 600 grit silicon carbide papers. The polished surfaces were then cleaned using de-ionized water and alcohol. After that, the corrosion coupon and WBE were partially coated with silicone rubber to avoid corrosion in the pre-rusting process, as shown in Figure 1c. It is seen from Figure 1d that the rubber-covered area was shiny, and the exposed area was covered by thick brown rust. Finally, all the specimens were sealed in a vacuum bag for future electrochemical tests.

2.2. The Experimental System

The schematic diagram of the experimental system is schematically shown in Figure 2. A conventional three-electrode system was used for the electrochemical measurements. The system consists of a WBE in which wires could work as the milli working electrodes, a saturated calomel electrode (SCE) works as the reference electrode, and a platinum electrode works as the counter electrode. The SCE was fixed near the surface of WBE by means of a Luggin capillary. The WBE was horizontally placed in the test cell. All electrochemical measurements were conducted at the room temperature of 25 ± 2 °C. Current distributions of the WBE were measured by using a multichannel zero resistance ammeter (YC-2200A, YunChi). During the default state of the instrument, all the wires of the WBE were electrically coupled together to restore a one-piece electrode that allows electrons to freely travel among the wires. The current distribution is mapped by measuring the galvanic current between the chosen wires and all the rest of the wires. In addition to the current mapping, 1–10 wires could be selected using the multiplexer for a period of continuous current monitoring. During the galvanic current monitoring, all the wires were also electrically connected. The electrochemical impedance spectroscopy (EIS) measurements could be conducted to obtain the interfacial reaction of a single wire using a Gamry-600 electrochemical workstation. A digital camera was placed on the top view of the WBE, which could provide photos of the WBE surface during the test.

The test procedure contains three steps, as shown in Figure 3. Firstly, the WBE was immersed in the 200 mL saturated Ca(OH)₂ solution for 72 h to reach a stable passive state. Excessive amounts of undissolved Ca(OH)₂ were kept in the SCPS to maintain a pH of around 12.6 during the passivation process. Secondly, the pH of the simulated pore solution was adjusted to 10.0 by the introduction of sodium bicarbonate (NaHCO₃) solution. After 12 h of immersion in the carbonated SCPS, the current distribution of WBE was measured with the coupled multi-electrode array current scanning system. Then, six selected wires were disconnected from the WBE in sequence. The EIS tests were performed on the independent wire around OCP with a 10 mV sinusoidal perturbation range from 10 kHz to 10 mHz. Thereafter, the galvanic current between these six selected wires and the rest of the wires was measured continuously with a sampling rate of 5 Hz. Finally, another 200 mL of SCPS solution containing 1 mol L⁻¹ NaCl was introduced into the test cell after 24 h of immersion in the carbonated SCPS. The current distribution of the WBE was measured at intervals to characterize the initiation and propagation process of pitting corrosion.

During the whole electrochemical test, steel coupons are immersed in the solution to provide a comparison of the WBE test. After the electrochemical test, the coupons were taken out from the test solution for observation. The porous corrosion products on the coupons were firstly washed with deionized water. Then, the surface morphologies of the coupons were observed by a digital camera. Thereafter, the dense corrosion products on the coupon surface were further cleaned using ASTM G1-03 solution. Finally, the local morphology of the coupons was further characterized by an Olympus OLS 5000 Infinite Microscope.

3. Results

3.1. The Chloride-Induced Corrosion Behavior of the Corrosion Coupons

The corrosion performances of the two pre-rusted coupons used in the parallel tests are shown in Figure 4. The original rust layer is located at the center of the coupons, while the margin areas of the coupons are kept fine polished. It is seen from Figure 4a that there are no additional corrosion products on the surface of the corrosion coupons except for the original rust layer in the central area. As shown in Figure 4b, the surface morphology changed dramatically after the introduction of Cl⁻ into the test solution for 6 h. Newly generated corrosion products of a dark green color appear on the fine polished region. It indicates that the Cl⁻ would lead to the local breakdown of the passive film and induce serious localized corrosion of carbon steel in the carbonated SCPS. The corrosion products could be easily identified from the original rust layer. Along with the immersion time increasing, the coverage area of the corrosion products gradually expanded, as shown in Figure 4b–e. The color of the corrosion products gradually changed from dark green to brown. Moreover, most of the newly generated corrosion products are propagating on the adjacent fine polished area. Nearly no newly generated corrosion products expanded to the pre-rusted region.

The surface morphologies of independent pre-rusted coupons after the removal of rust layers are shown in Figure 5. It is seen from Figure 5a that irregular pits appear on the coupon surface after the simple washing of the steel surface using deionized water. It is seen that newly generated corrosion products and the outer layer of the original corrosion products could be easily washed away, leaving the black inner rust layer remaining on the steel surface. The steel surfaces after being cleaned by the ASTM G1 03 solution are shown in Figure 5b. It is seen that the pitting damage would propagate on the fine polished area. The original rust layer inhibits the expansion of the localized corrosion. The boundary between the new pit and the pre-corroded area could be clearly seen from the local 3D profile, as shown in Figure 5c. The boundary further verifies that the pitting corrosion would be retarded by the rust layer, leading to the pit preferring to expand in the fine polished region.

3.2. The Passivation Behavior of WBE in Carbonated SCPS

The surface morphology and the distribution of macro-cell current on the WBE after being immersed in pure carbonated SCPS for 12 h are shown in Figure 6. It is seen from Figure 6a that the fine-polished region of WBE still has a metallic luster, which indicates that carbon steel can remain passivated in carbonated SCPS of pH 10. Due to the pre-rusted region on the WBE being covered by a layer of thick rust, it is hard to see whether the steel beneath the original rust layer can remain in a passive state only from the morphology observation. It is seen from Figure 6b that the current distribution shows an obvious difference between the fine polished region and the pre-rusted region. The current distribution in the fine polished region is uniform, while the current distribution in the pre-rusted region is highly scattered. Moreover, the major anodic areas and major cathodic areas are both distributed in the pre-rusted region. The maximum anodic current of fine polished wire (FP-wire) is only 0.09 μA/cm². Meanwhile, the maximum anodic current of pre-rusted wire (PR-wire) reaches 0.96 μA/cm². It is inferred that the rust layer can accelerate the electrochemical reaction rate. As shown in Figure 6b, three FP-wires (W_{2, 5(FP)}, W_{4, 5(FP)}, W_{6, 6(FP)}) and three PR-wires (W_{5, 1(PR)}, W_{7, 4(PR)}, W_{7, 8(PR)}) were selected for further investigating to understand the influence of rust layer on the passive behavior of rebar.

Figure 7 depicts the Nyquist and Bode plots of the six selected wires in carbonated SCPS. The protectiveness of passive film formed on the steel surface could be well revealed from the EIS measurement results. As shown in Figure 7a, it is observed from the Nyquist plots that the six curves can be classified into two groups. The radius of the semicircle which performed on FP-wires (W_{2, 5(FP)}, W_{4, 5(FP)}, W_{6, 6(FP)}) is much larger than those of PR-wires (W_{5, 1(PR)}, W_{7, 4(PR)}, W_{7, 8(PR)}). Additionally, the Bode-magnitude plots and Bode-phase plots (Figure 7b) also show that the impedance of the PR-wires at the low-frequency range was obviously lower than those of the FP-wires.

The EIS measurement results are fitted by the equivalent circuits shown in Figure 8. Since only a single time constant is seen from the Bode plots of the PR-wires, the equivalent circuit shown in Figure 8a is used for the EIS fitting, which indicates that the EIS performance is mainly determined by the passive film. It is seen that two time constants are found on the PR-wires, indicating that the EIS performances are influenced by both passive film and the rust layer. In this case, the equivalent circuit shown in Figure 8b is used to fit the EIS results. In both fitting circuits, R_s is the solution resistance, R_f is the resistance of the passive film, and R_r is the resistance of the rust layer. Since incomplete semicircles are presented in the Nyquist plots, constant phase elements (CPE) are used to represent the capacitors of both the passive film (Q_f) and rust layer (Q_r).

$$Z_{CPE}=\frac{1}{Y_0{\left(j\omega \right)}^n}$$

(1)

where Y₀ is the electrical constant of the CPE, which is proportional to the capacitance of pure capacitive electrodes, and n is an empirical exponent (0 ≤ n ≤1), which suggests the deviation from the ideal capacitive behavior. According to the Brug formula [39], the effective capacitance can be extracted from the CPE parameters.

$$C_{eff}=Y_0{}^{1/n}{R}_f{}^{\left(1-n\right)/n}$$

(2)

The fitted parameters of the two kinds of wires are listed in

Table 1. The fitted parameters of the EIS measurement results at different surface conditions.

	${\mathit{R}}_{\mathit{s}}$ (Ω·cm2)	${\mathit{R}}_{\mathit{f}}$ (kΩ·cm2)	${\mathit{Q}}_{\mathit{f}}$-Y0 (×10−5Ω−1·cm−2·sn)	${\mathit{Q}}_{\mathit{f}}$-n	${\mathit{Q}}_{\mathit{f}}$-Ceff (μF·cm−2)	${\mathit{R}}_{\mathit{r}}$ (kΩ·cm2)	${\mathit{Q}}_{\mathit{r}}$-Y0 (×10−5Ω−1·cm−2·sn)	${\mathit{Q}}_{\mathit{r}}$-n	χ2 (×10−4)
W2, 5(FP)	55.02	2227	3.589	0.955	44.12				7.84
W4, 5(FP)	57.01	2082	3.611	0.949	45.55				5.55
W6, 6(FP)	52.88	2724	3.231	0.951	40.69				8.03
W5, 1(PR)	42.57	38.06	19.51	0.849	185.0	257.9	2.600	0.551	2.02
W7, 4(PR)	45.42	142.9	10.15	0.889	79.76	684.2	2.122	0.734	2.42
W7, 8(PR)	59.41	89.57	8.713	0.889	112.6	717.8	1.586	0.660	1.36

. It is seen that the R_f of all the selected wires is higher than 30 kΩ·cm². According to the references [20,31], it is normally thought that the steel would step into passive state with the R_f higher than 10 kΩ·cm². The R_f of the PR-wires shows an order of magnitude less than those of FP-wires. It is inferred that the rust layer could weaken the resistance of the passive film. The Q_f-n of the FP-wires was maintained at 0.95, while the Q_f-n of PR-wires is lower than 0.9, which means the passive film formed beneath the rust layer is less uniform than that on the fine polished region. The effective capacitance of passive film (Q_f-C_eff) formed on the PR-wires is higher than that of the FP-wires. The increase in the capacitance agrees with the decrease in film thickness [39]. It suggests that the passive film beneath the rust layer is even thinner, since the polarization resistance (R_p) of steels includes R_f and R_r (R_p = R_f + R_r) [20]. In addition, the R_r of the PR-wires is quite high, ranging from 250 to 750 kΩ·cm², which means the rust layer could also suppress the charge transfer process, thus inhibiting the corrosion process. Overall, these EIS results reveal that the passive film formed on the surface of pre-rusted steel is less protective, which is possibly induced by the thickness decreasing of the passive film.

The measured galvanic currents between the selected wires and the rest of the wires on WBE are shown in Figure 9a. The positive current indicates that the wires acted as anodes, and the negative current suggests that the wires acted as cathodes. More detailed information on the current distribution of FP-wires can be seen from the local enlarged image in Figure 9a. The current amplitude is small, and the fluctuation is relatively stable. It suggests that the passive film formed on the surface of fine FP-wires is compact, where both the cathodic reaction and anodic reaction were inhibited. However, the galvanic currents of the PR-wires have larger amplitudes and higher fluctuations. A significant increase in the galvanic current level suggests that the rust layer weakens the compactness of the passive film. Especially for the pre-rusted wire W_5,1(PR), the anodic current exhibits a quick rise followed by exponential decay, which is associated with the breakdown and reparation of the passive film. In addition, the other two pre-rust wires (W_7,4(PR), W_7,8(PR)) appear as cathodes, and the current spikes also appear at the same time. It indicates that the cathodic reaction of metastable pitting mainly occurs in the pre-rusted region.

To obtain more useful information from the galvanic current measurement, the galvanic current is transferred to the frequency domain. Frequency domain analysis enables us to distinguish the different types of the corrosion process. In this work, the power spectra density (PSD) analysis was performed using Fast Fourier Transform (FFT), and the Hanning’s window function is applied to minimize artifacts in the spectra. Figure 9b shows the PSD plots on a log–log scale taken from the six selected current curves between 2.5 and 4.5 h. All the PSD plots show similar trends, in which there is an approximately horizontal jagged line in the lower frequency region, and the PSD value decreases continuously with frequency in the higher frequency region. Two important parameters are associated with the corrosion kinetics, including plateau amplitude (white noise level, W) in the low-frequency region and the slope (the roll-off slope, K) of the high-frequency linear region. The W value (f < 0.001 Hz) of three PR-wires (3 × 10⁻⁴ ~ 4 × 10 nA²/Hz) is considerably higher than those of FP-wires (4 × 10⁻⁷~3 × 10⁻⁵ nA²/Hz), suggesting that the rust layer will accelerate the dissolution of the passive film. The K values for current PSD plots can be used as a valuable indicator to discern whether a specific mode of anodic dissolution is general or localized [40]. The higher K values of the three PR-wires indicate that the passive film formed beneath the rust layer might undergo a local rupture process besides the uniform dissolution.

3.3. The Chloride-Induced Corrosion Behavior of WBE in the Carbonated SCPS

Figure 10 shows the dynamic progression of surface morphology and galvanic current distribution on the WBE after the introduction of Cl⁻ into the carbonated SCPS. After 6 h immersion in carbonated SCPS with Cl⁻, the W_1,6(FP) was completely covered by dark green corrosion products, indicating the occurrence of pitting corrosion on WBE. The corrosion performance on the WBE is almost the same as those of corrosion coupons. It suggests that the corrosion behavior of the one-piece electrode could be well restored by the WBE in this case. The pitting corrosion prefers to initiate from the fine polished steel surface. Meanwhile, an anodic current was found on W_1,6(FP) after 6 h of test, as shown in Figure 10b. The anode current reaches 2.4 mA/cm², which was much higher than the maximum anodic current before the addition of Cl⁻. In addition, except for W_1,6(FP), all the other wires act as cathodes. It is worth noting that the main cathodes are distributed in the pre-rusted region, while the cathode current in the fine polished region is negligible. It could be inferred that the rust layer would promote the cathode reaction, indicating that the rust layer might take part in the cathodic reduction. On the contrary, for the fine polished region, the cathodic reaction is inhibited because the passive film is relatively complete. After 12 h of immersion, the corrosion products expanded to the left side of the fine polished region. The maximum anode current is also transferred to W_1,5(FP), and the corrosion products on W_1,5(FP) gradually turn brown. After 24 h of immersion, the coverage area of corrosion products continues to increase. Moreover, the corrosion products expanded to the fine polished area above the initial pitting area. It indicates that the expansion path of the steel degradation is blocked by the original rust layer, which is similar to those of the steel coupons. Meanwhile, the anode sites would continually change along with the pitting propagation. After 48 h of the test, obvious cathode current also appears on the fine polished area without corrosion, while the amplitude of the current is still smaller than that of PR-wires.

4. Discussion

It is seen from the test results that the surface morphologies of the corrosion coupons and WBE are similar in carbonated SCPS in the presence of Cl⁻. The cathode sites and the anode sites are randomly distributed in the pre-rusted region in the absence of Cl⁻. After the addition of Cl⁻ into the carbonated SCPS, serious pitting corrosion occurs on the surface of the specimen. The pitting region would act as the anode sites, and all the remaining regions acted as the cathode sites. With the increase in immersion time, the pitting area continued to expand in the fine polished region, which is at the vicinity of the pre-rusted region. Meanwhile, no new pits formed in other areas of the specimen. It is interesting to find that pitting corrosion prefers to occur on the fine polished region, and the pitting damage prefers to expand on the fine polished area.

Many researchers consider that the protectiveness of the passive film and its initial breakdown are the critical factors of pitting corrosion [41,42,43]. From the passive film point of view, passive film properties such as film thickness, film structure and film composition are of paramount importance for good resistance to pitting corrosion. Therefore, the reason why pitting is not prone to occur in the pre-rusted region is that the original rust layer improves the corrosion resistance of the passive film, thus reducing the pitting sensitivity. However, the results of EIS show that the uniformity and the quality of the passive film formed on the pre-rusted region are dramatically lower than those of the passive film formed on the fine polished region. Obviously, the inhibition of pitting corrosion by the original rust layer could not be attributed to the enhanced quality of the passive film.

In carbonated SCPS without Cl⁻, the passive film might lose its passivity in some weak sites, and the metal matrix would be exposed to the solution. The bare metal would rapidly be oxidized to be cations, which could leave the metal substrate and then dissolve into the bulk solution. Thereafter, the Fe²⁺ would react with OH^- and precipitate at the defect, which hinders a further anodic reaction [44].

$$\mathrm{Fe}\to {\mathrm{Fe}}^{2+}+2{\mathrm{e}}^{-}$$

(3)

$${\mathrm{Fe}}^{2+}+2{\mathrm{OH}}^{-}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2\downarrow$$

(4)

With time lengthening, the hydroxides dehydrate and convert into stable oxides according to a series of following reactions [45]:

$$4\mathrm{Fe}{\left(\mathrm{OH}\right)}_2+{\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}\to 4\mathrm{Fe}{\left(\mathrm{OH}\right)}_3$$

(5)

$$4\mathrm{Fe}{\left(\mathrm{OH}\right)}_2+{ \mathrm{O}}_2\to 4\mathrm{FeOOH}+2{\mathrm{H}}_2\mathrm{O}$$

(6)

$$3\mathrm{Fe}{\left(\mathrm{OH}\right)}_2+2{\mathrm{OH}}^{-}\to {\mathrm{Fe}}_3{\mathrm{O}}_4+4{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$$

(7)

$$2\mathrm{Fe}{\left(\mathrm{OH}\right)}_3\to {\mathrm{Fe}}_2{\mathrm{O}}_3+3{\mathrm{H}}_2\mathrm{O}$$

(8)

In the early stage of the passivation, the OH⁻ directly participates in the reaction process. The original rust layer could hinder the diffusion process of OH⁻ from the solution to the steel surface, resulting in a local weakly alkaline environment [46]. Thus, the repair process of the passive film under the rust layer is slowed down. Moreover, the continuity of the passive layer would be locally interrupted by the immediate proximity of the points of contact between the corrosion products and the metallic surface. Furthermore, the native rust layer constitutes effective support for the cathodic reaction due to its porous nature [47]. The rust layer can also be the source of substances involved in this semi-reaction. All these factors contribute to the existence of many anodic spots and cathodic spots under the original rust layer, as shown in Figure 11a.

Along with the introduction of Cl⁻, the Cl⁻ would absorb into the defect of passive film, which catalytically enhances the transfer of metal cations from the oxide to the electrolyte [48]. Thus, the original slow equilibrium process of passivation film generation and dissolution was broken. In addition, soluble products will be produced at the anode, and they will not interfere with the corrosion process. The generated Fe²⁺ will hydrolyze in the solution near the defect reaction by the hydrolysis equilibrium:

$${\mathrm{Fe}}^{2+}+2{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{Fe}{\left(\mathrm{OH}\right)}_2+2{\mathrm{H}}^{+}$$

(9)

Hydrolysis of the steel cation enhances the local acidity and thus accelerates the dissolution of the steel matrix in the defect. Meanwhile, at the same time, the large quantity of the generated H⁺ could soon diffuse to the bulk solution, and thus, the rapid dissolution process was disrupted. Most of the initial tiny pits cease to grow. Only when enough H⁺ and Cl⁻ are accumulated at the defect and the dimension of anode area exceeds the critical value can the rapid anodic dissolution proceed. The anodic dissolution reaction and iron ion hydrolysis reaction will form a local acidification environment, leading to the pitting corrosion step into the autocatalytic process.

At the fine polished region, the H⁺ diffusing from the defect directly causes the restoration of the surrounding passive film [49]. Once the passive film is damaged, the corrosive electrolyte will directly react with the surrounding steel matrix. Rapid anodic dissolution also occurs at the area near the defect. The increase in anode area greatly accelerates the generation of Fe²⁺ and H⁺, which accelerates the formation process of local acidification. Meanwhile, in the pre-rusted region, the penetration of Cl⁻ from the bulk solution could be hindered by the dense original rust layer, as shown in Figure 11b. Due to the lack of sufficient Cl⁻, the anodic dissolution rate will slow down. Moreover, the dense rust layer would act as a barrier that prevents the acid solution from contacting the passive film and metal matrix. Thus, the enlargement of the anode area is interrupted. As a consequence, the process of pit initiation is inhibited by the original rust layer. It suggests that even if the passive film formed on the pre-rusted region is weaker than that formed on the fine polished steel surface, the pitting corrosion is still prone to occur on the fine polished region. Obviously, the initiation of the pitting corrosion on the steel bars could not be well predicted only from the quality of the passive film.

As shown in Figure 11c, when a macroscopic pit already forms on the fine polished region, a high concentration Fe²⁺ will migrate out and hydrolyze above the stable pit to form the porous corrosion products. The corrosion products cover the pit, resulting in a distinctively different local chemical environment beneath it. The enrichment of the corrosive agents in the stable pits would accelerate the diffusion of the H⁺ from the pit inside to its adjacent area, leading to a pH decrease in the pit vicinity. The decreasing of the local pH around the initial pit would induce the local breakdown of the passive film and facilitate the expansion of the pit. The local pit environment becomes depleted in the cathodic reactant (e.g., oxygen), which shifts most of the cathodic reaction to the pre-rusted region where this reactant is more plentiful. The corrosion current density within a growing pit would be very high, which draws Cl⁻ diffusion into the pit by electromigration to maintain charge neutrality. Due to the volumetric expansion of rust on the stable pit, the rust-covering area is larger than that of the pit cavity. The diffusion of the hydrogen ions from the stable pits to its vicinity would be retarded by the original rust layer. The acid solution firstly needs to dissolve the rust layer before it could further lead to the breakdown of the passive film. The rust layer would act as a physical barrier that could effectively inhibit the expansion of the pit. As a result, the pit would prefer to spread to the adjacent fine polished region. Although the rust layer can inhibit the initiation of pitting corrosion beneath it, the growth process of pitting corrosion is greatly accelerated due to the catalytic cathodic reaction of the rust layer.

5. Conclusions

(1)

The pre-rusted rebar could keep a passive state in carbonated SCPS without Cl⁻. Meanwhile, the passive film formed on the pre-rusted region is thinner and more inhomogeneous than that on the fine polished region. Moreover, the rust layer will accelerate the dissolution of the passive film and make it more fragile.

(2)

The rust layer can hinder the local acidification process by retarding the diffusion of Cl⁻ into the pit and slowing down the increase in pit area, thus inhibiting the initiation of pitting corrosion. Although the quality of the passive film formed on the pre-rusted region is lower than that of the fine polished region, the pitting corrosion still prefers to occur on the fine polished steel surface. The rust layer could play a more important role than the passive film in inhibiting the initiation of chloride-induced corrosion on rebar.

(3)

When the pitting corrosion continues to grow, the diffusion of the hydrogen ions from the stable pits to its vicinity would be retarded by the original rust layer. The pit would prefer to spread to the adjacent fine polished region. However, the growth process of pitting corrosion on rebar is greatly accelerated due to the catalytic cathodic reaction of the rust layer.