Introduction

Corrosion of metals and alloys, primarily carbon steels, is estimated to cost 3.4% of the annual gross domestic product of the world1. Barrier-type polymer coatings are commonly used to protect steel parts and facilities from corrosion. However, organic coatings are susceptible to weathering and degradation (e.g., sunlight UV, ozone, sandstorms); as a result, their protective ability deteriorates gradually. Furthermore, derived from petroleum and containing organic solvents, many polymer coatings involve environmental issues such as the release of volatile organic compounds (VOCs) and lack of sustainability2,3.

With generally complementary physicochemical characteristics to polymers, some inorganic materials have also proved effective in protecting steels. For example, zinc silicate coating has been applied on steel structures (e.g., pipelines, marine facilities) to provide durable protection3. However, the curing of this coating is relatively slow and sensitive to environmental conditions (e.g., temperature, humidity). Additionally, because of its shrinkage during curing, it is difficult to build a thick coating without cracking. Furthermore, zinc is increasingly recognized as a contaminant to the soil and ocean4. These disadvantages have limited its applications.

Magnesium phosphate cement (MPC) is a family of inorganic binders that use the acid–base reaction between an acidic phosphate salt [typically potassium or ammonium dihydrogen phosphate (KH2PO4 or NH4H2PO4) and dead-burned magnesia (MgO) to form a binding phase, struvite (MMgPO4.6H2O; M = K, NH4)5,Characterizations

The initial setting time of the slurry was measured by the Gilmore needle test. A needle (113.4 g, Φ2.12 mm) was periodically placed on the sample and lifted; when no indent was visible, the time elapsed from the starting of slurry mixing was recorded as the time of initial setting (i.e., loss of flowability). Surface morphology and phase composition were studied by scanning electron microscopy (SEM, Quanta 200) and X-ray diffraction (XRD, Panalytical X’Pert Pro), respectively.

Corrosion properties were evaluated by the linear polarization resistance (LPR) technique, electrochemical impedance spectroscopy (EIS), and the standard neutral salt spray test. The first two electrochemical tests were conducted in a three-electrode mode using a potentiostat (Interface 1010E, Gamry Instruments, Warminster, PA, USA), with a 15 × 15 mm panel embedded in epoxy resin serving as the working electrode, a platinum foil (20 × 20 mm) as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. For LPR, all electrodes were immersed in a 3.5% (w/w) sodium chloride (NaCl) solution, and the open-circuit potential (OCP) was monitored. After resting for ~ 30 min to allow the OCP to stabilize, the electrode potential of the panel was scanned from − 10 mV to + 10 mV (vs. OCP) at 0.167 mV  s−1, and the LPR was calculated from the slope of the voltammogram recorded. For EIS, after resting for ~ 1 h, a sinusoidal perturbation (amplitude: 20 mV) in the frequency range of 10–2–105 Hz was superimposed over the OCP, and the impedance spectrum was measured. EIS spectra were simulated with ZSimpWin 3.1 software. Neutral salt spray test was conducted with 150 × 70 mm panels according to ASTM B-119 (35 ± 1 °C; panel inclination: 30°; LP/YWX-250 spray chamber, Linpin Instruments, Shanghai, China).

Analyses of anti-corrosion mechanisms

To study the mechanisms of corrosion protection, immersion tests and Tafel polarization were performed. For the immersion tests, 15 × 15 mm panels were immersed separately in three liquids (see “Analyses of anti-corrosion mechanisms”): (1) 3.5% (w/w) NaCl solution, (2) 3.5% NaCl solution containing 10% (w/w) dead-burned magnesia particles (abbreviated as 3.5%NaCl-MgO), or (3) 3.5% NaCl containing 10% (w/w) of MPC particles prepared by grinding of hardened MPC (abbreviated as 3.5%NaCl-MPC). The panels were frequently examined for surface corrosion. For Tafel polarization, panels were immersed individually in the above liquids for 1 min, and the potential was scanned from − 0.3 V to + 0.3 V (vs. OCP) at 0.5 mV s−1. Throughout this study, each measurement included three parallel samples.

Results

Coating hardening and formulation selection

A series of slurries were tested by incrementally replacing dead-burned magnesia with Mg(OH)2 to adjust the workability (initial setting time) while kee** other component ratios constant. When Mg(OH)2 replaced 0–8% (w/w) of the dead-burned magnesia, the setting time (Fig. 1) decreased almost linearly, as the uncalcined Mg(OH)2 reacted more rapidly (vs. dead-burned magnesia) with KH2PO4 to form the binder phase. When Mg(OH)2 replacement exceeded 8%, the setting time kept decreasing but the slope also decreased gradually, forming a curve.

Figure 1
figure 1

Initial setting times of slurries prepared by incremental replacement of dead-burned magnesia with Mg(OH)2.

Full size image

The coating formulation was selected based on workability of the slurry. Considering several minutes were required for a thorough manual mixing of the components and the mixture became too viscous to handle near the setting time, a setting time of ~ 15 min [i.e., replacement of 6% of dead-burned magnesia] was found to give a reasonable time window (4 min) for mixing and applying the slurry. Therefore, this formulation was selected for subsequent experiments. Nevertheless, when different operation time-windows are required (e.g., different working temperature, equipment), the suitable formulation can also be determined based on the curve (Fig. 1).

Phase and morphology

XRD of the coating (Fig. 2) found strong peaks of magnesia and alumina (filler), and moderately strong peaks of struvite [Mg(NH4)PO4·6H2O]. SEM found that (Fig. 3a), the coating had a relatively compact microstructure with some micrometer-sized cracks. Under higher magnifications (Fig. 3b), the coating appeared to consist of particles bound by a matrix. Some coated samples were chiseled with a screw driver to induce brittle fracture-detachment of the coating (Fig. 3c). The cross-section thus exposed showed a visually intimate coating-substrate interface without evident defects (e.g., pores, separation). For comparison, several samples coated with the original unmodified mortar [i.e., 0% replacement of dead-burned magnesia] were also chiseled to expose the interface. In these samples, numerous pores (Fig. 3d) were seen at the coating-steel interface, indicating a defected interface.

Figure 2
figure 2

X-ray diffraction spectrum of MPC coating on mild steel.

Full size image
Figure 3
figure 3

(a,b) Scanning electron micrographs and (c) photograph of an MPC coating [6% of dead-burned magnesia replaced with Mg(OH)2)] on a mild steel panel; (d) a mild steel panel coated with original mortar [no replacement of dead-burned magnesia with Mg(OH)2].

Full size image

Anti-corrosion properties

Linear polarization resistance (LPR)

LPR (Rp) is inversely proportional to the corrosion rate of steel in aqueous media20. After immersion in 3.5% NaCl for 30 min (Fig. 4), the uncoated steel showed an Rp of 495 ± 55 Ω cm2. All uncoated panels corroded rapidly in the solution. After ~ 2 h, their surfaces were severely covered by rust and the NaCl solution was severely discolored (not shown). Additionally, the pH of the NaCl solution became increasingly acidic (i.e., hydrolysis of metal ions), preventing LPR monitoring under stable conditions. Therefore, the Rp of uncoated steel was not further monitored. In comparison, the MPC-coated panels showed an Rp of (1.8 ± 0.6) × 104 Ω cm2 after immersion for 30 min, followed by a decrease to a nadir of (8.2 ± 1.7) × 103 Ω cm2 at 24 h. Subsequently, it slowly increased till day 14 (end of experiment), reaching (1.9 ± 0.2) × 104 Ω cm2. After this 14-day immersion, no rust was visible on any MPC-coated panel.

Figure 4
figure 4

Linear polarization resistances of uncoated and coated mild steel panels during immersion in 3.5% NaCl solution.

Full size image

Electrochemical impedance spectroscopy

Figure 5 depicts the EIS spectra of uncoated steel panel immersed in 3.5% NaCl. After 1 h (Fig. 5). The Nyquist plot of the uncoated steel displayed a depressed semicircle at the high- to medium-frequency ranges followed by an inductive loop at the low-frequency end. The semi-circle represents the resistance–capacitance (R–C) characteristics of the steel-solution interface. The inductive loop is attributed to the adsorption of corrosion products (i.e., metal ions) on the electrode surface21, as frequently observed from carbon steels undergoing rapid corrosion22. The Bode plot (Fig. 5c) exhibited a strong peak of phase angle overlap** with a shoulder (marked by arrow), also indicating the presence of two time constants. Therefore, the spectrum was modeled with an equivalent circuit with two time constants (Fig. 5d), and the fitted parameters are summarized in Table 1. In the circuit, Rs represents the solution resistance; CPE is a constant phase element representing the steel-solution interfacial double layer; Rct represents the charge transfer resistance of the double layer; and L models the inductive characteristic of the interface during rapid corrosion. The fitted line deviated noticeably from the measured spectrum (Fig. 5a–c), presumably because of the insufficient accuracy in modeling of the inductive characteristic of the spectrum. The fitted charge transfer resistance (Rct) (Table 1), an indicator of the rate of electron transfer at the coating-steel interface (i.e., metal corrosion), was 3.8 × 102 Ω·cm2, approximately similar to the Rp measured by LPR (Fig. 4). Again, because of rapid corrosion in the solution, the EIS of uncoated steel was not measured after day 1.

Figure 5
figure 5

(a–c) Electrochemical impedance spectrum of uncoated mild steel in 3.5% NaCl solution (a: Nyquist plot; b,c: Bode plots); (d) equivalent circuit used to simulate the spectrum; (arrow: shoulder overlap** with major peak).

Full size image
Table 1 Fitting of EIS spectra of mild steel samples to equivalent circuit.
Full size table

The Nyquist plot of the MPC-coated panel (Fig. 6a,b) exhibited a depressed semicircle at the high- to medium frequency range followed by upward lines at the low-frequency range, a common pattern from coated metals. With the elapse of time, the semi-circles increased in diameter (Fig. 6b), indicating an increasing resistance to electron transfer (i.e., corrosion) at the steel-liquid interface. The upward lines were approximately 45° in slope, suggesting the presence of a diffusion-controlled process. The Bode plots (Fig. 6d) displayed two peaks of phase angles separated by a valley, indicating two time-constants. Therefore, the spectra were modeled with an equivalent circuit illustrated in Fig. 6c, and the fitted parameters are listed in Table 1. In the circuit, Rs represents the resistance of the bulk solution; CPE1 is a constant-phase element representing the capacitance of the MPC coating; Rpo represents the resistance attributed to the pores in the coating; W is a Warbug impedance describing the diffusion-controlled characteristic of the coating; CPE2 is a constant-phase element representing the capacitance of the coating-steel interface; Rct represents the charge transfer resistance of the interface. The fitted EIS spectra appeared relatively close to the measured spectra (Fig. 6c,d). The fitted Rct (Table 1) decreased between 1 h and 1 days, and increased subsequently until day 14. Throughout the study, the fitted Rct was at least one order of magnitude higher than that for uncoated steel, indicating a lower corrosion rate. These patterns are consistent with the trend of Rp (Fig. 4).

Figure 6
figure 6

(a–d) Electrochemical impedance spectra of MPC-coated mild steel immersed in 3.5% NaCl (a,b: Nyquist plots; c,d: Bode plots); (e) equivalent circuit used to model the spectra.

Full size image

Neutral salt spray

After salt spray for 2400 h (Fig. 7), no rust was visible on the coated panels, nor a sign of mechanical degradation of the coating (e.g., cracking, detachment, underlying bulging). During test, the coating appeared progressively whitish. A similar phenomenon (i.e., efflorescence) is frequently observed on hardened ordinate Portland cement. Is attributed to the transport of dissolved ions inside the cement to the surface by water and their subsequent crystallization. The underlying steel was exposed by chiseling off the coating (Fig. 7, middle panel), and the surface was observed to have millimeter-sized corrosion pits. The pits near the edges were larger, probably because the easier diffusion of chloride ions from the edges. In comparison, the uncoated panels rusted rapidly and became substantially covered by rust in several hours. As a result, they had to be removed from the chamber after 3 h.

Figure 7
figure 7

Photographs of a coated mild steel panel (150 × 70 mm) (upper panel) during salt spray for up to 2400 h, (middle panel) the panel chiseled to remove coating after 2400-h salt spray, and (lower panel) an uncoated panel before and after salt spray for 3 h.

Full size image

Anti-corrosion mechanisms

To probe the mechanisms of corrosion protection by the MPC coating, corrosion of the mild steel in its local medium and the underlying electrochemical processes were studied. Obviously, the steel is exposed to the pore fluid in the coating. The pore fluid can be simulated by suspending powdered MPC in 3.5% NaCl solution. However, considering the MPC contains multiple components that may influence the corrosion rate, it is to necessary to differentiate the contribution of each. Chemically, the hardened MPC comprises magnesia (stoichiometrically excess vs. KH2PO4), struvite, alumina, and potentially trace (unreacted) KH2PO4 (Eq. 1; Fig. 1). Magnesia is a mediumly strong base, which, according to the Pourbaix diagram, may passivate the steel surface by forming a hydroxide layer. Struvite and potentially trace KH2PO4 can liberate phosphate ions, which may form insoluble precipitates on the surface to retard corrosion. The alumina filler, calcined in nature, is commonly known to be unreactive. Therefore, panel corrosion and voltammograms were monitored in three liquids (Sect. 2.3) and compared.

After immersion in 3.5% NaCl for 24 h, the uncoated panels showed corrosion and the liquid became brown (Fig. 8). In comparison, those immersed in 3.5%NaCl-MgO underwent a more moderate corrosion, whereas those in 3.5%NaCl-MPC had further less corrosion. These indicate that, ions dissolved from magnesia acted as an effective corrosion inhibitor, and those from other phases in the hardened MPC further enhanced its protective ability.

Figure 8
figure 8

Photographs of uncoated mild steel panels immersed in three liquids for 0 and 24 h.

Full size image

Tafel polarization of the samples in the three liquids found that (Fig. 9), the anodic branches of the voltammograms decreased in the order: 3.5%NaCl > 3.5%NaCl-MgO > 3.5%NaCl-MPC whereas the cathodic branches changed moderately. The corrosion current densities (Icorr) followed the rank of: 3.5%NaCl (6.0 µA cm−2) > 3.5%NaCl-MgO (3.6 µA cm−2) > 3.5%NaCl-MPC (1.7 µA cm−2). The corrosion potentials (Ecorr) of the panels immersed in 3.5%NaCl-MgO and 3.5%NaCl-MPC shifted sequentially toward the positive direction (vs. 3.5%NaCl) by 73 mV and 157 mV, respectively. According to the mixed potential theory, these changes indicate a progressively suppressed anodic reaction [Fe(0) → Fe2+ + 2e]) in 3.5%NaCl-MgO and 3.5%NaCl-MPC relative to 3.5%NaCl.

Figure 9
figure 9

Tafel curves of uncoated mild steel panels immersed in three liquids.

Full size image

Discussion

These results indicate the MPC coating to be an effective anti-corrosive barrier for the mild steel. The coating was formulated by modification from an MPC mortar developed for road repair. Although suitable for fast road repair, the mortar sets too slowly (~ 25 min) for coating applications; more importantly, it is highly acidic (pH ~ 2) shortly after mixing due to the dissolution of KH2PO4. The acidic slurry reacts rapidly with the steel to form hydrogen gas, creating pores at the coating-substrate interface (Fig. 3d). Therefore, more reactive Mg(OH)2 was incrementally introduced to accelerating the slurry hardening and reducing the slurry acidity. Several other methods were also found effective for eliminating interfacial pores, including pre-phosphating of the steel, natural rusting in the air before coating, or partial replacement of KH2PO4 with K2HPO4. However, they were found to be inconvenient (pre-phosphating/rusting) or reduce the anti-corrosive capability (replacement with K2HPO4). Nevertheless, other innovative approaches to formulating MPC coatings may be possible and deserve exploration.

Even when the MPC coating was absent, suspension of MgO or ground MPC particles decelerated the steel corrosion in 3.5%NaCl (Fig. 8), indicating that magnesia and other components dissolved from the MPC coating can inhibit the steel corrosion. Tafel scans (Fig. 9) suggest that they act primarily by inhibiting the anodic reaction. These can be explained by the Pourbaix diagram. A mediumly strong base, magnesia (pH ~ 10.6) may passivate the steel surface by forming a protective iron hydroxide precipitate. This is analogous to the passivation of steel rebars in concrete, in which calcia (pH ~ 14) forms an iron hydroxide precipitate on the steel23. Struvite and potentially trace unreacted KH2PO4 release phosphate ions into the pore fluid, which may combine with Fe2+ liberated from sites of corrosion to form an iron phosphate precipitate, covering these sites. Phosphate salts are known effective corrosion inhibitors for carbon steels24. This chemical corrosion-inhibiting mechanism is desirable, as when small uncoated areas are present (e.g., defective coating operation) they are, to some degree, protected chemically. In fact, we observed that, when MPC-coated coupons were marked with X-shaped scribes and placed in the salt spray chamber shortly after coating hardening, the scribes became gradually partially filled. After salt spray for 240 h, many scribes were less noticeable (not shown). This reflects the migration of reactive species (e.g., Mg2+, PO43−) dissolved from the adjacent coating mass and local precipitation in the scribe, analogous to the self-healing of small cracks in concrete when water is available25.

The dense morphology of the coating (Fig. 2) and ~ 45° upward diffusion-controlled lines suggest a physical barrier effect. In 3.5% NaCl, Rp decreased between 1 h and 1 day, possibly because at 1 h the solution had not sufficiently penetrated the coating. With gradual infiltration of the solution, corrosion increased, and Rp reached the nadir on day 1. From day 1 to 14, Rp and the diameter of the semicircles in EIS kept increasing, likely because the corrosion product clogged some pores in the coating, impeding the ingression of reactants for corrosion (e.g., O2)26. Therefore, the combination of physical and chemical inhibition confined corrosion to local points (Fig. 7) and retarded its progression.

These results show that, MPC may be applied as effective anti-corrosive coatings for mild steel. This is expected to be valuable for structures serving under harsh conditions. For entering actual applications, more requirements must be satisfied, such as tolerance of temperature cycles and resistance to acid rain. Inspired by advances in cement technologies, various fillers and industrial wastes (e.g., flyash, mineral fibers) may be added into the coating to create products that are not only effective but also benign to the environment. These will be investigated systematically and reported in the future.

Conclusion

A MPC slurry suitable for coating mild steel was developed by incremental replacement of dead-burned magnesia in a road-repair mortar with Mg(OH)2. This modification yielded an acceptable working time, and reduced pore formation at the coating-steel interface. In 14-day electrochemical test, the coating increased the corrosion resistance of the steel. The coated steel underwent 2400-h salt spray without visible rusting. Immersion in liquids simulating pore fluid indicates magnesia to be a major contributor to its anti-corrosive ability. A barrier effect and dissolved phosphate ions further enhanced its protection. The coating is expected to be a promising durable and environmentally favorable anti-corrosive material for steel structures in some applications.