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Article

Calcium Sulfate Whiskers Dual-Enhance Mechanical and Anti-Corrosion Properties of Magnesium Phosphate Coatings

by
Yaxin Zheng
1,2,
Haoran Chen
1,2,
Yi Liu
1,2,* and
Xiang Gao
1,3,*
1
Institute for Composites Science Innovation (InCSI), School of Materials Science and Engineering, Zhejiang University, Hangzhou 310058, China
2
Zhejiang Key Laboratory of Advanced Composites & Structures, Zhejiang University, Hangzhou 310058, China
3
Ningbo Global Innovation Center, Zhejiang University, Ningbo 315100, China
*
Authors to whom correspondence should be addressed.
Materials 2026, 19(5), 1032; https://doi.org/10.3390/ma19051032
Submission received: 11 February 2026 / Revised: 4 March 2026 / Accepted: 6 March 2026 / Published: 8 March 2026
(This article belongs to the Special Issue Physical Metallurgy of Metals and Alloys (4th Edition))
Browse Figures
Versions Notes

Abstract

Inorganic magnesium potassium phosphate (MKP) coatings offer rapid, zero-volatile organic compound (VOC) corrosion protection for steel structures. However, their application is impeded by insufficient mechanical strength and limited barrier durability. This study integrates calcium sulfate whiskers (CSWs) into a sprayable MKP matrix. Unlike conventional polymeric or metallic fibers, CSWs demonstrate excellent chemical compatibility with the MKP matrix, enabling a dual-enhancement mechanism. The optimal formulation, containing 15 wt.% CSWs, boosts the 28-day compressive strength by 35% and the bond strength by 39%. Electrochemical analysis shows a 93.6% increase in coating resistance (Rf), indicating an improved physical barrier against corrosive species, along with a 52% reduction in corrosion current density. These improvements result from fiber bridging and a dissolution–reprecipitation process that densifies the whisker–matrix interface. Nevertheless, an excessive amount of CSW (20 wt.%) disrupts the matrix continuity and reduces performance. This work presents a high-strength, zero-VOC, spray-applied coating with a novel dual-enhancement mechanism for durable steel protection in aggressive environments.

1. Introduction

Steel infrastructure is continuously adapting to meet the demands of the rapidly growing marine economy and the global dual-carbon strategy [1,2]. This development faces a significant challenge in low-latitude coastal regions, where the synergistic action of chloride ions, sulfides, ultraviolet (UV) radiation, and hygrothermal cycles severely accelerates the corrosion of conventional carbon steel [3,4,5,6]. The resulting economic impact is profound, with studies indicating that direct losses from corrosion typically constitute approximately 3% of a country’s gross national product (GNP), representing a substantial global economic burden [7]. Protective coatings remain a widely adopted, cost-effective countermeasure. However, traditional organic coatings often fail to meet long-term performance requirements due to issues such as UV-induced degradation, emissions of volatile organic compounds (VOCs), and chalking in humid environments [8,9,10]. Meanwhile, metallic coatings have some limitations, including high energy consumption during application, challenges in treating process waste, and finite sacrificial protection cycles [11]. Consequently, inorganic coatings have gained considerable attention in recent corrosion protection research. Particularly, inorganic phosphate coatings have emerged as a promising alternative due to the inherent advantages like the ability to form chemical bonds at room temperature, zero VOC emissions, a thermal expansion coefficient compatible with steel substrates, and the potential to form a protective ceramic phase at elevated temperatures [12,13,14].
Phosphate coating systems are commonly categorized into three types based on their cationic species: calcium phosphate, aluminum phosphate, and magnesium phosphate [15,16,17,18,19]. These systems exhibit distinct properties and application profiles. Calcium phosphate coatings are renowned for their excellent biocompatibility, with research predominantly focused on bone repair and implants [16,18]. Conversely, while aluminum phosphate coatings demonstrate superior adhesion, good corrosion resistance, and high thermal stability, their widespread use is constrained by the necessity for high-temperature sintering during processing [19,20]. In contrast, magnesium phosphate coatings offer an optimal balance of excellent corrosion resistance and the ability to cure at ambient temperature. This unique set of characteristics underscores their significant application potential in the protection of metallic structures [15,16,21,22]. Currently, research on magnesium phosphate coatings is mainly centered on the magnesium phosphate cementitious (MPC) system. This system can achieve a dense structure at room temperature through the acid–base neutralization reaction between basic magnesium compounds (MgO or Mg(OH)2) and acidic phosphate salts [16,17,23]. The primary hydration products depend on the specific acid phosphate salts used: potassium dihydrogen phosphate (KH2PO4) yields K struvite (MgKPO4·6H2O), whereas ammonium dihydrogen phosphate (NH4H2PO4) produces struvite (MgNH4PO4·6H2O), as represented in Equations (1) and (2), respectively. Of the two systems, the potassium-based formulation is generally favored owing to its ability to eliminate ammonia release, generate K-struvite with reduced solubility at ambient temperature, and facilitate more thorough crystallization [24,25]. Given that MPC coatings offer multiple advantages, including zero VOC emissions, high early-age strength, chemical bonding to steel substrates, and ceramic transformation at elevated temperatures, they are widely employed for the rapid protection of steel structures in aggressive environments such as the marine and chemical industries, providing both physical barrier and chemical passivation functions [26,27].
M g O ( s ) + K H 2 P O 4 ( s ) + 5 H 2 O ( l ) →   M g K P O 4 · 6 H 2 O ( s )
M g O ( s ) + N H 4 H 2 P O 4 ( s ) + 5 H 2 O ( l ) →   M g N H 4 P O 4 · 6 H 2 O ( s )
To further enhance the mechanical properties and service durability of MPC coatings, various modification strategies have been explored, including chemical additives [28,29,30,31,32], mineral admixtures [33,34,35,36,37], and fiber reinforcement [38,39,40,41]. Among these, fiber reinforcement has proven to be an effective method for improving toughness and is widely applied in cementitious composites. Commonly used fibers such as steel [41,42,43], polypropylene [44], carbon [45], and glass fibers [46] can inhibit crack propagation through different mechanisms including crack bridging, fiber pull-out, and crack deflection, thereby improving the material’s flexural strength and toughness. However, conventional fibers are susceptible to corrosion, aging, or interfacial debonding in high-temperature, high-humidity, or highly corrosive environments, which can undermine their long-term reinforcing efficacy [47,48]. Therefore, developing high-performance fibers with superior thermal and corrosion resistance has become a critical research direction for advancing the overall performance of MPC coatings. Calcium sulfate whiskers (CSW) are inorganic fibrous materials known for their exceptional mechanical and chemical properties, including high strength, high modulus, excellent toughness, thermal stability, electrical insulation, and resistance to both acidic and alkaline environments [49,50]. Compared to previously reported MPC coatings reinforced with macroscopic, chemically inert fibers, the proposed CSW system is designed to address two major practical limitations. Traditional large fibers often rely on weak physical interlocking, and their macroscopic dimensions strictly limit their application to manual brushing, which fails to meet the demands of large-scale, rapid construction. In contrast, the micro-scale dimensions of CSW enable seamless integration into a scalable pneumatic spray system without nozzle blockage, while its slightly soluble nature is hypothesized to promote better chemical compatibility with the highly reactive MKP matrix. Moreover, CSW is cost-effective, readily available, and environmentally friendly, making it a highly promising reinforcement for magnesium phosphate cement (MPC) coatings.
To date, systematic research on fiber-reinforced MPC coatings remains scarce, and the specific effects of CSW on the performance of coatings have not yet been explored. Moreover, existing coating preparation processes largely rely on brush application, which hinders scalability for industrial use [46,51,52,53,54]. To address these issues, this study introduces CSW into a spray-applied MPC coating system and systematically investigates their influences on the coating’s microstructure, mechanical properties, and corrosion resistance. The underlying reinforcement and corrosion inhibition mechanisms are also elucidated. These findings can provide both theoretical insight and practical support for the large-scale application of CSW-reinforced MPC coatings in marine engineering, petrochemical infrastructure, and related industrial sectors.

2. Materials and Methods

2.1. Materials

Dead-burned magnesia (DBM, 98%, industrial grade) was obtained from Dashiqiao Tianyi Refractory Materials Co., Ltd., Dashiqiao, China. Potassium dihydrogen phosphate (KH2PO4, PDP, 99.5%) and magnesium hydroxide (Mg(OH)2, 99.5%) were supplied by Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The calcium sulfate whiskers (CSW, with an average diameter of 1–8 µm and an average aspect ratio of 10–200) were supplied by Techeng New Material Technology Co., Ltd., Zibo, China. Deionized water was used as the mixing water throughout the experiments. The microstructures of each raw material at different magnifications and the XRD diffraction patterns of CSW are shown in Figure 1. The mix proportions for each group are detailed in Table 1. Q235 steel plates (150 mm × 70 mm × 1.5 mm) served as the substrate.

2.2. Preparation of MKP Coatings

Prior to the application of the coating, the Q235 steel plates were subjected to a grit-blasting process using steel grit, with the objective of eliminating surface oil, oxide scales, and rust. In accordance with the mass ratios listed in Table 1, two slurries were formulated: component A (predominantly DBM and Mg(OH)2) and component B (predominantly KH2PO4). In this formulation, Mg(OH)2 was incorporated specifically to regulate the initial setting time of the coating and to effectively suppress the hydrogen evolution reaction during the early curing stage. To ensure uniform dispersion, CSW was pre-mixed into two components using high-speed mechanical stirring (1000 rpm for 4 min) before spraying. Subsequently, two components were concurrently dispensed onto the pretreated steel surface via a dual-component pneumatic spray gun, thereby yielding coated samples. To ensure consistent coating quality, typical spray-application parameters were applied. The pneumatic air pressure was maintained at approximately 0.35 MPa using a 2.0 mm nozzle, with a spraying distance of roughly 40 cm. The coatings achieved initial setting within 1 min and were subsequently cured under ambient conditions (25 ± 2 °C, 60 ± 5% RH) for the designated periods, resulting in a final dry-film thickness of 400 ± 50 µm. The preparation process of the coating is shown in Figure 2a.

2.3. Preparation of MKP Specimens

In order to facilitate the evaluation of compressive strength and hardness, MKP slurry was cast into 20 mm × 20 mm × 20 mm silicone molds and vibrated on a laboratory vibration table for a duration of 60 s, with the objective of removing any entrapped air. Subsequent to demolding, the cubic specimens were cured in parallel with the coated panels under conditions of identical temperature and humidity, ensuring direct comparability of the resulting mechanical properties.

2.4. Characterizations

The microstructure and elemental distribution of the MKP coating were characterized using an energy dispersive spectrometer (EDS) attached to a cold-field emission scanning electron microscope (SEM, SU8600, Hitachi, Tokyo, Japan). Particularly, large-area EDS elemental mapping was employed to systematically evaluate the dispersion uniformity of the CSW within the cured matrix. The phase composition of the MKP coating was analyzed using an X-ray diffractometer (XRD, Empyrean, Panalytical, Almelo, The Netherlands) with a scanning range of 2θ = 10–80° and a scanning rate of 10°/min.

2.5. Mechanical Properties

Compressive strength testing was conducted using a compressor with a maximum load capacity of 150 kN and a loading rate of 0.5 kN/s. Each test series utilized three specimens, with the final result being the average value. The tensile bond strength between the coating and the steel plate was determined according to ASTM D4541 [55], the schematic of which is shown in Figure 2b. A high-strength two-component structural epoxy adhesive (DP460, 3M, Maplewood, MN, USA) was used to bond the pull stubs to the coating surface. This adhesive offers a nominal bond strength exceeding 20 MPa, ensuring that failure occurs strictly at the coating-substrate interface or within the coating itself, rather than in the adhesive layer. The tensile bond strength test was conducted using three sets of samples, with the final result being the average value. The impact resistance of MKP coatings with different formulations was tested using a QCJ-120A paint film impact tester (Shanghai Qigong Instrument and Equipment Co., Ltd., Shanghai, China). The impact head consisted of an 8 mm diameter steel ball, and the weight of the impact hammer was 1.0 kg. During the impact resistance test, the hammer was dropped freely from a height of 50 cm, generating an impact energy of 4.9 J. Three replicate specimens were tested for each formulation, and representative images were captured to document surface damage.

2.6. Electrochemical Properties

Electrochemical testing was conducted using a coating test cell and a CHI660E electrochemical workstation. The structure of the coating test cell is shown in Figure 2c. The conventional three-electrode system was employed in the electrochemical experiments, featuring the Ag/AgCl electrode as the reference electrode, a cylindrical graphite electrode as the counter electrode, and the MKP-coated test plate as the working electrode. The cell container comprised a cylindrical glass tube with an inner diameter of 35 mm. A circular aperture of 11.28 mm diameter was cut at the interface with the test coating, maintaining a contact area of 1.0 cm2 between the electrolyte and the coating. The electrolyte used was a 3.5 wt.% NaCl solution. During polarization curve measurements, the potential scan range relative to open-circuit potential (OCP) was set from −300 mV to 300 mV. The scan rate was 1 mV/s, with a sampling frequency of 2 Hz and a potential interval of 0.5 mV. For electrochemical impedance spectroscopy (EIS) testing, the amplitude was set to 5 mV, the frequency range spanned 105 Hz to 10−2 Hz, and the sampling interval was 0.5 s. Following electrochemical testing, the obtained EIS spectra were fitted using ZSimp Win software (Version 3.3). All electrochemical measurements were performed in triplicate to ensure reproducibility, and representative curves are presented.

3. Results and Discussion

3.1. Phase Analysis

To investigate the influence of calcium sulfate whisker (CSW) content on the phase composition and hydration of the coating, XRD diffraction analysis was conducted on samples with different CSW contents (Figure 3). The XRD patterns show that the primary crystalline phases in all coatings are MgKPO4·6H2O (K-struvite) and residual MgO, which aligns well with the typical XRD patterns of MKP systems widely reported in the existing literature [16,17]. The results indicate that the addition of CSW does not alter the type of hydration products formed in the MKP coating. Notably, the absence of Mg(OH)2 characteristic peaks in all samples indicates its high reactivity and complete consumption during the hydration process. Meanwhile, it readily reacts with KH2PO4 during hydration and is almost entirely consumed in forming the crystalline products. Furthermore, the characteristic peaks of the pristine CSW (shown in Figure 1d) are not prominent in any coating. This observation, consistent with subsequent EDS analysis, confirms that the whiskers have chemically interacted with the coating matrix, leading to the disappearance of diffraction peaks. However, in the coating with 20 wt.% CSW, the intensity of the main MgKPO4·6H2O diffraction peaks is markedly reduced compared to the unmodified MKP and other composite samples. This suggests that an excessive amount of whiskers interferes with the complete progression of the acid–base reaction, resulting in a lower content of the hydration product within the coating.

3.2. Microstructure Analysis

To examine the effect of calcium sulfate whisker (CSW) content on the coating’s microstructure, scanning electron microscopy (SEM) analysis was employed. As shown in Figure 4a,b, the MgKPO4·6H2O (K-struvite) formed within the coating exhibits a well-developed prismatic crystal habit with tightly stacked morphology [56]. This indicates that the inclusion of CSW does not significantly alter the crystal morphology of MgKPO4·6H2O. Figure 4c illustrates the interfacial microstructure between the hydration products and the CSW. The columnar K-struvite crystals act as a binding phase, encapsulating the surfaces of the whiskers and establishing strong interfacial adhesion through mechanical interlocking. This cohesive microstructure facilitates efficient stress transfer and load distribution between the whiskers and the matrix under external load, thereby mitigating local stress concentrations. However, due to a size mismatch between the whiskers and the inherent pore structure of the matrix, some residual microvoids are still observed within the coating.
To further investigate the distribution of CSW and elucidate the cause of their attenuated diffraction peaks, energy dispersive X-ray spectroscopy (EDS) mapping was conducted on the coated surface (shown in Figure 4d,e). The results for the C3 sample (Figure 4e) confirm the presence of both Ca and S elements across the scanned area, with no pronounced agglomeration, indicating a generally uniform dispersion of the whiskers within the matrix. Particularly, the spatial distributions of Ca and S are not fully coincident. In certain regions, the Ca signal appears relatively enhanced, while the S signal is diminished. This suggests that the whiskers may have undergone a surface dissolution–reprecipitation process in the pore solution of the magnesium potassium phosphate cementitious material (MKPC). When integrated into the MKP slurry, the surface of calcium sulfate whiskers experiences partial dissolution, leading to the release of Ca2+ ions into the localized pore solution. Due to the elevated concentration of phosphate species (such as PO43− and HPO42−) resulting from the swift dissolution of the KH2PO4 precursor, a pronounced chemical gradient forms around the fibers. Subsequently, the released Ca2+ ions are strongly driven to react with the surrounding phosphate species, initiating the localized reprecipitation of an amorphous calcium phosphate (ACP) phase directly on the whisker surfaces [57,58]. In contrast to conventional inert fibers (e.g., glass or basalt) that depend exclusively on relatively weak physical-mechanical interlocking, the newly precipitated ACP functions as a chemical binder. It effectively mediates the interfacial gap between the residual unreacted CSW core and the evolving K-struvite (MgKPO4·6H2O) matrix. This localized chemical anchoring not only densifies the interfacial transition zone (ITZ) through the occlusion of intrinsic micro-voids but also fundamentally transforms the fiber–matrix interaction mechanism from mere physical contact to robust chemical bonding, thereby establishing the microstructural foundation for the enhanced mechanical toughness and superior barrier performance exhibited by the optimized coatings.
For a more intuitive comparison of the microstructural differences across all formulations, Table 2 systematically summarizes the key findings from both SEM imaging and EDS mapping, with a specific focus on matrix morphology, interfacial features, and defect status.

3.3. Mechanical Properties Analysis

Compressive strength is a fundamental mechanical property that evaluates the structural integrity and load-bearing capacity of coating materials, directly reflecting the mechanical reliability of coatings under service loads. Figure 5a presents the development curve of compressive strength for MKP-coated cubes as a function of calcium sulfate whisker (CSW) content and curing time. The results imply compressive strength initially increases and then decreases with increasing CSW content. At 1 day of curing, the control sample (Group B) exhibited a strength of 21.7 MPa. The strength peaked at 34.4 MPa for the sample with 15 wt.% CSW (Group C3), representing a 58% improvement over the control. However, further increasing the CSW content to 20 wt.% (Group C4), the strength decreased to 24.0 MPa. This pattern remained consistent at 3, 7, and 28 days. After 28 days, Group C3 achieved the highest strength of 42.1 MPa, which was 35% greater than that of Group B, whereas Group C4 decreased to 29.7 MPa. These findings demonstrate that an optimal amount of CSW can significantly enhance the mechanical strength of the coating, primarily through crack-bridging and matrix-densifying effects [51]. Conversely, excessive whiskers may trigger a surface dissolution–reprecipitation process that consumes PO43− and Ca2+ ions in the pore solution. This can interfere with the subsequent crystallization of K-struvite, ultimately reducing the matrix density and long-term strength [57,58].
Bond strength is a critical parameter that reflects the interfacial adhesion between the coating and substrate, thereby playing a decisive role in the coating’s long-term durability. Figure 5b displays the bond strength evolution of coatings containing different proportions of CSW at curing ages of 1, 3, 7, and 28 days. The results demonstrate that CSW addition significantly improves the bond strength, but the effect follows a nonmonotonic trend with respect to whisker content. At all tested ages, the bond strength first increased and then decreased as the CSW content was raised from 5 to 20 wt.%. The magnitude of improvement also showed a clear dependence on curing time. During early curing (1 and 3 days), all CSW modified groups exhibited substantially higher bond strength than the unmodified control (Group B). By 28 days, the coating with 15 wt.% CSW (Group C3) achieved the highest bond strength of 3.93 MPa, which was markedly superior to the 2.82 MPa measured for Group B and to all other modified groups at the same age. This optimal performance is attributed to two synergistic mechanisms: (1) appropriate whisker content enables effective fiber bridging and crack deflection within the matrix, which optimizes stress transfer and suppresses microcrack propagation [59], (2) the trace Ca2+ released from the partial surface dissolution of CSW promotes the re-precipitation of phosphate phases at the interface, enhancing chemical anchorage [57,58]. In contrast, when the CSW content increased to 20 wt.%, the excess whiskers impeded the complete filling of hydration products, resulting in higher internal porosity, reduced continuity of the cementitious network, and a consequent decline in bond strength [57,59]. Therefore, CSW function as an efficient reinforcing phase, with an optimal content of approximately 15 wt.% for the MKPC coating system. At this content, the bond performance can be synergistically optimized throughout the entire curing cycle.
Impact resistance reflects a coating’s ability to withstand sudden dynamic loads without cracking or interfacial delamination, serving as a key indicator for evaluating the mechanical durability of coating materials. Figure 6 displays the surface damage morphologies of MKP coatings containing varying CSW dosages after 28 days of curing, subjected to a drop-hammer impact of 4.9 J (release height: 50 cm). The unmodified MKP coating (Group B) exhibited fine white micro cracks around the impact crater. In contrast, the coating containing 15 wt.% CSW (Group C3) showed a relatively smooth crater surface with no visible crack propagation. This improvement is primarily attributed to the dispersed whiskers, which effectively dissipate and redistribute the impact stress, thereby inhibiting crack initiation and growth [59]. Conversely, the coating with 20 wt.% CSW (Group C4) displayed distinct crack propagation around the impact zone, indicating inferior impact resistance compared to both Group B and Group C3. The deteriorated performance is mainly due to the excessively high whisker content, which reduces the concentration and continuity of the binding phase in the matrix, leading to increased brittleness and compromised structural integrity [57].

3.4. Electrochemical Properties Analysis

To systematically assess the corrosion protection performance of the CSW-modified MKP coatings, the corrosion resistance of each coating group after 28 days of curing was studied using electrochemical techniques. The corresponding Tafel curves obtained from the tests are shown in Figure 7a. The results show that for all coated samples, the slope of the anodic polarization curve is significantly steeper than that of the cathodic curve. This characteristic indicates that the coatings can effectively inhibit the anodic corrosion process of the steel substrate, which is a characteristic of a protective barrier coating with excellent anticorrosion properties.
Fitting the measured Tafel curves produced the corrosion potential (Ecorr) and corrosion current density (icorr) for each specimen, which are summarized in Table 3. Compared to the control group (B), groups C1, C2, C3, and C4 had positive Ecorr values of 0.00253, 0.08242, 0.16820, and 0.09901 V, respectively. A shift in the corrosion potential to higher positive values indicates a larger suppression of the electrochemical anodic reaction, showing that the introduction of CSW at an optimum concentration can effectively inhibit anodic processes and suppress corrosion [15]. Furthermore, the icorr value of the ideal sample C3 (15 wt.% CSW) was 52% lower than that of control group B, indicating a significantly improved barrier effect against corrosive media within the MKP matrix. Notably, the icorr of sample C4 (20 wt.% CSW) increased significantly compared to C3. This reversal indicates that an excessive CSW content might introduce localized defects in the coating, thus impairing the corrosion resistance [59].
To further investigate the corrosion protection mechanism of the CSW-modified MKP coatings, electrochemical impedance spectroscopy (EIS) was performed to characterize their electrochemical behavior. The EIS data were fitted employing ZSimpWin (Version 3.3) software with the equivalent circuit model R(C(R(Q(RW)))), as presented in Figure 7b. This model consists of solution resistance (Rs), coating resistance (Rf), charge transfer resistance (Rct), coating capacitance (CPE), constant phase element (Q), and Warburg impedance (W).
Figure 7c displays the Nyquist plots for the coated samples. All samples exhibit similar characteristics, consisting of a capacitive arc in the high-frequency region and a linear slope in the low-frequency region. This behavior is indicative of a diffusion-controlled corrosion process, where the gradient of the low-frequency slope corresponds to the coating’s resistance against ion diffusion [10]. The capacitive arc observed at high frequencies represents the charge-transfer processes occurring at the interface, which are directly related to the kinetics of electrochemical surface reactions [60]. Among the samples, C3 displays the largest capacitive arc radius in the high-frequency region. Since a larger radius signifies higher charge-transfer resistance, this result demonstrates that the incorporation of 15 wt.% CSW effectively impedes the charge-transfer steps on the steel surface, thereby significantly reducing the corrosion rate.
Figure 7d shows the Bode plots for each group of coated samples. The impedance modulus (|Z|) of all samples with different ratios exhibits typical characteristics as a function of frequency. In the low-frequency region (<1 Hz), |Z| decreases with increasing frequency, while in the high-frequency region (>103 Hz), |Z| tends to stabilize. In the Bode diagram, the impedance modulus at 0.01 Hz (|Z|0.01 Hz) is an important parameter for determining the coating’s shielding performance, with higher values corresponding to better protection against corrosive media [61]. The tested samples’ |Z|0.01 Hz values are ordered as follows: B is lower than C1, C2, C4, and C3. Specifically, the |Z|0.01 Hz value of group C3 (15 wt.% CSW) is 124.8% higher than that of the unmodified MKP control B. Furthermore, when the CSW content was ≤15 wt.%, the MKP coating’s shielding performance improved with higher content. However, when the CSW content reaches 20 wt.% (group C4), the coating’s |Z|0.01 Hz value significantly decreased. This decline is attributed to an excessive amount of whiskers, which disrupts the continuity of the cementitious network in the coating and thereby reduces its ability to block corrosive species [57].
Table 4 summarizes the equivalent circuit parameters obtained by fitting the EIS data. A comparative analysis of coating resistance (Rf) and charge transfer resistance (Rct) reveals a dynamic interplay between physical barrier formation and interfacial passivation, which is profoundly dependent on the CSW content. For the unmodified coating (B), the abundant and unhindered phosphate ions rapidly react with the steel substrate to form a highly continuous iron phosphate passivation layer [62]. Consequently, sample B exhibits the highest initial Rct (808.8 Ω·cm2). However, lacking micro-scale reinforcement, its intrinsic physical barrier against sustained electrolyte permeation remains relatively limited (Rf = 81.09 Ω·cm2). The initial incorporation of 5 wt.% CSW (C1) leads to a marginal increase in Rf but a sharp plunge in Rct to 289.1 Ω·cm2. This phenomenon occurs because a low whisker dosage is insufficient to form a cohesive physical barrier. Instead, it physically disrupts the continuity of the interfacial passivation film and introduces isolated fiber–matrix interfaces that act as early permeation channels. Chemically, calcium ions released from the surface dissolution of CSW locally compete with the steel substrate for phosphate ions to precipitate amorphous calcium phosphate (ACP). This competitive reaction restricts the formation of a flawless iron phosphate layer, facilitating interfacial electrolyte penetration. As the CSW content increases to the optimal 15 wt.% (C3), the macroscopic barrier effect becomes overwhelmingly dominant. Although the intrinsic passivation layer remains slightly restricted by the aforementioned competitive mechanism, the coating resistance (Rf) surges to 157.03 Ω·cm2—a 93.6% increase over the control. At this optimal dosage, the whiskers construct a dense, three-dimensional interlocking network that severely tortures the diffusion pathways for corrosive species [63]. Concurrently, the precipitated ACP phase acts as a highly effective micro-sealant, densifying the whisker–matrix interface and healing initial micro-defects. This synergistic physical and chemical sealing effectively compensates for the altered passivation layer, yielding superior overall corrosion protection. However, excessive CSW addition (20 wt.%, C4) severely disrupts the continuity of the binder phase. The overwhelming volume of whiskers introduces macro-defects and re-opens permeation pathways, leading to a simultaneous decline in both Rf and Rct.
Based on these electrochemical insights, the refined corrosion protection mechanism is schematically illustrated in Figure 8, highlighting the critical transition from an intrinsic matrix barrier highly reliant on intact interfacial passivation in the unmodified coating to a robust, synergistically sealed 3D barrier network in the optimally modified system.
From an economic and practical application perspective, it is noteworthy that although the coating containing 20 wt.% CSW (C4) exhibits marginally diminished mechanical and electrochemical performance relative to the optimal 15 wt.% formulation, its overall properties still significantly surpass those of the unmodified MKP coating (B) and the low-content groups. Given that CSW serves as a readily available and cost-effective mineral filler relative to the primary MKP matrix components (dead-burned magnesia and potassium dihydrogen phosphate), the incorporation of elevated whisker loading (e.g., 20 wt.%) enables a substantial reduction in overall raw material expenditures. Consequently, such higher-dosage formulations remain a highly viable and economically efficient alternative for large-scale engineering projects necessitating a balance between acceptable comprehensive performance and stringent budgetary constraints.

4. Conclusions

In this study, calcium sulfate whiskers (CSW) were incorporated into spray-applied magnesium potassium phosphate (MKP) coatings. The optimal CSW content of 15 wt.% significantly improved the mechanical properties, increasing the 28-day compressive strength and bond strength by 35% and 39%, respectively. Furthermore, the optimal formulation demonstrated significantly enhanced impact resistance, characterized by substantially constrained crack propagation behavior under a 4.9 J impact load. Electrochemical results revealed a 52% reduction in corrosion current density alongside a 93.6% increase in coating resistance (Rf). Crucially, this 93.6% surge in Rf signifies a profound enhancement of the macroscopic physical barrier, as the 3D interlocking whisker network severely tortures the diffusion pathways of corrosive media. This barrier improvement is synergistically driven by physical bridging and a surface dissolution–reprecipitation process that seals interfacial micro-defects. However, excessive CSW addition (20 wt.%) disrupts binder continuity and deteriorates overall performance. Ultimately, this scalable and cost-effective system holds great broader significance for the robust protection of large-scale steel infrastructure, and future research will focus on exploring the synergistic effects of CSW with secondary functional fillers, as well as evaluating their long-term durability under simulated multi-field coupled environments.

Author Contributions

Conceptualization, Y.Z.; methodology, Y.L.; software, H.C.; validation, X.G.; formal analysis, X.G.; investigation, H.C.; resources, Y.L.; data curation, H.C.; writing—original draft preparation, Y.Z.; writing—review and editing, Y.L.; visualization, Y.Z.; supervision, Y.L.; project administration, Y.Z.; funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Key Research and Development Program of China (Grant No. 2024YFB3714804), National Natural Science Foundation of China (Grant No. 52171277), Baima Lake Laboratory Joint Funds of the Zhejiang Provincial Natural Science Foundation of China (Grant No. LBMHZ24E020001), Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (Grant No. 2022SZ-TD006), Start-up funding of Ningbo Global Innovation Center of Zhejiang University (Grant No. NBCL2023X006), and Ningbo Yongjiang Talent Program (Grant No. 2024A-393-G).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Materials 19 01032 g001
Figure 1. The morphology observations of DBM, PDP, CSW and phase analysis of CSW. (a–c) SEM micrographs of DBM, PDP and CSW, respectively; (d) XRD patterns of CSW.
Figure 1. The morphology observations of DBM, PDP, CSW and phase analysis of CSW. (a–c) SEM micrographs of DBM, PDP and CSW, respectively; (d) XRD patterns of CSW.
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Materials 19 01032 g002
Figure 2. The illustration of coating process and test method. (a) Schematic diagram of MKP coating preparation; (b) method of ASTM D4541 stub pull-off bond strength test; (c) equipment of electrochemical test.
Figure 2. The illustration of coating process and test method. (a) Schematic diagram of MKP coating preparation; (b) method of ASTM D4541 stub pull-off bond strength test; (c) equipment of electrochemical test.
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Materials 19 01032 g003
Figure 3. XRD patterns of coating samples with various proportions.
Figure 3. XRD patterns of coating samples with various proportions.
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Materials 19 01032 g004
Figure 4. The morphology observation on samples B and C3. (a,b) Morphology of hydration products in samples B and C3, respectively; (c) Cross-sectional observation of sample C3; (d,e) Chemical distribution of samples B and C3, respectively.
Figure 4. The morphology observation on samples B and C3. (a,b) Morphology of hydration products in samples B and C3, respectively; (c) Cross-sectional observation of sample C3; (d,e) Chemical distribution of samples B and C3, respectively.
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Materials 19 01032 g005
Figure 5. Mechanical properties of the coatings. (a) Effect of CSW dosage on compressive strength of MKP materials under different curing days; (b) Effect of CSW dosage on MKP coating adhesion at different curing days.
Figure 5. Mechanical properties of the coatings. (a) Effect of CSW dosage on compressive strength of MKP materials under different curing days; (b) Effect of CSW dosage on MKP coating adhesion at different curing days.
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Materials 19 01032 g006
Figure 6. Surface morphology of coatings after 4.9 J impact at 28 days.
Figure 6. Surface morphology of coatings after 4.9 J impact at 28 days.
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Materials 19 01032 g007
Figure 7. Electrochemical properties of the coatings. (a) Tafel curves of coating samples after 28 days of curing; (b) The equivalent circuit model; (c) Nyquist plots of coating samples and partial enlargement of the high-frequency area; (d) Bode plots of test samples.
Figure 7. Electrochemical properties of the coatings. (a) Tafel curves of coating samples after 28 days of curing; (b) The equivalent circuit model; (c) Nyquist plots of coating samples and partial enlargement of the high-frequency area; (d) Bode plots of test samples.
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Materials 19 01032 g008
Figure 8. Schematic diagram of corrosion mechanism of MKP coating. (a) unmodified MKP coating; (b) CSW-reinforced MKP coating.
Figure 8. Schematic diagram of corrosion mechanism of MKP coating. (a) unmodified MKP coating; (b) CSW-reinforced MKP coating.
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Table 1. The mix proportions of MKP coatings.
Table 1. The mix proportions of MKP coatings.
SamplesM/PMO/MHWater-to-Cement RatioCSW/MO
B450.250
C1450.255%
C2450.2510%
C3450.2515%
C4450.2520%
Notes: “M/P” refers to the molar ratio of the reacting alkaline raw materials, which include magnesium oxide (MO) and magnesium hydroxide (MH), relative to potassium dihydrogen phosphate (P); “MO/MH” indicates the molar ratio of magnesium oxide to magnesium hydroxide; “CSW/MO” denotes the mass ratio of calcium sulfate whiskers to magnesium oxide.
Table 2. Summary of microstructural features of MKP coatings with different CSW contents.
Table 2. Summary of microstructural features of MKP coatings with different CSW contents.
SamplesMatrix MorphologyInterfacial FeaturesDefect Status
BTypical prismatic K-struvite crystals; Lacks structural continuityN/AObvious microcracks
and
intrinsic pores
C1Denser crystalline structureLimited whisker–matrix contact; isolated whiskersSome interfacial micro-voids
C2Denser crystalline structureEnhanced whisker–matrix interlockingReduced porosity
C3Highly dense matrix with ACP sealing gapsStrong interfacial bonding with distinct ACP phaseMinimal defects;
sealed micro-pores
C4Disrupted matrix continuityDisrupted binder continuity; local whisker agglomerationIncreased porosity; micro-cracks
Table 3. Corrosion potential and corrosion current density of coating samples.
Table 3. Corrosion potential and corrosion current density of coating samples.
ParametersBC1C2C3C4
Ecorr/V−0.45084−0.44831−0.36842−0.28264−0.35183
icorr/A·cm−21.223 × 10–61.172 × 10–67.116 × 10–75.872 × 10–76.643 × 10–7
Table 4. Fitting results of electrochemical impedance spectra.
Table 4. Fitting results of electrochemical impedance spectra.
SamplesRf/Ω·cm2Q/sn·Ω−1·cm−2nRct/Ω·cm2
B81.092.662 × 10–30.4112808.8
C186.031.856 × 10–30.4672289.1
C290.901.489 × 10–30.4257341.6
C3157.031.156 × 10–30.4272460.2
C4116.906.236 × 10–30.4663336.5
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Zheng, Y.; Chen, H.; Liu, Y.; Gao, X. Calcium Sulfate Whiskers Dual-Enhance Mechanical and Anti-Corrosion Properties of Magnesium Phosphate Coatings. Materials 2026, 19, 1032. https://doi.org/10.3390/ma19051032

AMA Style

Zheng Y, Chen H, Liu Y, Gao X. Calcium Sulfate Whiskers Dual-Enhance Mechanical and Anti-Corrosion Properties of Magnesium Phosphate Coatings. Materials. 2026; 19(5):1032. https://doi.org/10.3390/ma19051032

Chicago/Turabian Style

Zheng, Yaxin, Haoran Chen, Yi Liu, and Xiang Gao. 2026. "Calcium Sulfate Whiskers Dual-Enhance Mechanical and Anti-Corrosion Properties of Magnesium Phosphate Coatings" Materials 19, no. 5: 1032. https://doi.org/10.3390/ma19051032

APA Style

Zheng, Y., Chen, H., Liu, Y., & Gao, X. (2026). Calcium Sulfate Whiskers Dual-Enhance Mechanical and Anti-Corrosion Properties of Magnesium Phosphate Coatings. Materials, 19(5), 1032. https://doi.org/10.3390/ma19051032

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