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Article

Establishment of Multivalent Molybdenum Salt System and Its Effect on the Anti-Corrosion Performance of Insulating Coatings for Oriented Silicon Steel

by
Ying Liu
1,2,†,
Xiaoyu Yang
2,3,†,
Lin Wu
2,3,
Jingwang Li
2,3,
Baoguo Zhang
2,3,
Yichen Wang
2,3,
Yuan Wang
2,3,
Shicheng Ruan
2,3,
Jiageng Sun
2,3,
Xueyang Gu
2,3,
Ya Hu
2,* and
Jun Han
1,4,5,*
1
College of Resources and Environment Engineering, Wuhan University of Science and Technology, Wuhan 430081, China
2
Institute of Applied Chemistry, Wuhan University of Science and Technology, Wuhan 430081, China
3
Academy of Green Manufacturing Engineering, Wuhan University of Science and Technology, Wuhan 430081, China
4
Hubei Key Laboratory for Efficient Utilization and Agglomeration of Metallurgic Mineral Resources, Wuhan University of Science and Technology, Wuhan 430081, China
5
Hubei Provincial Industrial Safety Engineering Technology Research Center, Wuhan University of Science and Technology, Wuhan 430081, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Coatings 2025, 15(2), 176; https://doi.org/10.3390/coatings15020176
Submission received: 20 December 2024 / Revised: 27 January 2025 / Accepted: 31 January 2025 / Published: 4 February 2025
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Abstract

:
Chromium salt fillers commonly used in current anti-corrosion coatings are highly toxic. However, due to the unique high–low valence transformation and passivation mechanisms of chromium-based functional fillers and their wide applicability, chromium-free coatings find it challenging to achieve the same performance and industry acceptance. This study introduces an innovative approach that uses zinc to reduce molybdate (MoO42−) in an acidic solution, thereby forming a multivalent MoO42− system (PMZ system), and applies it to chromium-free insulating coating for oriented silicon steel. The effects of reductant dosage on the valence composition of molybdenum in the PMZ system and the corrosion resistance of the coating were investigated. Additionally, the difference in the valence composition of molybdenum between the PMZ system and the multivalent phosphomolybdate system (PMNZ system) and its impact on corrosion resistance were studied. The results indicate that the PMZ system contains trivalent molybdenum and hexavalent molybdenum, while the PMNZ system contains pentavalent molybdenum and hexavalent molybdenum. The systems leverage the reactivity of lower-valence molybdenum to delay the corrosion by reacting with oxygen while maintaining the original mechanism of molybdenum salt fillers and forming sediment with iron ions to form a passivation layer. As the content of trivalent molybdenum in the PMZ system increases, the corrosion resistance of the insulating coating improves. When the amount of zinc added in the PMZ system is 0.006 g, the relative proportion of trivalent molybdenum reaches 20.52%, and the salt spray resistance of the coating developed with the PMZ system reaches 248 h with a corrosion area of less than 5%. When the contents of the main components and sodium molybdate in the PMZ coating and the PMNZ coating are the same, the corrosion resistance of the PMZ coating, which contains trivalent molybdenum, is better than that of the PMNZ coating, and the salt spray resistance exceeds 192 h.

Graphical Abstract

1. Introduction

Metal is an indispensable material in modern industry, but it is often plagued by corrosion. Research on the anti-corrosion technology of metals has received increasing attention, with environmentally friendly anti-corrosion coatings being the focus [1]. However, it is an undeniable fact that chromium-containing anti-corrosion coatings are still widely used in industry, despite their toxicity. The insulating coating for oriented silicon steel faces this dilemma. Due to the requirements of the rolling process, the coating needs to be both high-temperature- and corrosion-resistant. There has never been a commercially available chromium-free anti-corrosion coating in this field. The reason behind this is that the anti-corrosion technology of chromium-free coatings is less effective than that of chromium-containing coatings.
Anti-corrosion coatings are typically composed of film-forming substances and functional fillers. Defects can occur in the coating due to the release of small molecular substances during the curing process and cross-linking condensation. Although there have been studies using monomers to optimize the curing process and reduce defects [2], the curing mechanism of water-based coatings determines that coating defects are difficult to avoid. The use of functional fillers has become a common choice for researchers. The unique anti-corrosion mechanism of chromium salt fillers makes them effective in addressing the problem of coating defects, even though hexavalent chromium salts (Cr6+) used in chromium-containing coatings are highly toxic [3,4]. Cr6+ forms a three-dimensional anti-corrosion mechanism by intervening in cathodic and anodic reactions and generating passivating substances through valence changes. The anti-corrosion mechanism is based on the mixed oxides containing trivalent chromium (Cr3+) and Cr6+. During the curing of coating and at corrosion sites, Cr3+ rapidly forms a passivation layer with ferrous ions (Fe2+), iron ions (Fe3+), and O2−. When the coating is damaged, soluble Cr6+ is released, coupled with a reduction reaction at the cathode and a metal dissolution reaction at the anode, generating new Cr3+ and passivation substances to repair coating defects [5,6,7,8,9]. It is worth noting that although it is considered to have self-healing properties, the conversion of Cr6+ to Cr3+ is an irreversible behavior in the anti-corrosion mechanism. When Cr6+ is completely released, the coating will lose its self-healing performance [10]. It can be seen that the key to achieving the anti-corrosion effect of chromium-like salt is to use valence conversion to block both the cathodic and anodic reactions of electrochemical corrosion and form multiple anti-corrosion mechanisms.
Chromium-free functional fillers can also improve corrosion resistance by filling defects and preventing electrochemical reactions [5]. Based on this, functional fillers can be classified into filling and consumption types. The filling type can delay the penetration of corrosive media and usually has a low density and high stability, such as micro/nano-materials like silica [11], fiber material [12], and graphene [13,14,15]. However, the compatibility between functional fillers and the coating is limited, and excessive addition can lead to a deterioration in corrosion resistance. The consumption type can intervene in corrosion reactions, interrupting the original electrochemical corrosion process, such as by replacing anodes [16,17,18] or forming passivation layers through reactions with fFe2+ and Fe3+ [19,20]. The typical representative of the former is zinc (Zn) in zinc-rich coatings. Zn in zinc-rich coatings not only acts as a sacrificial anode but can also be converted into zinc oxide (ZnO) through reactions to fill coating defects. The Zn content is very important for the anti-corrosion performance of the zinc-rich coating. If the amount added is too small, a conductive path cannot be formed between the fillers to connect with the substrate, and it is impossible to ensure that the metal substrate is at the cathode. At this point, Zn will become the starting point of the pitting area. In reports related to fillers with passivation effects, the molybdate (MoO42−) and phosphomolybdate (PMo12O403−) are particularly noteworthy, as Mo is a congener of Cr. Nonetheless, current research suggests that their mechanism is different from that of Cr. The action mode of MoO42− is to compete with chloride ions (Cl−) in the corrosive medium through stronger cation adsorption, react with Fe2+ and Fe3+, form a passivation layer, and exhibit a certain degree of self-healing function [10,21,22]. However, its oxidation is weak, and the passivation layer cannot efficiently modify defects in the same manner as the chromium passivation layer. MoO42− can form both homopolyacid salts and heteropolyacid salts [23], among which PMo12O403− has attracted more attention [24,25]. Its mechanism is identical to that of MoO42− [26], and its corrosion resistance is better than that of MoO42−. This is because PMo12O403− has stronger oxidability than MoO42−, and the passivation layer forms more rapidly under acidic conditions [27]. Related studies also indicate that the formation of PMo12O403− requires specific reaction conditions [28,29]. To ensure that the structure of PMo12O403− is retained in the final coating, other researchers have first prepared PMo12O403− and then added it to the coating [30,31]. When comparing the mechanisms of molybdenum and chromium salts, the singularity of the molybdenum salt mechanism is the fundamental reason for the difference in effectiveness. However, considering oxidation as a key factor affecting the anti-corrosion performance of molybdenum salts is also one of the reasons limiting their performance improvement. The Mo in MoO42− and PMo12O403− is hexavalent (Mo6+), and this oxidation limit restricts the design of functional fillers. For this reason, previous research has adopted composite fillers to enhance the release of molybdenum salts [32,33,34]; however, the application system is limited, and the anti-corrosion performance still needs improvement. In short, compared with other chromium-free functional fillers with similar passivation mechanisms, the advantage of molybdenum salt fillers is that MoO42− and PMo12O403− have stronger cation adsorption, which can form a denser passivation layer and weaken the negative impact of Cl− on the anti-corrosion process. However, compared with chromium salt fillers, molybdenum salt fillers have a more passive anti-corrosion mechanism; they enter the passivation layer formation process after the formation of Fe2+ or Fe3+. The multiple anti-corrosion mechanisms of chromium salt fillers make them more active and able to actively intervene in corrosion electrochemical reactions after the penetration of corrosive media. Therefore, making the mechanism of molybdenum salt fillers more active by introducing low-valance Mo to consume the corrosive media would be an effective method to improve the anti-corrosion performance. In fact, multivalent molybdenum materials have been applied in fields such as energy [35,36], biological medicine [37,38], and information technology [39], and their mechanisms utilize the conversion between high and low valances in multivalent molybdenum systems. Some researchers have also observed that the presence of low-valance molybdenum contributes to the anti-corrosion performance of the molybdenum-based conversion coatings [40]. The low-valence molybdenum in their coatings is not the original intention of formulation design but rather formed in small amounts during passivation treatment. Unlike passivation treatment, the curing process of water-based coatings does not spontaneously produce low-priced molybdenum, so it is necessary to actively form multivalent molybdenum salt fillers.
Referring to the mechanism of chromium salts, the reductant was introduced during the preparation of the molybdenum salt to construct a multivalent molybdenum salt system in this manuscript. By introducing low-priced Mo, the mechanism of action of molybdenum salt fillers can be shifted from passive corrosion prevention by reacting with Fe2+ or Fe3+ to a more active mechanism that is similar to chromium salts, which simultaneously consumes corrosive media and forms a passive layer. The influence of the valence composition of molybdenum in multivalent MoO42− systems on the corrosion resistance of chromium-free insulating coatings on oriented silicon steel was studied. The valence composition of molybdenum in the multivalent MoO42− and multivalent PMo12O403− systems and their corrosion resistance were compared, which aids in attaining an in-depth understanding of the anti-corrosion mechanism of molybdenum.

2. Materials and Methods

2.1. Materials

The silica sol (SiO2·nH2O) was provided by Shandong Yinfeng Nano New Materials Co., Ltd., Jinan, Shandong, China. The content of silica (SiO2) is 30 wt%. The particle size of SiO2 is 10–25 nm. The pH is 3. The aluminum dihydrogen phosphate (Al(H2PO4)3) solution was purchased from Zhengzhou Yucai Phosphate Factory, Zhengzhou, Henan, China. The content of aluminum oxide (Al2O3) is 7.5–8.5 wt%, and that of phosphorus pentoxide (P2O5) is 32–34 wt%. The magnesium oxide (MgO, AR), sodium molybdate dihydrate (Na2MoO4·2H2O, AR), phosphoric acid (H3PO4, AR), potassium bromide (KBr, SP), and sodium chloride (NaCl, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China. The nitric acid (HNO3, AR) was provided by Wuhan Zhong Tian Chem Co., Ltd., Wuhan, China. The zinc powder (Zn, 99.99%) was provided by Hunan New Welllink Advanced Metallic Material Co., Ltd., Changsha, China. One-side-polished single-crystalline silicon wafers were purchased from the Beijing General Research Institute for Nonferrous Metals (GRINM). The deionized water and ultrapure water were homemade.

2.2. Multivalent Molybdenum Salt System and Coating Preparation

2.2.1. Multivalent MoO42− System and Coating

In this study, chromium-free inorganic insulating coating for oriented silicon steel was selected as the application target. The metal substrate used is oriented silicon steel. The usage method and curing requirements for the coating can be referred to in our previous research [41]. The coating primarily consists of Al(H2PO4)3 solution and SiO2·nH2O, with minor amounts of MgO and bentonite serving as additives, and is referred to as the base coating. The preparation of the solution system and coating in this manuscript was carried out at room temperature, and the coating thickness formed was 1.1–1.3 μm.
The reduction of Na2MoO4 requires the participation of hydrogen ions (H+). To investigate the effects of molybdenum valence composition in the multivalent MoO42− system on the coating’s corrosion resistance, modifications were made to the coating preparation process and the content of Al(H2PO4)3 solution in the formulation. Firstly, Na2MoO4·2H2O was dissolved in deionized water to prepare a 16.67 wt% Na2MoO4 aqueous solution. Subsequently, 0.9 g of the Na2MoO4 solution was added dropwise to 18.99 g of Al(H2PO4)3 solution under continuous stirring conditions until the solution became clear and transparent. Finally, Zn was added to the solution, followed by stirring for 5 min and ultrasonic dispersion for 1 h until the solution’s color stabilized and no bubbles formed. This result was named the PMZ system. The Zn was added at varying levels of 0 g, 0.003 g, 0.006 g, 0.009 g, 0.012 g, and 0.015 g, and the resulting mixtures were referred to as the PMZ0, PMZ0.03, PMZ0.06, PMZ0.09, PMZ0.12, and PMZ0.15 systems, respectively. It is noteworthy that upon the addition of Zn, the solution becomes turbid during stirring, accompanied by the gradual formation of bubbles. During ultrasonic dispersion, the solution transitions to a light brown color and exhibits the generation of a lot of bubbles. As the Zn content increases, the final color of the PMZ system changes from light brown to dark brown.
To prepare the anti-corrosion coating containing the multivalent MoO42– system (PMZ coating), the PMZ system was supplemented with SiO2·nH2O and additives in the same proportions used in the base coating. Deionized water was added to adjust the total mass to 100 g. Then, the mixture was stirred at 800 r/min for 30 min and filtered through a 250 mesh sieve to obtain the PMZ coating.

2.2.2. Multivalent PMo12O403− System and Coating

To compare the differences between the multivalent PMo12O403− and MoO42− systems, a multivalent PMo12O403− system was also prepared. The preparation of the PMo12O403− system is based on a method used in the industry [42].
First, 29.05 g of Na2MoO4·2H2O was dissolved in 64.09 g of deionized water to prepare a Na2MoO4 solution. Subsequently, 0.98 g of freshly opened H3PO4 was slowly added dropwise to the solution, with continuous stirring. Following this, 13.53 g of freshly opened HNO3 was slowly added to the solution to adjust the pH to 3–4. Finally, 1 g of Zn was added. The solution was stirred for 5 min, ultrasonically dispersed for 1 h, and filtered to obtain the PMNZ1 system. The solution prior to the Zn addition was designated as the PMNZ0 system. For application, 0.38 g of PMNZ1 system was mixed into 9 g of the Al(H2PO4)3 solution. The mixture was supplemented with SiO2·nH2O and additives in the same proportions as the base coating and adjusted with deionized water to a total mass of 100 g. After stirring at 800 r/min for 30 min and filtering through a 250 mesh sieve, the anti-corrosion coating with the multivalent PMo12O403− system (PMNZ1 coating) was obtained.
During this process, the addition of HNO3 caused the solution color to gradually change from colorless and transparent to yellow. Upon the addition of Zn, the solution color transitioned from yellow to green and finally to deep blue with increasing Zn content. Once the solution turned dark blue, further Zn addition did not alter the color, and solid residues were observed.

2.2.3. Control System and Coating

To facilitate comparison among the PMNZ1, PMZ, and base coating, the composition of the PMZ system was adjusted. The adjusted system, which was designated as PMZ’0.1, consisted of 1.17 g Na2MoO4·2H2O, 9.34 g deionized water, 95.49 g Al(H2PO4)3 solution, and 0.1 g Zn. The solution prior to Zn addition was designated as PMZ’0. For application, 10 g of PMZ’0.1 was mixed with SiO2·nH2O and additives in the same proportions as the base coating. Deionized water was then added to adjust the total mass to 100 g, yielding the PMZ’0.1 coating. Under these conditions, the contents of Al(H2PO4)3, SiO2·nH2O, and additives were consistent across the PMNZ1, PMZ’, and base coating, respectively.

2.3. Characterization

The chemical composition and valence states of molybdenum in the systems were analyzed using Fourier-transform infrared spectroscopy (FT-IR, Vertex 70, Bruker, Germany) and X-ray photoelectron spectroscopy (XPS, Al Kα, AXIS SUPRA+, Shimadzu, Japan).
The preparation method for infrared test samples is as follows: First, spectral purity KBr is thoroughly ground to ensure fine and uniform particles. Then, the KBr powder is pressed into a transparent pellet using a pellet press. Next, a small amount of the test liquid is dropped onto the center of the prepared KBr pellet, and the surface is gently blown to remove excess liquid. After completing these steps, the sample is ready for FT-IR analysis.
The preparation process for XPS samples is as follows: First, a one-side-polished single-crystal silicon wafer with a thickness of 0.5 mm is cut into 5 mm × 5 mm squares. These wafers are sequentially immersed in ultrapure water, acetone, ethanol, and ultrapure water, undergoing ultrasonic cleaning for 15 min in each solution. After cleaning, the silicon wafers are dried with compressed air and set aside. Glass slides are then prepared according to the number of samples needed, rinsed sequentially with ultrapure water, ethanol, and ultrapure water, and dried in an oven. The silicon wafer is placed onto the prepared glass slide, and 2 to 3 droplets of freshly prepared PMZ or PMNZ system are dispensed onto the wafer using a 1 mL rubber dropper. The sample is then placed in a vacuum drying oven, where the vacuum level is adjusted to –0.085 mPa and the temperature set to 80 °C. After vacuum drying for 24 h, the sample is ready for XPS analysis.
The corrosion resistance of the coating was tested using the neutral salt spray (NSS) method specified in GB/T 10125-2021. The concentration of NaCl solution used is 50 ± 5 g/L. The pH of the NaCl solution is 6.5–7.2. The ambient temperature is maintained at 33~37 °C. The sedimentation volume of salt spray is between 1 and 2 mL/h. The experiments utilized oriented silicon steel sheets measuring 150 mm × 100 mm. The edges and the uncoated backsides of the sheets were protected with transparent tape. Two methods were employed to evaluate the corrosion resistance; one was recording the salt spray duration required for the corrosion area to reach 5%, and the other was measuring the corrosion area after a predefined salt spray duration.

3. Results and Discussion

To investigate the corrosion resistance of the PMZ coating on oriented silicon steel sheets, the NSS test was conducted. The time required for the coating to reach a 5% corrosion area was recorded. Figure 1 illustrates the effect of the Zn mass fraction on the corrosion resistance duration of the PMZ coating. The corrosion resistance duration exhibits an oscillatory trend concerning the Zn mass fraction. In this experiment, the Na2MoO4 was consistently maintained at 0.15 wt%. When the mass of Zn added was 3 mg, the corrosion resistance of the coating was weaker than that of the coating without Zn. When the mass of Zn added was 6 mg, the best corrosion resistance was observed. Subsequently, the further increase in zinc content reduced the corrosion resistance of the coating. When the mass of Zn added exceeded 12 mg, the effect of zinc on corrosion resistance tended to stabilize without significant changes. When the mass of Na2MoO4 used was consistent, as designed, with the increase in Zn dosage, the amount of low-valence molybdenum in the coating would gradually increase. It can be hypothesized that when 3 mg of Zn was added, the small amount of low-valence molybdenum acted as a localized point of corrosion initiation, thus weakening the coating’s corrosion resistance. This phenomenon is similar to that observed in zinc-rich coatings, where insufficient Zn content fails to provide adequate corrosion protection. Obviously, this analysis has limitations. It can be postulated that, similar to zinc-rich coatings, insufficient amounts of low-valence molybdenum acting as sacrificial anodes may reduce the coating’s corrosion resistance. However, the observation that increasing the Zn dosage, which should increase the low-valence molybdenum, also detracts from corrosion resistance suggests that the initial hypothesis does not fully explain the experimental results.
To better understand the change in the corrosion resistance of this system, a more detailed analysis of the molybdenum species and valence states in the PMZ system is necessary. XPS spectra and Mo 3d high-resolution spectra were measured to investigate the molybdenum species and valence states in the PMZ system. Figure 2a,c display the XPS full spectra of the PMZ0 and PMNZ0.06 systems. It can be observed that the peak shapes for PMZ0 and PMNZ0.06 are similar, with prominent peaks for O 1s, C 1s, and P 2p. The Al 2p peak is also noticeable. However, the Mo 3d and Na 1s peaks are relatively weak, and a very weak Zn 2p peak is also detected in the PMNZ0.06 sample. These findings align with the composition of the PMZ system. The PMZ system was prepared using Al(H2PO4)3 solution, Na2MoO4, and water. Since Na2MoO4 was added in smaller amounts, the signal from Al(H2PO4)3 elements is stronger, while the signal from Na2MoO4 elements is weaker. Additionally, because the proportion of Al in the Al(H2PO4)3 solution is lower than that of P, the intensity of the Al 2p peak is weaker than that of the P 2p peak.
The fitting results are shown in Figure 2b,d. Despite the high spectral noise and large fluctuations due to the small amount of Na2MoO4− added to the PMZ system, the peak shapes of the PMZ0 and PMZ0.06 samples are clearly different. The binding energy range of the accompanying peaks in the PMZ0.06 spectrum is lower than that of the accompanying peaks in the PMZ0 spectrum. This indicates that the molybdenum species in PMZ0.06 differ from those in the PMZ0 system. In Figure 2b, the molybdenum species are identified as MoO42− and Na2MoO4·2H2O. In the PMZ0 system, molybdenum exists solely as MoO42−. During the vacuum drying process, some Na2MoO4 adsorbed crystallization water that was not fully expelled. This is reasonable considering that anhydrous Na2MoO4 is more difficult to prepare, and Na2MoO4·2H2O is a more commonly used reagent. It is plausible that crystallization water remains in the sample during the short-duration vacuum drying process. After calculation, the relative content of MoO42− in the PMZ0 sample is 41.13%, while the content of Na2MoO4·2H2O is 58.87%. Based on Figure 2d, the PMZ0.06 sample contains both MoO42− and Mo3+, indicating that the molybdenum in the PMZ0.06 system exists in both the MoO42− and Mo3+ forms. This also confirms that the addition of Zn causes the MoO42− in the PMZ system to be reduced to Mo3+, as shown in the following reaction equation [43](1):
2 MoO 4 2 - + 3 Zn + 16 H + → 2 Mo 3 + + 3 Zn 2 + + 8 H 2 O
The source of H+ in the reaction is the free H3PO4 present in the Al(H2PO4)3 solution. To analyze the effect of Zn addition on the composition of the PMZ system, the Mo 3d high-resolution spectra of all PMZ samples were fitted. The results are summarized in Table 1. As shown in Table 1, there is Mo3+ in all PMZ samples except PMZ0, and Na2MoO4·2H2O is no longer present. Based on the amount of Zn added to the PMZ system, the effect of Zn addition on the relative proportion of Mo3+ is illustrated in Figure 3. In Figure 3, the relative proportion of Mo3+ increases with the addition of Zn up to a certain point, then decreases. When the Zn addition exceeds 0.012 g, the relative proportion of Mo3+ stabilizes. The inflection point of the curve occurs at a Zn addition of 0.006 g, corresponding to the PMZ0.06 sample.
During the preparation of the PMZ system, it was observed that gas was generated after adding Zn. Due to the high viscosity of the Al(H2PO4)3 solution, the addition of a small amount of Zn tends to accumulate at the bottom of the container, and the color change is more easily observed than the gas generation. Although gas production is slower compared to the color change, it can be observed throughout the reaction. The endpoint of the reaction is marked not only by the cessation of the color change but also by the absence of further gas evolution. This indicates that Zn is involved in reactions beyond that mentioned in Reaction (1). Possible additional reactions in the system may include the following reaction:
Zn + 2 H + → Zn 2 + + H 2 ↑
Reactions (1) and (2) represent a competitive relationship. In the PMZ series systems, the amount of Na2MoO4 added is small. When the amount of Zn added exceeds a certain point, more Zn participates in the reaction described by Reaction (2), consuming H3PO4 and H+ in the system. This reduces the available H+, thereby slowing down Reaction (1). Consequently, as the amount of Zn increases, the Mo3+ content in the PMZ system decreases. It is noteworthy that the characteristics presented in Figure 3 explain the changes in the corrosion resistance of the PMZ series coatings. The corrosion resistance of the coating decreases with increasing Zn content when the Zn content exceeds 0.06 wt‰ due to the decrease in the Mo3+ content in the PMZ system and, consequently, in the PMZ coating. The reduction in the functional component (Mo3+) leads to a loss in the corrosion resistance of the coating.
Therefore, the use of Zn can partially reduce MoO42− in acidic systems, where Mo6+ is reduced to Mo3+. As the mass of Zn added to the PMZ0 system increases, the content of Mo3+ first increases and then decreases. The anti-corrosion performance of the coating increases with the increase in Mo3+ content. When the mass of Zn is 6 mg, the relative proportion of Mo3+ in the system reaches 20.52%, and the corrosion resistance time in NSS is as long as 248 h.
In the anti-corrosion coatings industry, PMo12O403− is also a typical representative of molybdenum salt fillers. For comparison, a PMNZ1 coating was prepared using the obtained multivalent PMo12O403− system. To ensure consistency in the amounts of Al(H2PO4)3 and Mo in the comparison system and facilitate the comparison of molybdenum salt species and valence states, the composition of PMZ was adjusted and labeled as PMZ’. The prepared PMZ’0.1 system was light brown, while the PMNZ1 system was dark blue. The resulting PMZ’0.1 coating was light yellow, while the PMNZ1 coating was light blue. Using an XPS analysis, the Mo composition in PMZ’0.1 was found to be 87.34% MoO42− and 12.62% Mo3+, which is consistent with the composition characteristics of the PMZ system (Figure S1). The primary components in both coatings, such as Al(H2PO4)3, SiO2·nH2O, and additives, were used in the same amounts, but the Zn content differed slightly. The Zn mass fraction in PMZ’0.1 was 0.0094%, while in PMNZ1, it was 0.0038%. Compared to other factors, the effect of Zn on the corrosion resistance of the coatings was relatively small. At this point, the comparison focuses on the impact of the form of molybdenum salt on the coating’s corrosion resistance. Figure 4 shows the morphology of the oriented silicon steel sheets coated with base, PMNZ1, and PMZ’0.1 coatings and their morphologies after 192 h of the NSS test. Figure 4a–c present the photos before the NSS test. It can be observed that although both the PMNZ1 and PMZ’0.1 coatings show some coloration, their steel sheets are similar to those with the base coating. The coatings are relatively thin, approximately 1 μm, and the markings from the rolling process are still visible. After 192 h of the NSS test, the base coating exhibited the largest corrosion area, reaching 16%. The corrosion area of the PMNZ1 coating is smaller but still exceeds 5%, reaching 8.5%. Most of the corrosion points are light yellow, which is characteristic of the early stages of corrosion. The PMZ’0.1 coating, however, shows a corrosion area of less than 5%, reaching 2.5%, indicating that the coating still possesses excellent corrosion resistance. Among the three coatings, the PMZ’0.1 coating demonstrated the best corrosion resistance. It is noteworthy that the areas of corrosion on both the PMNZ1 and PMZ’0.1 coatings did not develop along the laser-engraved marks, which were introduced during the rolling process to reduce iron loss. Typically, when the corrosion resistance of a coating is poor, these marks expose part of the metal substrate, and rust often propagates along the exposed substrate. However, in the case of the PMZ’0.1 and PMNZ1 coatings, the rust did not spread along the etched marks, indicating that, despite the metal substrate experiencing some exposure, the coatings still provided effective corrosion protection.
The PMo12O403− is generally considered to exhibit superior corrosion resistance [30]. However, in the comparison between the PMNZ1 and PMZ’0.1 coatings, the PMZ’0.1 coating demonstrated better corrosion resistance. To further understand the reason behind this, a compositional analysis of the PMNZ and PMZ’ systems was conducted. Initially, the FT-IR was used for a preliminary analysis of the PMNZ0 and PMZ’0 samples. The results are shown in Figure 5. The dark curve represents the PMZ’0 system, and the red curve represents the PMNZ0 system. Both spectra exhibit peaks at 3400 cm−1 and 1640 cm−1, corresponding to the stretching and bending vibrations of –OH groups. In the PMNZ0 spectrum, peaks at 1380 cm−1, 1059 cm−1, and 752 cm−1 are observed, which correspond to the asymmetric stretching, symmetric stretching, and in-plane bending vibrations of the nitrate ion (NO3−). Additionally, peaks at 929 cm−1 and 862 cm−1 represent Mo-O vibrations in the PMo12O403− structure [44,45,46,47,48]. The peak at 800 cm−1 is also believed to correspond to Mo-P-Mo bonding [49]. In contrast, in the PMZ’0 spectrum, peaks at 1127 cm−1, 987 cm−1, and 509 cm−1 are observed, which are attributed to the symmetric stretching of P-O2, symmetric stretching of P-O3, and asymmetric bending vibration of P-O, respectively [50]. Notably, there are no peaks corresponding to the PMo12O403− structure in the PMZ’0 spectrum, while these characteristic peaks are present in the PMNZ0 spectrum. This suggests that the forms of Mo in the two systems are different.
Based on these observations, XPS was used to further analyze the PMNZ system, as shown in Figure 6. By comparing Figure 6a with Figure 2a, it is observed that the main elements in the two spectra differ. In the PMNZ0 spectrum, strong Mo 3d, Na 1s, N 1s, C 1s, and O 1s peaks are evident, while the intensity of the P 2p peak is very low, and the Al 2p peak is not detected. This corresponds well with the composition characteristics of the PMNZ0 system, which is prepared from Na2MoO4, H3PO4, HNO3, and water. Since H3PO4 is used in small amounts, the signals from Na2MoO4 and HNO3 are stronger, while the signal from H3PO4 is weak, and the Al signal cannot be detected. In Figure 6b, the orange dot-dashed line representing the original data is smoother and less fluctuating compared to Figure 2b. This is because, in the PMZ system, Na2MoO4 is added in small amounts, resulting in weak detection signals and higher spectral noise, while in PMNZ0, Na2MoO4 is added in larger quantities, leading to stronger signals and smoother spectra. Additionally, in Figure 6b, the forms of Mo are MoO42− and PMo12O403−, with the peak area representing PMo12O403− being larger. This indicates that in the PMNZ0 system, Na3PMo12O40 was successfully synthesized, with PMo12O403− being the predominant species in the system, although MoO42− is also present. Figure 6c,d presents the XPS full spectrum and Mo 3d high-resolution spectrum for the PMNZ1 system. Comparing Figure 6a with Figure 6c, it can be observed that the elemental characteristics of PMNZ1 are consistent with those in Figure 6a. Since the amount of Zn added is relatively small compared to other components, the Zn 2p peak signal is weak. When comparing Figure 6c with Figure 2c, we notice that the Mo 3d original spectrum for PMNZ1 is smoother, whereas the Mo 3d spectrum for PMZ0.06 has more noise. This difference is influenced by the lower addition of Na2MoO4 in the PMZ0.06 system. Based on the fitted spectra, MoO42−, PMo12O403−, and Mo5+ are present in PMNZ1.
The phenomenon corresponding to the analysis above can also be observed in the P 2p high-resolution of the PMZ0.06 and PMNZ1 systems (Figure S2). In Figure S2a, the spectrum of P 2p is relatively smooth, consisting of peaks of P 2p3/2 and P 2p1/2 representing the hydrogen phosphate salt (H2PO4−). This is consistent with the composition characteristics of the PMZ0.06 system, where the addition of Al(H2PO4)3 solution is higher and exists in the form of H2PO4−. In Figure S2b, the spectrum of P 2p exhibits significant fluctuations, and two groups of peaks can be identified during fitting. One represents phosphate ions (PO43−), and the other represents PMo12O403−. This further confirms the presence of PMo12O403− in the PMNZ system. In addition, the spectral differences among Figure 6c, Figure 2c, and Figure S1b suggest that although the Mo is hexavalent in both MoO42− and PMo12O403−, the reduction of these species leads to different products. For PMo12O403−, the reaction is described in Reaction (3).
P M o 12 O 40 3 − + 4 Zn + 8 H + → P M o 12 O 36 3 − + 4 Zn 2 + + 4 H 2 O
PMo12O403− appears blue after reduction, also known as molybdenum blue. It is a complex mixture composed of PO43−, Mo6+, and Mo5+. PMo12O363− is a possible expression of the mixture. To clearly compare the relative proportions of Mo chemical states before and after reduction in the PMNZ system, the fitting results for the PMNZ0 and PMNZ1 systems are summarized in Table 2. In PMNZ0, the relative proportion of PMo12O403− reached 81.56%, which confirms that using the preparation method described in Section 2.2.2 yields a solution predominantly consisting of Na3PMo12O40. In contrast, according to Table 1, PMZ0 contains 41.13% MoO42− and 58.87% Na2MoO4·2H2O.
It is noteworthy that despite the PMNZ0 system containing more Na2MoO4, the fitting results show that the PMZ0 sample contains Na2MoO4·2H2O, whereas no Na2MoO4·2H2O was detected in the PMNZ0 sample. This difference can be attributed to the high viscosity of the Al(H2PO4)3 solution in the PMZ series, which makes it more difficult for water to evaporate during vacuum drying. The Al(H2PO4)3 in the outer layer of droplets forms a membrane, which further traps moisture inside, leading to the retention of crystallization water. In contrast, the PMNZ series primarily contains Na2MoO4 and water, which have lower viscosity and allow for the faster evaporation of moisture during vacuum drying. Additionally, there is no membrane formation during the drying of the outer layer of droplets in the PMNZ series, and this will not interfere with the escape of inner moisture, thereby preventing the presence of crystallization water. In PMNZ1, there are 37.12% MoO42−, 30.51% PMo12O403−, and 32.37% Mo5+. Compared to PMNZ0, the proportion of MoO42− has increased, the proportion of PMo12O403− has decreased, and Mo5+ has appeared. This indicates that Zn is indeed able to reduce Mo6+ in the PMNZ system to Mo5+. The increase in the proportion of MoO42− can be attributed to two factors: Firstly, the reduction reaction breaks the structure of PMo12O403−, and during this process, part of Mo6+ is reduced to Mo5+, while the remainder remains in the MoO42− form. Secondly, the addition of Zn increases the pH of the solution, which can also cause the structure of PMo12O403− to break down. Under the combined effects of these two factors, the final proportion of MoO42− in PMNZ1 is higher than that in PMNZ0.
Therefore, although Zn can also reduce the Mo in PMo12O403− systems, the obtained low-valence Mo is Mo5+. Under the conditions of the same main composition of the coating and the same amount of Na2MoO4 used, the PMZ system containing Mo3+ has better corrosion resistance. This is different from the common understanding of the anti-corrosion mechanisms of MoO42− and PMo12O403−, indicating that the mechanism of molybdenum salt functional fillers still needs to be improved.
The process of establishing and applying the multivalent molybdate system is illustrated in Figure 7. The addition of Zn to both the PMo12O403− and MoO42− systems indeed reduces Mo6+ to lower the valence, but the reduction products in the two systems are different. As shown in the figure, there is also a significant visual difference between the two systems. The multivalent PMo12O403− system appears dark blue, while the multivalent MoO42− system appears transparent brown. In the PMo12O403− system, Mo6+ is reduced to Mo5+, and in the MoO42− system, Mo6+ is reduced to Mo3+. Specifically, PMNZ1 contains MoO42−, PMo12O403−, and Mo5+, while the PMZ’0.1 coating contains MoO42− and Mo3+. Among these, PMo12O403− is considered to have a stronger oxidizing property than MoO42−. From the perspective of valence, Mo5+ is higher than Mo3+, indicating stronger oxidation. When the same amount of Na2MoO4 is used, based on the current mechanisms of MoO42− and PMo12O403−, the PMNZ1 coating is expected to exhibit better corrosion resistance. However, the aforementioned NSS experimental results suggest that the use of a multivalent molybdenum salt system can optimize the corrosion resistance, and the PMZ’0.1 coating performs better in terms of corrosion resistance. This further indicates that the oxidizing property should not be regarded as the core influencing factor of the mechanisms of MoO42− and PMo12O403−.
Several possible reasons for this discrepancy can be identified. The base coating used in this manuscript is composed of silica as the main structure, while additives and various condensation products of Al(H2PO4)3 form a network that fills the inter-particle pores of the silica, resembling the continuous phase in the coating. During service, the base coating delays the penetration of corrosive media through the “labyrinth effect”, and the electrochemical process of corrosion remains unchanged. Therefore, after the corrosive media penetrate, corrosion reactions occur and form pitting corrosion, and the corrosion products are usually hydroxides or oxides containing Fe2+. Under the action of chloride ions, this layer is usually relatively loose and unable to serve as a passivation layer, allowing the corrosion to gradually spread and deepen. In contrast, the low-valence Mo in the PMNZ1 and PMZ’1 coatings can react with the corrosive media during the formation of the passivation layer, oxidizing to Mo6+. This process, as shown by the possible reaction equation in the figure, implies that after the corrosive media penetrate, although corrosion occurs in the localized areas where the corrosive liquid accumulates, the corrosive media are also consumed by the oxidation reaction. The corrosion products precipitate with the PMo12O403− or MoO42− to form a passivation layer. As a result, both the anodic and cathodic reactions of the electrochemical corrosion process are hindered, thereby enhancing the corrosion resistance of the coating. Mo3+ has stronger reducibility compared to Mo5+, allowing it to react more effectively with the corrosive media, thus slowing down the corrosion during the passivation layer formation phase. This leads to the formation of a more compact passivation layer. Ultimately, the PMZ series coatings, which contain lower-valence Mo, exhibit better corrosion resistance. It is worth noting that the function of low-valence molybdenum is to consume oxygen, which is similar to the sacrificial anode mechanism of Zn in the zinc-rich coating. It can be found that low-valence molybdenum also has the defect of damaging the anti-corrosion performance of the coating when the addition amount is too small. This is reflected in the poorer anti-corrosion performance of PMZ0.03, PMZ0.12, and PMZ0.15 coatings compared to PMZ0 coating.
When compared with other types of chromium-free fillers and coatings, the PMZ system and coating also have prominent advantages. Table 3 presents the coatings mentioned in recent reports on chromium-free anti-corrosion coatings. The comparison mainly involves the types of coatings, types of functional fillers, filler content, film thickness, types of metal substrates, and NSS duration. The amount of filler added to the PMZ coating here is expressed in terms of the mass of Na2MoO4 and Zn used.
PMZ coating is a water-based inorganic coating. Compared with water-based semi-organic coatings containing resins, water-based inorganic coatings have better heat resistance and avoid the presence of volatile organic compounds (VOCs). Meanwhile, although PMZ coating does not have the longest corrosion resistance time, it uses fewer fillers and has a thinner film thickness, and the corrosion resistance of the oriented silicon steel used is inferior to other metal substrates. This indicates that PMZ coating is lower-cost and has more efficient anti-corrosion performance.

4. Conclusions

In this research, a multivalent molybdate system was successfully established by using Zn and applied to chromium-free inorganic coatings for oriented silicon steel. The results show that Zn can reduce Mo6+ to Mo3+ in the MoO42− system and reduce Mo6+ to Mo5+ in the PMo12O403− system. In the MoO42− system, the relative proportion of Mo3+ increases with the addition of Zn up to a certain point, then decreases and eventually stabilizes. Additionally, the establishment of the PMo12O403− system requires adhering to a 1:12 molar ratio of P to Mo during the preparation process. When the amounts of the main components, such as Al(H2PO4)3 and Na2MoO4, in the coating are consistent, the multivalent MoO42− system demonstrates better corrosion resistance than the PMo12O403− system. The corrosion resistance of the coating is enhanced with the increase in Mo3+ content. This is because the lower-valence molybdenum species can react with the corrosive media, optimizing the formation of the passivation layer and effectively delaying the corrosion process. The lower the valence state of Mo, the stronger its reducing ability, leading to higher reaction efficiency with the corrosive medium and a more significant delay in the corrosion process, thereby enhancing corrosion resistance. In the NSS test, the coatings with the multivalent MoO42− system showed a corrosion area of less than 5% after 192 h. Although the process of using Zn to reduce the molybdate system is complex, involving multiple possible side reactions, and further research is needed to master precise quantitative control methods for the valence composition of molybdenum, this study paves the path for the research of long-term anti-corrosion chromium-free coatings and provides a promising research approach for the development of chromium-free coatings.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings15020176/s1, Figure S1: XPS full spectrum and Mo 3d high-resolution spectrum of PMZ’0.1 system: (a) PMZ’0.1, full spectrum; (b) PMZ’0.1, Mo 3d; Figure S2: P 2p high-resolution spectra of PMZ0.06 system (a) and PMNZ1 system (b).

Author Contributions

Conceptualization, Y.L.; methodology, Y.L. and X.Y.; formal analysis, Y.L. and X.Y.; investigation, Y.L., B.Z. and Y.W. (Yuan Wang); resources, L.W. and Y.H.; data curation, S.R. and J.S.; writing—original draft preparation, Y.L., J.L. and X.G.; writing—review and editing, Y.L., X.Y. and Y.W. (Yichen Wang); supervision, Y.H. and J.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Cultivation Program of Wuhan Institute of Photochemistry and Technology (KHY2023KF009), Hubei Technological Innovation Special Fund (Grant Nos. 2023AFA004, 2023BCB106), National Foreign Experts Project (Grant Nos. G2023027001L), Natural Science Foundation of Wuhan (2024040701010057 and 2024010702030096), Science and Technology Research Project of Hubei Provincial Department of Education (F2023001), “14th five year plan” Hubei provincial advantaged characteristic disciplines project of Wuhan University of Science and Technology (2023C0108), and Key Research and Development Program of Hubei Province (Grant No. 2023BAB122).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We would like to thank the Analytical & Test Center of Wuhan University of Science and Technology for the help with the XPS analysis.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of this study, the collection, analysis, or interpretation of data, the writing of the manuscript, or the decision to publish the results.

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Coatings 15 00176 g001
Figure 1. Corrosion resistance time of PMZ coatings.
Figure 1. Corrosion resistance time of PMZ coatings.
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Figure 2. XPS spectra and Mo 3d high-resolution spectra of PMZ systems: (a) PMZ0, full spectrum; (b) PMZ0, Mo 3d; (c) PMZ0.06, full spectrum; (d) PMZ0.06, Mo 3d.
Figure 2. XPS spectra and Mo 3d high-resolution spectra of PMZ systems: (a) PMZ0, full spectrum; (b) PMZ0, Mo 3d; (c) PMZ0.06, full spectrum; (d) PMZ0.06, Mo 3d.
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Figure 3. Effect of Zn addition on the relative proportion of Mo3+ in PMZ.
Figure 3. Effect of Zn addition on the relative proportion of Mo3+ in PMZ.
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Figure 4. Morphology of base, PMNZ1, and PMZ’0.1 coatings after neutral salt spray test: (a) base coating, 0 h; (b) PMNZ1, 0 h; (c) PMZ’0.1, 0 h; (d) base coating, 192 h; (e) PMNZ1, 192 h; (f) PMZ’0.1, 192 h.
Figure 4. Morphology of base, PMNZ1, and PMZ’0.1 coatings after neutral salt spray test: (a) base coating, 0 h; (b) PMNZ1, 0 h; (c) PMZ’0.1, 0 h; (d) base coating, 192 h; (e) PMNZ1, 192 h; (f) PMZ’0.1, 192 h.
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Figure 5. FT-IR spectra of PMNZ0 and PMZ’0.
Figure 5. FT-IR spectra of PMNZ0 and PMZ’0.
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Figure 6. XPS spectra and Mo 3d high-resolution spectra of PMNZ systems: (a) PMNZ0, full spectrum; (b) PMNZ0, Mo 3d; (c) PMNZ1, full spectrum; (d) PMNZ1, Mo 3d.
Figure 6. XPS spectra and Mo 3d high-resolution spectra of PMNZ systems: (a) PMNZ0, full spectrum; (b) PMNZ0, Mo 3d; (c) PMNZ1, full spectrum; (d) PMNZ1, Mo 3d.
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Figure 7. Scheme of establishing and applying the multivalent molybdate system.
Figure 7. Scheme of establishing and applying the multivalent molybdate system.
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Table 1. Fit results of Mo 3d high-resolution spectra for PMZ samples.
Table 1. Fit results of Mo 3d high-resolution spectra for PMZ samples.
GroupMoO42− (%)Na2MoO4·2H2O (%)Mo3+ (%)
PMZ041.1358.870.00
PMZ0.0389.080.0010.92
PMZ0.0679.480.0020.52
PMZ0.0984.320.0015.68
PMZ0.1290.760.009.24
PMZ0.1590.120.009.88
Table 2. Fitting results of the chemical state and relative proportions of Mo in PMNZ0 and PMNZ1 systems.
Table 2. Fitting results of the chemical state and relative proportions of Mo in PMNZ0 and PMNZ1 systems.
GroupMoO42− (%)PMo12O403− (%)Mo5+ (%)
PMNZ018.4481.560.00
PMNZ137.1230.5132.37
Table 3. Comparison of anti-corrosion performance of chromium-free coatings.
Table 3. Comparison of anti-corrosion performance of chromium-free coatings.
Published TimeFilm-Forming MaterialsFunction FillerFiller Content (wt%)Film Thickness
(μm)		SubstrateNSS Duration (h)Reference Number
2022Waterborne epoxy resinSheet-like silica Janus1120 ± 15Q235 carbon steel336[11]
2023Waterborne epoxy resinFlake zinc35100Q235 carbon steel800[17]
2023Waterborne acrylic resinOctafluoropentyl methacrylate and phosphate functional monomer1120Iron plate168[2]
2024Waterborne epoxy resinModified chopped basalt fiber10300 ± 20Q235 carbon steel3000[12]
2024Waterborne epoxy resinModified graphene oxide0.1100Q235 carbon steel300[14]
-Al(H2PO4)3 solution and SiO2·nH2OMultivalent MoO42−0.11.1–1.3Oriented silicon steel192This study
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MDPI and ACS Style

Liu, Y.; Yang, X.; Wu, L.; Li, J.; Zhang, B.; Wang, Y.; Wang, Y.; Ruan, S.; Sun, J.; Gu, X.; et al. Establishment of Multivalent Molybdenum Salt System and Its Effect on the Anti-Corrosion Performance of Insulating Coatings for Oriented Silicon Steel. Coatings 2025, 15, 176. https://doi.org/10.3390/coatings15020176

AMA Style

Liu Y, Yang X, Wu L, Li J, Zhang B, Wang Y, Wang Y, Ruan S, Sun J, Gu X, et al. Establishment of Multivalent Molybdenum Salt System and Its Effect on the Anti-Corrosion Performance of Insulating Coatings for Oriented Silicon Steel. Coatings. 2025; 15(2):176. https://doi.org/10.3390/coatings15020176

Chicago/Turabian Style

Liu, Ying, Xiaoyu Yang, Lin Wu, Jingwang Li, Baoguo Zhang, Yichen Wang, Yuan Wang, Shicheng Ruan, Jiageng Sun, Xueyang Gu, and et al. 2025. "Establishment of Multivalent Molybdenum Salt System and Its Effect on the Anti-Corrosion Performance of Insulating Coatings for Oriented Silicon Steel" Coatings 15, no. 2: 176. https://doi.org/10.3390/coatings15020176

APA Style

Liu, Y., Yang, X., Wu, L., Li, J., Zhang, B., Wang, Y., Wang, Y., Ruan, S., Sun, J., Gu, X., Hu, Y., & Han, J. (2025). Establishment of Multivalent Molybdenum Salt System and Its Effect on the Anti-Corrosion Performance of Insulating Coatings for Oriented Silicon Steel. Coatings, 15(2), 176. https://doi.org/10.3390/coatings15020176

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