Preparation of New Vanadium Base Composite Conversion Coating on 6061 Aluminum Alloy Surface for Sports Equipment

Abstract

The 6061 aluminum alloy is a commonly used metal material for sports equipment but is vulnerable to the external environment and corrosion. A novel V-Zr-Ti composite conversion coating was successfully prepared on the surface of 6061 aluminum alloy, and a thorough investigation was conducted into the effect of the conversion parameters. Furthermore, the microstructure of the conversion coating, element contents of the coating surface, and dynamic evolution characteristics of the conversion solution were systematically investigated, and furthermore, the relationship among them was established. The results show that the optimal conversion time (CTI) and conversion temperature (CTE) for the VZrCC are 12 min and 45 °C. The VZrTiCC can gradually fill surface scratches during the coating-forming process, resulting in a relatively flat and even surface morphology. The conversion element contents on the VZrTiCC surface demonstrated a gradual increase, and the deposition rate was characterized by high Ti, medium Zr, and low V. The phase of the coating is predominantly constituted by metal oxides derived from conversion compositions, with a minor proportion of fluoride. Furthermore, the VZrTiCC can significantly enhance the corrosion resistance of an Al alloy matrix due to its low i_corr and average corrosion rate (ACR), and its corrosion resistance is about 5 times higher than that of the Al alloy matrix. Eventually, the formation process of the VZrTiCC with three key stages was proposed. In subsequent studies, to further establish a composition design framework for the conversion coating, a silane aqueous solution will be added to the existing V-Zr-Ti conversion solution, and a systematic study will be conducted on the V–organic composite conversion coating using computational molecular dynamics simulation combined with experimental characterization.

1. Introduction

With the rise of national fitness, sports have been welcomed by more and more people. Aluminum alloy is widely used in the manufacture of sports equipment and its parts due to its advantages of low density, high specific strength, good ductility, good thermal conductivity, and low price [1,2,3,4,5]. Aluminum alloy’s bare metal electrode potential is negative, and its corrosion resistance is poor. Achal Pandaya et al. studied the 3003 aluminum alloy and found that pollutants in the atmosphere would accelerate the corrosion of the aluminum alloy [6]. Especially in outdoor sports and other harsh environments (acid rain, high salt, high humidity, etc.), it is easy to cause various types of corrosion [7,8,9], such as pitting, crevice corrosion, and uniform corrosion, thus causing sports equipment to be scrapped in advance [10,11,12,13]. Therefore, it is urgent to find an aluminum alloy surface treatment method to improve the corrosion resistance of the aluminum alloy matrix and improve the life and reliability of sports equipment.

Generally speaking, surface modification techniques for aluminum alloys mainly contain anodic oxidation, electrodeposition, electrophoresis, physical vapor deposition (PVD), chemical conversion technology (CCT), etc. The oxide film generated via the anodic oxidation process has good corrosion resistance and friction resistance, yet this process has evident drawbacks, such as high energy consumption and being only applicable to some grades of aluminum alloy [14]. The electrodeposition process has problems such as easy detachment of the generated metal film and high sensitivity to impure metal ions in the plating solution. The electrophoresis process can prepare films with bright colors and good corrosion resistance, but this process is costly and has high cleaning requirements for workpiece surfaces [15]. The cost of the PVD process is relatively high, and it is powerless to handle workpieces with complex shapes [16]. CCT, a soak deposition technology, can form a uniform, dense, and corrosion-resistant compound isolation film on the surface of aluminum alloy. Compared with other surface treatment processes, CCT has the advantages of simplicity, low cost, wide application range, and good shape adaptability, and it is widely used in the surface treatment of aluminum alloys. Currently, the more mature process is the hexavalent chromium conversion process, but chromium ions can cause harm to the environment and human health, and thus, more and more countries have restricted the further promotion and use of this process [17,18,19,20,21]. Therefore, environmentally friendly chromium-free conversion processes have become a research focus for many scholars. Iannuzzi M et al. studied the corrosion inhibition mechanism of vanadate on AA2024-T3 and found that the critical concentration of vanadate was similar to that of reported Cr⁶⁺ [22]. Golru, S.S. et al. studied zirconium-based conversion coatings and found that the corrosion resistance was improved after Zr treatment [23]. Bhargava G et al. believed that titanium salt and chromium salt conversion films were similar and had certain self-repairing properties [24]. However, compared with the multi-transformation film, the corrosion resistance of the unit-transformation film still has some disadvantages. And the Zr/Ti conversion process is presently considered one of the most likely alternatives to the hexavalent chromium conversion process. Tongjin Zhang et al. found that the prepared Zr/Ti conversion coating has good corrosion resistance with the following conversion parameters: conversion temperature (CTE) of 50 °C, conversion concentration of 300 ppm, and conversion time (CTI) of 150 s [25]. Andreatta et al. found that with a Zr/Ti coating soaked in the conversion solution for 300 s, after immersion in 0.1 M NaCl for 72 h, localized corrosion occurred on the coating surface [26]. Nordlien et al. found that the aluminum alloy matrix affects the thickness of the Zr/Ti-based conversion film and then reduces its corrosion resistance [27]. In summary, Zr/Ti-based conversion coatings can improve the corrosion resistance of aluminum alloy substrates to a certain extent, but there is still a significant gap compared to hexavalent chromium.

The V element and Cr element are adjacent elements in the periodic table, and their chemical properties are similar. Additionally, V is one of the trace elements required by the human body, so theoretically, V is one of the most promising elements to replace Cr. Here, a novel ternary V/Zr/Ti conversion coating (VZrTiCC) was prepared, and subsequently, the dynamic evolution of its microstructure, element distribution, and corrosion resistance was systematically investigated. Ultimately, by elucidating the correlations among the micro-morphology, element distribution, and corrosion resistance, fundamental data and technical guidance are furnished to improve the service life of sports equipment.

2. Materials and Methods

2.1. Conversion Coating Preparation

The conversion coating (CC) deposition experiments were carried out on the specimen of commercially available 6061 aluminum alloy, and the Al alloy matrix (AAM) components are shown in

Table 1. Chemical composition of 6061 aluminum alloy (wt.%).

Element	Cu	Mg	Mn	Fe	Si	Zn	Cr	Al
Content/%	0.16	1.01	0.45	0.31	0.55	0.08	0.05	Bal.

. To improve the interfacial adhesion strength, the AAM surface required a two-step pretreatment. Firstly, all AAMs were polished with a series of sandpapers. Secondly, the polished specimens were washed with 5% ZHM-1026 (purchased from the Wuhan Research Institute of Materials Protection, Wuhan, China) to remove the naturally formed oxide film and grease.

Subsequently, the bare metal surfaces were rapidly immersed in conversion solution with the compositions shown in

Table 2. Composition of the conversion solution.

Type of Conversion Solution	NaVO3	H2ZrF6	H2TiF6	(NaPO3)6
V-Zr	4.0 g/L	2.8 mL/L	0 mL/L	0.4 g/L
V-Zr-Ti	4.0 g/L	3.0 mL/L	2.2 mL/L	0.4 g/L

. The pH value was adjusted to 5.0 with ammonia. The conversion times (CTIs) are set to 1, 2, 4, 8, 12, 16, 20, and 24 min, and the conversion temperature is 50 °C.

2.2. Characterization of Surface Morphology and Composition

The microscope morphology was observed using the Hitachi S-3400N (Hitachi, Tokyo, Japan) with an acceleration voltage of 15 kV, and the element distributions of the CC surface and conversion solution were characterized using energy dispersive X-ray spectroscopy (EDX) and inductively coupled plasma (Thermofisher iCAP Q, Thermo Fisher Scientific, Waltham, MA, USA), respectively. Meanwhile, X-Ray photoelectron spectroscopy (XPS; Thermo EscaLab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was used to thoroughly analyze the phase composition of VZrTiCC’s cross-section, and the step size of taken spectra is 0.05 eV. A C-C binding energy of 284.8 eV is used to adjust the spectra, and a series of spectra were obtained at an interval of 200 nm with a sputtering etching rate of about 20 nm/min.

2.3. Characterization of Wettability

A wet environment can accelerate the corrosion of sports equipment. The wetting performance of 3.5 wt.% NaCl solution on the Al alloy matrix and VzrTiCC surface was detected using the OCA 15Pro contact angle-measuring instrument (Dataphysics, Filderstadt, Germany), and the wetting process was captured and recorded with a Mini UX100 high-speed camera (Photron, Tokyo, Japan). Subsequently, Screen Protractor software was used to measure their wetting angle.

2.4. Characterization of Corrosion Resistance

Copper sulfate dropping test: In the complex use environment, sports equipment is extremely vulnerable to corrosion. To quickly determine the corrosion resistance of the Al alloy matrix and CC, a dropping solution was prepared according to the GB-6807-2001 standard, and its composition is shown in

Table 3. Composition of the dropping solution.

Composition	CuSO4∙5H2O	HCl	NaCl
Content	41 g/L	13 mL/L	35 g/L

. The corrosion solution was dripped onto five different parts of the sample surface, and the average time required for the corrosion solution to change from light blue to dark red was the dropping time. A longer dropping time represents better corrosion resistance.

Electrochemical test: The polarization curve was measured using a CHI660D electrochemical workstation with a three-electrode system, and the electrolyte solution was 3.5 wt.% NaCl solution. The auxiliary electrode and reference electrode were a platinum electrode and saturated calomel electrode, respectively, and the working electrode was the sample itself. The test samples with an exposed area of 1 cm² were pre-immersed in electrolyte solution for 15 min to obtain a stable open-circuit potential, and the measurement range of the polarization curve was from −2.0 to +0.5 V, with a scan rate of 0.01 V/s.

Immersion test: Before the experiment, the samples were weighed using an electronic balance and recorded as M₀ and then immersed into neutral 3.5 wt.% NaCl solution at room temperature for 168 h. The sampling interval was 24 h, and three samples of the Al alloy matrix and VZrTiCC were separately taken out at each sampling interval. Afterward, the macroscopic morphology of the surface was captured, and the loose corrosion products were removed using a soft bristled brush and then weighed to acquire the weight of M₁; lastly, the average corrosion rate (ACR) was calculated using Equation (1):

$$\mathit{ACR}={\displaystyle \frac{M_0-M_1}{A\cdot T}}$$

(1)

where M₀ is the original weight, M₁ is the weight after the removal of surface corrosion products, A is the surface area, and T is the immersion time.

3. Results and Discussion

3.1. Effect of Conversion Parameters

The conversion parameters affect the CC’s growth process and, thereby, directly determine its corrosion resistance, and the influence of the conversion time (CTI) and conversion temperature (CTE) on the dropping time are displayed in Figure 1. It can be seen that the dropping times of the VZrCC and VZrTiCC both presented a trend of increasing first and then decreasing with the increase in CTI. In general, an insufficient CTI cannot generate a complete CC, and an excessively long CTI can cause the already generated CC to be redissolved due to the low local pH on the surface. The optimal CTIs for the VZrCC and VZrTiCC are 16 and 12 min, and yet the dropping time for the VZrCC is not significantly different between the CTIs of 12 and 16 min. Therefore, the optimal CTI for both the VZrCC and VZrTiCC is confirmed as 12 min.

The CTE is related to the activity of converted ions in the conversion solution, mainly affecting the formation rate of the CC. It can be seen from Figure 1b that the dropping times of the VZrCC and VZrTiCC show a clear trend of first increasing and then decreasing with the increase in CTE, and both achieve the longest dropping time at a CTE of 45 °C. A low CTE can lead to a decrease in the activity of converted ions and, thus, reduce the formation rate of the CC. An excessive CTE can increase the collision probability of converted ions and decrease the effective adsorption probability on the Al matrix surface, thereby also reducing the film formation rate. Simultaneously, a high temperature generates high internal stress in the CC, which can cause the CC to crack and lose its corrosion resistance. In summary, the optimal CTI and CTE for the VZrCC and VZrTiCC are 12 min and 45 °C. The subsequent corrosion resistance tests of the CC in the paper were all the optimal conversion parameters.

3.2. Morphology and Composition Analysis

Figure 2a–c display the surface morphology of the AAM, VZrCC, and VZrTiCC, respectively. Clearly, numerous deep and wide scratches are generated on the surface of the AAM due to the sandpaper grinding during the pretreatment stage, and then the scale of these scratches changed after the CCT treatment. Despite the VZrCC surface still having some scratches, their depth is relatively shallow. Nevertheless, for the VZrTiCC surface, the number of scratches significantly decreased, and its surface was relatively flat.

The surface compositions of the AAM, VZrCC, and VZrTiCC are shown in Figure 2d,e, and the conversion elements (V, Zr, and Ti) and oxygen elements increased after CCT treatment in comparison to the AAM. On the contrary, the Al element, as a constituent element of the AAM, decreased in its content due to etching and dissolution during the CCT treatment. Simultaneously, it is also noteworthy that the Ti element content is highest among the conversion elements on the VZrTiCC surface, whereas the H₂TiF₆ content added to the conversion solution is the lowest.

3.3. Corrosion Resistance Analysis

3.3.1. Electrochemical Analysis

Figure 3a shows the potentiodynamic polarization curves of the AAM, VZrCC, and VZrTiCC, and their shapes are almost identical, with only slight differences in the position. The same shape implies the same electrochemical reaction pathway, and yet different positions reflect differences in their corrosion resistance. Therefore, the corrosion potential (E_corr) and corrosion current density (i_corr) values were acquired by fitting the curves, as shown in Figure 3b. The E_corr significantly increased after the CCT, wherein the VZrTiCC owned the highest E_corr, indicating the lowest possibility of corrosion. The i_corr is mainly used to characterize the corrosion rate, and the values of the AAM, VZrCC, and VZrTiCC are 5.2 × 10⁻⁴, 8.3 × 10⁻⁵, and 2.8 × 10⁻⁵ A∙cm⁻², respectively. The i_corr of the VZrTiCC is the lowest, which is one order of magnitude lower than that of the AAM, and thus, the VZrTiCC has the minimum corrosion rate. To sum up, the VZrTiCC has the lowest corrosion sensitivity and the best corrosion resistance.

3.3.2. Full Immersion Test

The full immersion test is commonly used to evaluate the corrosion resistance under harsh working conditions, and the relevant macroscopic corrosion morphology and ACR of the AAM and VZrTiCC are shown in Figure 4. In Figure 4a, the corrosion spots appeared on the AAM surface, and their size and coverage area increase with the extension of immersion time, covering about 80% of the surface area at an immersion time of 168 h. However, almost no corrosion spots were observed on the VZrTiCC surface. To quantify the degree of corrosion, the relationship between the ACR and immersion time is shown in Figure 4c, and the ACR of the VZrTiCC is significantly lower than that of the AAM, predictably. Taking 168 h as an example, the ACRs of the AAM and VZrTiCC are 0.569 and 0.093 mg/cm²∙h, respectively, and the corrosion resistance of the VZrTiCC is about 5 times higher than that of the AAM.

In addition, the wetting angle test is mainly used to illustrate the wetting characteristics between the corrosive medium and samples surface. The wetting angle on the AAM surface is 54°, whereas that on the VZrTiCC surface is 104°. A large wetting angle indicates that the corrosive medium cannot form effective adsorption and permeation on the VZrTiCC surface, and thus, the corrosive ions cannot form continuous and effective corrosion attacks on the surface, which is one of the important reasons for the VZrTiCC surface without corrosion spots.

3.4. Microscopic Morphology Evolution of VZrTiCC

Figure 5 reveals the microscopic morphology evolution of the VZrTiCC with different CTIs. Because of the sandpaper polishing in the pretreatment process, many scratches are introduced on the AAM surface (Figure 5a), and their depth and width gradually decrease along with the increase in CTI. As the CTI reaches 12 min, the scratches almost disappear, except for some deep and wide scratches. After a CTI of 12 min, the depth and width of the scratches slightly increase again, possibly due to the local redissolution. In summary, the microscopic morphology with a CTI of 12 min is relatively flat, and small scratches have basically disappeared; therefore, its corrosion resistance is also better (Figure 5j).

3.5. Elemental Composition Evolution of VZrTiCC

To further investigate the elemental composition evolution during the deposition process (in the cross-section direction), the different etching depths’ total XPS spectra of the VZrTiCC with a CTI of 12 min were measured and are shown in Figure 6. Clearly, all the XPS curves have roughly the same shape and peak positions, except for some differences in peak intensity. The different etching depths mainly contain the same elements, such as Al, V, Ti, Zr, O, F and C elements, but the C element is mainly pollutants and will not be discussed.

Ordinarily, the etching depth direction is opposite to the growth direction, and the main element contents at different etching depths are exhibited in Figure 7. During the growth process, the Al element content gradually decreased due to the dissolution of the AAM and the continuous deposition of conversion elements (V, Zr, and Ti), whilst the conversion element contents accompanied by F and O contents present an overall upward trend. It can be speculated that the AAM surface continuously adsorbs V, Zr, and Ti ions and then react with oxygen and fluoride ions to form corresponding oxides and fluorides. Nevertheless, the growth rates of the V, Zr, and Ti contents are inconsistent. The amount of H₂TiF₆ added is the smallest (2.2 mL/L), yet the adsorption content is the highest. The adsorption content of V is merely between Ti and Zr, even if the addition amount of NaVO₃ is relatively large (4 g/L). Accordingly, the AAM exhibits significant absorption characteristics of high Ti, medium Zr, and low V.

As a supplement to the XPS results, the metal ion contents in the conversion solution with different CTIs were detected using ICP, as shown in Figure 8. As the two main constituent elements of the AAM, the Mg content is almost unchanged, whereas the Al content significant increases with the extension of the CTI due to the dissolution of the AAM. However, the increase in Al content gradually slowed down after the CTI of 12 min, and the reason is that the formed VZrTiCC inhibited the further dissolution of the internal AAM. Conversely, the conversion ions adsorb and react on the AAM surface to produce the VZrTiCC, thus causing a decrease in their content in the conversion solution. The ICP results echo the XPS results well.

To clarify the valence state and compound composition of the VZrTiCC, the high resolution XPS spectra of different elements were fitted, as shown in Figure 9. The peak of the Al 2p spectrum (Figure 9a) can be deconvoluted into three peaks: the two secondary peaks centered at 73.65 and 76.67 eV are related to Na₃AlF₆ and AlF₃·3H₂O, respectively, and the dominant peak centered at 73.91 eV is related to Al₂O₃. The O 1s spectrum (Figure 9b) can be fitted into two peaks. The peak centered at 531.78 eV is related to V₂O₃, TiO₂, and ZrO₂, and the other peak centered at 532.89 eV is related to Al₂O₃.

The V 2p spectrum (Figure 9c) can be deconvoluted into three peaks: the peaks centered at 517.49 and 517.73 eV are ascribed to V₂O₅, and the remaining peak centered at 516.45 eV is related to V₂O₃ [28]. The two peaks of Ti 2p (Figure 9d) centered at 458.82 and 464.58 eV are both associated with TiO₂ [29], and the two peaks (Figure 9e) of Zr 3d centered at 182.75 and 185.16 eV belong to ZrO₂ and ZrF₄, respectively. Furthermore, the F 1s spectrum (Figure 9f) also contains two peaks: the peak centered at 684.59 is related to Na₃AlF₆; the other peak centered at 685.78 is ascribed to ZrF₄. According to above-mentioned fitting results, the phase composition of the VZrTiCC is mainly metal oxides (V₂O₃, V₂O₅, TiO₂, ZrO₂, and Al₂O₃) and a few metal fluorides (Na₃AlF₆, ZrF₄, and AlF₃∙3H₂O).

3.6. Formation Mechanism of VZrTiCC

The AAM is polished with a series of sandpaper and then cleaned with 5% ZHM-1026 to remove the surface oxide layer and grease, as shown in the pretreatment stage in Figure 10. Immediately, the samples to be sedimentated are added into the conversion solution to enter the film formation stage, and this stage mainly consists of three steps. The AAM structure contains Al-based solid solution and various intermetallic compounds and, thus, form a large number of micro-anodes and micro-cathodes due to their potential difference. Usually, Al-based solid solutions, as micro-anodes, are easily unaffected by the Coulomb attraction to generate Al³⁺ and free electrons, as shown in Equation (2). In parallel with the micro-anode reaction, the intermetallic compound region serving as the micro-cathode mainly undergoes oxygen absorption reactions, as shown in Equation (3). Moreover, a few bubbles observed during the film formation process suggest that hydrogen evolution reactions also occurred in the micro-cathode region, as shown in Equation (4).

Micro-anode:

Al − 3e⁻→ Al3⁺

(2)

Micro-cathode:

O₂ + 2H₂O + 4e⁻ → 4OH⁻

(3)

2H⁺ + 2e⁻→ H₂ ↑

(4)

As the oxygen absorption reaction continues, the OH⁻ ions are constantly generated at the interface. The Al³⁺ released by the micro-anode, as well as TiF₆²⁻ and ZrF₆²⁻ in the conversion solution, rapidly react with OH⁻ ions to generate their respective hydroxides and ultimately dehydrate to produce the corresponding oxides, as shown in Equations (5)–(7), whereas the VO₃⁻ in the conversion solution only reacts actively with weak H⁺ to form vanadium oxide, as shown in Equations (8) and (9). The generated various metal oxides adsorb on the surface to form the nucleation center, as shown in step 2 of the film formation stage in Figure 10. Furthermore, the F⁻ generated by Equations (6) and (7) has a small radius, strong electronegativity, and, thus, is easily adsorbed and reacts with Al³⁺, Zr⁴⁺, and Na⁺ to form a series of fluorides, as shown in Equations (10)–(12). Fluorides are mainly used to assist in the construction of the VZrTiCC; nevertheless, they will not be discussed in this article due to their low content.

Al³⁺ + 3OH⁻ → Al(OH)₃ → Al₂O₃

(5)

TiF₆²⁻ + 4OH⁻ → TiO₂∙2H₂O + 6F⁻

(6)

TiZr₆²⁻ + 4OH⁻ → ZrO₂∙2H₂O + 6F⁻

(7)

2VO₃⁻ + e⁻ + 6H⁺ → V₂O₃ + 3H₂O

(8)

2VO₃⁻ + 2H⁺ → V₂O₅ + H₂O

(9)

Al³⁺ + 3F⁻ + 3H₂O → AlF₃∙3H₂O

(10)

Al³⁺ + 6F⁻ + 3Na⁺ → Na₃AlF₆

(11)

Zr⁴⁺ + 4F⁻ → ZrF₄

(12)

The various oxides formed above aggregate and grow and then reach a certain size to form stable crystal nucleus, as shown in step 3 of the film formation stage in Figure 10. So as to reduce the surface energy and interface energy, the crystal nucleus continuously adsorbs newly produced metal oxides, resulting in their rapid growth. The growth mode of the VZrTiCC is mainly two-dimensional plane growth, and the growth rate in the thickness direction is slow due to the high internal stress. After an appropriate growth period, a complete and corrosion-resistant VZrTiCC is formed on the surface.

After the film formation stage, continued soaking enters the over-sedimentation stage, and this stage may have two pathways. One path is that the continued growth accompanied by high internal stress leads to the surface cracking; the other path is that the formation of metal oxides implies a decrease in the local pH values due to the continuous consumption of OH⁻, which will result in the local redissolution of the already formed VZrTiCC. Therefore, excessive sedimentation can easily form gaps on the surface, causing a decrease in its corrosion resistance. In summary, the formation of the VZrTiCC mainly includes three stages: pretreatment stage, film formation stage, and over-sedimentation stage.

4. Conclusions

(1)

The optimal conversion time (CTI) and conversion temperature (CTE) for the VZrTiCC are 12 min and 45 °C. The VZrTiCC can significantly fill the scratches on the AAM surface, and its surface is relatively flat. The combination of a high E_corr and the lowest i_corr in the VZrTiCC underscores its exceptional electrochemical corrosion resistance, as evidenced from both thermodynamic and kinetic viewpoints. The higher wetting angle for the VZrTiCC makes the corrosive medium unable to corrode its surface efficiently. The VZrTiCC significantly reduces the ACR, with almost no corrosion spots on its surface, and its corrosion resistance is approximately 5 times higher than that of the AAM. This plays an important role in improving the service life of sports equipment.

(2)

The VZrTiCC primarily comprises six elements—V, Zr, Ti, Al, F, and O—and its surface phases mainly consist of metal oxides of corresponding elements and a small amount of fluoride. As the VZrTiCC grows from the inside out, the contents of the V, Zr, Ti, and O elements gradually increase, whereas the content of the Al element gradually decreases. The entire growth and sedimentation process shows significantly high Ti, medium V, and low Zr, and yet the unit sedimentation amount of the three conversion elements is Ti > Zr > V. The metal ion contents measured using ICP complemented the XPS results well.

(3)

The formation process of the VZrTiCC mainly includes three key stages: a pretreatment stage, film formation stage, and over-sedimentation stage. During the film formation stage, various metal oxide crystal nuclei are formed and undergo rapid two-dimensional growth. After the optimal CC is formed, it progresses to the sedimentation stage, where it disrupts the dynamic equilibrium of sedimentary reactions. In this stage, the conversion reaction results in the occurrence of cracks or dissolution due to the excessive internal growth stress or low local pH values.