MoTiCo Conversion Coating on 7075 Aluminium Alloy Surface: Preparation, Corrosion Resistance Analysis, and Application in Outdoor Sports Equipment Trekking Poles

Abstract

The problem of protecting 7075 Al alloy trekking poles from corrosion in complex outdoor environments was addressed using a composite conversion coating system. This system comprised Na₂MoO₄, NaF, CoSO₄·7H₂O, ethylenediaminetetraacetic acid-2Na, and H₂(TiF₆). The influences of this system on the properties of the coating layer were systematically studied by adjusting the pH of the coating solution. The conversion temperature and pH were the pivotal parameters influencing the formation of the conversion coating. The pH substantially influenced the compactness of the coating layer, acting as a regulatory agent of the coating kinetics. When the conversion temperature and pH were set to 40 °C and 3.8, respectively, the prepared coating layer displayed optimal performance in terms of compactness and protective properties. Therefore, this parameter combination favours the synthesis of high-performance conversion coatings. Microscopy confirmed the formation of a continuous, dense composite oxide film structure under these conditions, effectively blocking erosion in corrosive media. Furthermore, the optimised process led to substantial enhancements in the environmental adaptabilities and service lives of the components of trekking poles, thus establishing a theoretical foundation and technical reference for use in the surface protection of outdoor equipment.

1. Introduction

A nationwide fitness campaign, which has been extensively implemented, and the robust development of the sports tourism industry have contributed to the increased popularity of hiking and mountain climbing as outdoor activities. Trekking poles are considered essential in hiking and mountain climbing, as they provide crucial mechanical support [1,2,3]. The levels of effectiveness of these poles directly affect the safety of the user and energy utilisation efficiency during mountain climbing. In the field of materials engineering, the 7075 Al alloy has emerged as a preferred option for use in fabricating critical load-bearing components, such as the bodies of trekking poles, effectively yielding lightweight equipment [4,5]. Its use is primarily attributed to its exceptional specific and outstanding compressive strengths, favourable processability, and comparatively modest cost. However, complex, harsh, corrosive media (including rain and snow infiltration, a high humidity, salty fog, and even residues of snow-melting agents) prevail in the mountainous outdoor environment. These factors can readily induce local damage, such as in the forms of pitting and intergranular and exfoliation corrosion, in Al alloy materials [6,7,8,9]. Consequently, maintaining the core mechanical properties of Al alloys during service along with their lightweight characteristics and developing long-term high-efficiency green corrosion-resistant surface protection technologies are highly salient and challenging research topics in the field of materials corrosion and protection.

Presently, the major corrosion protection technologies for Al alloys comprise anodic oxidation, physical and chemical vapour deposition (PVD and CVD, respectively), chemical conversion technologies, and others [10]. Anodic oxidation generates a porous Al₂O₃ layer via electrolytic oxidation, which improves the corrosion and wear resistances of Al alloy surfaces [10]. However, this process is energy-intensive, with limited adaptability to complex-shaped workpieces. In contrast, PVD and CVD produce ultrathin, dense protective layers; however, their large-scale industrial application is hindered by the required vacuum environment and low deposition rates [11]. Conversely, chemical conversion entails the formation of a compound coating layer characterised by exceptional corrosion resistance and robust adhesion on the metal surface within a designated medium. This process does not require the use of sophisticated equipment and necessitates minimal financial expenditure; it is also highly adaptable to different conditions. Therefore, it has been widely applied in protecting the surfaces of Al alloys [12,13,14]. Currently, the predominant chemical conversion process is the hexavalent Cr (Cr⁶⁺) conversion process. This process enhances the corrosion resistance of Al alloys, and it exhibits a distinctive self-healing property [12]. However, the Cr⁶⁺ conversion process generates highly toxic Cr⁶⁺ ions during coating formation. These ions display strong levels of carcinogenicity and ecological accumulation; thus, the European Union (EU, Brussels, Belgium) Restriction of Hazardous Substances Directive (2011/65/EU) and International Organization for Standardization (Geneva, Switzerland) 14001 environmental management system impose strict restrictions on the use of Cr⁶⁺ [15,16]. Furthermore, the formation of Cr⁶⁺ conversion coatings is subject to stringent requirements in terms of process parameters. Minor deviations from these parameters can result in the production of coating layers with uneven surfaces and compromise the anti-corrosion effect [17].

Recently, transition metal composite conversion systems emerged as a focal point in research due to their levels of environmental friendliness. Zhang et al. developed a novel ZrTiCC on 6061 Al alloy, which exhibited high corrosion resistance, and investigated its microstructure, growth mechanism, and corrosion resistance [18]. In a recent study, Zhan et al. prepared a TiZrMoCC on 6063 Al alloy, which possessed the capacity to replace Cr⁶⁺ [19]. Wang developed a novel chemical conversion coating by incorporating Ce(NO₃)₃·6H₂O and salicylic acid into the treatment solution, which already contained Ti and Zr ions [20]. Via rigorous experimentation, they identified the optimal formula, which exhibited a substantially enhanced corrosion resistance in relation to that of the Al substrate. Liu prepared Zr/Ti-based conversion coatings on Al alloy AA 5083-H116 and analysed the elements and composition of the conversion coatings [21]. Moreover, the conversion coatings exhibited relatively dense structures, and they could effectively prevent corrosion. The contributions from Du included the preparation of Zn₃(PO₄)₂ coatings on Ti and Al alloys, in addition to the evaluation of the coatings [22]. The properties of the substrate substantially influenced the crystal microstructure, thickness, and adhesion strength of the coating. A considerable body of research is devoted to studying Cr-free conversion coating systems; however, these systems are not without limitations. Although Ti-Zr conversion coatings exhibit specific corrosion resistance properties, they display lower corrosion resistance than chromate conversion coatings. The corrosion resistance of rare earth conversion coatings is significantly influenced by their application in complex environments. Zn₃(PO₄)₂ conversion coatings present certain challenges, including elevated levels of energy consumption and the complexity of precisely controlling the thicknesses of their coating layers. Mn-based conversion coatings exhibit relatively high porosity, which results in coating layers with inadequate degrees of compactness and subsequent effects on their corrosion resistance.

The presence of elements such as Mo, Ti, and Co promotes the formation of dense structures within conversion coatings via synergistic effects, thus enhancing their chemical stabilities and significantly improving the corrosion resistance of their metal substrates. In this study, the 7075 Al alloy was utilised as the substrate, and the effects of the pH of the conversion solution on the microstructure and corrosion resistance of the Mo/Ti/Co conversion coating (MoTiCoCC) were systematically investigated. The growth and corrosion protection mechanisms of the conversion coating were studied by integrating scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electrochemical techniques. This study provides a critical theoretical basis and technical support for the application of Al alloy conversion.

2. Materials and Methods

2.1. Preparation of Conversion Coatings

The 7075 Al alloy was utilised in this study, and the material was converted to cylindrical samples measuring Φ18mm × 18 mm. The composition of the aluminium alloy substrate is shown in

Table 1. Chemical composition of 7075 aluminium alloy.

Element	Al	Zn	Mg	Cu	Fe	Mn	Cr	Si	Ti	Other
Content (%)	89.33	5.51	2.39	1.38	0.36	0.23	0.21	0.26	0.18	0.15

. The surfaces were then subjected to sanding procedures, employing sandpapers with grit sizes of 600, 1000, and 1200, in that order, until the desired level of polish was attained. Subsequently, they were soaked in the prepared mixed cleaning solution (ZOL) for 5 min to remove surface oxides and grease, and then washed with deionised H₂O. After cleaning, the samples were immersed in a solution that was prepared in advance. The composition of the conversion solution is as follows: Na₂MoO₄ (1.5 g/L), NaF (0.6 g/L), CoSO₄·7H₂O (3 g/L), ethylenediaminetetraacetic acid-2Na (EDTA-2Na, 0.2 g/L), and H₂(TiF₆) (2.4 g/L). The pH of the conversion solution was set to 2.8, 3.8, 4.8, 5.8, or 6.8, and the temperature was set to 30, 40, 50, or 60 °C. Conversion was completed within 15 min, and the experimental process is shown in Figure 1.

2.2. CuSO₄ Spot Test

The CuSO₄ spot test is a rapid, effective experimental method of analysing the corrosion resistance of coatings. The titrant prepared for use in this test comprised HCl (13 mL/L), CuSO₄·5H₂O (41 g/L), and NaCl (35 g/L). The corrosion resistance of the coating was evaluated by meticulously selecting three micro-regions on the sample surface and administering the CuSO₄ titration solution. The duration wherein the titration solution transitioned from light blue to light red was designated as the spot time. An increased spot time corresponded to enhanced corrosion resistance.

2.3. Microscopic Morphology and Component Analysis

The microscopic morphologies of the 7075 Al alloy substrate and MoTiCoCC on the surface were characterised using an S-3400N scanning electron microscope (Hitachi, Tokyo, Japan). The experiment was conducted at an acceleration voltage and working distance of 15 kV and 10 mm, respectively. Elemental composition analysis was performed using EDS (Hitachi, Tokyo, Japan).

2.4. Electrochemical Study

In the electrochemical study, a CHI660D electrochemical analyser (CH Instruments, Austin, TX, USA) was used to assess the potentiodynamic polarisation curves of the samples. Because trekking poles are frequently contaminated by the perspiration of the user during usage, the test samples were immersed in a 3.5 wt.% NaCl solution at ambient temperature for 15 min, with an exposed surface area of 1 cm², to yield a stable potential. The initial and final potentials used in polarisation curve measurement were set to −2.0 and 0.5 V, respectively, and the scan rate was 0.01 V/s.

2.5. Salt Spray Test

The corrosion performance under a salt spray was determined via accelerated evaluation using a cyclic corrosion test chamber. The samples were meticulously cut to dimensions of 150 mm × 70 mm × 1 mm according to the ASTM B117 standard [23]. Subsequently, the samples were thoroughly cleaned using anhydrous ethanol, followed by drying to ensure the complete removal of residual moisture. The experimental apparatus comprised a continuous spray system, employing a solution of 5.0% ± 0.5% NaCl at a constant temperature and deposition rate of 35 ± 1 °C and 1.5 mL/(h·cm²), respectively. The total duration of the test was 144 h. After the test, the corrosion morphology on the surface of each sample was comparatively analysed, with a particular emphasis on the corrosion and peeling characteristics of the coating.

2.6. X-Ray Photoelectron Spectroscopy (XPS)

The chemical states of the elements on the surface of the conversion coating were characterised via XPS (Thermo EscaLab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). Prior to evaluation, the binding energy scale was calibrated using the C 1s peak (284.8 eV). The elemental composition was determined using full-spectrum scanning and high-resolution narrow-spectrum analysis. The chemical states of each element were then rigorously analysed via peak fitting.

3. Results and Discussion

3.1. Effects of Conversion Time and Temperature

The conversion temperature and pH of the conversion solution are two critical conversion parameters that directly affect the formation of the conversion coating. As shown in Figure 2, the conversion temperature influences the activity of the conversion ions. An increased conversion temperature is associated with elevated ion activity and increased frequency of ion motion. A relatively low conversion temperature of 30 °C affects the formation of the conversion coating, resulting in a conversion coating with a shorter spot time. When the conversion temperature is relatively high, the local reaction is more intense, and the formation of the conversion coating is prone to generating a higher internal stress, thus also reducing its spot time. Consequently, the optimal conversion temperature is 40 °C.

At the respective conversion temperature and pH of 40 °C and 3.8, the spot time of the resulting conversion coating is maximised, and its corrosion resistance is optimised. A follow-up increase in the pH of the conversion solution results in a corresponding decline in the protective performance of the coating layer. This nonlinear change is attributable to the influences of the pH on the kinetics of coating formation. At a low pH, the concentration of free active H ions (H⁺) within the conversion solution is high. This accelerates the anodic dissolution of the base metal, causing the protective film that already formed to undergo secondary dissolution. Conversely, the concentration of active H⁺ within the conversion solution decreases when the pH is high. This can inhibit the dissolution of the base metal, but it reduces the deposition rates of the coating-forming components, which affects the structural integrity of the conversion coating. In summary, the optimal conversion temperature and pH are 40 °C and 3.8, respectively.

3.2. Effects of Conversion Solution pH on Morphology and Composition of Conversion Coating

The micrographs of the 7075 Al alloy substrate and conversion coatings formed using the conversion solutions with varying pH values are shown in Figure 3. According to Figure 3a, the surface of the Al alloy substrate exhibits numerous fine scratches, primarily owing to sandpaper grinding during pretreatment. After treatment with the conversion solutions of varying pH values, the surface scratches of the conversion coatings are gradually filled. At pH 2.8 (Figure 3b), the fine scratches on the surface are almost imperceptible. However, the surface of the conversion coating exhibits numerous networks of continuous cracks. These networks permit the facile penetration and corrosion of the Al alloy substrate by the corrosive media. When the pH increases to 3.8, the surface of the conversion coating is relatively flat and dense, and no clear cracks are observed. Notably, as the pH continues to increase, the local coating layer dissolves, resulting in the re-emergence of micro-cracks on the surface. This may be attributed to the disruption of the dynamic equilibrium that governs the reaction through which the coating is formed.

EDS was conducted to analyse the elemental composition of the conversion coatings prepared using the conversion solutions with different pH values; the corresponding results are shown in Figure 4. Following conversion treatment, the contents of the base elements Al, Mg, and Cu decrease, as shown in Figure 4, whereas those of O and the conversion atoms Mo, Ti, and Co increase. This may be primarily attributed to the formation of the MoTiCoCC on the surface of the Al alloy substrate. Concurrently, Al atoms undergo etching and dissolution during the conversion reaction, resulting in a concomitant decrease in the Al content. Subsequently, a comparison of the contents of the three conversion atoms yields a distinct “high Mo, medium Ti, low Co” pattern.

At pH 3.8, the maximum O content of the conversion coating is determined at 30.22%, concurrently marking the most substantial cumulative content of the three conversion elements, i.e., Mo, Ti, and Co. The respective contributions of Mo, Ti, and Co are 5.21%, 2.4%, and 0.83%. The findings suggest that the surface accumulates the highest quantities of Mo, Ti, Co, and O for the conversion reaction at pH 3.8, with the highest contents of the resulting oxides also being observed. Consequently, the superficial imperfections can be adequately restored, yielding a relatively smooth and dense surface, which is consistent with the SEM results.

3.3. Electrochemical Polarisation Curves

The potentiodynamic polarisation curves of the Al alloy substrate and conversion coatings prepared using the conversion solutions with different pH values are shown in Figure 5. The shapes of the electrochemical curves of the different samples do not differ significantly, with only shifts in position observed. This suggests that the conversion coating does not influence the pathways of the anodic and cathodic reactions in electrochemical processes—it merely affects the rate of electrochemical corrosion. The self-corrosion potentials (E_corr) and current densities (i_corr) were fitted using the polarisation curves of the different samples; the values are listed in

Table 2. Electrochemical test data of 7075 aluminium alloy.

Sample	icorr/µA	Ecorr/V
7075 aluminium alloy substrate	79.43	−0.92
pH 2.8	33.88	−0.91
pH 3.8	7.93	−0.75
pH 4.8	24.55	−0.87
pH 5.8	27.54	−0.79
pH 6.8	38.02	−0.78

.

E_corr is a thermodynamic parameter that primarily serves to evaluate the corrosion sensitivity of the sample, whereas i_corr is a kinetic parameter that mainly determines the electrochemical corrosion rate. At pH 3.8, E_corr displays a more positive response, whereas i_corr exhibits the minimum response. This suggests that the generated conversion coating displays low corrosion sensitivity and the lowest electrochemical corrosion rate, which is 10% of that of the 7075 Al alloy substrate.

3.4. Salt Spray Test Analysis

Salt spray tests were performed using the conversion coatings prepared at different pH values to further verify their corrosion resistance. The salt spray test is an accelerated corrosion test. The 7075 Al alloys were prepared as coupons measuring 150 mm × 70 mm × 1 mm. The substrates and samples prepared using solutions with pH values of 2.8, 3.8, 4.8, 5.8, and 6.8 were positioned at angles of 25° relative to the vertical line. The conversion coatings were exposed to continuous spraying, and the surface morphologies were observed after 144 h of salt spray testing; the results are shown in Figure 6. The conversion coating developed at pH 3.8 exhibits the smallest corroded area on the surface following the salt spray test. The SEM and EDS results indicate that the conversion coating produced during this conversion process possesses the highest conversion atom content. The surface of the conversion coating is relatively flat and dense, which suggests that it can effectively resist the penetration of corrosive media and thus exhibit enhanced corrosion resistance. Conversely, the conversion coatings produced using solutions with other pH values exhibit larger corroded areas after the salt spray test. Furthermore, numerous bubbles are observed on the coupons, accompanied by discernible alterations in surface morphology. The results of the salt spray test are consistent with those of the spot and electrochemical corrosion tests.

3.5. XPS

XPS was performed using the 7075 Al alloy substrate and sample prepared using the conversion solution with a pH of 3.8 to further elucidate the elemental composition and valence state of the conversion coating. The full spectra are shown in Figure 7. Compared to the Al alloy substrate, the composite conversion coating layer primarily contains elements such as O, Mo, Ti, Co, and F after conversion treatment, which is consistent with the EDS results.

Detailed chemical analyses of the narrow spectra of elements such as Al 2p, O 1s, Ti 2p, Co 2p, Mo 3d, and F 1s were performed; the fitted peaks are shown in Figure 8a–f. The Al 2p narrow spectrum (Figure 8a) displays a broad peak that can be deconvoluted into two main peaks at 75.1 and 75.8 eV, corresponding to Al₂O₃ and AlF₃∙3H₂O, respectively. The O 1s narrow spectrum (Figure 8b) also displays a broad peak that can be deconvoluted into two main peaks at 530.3 and 532.4 eV, which are, respectively, assigned to the metal oxides Me_xO_y and Al₂O₃. The Ti 2p peak (Figure 8c) can be deconvoluted into two peaks at 458.5 and 464.4 eV, and both peaks clearly correlate significantly with TiO₂. The Co 2p spectrum (Figure 8d) displays four discernible peaks. Those at approximately 780.2 and 785.8 eV are attributed to Co₃O₄, whereas those at approximately 796.7 and 798.2 eV are ascribed to CoO. The peak in the Mo 3d narrow spectrum (Figure 8e) can be adequately deconvoluted into two peaks. The peak at 235.7 eV is attributed to MoO₃, whereas that at 232.6 eV corresponds to Mo₂O₅. The F 1s narrow spectrum (Figure 8f) displays a broad peak that can be deconvoluted into two main peaks: the peak at 685.5 eV is ascribed to Na₃AlF₆ and the other peak corresponds to AlF₃∙3H₂O. In summary, the conversion coating is primarily constituted by metal oxides, including Co₃O₄, CoO, MoO₃, Mo₂O₅, TiO₂, and Al₂O₃, along with trace amounts of fluorides such as Na₃AlF₆ and AlF₃∙3H₂O.

3.6. Formation Mechanism of MoTiCoCC

Under typical conditions, the Al alloy substrate exhibits high activity, and it is susceptible to oxidation in ambient air, resulting in the formation of a thin, discontinuous oxide layer, as shown in Figure 9. This oxide layer exhibits limited corrosion resistance, and its capacity to prevent corrosion is inadequate, thus compromising the adhesion between the subsequent conversion coating and Al alloy substrate. Consequently, the ZHM-1026 mixed cleaning agent was employed in pre-treating the Al alloy substrate to remove the surface oxide layer and grease.

Subsequently, the bare metal surface is rapidly immersed in the conversion solution, which facilitates soaking and deposition. In general, following solution heating and ageing, the 7075 Al alloy contains Al-based solid solutions and substantial amounts of secondary phases that precipitate during ageing. These phases include η (MgZn₂), T (Al₂Mg₃Zn₃), S (Al₂CuMg), and θ (Al₂Cu). The potential differences between the Al-based solid solutions and these secondary phases result in the formation of micro-anodes within the Al-based solid solutions and micro-cathodes within the secondary phases. At this point, the micro-anodes dissolve, generating Al³⁺ and releasing electrons, whereas the micro-cathodes absorb electrons and undergo oxygen absorption or hydrogen evolution reactions. The reaction pathways are shown in Figure 9 and the reaction Equations (1)–(3):

Micro-anode:

Al → Al³⁺ + 3e⁻

(1)

Micro-cathode:

2H⁺ + 2e⁻ → H₂ ↑

(2)

O₂ + 2H₂O + 4e⁻ → 4OH⁻

(3)

The hydrogen evolution and oxygen absorption reactions within the micro-cathode region result in the augmentation of the local OH⁻ concentration and the alteration of the pH on the bare metal surface of the Al alloy. Upon attaining a specified OH⁻ concentration, the substance combines with the Al³⁺ ions emanating from the micro-anode, resulting in the formation of insoluble Al(OH)₃ on the surface. The subsequent dehydration of this compound leads to its transformation into Al₂O₃, as shown in the reaction Equation (4). Furthermore, the presence of Co²⁺ and TiF₆²⁻ in the conversion solution results in their adsorption on the surface of the Al alloy. This process subsequently leads to a reaction with OH⁻, thus generating the nucleation centres of the corresponding metal oxides in proximity to the intermetallic compound region. The conversion process is shown in the reaction Equations (5)–(7). Concurrently, the local H⁺ content is notably prevalent in the micro-cathode region, undergoing reactions with MoO₄²⁻ to generate the nucleation centres of the corresponding Mo metal oxides. The reaction processes are shown in the reaction Equations (8) and (9). A schematic of nucleation associated with diverse metal oxides is shown in Figure 9.

2Al³⁺ + 6OH⁻ →2Al(OH)₃ → Al₂O₃ + 3H₂O

(4)

Co²⁺ + 2OH⁻ → Co(OH)₂ → CoO + H₂O

(5)

6Co²⁺ +12OH⁻ + O₂ → 2Co₃O₄ +6H₂O

(6)

TiF₆²⁻ + 4OH⁻ → TiO₂∙2H₂O + 6F⁻

(7)

2MoO₄²⁻ + 2e⁻ + 6H⁺ → Mo₂O₅ + 3H₂O

(8)

MoO₄²⁻ + 2H⁺ → MoO₃ + H₂O

(9)

As the conversion time increases, the metal oxide core continuously absorbs newly deposited metal oxides for growth. The growth mode is primarily 2D planar growth (the growth scale in the thickness direction is relatively small, primarily due to the formation of a larger internal stress during vertical growth), and finally, the complete MoTiCoCC is formed on the surface, as shown in Figure 9. In summary, the MoTiCoCC is principally formed in three stages: pretreatment, surface micro-dissolution, and coating formation.

4. Conclusions

(1)

When the conversion temperature and pH of the conversion solution were elevated or diminished, the coating layer was susceptible to complications, including a lack of structural integrity, disproportionate coating formation, and the generation of surface micro-cracks. When the conversion temperature and pH were 40 °C and 3.8, respectively, the prepared MoTiCoCC surface had the highest contents of O and conversion atoms and was relatively flat and dense.

(2)

The XPS results indicated that the MoTiCoCC primarily comprises elements such as Ti, Mo, Co, O, Mg, and F. The metal oxides within the coating primarily comprised Co₃O₄, CoO, MoO₃, Mo₂O₅, TiO₂, and Al₂O₃, and trace amounts of fluorides, including Na₃AlF₆, and AlF₃·3H₂O, were also present. The presence of these metal oxides and fluorides within the coating enhanced its physical compactness and electrochemical stability. Based on mechanistic analysis, the formation of the MoTiCoCC involved three primary stages: pretreatment, surface micro-dissolution, and coating formation.

(3)

The findings of the spot, electrochemical, and neutral salt spray tests indicated that the conversion coating prepared at a conversion temperature and pH of 40 °C and 3.8, respectively, exhibited the most exceptional corrosion resistance. The longest spot time and lowest i_corr were 69 s and 7.93 μA/cm², respectively, and the sample displayed the highest salt spray resistance. Using a conversion solution with a pH of 3.8 for the surface treatment of a 7075 Al alloy trekking pole can result in long-term corrosion resistance.