Improving the anticorrosion property of outdoor fitness equipment via a novel Ti/Zr/Mo composite conversion coating

In recent years, a large number of outdoor fitness equipment have been newly built in China. In order to improve the anticorrosion property of outdoor fitness equipment, a new type of Ti/Zr/Mo conversion coating (TiZrMoCC) has been prepared on its surface. The surface morphology, element distribution, phase composition, and anticorrosion property were systematically measured, and the relationships between them were established. The results show that the optimal conversion temperature and time were 30 °C and 20 min, respectively. Under the optimal conversion parameters, the surface morphology of the TiZrMoCC is relatively flat and dense, and its phase compositions mainly consisted of TiO2, ZrO2, MoO3, Mo2O5 and Fe2O3, as well as a small quantity of Na3FeF6 and ZrF4. The TiZrMoCC could significantly reduce corrosion rate with the lower i corr , which is also confirmed by EIS results. The ACR of the TiZrMoCC decreased by nearly 63.5% compared to the Q235 matrix, and it has positive significance for protecting outdoor fitness equipment.


Introduction
In recent times, with an escalating preoccupation towards personal well-being, a remarkable surge has been witnessed worldwide in the construction of outdoor fitness equipment (OFE), emblematic of a growing commitment to health-conscious lifestyles [1,2]. Primarily composed of Q235 steel, OFE capitalizes on its impressive amalgamation of robust strength, pliability, tenacity, and malleability [3][4][5], rendering it an ideal choice for materializing these fitness installations. However, because of its poor anticorrosion property [6], OFE is highly susceptible to corrosion, especially in some coastal areas or acid rain-prone areas [7]. It is reported that there were 134,000 accidents caused by OFE corrosion in 2018 alone. Therefore, it is particularly important to improve the anticorrosion property of Q235 steel to increase its service life.
Currently, the common methods used to improve the anticorrosion property of Q235 steel are cathodic protection [8], electroplating [9], physical vapor deposition [10], chemical vapor deposition [11], plasma overlay welding [12,13] and Laser Cladding [14]. However, a meticulous scrutiny of these surface treatment techniques reveals that each process has its own intrinsic flaws, rendering them ill-suited for the demanding of outdoor fitness equipment (OFE) with complex shapes. Cathodic protection is a relatively simple method, but it is easy to cause the inherent risk of hydrogen embrittlement, posing a formidable obstacle to its widespread adoption [15]. Electroplating has a variety of defects, encompassing leakage, blistering, and sanding, thus impeding its effectiveness [16]. Physical vapor deposition (PVD) is marked by low deposition rate and exorbitant equipment costs, and even more importantly, it is powerless for OFE's complex shapes [17,18]. Chemical vapor deposition (CVD) can cause excessive temperature in the vacuum chamber, and then resulting in the deformation of the matrix material [19,20]. Plasma overlay welding and Laser Cladding can also not be applied to OFE's complex shape structure, which limits its application. Therefore, it is of great significance to seek a appropriate surface treatment method, which can not only improve the anticorrosion property, but also deal with the complex shape structures.
To address the above problems, conversion coating process (CCP) is one of the most effective solutions due to its simple [21,22], low cost, and enable to adapt to OFE's complex shapes [22,23]. Currently, the most commonly used CCP is the chromate (Cr) CCP [24]. However, the toxic and carcinogenic nature of Cr 6+ has led to restrictions on its usage imposed by numerous countries [25]. As a result, the development of a new CCP to replace the conventional chromate CCP assumes critical importance. It is now well accepted that Ti/Zr conversion coating (TiZrCC) is the most promising alternative to CrCC [26], nevertheless its anticorrosion property, self-healing and adhesion are far from Cr 6+ coatings, thereby hampering its industrial application. Molybdenum (Mo) has low toxicity and is even an essential trace element for human body [27], and also it has the similar chemical properties with Cr due to the same family. Therefore, in this paper, we explored a new type of Ti/Zr based composite conversion coating assisted by Mo element, and its morphology, elements distribution, phase compositions and anticorrosion property were discussed. So as to provide the important theoretical and technical support for the safe service of OFE.

Materials and preparation of TiZrMoCC
The experimental matrix material is commercially available Q235 steel, and the specimens are cylindrical. The chemical composition of Q235 steel is shown in the following table 1.
To enhance the film forming ability, a three-step pretreatment process was employed on the specimen surface. Initially, the specimens underwent sequential polishing using sandpaper with grits ranging from 400 to 1200. Subsequently, a solution of 5% ZHM-1026 (procured from Wuhan Research Institute of Materials Protection, China) was used to eliminate the naturally formed oxide film and grease, followed by thorough rinsing with distilled water three times. Following the pretreatment stage, the specimens were immersed in the conversion solution to facilitate the creation of the conversion coating (CC). The formulation details for TiZrCC and the optimized TiZrMoCC conversion solutions can be found in table 2. The conversion parameters were that the conversion time (CTI) was 1, 3, 5, 10, 15, 20, 30 min; the conversion temperature (CTE) was in the range of 20°C-60°C; the pH value was 4.5 adjusted by ammonia.

Morphology and composition analysis
The micromorphology of TiZrMoCC on Q235 steel surface has been observed under S-3400N scanning electron microscope (SEM) with an acceleration voltage of 15 kV, and the chemical composition, distribution and valence state of the elements were qualitatively analyzed by Energy Dispersive x-ray Spectroscopy (EDX) and the x-ray photoelectron spectroscopy (XPS). Wherein, the spectra of XPS are corrected by C 1s with the binding energy of 284.8 eV.

Characterization of anticorrosion property 2.3.1. Copper sulfate dropping corrosion test (CSD)
The Copper Sulfate Dropping Corrosion Test (CSD) is a straightforward experiment designed to rapidly evaluate the anticorrosion properties of the conversion coating (CC). The test involves using a dropping solution composed of HCl (13 ml l −1 ), CuSO 4 ·5H 2 O (41 g l −1 ), and NaCl (35 g l −1 ). The duration required for the droplets to transition from sky blue to light red is recorded, with a longer dropping time indicating a more favorable anticorrosion property. Sample H 2 TiF 6 (mL/L) H 2 ZrF 6 (mL/L) ( NaPO 3 ) 6 (g/L) Na 2 MoO 4 ·2H 2 O (g/L)

Electrochemical test
An electrochemical test was conducted using a CHI660D electrochemical workstation. Stable electrodes were prepared by immersing them in a 3.5% NaCl solution for 15 min at room temperature, with an exposed area of 1 cm 2 . Subsequently, Electrochemical Impedance Spectroscopy (EIS) spectra were recorded within a frequency range spanning from 10 5 to 10 −2 Hz, using a sinusoidal AC voltage with an amplitude of 0.005 V. Following the EIS test, potentiodynamic polarization curves were generated by scanning the voltage from −2.0 to +0.5 V at a scan rate of 0.01 V s −1 . Fitting analysis techniques were then employed to determine the self-corrosion potential (E corr ) and the self-corrosion current density (i corr ).

Full immersion accelerated test
To address the challenges posed by the high humidity in the Huangshan area and the potential corrosion caused by human sweat, a Full immersion accelerated test was employed. This test aimed to simulate and quantify the average corrosion rate (ACR) of TiZrMoCC and Q235 steel. The test samples were immersed in a 3.5 wt% NaCl solution at room temperature for various durations: 24, 48, 72, 96, 120, and 144 h. Three samples were taken for each immersion time. Following the immersion, any loose corrosion products on the surface were carefully removed, weighed, and subsequently used to calculate the average corrosion rate (ACR) using equation (1).
where M 0 is original weight, M 1 is weight after removal of surface corrosion products, ρ is density of Q235 steel, T is immersion time, S is the corrosion area.

Effect of conversion solution parameters
Through an extensive series of experiments and meticulous screening processes, the main film-forming agent was determined as 3.0 ml l −1 H 2 TiF 6 , 1.3 ml l −1 H 2 ZrF 6 , and 0.5 g l −1 (NaPO 3 ) 6 , and the coupling effects ofconversion time (CTI) and conversion temperature (CTE) on dropping time were shown in figure 1. In general, CTI and CTE are closely related to the formation process of CC, and thus affect the anticorrosion property. CTI, as a pivotal conversion parameter, holds the key to the formation of a complete and robust CC. It becomes evident that a brief CTI fails to engender a complete CC, and thus causing a decrease in anticorrosion property. Conversely, an excessively protracted CTI can cause the CC formed on the surface to be dissolved, which will also reduce the anticorrosion property. Therefore, the optimal CTI for attaining the utmost anticorrosion property stands at 20 min. Based on the Arrhenius theory, CTE bears a direct relationship with the activity of conversion ions. A lower CTE curtails the reactivity of the ions, thereby impeding the coating-forming deposition rate. In instances where the CTI is brief, the complete CC can not be formed, and thus leading to the subpar anticorrosion property. However, while a higher CTE enhances ion activity and deposition rate, it concurrently engenders significant internal stress. Consequently, the CC formed is susceptible to degradation as the CTI is prolonged, ultimately compromising its anticorrosion property. Moreover, elevated CTE intensifies the cathode reaction, prompting the release of copious amounts of hydrogen. The ensuing hydrogen gas flow poses a hindrance to the formation of the conversion coating, exacerbating the deterioration of coating quality and undermining its anticorrosion property. To sum up, the optimum CTI and CTE are determined to be 20 min and 30°C respectively.

The effect of sodium molybdate
The longevity of outdoor fitness equipment (OFE) hinges directly on the strength of its anticorrosion property. In pursuit of further enhancing the anticorrosion property, sodium molybdate was introduced alongside the aforementioned optimal conversion parameters, and the relationship between its addition amount and anticorrosion property is graphically depicted in figure 2. Evidently, the anticorrosion property exhibit a rapid increase as the content of sodium molybdate rises from 0 to 0.5 g l −1 , and the best anticorrosion property corresponding to the addition is 0.5 g l −1 . However, once the content surpasses 0.5 g l −1 , the anticorrosion property witness a marked decline, reaching a nadir at 1.5 g l −1 . Subsequently, as the quantity of sodium molybdate continues to increase, the anticorrosion property remains consistently low, with only minor fluctuations. Consequently, the most effective addition amount of sodium molybdate is determined to be 0.5 g l −1 .

Electrochemical analysis
The corrosion characteristics of Q235, TiZrCC, and TiZrMoCC were investigated through the analysis of potentiodynamic polarization curves. These curves were obtained in a neutral 3.5 wt.% NaCl solution, as visually depicted in figure 3 for each material. While the curves demonstrate minimal disparity in terms of shape, signifying that the conversion coating did not alter the reaction mechanisms of the anodic and cathodic reactions. Notable distinctions are observed in the positions of the three curves. These variations account for the divergent anticorrosion property exhibited by each specimen.
The anticorrosion properties of Q235, TiZrCC, and TiZrMoCC were analyzed by determining the corrosion potential (E corr ) and corrosion current density (i corr ) through data fitting, and the corresponding fitting results are shown in figure 4. E corr is a crucial thermodynamic parameter that indicates sensitivity to corrosivity. The E corr values for Q235, TiZrCC, and TiZrMoCC were found to be −0.728 V, −0.668 V, and −0.607 V, respectively. Notably, the E corr values of TiZrMoCC shifted towards a more positive direction, indicating lower susceptibility to corrosion. On the other hand, i corr , an important kinetic parameter, serves as an estimation of the anticorrosion property. A higher i corr value signifies a faster corrosion rate. For Q235 and TiZrCC, the i corr values were 8.97 × 10 −4 A cm −2 and 2.15 × 10 −4 A cm −2 , respectively, while that for TiZrMoCC was 6.13 × 10 −5 A cm −2 . Comparing TiZrMoCC to the Q235 matrix, the i corr value decreased by approximately 14 times, indicating that TiZrMoCC significantly reduces the corrosion rate and improves its anticorrosion property.
In order to gain a deeper understanding of the interaction between the electrode surface and the studied compounds (charge transfer, diffusion, adsorption), Nyquist diagrams were employed as a complementary analysis alongside potentiodynamic polarization curves. The Nyquist diagrams, as shown in figure 5, exhibited semicircular capacitive behavior across the entire frequency range for all specimens. To accurately interpret the electrochemical impedance spectroscopy (EIS) data, the appropriate R(C(R(CR))) Randle equivalent circuit model (ECM) was used, and the fitted model is presented in the inset.
In the ECM, the parameters R s represent the solution resistance, R f represents the film resistance, which is connected in parallel with a constant phase element (CPE f ), and R ct represents the charge transfer resistance on the metal surface, which is connected in parallel with a constant phase element (CPE dl ). The corresponding values of these parameters have been calculated and are provided in table 3. According to table 3, the R s values for Q235, TiZrCC, and TiZrMoCC were determined to be 13.48 Ω cm −2 , 12.13 Ω cm −2 , and 9.33 Ω cm −2 , respectively. These results indicate that the tested electrochemical solution of 3.5 wt% sodium chloride has good conductivity.
The R f values for Q235 and TiZrCC is 36.16 and 59.25 Ω cm −2 respectively, whereas that of TiZrMoCC has the largest R f value of 148.70 Ω cm −2 , which indicate that TiZrMoCC has a favorable anticorrosion property itself. Another important parameter, R ct , characterizes the charge transmission between the surface and the corrosive medium. It is evident that the R ct value of Q235 matrix after CCP is also greatly improved, especially for TiZrMoCC, where it is nearly three times higher than that of the Q235 matrix alone. This outcome suggests that TiZrMoCC effectively impedes charge transfer between the surface and the corrosive medium. In summary, TiZrMoCC possesses favorable anticorrosion property and effectively controls the occurrence of interfacial electrochemical reactions, thereby providing substantial protection to the Q235 matrix.

Full immersion accelerated test
OFE may encounter various adverse conditions under natural conditions, and full immersion accelerated test mainly used to simulate its accelerated corrosion behavior and then rapidly evaluate its comprehensive anticorrosion property. Figure 6 illustrates the average corrosion rate (ACR) of the Q235 matrix and TiZrMoCC immersed in a neutral 3.5 wt% NaCl solution at room temperature for different durations. As can be seen that the ACR of TiZrMoCC is significantly lower than that of the Q235 matrix, indicating that TiZrMoCC can effectively prevent chloride ion erosion and protect the Q235 matrix from corrosion. For instance, after an immersion time of 144 h, the ACR values for the Q235 matrix and TiZrMoCC are 0.167 and 0.061 mm a −1 ,    respectively. This demonstrates a remarkable decrease of approximately 63.5% in ACR for TiZrMoCC compared to the Q235 matrix. Moreover, due to the short immersion time, the Q235 matrix and TiZrMoCC both exhibit an overall increasing trend in ACR with prolonged immersion time. Figure 7 shows morphology evolution of the TiZrMoCC with better anticorrosion property, and there are many grooves on the surface in the first 10 min of the conversion reaction. These grooves primarily result from the sanding process during sample pretreatment, and the generated CC with a short CTI is too thin to completely cover the entire groove. Notably, after a CTI of 3 min, irregularly shaped white flocculent substances become apparent on the surface. To further investigate these white flocculent substances, area scanning was conducted on the surface of TiZrMoCC with a CTI of 3 min, as depicted in figure 8. From the analysis of figure 8, it is evident that the white flocculent material region exhibits a low number of iron atoms while having a relatively high number of oxygen atoms, accompanied by a certain presence of titanium (Ti), zirconium (Zr), and molybdenum (Mo) atoms. Consequently, it can be concluded that the white flocculent material mainly consists of aggregated metal oxides, resulting in the formation of conversion coating microregions. As the CTI exceeds 10 min, the grooves on the surface gradually become filled out; as the CTI  reaches 20 min, the surface appears relatively flat, and the grooves become almost indiscernible. However, with a further increase in the CTI to 30 min, slight grooves reappear on the surface. This phenomenon may be attributed to the reverse dissolution of the generated conversion coating. Figure 9 shows the atomic percentages of TiZrMoCC with different CTIs, revealing that the surface primarily consists of Ti, Zr, Mo, Fe, and O atoms. During the conversion reaction process, Ti, Zr and Mo ions (figure 9(a)) in the conversion solution accompanied by the O atom generated in the anode reaction ( figure 9(b)) continuously deposit on the metal surface, leading to an increase in their concentrations, and the concentration peak occurs at the CTI of 20 min. Conversely, the concentration of iron atoms exhibits an opposite trend, mainly due to the growth of TiZrMoCC and the dissolution of the Q235 matrix on the surface. It is worthy to point out that the peak concentration, relatively flat micromorphology, and the optimal anticorrosion property all correspond to the CTI of 20 min This is primarily because sufficient conversion ions are deposited on the surface, effectively filling the surface grooves and resulting in a flat surface. As a result, the invasion of corrosion ions is prevented, ultimately enhancing its overall anticorrosion property.

XPS analysis
To delve deeper into the elemental compositions and valence states of TiZrMoCC, comprehensive x-ray photoelectron spectroscopy (XPS) analysis was conducted. The full XPS spectrum is presented in figure 10, while high-resolution XPS spectra are displayed in figure 11. Figure 10 reveals that the surface of TiZrMoCC consists of elements such as Ti, Zr, Mo, Fe, O, F, and C. This observation aligns closely with the findings obtained from Energy-Dispersive x-ray (EDX) analysis, thus confirming the consistency of the results. Wherein the C element  (tiny C1s peak of 284.8 eV) can be attributed to impurities and surfactants. Since the primary focus is on the detailed analysis of other elements, we have omitted a comprehensive examination of the C element, which is primarily used to correct the full XPS spectrum. Figure 11 exhibits the detailed high-resolution XPS spectra of O 1s, Fe 2p, Ti 2p, Zr 3d, Mo 3d, and F 1 s spectra and their fitting peaks are shown in figures 11(a)∼(f), respectively. The peaks of O 1s spectra ( figure  11(a)) can be fitted into two peaks: the peak located at 529.64 eV is assigned to MoO 3 , TiO 2 , ZrO 2 ; the other peak located at 531.27 eV corresponds to Fe 2 O 3 . The Fe 2p spectrum ( figure 11(b)) contains two fitting peaks located at about 711.49 and 724.84 eV, and the two peaks are both assigned to Fe 2 O 3 .
The Ti 2p spectrum (figure 11(c)) displays two distinct peaks located at 458.44 and 464.12 eV, which can be attributed to TiO 2 . Similarly, the Zr 3d spectra (figure 11(d)) exhibit two peaks: the peak at 185.11 eV corresponds to ZrF 4 , while the peak at 182.76 eV corresponds to ZrO 2 . Moving on to the Mo 3d spectra ( figure  11(e)), four peaks are observed: the peaks at 235.58 and 232.53 eV are attributed to MoO 3 , while the peaks at 235.61 and 232.06 eV are attributed to Mo 2 O 5 . Analysis of the F 1 s spectra (figure 11(f)) reveals two peaks: the Figure 11. The high-resolution XPS spectra of TiZrMoCC. peak at 683.98 eV is assigned to Na 3 FeF 6 , while the other peak at 684.53 eV is assigned to ZrF 4 . In summary, the composition of TiZrMoCC primarily comprises various metallic oxides, including TiO 2 , ZrO2, MoO 3 , Mo 2 O 5 , Fe 2 O 3 , along with a few metallic fluorides such as Na 3 FeF 6 and ZrF 4 .

Formation process of TiZrMoCC
Normally, OFE expose to wind, rain, and Sun in natural environment, the surface of Q235 is prone to generate a reddish brown metal rust layer, which is loose and not dense, as shown in figure 12(a). In order to ameliorate the surface activity and further improve the adhesion between the coating and the matrix materials, the Q235 matrix underwent an abrasion process using emery paper, followed by washing with 5% ZHM-1026 to eliminate the naturally formed oxide film and grease. Subsequently, the freshly exposed bare surface was immersed in the conversion solution, initiating a complex film-forming process.
Q235 steel, a typical low-carbon steel, consists of ferrite and pearlite phases. The pearlite phase is a mixed structure with a lower potential compared to ferrite. As a result, ferrite acts as the micro-anode, while pearlite functions as the micro-cathode in this electrochemical corrosion system. In this system, ferrite tends to undergo electron loss and dissolve in the electrolyte, resulting in the generation of Fe 3+ ions (as depicted in equation (2)). On the other hand, reactions in the pearlite region involve two main pathways: the H + ions accept electrons to form H 2 (g) (as shown in equation (3)), and the H 2 O and dissolved oxygen in the electrolyte accept electrons to produce OH − ions (as shown in equation (4).
- ( ) The presence of OH − ions trigger a positive reaction with Fe 3+ ions, resulting in the formation of poorly soluble Fe(OH) 3 on the surface. Subsequently, Fe(OH) 3 undergoes dehydration and transforms into Fe 2 O 3 , as illustrated in equation (5). Simultaneously, TiF6 2− , ZrF6 2− , and MoO4 2− ions present in the conversion solution adsorbed on the surface and participate in reactions, leading to the creation of nucleation centers for the corresponding metal oxides. These reactions can be represented by equations (6)- (9). The nucleation process associated with the different metal oxides is depicted schematically in figure 11( ( ) Figure 12. The schematic diagrams of the growth process.