P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... 707–717 EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE PHOSPHATE CONVERSION COATING ON 65Mn SPRING STEEL RAZISKA V A KOROZIJSKE ODPORNOSTI IN MIKROSTRUKTURE KONVERZIJSKE PREVLEKE NA OSNOVI MANGAN FOSFATA, NANE[ENE NA VZMETNO JEKLO VRSTE 65Mn Peng-fei Zhang 1,2,3* , Ke-yao Li 2 , Gang Ji 2 ,QiLiu 2 , Long-fei Xie 2 , Lin-Feng Wei 2 , De-cheng Wang 3 1 University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, 100083, P.R. China 2 China Xi’an Rare Metal Materials Institute Co., Ltd., 96 Weiyang Road, Xi’an, 710016, P.R. China 3 China Academy of Machinery Science and Technology, 2 Shouti Road, Haidian District, Beijing, 100044, P.R. China Prejem rokopisa – received: 2024-05-20; sprejem za objavo – accepted for publication: 2024-09-25 doi:10.17222/mit.2024.1198 Manganese phosphate conversion coatings were prepared on 65Mn spring steel. The surface morphologies and compositions of the coatings were examined through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion resis- tance of the coatings was investigated using a copper sulfate titration experiment, salt spray test and electrochemical corrosion test. The results showed that a uniform and dense coating was obtained when the acid ratio was 10, the phosphating temperature was 85 °C, and the phosphating time was 5 min. There was no obvious rust on the manganese phosphate coating surface after 48 h of salt spray. Ultimately, the growth mechanism and corrosion behavior of the manganese phosphate conversion coating are explained. Keywords: 65Mn spring steel, manganese phosphate, microstructure, corrosion resistance Avtorji so pripravili konverzijsko prevleko na osnovi mangan-fosfata na vzmetnem jeklu vrste 65Mn. Morfologijo povr{ine in sestavo prevleke so analizirali s pomo~jo vrsti~nega elektronskega mikroskopa (SEM) in rentgenske difrakcije (XRD). Korozijsko odpornost prevleke so preiskovali s pomo~jo titracije bakrovega sulfata, testa s sprejem slanice in elektro-kemijskih korozijskih preiskav. Rezultati analiz so pokazali, da so izdelali enovito (homogeno) in gosto prevleko pri kislinskem razmerju 10, temperaturi fosfatiranja 85 °C in ~asu fosfatiranja 5 min. Po 48 urni obdelavi s sprejem slanice avtorji niso opazili nobenega znaka o~itnega rjavenja prevleke. Avtorji v tem prispevku na koncu razlo`ijo {e mehanizem rasti in korozijsko obna{anje izdelane mangan-fosfatne konverzijske prevleke. Klju~ne besede: vzmetno jeklo vrste 65Mn, mangan-fosfat, mikrostrukture, korozijska odpornost 1 INTRODUCTION 65Mn steel is one of the famous spring materials with high strength and good capability of plastic deforma - tion. 1 It is widely used for automobile suspension springs, leaf springs, 2 mechanical springs and so on. 3–4 However, springs in the process of manufacture, storage and use are often eroded by a moist atmosphere, salt fog in the sea atmosphere, and industrial waste gas and so on, 5–7 which makes its use limited. Thus, it is essential to enhance the corrosion resistance of the 65Mn steel sur- face and extend the fatigue life of springs. At present, to improve the corrosion resistance, spring surfaces use anodic plating, polymer coating, 8 ox- idation treatment, 9 micro-arc oxidation, 10 and so on. The chemical method allows us to make something promi- nent due to its simple operation and stability. Chromium salt treatment is an effective method of anticorrosion in traditional approaches, but it significantly increases car- cinogenicity and is environmentally not friendly due to chromium being virulent. 11,12 The choice of environmen- tally friendly non-chromium coatings has aroused wide attention. Phosphate coating is widely used in the auto- motive, marine, aerospace, and manufacturing industries because of its low cost and high corrosion resistance. 13 Manganese phosphate conversion coating is used for springs due to its low cost, and rapid action combined with corrosion-resistance and self-lubricating properties. Shanmugam et al. improved the quality of manganese phosphate coating on mild steel using permanganate as an additive to increase iron dissolution. 14 Guo et al. en- hanced the corrosion resistance of Mn-P coating on Q235 steel by adjusting the phosphorus-manganese ra - tio. 15 Nikdehghan et al. investigated the surface morphol- ogy and corrosion resistance of Zn-Mn phosphate coat- ing on AISI 4130 steel under different heat treatments. 16 A thin and dense phosphate coating was obtained when Mn 2+ : PO 4 3 − = 1 : 7. There was no obvious corrosion af- ter 30-h salt spray. In order to improve the corrosion re- sistance of phosphate conversion coating on an Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 707 UDK 669.018.27:57.012.3 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek Mater. Tehnol. *Corresponding author's e-mail: zhangpf1007@163.com (Peng-fei Zhang) electrogalvanized steel sheet, Ni 2+ and Mn 2+ ions were added to the phosphate solution by Su. 17 Zang et al. re- searched the tribological properties of manganese phos- phate conversion coatings on the AISI 52100 steel sur- face. 18 The results showed that the friction coefficients of Mn-PH coatings were reduced by 15 %. However, the formation of manganese phosphate conversion and the anti-corrosion properties of the coating on 65 Mn spring steel have been seldom reported. In this study, the surface phosphating process (acid ratio, phosphating temperature and phosphating time) of 65 Mn spring steel was studied using the orthogonal test method. The anti-corrosion of phosphating conversion coating is researched using a copper sulfate titration ex- periment, salt spray test and electrochemical corrosion test. The microstructures and compositions of coatings applied using different phosphating processes on a 65Mn spring steel substrate were researched, and the corrosion resistance was evaluated. Simultaneously, the growth mechanism and corrosion behavior of manganese phos- phate on the surface of 65 Mn spring steel were deter- mined. Finally, the phosphating-process parameter com- bination was optimized to improve the corrosion resistance of the spring surface. 2 EXPERIMENTAL PART Commercial 65Mn steel (composition in w/%: 0.64 C, 0.22 Si, 0.99 Mn, 0.003 S, 0.022 P, 0.16 Cr, 0.02 Ni, 0.04 Cu, with the balance being Fe) was cut into (20 × 12 × 3) mm samples. The substrate was ground with #400 to #1500 SiC abrasive paper and rinsed with deionized water. The specimens were degreased with an alkaline solution (10 w/% KOH) for 6 min at room tem- perature (RT), thereafter, they were cleaned with deionized water. The BT-320 powder of titanium phos- phate salt at a concentration of 3–5 g/L (Dongguan Dingrun Chemical Technology Co., LTD.) was used as pre-dip for 1 min. After cleaning with deionized water, the sample surface was treated by manganese phosphat- ing. The phosphating solution was mainly composed of Mn 2+ ,N O 3– , PO 4 3 − ,H 3 PO 4 , and other additives. The acid ratio was defined as the ratio of free acidity to total acid- ity. Orthogonal experiments with three factors and four levels were conducted, considering the following process parameters: acid ratio (4, 6, 8, and 10), phosphating tem- perature (65, 75, 85, and 95) °C, phosphating time (3, 5, 7, and 9 min). Phosphating temperature was controlled by a water bath kettle. The H 2 Cr 2 O 7 (0.3–0.45 g/L, pH 2–4) solution was used for phosphating post-treatment passivation at 50 °C for 2 min. After the phosphating experiments, the sample sur- faces were analyzed using scanning electron microscopy (SEM, GeminiSEM 500). The phase constitution of the coatings were analyzed using X-ray diffraction (XRD, Rigaku D/max-2500/PC) with CuK radiation. The cor- rosion resistance of phosphate coatings was determined using the CuSO 4 drop method in accordance with the GB6807-86 standard. When 1–2 drops of the solution are applied to the phosphate coating, a color change from blue to yellow or light red is observed. The solution is composed of 40 mL CuSO 4 (0.25 mol/L), 0.8 mL HCl (0.10 mol/L) and 20 mL NaCl (10 %). The phosphating coatings were removed by a 50 g/L CrO 3 solution at 75 °C for 15 min in accordance with the GB/T 9792-2003 standard, and then cleaned with deionized water. Each sample used a new solution. The mass of the samples and removal of the coatings were denoted as m 1 and m 2 , respectively. A was the phosphat- ing-coating surface area. The weight of the phosphating coating was calculated with the following formula: m mm A A = − × 12 4 10 (1) A salt-spray test was performed in a salt-spray test chamber (Beijing Donggong Lianhua Scientific Instru- ment and Equipment Co., LTD, YWX-1200) in accor- dance with the GB/T 10125-1997 standard. The 5 % NaCl solution was used as the corrosion medium spray, and the salt spray volume was 1–2 mL/min. The test chamber temperature was 35 °C for 48 h. The corrosion resistance of the coatings was tested using a standard three-electrode cell (CHI660E) in a 3.5 w/% NaCl aque- ous solution. The auxiliary electrode and reference elec- trode were a platinum electrode and saturated calomel electrode (SCE), respectively. When the open circuit po- tential was stable, a dynamic polarization curve with a scan rate of 1 mV/s was obtained. Electrochemical im- pedance spectroscopy (EIS) was performed at a fre- quency range of 10 –2 –10 5 Hz. The EIS data were fitted using ZSimDemo 3.30 software. The corrosion morphol- ogy surface was observed with SEM. 3 RESULTS 3.1 Surface morphology of the coating Results of the three-factor and four-level orthogonal tests, considering also macroscopic morphology, coating weight and copper sulfate corrosion resistance time are shown in Table 1. It can be seen that the macroscopic morphology of a coating appears to be gray, ash black and black. The phosphate coating is partially non- phosphated on Samples 1 and 2. This is due to the low acid ratio and slow reaction progress. With an increase in the phosphating temperature and time, the surface of phosphate coating (Samples 3 and 4) appears gray black and smooth. Meanwhile, the surface coverage of phos- phate coating is low on Samples 5 and 6. Thus, the weight of their phosphating coating is low. The phosphating reaction is slow and the phosphate coating is thin when the acid ratio is low. The acid ratio can con- trol the equilibrium state of ionization and hydrolysis in the phosphating solution. It can be seen that Sample 8 P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... 708 Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 has a gray and smooth surface, and its phosphating coat- ing weight is 19.54 g/m 2 . As we know, the phosphating temperature is an im- portant factor, which directly affects the speed of phosphating and the performance of phosphate coating. 19 The surface of Sample 9 is black and smooth, and its copper sulfate corrosion resistance time is 120 s. How- ever, the surface of Sample 11 is dense, grayish black, its phosphate coating weight is 27.58 g/m 2 , and its copper sulfate resistance time is 115 s. Besides, the phosphating temperature can improve the thermodynamics and dy- namics of phosphate-solution molecules, which can im- prove the phosphating process. It can be found that the phosphate-coated surface of Sample 13 appears to ex- hibit an ash-like phenomenon. This may be because the acid ratio is too large, making the anodic oxidation of the metal surface too slow and the etching of the metal sur- face insufficient. There is a large amount of precipitation and turbidity of the phosphating solution when there is excessive hydrolysis of dihydrogen phosphate salt. Finally, the sediment floats in the phosphating solution and adheres to the 65Mn substrate to form the hanging ash phenomenon. P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 709 Table 1: Macroscopic morphology, coating weight and copper sulfate corrosion resistance time with different process parameters No. Acid ratio Phosphating temperature (°C) Phosphating time (min) Macroscopic morphology Evaluation Coating weight (g/m 2 ) Corrosion resis- tance time of cop- per sulfate (s) 146 53 Gray, partially non-phosphated 12.21 48 247 55 Gray, partially non-phosphated 18.44 60 3 4 85 7 Black, rich in grains 20.25 68 4 4 95 9 Light black 19.02 75 566 55 Gray, partially non-phosphated 12.5 54 667 53 Gray, partially non-phosphated 13.87 58 7 6 85 9 Black, rich in grains 22.58 78 869 57 Dark gray, smooth 19.54 98 9 8 65 7 Black, smooth 21.78 120 10 8 75 9 Ash black, rough sur- face 24.91 90 11 8 85 3 Ash black, smooth 27.58 115 12 8 95 5 Gray, partially non-phosphated 12.85 63 13 10 65 9 Off-white, hanging ash 11.54 85 14 10 75 7 Ash black 13.05 75 15 10 85 5 Black, smooth 22.25 132 16 10 95 3 Partially non-phosphated 14.86 53 In short, the microstructures and phase compositions of the phosphating surfaces were determined for Samples 4, 7, 9, 10, 11 and 15. The corrosion resistance of phosphating surfaces was examined using electrochemi- cal corrosion and salt spray tests. Figure 1 shows SEM morphologies of samples 4, 7, 9, 10, 11 and 15. It can be seen from Figure 1a that the grain gap is large and the surface coverage of phosphate coating is low when the process parameters include an acid ratio of 4, phosphating temperature of 95 °C and phosphating time of 9 min. This indicates that the phos- phate coating has not been fully grown. When the phosphating temperature is reduced to 75 °C (Sample 7), the phosphate coating exhibits blocky grain crystals (of approximately 135.3 μm). It can be seen from Figure 1d that the phosphate coating surface is complete. Neverthe- less, the crystal grain size is non-uniform, and some small grains cluster together. The phosphate coating has a clear, large, oblique prismatic structure when the acid ratio is 8, the phosphating temperature is 75 °C, and the phosphating time is 9 min (Sample 10). Moreover, the microstructure of Sample 11 shows a massive accumula- tion of crystals with a large grain size, while the cover- age rate is approximately 90 %. As shown in Figure 1h, the non-phosphating area, indicated by the yellow arrow, is more prone to corrode. Finally, the phosphate coating grains of Sample 15 (Figure 1j) are fine and the grain size is about 9.98 μm. Meanwhile, Sample 15 shows a high surface coverage. In accordance with the above analysis, Figure 2 shows SEM morphologies and the distributions of C, Fe, Mn, O and P elements in typical phosphate coatings of Samples 7, 10 and 15. It can be seen from Figure 2a 1 –2f 1 that the distribution of the C element is low and the grain size is inconsistent, while the O and Mn ele- ments are distributed in the phosphate coating and the substrate. This indicates that the Mn and P elements in- filtrated into the substrate. Figure 2a 2 –2f 2 shows the dis- P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... 710 Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 Figure 1: Scanning electron micrographs of manganese phosphate conversion coating after different phosphating processes: a) Sample 4, b) Sam- ple 7, d) Sample 9, f) Sample 10, h) Sample 11, j) Sample 15; c), e), g), i), k) are the magnified images of the red quadrangles Figure 2: Surface morphology and surface element distribution of typ- ical phosphating coating: (a 1 –f 1 ) for Sample 7; (a 2 –f 2 ) for Sample10; (a 3 -f 3 ) for Sample15 tribution of elements on the phosphate coating surface of Sample 10. It can be seen that the Fe element is mainly distributed in the ravine-like grains in the bottom, indi- cating that the 65Mn substrate partial region has no phosphate coating. As can be seen from Figure 2a 3 –2f 3 (Sample 15), the surface grains are fine and elements C, Fe, Mn, O and P are evenly distributed. The coverage rate is higher than that of other samples. Thus, the opti- mal process parameters for the phosphating treatment of 65Mn spring steel are an acid ratio of 10, a phosphating temperature of 85 °C and a phosphating time of 5 min. Figure 3 presents SEM cross-section morphologies of samples 7, 10, and 15. It can be seen that the phos- phate coating is well combined with the substrate. The thickness of the phosphate coating was calculated as the average of the values obtained at three different positions using Image-pro Plus 6.0 software. The thickness of phosphate coating is about 13.82 μm (see Figure 3a) when the phosphating solution acid ratio is 6, phosphat- ing temperature is 85 °C and phosphating time is 9 min. This coating thickness is due to a loose intergranular ar- rangement and spherical particle accumulation. As can be seen on Figure 3b, the phosphate coating of sample 10 has an obvious, oblique, prismatic structure and its thickness is about 9.93 μm. When the acid ratio is 10, the phosphating temperature is 85 °C and the phosphating time is 5 min, the thickness of phosphate coating (Sam- ple 15) is 10.14 μm. 3.2 Phase composition of the coating Figure 4 shows the XRD patterns of 65Mn steel sub- strate and Samples 7, 10, and 15. Based on Figure 4a, the 2 values of 44.67°, 65.02° and 82.33° are character- istic peaks of Fe corresponding to diffraction peaks of the substrate. The manganese phosphate conversion coat- ing is composed of Mn 5 (PO 4 ) 2 (PO 3 (OH)) 2 ·4H 2 Oa n d MnHOP 4 ·2.25H 2 O insoluble phosphate. The diffraction peaks of the Mn 5 (PO 4 ) 2 (PO 3 (OH)) 2 ·4H 2 O phase can be observed at 10.11°, 26.24°, 27.61°, 30.67°, 35.34° and 36.62°, and the diffraction peaks of the MnHOP 4 ·2.25H 2 O phase are at 10.99°, 34.06°, 35.02°, 56.02° and 58.35°. The XRD peaks of phosphate coating are basically the same, indicating that the phosphate coating has little influence on the acid ratio, phosphating temperature and phosphating time. Thus, different pro- cess parameters affect the crystallinity of the phosphate coating without changing the phase composition of the coating. P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 711 Figure 4: XRD patterns of 65Mn substrate and coated samples: a) 65Mn spring steel, b) Sample 7, c) Sample 10, d) Sample 15 Figure 3: Cross-section morphologies of coated samples: a) Sample 7, b) Sample 10, c) Sample 15 3.3 Corrosion resistance performance 3.3.1 Salt spray test Figure 5 shows the macroscopic morphologies of 65Mn substrate and Samples 7, 10 and 15 after the salt spray test for 48 h. It can be seen that there are large ar- eas of rust on the 65Mn spring steel substrate. Neverthe- less, the surface of Sample 7 includes yellow rust. Salt water is a typical electrolyte. Cl – adsorbed on the surface of the spring steel formed a strong electric field, which accelerated the anodic dissolution of the substrate. It can also be seen on Figure 5c that Sample 10 has some yel- low spots with a rough surface, indicating that the coat- ing suffered local corrosion. On Figure 5d,i tc a nb e clearly seen that the surface of the sample is perfect, and the phosphate coating remains ash black. Phosphate coating is a non-conductive and insoluble phosphate con- version coating. It can isolate the corrosive medium from the substrate to improve the corrosion resistance of the substrate. 3.3.2 Electrochemical corrosion test Figure 6 shows the electrochemical corrosion polar- ization curves of 65Mn spring steel substrate, and coated samples 7, 10 and 15 in the 3.5 w/% NaCl solution. The anode slope ( a ), cathode slope ( c ), corrosion potential (E corr ) and corrosion current density (I corr ) 20 were obtained using the Tafel curve extrapolation method, and the po- larization resistance (R p ) was expressed with the Stern-Geary equation. 21,22 R I p ac corr a c 2.303 = + () (2) Electrochemical data for different specimens were calculated with the Tafel extrapolation method, as shown in Table 2. The electrochemical corrosion potential (E corr ) of the phosphate coating is higher than that of the 65Mn spring steel substrate, and the I corr is 3.893 × 10 –6 A/cm 2 . This indicates that the corrosion resistance of the phosphate coating is better than that of the 65Mn spring steel. In addition, it can be seen from Table 2 that the polarization resistance (R p ) of the phosphate coating is greater than that of the substrate. Table 2: Parameters of the potentiodynamic polarization curves E coor (V) I corr (A/cm 2 ) R p ( ·cm 2 ) 65Mn steel –0.837 1.210×10 –5 3744 Sample 7 –0.770 7.552×10 –6 5399 Sample 10 –0.703 4.443×10 –6 9894 Sample 15 –0.649 3.893×10 –6 11279 Figure 7 shows the electrochemical impedance spec- troscopy (EIS) of 65Mn substrate and Samples 7, 10, 15 in the format of both Nyquist plots (Figure 7a) and Bode plots (Figure 7b and 7c). It can be seen on Figure 7a that the semicircle diameters of Samples 15, 10, 7 and the 65Mn substrate are successive. Meanwhile, it is worth noting that the semicircle diameter of Sample 15 is about four times that of the 65Mn substrate. In Fig- ure 7b, Sample 15 has the maximum Z value in the low-frequency region. The phase angle at a high fre- P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... 712 Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 Figure 6: Tafel polarization curves of the samples measured in 3.5% NaCl solution: a) 65Mn spring steel, b) Sample 7, c) Sample 10, d) Sample15 Figure 5: Surface morphologies of the samples tested using a 48-h salt spray test: a) 65Mn spring steel, b) Sample 7, c) Sample 10, d) Sample 15 quency can be used to explain the integrity of the coat- ing. 23–24 As shown in Figure 7c, Sample 15 shows higher values than the 65Mn substrate in the high-frequency re- gion, indicating that the manganese phosphating treat- ment can improve the corrosion resistance of the phos- phate coating and reduce its defects. To obtain impedance parameters, the equivalent circuit (EC) was analyzed using ZSimDemo 3.30 software, as shown in Figure 7d. The fitting parameters are listed in Table 3. R s values are lower than 20 ·cm 2 , indicating reliable fit- ting data. All chi-square ( 2 ) values are almost less than 0.01, indicating a high fitting degree. 25,26 Here, R s and R f are solution resistance and phosphate coating resistance, respectively. R ct is the charge transfer resistance, while CPE d represents a bidirectional capacitor. The slower the dissolution rate of the substrate, the greater is the R ct value. In Table 3, the R f value of Sample 15 is twice that of the 65Mn substrate, indicating better corrosion resis- tance. 3.3.3 Corrosion morphology Figure 8 shows the surface morphology of the 65Mn substrate and phosphate coating with different process parameters after immersion in the 3.5 % NaCl solution. It can be seen from Figure 8a that the substrate is se- verely corroded, having large white corrosion pits. When the phosphating solution acid ratio is 8, the phosphating temperature is 75 °C, and the phosphating time is 9 min, the substrate suffers spot corrosion when exposed to the NaCl solution. Nevertheless, it can be seen that Sample 15 maintains fine gray-black grains with some NaCl par- ticles. Phosphate conversion coating is insoluble and non-conductive, and can prevent the corrosion of 65Mn spring steel. P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 713 Table 3: Electrochemical data obtained by equivalent circuit fitting of the EIS spectra 65Mn Sample 7 Sample 10 Sample 15 R s ( ·cm 2 ) 6.268 9.424 9.888 16.07 CPE f 0.0008154 0.0001171 5.715×10 –5 3.906×10 –5 n f 0.6905 0.516 0.6645 0.5041 R f ( ·cm 2 ) 1972 43.53 41.15 4138 R ct ( ·cm 2 ) 1.182×10 5 3988 5558 6723 CPE d 4.084×10 10 2.661×10 –5 6.326×10 –5 1.555×10 –4 n d 0.7919 0.6738 0.6968 0.5688 2 3.356×10 –3 8.774×10 –4 6.434×10 –4 7.511×10 –4 Figure 7: Electrochemical impedance: a) Nyquist plots, b) and c) Bode plots, d) equivalent circuit 4 DISCUSSION 4.1 Formation mechanism of the manganese phos- phate coating In the present study, the manganese phosphate coat- ing was prepared on the 65Mn spring steel. Phosphating is a complex chemical reaction process, which is also an electrochemical reaction. To further understand the phos- phate coating forming process, the formation mechanism can be illustrated with the schematic diagram shown in Figure 9. The formation of the manganese phosphate coating consists of two parts. 27 The first step is the mi- cro-battery reaction (Figure 9a), which includes the dissolution of the 65Mn spring steel anode and the reac- tion of cathode hydrogen evolution. When the 65Mn spring steel is immersed in a phosphating solution, Fe begins to dissolve and H + releases hydrogen gas. The chemical reactions are as follows: anode reaction: Fe Fe 2+ +2 e – (3) cathode reaction: 2H 2 O+2 e – 2OH – +H 2 (4) P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... 714 Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 Figure 8: Corrosion morphologies of the samples after immersion in 3.5 w/% NaCl solution: a) 65Mn spring steel, b) Sample 7, c) Sample 10, d) Sample 15 Figure 9: Diagram of phosphate coating on the 65Mn spring steel surface overall reaction: Fe + 2H 2 O Fe 2+ +H 2 + 2OH – (5) The phosphoric acid solution dissociates into PO 4 3 − , HP O 24 3 − , HPO 4 2 − , and other ions. Therefore, there is an increase in the pH value of the solution. Fe 2+ , PO 4 3 − , OH − , HPO 4 2 − and Mn 2+ can react and form an insoluble phosphate. Mn 2+ and HPO 4 2 − are involved in the reaction to form MnHOP 4 ·2.25H 2 O crystals. Mn 2+ , PO 4 3 − and HPO 4 2 − react to form Mn 5 (PO 4 ) 2 (PO 3 (OH)) 2 ·4H 2 O (see Figure 4). The responses are as follows: Mn 2+ + HPO 4 2 − + 2.25H 2 O MnHPO 4 · 2.25H 2 O (6) 5Mn 2+ +2 PO 4 3 − + 2(PO 3 (OH)) 2– +4 H 2 O Mn 5 (PO 4 ) 2 (PO 3 (OH)) 2 ·4 H 2 O (7) There is an insoluble phosphate and a dense coating structure composed of Mn 5 (PO 4 ) 2 (PO 3 (OH)) 2 ·4 H 2 O and MnHOP 4 · 2.25H 2 O. 28–30 As mentioned above, the den- sity of the coating is the key factor affecting the acid ra- tio and phosphating temperature. 4.2 Corrosion behavior of the manganese phosphate coating Figure 10 shows the diagram of the corrosion of 65Mn spring steel and manganese phosphate coating im- mersed in the 3.5 % NaCl solution. This is the Pourbaix diagram of the Fe - H 2 O system. 31 The cathodic reaction of the sample is mainly an oxygen reduction reaction and the anode solution of Fe into Fe 2+ . The reaction equation is given below: O 2 +2 H 2 O+4 e – 4OH – (9) Fe Fe 2+ +2 e – (10) During the anode reaction, Fe 2+ combines with OH – to from Fe(OH) 3 , which then continues to hydrolyze into Fe(OH) 3 and becomes Fe 2 O 3 · xH 2 O rust. The relevant equations are expressed as follows: Fe 2+ + 2OH – Fe(OH) 2 (11) 4Fe(OH) 2 +2 H 2 O 4Fe(OH) 3 (12) 2Fe(OH) 3 Fe 2 O 3 · xH 2 O+( 3–x)H 2 O (13) When 65Mn spring steel is directly immersed in the NaCl solution, E corr exceeds the corrosive pitting of the material, and corrosive pitting begins to occur on the fer- rite. Cl – corrosive anions continuously enter the corro- sion pits, causing the pitting growth to expand. 32–34 Even- tually, large areas of corrosion are formed (Figure 8a). Figure 10b shows the diagram of the corrosion process of manganese phosphating coating. The corrosion is due to the low coverage of 65Mn spring steel exposed to the NaCl solution (see Figures 1f-1h and 2a 2 -2f 2 ). Cl – ions erode 65Mn steel, resulting in the substrate being cor- roded. According to the above, manganese phosphate coating can guarantee good corrosion resistance when it has high coverage and a fine grain size. Meanwhile, it is classified as an insoluble phosphate. 35,36 Thus, the phos- phate coating on the surface of Sample 15 has a fine grain size, enhancing the anti-corrosion of the substrate. 5 CONCLUSIONS Manganese phosphate conversion coatings were pre- pared on 65Mn spring steel. The surface morphology, microstructure, and corrosion behavior of the coating were investigated. The main conclusions are as follows: The macroscopic morphology of the coating presents gray, ash black and black colors. With an increase in the phosphating temperature and time, the surface of phos- phate coating becomes gray black and smooth. The microstructure shows blocky grain crystals, mas- sive grain accumulation crystals, and an oblique pris- matic structure, all resulting from different phosphating process parameters. Elements C, Fe, Mn, O and P are evenly distributed, and the surface grain size is well dis- tributed. The XRD pattern shows that manganese phosphating coating is mainly composed of the Mn 5 (PO 4 ) 2 (PO 3 (OH)) 2 ·4H 2 O and MnHOP 4 ·2.25H 2 O phase. Furthermore, the phase composition of the coat- ing is not affected by the phosphating process parame- ters. P. ZHANG et al.: EXPLORING CORROSION RESISTANCE AND MICROSTRUCTURE OF MANGANESE ... Materiali in tehnologije / Materials and technology 58 (2024) 6, 707–717 715 Figure 10: Diagram of the corrosion of 65Mn spring steel and manganese phosphate coating There was a large area of yellow rust on the 65Mn steel substrate after salt spray for 48 h. However, there were no obvious signs of corrosion on the phosphating-coated surface when the acid ratio was 10, the phosphating temperature was 85 °C, and the phosphating time was 5 min. Meanwhile, the coating maintained fine gray-black grains. Acknowledgment This work is supported by the Qin Chuang Yuan In- novation Talent Program of Shaanxi (QCYRCXM- 2023-036, QCYRCXM-2023-032 and QCYRCXM- 2022-290), Natural Science Basic Research Program of Shaanxi (2024JC-YBQN-0461 and 2024JC-YBQN- 0114), and Key Research and Development Program of Shaanxi (No. 2024GX-YBXM-343). 6 REFERENCES 1 Y . J. Wang, J. J. Sun, T. Jiang, C. Yang, Q. Tan, S. W. Guo, Y . N. Liu, Super strength of 65Mn spring steel obtained by appropriate quench- ing and tempering in an ultrafine grain condition, Mater. Sci. Eng. A, 754 (2019) 1–8, doi:10.1016/j.msea.2019.03.059 2 P. F. Zhang, D. C. Wang, P. Cheng, C. X. Shao, J. Y . Zhou, J. H. Huang, Warm-deformation behavior of 65Mn spring steel, Mater. 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