The Effect of Electrolytic Solution Composition on the Structure, Corrosion, and Wear Resistance of PEO Coatings on AZ31 Magnesium Alloy
Abstract
:1. Introduction
2. Materials and Methods
2.1. Preparation of Specimens, Electrolyte Solutions, and Coatings
2.2. Characterization, Electrochemical, and Tribological Evaluations
3. Results and Discussions
3.1. Cell Current Response Evolution during Peo Treatment
3.2. Structural Features of the Coatings
3.2.1. Surface Morphology and Cross-Sectional Observations
3.2.2. Phase Analysis and Chemical Composition
3.3. Corrosion Resistance of the Coatings
3.4. Wear Resistance of the Coatings
4. Conclusions
- (1)
- The surface morphology of obtained coatings is strongly affected by the electrolyte composition. Aluminate containing coating showed a volcano-like morphology with a combination of nodular particles and craters distributed over the surface. Phosphate comprising coating exhibited a sintering-crater structure, with non-uniform distribution of micro-pores and micro-cracks connected and to porosities. Silicate containing coating illustrated a highly porous scaffold surface formed by a network of micro-pores and oxide granules.
- (2)
- The cross-sectional image obtained in aluminate-based electrolyte included tiny pores due to occurrence of A and C-type discharges and formed more compact coating. The B, D, and E-type discharges were responsible for deep channels and open pores (pancakes) in phosphate and silicate containing coatings. Hence, a larger porosity percentage was related to phosphate and silicate containing coatings.
- (3)
- The coatings contained MgO and MgF2 phases, along with stoichiometric and non-stoichiometric MgAl2O4, Mg2SiO4, MgSiO3, and Mg2P2O7 in aluminate, silicate, phosphate, electrolytes, respectively.
- (4)
- After 2 days of exposure to 3.5 wt.% NaCl solution, the silicate specimen showed the highest corrosion resistance, due to the presence of the most MgF2 amount in the inner barrier layer. However, after 6 days, the substrate was intensively attacked, which stemmed from the low thickness and its porous structure. For the phosphate-containing coating, the substrate was exposed to the corrosive solution after 9 days of exposure. Aluminate-containing coating endured 10 days of exposure, despite exhibiting an inductive loop after 2 days of exposure. Therefore, the highest corrosion resistance at short-immersion time is shown by the silicate-containing coating, while at long-immersion time, this was provided by the aluminate-containing coating.
- (5)
- The highest wear resistance was given by the coating produced in aluminate solution, with revealing a much lower track width and volume loss in all evaluations. The lowest wear resistance was shown by the silicate-containing coating with the highest volume loss, track width and depth before 15 min. At the same time, the phosphate-containing coating with higher thickness delayed the degradation process, but as the sliding continued to 60 min, generated debris filled the larger-sized flaws, and the rest of the spalling particles engaged in the wear process as third-bodies, which increased the wear rate over time.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
References
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Sample Code | Electrolyte | Voltage Waveforms | |||
---|---|---|---|---|---|
Composition (g L−1) | pH | Conductivity (mS cm−1) | First Half at 1 kHz | Second Half at 2 kHz | |
Al | 10 NaAlO2 + 8 NaF + 1 KOH | 12.96 | 20.45 | ||
Ph | 10 Na3PO4 + 8 NaF + 1 KOH | 12.50 | 14.28 | ||
Si | 6 Na2SiO3 (liquid glass, 37%) + 8 KF + 8 KOH | 10.58 | 11.94 |
Sample | Thickness | Roughness | Porosity Percentage (%) | |||
---|---|---|---|---|---|---|
Measured by Eddy Current (µm) | Measured by Cross-Sectional Analysis (µm) | Ra (µm) | Rz (µm) | Surface | Cross-Section | |
Al | 16.50 ± 1.10 | 15.04 ± 0.61 | 0.84 ± 0.01 | 5.98 ± 0.20 | 7.03 ± 1.15 | 1.75 |
Ph | 30.90 ± 3.90 | 30.40 ± 4.75 | 1.92 ± 0.15 | 12.49 ± 0.33 | 3.87 ± 0.49 | 7.25 |
Si | 16.70 ± 1.90 | 14.07 ± 2.69 | 1.10 ± 0.02 | 7.29 ± 0.23 | 14.07 ± 2.19 | 4.86 |
Sample | Mg | O | F | Al | P | Si | ||
---|---|---|---|---|---|---|---|---|
Al | Cross-section | Close to substrate | 60.38 | 16.29 | 12.44 | 10.90 | - | - |
Close to the surface | 60.66 | 22.70 | 2.77 | 13.87 | - | - | ||
Ph | Cross-section | Close to substrate | 54.17 | 19.86 | 15.03 | - | 9.13 | - |
Close to the surface | 44.96 | 25.49 | 14.27 | - | 9.79 | - | ||
Si | Cross-section | Close to substrate | 46.82 | 17.93 | 23.96 | - | - | 11.04 |
Close to the surface | 32.77 | 32.25 | 9.62 | - | - | 23.03 |
Specimen | Immersion Time (EC Model) | Outer Layer | Inner Layer | Substrate | |||||
---|---|---|---|---|---|---|---|---|---|
CPE (μF cm−2 Sn−1) | n | Rout (kΩ cm2) | CPE (μF cm−2 Sn−1) | n | Rin (kΩ cm2) | RL (kΩ cm2 | L (kH cm2) | ||
Al | 2 days (a) | 0.21 ± 0.01 | 0.79 ± 0.02 | 0.62 ± 0.05 | 0.11 ± 0.02 | 0.92 ± 0.06 | 23.99 ± 1.62 | 24.37 ± 1.21 | 8.15 ± 0.51 |
6 days (a) | 0.33 ± 0.01 | 0.82 ± 0.02 | 0.24 ± 0.03 | 0.24 ± 0.02 | 0.96 ± 0.07 | 17.84 ± 0.43 | 18.64 ± 0.95 | 6.02 ± 0.14 | |
9 days (a) | 0.41 ± 0.02 | 0.86 ± 0.03 | 0.14 ± 0.01 | 0.41 ± 0.04 | 0.96 ± 0.07 | 11.56 ± 0.22 | 13.68 ± 0.37 | 4.00 ± 0.10 | |
10 days (a) | 0.48 ± 0.04 | 0.86 ± 0.03 | 0.11 ± 0.01 | 0.46 ± 0.03 | 0.96 ± 0.06 | 5.90 ± 0.01 | 9.89 ± 0.12 | 2.64 ± 0.04 | |
Ph | 2 days (a) | 0.48 ± 0.02 | 0.91 ± 0.03 | 0.20 ± 0.01 | 1.67 ± 0.02 | 0.77 ± 0.03 | 71.11 ± 3.22 | 80.27 ± 3.88 | 24.49 ± 1.58 |
6 days (b) | - | - | - | 5.44 ± 0.07 | 0.92 ± 0.02 | 11.65 ± 0.34 | 20.07 ± 1.07 | 17.17 ± 0.64 | |
Si | 9 days (b) | - | - | - | 10.20 ± 0.09 | 0.93 ± 0.03 | 1.96 ± 0.03 | 5.56 ± 0.23 | 0.92 ± 0.01 |
2 days (c) | 0.81 ± 0.04 | 0.99 ± 0.03 | 0.88 ± 0.02 | 1.01 ± 0.02 | 0.72 ± 0.03 | 89.01 ± 6.32 | - | - | |
6 days (b) | - | - | - | 5.84 ± 0.16 | 0.90 ± 0.03 | 4.31 ± 0.61 | 5.72 ± 0.19 | 0.68 ± 0.02 |
Sample Code | Worn Volume (×10−3 mm3) | Track Width (µm) | Maximum Depth (µm) |
---|---|---|---|
Al-15 min | 3.1 ± 0.4 | 322.04 ± 16.57 | 3.13 ± 0.29 |
Ph-15 min | 7.5 ± 1.1 | 398.52 ± 35.81 | 6.44 ± 0.52 |
Si-15 min | 63.4 ± 5.1 | 579.80 ± 47.11 | 33.17 ± 2.01 |
Al-30 min | 7.6 ± 0.8 | 413.72 ± 21.33 | 8.84 ± 0.32 |
Ph-30 min | 33.9 ± 2.9 | 514.05 ± 28.50 | 13.83 ± 0.50 |
Al-60 min | 13.4 ± 2.4 | 503.84 ± 33.31 | 12.43 ± 0.48 |
Ph-60 min | 117.9 ± 12.6 | 842.19 ± 46.21 | 31.42 ± 1.33 |
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Toulabifard, A.; Rahmati, M.; Raeissi, K.; Hakimizad, A.; Santamaria, M. The Effect of Electrolytic Solution Composition on the Structure, Corrosion, and Wear Resistance of PEO Coatings on AZ31 Magnesium Alloy. Coatings 2020, 10, 937. https://doi.org/10.3390/coatings10100937
Toulabifard A, Rahmati M, Raeissi K, Hakimizad A, Santamaria M. The Effect of Electrolytic Solution Composition on the Structure, Corrosion, and Wear Resistance of PEO Coatings on AZ31 Magnesium Alloy. Coatings. 2020; 10(10):937. https://doi.org/10.3390/coatings10100937
Chicago/Turabian StyleToulabifard, Amirhossein, Maryam Rahmati, Keyvan Raeissi, Amin Hakimizad, and Monica Santamaria. 2020. "The Effect of Electrolytic Solution Composition on the Structure, Corrosion, and Wear Resistance of PEO Coatings on AZ31 Magnesium Alloy" Coatings 10, no. 10: 937. https://doi.org/10.3390/coatings10100937