Table 2.

Characteristics of different HF-coated magnesium alloys prepared under various parameters.

Reference	Alloys	Treatment	Thickness of the coatings	Special structure	Composition of coating	E corr (V)	i corr (A/cm2)	The reference electrode and the electrolyte
[69]	AZ61	Treating by unconventional fluoride conversion in Na[BF4] molten salts at 410, 420, 430, 440, and 450°C for 0.5, 1, 2, 4, 8, 12, 24, and 36 h	Shown in Figure 4 [69].	No porosity or structural defects	Primary layer: Mg-F; secondary layer: Na[MgF3]	—	—	—
[24]	AZ31	Treating by ultrasonic immersion fluorination in HF 46% solution for 24 h	7.7 μm	Nanocrystalline structure	MgF2	HFU: −1.298	HFU: 9.231 × 10−7	Reference electrode: Ag/AgCl/Sat-KCl (+197 mV). Electrolyte: HBSS
[27]	Pure Mg	Treating by microarc fluorination (MAF) in saturated NH4HF2 solution by constant voltages of 120, 160, 200, and 210 V for 3 min	MAF120: 2.5 μm	MAF120: long-slot shape structure	MgF2	−1.573 (the pure Mg group is −1.842)	0.301 × 10−6 (the pure Mg group is 5.064 × 10−6)	Reference electrode: Ag/AgCl/Sat-KCl (+197 mV). Electrolyte: SBF
			MAF160: 3.5 μm	MAF160: uniform and porous		−1.558	0.238 × 10−6
			MAF200: 5.5 μm	MAF200: uniform and porous		−1.547	0.187 × 10−6
[26]	AZ31	Treating by plasma electrolytic fluorination in the pure NH4HF2 (150°C) by voltages of 100, 110, 120, 130, and 140 V for 30 s	PEF100: 1 and 3 μm	PEF100, PEF110, and PEF120: a rough structure with a nonuniform texture. PEF130 and PEF140: porous and uniform structure	MgF2	−1.363 (the bare group is −1.543)	6.811 × 10−6 (the bare is 2.470 × 10−5)	Reference electrode: Ag/AgCl/Sat-KCl (+197 mV). Electrolyte: HBSS
			PEF110: 2.7 μm			−1.403	6.498 × 10−6
			PEF120: 5.6 μm			−1.388	3.975 × 10−6
			PEF130: 13.6 μm			−1.358	8.533 × 10−7
			PEF140: 13.9 μm			−1.334	4.360 × 10−6
[32]	Pure Mg	Treating by anodic fluorination (AF) at 0.1 mol/L NH4HF2 solution by direct current (CD) power supply at 10, 20, 30, 40, 50, 60, 70, 80, and 90 V for 3 min	Shown in Figure 5 [32]	AF10: a dot-like morphology. AF30 and AF40: a homogeneous matte-like appearance (as the voltage increased, the coral-like shape became coarser and shale-like)	MgF2	No specific data are mentioned in the article	AF10: 6.37 × 10−6 (the pure Mg group is 2.25 × 10−5)	Reference electrode: Ag/AgCl/Sat-KCl (+197 mV). Electrolyte: HBSS
							AF20: 4.13 × 10−6
							AF30: 7.15 × 10−6
							AF50: 2.61 × 10−5
							AF60: 3.83 × 10−5
							AF70: 8.55 × 10−5
							AF80: 6.98 × 10−5
							AF90: 7.72 × 10−5
[25]	AZ31	Treating by microarc fluorination (MAF) anodized by constant voltage at 100, 150, 200, 250, and 300 V for 30 s in 46% HF solution	150 V: 0.5 μm	The coral-like structure appeared	MgF2	−1.318 (the bare AZ31 group is −1.501)	0.228 × 10−6 (the bare AZ31 group is 342.4 × 10−6)
			200 V: 0.6 μm	The coral-like structure		−1.262	0.177 × 10−6
			250 V: 0.7 μm	The coral-like structure disappeared		−1.293	0.199 × 10−6
[70]	AZ91	Treating by plasma electrolytic oxidation (PEO) at an aluminate-based electrolyte containing NaAlO2, NaF, and KOH at pH 12.20 at 32 ± 2°C for 10 min at two constant anodic voltages of 350 and 400 V using three different waveforms of unipolar, bipolar with 20% cathodic duty cycle and bipolar with 40% cathodic duty cycle for 10 min	The coatings are all thick on the outside and thin on the inside	Obvious microcracks and microporosity were observed on the surface. Double-layer structure of coating: porous outer layer and dense inner layer	MgO, MgAlO4, and MgF2	—	—	—
			Unipolar waveform: 4 μm	Uniformly distributed pores were observed on the surface, showing regular circular holes with different sizes
			Bipolar waveform: 15 μm	Surface cavities with a crater-like morphology along with some granules of oxide were observed
[71]	AZ31	Treating by potentiostatic polarization measurements using a potentiostat/Galvanostat 273 A at −1.4 V in 0.1M KF solution at room temperature	The inner layer: 300 nm	Compact	KMgF3, Mg(OH)2, and MgF2 are not detected	—	—	—
			The outer layer: 260 nm	Rough