Table 3.

Surface modification methods applied for improvement of mechanical or corrosion properties of Mg or its alloys.

Category	Sub-category	Surface composition	Method	Testing environment	Results (post-treatment vs. pre-treatment)	Shortcomings	References
Physical deposition	Polymer coatings	PCL	Deposition layer by layer by a custom designed spraying device	In vitro	60%–80% decrease in corrosion rate while 50% increase in the remaining compressive strength after 60 days of immersion	Insufficient bonding strength between the substrate and coating layer	[97]
		PLLA, PHB, PHBV, or PLGA	Spin coating	In vitro	Over a half reduction of weight loss for the coated samples after 4-week immersion test		[110]
		Chitosan-based nanofibers with incorporated silver sulfadiazine and carbon nanotubes	Electrospinning technique	In vitro	Two times higher in charge transfer resistance		[111]
		PLLA/AKT-DOXY	Electrospinning method	In vitro	Over 60% reduction in corrosion rate		[112]
	Inorganic or hybrid (polymer + inorganic) coatings	HA	Deposition with Ca chelate compound in solution	In vivo	25% decrease in corrosion rate		[113]
		CeO2	Sol-gel method on a fluorinated surface	In vitro	Approximate 90% reduction in corrosion current density		[114]
		Fe-substituted tricalcium phosphate	Pulsed laser deposition	In vitro	50% reduction in corrosion current density		[115]
		Si-rich oxide and Si-rich layer	Ion implantation	In vitro	Over 90% reduction in corrosion current density		[116]
		TiOx or AlOx film	Plasm vapor deposition	In vitro	Approximate 50% reduction in corrosion current density		[117]
		PCL/bioglass nanoparticle composite	Spin coating	In vitro	Significant reduction in corrosion current density		[98]
	Refinement in microstructure	Homogenous surface	Laser surface melting	In vitro	Over 60% reduction in corrosion current density after treatment and grinding	Residual stress exists after LSM treatment	[100]
		Nano-crystallization	Laser shock processing	In vitro	86% increase in surface hardness	Impairment in corrosion resistance	[99]
		Mesoporous silica	Selective laser melting	In vitro	57% reduction in corrosion rate	Potential inhibition of bioactivity of Mg ions by the inert coating	[118]
		Supra-nano-dual-phase alloy membrane	Mg–Cu–Y alloy	In vitro	Increase in the ultimate yield stress by nearly 8 times	Potential electrochemical reactions between membrane and Mg-based substrate	[101]
Chemical conversion coatings	Alkaline derived layers	Magnesium oxide layers	Alkaline-heat treatment	In vitro	Approximate 85% decrease in corrosion rate	Concerns in long-term corrosion performance caused by porosity and detachment of coatings	[88]
	Acid derived layers	Mg3(PO4)2	Phosphoric acid treatment	In vitro	Over 50% reduction in corrosion rate		[90]
	Fluorinated layers	MgF2	HF treatment	In vitro	50%–70% reduction in corrosion current density		[89]
	Electrochemical deposition	Ceramic coatings with dense micropores	Micro-arc oxidation	In vitro	Over 80% reduction in corrosion current density		[93]
		Hydroxyapatite	Electrophoresis	In vitro	Slight increase in corrosion resistance depending on thickness		[92]
	Others	DAHP/PEI	Chemical conversion and spin coating	In vitro	Reduction in corrosion current density by five orders of magnitude		[119]
Mechanical treatment	surface mechanical attrition treatment (SMAT)	Gradient nanostructures	Repeated multidirectional impact of flying balls on the surface	In vitro	Approximate 50% increase in tensile strength	Weaker corrosion resistance of alloy caused by high density of crystalline defects	[102]
	Combination of SMAT and dual-phase metallic glass film	Hybrid nanostructures	Surface impact by ZrO2 balls and magnetron sputtering	In vitro	Improvement in both strength and ductility	Potential electrochemical reactions between the two layers for impairment of corrosion resistance	[120]