Table 6.

Advantages and disadvantages associated with various coatings on Mg alloys.

Coatings	Advantages	Disadvantages
Phosphate conversion	Improved osteointegration, osteoconduction response, Biocompatible High adhesion strength	Inevitable spatial defects due to evaporation of organic residues, Structural destabilization and enhanced solubility of Ca–P due to Mg substitution.
Rare earth conversion	chemically stable and insoluble during long-term physiological immersion, Superior anti-bacterial response, Significant antitumor activity	Prolonged exposure to human body can decline the locomotor frequency, induce down-expressions and other behavioural deficits
Fluoride conversion	Promotes Ca–P mineralization, Highly insoluble, significant corrosion resistance, blood compatibility, cytocompatibility	Acceptable to micro-pores due to H2 generation, internal stresses generation and cracking
Organic conversion	Biodegradable, remarkable chelation ability to mineralize Ca2+, PO43− and Mg2+ into stable compounds, Excellent self-healing and superhydrophobic characteristics.	Substandard corrosion resistance, Poor adhesion strength with Mg alloys.
MAO	Superior bonding strength, wear resistance, and cell-anchoring response.	Poor corrosion resistance, biocompatibility.
Ion Implantation	Selective surface modification, low temperature conditions, and reproducibility.	Residual stress generation, increased surface roughness, and uneven thickness of coating layer.
Layered Double hydroxides	High corrosion resistance, Self-repairing, biocompatible, anti-bacterial, and anti-inflammatory response	Unfavorable processing parameters including high temperature, and pressure conditions, and poor ion-exchangeability of bioactive molecules.
Physical vapor deposition	Highly uniform and crystalline coating structure, significant bone cell attachment, improved elastic strain to failure and plastic deformation resistance.	Difficult to control deposition temperature, complex equipment requirements and high cost.
Atomic layer deposition	excellent corrosion resistance, conformal and facile thickness control, and pin-hole free structure, high surface finish	Difficult to control deposition temperature, poor adhesion strength and lower growth rate.
Cathodic Electrodeposition	Versatility, short deposition time, room temperature conditions, and slight limitation on substrate shape	Poor adhesion strength, accumulated H2 bubbles on the cathode (i.e., substrate) severely affects the coating deposition.
Layer-by-layer	Sustained and target drug-delivery, Self-healing, corrosion resistance	Time consuming, weak adhesion strength
Sol-gel	Superior corrosion resistance, mild temperature, and pressure conditions, facile multilayer arrangement incorporating organic and inorganic components.	Poor adhesion strength and thickness control, prone to spatial defects.