Relationship between microstructure and formation-biodegradation mechanism of fluoride conversion coatings synthesised on the AZ31 magnesium alloy

Fluoride Coatings on Magnesium Alloy Implants

“…Because Mg(OH) 2 is extremely unstable under acidic conditions, it can undergo an exchange reaction (equation ( 7 )), in which the OH − within the Mg(OH) 2-x F x coating is replaced by F − ( Figure 1 ) [ 31 ]. The above reaction was also accelerated by increasing the concentration of HF in the conversion solution [ 31 ]. …”

“…However, an overly high concentration of HF may lead to thinning of the coating, which may be attributed to the fact that the dissolution rate of the magnesium substrate is faster than the generation rate of the conversion coating [ 55 ]. Additionally, treatment time has also been proven to affect the coating thickness [ 31 , 44 ], which in turn affects the corrosion resistance of magnesium alloys by changing the probability of defects or “active spots” on the surface of the magnesium substrate, that is, the number of through-holes [ 56 ]. Usually, the thickness curve rises with treatment time and eventually flattens out ( Figure 2 ) [ 31 ], but the formation rate of the coating gradually decreases, which may be related to the thickening of the coating that prevents the HF from reacting with the internal magnesium [ 57 ].…”

“…Additionally, treatment time has also been proven to affect the coating thickness [ 31 , 44 ], which in turn affects the corrosion resistance of magnesium alloys by changing the probability of defects or “active spots” on the surface of the magnesium substrate, that is, the number of through-holes [ 56 ]. Usually, the thickness curve rises with treatment time and eventually flattens out ( Figure 2 ) [ 31 ], but the formation rate of the coating gradually decreases, which may be related to the thickening of the coating that prevents the HF from reacting with the internal magnesium [ 57 ]. da Conceicao et al [ 54 ] treated AZ31 at HF acid concentrations at a concentration gradient from 12 to 49 vol%.…”

“…Conversely, the microstructure of Elektron 21 alloys was more homogeneous, and the intermetallic compound Mg12(NdxGd1-x) phase remained stable, allowing the formation of continuous and homogeneous coatings. The microstructure of fluoride conversion coatings on AZ31 in relation to the degradation mechanism was studied by Barajas et al The authors performed chemical conversion coating of AZ31 Mg alloy at 4 and 10% HF concentration with an immersion time of 24–168 hours [ 31 ]. During the conversion process, most of the metal particles in the α -Mg matrix on the surface of AZ31 were dissolved, but SEM observation revealed undissolved metal particles, and then EDX analysis revealed that the undissolved metal particles corresponded to rare earth-containing dispersions (La, Ce, and Nd).…”

“…Anodic polarization experiments were performed on untreated and 4.10 vol% HF immersion treated AZ31 magnesium alloy, and the electrochemical parameters were extracted as shown in Table 3 [ 31 ]; the majority of the coatings provided a scope of protection (Epit-Ecorr) to the metal substrate, and the fluoride coatings reduced the corrosion current density and enhanced the corrosion resistance of the alloy. Compared to previous studies [ 26 ], Dai et al [ 32 ] used a low-voltage fluorination method to obtain coatings with controlled corrosion rates under safer conditions.…”