Effect of the second-step voltages on the structural and corrosion properties of silicon–calcium–phosphate (Si–CaP) coatings on Mg–Zn–Ca alloy

Abstract

The applications of magnesium (Mg) alloys as biodegradable orthopedic implants are mainly restricted due to their rapid degradation rate in the physiological environment. In this study, Si–CaP micro-arc oxidation (MAO) coatings were prepared on a Mg–Zn–Ca alloy by a second-step MAO process at different voltages in order to decrease the degradation rate and increase the bioactivity of the alloy. The microstructure and morphology of the samples were characterized using XRD, FT-IR SEM and EDS. The degradation behaviours of samples were evaluated using electrochemical techniques, and immersion tests in simulated body fluid (SBF). The results indicate that the morphology of the Si–CaP coatings changed significantly with the increase in Ca/P ratio as the second-step voltage increased. The Si–CaP containing coating produced at 450 V could significantly decrease the degradation rate of Mg and caused a slow increase in pH of the SBF solution. The haemolysis test concluded that the coating C3 did not cause a haemolytic reaction. The corrosion resistance of Mg alloy was greatly improved with the Si–CaP coatings, and the Mg alloy with Si–CaP coating prepared at 450 V had the best corrosion resistance, which indicates that the Si–CaP coatings are promising for improving the biodegradation properties of Mg-based orthopedic implants. Haemolysis tests indicated that the Si–CaP coating prepared at 450 V conforms to the given standard (YY/T0127.1-93).

Effect of V2O5 Additive on Micro-Arc Oxidation Coatings Fabricated on Magnesium Alloys with Different Loading Voltages

“…In addition, with an increase of the amount of V 2 O 5 added, the conductivity of the electrolyte also decreases. It is generally believed that the MAO process involves the dissolution of old coatings and the formation of new coatings, resulting in a competition in both processes [27,37]. The decrease of pH and conductivity changes the properties of the electrolytes, which affects the formation of passivation film in the initial stage of MAO and the discharge characteristics of the coatings.…”

“…In addition, with an increase of the amount of V2O5 added, the conductivity of the electrolyte also decreases. It is generally believed that the MAO process involves the dissolution of old coatings and the formation of new coatings, resulting in a competition in both processes [27,37].The decrease of pH and conductivity changes the properties of the electrolytes, which affects the formation of passivation film in the initial stage of MAO and the discharge characteristics of the coatings. Therefore, the addition of V2O5 to the solution accelerates the dissolution process due to the reduced alkalinity and conductivity, and may result in the decreased growth rate and a weak state in a certain local area on the coating surface where the continual breakdown occurs, inducing the emergence of the large-size micropores and microcracks, which in turn become the access for corrosive media entering into the coating, and thus affects the corrosion resistance of the coating.…”

“…Furthermore, the voltage, as the most important electrical parameter, exhibits a serious effect on the property of the coating by affecting the thickness and microstructure of the coating. With an increase of the voltage, the coating thickness increases but the pore size also enlarges [2,26,27], so the corrosion resistance of the coating maynot be always improved.…”

Comparative Study of the Structure, Properties, and Corrosion Behavior of Sr-Containing Biocoatings on Mg0.8Ca

“…Rau et al [19] used the glass-ceramic coatings on Mg-Ca alloys to reduce its biodegradation rate under physiological conditions. In addition, Dou et al [44] demonstrated the influence of the second-step voltages on the structural and properties of the coatings on Mg-Zn-Ca alloy.…”

Calcium Doped Flash-PEO Coatings for Corrosion Protection of Mg Alloy

“…Ca is sometimes used in protective coatings produced by anodizing [5][6][7][8][9][10][11][12][13][14][15][16][17], electrodeposition [18][19][20], conversion [21,22] or by mixed strategies (e.g., PEO with electrophoretic deposition [23] or hydrothermal treatment [24]); the formation of calcium phosphates, hydroxyapatite and other Ca-rich compounds typically leads to an improvement in the corrosion resistance of coated Mg alloys. The current literature considers PEO coatings that contain Ca species mostly for biological implant applications due to their great biocompatibility, especially in terms of hydroxyapatite-forming ability [25].…”