Technical Note: A High Corrosion-Resistant Al2O3/MgO Composite Coating on Magnesium Alloy AZ33 by Chemical Conversion
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An Al2O3/MgO composite coating was synthesized on magnesium alloy AZ33 using a chemical conversion process for the first time. EDTA-2Na was introduced in the Al conversion solution to chelate Al3+,...Keywords:
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Magnesium and its alloys are explored as potential biomedical materials for being lightweight, bio-absorbable, and having attractive biological properties. A major hindrance for their use is their high corrosion rate, in particular when exposed to body fluids. This study aims at suppressing the corrosion rate of a magnesium alloy (Mg1.0Ca) by coating it with magnesium fluoride (MgF2). The coating was done by immersion of the work-piece in hydrofluoric acid solution. For comparison, pure magnesium was also coated with MgF2. The MgF2 coated magnesium exhibits significantly lower corrosion rate than pure magnesium. The MgF2 coated magnesium alloy shows even lower corrosion rate. The MgF2 coating works in inhibiting corrosion on magnesium alloy Mg1.0Ca. The corrosion inhibition was also contributed by other compound formed during reaction between Mg1.0Ca and hydrofluoric acid and the alloy in Mg1.0Ca.
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Abstract Magnesium and its alloys are explored as potential biomedical materials for being lightweight, bio-absorbable, and having attractive biological properties. A major hindrance for their use is their high corrosion rate, in particular when exposed to body fluids. This study aims at suppressing the corrosion rate of a magnesium alloy (Mg1.0Ca) by coating it with magnesium fluoride (MgF 2 ). The coating was done by immersion of the workpiece in hydrofluoric acid solution. For comparison, pure magnesium was also coated with MgF 2 . The MgF 2 coated magnesium exhibits significantly lower corrosion rate than pure magnesium. The MgF 2 coated magnesium alloy shows even lower corrosion rate. The MgF 2 coating works in inhibiting corrosion on magnesium alloy Mg1.0Ca. The corrosion inhibition was also contributed by other compound formed during reaction between Mg1.0Ca and hydrofluoric acid and the alloy in Mg1.0Ca.
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Magnesium and its alloys are explored as potential biomedical materials for being lightweight, bio-absorbable, and having attractive biological properties. A major hindrance for their use is their high corrosion rate, in particular when exposed to body fluids. This study aims at suppressing the corrosion rate of a magnesium alloy (Mg1.0Ca) by coating it with magnesium fluoride (MgF2). The coating was done by immersion of the work-piece in hydrofluoric acid solution. For comparison, pure magnesium was also coated with MgF2. The MgF2 coated magnesium exhibits significantly lower corrosion rate than pure magnesium. The MgF2 coated magnesium alloy shows even lower corrosion rate. The MgF2 coating works in inhibiting corrosion on magnesium alloy Mg1.0Ca. The corrosion inhibition was also contributed by other compound formed during reaction between Mg1.0Ca and hydrofluoric acid and the alloy in Mg1.0Ca.
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Conversion coating by immersion in solution including Mg(NO3)2 and/or La(NO3)3 was applied to commercial Mg alloys: AZ31, AZ61, AZ91 and AM60. The formed layer was analyzed, and the corrosion resistance of the specimen against salt water was investigated. The influences of the coating conditions on the characteristics of the layer and on the corrosion behavior were discussed. A thin oxide layer was formed on the surface of the specimen by the conversion coating, and La was contained in the layer obtained in the solution including La(NO3)3. The corrosion resistance of Mg alloys wasn't remarkably improved by the conversion coating using either Mg(NO3)2 solution or La(NO3)3 solution, while it was particularly bettered by the coating using the solution including both Mg(NO3)2 and La(NO3)3. The good corrosion resistance was obtained under the wide concentration ratio of La(NO3)3 to Mg(NO3)2, and the optimum coating conditions for each alloy was specified. The pretreatment, such as the alkaline degrease, the acid pickling and the surface activation, was effective for the uniform coating on large specimen.
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Magnesium alloys are nontoxic and promising as orthopedic metallic implants, but preparing a biocompatible Mg(OH)2 layer with high corrosion protection ability remains challenging. It is generally believed that the Mg(OH)2 layer, especially that formed in a natural condition, cannot provide desirable corrosion resistance in the community of corrosion and protection. Here, several Mg(OH)2 coatings were prepared by changing the pH values of sodium hydroxide solutions. These coatings were composed of innumerable nanoplatelets with different orientations and showed distinguished capability in corrosion resistance. The nanoplatelets were well-oriented with their ab-planes parallel to, instead of perpendicular to, the magnesium alloy surface by raising the pH value to 14.0. This specific orientation resulted in the optimal coating showing long-term corrosion protection in both in vitro and in vivo environments and good osteogenic capability. These finds manifest that the environment-friendly Mg(OH)2 coating can also provide comparable and better corrosion protection than many traditional chemical conversion films (such as phosphate, and fluoride).
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Abstract Magnesium based alloys are very promising material to be used mainly for biodegradable implants in medical applications. However, due to their very low corrosion resistance in the environment of in vivo is their use limited. Increase of the corrosion resistance of magnesium alloys in vivo can be achieved, for example, by a suitable choice of surface treatment while the biocompatibility must be ensured. Fluoride conversion coatings meet these requirements. Unconventional fluoride conversion coating was prepared on ZE41 magnesium alloy by dipping the magnesium alloy into the Na[BF 4 ] salt melt at 450 °C for 0.5; 2 and 8 h. The morphology and thickness of the prepared fluoride conversion coatings were investigated as well as the corrosion resistance of the treated and untreated ZE41 magnesium alloy specimens. The corrosion resistance of the untreated and treated ZE41 magnesium alloy was investigated using electrochemical impedance spectroscopy in the environment of the simulated body fluids at 37 ± 2 °C. The obtained results showed a positive influence of the fluoride conversion coating on the corrosion resistance of the ZE41 magnesium alloy.
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