Abstract:The methodology of deposition of fluoride conversion coatings is modified with the use of galvanic coupling, agitation of the electrolyte solution, and addition of K2CO3, which helps to provide a better understanding of the mechanism and new avenues to tailor the composition of the coating.
“…Besides chemical conversion coatings [ 10 , 11 ] and degradable polymer coatings [ 12 , 13 ], plasma electrolytic oxidation (PEO) coatings have been widely investigated because of their high adhesion strength to the substrate and their adjustable compositions [ 14 ]. In physiological environment, as magnesium fluoride is more chemically stable than magnesium oxide, the corrosion resistance of magnesium substrate could be enhanced by increasing the F/O ratio of the surface coating [ 15 ]. The addition of fluoride into the PEO coatings to form magnesium fluoride could effectively improve their corrosion resistance [ 16 , 17 ].…”
Fluoride-incorporated plasma electrolytic oxidation (PEO) coating was fabricated on biodegradable AZ31 alloy. The surface morphologies and phases were investigated by scanning electron microscopy and X-ray diffraction. The effect of fluoride incorporation in coatings on corrosion behaviour was investigated in simulated body fluid and in vitro cytocompatibility of the coatings was also studied by evaluating cytotoxicity, adhesion, proliferation and live–dead stain of osteoblast cells (MC3T3-E1). Furthermore, the corrosion morphologies in vivo were examined. The results showed that the fluoride could be incorporated into the coating to form MgF2 phase. In vitro and in vivo degradation tests revealed that the corrosion resistance of the coating could be improved by the incorporation of fluoride, which may attribute to the chemical stability of MgF2 phase. Moreover, good cytocompatibility of fluoride-incorporated coating was confirmed with no obvious cytotoxicity, enhanced cell adhesion and proliferation. However, when the fluoride content was high, a slight inhibition of cell growth was observed. The results indicate that although fluoride incorporation can enhance the corrosion resistance of the coatings, thus resulting a more suitable environment for cells, the high content of fluoride in the coating also kill cells ascribed to the high released of fluorine. If the content of fluoride is well controlled, the PEO coating with MgF2 phase is a promising surface modification of Mg alloys.
“…Besides chemical conversion coatings [ 10 , 11 ] and degradable polymer coatings [ 12 , 13 ], plasma electrolytic oxidation (PEO) coatings have been widely investigated because of their high adhesion strength to the substrate and their adjustable compositions [ 14 ]. In physiological environment, as magnesium fluoride is more chemically stable than magnesium oxide, the corrosion resistance of magnesium substrate could be enhanced by increasing the F/O ratio of the surface coating [ 15 ]. The addition of fluoride into the PEO coatings to form magnesium fluoride could effectively improve their corrosion resistance [ 16 , 17 ].…”
Fluoride-incorporated plasma electrolytic oxidation (PEO) coating was fabricated on biodegradable AZ31 alloy. The surface morphologies and phases were investigated by scanning electron microscopy and X-ray diffraction. The effect of fluoride incorporation in coatings on corrosion behaviour was investigated in simulated body fluid and in vitro cytocompatibility of the coatings was also studied by evaluating cytotoxicity, adhesion, proliferation and live–dead stain of osteoblast cells (MC3T3-E1). Furthermore, the corrosion morphologies in vivo were examined. The results showed that the fluoride could be incorporated into the coating to form MgF2 phase. In vitro and in vivo degradation tests revealed that the corrosion resistance of the coating could be improved by the incorporation of fluoride, which may attribute to the chemical stability of MgF2 phase. Moreover, good cytocompatibility of fluoride-incorporated coating was confirmed with no obvious cytotoxicity, enhanced cell adhesion and proliferation. However, when the fluoride content was high, a slight inhibition of cell growth was observed. The results indicate that although fluoride incorporation can enhance the corrosion resistance of the coatings, thus resulting a more suitable environment for cells, the high content of fluoride in the coating also kill cells ascribed to the high released of fluorine. If the content of fluoride is well controlled, the PEO coating with MgF2 phase is a promising surface modification of Mg alloys.
“…2). The bare alloy display only Mg peaks whereas the PD coated and HF PD coated samples show the MgO and MgF 2 peaks 40,41 . This is due to the formation of passive oxide layer, and MgF 2 on the surface of bare alloy due to HF acid treatment as described below 41 :Figure 2XRD patterns of ( A ) Bare alloy, ( B ) PD coated sample, ( C ) HF- PD coated Sample, ( D ) Ag HF-PD sample, and ( E ) Au HF-PD sample.…”
Magnesium (Mg) and its alloys have attracted much attention as a promising candidate for degradable implant applications however the rapid corrosion of magnesium inside the human body greatly limits its use as an implant material. Therefore, coating the alloy surface with a multifunctional film is a promising way to overcome the drawbacks. Here we propose for the first time a multifunction layer coating to enhance the cell viability, antibacterial property and decelerated corrosion rates to act as a novel material to be used for degradable implant Applications. For that, the magnesium alloy (AZ31) was first treated with hydrofluoric acid (HF) and then dopamine tris Hydrochloric acid (tris-HCL) solution. The reducing catechol groups in the polydopamine (PD) layer subsequently immobilize silver/gold ions in situ to form uniformly dispersed Ag/Au nanoparticles on the coating layer. The successful formation of Ag/Au nanoparticles on the HF-PD AZ31 alloy was confirmed using XPS and XRD, and the morphology of all the coated samples were investigated using SEM images. The alloy with HF-PDA exhibit enhanced cell attachment and proliferation. Moreover, the nanoparticle immobilized HF-PD alloy exhibited dramatic corrosion resistance enhancement with superior antibacterial properties and accountable biocompatibility. Thus the result suggest that HF-PD Ag/Au alloy has great potential in the application of degradable implant and the surface modification method is of great significance to determine its properties.
“…However, EDS mapping of the coating surface revealed that the defects do not penetrate to the coated alloy. On the other hand, there is an assumption that the coating will contain microcavities (not observable by SEM) created during the coating preparation due to the gas evolution as observed by Sankara Narayanan et al (2014).…”
Purpose
The purpose of this paper is to compare electrochemical corrosion characteristics of conventional and unconventional fluoride conversion coating prepared on magnesium alloy.
Design/methodology/approach
The chemical reaction of AZ61 with 38 wt.% hydrofluoric acid (HF) for 24 h was used as a conventional way of fluoride conversion coating preparation. The unconventionally prepared coating was created in Na[BF4] salt melt at 450°C for 2 h. Morphology and chemical composition of prepared fluoride conversion coatings were studied with scanning electron microscopy and energy-dispersive X-ray spectroscopy. Electrochemical corrosion characteristics of the coatings were analyzed in Hank’s solution using potentiodynamic tests.
Findings
Both the coating preparation ways resulted in the creation of uniform conversion coatings with the same thickness (1.3 ± 0.1 μm). Some defects were observed on the coatings surface; however, the defects did not reach the AZ61 surface. Electrochemical tests performed in Hank’s solution at 37°C showed an improvement of corrosion resistance of AZ61 treated with fluoride conversion coatings when compared to the untreated material. Unconventionally prepared coating reached better electrochemical corrosion characteristics when compared to the conventionally prepared coating.
Originality/value
Electrochemical corrosion characteristics of AZ61 magnesium alloy can be improved with fluoride conversion coatings. Two methods are used in the literature for the coatings preparation. The conventional method is based on dipping of the coated material to the HF, and the unconventional method lies in dipping of the sample to the Na[BF4] salt melt. The main purpose of the present study is to analyze the conventionally and unconventionally prepared coatings in terms of chemical analysis, morphology and material corrosion protection (electrochemical corrosion characteristics), while the data are not provided in the literature, according to the authors’ knowledge. Very similar coatings were prepared using both the methods from the morphological and chemical composition point of view. However, unconventionally prepared coating created in Na[BF4] salt melt reached better electrochemical corrosion characteristics compared to the coating prepared in HF.
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