Corrosion of an Additively Manufactured Ti6Al4V Alloy in Saline and Acidic Media.

The present work aims to provide corrosion performance data for an additively manufactured Ti6Al4V alloy in saline and polluted environments. The as-received additively manufactured material underwent heat treatment at 850 °C for 3 h to transform the acicular α' microstructure into a lamellar α microstructure. Comparative corrosion assessments were conducted between the heat-treated substrates, the as-received condition, and a conventionally mill-annealed alloy. Potentiodynamic polarization experiments were carried out in saline (3.5 wt.% NaCl) and acid aqueous media ((NH4)2SO4 containing Harrison's solution). The corrosion performance of additively manufactured substrates matched or surpassed that of the conventional alloy in Harrison's solutions while remaining inferior in saline medium, despite forming a thicker passive film. Overall, the XY plane showed better corrosion performance, particularly after the elimination of the acicular α' martensite by the applied heat treatment. The results also suggested that the presence of the coarse ß phase was beneficial in 3.5 wt.% NaCl solution and detrimental in Harrison's solutions, more so in acidified and fluorinated conditions.

Hierarchical Hybrid Coatings with Drug-Eluting Capacity for Mg Alloy Biomaterials.

A hierarchical hybrid coating (HHC) comprising a ceramic oxide layer and two biodegradable polymeric (polycaprolactone, PCL) layers has been developed on Mg3Zn0.4Ca cast alloy in order to provide a controlled degradation rate and functionality by creating a favorable porous surface topography for cell adhesion. The inner, ceramic layer formed by plasma electrolytic oxidation (PEO) has been enriched in bioactive elements (Ca, P, Si). The intermediate PCL layer sealed the defect in the PEO layer and the outer microporous PCL layer loaded with the appropriate active molecule, thus providing drug-eluting capacity. Morphological, chemical, and biological characterizations of the manufactured coatings loaded with ciprofloxacin (CIP) and paracetamol (PAR) have been carried out. In vitro assays with cell lines relevant for cardiovascular implants and bone prosthesis (endothelial cells and premyoblasts) showed that the drug-loaded coating allows for cell proliferation and viability. The study of CIP and PAR cytotoxicity and release rate indicated that the porous PCL layer does not release concentrations detrimental to the cells. However, complete system assays revealed that corrosion behavior and increase of the pH negatively affects cell viability. H2 evolution during corrosion of Mg alloy substrate generates blisters in PCL layer that accelerate the corrosion locally in crevice microenvironment. A detailed mechanism of the system degradation is disclosed. The accelerated degradation of the developed system may present interest for its further adaptation to new cancer therapy strategies.

Biological Performance of Duplex PEO + CNT/PCL Coating on AZ31B Mg Alloy for Orthopedic and Dental Applications.

To regulate the degradation rate and improve the surface biocompatibility of the AZ31B magnesium alloy, three different coating systems were produced via plasma electrolytic oxidation (PEO): simple PEO, PEO incorporating multi-walled carbon nanotubes (PEO + CNT), and a duplex coating that included a polycaprolactone top layer (PEO + CNT/PCL). Surfaces were characterized by chemical content, roughness, topography, and wettability. Biological properties analysis included cell metabolism and adhesion. PEO ± CNT resulted in an augmented surface roughness compared with the base material (BM), while PCL deposition produced the smoothest surface. All surfaces had a contact angle below 90°. The exposure of gFib-TERT and bmMSC to culture media collected after 3 or 24 h did not affect their metabolism. A decrease in metabolic activity of 9% and 14% for bmMSC and of 14% and 29% for gFib-TERT was observed after 3 and 7 days, respectively. All cells died after 7 days of exposure to BM and after 15 days of exposure to coated surfaces. Saos-2 and gFib-TERT adhered poorly to BM, in contrast to bmMSC. All cells on PEO anchored into the pores with filopodia, exhibited tiny adhesion protrusions on PEO + CNT, and presented a web-like spreading with lamellipodia on PEO + CNT/PCL. The smooth and homogenous surface of the duplex PEO + CNT/PCL coating decreased magnesium corrosion and led to better biological functionality.

Ciprofloxacin Release and Corrosion Behaviour of a Hybrid PEO/PCL Coating on Mg3Zn0.4Ca Alloy.

In the present work, a hybrid hierarchical coating (HHC) system comprising a plasma electrolytic oxidation (PEO) coating and a homogeneously porous structured polycaprolactone (PCL) top-coat layer, loaded with ciprofloxacin (CIP), was developed on Mg3Zn0.4Ca alloy. According to the findings, the HHC system avoided burst release and ensured gradual drug elution (64% over 240 h). The multi-level protection of the magnesium alloy is achieved through sealing of the PEO coating pores by the polymer layer and the inhibiting effect of CIP (up to 74%). The corrosion inhibition effect of HHC and the eluted drug is associated with the formation of insoluble CIP-Me (Mg/Ca) chelates that repair the defects in the HHC and impede the access of corrosive species as corroborated by FTIR spectra, EIS and SEM images after 24 h of immersion. Therefore, CIP participates in an active protection mechanism by interacting with cations coming through the damaged coating.

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys-A Review: Part III-Corrosion Inhibitors and Combining Them with Other Protection Strategies.

Owing to the unique active corrosion protection characteristic of hexavalent chromium-based systems, they have been projected to be highly effective solutions against the corrosion of many engineering metals. However, hexavalent chromium, rendered a highly toxic and carcinogenic substance, is being phased out of industrial applications. Thus, over the past few years, extensive and concerted efforts have been made to develop environmentally friendly alternative technologies with comparable or better corrosion protection performance to that of hexavalent chromium-based technologies. The introduction of corrosion inhibitors to a coating system on magnesium surface is a cost-effective approach not only for improving the overall corrosion protection performance, but also for imparting active inhibition during the service life of the magnesium part. Therefore, in an attempt to resemble the unique active corrosion protection characteristic of the hexavalent chromium-based systems, the incorporation of inhibitors to barrier coatings on magnesium alloys has been extensively investigated. In Part III of the Review, several types of corrosion inhibitors for magnesium and its alloys are reviewed. A discussion of the state-of-the-art inhibitor systems, such as iron-binding inhibitors and inhibitor mixtures, is presented, and perspective directions of research are outlined, including in silico or computational screening of corrosion inhibitors. Finally, the combination of corrosion inhibitors with other corrosion protection strategies is reviewed. Several reported highly protective coatings with active inhibition capabilities stemming from the on-demand activation of incorporated inhibitors can be considered a promising replacement for hexavalent chromium-based technologies, as long as their deployment is adequately addressed.

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys-A Review: Part II-PEO and Anodizing.

Although hexavalent chromium-based protection systems are effective and their long-term performance is well understood, they can no longer be used due to their proven Cr(VI) toxicity and carcinogenic effect. The search for alternative protection technologies for Mg alloys has been going on for at least a couple of decades. However, surface treatment systems with equivalent efficacies to that of Cr(VI)-based ones have only begun to emerge much more recently. It is still proving challenging to find sufficiently protective replacements for Cr(VI) that do not give rise to safety concerns related to corrosion, especially in terms of fulfilling the requirements of the transportation industry. Additionally, in overcoming these obstacles, the advantages of newly introduced technologies have to include not only health safety but also need to be balanced against their added cost, as well as being environmentally friendly and simple to implement and maintain. Anodizing, especially when carried out above the breakdown potential (technology known as Plasma Electrolytic Oxidation (PEO)) is an electrochemical oxidation process which has been recognized as one of the most effective methods to significantly improve the corrosion resistance of Mg and its alloys by forming a protective ceramic-like layer on their surface that isolates the base material from aggressive environmental agents. Part II of this review summarizes developments in and future outlooks for Mg anodizing, including traditional chromium-based processes and newly developed chromium-free alternatives, such as PEO technology and the use of organic electrolytes. This work provides an overview of processing parameters such as electrolyte composition and additives, voltage/current regimes, and post-treatment sealing strategies that influence the corrosion performance of the coatings. This large variability of the fabrication conditions makes it possible to obtain Cr-free products that meet the industrial requirements for performance, as expected from traditional Cr-based technologies.

Chromate-Free Corrosion Protection Strategies for Magnesium Alloys-A Review: PART I-Pre-Treatment and Conversion Coating.

Corrosion protection systems based on hexavalent chromium are traditionally perceived to be a panacea for many engineering metals including magnesium alloys. However, bans and strict application regulations attributed to environmental concerns and the carcinogenic nature of hexavalent chromium have driven a considerable amount of effort into developing safer and more environmentally friendly alternative techniques that provide the desired corrosion protection performance for magnesium and its alloys. Part I of this review series considers the various pre-treatment methods as the earliest step involved in the preparation of Mg surfaces for the purpose of further anti-corrosion treatments. The decisive effect of pre-treatment on the corrosion properties of both bare and coated magnesium is discussed. The second section of this review covers the fundamentals and performance of conventional and state-of-the-art conversion coating formulations including phosphate-based, rare-earth-based, vanadate, fluoride-based, and LDH. In addition, the advantages and challenges of each conversion coating formulation are discussed to accommodate the perspectives on their application and future development. Several auspicious corrosion protection performances have been reported as the outcome of extensive ongoing research dedicated to the development of conversion coatings, which can potentially replace hazardous chromium(VI)-based technologies in industries.

Anticorrosion and Cytocompatibility Assessment of Graphene-Doped Hybrid Silica and Plasma Electrolytic Oxidation Coatings for Biomedical Applications.

Magnesium AZ31 alloy substrates were coated with different coatings, including sol-gel silica-reinforced with graphene nanoplatelets, sol-gel silica, plasma electrolytic oxidation (PEO), and combinations of them, to improve cytocompatibility and control the corrosion rate. Electrochemical corrosion tests, as well as hydrogen evolution tests, were carried out using Hanks' solution as the electrolyte to assess the anticorrosion behavior of the different coating systems in a simulated body fluid. Preliminary cytocompatibility assessment of the different coating systems was carried out by measuring the metabolic activity, deoxyribonucleic acid quantification, and the cell growth of premyoblastic C2C12-GFP cell cultures on the surface of the different coating systems. Anticorrosion behavior and cytocompatibility were improved with the application of the different coating systems. The use of combined PEO + SG and PEO + SG/GNP coatings significantly decreased the degradation of the specimens. The monolayer sol-gel coatings, with and without GNPs, presented the best cytocompatibility improvement.

Hierarchical Functionalized Polymeric-Ceramic Coatings on Mg-Ca Alloys for Biodegradable Implant Applications.

Magnesium-based implants present several advantages for clinical applications, in particular due to their biocompatibility and degradability. However, degradation products can affect negatively the cell activity. In this work, a combined coating strategy to control the implant degradation and cell regulation processes is evaluated, including plasma electrolytic oxidation (PEO) that produces a 13 µm-thick Ca, P, and Si containing ceramic coating with surface porosity, and breath figures (BF) approach that produces a porous polymeric poly(ε-caprolactone) surface. The degradation of PCL-PEO-coated Mg hierarchical scaffold can be tailored to promote cell adhesion and proliferation into the porous structure. As a result, cell culture can colonize the inner PEO-ceramic coating structure where higher amount of bioelements are present. The Mg/PEO/PCL/BF scaffolds exhibit equally good or better premyoblast cell adhesion and proliferation compared with Ti CP control. The biological behavior of this new hierarchical functionalized scaffold can improve the implantation success in bone and cardiovascular clinical applications.

Doped TiO2 anodic layers of enhanced antibacterial properties.

Ti-6Al-4V joint replacement implants foster uncemented fixation in orthopaedic surgery. However, bacterial colonization competes with host cells and ultimately may produce implant-related difficult-to-treat infections, justifying the efforts to obtain infection-resistant materials. In a previous work, the authors demonstrated the antibacterial properties of anodic fluoride-TiO2 nanostructured layers on Ti-6Al-4V alloy. In this work, the anodizing bath has been modified in order to grow fluoride-TiO2 barrier layers (FBL). A bacterial adherence protocol, run with reference and six different clinical strains of Staphylococcus aureus and Staphylococcus epidermidis, showed a statistically significant decrease in the percentage of covered surface (p<0.0001, Kruskal-Wallis test) for FBL specimens when compared with non fluoride-containing specimens, i.e. chemically polished Ti-6Al-4V and F-free TiO2 barrier layers. The results obtained on the F-barrier layers allowed discrimination between the effects of the presence of fluoride in the layer and the layer nanostructure on bacterial adhesion.

In vitro evaluation of cell proliferation and collagen synthesis on titanium following plasma electrolytic oxidation.

Titania-based coatings produced by plasma electrolytic oxidation are being investigated as bioactive surfaces for titanium implants. In this study, plasma electrolytic oxidation was performed in calcium- and phosphorus-based electrolytes under DC conditions, resulting in coatings of thickness of approximately 8-15 mum. Coating morphologies, microstructures, and compositions were examined by scanning electron microscopy with energy-dispersive X-ray analysis, X-ray diffraction, and electron probe microanalysis. The coatings revealed a cratered morphology, with incorporated calcium and phosphorus species. Proliferation rates of primary human osteoblasts cells on the coatings were up to approximately 37% faster than those for uncoated titanium and 316L stainless steel reference materials. Further, the coatings assisted cell adhesion and generation and anchorage of collagen. The amount of collagen was upto approximately 2.4 times greater than for the reference substrates. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.