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  • Open access
  • 110 Reads
Degradation rate control of MgxZnyCa alloys by PEO coatings

Magnesium alloys are actively researched for biodegradable implants in order to avoid an implant-removal surgery after completion of the healing process. However, high degradation rate of Mg leads to hydrogen evolution and may increase the pH of the environment, causing inflammatory response. In the present work, bioactive plasma electrolytic oxidation (PEO) coatings on three Mg­xZnyCa alloys (two cast alloys and one extruded; x=1, 3 or 0.5; y=1 or 0.3), manufactured by Helmholtz-Zentrum Geesthacht, were generated in order to enhance the corrosion resistance, bioactivity and cytocompatibility of the alloys. AC PEO process was carried out in two environmentally friendly alkaline electrolytes containing Ca, P and Si as bioactive elements. The electrolytes were a true solution and a particle suspension. F-free electrolyte design was employed to ensure cytocompatibility of the coatings with different types of cells. The materials were characterized by SEM, EDS, XRD and optical profilometry. The corrosion behavior was evaluated by EIS and hydrogen evolution measurements during 5 days of immersion in 0.9% NaCl and α-MEM solutions, a complete one and an inorganic part only, at 37ºC. PEO coatings (7-13 µm-thick, Sa= 1.85-4.19 µm) were constituted by MgO, Ca3(PO4)2, Ca5(PO4)3(OH) and Mg2SiO4 phases. Both PEO coatings decreased the degradation rate of Mg alloys; corrosion resistance of coated samples in inorganic α-MEM increased by more than an order of magnitude (|Z|10mHz, (Ω·cm2): MgZnCa = 746, MgZnCa/PEO = 8544…28277). All materials exhibited considerably greater corrosion rates in 0.9% NaCl than in α-MEM, where phosphate-based additives acted as corrosion inhibitors. Corrosion rates were slightly greater in complete α-MEM than in inorganic α-MEM due to the presence of complexing aminoacids. The developed coatings are considered suitable candidates for the subsequent development of hybrid hierarchical ceramic/biodegradable polymer systems.

  • Open access
  • 94 Reads
Electrochemical determination of corrosion properties of iron foams prepared by using urea as a spaceholder

Porous biodegradable materials have the potential to serve as temporary orthopedic scaffolds with the ability to degrade spontaneously in the human body environment. Several production methods may be used for their manufacturing, including replication methods, additive manufacturing, or casting. The space-holder method represents a cost-effective production way that can produce highly porous materials with an easily controllable porosity ratio. Since porosity plays an important role in the evaluation of material corrosion and mechanical properties, a series of samples with increasing space-holder (urea) ratio (Fe:Urea - 95: 5, 90:10, 85:15, and 80:20) to the iron were fabricated and tested for their electrochemical corrosion properties. Iron powder and spherical urea particles were used as a starting material, compressed into cylindrical shapes, and sintered at 1120 °C for one hour in a reducing hydrogen atmosphere. Potentiodynamic polarization curves of Fe samples were obtained in Hanks´ solution at 37 °C and pH = 7.4±0.2. A decrease of the corrosion potential (Ecorr) with increasing porosity was confirmed in simulated body fluids, suggesting the importance of porosity and its influence on the degradation susceptibility of iron-based biomaterials. Smaller urea addition (5 wt.%) did not significantly affect the corrosion potential when compared to the pure iron prepared without the space-holder addition. However, the mechanical integrity of the samples with a higher Fe-urea ratio (80:20) was not maintained after sample sintering, which also must be addressed during future fabrication of such material.

  • Open access
  • 111 Reads
Biofilms of sulfate-reducing bacteria on polyethylene terephthalate: the effect of bacillibactin-producing strains of Bacillus velezensis

Poly(ethylene terephthalate) (PET) is very resistant to degradation by microorganisms, the ability to biodegrade PET is inherent in a small number of species. Among the microorganisms isolated from the plastic surface and biofilms there are representatives of sulfate-reducing bacteria of the genus Desulfovibrio. Bacteria Desulfovibrio oryzae were identified, which are participants in the damage of metal construction in soil. We used D. oryzae as a model organism to study biofilm formation on PET, although they did not have a degrading ability with respect to PET (according to the results of gravimetric analysis). Antibiofilming properties are known for Bacillus velezensis bacteria as their ability to produce siderophore bacillibactin. Siderophores are Fe(III) chelators, and chelating agents affect the stages in biofilm development. The ability to form bacillibactin is characteristic of B. velezensis NUChC C1 and NUChC C2b. The aim of this study was to investigate the intensity of biofilm formation of D. oryzae on the PET surface by the influence of bacillibactin-producing B. velezensis. The duration of exposure of PET samples (10×10 mm) in Postgate’s “C” medium with culture of D. oryzae NUChC SRB1 under the influence of the supernatant from MPB cultures of B. velezensis NUChC C1 and NUChC C2b was 50 days. A biofilm assay (indirect measurement of bacterial biofilm biomass by crystal violet adsorption/desorption) was used. Siderophore-producing strains of Bacillus velezensis inhibited (2 times) the formation of bacterial biofilms on the polymeric material PET with its long-term exposure in a culture of sulfate-reducing bacteria under conditions of sufficient iron supply. Bacillibactin-producing strains prevent the development of bacterial biofilms on the poly(ethylene terephthalate) surface. This may be one of the reasons for the prolongation of the process of PET biodegradation in natural ecosystems.

  • Open access
  • 83 Reads
Early stages of crack development in environmentally assisted cracking

The development of damage due to environmentally assisted cracking is commonly described by the broad stages: precursor development; transition to a crack; small crack growth; long crack growth. There still remains a degree of ignorance with respect to the role of the surface preparation method in the earliest stages of damage development; in particular, the effect of machining and grinding on the formation of surface defects, residual stress, and nanocrystalline and heavily deformed surface layers. To increase awareness of the potential impact of the surface state, examples will be illustrated briefly in relation to pitting of 316L and formation of dealloyed layers on duplex stainless steel.

Corrosion pits are the most commonly observed precursor to cracks in aqueous chloride environments, and the transition from pit to crack perhaps the most studied. A perspective on the loci of sites of crack initiation and the criteria for the pit-to-crack transition will be presented and the implications for the optimum methodology for quantifying the early stages of crack growth discussed. The subsequent application will be highlighted using examples from NPL research on stress corrosion cracking and low frequency corrosion fatigue of 12 Cr steam turbine blade steel. A key feature to emerge from the research is the observation of remarkedly enhanced crack growth rates for small and short cracks, which is rationalised on the basis of the solution conductivity dependent electrochemical crack size effect.

  • Open access
  • 78 Reads
Influence of ceramic coating on corrosion properties of sintered iron materials

Biomaterials have become an essential part of modern medicine as components of orthopedic implants. Metals such as iron or magnesium are most used for this purpose. By applying different coatings to metals, it is possible to change, improve and adjust the properties of metallic materials to the desired applications. Hydroxyapatite is often used as a bioactive coating in the field of biomaterials mainly due to good biocompatibility and osteoconductivity. The paper is focused on the study of the degradation properties of iron-based biomaterials. The iron samples were prepared from carbonyl iron powder by cold pressing into pellets followed by sintering in a reductive atmosphere for 1 hour. A bioceramic hydroxyapatite (HAp) coating was applied to the surface of the sintered iron samples by electrochemical deposition for 15, 30, and 50 minutes. The corrosion properties of samples were studied using an anodic polarization method in Hanks´s solution. Electrochemical impedance spectroscopy measurements were performed after immersing the samples for 60 minutes in Hanks's solution. Based on dynamic tests, a shift to a more negative value of the potential was observed for the sample with a longer time of HAp deposition, which indicates a stronger restriction for the transport of dissolved oxygen to the iron surface. Compared to the iron sample without the ceramic coating, a shift to a positive efficiency value was observed for the HAp coated samples.

  • Open access
  • 95 Reads
In situ incorporation of corrosion inhibitors into Plasma electrolytic oxidation coatings on 2024-T3 alloy

Plasma Electrolytic Oxidation (PEO) has been targeted as an eco-friendly alternative to Chromic acid anodizing (CAA) as it results in the formation of multifunctional ceramic-like coatings with similar corrosion resistance to CAA. Nevertheless, the PEO process usually produces thick coatings (50-100) µm, thus limiting their use in fatigue-sensitive applications in the aircraft industry. A possible way to overcome this limitation is the “flash” PEO process, producing thin coatings (≤ 8 μm) in treatment times as short as 1–3 min.

The present work studies the in situ incorporation of corrosion inhibitors (V-, Mo-, La-, Sn-, W-, and Ce-salts with and without EDTA in thin (< 8 µm) PEO coatings.

The screening process to select the best flash-PEO coating was according to the higher corrosion resistance (EIS, 1h)-thickness ratio. The coatings generated in W- and Ce/EDTA-containing electrolytes were selected for further analysis. The morphology, composition, corrosion resistance and paint adhesion (ISO 2409) were evaluated using conventional characterization techniques (SEM, XRD, WCA, salt spray, etc). The most promising results were obtained for the coating formed in Ce-Na2EDTA electrolyte since it showed an optimum paint adhesion capacity and no signs of corrosion in painted condition after 1000 h of exposure in a salt spray chamber.

  • Open access
  • 50 Reads
Nanocharacterizations of dental materials by atomic force microscopy and their thermal degradation evaluation

The restorative dental materials must be produced with special characteristics because these are operating in an environment medium with different humidity and temperature. These day-to-day factors play an important role in the lifetime of such dental restorative materials. Resin composites have been by far the most successful in dental applications by meeting several stringent design requirements difficult to achieve with homogeneous materials such as ceramics and metal alloys. Mechanical and tribological properties of direct restorative filling materials are crucial not only to serve and allow similarity with human enamel and dentine but also to compare composites between them and determine objective criteria for their selection. The objective of this research work is to investigate the mechanical and tribological properties of some commercial restorative materials using the atomic force microscopy technique as a function of the operating temperature. Therefore, restorative materials are expected to replace and perform as natural tooth materials. The demand of achievement it is so great that most of the times restorative filling materials replace enamel and dentin, which have very different mechanical properties, namely hardness and elastic modulus. The scope is to estimate the lifetime of such materials starting from their nano-behaviors as nano-wear, nano-friction, nano-mechanical tests. Concluding, the nanoindentation is an attractive method for measuring the mechanical behavior of small specimen volumes in dental hard materials. Using this technique, the mechanical and tribological properties of nanocomposite resins were investigated. This technique evaluates only the tribo-mechanical properties of a very shallow surface region of a specimen that may have undergone damage associated with mechanical preparation required to achieve a satisfactory flat sample for testing. Experimentally study has been carried out with several normal loads and time-duration tests i.e., representing several steps of severity conditions for materials under investigation.

  • Open access
  • 113 Reads
Corrosion phenomena of plasma nitrided steel, duplex CrN and TiAlN coatings subjected to molten Al-alloy

High pressure die casting (HPDC) is a progressively developing technology used for mass production of complex, near-net shape and thin-walled components of light alloys. Its production efficiency greatly depends on a die quality and its endurance. Nowadays this is usually improved by application of ceramic coatings produced by physical vapor deposition (PVD) on its surfaces. However, future development of these coatings still requires fundamental knowledge about oxidation and corrosion mechanisms acting in these specific cases. Therefore, we investigated the performance of plasma nitrided steel, duplex CrN and TiAlN PVD coatings prepared to different degrees of surface roughness. Corrosion behaviour in Al-Si-Cu cast alloy was evaluated by ejection test, performed with conventional (CS) and delayed cast alloy solidification (DS) for 5 and 20 min. Beside corrosion in a casting process this test simulates the ejection process of a die core from a casting. The force required for ejection is a measure of cast alloy soldering (corrosion) tendency toward pin material. Different microscopy and analytical techniques were employed for the analysis of samples surfaces after the ejection tests. On all samples subjected to CS tests cast alloy built-up layer formed due to galling and mechanical soldering effects. In conditions of DS tests, plasma nitrided sample was attacked by aluminium, a corroded layer formed which easily sheared under lower force during the ejection process. On the other side, lower ejection force recorded in DS tests for smoother coated samples is attributed to the thickening of the casting oxide scale that occurred due to the consumption of oxygen from coatings oxide layers. For both coatings, in DS tests, a nondetrimental corrosion of underlying nitrided substrate occurred through the coating growth defects. Results of focused ion beam analysis of different corrosion sites revealed the morphology and chemistry of corrosion products which suggested possible mechanisms that act in such processes.

  • Open access
  • 144 Reads
Exploring chemical space using computational methods

As the lightest structural engineering metal, magnesium (Mg) is a promising base material for the development of advanced technologies in transport, medical as well as in battery applications. A prerequisite to unlock the full potential of Mg–based materials is gaining control over their corrosion behaviour due to the relatively high chemical reactivity of Mg whereas each application field imposes unique requirements on this challenge. Corrosion prevention is essential in transport applications to avoid material failure. Bone implants require a degradation rate that is tailored to a specific injury whereas constant dissolution of the anode material is required to boost the efficiency of Mg-air primary batteries. Fortunately, small organic molecules have shown great potential to control the dissolution properties of pure Mg materials and its alloys.1 However, the vast space of small molecules with potentially useful dissolution modulating properties (inhibitors or accelerators) renders conventional experimental discovery methods too time- and resource-consuming. Consequently, computer-assisted selection prior to experimental investigations of the most promising candidates is of great benefit in the search for effective corrosion modulating additives.

Here, we demonstrate how unsupervised clustering of potential Mg dissolution modulators based on structural similarities and sketch-maps2 can quantitatively predict their experimental performance when combined with a kernel ridge regression (KRR) model.3 The prediction accuracy of the KRR model is compared to an artificial neural network (ANN) model that was trained on a combination of atomistic and structural molecular descriptors.4 Furthermore, we confirm the robustness of our data-driven model by blind prediction of the dissolution modulating performance of 10 untested compounds. Finally, a workflow is presented that facilitates an automated selection of compounds with promising properties by screening of a large database comprised of commercially available substances.

[1] S. V. Lamaka, B. Vaghefinazari, D. Mei, R.P. Petrauskas, D. Höche, M. L. Zheludkevich, Corros. Sci. 2017, 128, 224-240.

[2] M. Ceriotti, G. A. Tribello, M. Parrinello, Proc. Natl Acad. Sci. USA 2011, 108, 13023.

[3] T. Würger, D. Mei, B. Vaghefinazari, D. A. Winkler, S. V. Lamaka, M. L. Zheludkevich, R. H. Meißner, C. Feiler, npj. Mater. Degrad. 2021, 5, 2.

[4] C. Feiler, D. Mei, B. Vaghefinazari, T. Würger, R. H.Meißner, B. J. C. Luthringer-Feyerabend, D. A. Winkler, M. L. Zheludkevich, S. V. Lamaka, Corros. Sci. 2020, 163, 108245.

  • Open access
  • 276 Reads
The virtual corrosion engineer

At an industrial plant, the conventional corrosion management process consists of defining the expected process conditions, identifying potential corrosion threats and estimating their likely rate, then using that information to develop mitigation plans and inspection schedules. Eventually, over periods of years, inspection results are fed back to corrosion engineers, initial threat assessments are updated, and then the cycle is repeated. The Virtual Corrosion Engineer (VCE) project aims to improve the accuracy of this process and speed up the feedback cycle by taking advantage of the enormous quantity of process information that is already collected and digitally stored every day. The VCE utilizes actual monitoring data, automates running of the best available corrosion models and provides a continuously updated dashboard in real time. The overall goal is not to remove the need for real engineers, but to arm those people with the most accurate and up to date information in the most efficient possible way, thus freeing up their time to focus on decision-making and continuous improvement of the underlying models. In this paper, we provide an overview of the VCE together with more detailed discussion of the underlying models for specific exemplar damage mechanisms, including High Temperature Hydrogen Attack (HTHA) and Under Deposit Corrosion (UDC) in steam generators.