The extensive use of composite materials in industrial applications such as aeronautics, automobile, wind energy necessitates a sound understanding of the mechanical and damage behaviors of composites. It is well known that the service environment (temperature and humidity) influences the durability of composite structures, therefore environmental effects on tolerance design must be considered to guarantee their structural integrity. In particular, low velocity impact is assumed to be a severe threat to composite materials as significant internal damage can be generated that can be easily go undetected during a visual inspection. Therefore, the assessment of residual strength of composite structures after impact while considering the various environmental effects is a complex and not so common problem. In this regard, this work aims at highlighting the environmental effects on the impact behavior of composite laminates based on both synthetic and natural fibers, a topic that has received a limited coverage in the available literature, with a description of future research areas to bridge this gap of knowledge.

Cork is a natural raw material with unique properties. It is characterized by its low density, elasticity, compressibility, impermeability, and is also an excellent thermal and acoustic insulator as well as a fire-retardant material, highly resistant to abrasion. Additionally, it has a remarkable environmental footprint since it is completely biodegradable, renewable, and recyclable. Therefore, this versatile material and its composites have many different applications, including flooring, wall coverings and building facades, among many others where this material faces possible degradation caused by exposure to environmental conditions. To the authors best knowledge, the effects of this exposure on cork composites properties were never addressed. Thus, in order to determine theses effects on cork’s mechanical properties, thermal properties and aesthetics, cork composites were subjected to accelerated ageing by exposure to ultraviolet radiation simulating natural weather conditions, followed by mechanical and thermal testing, as well as scanning electron microscopy and Fourier-transform infrared spectroscopy. The results show how prolongated exposure to ultraviolet radiation degrade cork composites, including its aesthetics, as well as its impact on the mechanical behavior and thermal properties.

Evaluation of NiTi alloy corrosion behavior in conditions that exist in the oral cavity still remains a great characterization challenge. Such characterization is commonly simplified by avoiding the use of non-accelerated corrosion tests. Accordingly, the difficulties in characterization of material changes on a nano level are avoided and results do not sufficiently resemble the real situation. Therefore, the motivation of this work was to perform a non-accelerated corrosion test, characterize the nano topographic changes and to evaluate the obtained results by statistical methods. In this study, we examined the behavior of NiTi alloy (50% Ni, 50% Ti) archwires exposed for 21 days to different corrosive mediums: artificial saliva, Eludril®, Aquafresh® and Listerine®. The corrosion was characterized by means of changes in surface topography. This was done by contact mode atomic force microscopy on all samples at 5 locations of 10x10 µm areas, before and after the corrosion tests. Image analysis software was used for analysis of topographic images and calculation of surface roughness parameters Sa and S10z. The changes of roughness parameters were statistically analyzed by Anova. Sa and S10z parameters displayed changes with trend for all treatments. However, the confidence interval for all cases was overlapped. Statistically analysed data revealed that all samples exposed to mouthwashes displayed significant changes in parameter S10z, while only samples exposed to Aquafresh® and Eludril® displayed significant changes in parameter Sa. On the other side, sample exposed to artificial saliva did not display significant changes in any parameter. As such it is implied that mouthwashes have significantly higher effect on surface topography. Differences in confidence interval of Sa parameter indicate that changes in roughness parameters caused by corrosion does not depend on the initial surface roughness. In this study statistical analysis methods have been proven as a useful tool in characterization of nano topographic changes caused by corrosion in real conditions.

A simultaneous improvement of the corrosion resistance and bioactivity of magnesium implants in order to ensure their uniform biodegradation and cells adhesion during the process of the bone tissue formation is one of the current challenges of modern biomedicine [1–4].

This paper presents the results of investigation of self-healing polymer-containing bioresorbable hybrid coatings, allowing to control the corrosion rate of magnesium implant in physiological liquids. To ensure the bioactivity and improve the adhesive strength between the hybrid inhibitor/polymer layer and the metallic substrate, the hydroxyapatite-containing coating was obtained on Mg-Mn-Ce alloy by plasma electrolytic oxidation (PEO). It was established that the heterogeneity of the formed oxide layer, expressed by the presence of pores and microdefects, is suitable for further corrosion properties improving. The following methods of PEO-coating modification were developed to increase the corrosion resistance of a magnesium alloy:

CC-1 – PEO-layer, treated in the 6 wt. % solution of polycaprolactone (PCL) in dichloromethane;

CC-2 – hybrid coating, formed by impregnation of PEO-layer with 8-hydroxyquinoline (8-HQ) from the alkaline solution (3 g/l), followed by application of PCL (6 wt. %) from the dichloromethane solution;

CC-3 – hybrid coating, obtained by a treatment of PEO layer with 8-HQ (15 g/L) and PCL (6 wt. %) from the dichloromethane solution.

The accelerated corrosion tests of formed surface layers in 3 wt. %NaCl solution during 24 h allowed to reveal the best protective performance for the samples with CC-3. The corrosion current densities I_c for these samples were by 6 and 2 fold lower, than those for the samples with CC-1 and CC-2, respectively. The inhibitor efficiency (η_i) of CC-3 hybrid coating was 80 %. After 24 h of samples immersion in NaCl, η_i for CC-3 increased up to 84 %.

This work was supported by the Grant of Russian Science Foundation (project no. 20-13-00130).

References

[1] N. Sezer, Z. Evis, S.M. Kayhan, A. Tahmasebifar, M. Koç, Review of magnesium-based biomaterials and their applications, J. Magnes. Alloy. (2018). https://doi.org/10.1016/j.jma.2018.02.003.

[2] S. V. Dorozhkin, Calcium orthophosphate coatings on magnesium and its biodegradable alloys, Acta Biomater. (2014). https://doi.org/10.1016/j.actbio.2014.02.026.

[3] V.K. Bommala, M.G. Krishna, C.T. Rao, Magnesium matrix composites for biomedical applications: A review, J. Magnes. Alloy. (2019). https://doi.org/10.1016/j.jma.2018.11.001.

[4] A.S. Gnedenkov, S. V. Lamaka, S.L. Sinebryukhov, D. V. Mashtalyar, V.S. Egorkin, I.M. Imshinetskiy, M.L. Zheludkevich, S. V. Gnedenkov, Control of the Mg alloy biodegradation via PEO and polymer-containing coatings, Corros. Sci. (2021). https://doi.org/10.1016/j.corsci.2021.109254.

Microbiologically Influenced Corrosion is a major concern to a wide range of industries, with claims that it contributes 20% of the total annual corrosion cost. While there is moderate understanding of aerobic MIC mechanisms, the picture is les clear for anoxic conditions. Furthermore, complications arise due to divergent terminologies used by microbiologists and electrochemists, e.g. “anaerobic” and “direct electron transfer”, yet it is vital for these two fields to work together if we are to not only understand MIC, but also control / predict it without reverting to excessive use of toxic biocides. The focus of this presentation is to review proposed for MIC mechanisms, with particular emphasis on whether or not these make sense in terms of their electrochemistry.

Magnesium is very light metal, which possesses many useful properties such as high strength to weight ratio and good electrical and thermal conductivity. Magnesium and its alloys can be used in implant surgery as biodegradable materials. Taking into account the rapid degradation process of Mg alloys in chloride-containing solutions and very complex composition of the human body media, it is worth to study the mechanism and kinetics of the Mg alloys corrosion in solutions, which closely resemble that of human body fluids.

A comparative analysis of the corrosion activity of MA8 magnesium alloy (intended as bioresorbable material) in a medium for cultivation of mammalian cells (minimum essential medium, MEM) and 0.83% NaCl solution was performed using scanning vibrating electrode technique, local pH measurements, hydrogen evolution tests, OCP, PDP and EIS tests. Corrosion products formed on the alloy surface are characterized using XRD and SEM-EDX analysis, Raman spectroscopy. Hydrogen evolution rate is higher for samples in NaCl solution in comparison with MEM. The impedance modulus in the frequency range from 10⁵ Hz down to 10^-1 Hz for the sample immersed in MEM was higher than that for the sample immersed in NaCl. This indicates higher protective ability of the corrosion film formed in MEM compared to that formed during immersion in NaCl solution. Ca and P rich deposits were formed in the corrosion layer.

The model of corrosion mechanism of MA8 magnesium alloy in MEM, which includes three stages of the development of corrosion product film, is proposed. The formation on the surface of magnesium alloy sample in MEM of corrosion product layer, including magnesium-substituted hydroxyapatite, stabilizes the local pH below 9.0 and, along with organic acids, does not allow increasing the pH during corrosion of the Mg alloy. The obtained results indicate the prospect of using bioresorbable magnesium in implant surgery [1, 2].

This work was supported by the Grant of Russian Science Foundation, Russia (project no. 19-73-00078).

[1] A.S. Gnedenkov, D.Mei, S.V. Lamaka, S.L. Sinebryukhov, D.V. Mashtalyar, I.E. Vyaliy, M.L. Zheludkevich, S.V. Gnedenkov, Localized currents and pH distribution studied during corrosion of MA8 Mg alloy in the cell culture medium, Corros. Sci. 170 (2020) 108689.

[2] A.S. Gnedenkov, S.V. Lamaka, S.L. Sinebryukhov, D.V. Mashtalyar, V.S. Egorkin, I.M. Imshinetskiy, A.G. Zavidnaya, M.L. Zheludkevich, S.V. Gnedenkov Electrochemical behaviour of the MA8 Mg alloy in minimum essential medium, Corros. Sci. 168 (2020) 108552.

Despite the over 140 years of research with over 38,000 peer-reviewed publications, hydrogen embrittlement has remained little understood. That is because hydrogen is not trivial to detect, and it operates over various time and space scales. The detection of hydrogen, i.e., its position in the material, and its instantaneous effect on the microstructure, which creates precursor events for the evolution of cracks, are needed to be investigated under operando (and ideally non-destructive) conditions for understanding hydrogen-induced material degradation.

In this talk, recently obtained results from operando synchrotron high-energy x-ray diffraction measurements on a commercial 25Cr-7Ni super duplex stainless steel during electrochemical hydrogen charging will be presented. Miniature-sized tensile specimens were subjected to uniaxial mechanical loading and hydrogen absorption while simultaneously mapping local diffraction patterns with a local resolution of 20 µm across the entire sample thickness. The infusion of hydrogen resulted in the evolution of tensile lattice strains in austenite grains at the surface region parallel to the loading axis, more extensive than those developed in the ferrite, which increased gradually with charging time. Most strains were developed perpendicular to the loading direction and the surface, with the strain evolution in the austenite being more pronounced than the ferrite. The strain evolution in the bulk interior had an alternating character between compensating compressive and tensile strains, with their magnitude increasing to levels beyond the yield point. The same experiment was repeated under an optical microscope, and a set of micrographs from the surface were recorded in time-lapse. Processing these images using Digital Image Correlation revealed numerous strain hot-spots developing in the austenite and interphase boundaries corroborating the synchrotron diffraction data. This talk will introduce a new way of testing hydrogen-microstructure interactions. It will showcase the need to test under operando conditions to better understand hydrogen embrittlement.

Complex concentrated alloys (CCAs) are new types of materials, where the equimolar rule proposed by high entropy alloys (HEAs) is modified in relation to the potential of the obtained structures. CCAs expend the compositional space of the conventional alloys, revealing new pathways for material design. The Al₇Cu_0.2Si_0.2Zn_0.2Mg_0.1 alloy was prepared in an induction furnace, in controlled atmosphere and was cast in a copper ingot mold. The resulted samples of Al₇Cu_0.2Si_0.2Zn_0.2Mg_0.1 were analysed by chemical, structural, and corrosion resistance. Also, the alloy has been subjected to mechanical tests of hardness, elongation and tensile strength. The corrosion immersion tests, were performed in 3wt% and 5wt% NaClsolution, and corrosion indices were measured periodically. The obtained corrosion film was analized by SEM-EDS to determine the composition and structural behaviour. Depending on the adhesion level, the corrosion film remained stable or partially broken and separated in the solution. The sample weight loss presented large variations between the various experimental conditions, but the general tendency was the decrease in the weight of the samples during the corrosion tests. The formation of oxide and chloride layers, during the corrosion process, determined only the dealloying in Al. Other elements remained in initial concentrations. Overall, the resistance of the alloy in saline environment seems to be promising, with significant improvement over the comparable compositions of 2000 and 7000 series alluminum alloys.

Museum environmental monitoring is an important issue for artefact conservation and requires the development of systems able to measure temperature, relative humidity, as well as to detect very low concentrations of different atmospheric pollutants. Improper environmental conditions accelerate indeed corrosion and degradation of many materials including metals, papers and textiles. This effect is even amplified in closed environment, like in exhibition rooms and showcases, where temperature and humidity gradients introduce micro-climatic problems. While numerous systems exist for monitoring temperature and humidity, developing low cost sensors sensitive to the presence of pollutants and hence to the environmental corrosivity is still a challenge. Within the EU-SensMat project (https://www.sensmat.eu/), we recently demonstrated the interest of the RFID technology for the realization of such type of sensors. The method is based on an RFID tag which electromagnetically interacts with a thin metallic sensitive layer. Corrosion of such layer by pollutants modifies the signal received by the RFID reader leading to the possibility of extracting the corrosivity index via standard (ISO 11844). This type of sensors will be described in details at the conference. For this purpose, an application to the monitoring of pollution emitted by archeological woods containing pyrite (FeS₂) at ARC-Nucléart, an institute specializing in the conservation-restoration of archeology and historical wood, will be presented. Due to the possibility of extending the number of such low cost sensors, the monitoring was made in several rooms/locations within the building. As it will be shown, detection of pollutants by silver sensors is correlated to the formation of silver sulphide on the sensitive layers.

The bio-degradation of bio-based composite materials is an unwanted process. Excess humidity accelerates it, therefore hydrophobic protective coatings are used. In this work, the hydrophobic protective coatings were formed by the autooxidation and polycondensation mechanisms using drying oils such as a tung tree, linseed, and semi-drying hempseed oil. The bio-based composite boards were made of hemp shives, corn starch binder and flame retardants such as expanded graphite or multifunctional aqueous mixture based on phosphorus and nitrogen organic compounds. The hydrophobic coatings were formed by covering the bio-based composite boards (BcBs) with selected oils in the vacuum desiccator. After this procedure, BcBs covered with oil were maintained at 40^oC, 90^oC, or 120^oC temperature, respectively, until the protective film was formed. It was demonstrated that the compressive strength (at 10% of relative deformation) of the BcBs with the oil protective coatings compared to untreated BcB increased up to 4.5 fold and could reach up to 14 MPa, bending strength increased by 2.78 fold (up to 19 MPa), the water absorption decreased around 4-fold (from 1.34 kg/m2 to 0.37 kg/m2), swelling in thickness decreased from 7.51% to 4.59%, while the thermal conductivity remained unchanged and was around 0.085 W/(m·K) [1]. It was concluded that the most desired properties of such materials were obtained when the tung tree oil was used for coating.

References:

1. Vasiliauskienė, D.; Balčiūnas, G.; Boris, R.; Kairytė, A.; Kremensas, A.; Urbonavičius, J. The effect of different plant oil impregnation and hardening temperatures on physical-mechanical properties of modified biocomposite boards made of hemp shives and corn starch. Materials (Basel). 2020