Sebastian Feliu

Corrosion is a natural and spontaneous process of degradation or failure that occurs in almost all metal materials in contact with the surrounding environment

Additive manufacturing, heralded for its ability to produce intricate geometries with unparalleled precision while potentially minimizing material and energy consumption compared to traditional methods, has garnered significant attention in recent years. Magnesium, with its density of around 1.74 g/cm³ and appropriate mechanical strength, is an appealing lightweight structural material widely employed in industries such as automotive and aerospace. Despite these advantages, the corrosion behavior of additively manufactured magnesium alloys remains relatively unexplored, posing challenges for their widespread adoption. In this work, the corrosion behavior of AZ91 Mg alloy obtained through additive manufacturing was investigated. Cylindrical AZ91 Mg alloy samples were fabricated using the Powder Bed Laser Fusion (PBLF) technique. Analysis was conducted in both the transverse and longitudinal planes, where the transverse plane represents the top plane of the cylinder and the longitudinal plane represents a section through the center of the cylinder. This comparative approach, allowed a comprehensive examination of corrosion kinetics and behavior across different orientations. Furthermore, cast AZ91 Mg alloy was also studied for comparative purposes. Potenciodynamic polarization (PDP) tests, including both cathodic and anodic polarization, revealed similar kinetics for both the longitudinal and transverse planes studied, indicating consistent corrosion behavior across different orientations. Simultaneous gravimetric hydrogen collection and electrochemical impedance spectroscopy measurements (EIS) were performed to elucidate the underlying mechanisms governing corrosion processes. Furthermore, galvanodynamic polarization tests were employed to investigate the anomalous hydrogen evolution (AHE) intrinsic to Mg alloys. These tests were conducted in both planes, with concurrent hydrogen collection. Additionally, the role of the beta-Mg17Al12 phase on the electrochemical response of the additively manufactured AZ91 Mg alloy was investigated. The results of this study have significant implications for various industries, particularly automotive and aerospace applications, where Mg corrosion resistance is crucial for ensuring structural integrity and component longevity. Understanding the corrosion behavior of additively manufactured Mg alloys is paramount for optimizing their performance and facilitating widespread adoption in critical applications.

The urgent need to address the challenges posed by climate change has led to increased research and development efforts aimed at creating lightweight materials to reduce greenhouse gas emissions. This study focuses on corrosion protection of the commercial magnesium alloy AZ61, valued for its wide-ranging applications in engineering and industry, including automotive, aeronautics, and biomedical engineering. Despite its interesting mechanical properties, the AZ61 alloy exhibits poor corrosion resistance in saline aqueous environments, prompting significant research into coatings to enhance its durability. This research aims to design and prepare organic-inorganic hybrid sol-gel coatings for corrosion protection of the AZ61 alloy. The inorganic phase, mainly based on tetramethyl orthosilicate (TMOS), serves as a cross-linking agent, while four organofunctionalized silanes containing hydrolysable methoxy groups and functional organic groups into the same molecule are used as precursors for the organic component of these hybrid coatings. Specifically, the organofunctionalized silanes selected are γ-methacryloxypropyltrimethoxysilane (MAPTMS), γ -glycidoxypropyltrimethoxysilane (GPTMS), γ -aminopropyltrimethoxysilane (AMPTMS), and γ -mercaptopropyltrimethoxysilane (MPTMS). These silanes are chosen for their ability to create organopolysiloxane coatings with sufficient chemical compatibility and flexibility to accommodate other species without phase segregation or cracking. These properties are essential to create a suitable structural environment to encapsulate the selected ecofriendly corrosion inhibitors that are added in later stages of the study, thereby yielding a processable system that serves as a functional phase of active corrosion protection coatings for magnesium alloys. Physicochemical characterization techniques are employed to understand the chemical environment within the hybrid networks and confirm the successful formation of organopolysiloxane networks. Thermogravimetry and differential thermal analysis (TG/DTA), Fourier transform infrared spectroscopy (FTIR), and high-resolution solid-state ¹³C and ²⁹Si nuclear magnetic resonance spectroscopies are used for these purposes. The performance of coated AZ61 samples is analysed using the aforementioned organic-inorganic hybrid gels. These sol-gel coatings are applied using immersion techniques (dip-coating) on unpolished (as-received) and polished AZ61 samples, to determine the influence of surface conditions on their behaviour during immersion tests in 0.6, 0.06 and 0.006 M NaCl aqueous solutions. Open circuit potential (OCP) measurements and global and localized electrochemical impedance spectroscopies (EIS, LEIS) are applied with this purpose. The microstructure and texture of the coatings, both before and after the corrosion tests, are observed by optical microscopy (OM) and scanning electron microscopy (SEM). Moreover, Energy dispersive X-ray (EDX) microanalyses are performed on several areas of the coated samples after the corrosion tests. In the last phase of the study, modifications to the sol-gel formulations are explored by incorporating environmentally friendly corrosion inhibitors such as cysteine (L-Cys) and benzotriazole (BTA), as well as cross-linking agents like hexamethoxymethylmelamine (HMMM) in presence of an acid catalysts, specifically p-toluenesulfonic acid (p-TSA), to facilitate the cross-linking reaction within the organosilicon network. These modifications aim to enhance the protective properties of the sol-gel coatings. EIS and LEIS are used to assess their effectiveness. Films formulated with HMMM demonstrate robust passive protection against corrosion, while those doped with L-Cys and BTA exhibit self-healing and active protection properties, offering a promising alternative to traditional chemical conversion pretreatments based on hexavalent chromium. Funding Sources This work has been supported by the Project PID2022-139920OB-I00 (Ministry of Science, Innovation and Universities, MICINN, Spain).

The use of magnesium and its alloys has increased in recent years due to their low weight, high strength, excellent machinability, and high dimensional durability. Therefore, they have found application in communications, automotive and aerospace industries, among others. However, their high negative reduction potentials, low chemical stability, and the unstable native oxide films have limited their use, particularly in applications that require high corrosion resistance.^{1 -3} The application of sol-gel coatings has shown to be an effective method to overcome this problem. In this context, the aim of this study was to develop sol-gel thin-films doped with ecological corrosion inhibitors for corrosion protection of AZ61 alloy surfaces. The objective was to determine whether these new systems could provide an alternative to the replacement of chromates, surface pre-treatments and chemical conversion coatings that contain conventional corrosion inhibitors, which are currently being questioned for their harmfulness to health and the environment.⁴ Methyltriethoxysilane (MTEOS) and tetraethylorthosilicate (TEOS) were used as precursors to produce sols, which were then modified with active corrosion protection dopants. Eco-benign corrosion inhibitors, such as L-cysteine (L-Cys), lanthanum (III) acetate hydrate, and lanthanum (III) isopropoxide were incorporated in the sol-gel matrix as dopants. A set of sol-gel coatings was modified with a common dopant, benzotriazole (BTA), for comparison purposes, as it is a well-known effective corrosion inhibitor. However, this chemical compound is known to be carcinogenic and toxic to flora and fauna, and its use is currently being restricted today in compliance with environmental protection regulations.^5,6 The resulting sols were processed and deposited on AZ61 substrates by dip-coating technique, producing transparent sol-gel thin-films. The thickness of the films was evaluated by interference of reflection spectra. Their chemical composition was characterized by X-ray fluorescence (XRF), while surface chemical composition and oxidation state of elements present at the outermost surface nanolayers was also analysed by X-ray photoelectron spectroscopy (XPS). The structural and thermal characterization of the doped thin-films were respectively analysed by Fourier transformed infrared spectroscopy (FTIR) and thermogravimetry and differential thermal analysis (TG/DTA). The corrosion protection behaviour of the sol-gel coatings during immersion tests in 0.006 M and 0.6 M NaCl aqueous solutions was studied using a multiscale electrochemical approach. Global electrochemical impedance spectroscopy (EIS) was used for macroscopic scale characterisation. Localised electrochemical impedance spectroscopy (LEIS and LEIM) was used for the characterisation at micro- and sub-microscopic scales using an electrochemical minicell system and a scanning electrochemical workstation. The texture and microstructure of coated samples before and after corrosion tests were observed by optical and scanning electron microscopies (OM and SEM), while topography and coating roughness were analysed by atomic force microscopy (AFM). In terms of corrosion resistance, similar results were obtained with sol-gel thin-films doped with lanthanum acetate or lanthanum isopropoxide. The synthesis method of the acetate-doped gels is much simpler and cheaper, so from a practical standpoint, these sol-gel thin-films could be more interesting. On the other hand, highly satisfactory results were obtained with gels doped with non-toxic L-Cys compared with those doped with BTA. Finally, it is noteworthy that an interesting synergistic effect was observed in the corrosion protection of the AZ61 alloy in sol-gel coatings that were doped with metal-organic inhibitors and loaded with organic inhibitors (L-Cys or BTA). As a concluding remark, this study has provided an effective and environmentally friendly solution for the active corrosion protection of the AZ61 alloy. Sol-gel thin-films doped with eco-friendly corrosion inhibitors have showed promising results in extending the durability of AZ61 alloy while being sustainable. The multiscale electrochemical approach used has provided a comprehensive understanding of the active corrosion protection behaviour and the self-healing properties of sol-gel coatings, which can be extended to other materials and systems. Funding Sources This work has been supported by the Ministry of Science and Innovation (MCINN, Spain) through the Project PID2019-104717RB-I00. References Feliu Jr., S., Maffiotte, C., Samaniego, A., Galván, J.C., Barranco, V. Acta, 56 (12) (2011) 4554-4565 Stojadinović, S., Vasilić, R., Radić-Perić, J., Perić, M. Coat. Technol., 273 (1) (2015), pp. 1-11 Toorani, M., Aliofkhazraei, M., Naderi, R., Golabadi, M., Sabour Rouhaghdam, A. Ind. Eng. Chem., 53, (2017) pp. 213-227 Vaghefinazari, B., Wierzbicka, E., Visser, P., Posner, R., Arrabal, R., Matykina, E., Mohedano, M., Blawert, C., Zheludkevich, M., Lamaka, S. Materials, 15 (23) (2022), art. no. 8676 Albini, M., Letardi, P., Mathys, L., Brambilla, L., Schröter, J., Junier, P., Joseph, E. Corrosion Sci., 143 (2018), pp. 84-92 Tan, L., Sun, Y., Li, J., Han, S., Zhou, X., Tang, Y., Zeng, X. Langmuir, 39 (2023), 2579-2588

Stainless steel SS430 and carbon steel B450C were exposed for 30 days to the aqueous extract of sodium silicate-modified limestone-Portland cement as an alternative for the partial replacement of the Portland cement clinker. The initial pH of 12.60 was lowered and maintained at an average of 9.60, associated with air CO₂ dissolution and acidification. As a result, the carbon steel lost its passive state, and the corrosion potential (OCP) reached a negative value of up to 296 mV, forming the corrosion layer of FeO, and FeOOH. In the meaning time, on the stainless steel SS430 surface, a passive layer of Cr₂O₃ grew in the presence of FeO, Fe₂O₃ and Cr(OH)₃ corrosion products; thus, the OCP shifted to more positive values of +150 mV. It is suggested that a self-repassivation process took place on the SS430 surface due to the accumulation of alkaline sulfates on the interface. Because of the chloride attack, SS430 presented isolated pits, while on B450C, their area was extended. The quantitative analysis of EIS Nyquist and Bode diagrams revealed that the Rp of the corrosion process for SS430 was 2500 kΩcm², ≈32 times lower in magnitude than on B450C, for which the passive layer tended to disappear, while that on SS430 was ≈0.82 nm.

Carbon steel B450C and low-chromium stainless steel SS430 were exposed for 30 days to supersulfated “SS1” cement extract solution, considered as a “green” alternative for partial replacement of the Portland cement clinker. The initial pH of 12.38 dropped since the first day to 7.84, accompanied by a displacement to more negative values of the free corrosion potential (OCP) of the carbon steel up to ≈−480.74 mV, giving the formation of γ-FeOOH, α-FeOOH and Fe₂O₃, as suggested by XRD and XPS analysis. In the meantime, the OCP of the SS430 tended towards more positive values (+182.50 mV), although at lower pH, and XPS analysis revealed the presence of Cr(OH)₃ and FeO as corrosion products, as well the crystals of CaCO₃, NaCl and KCl. On both surfaces, a localized corrosion attack was observed in the vicinity of local cathodes (Cu, Mn-carbides, Cr-nitrides, among others), influenced by the presence of Cl⁻ ions in the “SS1” extract solution, originating from the pumice. Two equivalent circuits were proposed for the quantitative analysis of EIS Nyquist and Bode diagrams, whose data were correlated with the OCP values and pH change in time of the “SS1” extract solution. The thickness of the corrosion layer formed on the SS430 surface was ≈0.8 nm, while that on the B450C layer was ≈0.3 nm.

The carbon steel B450C and low chromium SS 430 ferritic samples were exposed for 30 days to chloride-containing (5 g L⁻¹ NaCL) cement extract solution. The initial pH ≈ 13.88 decreased to pH ≈ 9.6, associated mainly with the consumption of OH⁻ ions and the formation of γ-FeOOH, α-FeOOH, Fe₃O₄ and Cr(OH)₃, as suggested by XRD and XPS analysis, in the presence of CaCO₃ and NaCl crystals. The deep corrosion damages on B450C were observed around particles of Cu and S as local cathodes, while the first pitting events on the SS 430 surface appeared after 30 days of exposure. The change in the activity of each type of steel was provided by the potentiodynamic polarization curves (PDP). Two equivalent electrical circuits (EC) were proposed for quantitative analysis of EIS (Nyquist and Bode diagrams). The calculated polarization resistance (R_p), as an indicator of the stability of passive films, revealed that SS 430 presented relatively constant values, being two-three orders of magnitude higher than those of the carbon steel B450C. The calculated thickness (d) of the SS 430 passive layers was ≈0.5 nm and, in contrast, that of the B450C passive layers tends to disappear after 30 days.

This work compares the degradation of Mg and Mg-Ca0.3 alloy when they are exposed for 14 days to Hank’s solution at 37 °C. A combination of immersion test, electrochemical techniques (PDP, EIS, EN), and surface characterization methods (SEM-EDS, XRD, and XPS) were carried out. The pH change over time, the lower mass loss (≈20%), and the lower concentration of the released Mg²⁺ ions (≈3.6 times), as well as the lower level of the surface degradation, allowed to consider the positive effect of Ca, presenting Mg-Ca0.3 alloy with lower electrochemical activity than that of Mg. The positive effect of Ca may be due to the formed layer characteristics on the alloy surface, which impedes the cathodic hydrogen evolution and Mg-ions release. The electroless deposited Ag-nano-particles (Ag-NPs) on Mg-Ca0.3 surface were characterized by SEM-EDS, XRD, UV-Vis, and contact angle. The agar-diffusion test was used to compare the growth of Staphylococcus aureus and Escherichia coli bacteria on Mg-Ca0.3 in the presence of Ag-NPs deposits in different size. Zeta-potential of the bacteria was negative, with respect to pH of the Mueller-Hinton culture broth. The greater antibacterial effect of S. aureus was attributed to its more negative zeta-potential, attracting more released Ag⁺ ions.