A critical look at interpretation of electrochemical impedance spectra of sol-gel coated aluminium

A proper assignment of time constants is important for the correct interpretation of the electrochemical impedance spectra of coated substrates. In this work, several model aluminium samples with different anodic oxide thicknesses were prepared and characterised. The aluminium samples were coated by a hybrid sol-gel formulation and studied by electrochemical impedance spectroscopy (EIS) during immersion in NaCl solution. A hierarchical model containing three R-C elements adequately described the impedance spectra of the coated samples. The R-C elements described the properties of sol-gel coatings, aluminium oxide layers and corrosion process. The contribution of the aluminium oxide layer to impedance was unambiguously proven to be at middle frequencies on the spectra. Furthermore, the “real” capacitances of oxide layers were obtained from two capacitance–CPE equations and their applicability was scrutinized. The obtained results presented an opportunity to determine the adhesion behaviour of the sol-gel coatings to the oxide layers, which was studied by analysing the ratio of the oxide layer capacitance of the sol-gel coated samples to the capacitance of the uncoated ones. It was demonstrated that the exposed surface area of the oxide to the electrolyte was about 60–68% and was not changing during immersion. Based on these findings, a degradation model of the sol-gel coating on anodised aluminium was proposed.publishe

Um roteiro para a produção de metal duro com maior resistência à corrosão

RESUMO: Com o objetivo de racionalizar o desenvolvimento de compósitos de metal duro com maior resistência à corrosão, realizou-se um estudo sistemático da influência de vários parâmetros que determinam o seu desempenho. Neste trabalho apresentam-se resultados da influência de parâmetros relacionados com a constituição do material (tipo de ligante e tamanho de grão de carboneto de tungsténio - WC) e de parâmetros do ambiente de serviço (pH e temperatura). O estudo socorreu-se principalmente de métodos electroquímicos e de análise da superfície.info:eu-repo/semantics/publishedVersio

Smart coating for detection of early-stage corrosion of steel

ABSTRACT: The work describes the investigation of LDH-based nanoadditives for early-stage corrosion detection of steel, and subsequent development of a multi-layer functional protection coating. A systematic study on the level of degradation and the detected colorimetric signal was performed using electrochemical characterisation. The protection properties and detection functionality were also studied in conditions relevant to exploitation of metallic structures.info:eu-repo/semantics/publishedVersio

Redução electroquímica de iões de terras raras e o seu efeito na corrosão de metais

RESUMO: Os iões de terras raras estão entre os inibidores de corrosão de nova geração mais promissores. No entanto, foram encontradas condições onde a corrosão é acelerada na presença destes iões [1]. O caso mais relevante devido à sua importância é o aumento da corrosão do zinco no par galvânico zinco-ferro. Esse aumento está associado a uma reacção catódica adicional inesperada que é observada nas curvas de polarização obtidas experimentalmente. Esta nova redução corresponde a corrente catódica adicional que aumenta a oxidação do ânodo do par galvânico. Nesta comunicação analisa-se o impacto prático desta aceleração no par Zn-Fe e no aço galvanizado e procura-se identificar a natureza da nova reacção catódica.info:eu-repo/semantics/publishedVersio

Corrosion behaviour of WC hardmetals with nickel - based binders

Cobalt is the standard binder in tungsten carbide (WC) hardmetals due to its compatibility with the WC phase, resulting in composites with exceptional hardness and wear resistance. However, their corrosion resistance is not satisfactory in many applications, leading to the early deterioration and failure of tools and equipment. In this work, the corrosion of WC hardmetals with three alternative binders (FeCoNi, NiCrCoMo and NiCrMo) is compared with a benchmark WC-Co composite, using electrochemical techniques such as open circuit potential (OCP) monitoring, polarisation curves and electrochemical impedance spectroscopy (EIS), assisted by scanning electron microscopy (SEM).publishe

Layered double hydroxide based active corrosion protective sealing of plasma electrolytic oxidation/sol-gel composite coating on AA2024

This work reports a novel approach for growing layered double hydroxide (LDH) films on any plasma electrolytic oxidation (PEO) coated AA2024 independently of the nature of the PEO coating. The specific PEO coating chosen to carry out this work is considered to be not suitable for direct LDH growth because of phase composition and morphological features. In this paper, we describe a new methodology that consists of covering the PEO coating with a thin layer of aluminum oxide based xerogel as the source of aluminate ions for subsequent in-situ LDH growth. X-ray diffraction (XRD) and scanning electron microscope (SEM) images showed a successful formation of LDHs on the surface. An improvement in terms of active corrosion protection was also demonstrated by electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET).publishe

The use of nanostructured materials loaded with pH indicating molecules for corrosion sensing

In this work we report the synthesis of different materials for immobilization of well-known pH indicators, to be used as additives in coatings for corrosion sensing: inorganic materials such as layered double hydroxides, silica nanocapsules and polymeric microcapsules (chitosan). The materials obtained were characterized by X ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopies. Furthermore, the release profile of the obtained materials was performed using UV-Visible spectrophotometry under conditions relevant for the onset of corrosion processes, namely in the presence of NaCl and different pH conditions. The release and color changes were correlated with pH maps obtained by the scanning ion-selective technique for different galvanic couples (Al-Cu, Fe-Zn) and magnesium alloy AZ31. The results show that the materials obtained are able to change color, associated with the pKa of selected indicators and ascribed to pH changes in the alkaline direction as result of cathodic activity in different metals. These findings open prospects for the use of the prepared additives in functional coatings for corrosion sensing.publishe

Insights into corrosion behaviour of uncoated Mg alloys for biomedical applications in different aqueous media

MgCa and MgGd series of alloys are often reported as promising candidates for biomedical applications. In the present study, cytotoxicity and corrosion behavior of Mg1Ca and Mg10Gd alloys in different electrolytes (NaCl, PBS, MEM) have been investigated in order to make a direct comparison and understand the mechanisms behind their performance. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were employed to analyze corrosion processes depending on media composition, whereas X-Ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate crystalline structure, phase composition and surface morphology of the corroded substrates after immersion in the different electrolytes. Moreover, cytotoxicity of the Mg alloys was assessed using the WST-1 reduction and lactate dehydrogenase (LDH) release assays in L929 mouse fibroblasts. The electrochemical results showed that Mg1Ca has a lower degradation rate when compared to Mg10Gd, due to the lower microgalvanic effects and the presence of Ca as an alloying element. Furthermore, the corrosion activity is reduced in MEM, for both alloys, when compared to NaCl and PBS. The cytotoxicity assays revealed that Mg10Gd was cytotoxic in all the conditions tested, while the toxicity of Mg1Ca was low. Overall, these findings show that Mg1Ca alloy presents a higher corrosion resistance and biocompatibility and is a promising material to be used in biomedical implants.This work was financed by Portugal 2020 through European Regional Development Fund (ERDF) in the frame of Operational Competitiveness and Internationalization Programme (POCI), in the scope of the project MAGICOAT POCI-01-0145-FEDER016597/PTDC/CTM-BIO/2170/2014 and in the scope of the project CICECO - Aveiro Institute of Materials, UIDB/50011/2020 & UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/ MCTES. Furthermore, thanks are due to Portuguese Foundation for Science and Technology/MCTES for the financial support through national funds to EPI Unit (UIDB/04750/2020).info:eu-repo/semantics/publishedVersio

Cast iron corrosion protection with chemically modified Mg Al layered double hydroxides synthesized using a novel approach

Layered double hydroxides (LDHs) intercalated with corrosion inhibitive species are considered as promising additives to protection coatings. However, the conventional method of LDH preparation via co-precipitation followed by anion exchange is a water consuming and slow process hardly applicable to industrial use. In this work, a novel approach to LDH synthesis via hydration of sol-gel prepared mixed metal oxides and two-step anion exchange, all assisted by high-power sonication, was applied. Mg—Al and Mg-Al-Ce LDH with cations ratios 2:1 and 2:0.9:0.1, respectively, intercalated with corrosion inhibitive dihydrogen phosphate anion were successfully prepared. The obtained LDH were characterized by X-ray diffraction and scanning transmission electron microscopy. Anion release from these LDH in NaCl solutions and their corrosion inhibitive action on cast iron samples were monitored by electrochemical impedance spectroscopy. The results show that the dihydrogen-phosphate-intercalated LDHs produced using the novel technique are efficient in corrosion protection.publishe

Nanostructured corrosion sensing coatings for aeronautical applications

It is critical for the aeronautical industry that the next generation of smart coatings allows the early detection and continuous monitoring of corrosion. Once corrosion is detected, preventive actions can be taken in order to mitigate its costs. Our strategy relies on functional coatings capable of detecting metallic corrosion early on. After appropriate selection of sensing compounds and subsequent loading into nanostructured materials, these are incorporated into coating formulations giving them corrosion sensing functionality. Based on this concept we focused on the compatibility between nanocontainers and coating formulations. Thus, a new sensing coating was investigated using immersion and salt-spray tests, release and leaching studies, viscoelastic properties, curing, thermal stability, hardness, mechanical properties and corrosion resistance. The results embody a new generation of coatings with sensing ability, and have implications for self-healing and anti-fouling coatings as well.publishe