28 research outputs found
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
In situ surface film evolution during Mg aqueous corrosion in presence of selected carboxylates
Mechanisms of inhibition of Mg aqueous corrosion in presence of chloride by sodium salicylate (Sal), 2,5-pyridinedicarboxylate (PDC) and fumarate (Fum) were studied by in situ Raman spectroscopy, ATR-FTIR, GD-OES and hydrogen collection. In situ detected surface films were composed by Mg(OH)2 nano-crystals and included inhibitors. All carboxylates significantly modified Mg(OH)2 growth kinetics as well as pevented chloride incorporation in the film. Vibrational spectra of the surface films demonstrated specific interactions between the carboxylates and the surface: adsorption of Sal and Fum on the oxide/hydroxide, precipitation of coordination polymer by PDC, dissolution of iron inclusions via formation of iron-Sal soluble complexes.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
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
A new concept for corrosion inhibition of magnesium: suppression of iron re-deposition
In our recent work (Höche et al. 2016) we proposed that non faradaic dissolution of Fe impurities and/or iron
containing phases with subsequent re deposition of thin film of pure (“in statu nascendi”) ironenlarges cathodically
active sites at the surface of corroding magnesium. The effect drastically accelerates corrosion of impurity containing
Mg. In the presentwork we assume that if Fe re deposition is prevented, the area of cathodic sites can be drastically
decreased and hence corrosion of Mg can be suppressed. In this proof of concept work we use strong Fe3+
complexing agents in order to remove dissolved iron cations from corrosion sites and prevent iron re deposition.
All used iron complexing agents efficiently lowered the corrosion rate ofMg. Direct correlation of complex stability
with its inhibiting efficiency was established. It was shown that cyanide, salicylate, oxalate, methylsalicylate and
thiocyanate efficiently reduce hydrogen evolution and suppress critical dark area formation