Search

The present study investigates the influence of organic acids, added to the electrolytic solution, on the structure, morphology, and corrosion behaviour of plasma electrolytic oxidation (PEO) coatings produced on titanium grade 2. Particular attention is paid to the role of functional groups in the modification of the oxide's properties. For this reason, all three selected acids, namely glutaric, glutamic, and tartaric acid, display two carboxylic groups, thus they interact with the substrate material mainly through –COO− adsorption. However, glutamic acid also has an amine group, while tartaric acid has two hydroxyl groups. The presence of such additional functional groups is found to impact the formation of the PEO coatings. According to scanning electron microscopy (SEM) analyses, the number of defects and their dimension increase with an increasing number of active groups present in the organic molecules. Then, when glutaric acid with only two carboxyl groups, is employed as an additive, smaller pores are produced. The dimension of defects increases when glutamic and tartaric acid are used. X-ray diffraction (XRD) testing demonstrates that rutile and anatase are present in all the coatings and that when using tartaric acid, a relatively high level of amorphism is reached. The electrochemical and corrosion behaviours are evaluated by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) performed in a heated sulphuric acid solution. It is found that all types of coatings provide protection against corrosion, with oxides produced using glutamic acid showing the lowest corrosion current density (0.58 mA·m−2) and low corrosion rate (1.02 μm·y−1). [ABSTRACT FROM AUTHOR]

Inhibitors for the prevention of corrosion in reinforced concrete are chemical substances able to reduce carbon steel reinforcements corrosion without altering the overall properties of concrete. Today, many commercially available substances have a negative impact on human safety during either the inhibitor synthesis, their handling or application in field. Green corrosion inhibitors are nontoxic, biodegradable and environmentally biocompatible substances. They are generally made of extracts from natural plants or waste, which are abundantly available in several countries. The majority of green inhibitor molecules usually contain multiple bonds, aromatic rings, polar functional groups and electronegative atoms as P, N, S or O; the latter are able to coordinate with metal cations to form protective layers on the metallic surface of the reinforcements, so as to inhibit the development (initiation and/or propagation) of the corrosion process. In this review, the most recent achievements on the study and investigation of green corrosion inhibitors for concrete structures are presented and discussed. Inhibitors are classified based on their nature and inhibition mechanism. The inhibition effectiveness of the substances is compared with the well-established effective nitrite-based inhibitor, distinguishing between accelerated and long-term tests. Based on the available data, a summary of corrosion inhibitors efficiency is reported. [ABSTRACT FROM AUTHOR]

This research describes the influence of two types of particles, namely rutile and anatase microparticles (average d < 5 µm), on the morphology, structure, and anticorrosive properties of PEO coatings on titanium produced in an alkaline solution based on NaOH and sodium metasilicates. The paper reports the experimental results relating to the study of the influence of the electrical regime and working frequency of the anodizing treatment on the interaction between the particles, the substrate, and the oxide to determine the optimal conditions that favour the incorporation of the particles and the production of a thick oxide. PEO coatings are characterized by using a scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) analysis, and X-ray diffraction (XRD) testing. The electrochemical behaviour is evaluated by free corrosion potential monitoring and electrochemical impedance spectroscopy analysis (EIS) performed in a sulphuric acid solution. The particles are successfully incorporated into the coating under any electrical condition and at any frequency. However, only treatments carried out at 1000 Hz allow the production of coatings that combine a large thickness (up to 50 µm) and improved anticorrosion behaviour. In contrast, oxide layers produced at 20 Hz and in DC show a quite damaged structure, affecting their anticorrosion behaviour and resulting in lower corrosion potential and impedance values. [ABSTRACT FROM AUTHOR]

The present research investigates the influence of surface roughness imparted by cold surface finishing processes on the localized corrosion resistance of stainless steel. Five different alloys were studied: ferritic AISI 430, martensitic AISI 430F, austenitic AISI 303, AISI 304L, and AISI 316L. It was demonstrated that the grinding process, executed on previously cold drawn bars, leads to an improvement in corrosion resistance according to the results obtained with electrochemical tests, namely, potentiostatic and potentiodynamic tests in chloride-rich environments, the salt spray test, and long-term exposure in urban and marine atmospheres. This allowed us to establish a trend among the different alloys regarding the resistance to pitting corrosion, which was assessed according to pitting potentials, critical chloride contents, and pitting initiation time. All the tests confirmed that surface finishing, as well as alloy chemical composition, is an important factor in controlling the corrosion resistance of stainless steel. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation is a powerful technique allowing the formation of ceramic coatings with a high degree of functionalization. Plasma–chemical interactions, resulting from the application of a high voltage in a conductive electrolyte, like 0.5 M NaOH, favor the development of a porous surface, which deserves to be sealed to obtain proper corrosion resistance. In the present work, the effect of temperature employed during sealing in calcium acetate is investigated, finding 85°C as an optimum temperature to allow pores closure. Furthermore, a comparison with sealing performed in hot water is presented, which demonstrated similar sealing efficiency. Scanning electron microscope and electrochemical impedance spectroscopy are the tools selected to investigate sealing effects. Impedance spectra are fitted according to feasible electrochemical equivalent circuits finding two‐time constants when applied to sealed samples indicative of the presence of barrier and porous layer, respectively. [ABSTRACT FROM AUTHOR]

Titanium has a good corrosion resistance, owing to a thin, compact oxide layer that is formed spontaneously when the metal is exposed to the environment. However, even titanium can be subject to corrosion, for example, when exposed to reducing strong acids. This paper aims to analyze the differences in corrosion resistance of commercially pure titanium (Ti grade 2) and the most resistant Ti–Pd alloy (Ti grade 7, ∼0.2% Pd) in a strong reducing acid environment. Immersion tests, with results collected in the form of corrosion rates, are provided in the first part of the present work. Corrosion morphology is investigated using optical microscopy and laser profilometry. Then, an electrochemical characterization of both metals has been performed by means of cathodic polarization tests, electrochemical impedance spectroscopy, and linear polarization resistance. Weight loss data, electrochemical tests, and corrosion morphology are in good agreement: H2SO4, HCl, and oxalic acid are the most aggressive chemicals for commercially pure titanium. In such conditions, the use of Ti grade 7 appears mandatory. [ABSTRACT FROM AUTHOR]

This study deals with the anodisation of titanium grade 2 in 0.5‐M sulphuric acid using a pulsed signal in a unipolar regime. The electrical parameters investigated are voltage, frequency and duty cycle. The use of duty cycles with a high percentage of anodic polarisation (90%), combined with high frequencies (1000 Hz) and the higher voltage tested (220 V), favoured the establishment of a plasma regime involving strong dielectric discharges, allowing the growth of thicker oxides but with rough architecture. The corrosion resistance of the formed film has been characterised by potentiodynamic tests in 0.5‐M NaBr for localised corrosion resistance and by immersion tests in 10% v/v sulphuric acid solution for a uniform corrosion assessment. Current–time curves, visual observations and electron microscope analysis (scanning electron microscopy, energy‐dispersive X‐ray spectroscopy) were the tools selected to provide a correlation between technological parameters and oxide growth mechanism. For localised and uniform corrosion, anodisation at 220 V with a high level of anodic polarisation (90%) and frequency (1000 Hz) was verified to be particularly advantageous. [ABSTRACT FROM AUTHOR]

This research study describes recent advances in understanding the effects of the addition of organic acids, such as acetic, lactic, citric and phytic acids, on the process of plasma electrolytic oxidation (PEO) on Ti using an alkaline bath. As the plasma developed over the workpiece is central to determine the particular morphological and structural features of the growing oxide, the focus is then on the inter-relationships between the electrolyte and the resultant plasma regime established. In situ optical emission spectroscopy (OES) allowed us to verify a marked plasma suppression when adding low-molecular-weight anions such as acetates, resulting in short-lived and well-distributed discharges. Conversely, when more bulky anions, such as lactates, citrates and phytates, were considered, a less efficient shielding of the electrode caused the build-up of long-lasting and destructive sparks responsible for the formation of thicker coatings, even >30 µm, at the expense of a higher roughness and loss of compactness. Corrosion resistance was tested electrochemically, according to electrochemical impedance spectroscopy (EIS), and weight losses evidenced the coatings produced in the solution containing acetates to be more suitable for service in H2SO4. [ABSTRACT FROM AUTHOR]