Diamond microelectrodes for corrosion studies

Este trabalho teve como objetivos a produção, caracterização e aplicação de microelétrodos (MEs) de diamante como sensores amperométricos e potenciométricos em sistemas de corrosão nos quais a agressividade do meio e a presença de produtos de corrosão, constituem obstáculos que podem diminuir o desempenho, ou inviabilizar a utilização, de outros tipos de sensores. Os microeléctrodos são baseados em filmes finos de diamante dopado com boro (BDD – Boron Doped Diamond) depositados sobre fios de tungsténio afiados, através do método de deposição química a partir da fase vapor, assistida por filamento quente (HFCVD – Hot Filament Chemical Vapor Deposition). A otimização das diversas etapas de fabricação dos MEs deu origem ao desenvolvimento de um novo sistema de afiamento eletroquímico para obtenção destes fios e a várias opções para a obtenção dos filmes de diamante condutor e seu isolamento com resinas para exposição apenas da ponta cilíndrica. A qualidade cristalina dos filmes de diamante foi avaliada por espectroscopia de Raman. Esta informação foi complementada com uma caracterização microestrutural dos filmes de diamante por microscopia eletrónica de varrimento (SEM), em que se fez a identificação da tipologia dos cristais como pertencendo às gamas de diamante nanocristalino ou microcristalino. Os filmes de BDD foram utilizados na sua forma não modificada, com terminações em hidrogénio e também com modificação da superfície através de tratamentos de plasma RF de CF4 e O2 indutores de terminações C-F no primeiro caso e de grupos C=O, C-O-C e C-OH no segundo, tal como determinado por XPS. A caracterização eletroquímica dos MEs não modificados revelou uma resposta voltamétrica com elevada razão sinal/ruído e baixa corrente capacitiva, numa gama de polarização quasi-ideal com extensão de 3 V a 4 V, dependente dos parâmetros de crescimento e pós-tratamentos de superfície. Estudou-se a reversibilidade de algumas reações heterogéneas com os pares redox Fe(CN)6 3-/4- e FcOH0/+ e verificou-se que a constante cinética, k0, é mais elevada em elétrodos com terminações em hidrogénio, nos quais não se procedeu a qualquer modificação da superfície. Estes MEs não modificados foram também testados na deteção de Zn2+ onde se observou, por voltametria cíclica, que a detecção da redução deste ião é linear numa escala log-log na gama de 10-5-10-2 M em 5 mM NaCl. Realizaram-se também estudos em sistemas de corrosão modelares, em que os microeléctrodos foram usados como sensores amperométricos para mapear a distribuição de oxigénio e Zn2+ sobre um par galvânico Zn-Fe, com recurso a um sistema SVET (Scanning Vibrating Electrode Technique). Foi possível detetar, com resolução lateral de 100 μm, um decréscimo da concentração de O2 junto a ambos os metais e produção de catiões de zinco no ânodo. Contudo verificou-se uma significativa deposição de zinco metálico na superfície dos ME utilizados. Os MEs com superfície modificada por plasma de CF4 foram testados como sensores de oxigénio dissolvido. A calibração dos microeléctrodos foi efetuada simultaneamente por voltametria cíclica e medição óptica através de um sensor de oxigénio comercial. Determinou-se uma sensibilidade de ~0.1422 nA/μM, com um limite de deteção de 0.63 μM. Os MEs modificados com CF4 foram também testados como sensores amperométricos com os quais se observou sensibilidade ao oxigénio dissolvido em solução, tendo sido igualmente utilizados durante a corrosão galvânica de pares Zn-Fe. Em alguns casos foi conseguida sensibilidade ao ião Zn2+ sem que o efeito da contaminação superficial com zinco metálico se fizesse sentir. Os microeléctrodos tratados em plasma de CF4 permitem uma boa deteção da distribuição de oxigénio, exibindo uma resposta mais rápida que os não tratados além de maior estabilidade de medição e durabilidade. Nos MEs em que a superfície foi modificada com plasma de O2 foi possível detetar, por cronopotenciometria a corrente nula, uma sensibilidade ao pH de ~51 mV/pH numa gama de pH 2 a pH 12. Este comportamento foi associado à contribuição determinante de grupos C-O e C=O, observados por XPS com uma razão O/C de 0,16. Estes MEs foram igualmente testados durante a corrosão galvânica do par Zn-Fe onde foi possível mapear a distribuição de pH associada ao desenvolvimento de regiões alcalinas causadas pela redução do oxigénio, acima da região catódica, e de regiões ácidas decorrentes da dissolução anódica do ânodo de zinco. Com o par galvânico imerso em 50 mM NaCl registou-se uma variação de pH aproximadamente entre 4,8 acima do ânodo de zinco a 9,3 sobre o cátodo de ferro. A utilização pioneira destes MEs como sensores de pH é uma alternativa promissora aos elétrodos baseados em membranas seletivas.

Electrochemical impedance spectroscopy and surface characterization techniques to study carbon dioxide corrosion product scales

Electrochemical impedance spectroscopy (EIS) was used to study carbon dioxide (CO2) corrosion product scales and their effects on further CO2 corrosion. Objectives were to determine the suitability of EIS for studying corrosion scales and to investigate the influence of environmental factors on scale formation. EIS provided useful information about protective abilities and electrochemical properties of corrosion scales. CO2 corrosion scales formed at high-temperature and pressure provided better protection than those formed at low-temperature and pressure. The level of protection of the scale formed at higher temperature and pressure increased with exposure time. EIS results were compared with coupon weight-loss measurements. Scales were analyzed using a combination of Fourier transform infrared (FTIR) analysis, x-ray diffraction (XRD), and electron microscopy.

Wire beam electrode : a new tool for studying localised corrosion and other heterogeneous electrochemical processes

Heterogeneous electrochemical processes are very common in industry and are important, but difficult topics in electrochemical and corrosion science studies. Traditional electrochemical techniques which employ a conventional one-piece electrode have major limitations in studying heterogeneous electrochemical processes since the one-piece electrode has major difficulties in measuring electrochemical parameters from local areas of the electrode surface. In order to overcome this problem, a multi-piece electrode, namely the wire beam electrode, has been developed. This new electrode enables the measurement of electrochemical parameters from local areas over a working electrode surface and thus it can be used to study heterogeneous electrochemical processes. This paper describes how this new electrode was applied in studying several typical heterogeneous electrochemical processes including water-drop corrosion, corrosion under non-uniform organic films and cathodic protection.

Studies of CO2 corrosion product scales using electrochemical impedance spectroscopy and surface characterisation techniques

Electrochemical impedance spectroscopy (EIS) was used to study carbon dioxide (CO2) corrosion product scales and their effects on further CO2 corrosion. Objectives were to determine the suitability of EIS for studying corrosion scales and to investigate the influence of environmental factors on scale formation. EIS provided useful information about protective abilities and electrochemical properties of corrosion scales. CO2 corrosion scales formed at high-temperature and pressure provided better protection than those formed at low-temperature and pressure. The level of protection of the scale formed at higher temperature and pressure increased with exposure time. EIS results were compared with coupon weight-loss measurements. Scales were analyzed using a combination of Fourier transform infrared (FTIR) analysis, x-ray diffraction (XRD), and electron microscopy

Quantifying the efficiency and understanding the mechanism of localised corrosion inhibition using the wire beam electrode

Parameters extracted from the wire beam electrode (WBE) galvanic current maps have been used in conjunction with electrochemical noise patterns to directly quantify the degree of localised corrosion inhibition provided by inhibitors and to understand the mechanism of localised corrosion inhibition. The behaviour of two traditional localised corrosion inhibitors has been assessed by their effects on the maximum anodic current density (imax), total anodic current density (itot), the number of anodic sites (Na) and the localised corrosion intensity index (LCII). Typical experiments are presented to illustrate the application of these parameters in providing useful information on the efficiency and mechanism of localised corrosion inhibition.

Microstructures, mechanical properties and in vitro corrosion behaviour of biodegradable Mg-Zr-Ca alloys

The microstructures, mechanical properties, corrosion behaviour and biocompatibility of the Mg-Zr-Ca alloys have been investigated for potential use in orthopaedic applications. The microstructures of the alloys were examined using X-ray diffraction analysis, optical microscopy and scanning electron microscopy. The mechanical properties of Mg-Zr-Ca alloys were determined from compressive tests. The corrosion behaviour has been investigated using an immersion test and electrochemical measurement. The biocompatibility was evaluated by cell growth factor using osteoblast-like SaOS2 cell. The experimental results indicate that the hot-rolled Mg-Zr-Ca alloys exhibit much finer microstructures than the as-cast Mg-Zr-Ca alloys which show coarse microstructures. The compressive strength of the hot-rolled alloys is much higher than that of the as-cast alloys and the human bone, which would offer appropriate mechanical properties for orthopaedic applications. The corrosion resistance of the alloys can be enhanced significantly by hot-rolling process. Hot-rolled Mg-0.5Zr-1Ca alloy (wt %) exhibits the lowest corrosion rate among all alloys studied in this paper. The hot-rolled Mg-0.5Zr-1Ca and Mg-1Zr-1Ca alloys exhibit better biocompatibility than other studied alloys and possess advanced mechanical properties, corrosion resistance and biocompatibility, suggesting that they have a great potential to be good candidates for orthopaedic applications. © 2012 Springer Science+Business Media New York.

Effects of high salt concentration and residue on copper and aluminum corrosion

Traditional researches on metal corrosion under salt solutions deposit conditions are usually carried out by visual, electron microscopic observations and simple electrochemical measurement via a traditional one-piece electrode. These techniques have difficulties in measuring localized corrosion that frequently occur in inhomogeneous media. This paper reports the results from the experiments using specially shaped coupons and a relatively new method of measuring heterogeneous electrochemical processes, namely, the wire beam electrode(WBE). Preliminary results from copper and aluminum corrosion in highly concentrated sodium chloride solutions with and without solid deposits show that the method is useful in simulating and studying corrosion especially localized corrosion in pipelines.

Microstructures, mechanical and corrosion properties and biocompatibility of as extruded Mg-Mn-Zn-Nd alloys for biomedical applications

Extruded Mg-1Mn-2Zn-xNd alloys (x=0.5, 1.0, 1.5 mass %) have been developed for their potential use as biomaterials. The extrusion on the alloys was performed at temperature of 623K with an extrusion ratio of 14.7 under an average extrusion speed of 4mm/s. The microstructure, mechanical property, corrosion behavior and biocompatibility of the extruded Mg-Mn-Zn-Nd alloys have been investigated in this study. The microstructure was examined using X-ray diffraction analysis and optical microscopy. The mechanical properties were determined from uniaxial tensile and compressive tests. The corrosion behavior was investigated using electrochemical measurement. The biocompatibility was evaluated using osteoblast-like SaOS2 cells. The experimental results indicate that all extruded Mg-1Mn-2Zn-xNd alloys are composed of both α phase of Mg and a compound of Mg7Zn3 with very fine microstructures, and show good ductility and much higher mechanical strength than that of cast pure Mg and natural bone. The tensile strength and elongation of the extruded alloys increase with an increase in neodymium content. Their compressive strength does not change significantly with an increase in neodymium content. The extruded alloys show good biocompatibility and much higher corrosion resistance than that of cast pure Mg. The extruded Mg-1Mn-2Zn-1.0Nd alloy shows a great potential for biomedical applications due to the combination of enhanced mechanical properties, high corrosion resistance and good biocompatibility.

Electrochemical and DFT studies of quinoline derivatives on corrosion inhibition of AA5052 aluminium alloy in NaCl solution

Two quinoline derivatives, 8-aminoquinoline (8-AQ) and 8-nitroquinoline (8-NQ), have been used as inhibitors to examine their corrosion protection effect on AA5052 aluminium alloy in 3% NaCl solution. The weight-loss and electrochemical measurement have indicated that 8-AQ and 8-NQ play as anodic inhibitor to retard the anodic electrochemical process. SEM/EDS analysis clearly shows that 8-AQ and 8-NQ form a protective film on the AA5052 alloy surface. Density functional theory (DFT) calculation confirmed the formation of strong hybridization between the p-orbital of reactive sites in the inhibitor molecules and the sp-orbital of the Al atom. 8-aminoquinoline and 8-nitroquinoline may be useful as effective corrosion inhibitors for aluminium alloys.

Effects of the high-temperature plasma immersion ion-implantation treatment on corrosion behavior of Ti-6Al1-4V

Nitrogen implantation into Ti alloys at higher temperatures improves their mechanical and corrosion resistance properties by forming a thicker nitride layer. In this paper, two different sets of Ti-6Al-4V samples were plasma immersion ion implantation (PIII)-treated using nitrogen plasma, varying the treatment time from 30 to 150 min (800 degrees C) and the process temperature from 400 degrees C to 800 degrees C (t = 60 min). Nanoindentation measurements of the PIII-treated samples at 800 C during 150 min showed the highest hardness value, 24 GPa, which is about four times bigger than untreated sample hardness. The N penetration at these conditions reached approximately 150 nm as analyzed by Auger spectroscopy. on the other hand, the lowest passive current density (3 x 10(-7) A. cm(-2)) was obtained for a PIII-treated sample during 30 min at higher temperature (800 degrees C). The corrosion resistance of this sample is almost the same as for the untreated specimen. Corrosion behavior evidenced that in strong oxidizing media, all PIII-treated samples are more corrosion resistant than the untreated one. PIII processing at higher temperatures promotes smoothing of the sample surface as observed by scanning electron microscopy (SEM). Grazing incidence X-ray diffraction analyses of the untreated samples identified the two typical Ti phases, Ti alpha and Ti beta. After the implantation, Ti2N and TiO2 phases were also detected.

Carbon nanotube-reinforced siloxane-PMMA hybrid coatings with high corrosion resistance

Siloxane-polymethyl methacrylate hybrid films containing functionalized multiwall carbon nanotubes (CNTs) were deposited by dip-coating on carbon steel substrates from a sol prepared by radical polymerization of methyl methacrylate and 3-methacryloxy propyl-trimethoxysilane, followed by hydrolytic co-polycondensation of tetraethoxysilane. The correlation between the structural properties and corrosion protection efficiency was studied as a function of the molar ratio of nanotubes carbon to silicon, varied in the range between 0.1% and 5%. 29Si nuclear magnetic resonance and thermogravimetric measurements have shown that hybrids containing carbon nanotubes have a similar degree of polycondensation and thermal stability as the undoped matrix and exhibit and excellent adhesion to the substrate. Microscopy and X-ray photoelectron spectroscopy results revealed a very good dispersion of carbon nanotubes in the hybrid matrix and the presence of carboxylic groups allowing covalent bonding with the end-siloxane nodes. Potentiodynamic polarization curves and electrochemical impedance spectroscopy results demonstrate that CNTs containing coatings maintain the excellent corrosion protection efficiency of the hybrids, showing even a superior performance in acidic solution. The nanocomposite structure acts as efficient corrosion barrier, increasing the total impedance by 4 orders of magnitude and reducing the current densities by more than 3 orders of magnitude, compared to the bare steel electrode. © 2013 Elsevier B.V. All rights reserved.

Development of a method for sampling and determination of corrosion inhibitors in modified atmospheres

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Corrosion of magnesium alloys in commercial engine coolants

A number of magnesium alloys show promise as engine block materials. However, a critical issue for the automotive industry is corrosion of the engine block by the coolant and this could limit the use of magnesium engine blocks. This work assesses the corrosion performance of conventional magnesium alloy AZ91D and a recently developed engine block magnesium alloy AM-SC1 in several commercial coolants. Immersion testing, hydrogen evolution measurement, galvanic current monitoring and the standard ASTM D1384 test were employed to reveal the corrosion performance of the magnesium alloys subjected to the coolants. The results show that the tested commercial coolants are corrosive to the magnesium alloys in terms of general and galvanic corrosion. The two magnesium alloys exhibited slightly different corrosion resistance to the coolants with AZ91D being more corrosion resistant than AM-SC1. The corrosivity varied from coolant to coolant. Generally speaking. an oraganic-acid based long life coolant was less corrosive to the magnesium alloys than a traditional coolant. Among the studied commercial coolants. Toyota long, life coolant appeared to be the most promising one. In addition. it was found that potassium fluoride effectively inhibited corrosion of the magnesium alloys in the studied commercial coolants. Both general and galvanic corrosion rates were significantly decreased by addition of KF, and there were no evident side effects on the other engine block materials, such as copper, solder. brass. steel and aluminium alloys, in terms of their corrosion performance. The ASTM D 1384 test further confirmed these results and suggested that Toyota long life coolant with 1%wt KF addition is a promising coolant for magnesium engine blocks.