L.F.P. Dick

Initiation and Growth of Self-Organized TiO2 Nanotubes Anodically Formed in NH4F ∕ ( NH4 ) 2SO4 Electrolytes

The anodic formation of self-organized porous TiO 2 on titanium was investigated in 1 M (NH 4 ) 2 SO 4 electrolytes containing 0.5 wt % NH 4 F by potential sweeps to 20 V S C E . By a combination of electrochemical, morphological, and compositional information we show that the sweep rate has a significant influence on the initiation and growth of the porous structures. In the first phase of the anodization process, a precursor barrier type of oxide film is formed; underneath this film pores then start growing first randomly and then self-organize. High-aspect-ratio TiO 2 nanostructures can be obtained under optimized electrochemical conditions. These nanotubular oxide layers have single-pore diameter ranging from 90 to 110 nm, average spacing of 150 nm, and porosity in the order of 37-42%. The current work indicates that the nature of the initial barrier-type layer has a strong influence on establishing optimized pore growth conditions.

Novel Inorganic Host Layered Double Hydroxides Intercalated with Guest Organic Inhibitors for Anticorrosion Applications

Zn-Al and Mg-Al layered double hydroxides (LDHs) loaded with quinaldate and 2-mercaptobenzothiazolate anions were synthesized via anion-exchange reaction. The resulting compounds were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy/energy-dispersive X-ray spectroscopy. Spectrophotometric measurements demonstrated that the release of organic anions from these LDHs into the bulk solution is triggered by the presence of chloride anions, evidencing the anion-exchange nature of this process. The anticorrosion capabilities of LDHs loaded with organic inhibitors toward the AA2024 aluminum alloy were analyzed by electrochemical impedance spectroscopy. A significant reduction of the corrosion rate is observed when the LDH nanopigments are present in the corrosive media. The mechanism by which the inhibiting anions can be released from the LDHs underlines the versatility of these environmentally friendly structures and their potential application as nanocontainers in self-healing coatings.

Voltage Oscillations and Morphology during the Galvanostatic Formation of Self-Organized TiO2 Nanotubes

The galvanostatic formation of self-organized nanotubular TiO 2 layers on titanium was investigated in 1 M (NH 4 ) 2 SO 4 + 0.5 wt % NH 4 F electrolytes. Under specific electrochemical conditions (anodization above a critical current density) large potential oscillations were observed. The electrochemical and morphological investigations show that these oscillatory phenomena are coupled with sequential growth of the TiO 2 nanotube layer and a subsequent lift-off of these layers. The origin of the detachment process is related to the formation of disordered porous film underneath the first tubular layer at higher voltages. Under galvanostatic conditions that do not lead to lift-off, a nanotubular TiO 2 film with high aspect ratio tubes of medium pore diameter 60 nm and thickness up to 1 μm can be obtained.

The negative-difference effect during the localized corrosion of magnesium and of the AZ91HP alloy

Foram realizados testes galvanostaticos em solucoes de NaCl com magnesio puro e com a liga AZ91HP, tendo sido medido o volume de hidrogenio desprendido durante os testes. A fim de estudar o efeito da diferenca negativa (NDE), uma ideia foi desenvolvida para separar o desprendimento de hidrogenio (H.E) na superficie passiva do desprendimento dentro dos pites. A analise dos dados volumetricos mostrou que a corrente de reducao de hidrogenio dentro dos pites, sobre o Mg, e muito mais alta do que nos pites sobre a liga AZ91HP. Supondo que a razao entre a taxa de reducao de hidrogenio e a taxa de dissolucao de magnesio, expressa pelo fator k, nao muda com a polarizacao, foi encontrado um valor de aproximadamente 0,5 para o Mg puro. Para a liga AZ91HP foi alcancado somente um valor em torno de 0,3. Galvanostatic tests with pure magnesium and with the AZ91HP alloy were performed in NaCl solutions, and the volume of hydrogen evolved during the tests was measured. In order to examine the negative-difference effect, a concept was developed to separate the hydrogen evolution (H.E.) on the passive surface and the H.E. inside the pits. An analysis of the volumetric data showed that the current density of H.E. inside the pits is much higher for Mg than for the AZ91HP alloy. With the assumption, that the ratio between the hydrogen reduction rate and the rate of the anodic metal dissolution, expressed by the factor k, does not change with the polarization, a k value of ca. 0.5 for pure Mg was found, while for the AZ91HP alloy this value was only ca. 0.3.

Influence of Humic Substances on the Corrosion of the API 5LX65 Steel

Aiming to simulate the underground corrosion of pipeline steels in tropic soils, the electrochemical behavior and corrosion morphology of the API 5LX65 steel in 0.01 mol/L sodium sulfate (Na 2 SO 4 ) solutions were studied,with additions of humic (HA) or fulvic (FA) acids, extracted from a peat soil. The corrosion tests showed that FA additions improve the passivity of the steel, whereas HA has the opposite effect. The nucleation of pits is always associated with the presence of rounded complex inclusions, containing sulfides and oxides. Raman spectra of the corrosion products covering the pits and inclusions suggest the formation of organic Fe compounds. The influence of humic substances (HS) on pit nucleation was attributed to a passivity breakdown on the interface between Fe and inclusions, through the formation of partially soluble compounds ofFe and HS.

Microcharacterization of Colored Films Formed on AISI 304 by Different Electrochemical Methods

The colored oxide films grown on AISI 304 stainless steels by three different methods chemical treatment, square-ware potential pulse, and triangular current scan in electrolytes containing chromic and sulfuric acids have been studied by transmission electron microscopy, selected area diffraction, energy dispensive X-ray spectrometry, micro-Raman spectrometry, and scanning electron microscopy. The morphology and structure of films formed by all coloration processes are relatively similar. The oxide layers are constituted of elongated nanocrystalline grains with dimensions ranging from 11 5t o 13 8 nm of the spinel type Ni,Fe Fe,Cr2O4. The composition is not uniform, with the Fe content and hydration grade increasing from the inner metal/oxide interface to the outer oxide surface, while the Ni content is constant and similar to its value in the alloy substrate. A higher Cr content is obtained by the E-pulse process. This allied to the more pronounced grain boundary attack caused by the chemical process, points out to a higher chemical stability and corrosion resistance of samples produced by electrochemical methods studies here, especially the E-pulse process.

A Comparative Study of the Mott-Schottky Behavior of Oxide Films on Stainless Steels in Ionic Liquids and in Aqueous Solutions

The semiconducting properties of passive films formed on AISI304 steel, thermally at 400oC or anodically in a borate buffer solution, were comparatively studied by capacitance measurements and the usual Mott-Shottky (MS) approach in an aqueous borate buffer solution of pH 9.2 and in BMMITFSI, a room temperature ionic liquid. This ionic liquid was used due to its wide electrochemical window of circa 4.3 V, enabling the verification of the accuracy of the MS approach. Two regions corresponding to p and n-type semiconduction were observed in both electrolytes, but calculated dopant concentrations were not always equal. In the ionic liquid a second deeper donor level was always present, which must be further investigated.

AFM Study of the Corrosion of Pipeline Steel in Organic Compounds Extracted from Soil

Aiming to simulate the external corrosion of pipelines, the first stages of the inclusions attack and nucleation of pits were studied by AFM on the API 5LX56 steel in pure 10 mM Na2SO4 solutions with additions of humic and fulvic acids, both extracted from a Brazilian peat soil. For this, selected inclusions were first marked with micro indentations, analyzed in the SEM in the backscattering mode and chemically mapped with EDS. Pit formation was then investigated by scanning Kelvin probe force microscopy (SKPFM). The SKPFM evidenced that HA added to sulfate solutions promotes the fast dissolution of the sulfides of complex inclusions, resulting in pit nucleation, while FA additions slow this attack, acting as a natural corrosion inhibitor.

Evaluation of Corrosion Protection of Sol-Gel Coatings on AZ31B Magnesium Alloy

Magnesium is one of the lightest metals and magnesium alloys have good strength to weight ratio making them very attractive for many particular applications [1]. The main drawback of magnesium alloys is their high corrosion susceptibility. Improving the corrosion protection by deposition of thin hybrid films can expand the areas of applications of relatively cheap magnesium alloys. This work aims at investigation of new anticorrosion coating systems for magnesium alloy AZ31B using hybrid sol-gel films. The sol-gels were prepared by copolymerization of 3- glycidoxypropyltrimethoxysilane (GPTMS), titanium alcoxides and special additives which provide corrosion protection of magnesium alloy. Different compositions of sol-gel systems show enhanced long-term corrosion protection of magnesium alloy. The sol-gel coatings exhibit excellent adhesion to the substrate and protect against the corrosion attack. Corrosion behavior of AZ31B substrates pre-treated with sol–gel derived hybrid coatings was tested by Electrochemical Impedance Spectroscopy (EIS). The morphology and the structure of sol-gel films under study were characterized with SEM/EDS techniques.

Selective Dissolution of Ni from Nitinol for Increasing the Biocompatibility

Different electrochemical surface treatments were applied to the Ni-Ti shape memory alloy and to pure Ti and Ni for comparison. Electrolytes containing the halogen ions F and Cl produced localized corrosion on Ni-Ti. However, by Rutherford backscattering spectroscopy and scanning electron microscopy a loss of nickel and morphological modifications of the surface were verified for Ni-Ti polarized in 1M H3PO4 or 1M Na2SO4. Corrosion resistance improvement in Hank’s solution was observed however, for the Ni-Ti alloy polarized at 3V in 1M H3PO4.