- Porto Alegre, Rio Grande do Sul, Brazil
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The electronic properties of thick oxide films formed on AISI 304L by hot acid anodizing was investigated using the Mott-Schottky approach. All modified films showed n-type and p-type behavior, depending on the applied potential. The... more
The electronic properties of thick oxide films formed on AISI 304L by hot acid anodizing was investigated using the Mott-Schottky approach. All modified films showed n-type and p-type behavior, depending on the applied potential. The incorporation of molybdate and niobil oxalate during the anodization affected mainly the acceptor concentration. The apparent increase in the dopant concentrations is mainly explained by surface area increase associated to the treatments.
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ABSTRACT Aiming to simulate the underground corrosion of pipeline steels in tropic soils, the electrochemical behavior and corrosion morphology of API 5LX65 steel were studied in 0.01molL−1 Na2 SO4 solutions with additions of humic (HA)... more
ABSTRACT Aiming to simulate the underground corrosion of pipeline steels in tropic soils, the electrochemical behavior and corrosion morphology of API 5LX65 steel were studied in 0.01molL−1 Na2 SO4 solutions with additions of humic (HA) or fulvic (FA) acids extracted from a peat soil. These humic substances have a higher carboxylic and nitrogenated character for HA, and a higher concentration of oxidized groups for FA, as determined by infrared (FTIR) and Raman spectroscopies. The corrosion tests showed that HA and FA enhance the pitting corrosion of the API 5LX65 steel. The observed pits are always associated to the presence of the rounded calcium aluminate inclusions of API 5LX65. Raman spectra of the corrosion products partially covering pits and inclusions suggest the formation of organic Fe compounds. The influence of humic substances on pit nucleation was attributed to passivity breakdown on the interface between Fe and inclusions, by the formation of partially soluble compounds of Fe and humic substances.
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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... more
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.
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Abstract The porous anodization of pure Ti and arc melted Ti2Si, and Ti4Si alloys were studied, aiming to produce novel anodes for lithium-ion batteries. The annealed and quenched Ti2Si alloys were homogeneous β-martensite, while Ti4Si... more
Abstract The porous anodization of pure Ti and arc melted Ti2Si, and Ti4Si alloys were studied, aiming to produce novel anodes for lithium-ion batteries. The annealed and quenched Ti2Si alloys were homogeneous β-martensite, while Ti4Si close to the maximal Si solubility presented Ti3Si and Ti5Si3 precipitates. The alloys anodized in standard HF + NH4F electrolyte showed an increasing tendency to detach the oxide layer with the Si content increase. The slope of the cell voltage transient ∂E/∂t was used to evaluate each material's current efficiency. Si addition decreases the oxide's electronic conductivity and the oxygen evolution reaction but increases the oxide dissolution rate due to a higher Si dissolution rate. Consequently, higher current efficiency of oxide formation is observed during the barrier layer and a lower one during the porous layer formation. Nevertheless, the specific Li mass capacity increased by around 15% with 2 at% Si addition to Ti.
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Sulfide inclusions are preferred nucleation sites for the localized corrosion of carbon and stainless steels. Thus, inclusions distribution and composition are important for the occurrence of pitting, crevice and the different forms of... more
Sulfide inclusions are preferred nucleation sites for the localized corrosion of carbon and stainless steels. Thus, inclusions distribution and composition are important for the occurrence of pitting, crevice and the different forms of stress corrosion cracking. Amines are usually used for the removal of sulfides, with the precipitation of carbamates by the production of environmental friendly fuels with low S contents. On the other hand, amines are also used associated to formation/injection waters simultaneously to CO2 reinjection by the offshore oil production at high pressures, usually under the assumption that it will act as a corrosion inhibitor, as well. In the present work, we show that amines, usually seen as corrosion inhibitors may enhance the attack to sulfide inclusions in unalloyed carbon steels. For this, artificial galvanic elements with carbon steel (AISI 1006) and MnS tablets (Figure 1.a and 1.c), simulating an inclusion, were studied by SVET (scanning vibration electrode technique) (Figure 1.b and1.d) and other techniques to evaluate the influence of amine (1% tetraethylenepentamine-TEPA) on the corrosion in Na2SO4solutions. Carbon steels (e.g. SAE1006) exposed to dilute sulfate solutions show the typical passive behavior with pitting at higher potentials. The amine “TEPA” shifts the pitting potential to higher values, as expected (Fig. 3a). MnS inclusions show the opposite behavior and its dissolution is promoted when TEPA is added in significant concentrations (Fig. 3b). Accordingly, SVET measurements show that the addition of TEPA can invert the galvanic pair formed by Fe-MnS (Fig. 1). At OCP, a synthetic MnS inclusion is attacked progressively by TEPA as Fig.2 shows for different exposure times. This has significant consequences for the initiation of pitting and stress corrosion cracking of steels, as verified by SVET measurements under simultaneous mechanical strain. Figure 1
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ABSTRACT a b s t r a c t Cellulose acetate films doped with amoxicillin were deposited onto AA2024-T3 aluminum alloy and the corrosion protection in 0.05 M NaCl was evaluated by Electrochemical Impedance Spectroscopy (EIS) and the... more
ABSTRACT a b s t r a c t Cellulose acetate films doped with amoxicillin were deposited onto AA2024-T3 aluminum alloy and the corrosion protection in 0.05 M NaCl was evaluated by Electrochemical Impedance Spectroscopy (EIS) and the Scanning Vibrating Electrode Technique (SVET). The doping of the cellulose acetate film with amox-icillin resulted in a significant increase in the high frequency resistance and a decrease in the capacitance of the material. The protective effect could be observed under anodic polarization by way of a marked decrease in the anodic current. These results show the promising potential for the deposition of cellulose acetate films doped with amoxicillin onto AA2024-T3.
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The electrochemical behavior and the electronic properties of passive films formed on a super martensitic stainless steel (SMSS) used in oil and gas industries were investigated in aqueous 0.6 M and 2.1 M NaCl solutions with additions of... more
The electrochemical behavior and the electronic properties of passive films formed on a super martensitic stainless steel (SMSS) used in oil and gas industries were investigated in aqueous 0.6 M and 2.1 M NaCl solutions with additions of sodium acetate and acetic acid (pH 4.5). Open-circuit potential transients, electrochemical impedance spectroscopy, cyclic voltammetry, and x-ray photoelectron spectroscopy were measured to characterize the passive film formed on SMSS. The electrochemical behavior of the steel in an aqueous solution of 0.6 M NaCl presented the highest pitting potential and the highest polarization resistance in relation to the NaCl/NaAc solution. The passive film of SMSS in an aqueous solution of NaCl presented a thickness of 18.40 nm, three times the thickness of the oxide film in NaCl/NaAc, and consisted of FeO, Cr2O3, MoO2, and spinels such as FeCr2O4 species that are a p-type semiconductor, but may also contain a small fraction of the Fe2O3 and MoO3 oxides. Addi...
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Abstract Aiming to simulate the corrosion of Zn-coated steels in soils containing high quantities of organic matter, we studied the corrosion of Zn in 0.1 M NaCl containing 2 g•L−1 humic acid (HA). Up to around 4 h exposure, HA slightly... more
Abstract Aiming to simulate the corrosion of Zn-coated steels in soils containing high quantities of organic matter, we studied the corrosion of Zn in 0.1 M NaCl containing 2 g•L−1 humic acid (HA). Up to around 4 h exposure, HA slightly promotes corrosion, while after 20 h, it acts as an inhibitor reaching efficiencies around 90%, as inferred from EIS and mass loss data. Voltammetry and EIS evidenced the slow formation of a secondary passive layer that explains the HA inhibition. FTIR and XPS spectroscopy showed that the additional layer contains Zn humate, which blocks the lateral growth of pits, as observed by SVET (scanning vibrating electrode technique).
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The uniform and localized corrosion of Zn in chloride solution were analyzed using the scanning vibrating electrode technique (SVET). Initially, shallow crystallographic pits nucleate, which then spread laterally assuming prismatic... more
The uniform and localized corrosion of Zn in chloride solution were analyzed using the scanning vibrating electrode technique (SVET). Initially, shallow crystallographic pits nucleate, which then spread laterally assuming prismatic hexagonal orientations (100), and finally cover the whole surface. Anomalous cathodic currents are observed by SVET over the whole surface after the corrosion type changes from localized to uniform corrosion of the metal surface due to the lateral growth of pits. These anomalous residual cathodic currents are unreal and were assigned to pH and oxygen concentration variations near the electrode surface, changing the open circuit potential (OCP) of the vibrating Pt microelectrode of the SVET system in the negative direction toward the surface. The combined action of the /Zn 2+ hydrolysis and oxygen reduction causes a pH increase toward the metal surface. A similar effect is expected in the case of oxygen depletion near the surface by the cathodic reaction. For a corrosion process under diffusion control, the concentration gradients will form in the region of the μ-electrode vibration, as confirmed by measuring the OCP of the μ-electrode versus the distance to the electrode surface.
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Friction Stir Welding (FSW) is a solid-state joining technique with various advantages when compared to other welding methods, being therefore a very important and promising process for the aerospace industry. This technique is... more
Friction Stir Welding (FSW) is a solid-state joining technique with various advantages when compared to other welding methods, being therefore a very important and promising process for the aerospace industry. This technique is particularly well suited for high-strength aluminum alloys based on the Al-Cu-Mg and Al-Zn-Cu systems, which undergo well defined precipitation hardening heat treatments and suffer loss of mechanical properties and occurrence of defects if welded by methods that involve fusion. The alloying elements and the heat treatments applied to these alloys also make them highly susceptible to localized corrosion, and thus a coating process such as anodizing becomes necessary. Considering that the existence of second phase particles and flaws in the substrate can affect the quality of the oxide coating obtained by anodizing, the influence of FSW on a subsequent anodizing process must be assessed. In this work, we studied the impact of the microstructure created by FSW o...
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Abstract Porous anodic oxides on AA2024-T3, grown in Cr(VI)-free mixtures of H2SO4 with carboxylic acids (malic, tartaric, oxalic, malonic, citric) under identical experimental conditions were comparatively studied. The corrosion... more
Abstract Porous anodic oxides on AA2024-T3, grown in Cr(VI)-free mixtures of H2SO4 with carboxylic acids (malic, tartaric, oxalic, malonic, citric) under identical experimental conditions were comparatively studied. The corrosion resistance determined by pit initiation times in 0. 1 M NaCl is improved by the addition of malic, tartaric, and citric acid and impaired by malonic and oxalic acid. It is shown that the current efficiency of oxide formation can be obtained from the ∂Ecell/∂t value verified during barrier layer formation. The ranking of the corrosion performance can be deduced from electrochemical parameters of anodic oxide formation, morphology, structure, and composition of the oxide films. Beneficial for the corrosion resistance are higher voltage slopes ∂Ecell/∂t during barrier layer formation, which also indicates higher formation efficiency, as well as smaller flaws, denser oxides, lower OH− and S-content, as well as higher Cu content of the anodic oxide.
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Direct incorporation of Li during the growth of anodic TiO2 was achieved by a one-step fabrication process.
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Abstract The corrosion of laser marked M340 martensitic stainless steel for biomedical applications was investigated using the scanning vibrating electrode technique (SVET) under simultaneous polarization and cyclic voltammetry in a 0.1 M... more
Abstract The corrosion of laser marked M340 martensitic stainless steel for biomedical applications was investigated using the scanning vibrating electrode technique (SVET) under simultaneous polarization and cyclic voltammetry in a 0.1 M NaCl solution. The laser marking greatly increases the active dissolution of the affected region and decreases the pitting potential due to Cr loss by volatilization and oxidation on the laser-melted zone. Laser marking also increases cathodic currents on the formed oxide resulting in a higher susceptibility to active dissolution and pitting corrosion.
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Aiming to elucidate the chemical descaling of thermally produced oxides during the hot rolling of steels, laboratory and industrially produced scales were analyzed by Scanning Vibrating Electrode Technique (SVET) and voltammetry in... more
Aiming to elucidate the chemical descaling of thermally produced oxides during the hot rolling of steels, laboratory and industrially produced scales were analyzed by Scanning Vibrating Electrode Technique (SVET) and voltammetry in chloride containing electrolytes. The scales were characterized by SEM and low energy EDS with the use of mineral standards and µ-Raman spectroscopy. The FeO|Fe inetrface was observed to be electrochemically very active and related to the delamination mechanism of scales.
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In refrigeration systems, aluminum sheets are continually submitted to condensation of water, which provides a corrosive environment and eventually results in the formation of ice and impairment of heat exchange. Therefore, hydrophobic... more
In refrigeration systems, aluminum sheets are continually submitted to condensation of water, which provides a corrosive environment and eventually results in the formation of ice and impairment of heat exchange. Therefore, hydrophobic surfaces are highly desirable, in order to extend life and improve performance of refrigeration systems. In this work, we propose a novel anodizing method combining porous anodic oxidation with simultaneous incorporation of hydrophobic organosilanes. Commercially pure aluminum (AA1100) was used as substrate and anodizing baths consisted of different portions of sulfuric acid, water, ethanol and hexadecyl-trimethoxysilane (HDTMS). In some cases, HDTMS hydrolysis was started in controlled environment before anodizing. Anodizing was performed galvanostatically, starting at different temperatures with a current density of 15 mA/cm2, for 25 minutes. The anodic layers obtained were characterized by scanning electron microscopy, Raman spectroscopy and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). Contact angles were measured by the sessile drop technique. The anodic layers obtained are circa 10 µm thick with incorporation of silanes inside pores and on the surface, partially blocking the pores. Hydrophobicity was significantly enhanced by adding organosilanes, achieving contact angles greater than 130°. The effects of ethanol concentration, HDTMS concentration and curing process on contact angles and potential transients during anodizing were studied. Corrosion resistance of anodic layers was evaluated using potentiostatic methods and standard salt spray exposure and compared to other traditional anodizing methods. Corrosion resistance was greatly increased by the developed process.
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Aiming to develop hexavalent chromium (CrVI)-free anodizing baths for Al-alloys, the porous anodizing of AA2024 was studied in different mixtures of sulphuric acid-organic acid formulations with similar concentrations. Mixtures of... more
Aiming to develop hexavalent chromium (CrVI)-free anodizing baths for Al-alloys, the porous anodizing of AA2024 was studied in different mixtures of sulphuric acid-organic acid formulations with similar concentrations. Mixtures of sulphuric with dicarboxylic acids as malic, malonic, oxalic and tartaric and with the tricarboxylic citric acid were tested. The corrosion behavior of the unsealed anodized layers exposed to 0.1M NaCl were compared by voltammetry and chronoamperometry at polarizations slightly above the pitting potential of blank AA2024 with the behaviour of standard chromic acid anodizing. The obtained oxide layers were characterized by SEM, FTIR and micro-Raman spectroscopy, which confirmed the incorporation of organic acids in relative different amounts. Best corrosion resistances expressed in terms of pitting nucleation time were observed for malic-sulphuric, followed by tartaric-sulphuric anodizing. However, none of the formulations achieved the resistance conferred b...
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Research Interests: Engenharias and BATERIAS
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Estudou-se, através do método eletroquímico de permeação, a difusão de hidrogênio no ferro puro e no aço DIN 90MnV8. Observou-se nestes materiais, que o coeficiente aparente de difusão de hidrogênio diminui no decorrer dos transientes de... more
Estudou-se, através do método eletroquímico de permeação, a difusão de hidrogênio no ferro puro e no aço DIN 90MnV8. Observou-se nestes materiais, que o coeficiente aparente de difusão de hidrogênio diminui no decorrer dos transientes de permeação, tendo-se interpretado este comportamento como causado pela presença de armadilhas reversíveis. Deduziu-se, para transientes de permeação obtidos entre dois estados estacionários de difusão, que a variação da concentração de hidrogênio preso a armadilhas reversiveis é dada por: Delta Ca = Na. In [(1+alfa)1/2 dividido por (1+beta)1/2], alfa = K.Co/p beta=K.Cl/p sendo Na a densidade de armadilhas; k e p, os parâmetros de captura e de liberação de uma armadilha e Co e Cl, a concentrasão inicial e final de H. Mediu-se, no ferro recozido, um coeficiente médio de difusão de hidrogênio de 6,4.10-5 cm²/s. A deformação com 50% de trabalho a frio no ferro reduz este valor em aproximadamente uma ordem de grandeza. Verificou-se no aço DIN 90MnV8 que o coeficiente de difusão de hidrogênio aumenta com o aumento dn temperatura de revenimento após a têmpera, atingindo o valor máximo de 7,0.10-6 cm²/s no aço esferoidizado. A dependência térmica do coeficiente de difusão no aço DIN 90MnV8 esferoidizado pode ser expressa por: D = 1,95 (+0,49) exp (-30,43kJ/mol.R.T.) (cm²/s) e neste mesmo aço temperado e revenido com 55RC de dureza por : D=0,29 (+- 0,05 ) exp (-28,47kJ/mol.R.T.) ( cm²/s). Uma nova metodolagia fractomecânica foi introduzida, a qual permite a determinação da cnncentração crítica de hidrogênio necessária para provocar o avanço de uma trinca estacionária. Observou-se no aço DIN 9OMnV8, que a concentração crítica de hidrogênio diminui com o aumento da dureza do aço e é menor quando um valor maior de KI é aplicado. O modo de fratura apresentado Por este aço revelou ser independente da concentração de hidrogênio e do valor de KI, sendo função unicamente de sua dureza.The hydrogen diffusion in pure iron and DIN 90MnV8 steel was studied by means of the electrochemical perrneation method. It was observed that the apparent hydrogen diffusion coefficient decreased during the pemeation transients, such behavior beeing interpreted as due to reversible traps. It was shown that for permeation transients obtained between two steady states, the concentration variation of hydrogen bound to reversible traps is given by: Delta Ca = Na. In [(1+alfa)1/2 dividido por (1+beta)1/2], alfa = K.Co/p beta=K.Cl/p beeing Na, the trap density; k and p, the capture and release parameters of a trap; and Co and Cl, the initial and final hydrogen concentrations. A medium hydrogen diffusion coefficient of 6,4.10-5 cm²/s was measured for recristalized iron. The deformation due to 50% cold work reduces this value in one order of magnitude. For DIN 90MnV8 steel, the hydrogen diffusion coefficient increases as the tempering temperature increases, attaining a maximal value of 7,0.10 -6 cm2/sec on the spherodized state. The temperature dependance of the hydrogen diffusion coefficient, on the spherodized 90MnV8 steel was: D=1,95 (-0,49) exp (-30,43kJ/mol.R.T) (cm²/sec) and on this steel tempered to a 55RC hardness: D=0,29 (+-0,05) exp (-28,47kJ/mol.R.T) (cm²/sec) A new fractomechanical methodology is intsaduced, wich allows the determination of the critical hydrogen concentration necessasy to propagate a stationary crack. For DIN 90MnV8 steel, this critical hydrogen concentration decreases as the steel hardness increases, beeing lower as a greater value of KI is aplied. The fracture mode of this steel was found to be independent on the applied value of KI, beeing a function only of its hardness
Research Interests: Humanities and Hidrogênio
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Different electrochemical surface treatments were applied to the Ni-Ti shape memory alloy and to pure Ti and Ni for comparison. Electrolytes containing complexing ions, as F- or Cl-, produced localized corrosion on Ni-Ti. However, by... more
Different electrochemical surface treatments were applied to the Ni-Ti shape memory alloy and to pure Ti and Ni for comparison. Electrolytes containing complexing ions, as F- or 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 H2SO4. Corrosion resistance improvement in Hank's solution was observed for the Ni-Ti alloy polarized at 3V in 1M H3PO4.
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The corrosion of metal structures in large facilities such as AC power transmission lines results in economic losses due to the necessary replacement of these structures and the interruption of power supply. To evaluate soil corrosion of... more
The corrosion of metal structures in large facilities such as AC power transmission lines results in economic losses due to the necessary replacement of these structures and the interruption of power supply. To evaluate soil corrosion of the buried part of the structure the corrosion rate of AISI1020 steel samples was determined by weight loss measurements as a function of the exposure time to the soil close to a power transmission tower. The same measurements were made with samples which were connected to the tower while buried in the soil. By comparison of the two measurement series the influence of the AC power signal on soil corrosion can be studied.
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ABSTRACT Colored oxide films have been grown at room temperature on ferritic 430 stainless steel samples by a two-step photocatalytic coloring method. This coloration process consists in treating a stainless steel sample in a... more
ABSTRACT Colored oxide films have been grown at room temperature on ferritic 430 stainless steel samples by a two-step photocatalytic coloring method. This coloration process consists in treating a stainless steel sample in a sulphochromic electrolyte solution during a series of cyclic (triangular wave) galvanodynamic polarization scans (electrochemical step), followed by a period of further chemical reaction in the same electrolyte solution at open-circuit potential and under visible light illumination (photochemical step). The electrochemical step produces thin, passive oxide films that are translucent to light interference, whereas the photochemical step produces interference-colored chromium-rich oxide films whose thicknesses depend on the immersion time in the electrolyte solution. SEM-EDS and Raman spectroscopy analyses revealed that both the composition and the structure of the colored films also change with the duration of the immersion time.
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ABSTRACT 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... more
ABSTRACT 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 of the spinel type . 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.