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Copper-enriched layers were developed onto aluminum-copper alloys using alkaline etching in sodium hydroxide, for both, sputter deposited and bulk conditions. Enriched alloys were evaluated by potentiodynamic polarization in sodium chloride solution in order to determine the effect of the enriched layers on the pitting potential of the alloys. Rutherford backscattering spectroscopy was employed to quantify the enrichments and their locations just beneath the alumina-based oxides remaining from the etching. For the sputter deposited aluminum-copper alloys, the results show some scattering of the pitting potential data, and no correlation between pitting potential and the alloy enriched layer. In the case of bulk Al-2wt.%Cu alloy, with the copper in solid-solution, the pitting potential increased for the enriched specimens, indicating also a different pit morphology, with respect to the non-enriched alloy. Resumen Fueron desarrolladas capas de enriquecimiento de cobre mediante ataque alcalino en hidróxido de sodio sobre aleaciones aluminio-cobre. Se usaron aleaciones tanto depositadas por pulverización catódica, como vaciadas convencionalmente. Las aleaciones con las capas de enriquecimiento de cobre fueron estudiadas mediante polarización potenciodinámica en una solución de cloruro de sodio, para poder determinar si existe alguna correlación entre las capas de enriquecimiento de cobre y el potencial de picadura de las aleaciones. Los resultados no son concluyentes en el caso de las aleaciones depositadas por pulverización catódica; sin embargo, en el caso de la aleación en condición de vaciado, con el cobre en solución sólida, el potencial de picadura aumenta por la presencia de la capa de enriquecimiento, mostrando además una morfología diferente. Resumo Foram desenvolvidas camadas de enriquecimento de cobre mediante ataque alcalino em hidróxido de sódio sobre ligas alumínio-cobre. Usaram-se ligas tanto depositadas por pulverização catódica, como esvaziadas convencionalmente. As ligas com as camadas de enriquecimento de cobre foram estudadas mediante polarização potenciodinâmica em uma solução de cloreto de sódio, para poder determinar se existe alguma correlação entre as camadas de enriquecimento de cobre e o potencial de picadura das ligas. Os resultados não são concluintes no caso das ligas depositadas por pulverização catódica; porém, no caso da liga em condição de esvaziado, com o cobre em solução sólida, o potencial de picadura aumenta pela presença da camada de enriquecimento, mostrando além disso, uma morfologia diferente.

Fluoride ions are commonly present in solutions used in pre-treatments and conversion treatments of aluminium alloys. Because of the intrinsically reactive nature of aluminium metal, alumina layers are either already present on an aluminium surface or form within a matter of seconds on pre-etched aluminium. Hence fluoride ion transport through an alumina layer is an integral part of conversion coating formation. In order to understand the behaviour of fluoride ions in alumina films, the present study investigated the behaviour of fluoride ions during the growth of barrier-type anodic films formed on aluminium at 5 mA cm-2 in 0.1 M ammonium pentaborate solution. The films were examined by analytical transmission electron microscopy, scanning electron microscopy and glow discharge optical emission spectroscopy in order to determine their composition and morphology. Fluoride ions were incorporated into the films by adding sodium fluoride to the electrolyte. Additions of up to 3.5 x 10-3 M sodium fluoride had a negligible influence on the film growth, which occurred at a high efficiency. In contrast, additions of 3.5 x 10-2 M sodium fluoride reduced the efficiency to about 60%. Dissolution studies demonstrated that fluoride ions had promoted the field-assisted ejection of Al3+ ions from the film surface. Using sequential anodizing, the fluoride ions were shown to migrate inwards in the film at a rate about twice that of O2- ions. Thus, a thin fluoride-rich layer of the film was formed next to the aluminium/film interface. Increasing the sodium fluoride content to 3.5 x 10-1 M led to a porous film formed at a low voltage.

In this work, a trivalent chromium conversion coating applied on AA2024-T351 alloy has been optimized for corrosion protection in sodium chloride solutions. Scanning electron microscopy, energy dispersive X-ray spectroscopy and electrochemical measurements were employed to characterize the coating. An immersion post-treatment of the coated alloy in 40 ºC deionized water for 120 s considerably enhanced the corrosion protection properties, compared with a post treatment at 20 °C, or the absence of a post-treatment. Electrochemical noise measurements, combined with real time imaging, and potentiodynamic polarization experiments indicated a conversion treatment for 300 or 600 s provides optimal corrosion protection. Relatively long conversion treatments decreased the corrosion protection due to an increase in coating defects and cracks, especially around the second-phase particles.

A gravimetric method based on changes in buoyancy force of a submerged gas-collecting container has been optimized to measure the evolution of oxygen during anodizing of an aluminium alloy. Previously, the gravimetric method has been used to measure hydrogen evolution from magnesium and aluminium during corrosion processes, either at the free corrosion potential or under relatively low polarization. However, during anodizing, the comparatively higher values of current applied and the heating effects associated with power dissipation might introduce artefacts in the gas measurement. Optimization of the experimental setup enabled reduction or elimination of such artefacts, so that a reliable measurement could be obtained. The results show that typically about 15 to 20% of the current applied to an AA 2024-T3 aluminium alloy during anodizing in sulphuric acid under the present conditions was used in generating oxygen, which is in reasonable agreement with estimates in the literature using less direct methods.

The influence of a low concentration of strontium chromate on the corrosion inhibition of superpure aluminium and AA2014-T6 aluminium alloy in 0.6 m chloride solution has been investigated to simulate the leaching process of inhibitors from coatings. The potential-time and polarisation behaviour show influences on both the anodic and cathodic kinetics on the superpure aluminium surface. However, predominant cathodic inhibition was observed for the AA2014-T6 aluminium alloy. It was evident that chromium species were reduced at cathodic second phase particles, forming a thin passive film at the cathodic sites, which blocks the oxygen reduction reaction and, consequently, provides effective corrosion inhibition

A través de espectroscopía de impedancia electroquímica se estudió la influencia de la densidad de corriente en la formación de películas anódicas de aluminio de alta pureza en una solución 0,25M de acido crómico a 40°C. Selectas muestras fueron observadas por microscopia óptica y electrónica. Las películas anódicas están compuestas de una capa barrera de espesor uniforme adyacente a la interface metal/óxido y una capa externa porosa de varios micrómetros de espesor en la interface óxido/electrolito. Los resultados muestran que la morfología de las películas anódicas cambia con el aumento de la densidad de corriente. A bajas densidades de corriente se forman poros con una morfología irregular y no ordenada, características de las películas anódicas formadas en ácido crómico. Mientras que a altas densidades de corriente las películas anódicas están compuestas de una capa barrera de espesor considerablemente alto y una capa externa porosa más delgada. Se concluyó que el mecanismo de formación de las películas corresponde a un proceso dinámico de formación y disolución química del material anódico en la base de los poros. The effect of current density on the mechanism of formation of porous anodic films on aluminum is investigated by impedance spectroscopy. The study employs porous anodic films formed in 0.25M chromic acid solution at 40°C. Selected samples were analyzed by scanning electron microscopy (SEM) and optical microscopy. The anodic films are composed for a thin barrier layer of constant thickness close to the metal/oxide interphase and an external porous layer of several microns at oxide/electrolyte interphase. The increase in current density affects the morphology of the anodic films. At lower current densities feathered porous structure were developed. Meanwhile at high current densities a thick barrier layer is formed. The formation of porous layers is the result of the dynamic process of growth and dissolution of anodic alumina at the pore base.

6 p. Los tratamientos de modificación superficial por fusión láser (LSM siglas en inglés) que utilizan fuentes de radiación de láser excímero pulsado, han surgido en la actualidad como métodos para la mejora de ciertas propiedades superficiales de materiales metálicos, tales como la resistencia a la corrosión y al desgaste. Este hecho es asociado a las elevadas velocidades de enfriamiento que se pueden alcanzar, y que favorecen la formación de películas superficiales cuya microstructura presenta una extraordinaria homogeneidad y refinamiento y virtualmente libres de precipitados de naturaleza corrosiva. Sin embargo, todavía existen ciertas lagunas de conocimiento con relación a cómo afectan ciertos parámetros del tratamiento en la microstructura de las películas superficiales resultantes. En el presente trabajo tratamientos LSM utilizando un láser excímero pulsado KrF (248 nm) fueron desarrollados sobre la aleación de aluminio AA2050-T8 (de aplicación en la industria aeronáutica), con objeto de evaluar la influencia del número de pulsos láser aplicados en la microstructura de la película superficial. Para la caracterización microstructural se dispuso de técnicas microscópicas e interferometría. En general, los tratamientos LSM sobre la aleación de aluminio estudiada dieron como resultado la formación microstructuras superficiales altamente homogéneas con completa disolución de precipitados. Además, un aumento del número de pulsos láser aplicados favoreció la producción de películas de mayor espesor pero con mayor grado de porosidad, así como la formación de películas superficiales de óxido de aluminio. Laser fusion surface modification treatments of (LSM) which use pulsed excimer laser radiation sources, have emerged today as methods for the improvement of certain surface properties of metals, such as resistance to corrosion and wear. This fact is associated with high rates of cooling that can be achieved, which favor the formation of surface films whose microstructure poses an extraordinary homogeneity and refinement, and are virtually free of corrosive precipitates. However, there are still some knowledge gaps as to how they affect certain parameters of treatment on the microstructure of the resulting surface films. In the present paper LSM treatments using a KrF (248 nm) pulsed excimer laser were developed on the AA2050-T8 alloy of aluminum (implemented in the aviation industry), in order to evaluate the influence of the number of laser pulses applied in the microstructure of the surface film. As far as microstructural characterization, microscopic techniques and interferometry were used. In general, LSM treatments on the aluminum alloy studied, resulted in the formation of highly homogeneous surface microstructures with complete dissolution of precipitates. In addition, an increase in the applied number of laser pulses favored the production of thicker films but with a higher degree of porosity, as well as the formation of aluminum oxide surface films. Bucaramanga

In the present study, the effects of mechanical polishing on the microstructure and corrosion behaviour of AA7075 aluminium alloy are investigated. It was found that a nano-grained, near-surface deformed layer, up to 400 nm thickness, is developed due to significant surface shear stress during mechanically polishing. Within the near-surface deformed layer, the alloying elements have been redistributed and the microstructure of the alloy is modified; in particular, the normal MgZn2 particles for T6 are absent. However, segregation bands, approximately 10-nm thick, containing mainly zinc, are found at the grain boundaries within the near-surface deformed layer. The presence of such segregation bands promoted localised corrosion along the grain boundaries within the near-surface deformed layer due to microgalvanic action. During anodic polarisation of mechanically polished alloy in sodium chloride solution, two breakdown potentials were observed at −750 mV and −700 mV, respectively. The first breakdown potential is associated with an increased electrochemical activity of the near-surface deformed layer, and the second breakdown potential is associated with typical pitting of the bulk alloy.

The influences of pre-treatments on the composition of Al–Zn alloys, containing 0.6–1.9 at.%Zn, are investigated using scanning and transmission electron microscopies, glow discharge optical emission spectroscopy and ion beam analysis. Alkaline etching, anodic alkaline etching and electropolishing result in zinc enrichments, just beneath the oxide/hydroxide films on the alloy surfaces. The enriched alloy, about 4–5 nm thick, contains zinc in solid-solution with aluminium, up to about 18 at.%. The highest and lowest enrichments are produced by alkaline etching and electropolishing respectively. For a given pre-treatment, the enrichment increases with increase in zinc content of the bulk alloy.