V. López

Characterization of porous aluminium oxide films from a.c. impedance measurements

An equivalent circuit (EC) that reproduces the a.c. impedance of porous aluminium oxide films in a highly approximate manner is proposed. The results reveal that electrochemical impedance spectroscopy (EIS) is a powerful tool for obtaining detailed information on the electrochemical properties of both the porous and barrier layer on which the corrosion resistance of aluminium depends. The impedance at a given frequency can be used for accurate calculation of the electrochemical parameter for the oxide film represented by each element of the EC. In this way, the effects of any factor on sealing and ageing of anodized aluminium oxide films can be precisely analysed. The EIS technique provides an effective, advantageous alternative to existing seal quality control tests.

Metallographic techniques for the determination of the austenite grain size in medium-carbon microalloyed steels

Abstract Different techniques have been investigated to seek the best procedure to reveal the prior-austenite grain boundaries in three medium-carbon microalloyed steels. This study has been carried out over a wide range of temperatures (950–1250°C) and it has been found that thermal etching (TE) is the best technique to reveal the prior-austenite grain boundaries in these steels.

Atmospheric corrosion of bare and anodized aluminium in a wide range of environmental conditions. Part I: Visual observations and gravimetric results

Abstract This study compares the behaviour of aluminium in the bare condition and when protected with anodic films of approximately 7, 17 and 28 μm thickness for 42 months of exposure in 11 atmospheres of very different aggressivities, with salinity values ranging from between 2.1 and 684 mg Cl − m −2 d −1 . The anodic films, obtained in a sulfuric acid bath and sealed for 60 min in boiling deionized water, were characterised before and after different exposure times by means of: visual inspection; observation with a magnifying glass; and using classic gravimetric techniques. Optical and electron microscopy were occasionally used with some specimens that showed symptoms of localised corrosion. Aluminium behaves as a passive material in atmospheres of low salinity but exhibits pitting corrosion after 1 year at chloride pollution levels of ≥50 mg Cl − m −2 d −1 and after 2 years at levels of ≥10 mg Cl − m −2 d −1 . Anodising and sealing prevents the risk of pitting corrosion even in the most aggressive atmospheres, except in the case of the coatings of the lowest thickness.

Postsealing Changes in Porous Aluminum Oxide Films Obtained in Sulfuric Acid Solutions

The behavior of sealed and unsealed porous aluminum oxide films during aging and autosealing-aging in various media is studied. The quality of the anodic layers and the degree of sealing and aging are assessed. The results of electrochemical impedance spectroscopy are compared with those provided by standard quality control tests, scanning and transmission electron microscopies, gas adsorption porosimetry, and microhardness tests involving specimens sealed to a different extent. These techniques provide complementary information on the different stages of the sealing and autosealing processes (viz. pore plugging; pore wall dissolution; precipitation, crystallization, and agglomeration of hydrated alumina; formation of the intermediate layer), and on the aging stage and its duration.

Atmospheric corrosion of bare and anodised aluminium in a wide range of environmental conditions. Part II: Electrochemical responses

Abstract This study compares the behaviour of aluminium in the bare condition and when protected with anodic films of approximately 7, 17 and 28 μm thickness for up to 42 months of exposure in 11 atmospheres of very different aggressivities, with salinity values ranging between 2.1 and 684 mg Cl− m−2 d−1. The results of a previous study based on visual inspection and traditional gravimetric techniques are complemented by the application, essentially, of electrochemical impedance spectroscopy (EIS). The specimens were characterised by EIS when recently obtained and after 12, 24 and 42 months of exposure in the different testing stations. Exposure leads to an improvement in all the quality indices of anodic films, irrespective of their thickness or the degree of environmental pollution. Anodising and sealing prevents localised corrosion — which is characteristic of aluminium in marine or industrial atmospheres — even in the most unfavourable situations, except in the case of 7 μm films, the lowest thickness tested.

Laser welding of AA 5083 samples by high power diode laser

Abstract Laser welding is a very attractive technique to join different alloys at the industrial level, due to its low heat input, high flexibility, high weld quality and high production rate. In this work, the weldability of the aluminium alloy AA 5083 with a high power diode laser has been tested. Concisely, samples were subjected to lineal treatments of laser radiation, with the objective of studying the properties of the bead on plate welds generated. The main objective of the present work has been to study the influence of both the processing rate and the superficial treatment of the AA 5083 samples, on the morphological, microstructural and corrosion properties of the laser weld beads. The sizes of the welds were higher as the processing rate was decreased. The weld beads were seen to have better behaviour against corrosion than the base metal due to the microstructural refinement. It was also verified that a blasting process before processing gave beads with lower size but better corrosion resistance than the application of a black layer, due to the minimisation of the magnesium evaporation in this former superficial treatment.

Comparison by SEM, TEM, and EIS of Hydrothermally Sealed and Cold Sealed Aluminum Anodic Oxides

A comparative study is made of the changes caused to the microstructure of anodic films by traditional hydrothermal sealing (HTS) and cold sealing (CS) processes, using the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. SEM and TEM make it possible to visualize the different stages of the very complex sealing and aging mechanism of aluminum anodic oxide coatings indirectly determined by other techniques, showing evidence of the hexagonal structure of anodic coatings, plugging of the pore mouth with the formation of acicular pseudobohemite and the intermediate sublayers in HTS. It is also possible to verify, both in the coatings resulting from CS and in those subjected to HTS, the stages of dissolution-precipitation, with prior enlargement of the pore diameter and subsequent plugging of the entire pore length. For its part, electrochemical impedance spectroscopy (EIS) provides precise information on the effects of the aging process of anodic layers and their persistence over years and decades.

Influence of extrusion temperature on the aging behavior of 6061Al-15vol%SiCw composites

It is known that some discontinuously reinforced metal matrix composites, MMCs, with precipitation hardenable matrices, like 6061Al with SiC whiskers or particles, show faster aging behavior than the unreinforced alloys. Regarding the influence of processing techniques on the aging behavior, some investigations have found acceleration on aging of materials consolidated by powder metallurgy (PM) with respect to materials consolidated by ingot metallurgy (IM). This was attributed to the higher presence of oxide inclusions in the PM than in the IM materials. For a given composite preparation technique, however, the influence of the processing variables on the aging response has been barely studied. In this work, the effect of extrusion temperature on the aging of 6061Al-15vol%SiC{sub w} composites processed by a powder metallurgy route is investigated. The results obtained in this research go deep into the influence of the dislocation density on both the accelerated aging and the increase in strength of composites. Whereas the effect of dislocation density on accelerated aging is well documented, its effect on the strength of MMCs is not yet clear.

Overaging of sealed and unsealed aluminium oxide films

The implications of aging on the properties of unsealed and poorly and properly sealed anodized aluminium exposed to the atmosphere of the University Campus of Madrid for 0, 1, 2, 3, 17 and 20 years were investigated. Aging is shown to occur for many years and to lead to a sealing quality well above that required by industrial standards.

Sealing of anodic films obtained in oxalic acid baths

The response to sealing quality control tests of anodic films obtained in oxalic and sulphuric acid baths has been compared. Unsealed films obtained in oxalic acid remain virtually unaltered in humid atmospheres, contrary to unsealed films obtained in sulphuric acid which tend to autoseal. The former also appear to take a shorter time to reach the quality thresholds of all the usual control tests on traditional sealing in boiling deionized water. However, after sealing, both types of films age in a qualitatively identical way. Transmission electron microscopy revealed the presence of a complex structure of hexagonal cells, comprising three distinct zones in the films formed in oxalic acid, the pores in which were found to be about five times larger in diameter than those in the films obtained in sulphuric acid. This considerably facilitates microstructural examination and elucidation of the underlying sealing mechanism. Furthermore, the transformation of the film morphology under the electron beam is slower in films obtained in oxalic acid.