E. Escudero

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.

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.

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.

Nanostructural changes in porous anodic films on aluminum during aging

Abstract In the microstructure of sealed porous anodic films on aluminum, prepared in an oxalic acid solution, a substructure with three regions can be clearly seen: the cell boundary bands; the cell walls containing acid anions; and the central pore fill material with abundant retained water. Under the electron beam the first region remains practically unaltered, while rapid changes take place in the substructure of the other two regions, leading to their progressive homogenization. The results provided by transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) show a clear similarity between the transformations that take place in a few minutes under the effect of the electron beam, in a few days at moderate temperatures of 70–100 °C, and over months and years of atmospheric exposure at ambient temperature. Retained water plays a key role and unsealed anodic films, which are practically free of water, hardly experience any transformation under the electron beam; giving rise to identical impedance plots when recently obtained, after 24 h of aging in an oven at 100 °C, and after months and years of exposure in an atmosphere of low relative humidity.