M.J. Cano

Localized alkaline corrosion of alloy AA5083 in neutral 3.5% NaCl solution

The corrosion process of the alloy AA5083 in an aerated solution of NaCl at 3.5% has been studied. The results obtained indicate that the main process that this alloy undergoes, under the conditions of exposure studied, is related to localized corrosion that takes place as a consequence of the process of alkalinization around the cathodic precipitates existing in the alloy. The pitting formed presents a hemispherical morphology that is clearly different from crystallographic pitting. The formation of crystallographic pitting has not been observed, even in samples submitted to tests of very long duration. In order to obtain the formation of crystallographic pitting, it is necessary to polarize the alloy at the nucleation potential of pitting and, in addition, the density of the current must be above a critical value. Only when the layer of oxide is eliminated does the formation of crystallographic pitting take place by simple exposure in an aerated solution of NaCl at 3.5%.

Influence of the cathodic intermetallics distribution on the reproducibility of the electrochemical measurements on AA5083 alloy in NaCl solutions

The corrosion behavior of alloy AA5083 in solutions of NaCl at 3.5% is controlled by the density of the cathodic precipitates of Al(Mn,Fe,Cr) present in the alloy. These precipitates are not distributed homogeneously over the surface of the alloy. Further, their presence conditions the electrochemical response of the alloy. For these reasons, in order to guarantee the reproducibility of electrochemical tests on this alloy, it is necessary to determine the minimum surface area exposed to the medium which represents the average behavior of the system. In the present study, a systematic analysis has been conducted on the degree of reproducibility of electrochemical tests as a function of the area of surface exposed, for the alloy AA5083 in solutions of NaCl at 3.5%. A high dependence between the dispersion of the electrochemical data and the exposed area has been established. This behavior contrast with those of other alloys, like AA1050, which is not conditioned by the values of exposed area. This has been related to the much lower cathodic precipitates density in this alloy.

Inhibition of the corrosion process of alloy AA5083 (Al‐Mg) in seawater by cerium cations. An EIS study

The principal corrosion process of the alloy AA5083 in aerated NaCI solutions mainly affects the zones occupied by Al(Mn,Fe,Cr) cathodic precipitates, over which the oxygen reduction reaction takes place. Thus, the design of an effective protection system could be based on the use of cathodic inhibitors. In previous papers, the inhibition efficiency of various lanthanide chlorides, particularly CeCl 3 , has been proved. These compounds act as cathodic inhibitors, blocking these cathodic sites by means of the precipitation of a lanthanide oxide or hydroxide film. In this paper, EIS has been used to detect the blocking effect of the cathodic sites. This fact is reflected in the electrical response of the system through the minimisation of the values of those elements of the electrical loop that are related to the response of the cathodic intermetallics. In addition, the dielectric properties of the anodic film have been evaluated by measuring the value of the associated capacitance. From these values, it can be concluded that the thickness of the film produced over the matrix when the inhibitor is added is much lower than that produced in the bare solution. This finding is in good agreement with the results obtained with other techniques and may be related to the minimisation of the anodic reaction as a consequence of the decrease in the intensity of the cathodic process by the action of CeCl 3 .

On the mixed nature of cerium conversion coatings

Alternative pretreatments are currently under development in order to avoid the environmental impact produced by using surface finishing processes based on chromates. Some of the environmentally friendly alternatives proposed involve the use of lanthanide based compounds. In this study, conversion coatings on AA5083 (Al-Mg) samples developed using full immersion pretreatments in 500 ppm CeCl 3 aqueous solutions have been investigated. Their microscopic and compositional features have been analyzed using SEM images and EDS spectra. From this analysis it has been determined that this layer over the surface of the samples is of a heterogeneous composition. This coating is formed by an alumina coating covering the aluminum matrix and dispersed cerium-rich islands deposited over the cathodic sites of the alloy. A characterization methodology is proposed based on the utilization of different electrochemical techniques, such as Open Circuit Potential monitoring (OCP), Linear Polarization (LP) and Electrochemical Impedance Spectroscopy (EIS).

Degradation mechanism of an acrylic water-based paint applied to steels

The behaviour, in conditions of total immersion, of an acrylic water-based paint applied to a St 35.8 steel, has been studied using electrochemical techniques. The data obtained have enabled a three-step mechanism to be proposed. In the first step, there is an increase in the activity of the system, as a consequence of the entry of water. In the second step, once a critical level in the water content has been passed, the inhibitor pigments are activated, and this leads to a decrease in the electrochemical activity. Finally, once that the pigments have been exhausted, there is a loss of the protective properties of the paint, with the result that the activity detected is similar to that of the bare metal.

Dry Drilling of Fiber Metal Laminates CF/AA2024. A Preliminary Study

Carbon Fiber (CF)/Metallic Alloy (MA) laminar structures, also known as Fiber Metal Laminates (FML) allow diminishing the airship weight. Because of that the use of these materials is growing continuously in the aerospace industry. These composites materials need to be drilled because of the assembly requirements in the different airship elements. The most common problems that can appeared when those structures are machined are related with the interaction of the tool with dissimilar materials, which need different cutting parameters for the optimized machining process. This work reports on the results about a study of the dry drilling processes of hybrid composites Carbon Fiber/aluminum alloy, and especially CF/AA2024.

Protection by Thermal and Chemical Activation with Cerium Salts of the Alloy AA2017 in Aqueous Solutions of NaCl

A wide variety of anticorrosive treatments for aluminum alloys that can be employed as “green” alternatives to those based on Cr(VI) are currently under development. This article reports a study of the morphological and anticorrosive characteristics of surface layers formed on the Al–Cu alloy AA2017 by immersion treatment in baths of cerium salt, accelerated by increased temperature and the employment of hydrogen peroxide. Scanning electron microscopy (SEM)/X-ray energy dispersive spectroscopy (XEDS) studies of the samples treated have demonstrated the existence of a heterogeneous layer formed by a film of aluminum oxide/hydroxide on the matrix, and a series of dispersed islands of cerium over the cathodic intermetallics. The protective efficacy has been evaluated using electrochemical techniques, linear polarizations (LP) and electrochemical impedance spectroscopy (EIS), and salt spray tests. The results obtained indicate that the layer provided good resistance to corrosion in media with chlorides, and the method gives a considerable reduction of the time required for the immersion treatments.