Corrosion Research Group

University of Cádiz. Department of Material Science,  Metallurgical Engineering and Inorganic Chemistry

Lanthanide salts as corrosion inhibitors of aluminium alloys

Low  environmental impact surface pretreatments. Lanthanide elements based systems

Surface finishing industries give rise to wastes whose management implies a severe environmental hazard. As it is well known, pre-treatments based on Cr (VI) compounds are among the most often employed, due to their high efficiency and low costs. Nevertheless, because of their high both toxicity and environmental impact, an intensive research effort is being undertaken to look for the replacement of chromate by more environmentally friendly compounds. In this sense, lanthanide compounds have been tested as corrosion inhibitors of different metallic alloys.

This project is being developing in a co-ordinated way by two research institutes with previous experience in this research field (MAT-97-1075-C03) and supervised by FABA at San Fernando (Spain). This Factory has a long experience in the field of metallic alloys surface pre-treatments. The main targets of this Project (defined in accordance with the interest of FABA) emphasising the search of new “green” surface pre-treatment that could be used at industrial scale. For this purpose, new protective films have been promoted by the action of lanthanide compounds. Corrosion evaluation is as necessary as an analytical control of the effluents in order to evaluate the new coatings environmental influence.

In our laboratories, some lanthanide compounds, mainly LnCl3, have been tested successfully as corrosion inhibitors and as components in conversion coatings for Al-Mg alloys and stainless steels. In these studies, the best performance has been obtained using CeCl3 when the corrosive media was NaCl solutions.

A variety of techniques are employed in order to obtain information about the corrosion mechanism of the alloys and the anticorrosive effects of lanthanide inhibitors: Full immersion, Open Circuit Potential (OCP), Linear (LP) and Cyclic (CP) Polarisations, Galvanostatic Polarisations (GP) and Electrochemical Impedance Spectroscopy (EIS). In addition, SEM, EDS and Optical Microscopy are employed to get microestructural and analytical information about the corrosion and inhibition processes, Figure 1.

Figure 1. (a) SEM image corresponding to Al(Mn,Fe,Cr) intermetallic of a AA5083 sample immersed in 3.5% NaCl solution for 1 day. (b) EDS spectrum recorded on the point marked in (a).

Linear polarisation curves similar to those shown in Figure 2 show that lanthanide chlorides behave as cathodic inhibitors. They act by blocking the cathodic sites of metallic alloys. The hydroxyl groups formed over the cathodic sites would react with lanthanide ions present in the solution. This reaction gives rise to the formation of lanthanide island observed in Figure 3. The blockage of the cathodic sites decreases the available cathodic current and, therefore, reduces the overall corrosion rate.

Figure 2. Linear Polarisation diagrams for AA5083 alloy sample in bare NaCl solution and with 500-ppm addition of CeCl3 to the solution.
Figure 3. (a) SEM image recorded on a sample of AA5083 alloy after a 48-hours immersion in 3.5% NaCl solution with 500 ppm of CeCl3. (b) EDS spectrum recorded on the point marked in (a).

In a variety of practical applications in which lanthanide compounds are used, it has been shown that using formulations based on mixtures of lanthanide elements can enhance the performance of pure compounds. Nevertheless, no data are presently available regarding the use of mixtures of rare earth ions as corrosion inhibitors.

The first results obtained in the study of LnCl3 binary solutions to decrease the corrosion rate of Al-Mg alloy (AA5083) in NaCl solutions point out that Ce/La binary solutions show a better performance than those containing solely LaCl3 or CeCl3 (Figure 4). As a consequence, a synergistic effect seems to exist when cerium and lanthanum ions act simultaneously as inhibitors. Nowadays, we are working in this sense.

Figure 4. Inhibition power (%) obtained for different lanthanide combinations.