On the mixed nature of cerium conversion coatings

Abstract

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 CeCl3 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).

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

“…Both events bring about a reduction in the activity of the system. In accordance with the previous observations, it is proposed in [30,31] to use the value of the slope of the passivity section of the LP curves, on a linear scale, as a measure of the resistance of the mixed layer R cp (Eq. [2]).…”

“…This has enabled the temperatures previously employed to be reduced. After validating the design of the protection systems based on the combined action of temperature and H 2 O 2 in solutions of cerium on the Al-Mg alloy AA5083, [29,31] these treatments have been applied to the alloy under study here, which is AA2017.…”

“…In previous studies, [25,26] it was proposed to use the parameter passive layer resistance (R cp ) for the discriminated evaluation of the protective layer formed by direct current; this parameter corresponds to the slope on a linear scale of the passive section of the passive region of the LP. Although this is also an average term, it gives us information on the resistance of the layer to polarization.…”

Coatings for corrosion prevention based on rare earths

“…Other bodies of work in this area include the work of Mansfeld and co-workers, 154,[167][168][169][170][171][172] as well as those of Botana, Abelle, Bethencourt and co-workers 68,102,173,174 who developed boehmite-like coatings on the matrix of alloys using conversion coating solutions containing cerium nitrate. It is diffi cult to draw general conclusions from the literature in this area as a variety of pretreatments are used.…”

Underfilm corrosion on polyurethane-coated aluminum alloy 2024-T3 containing dissimilar metal fasteners

“…18 Cerium-based coatings were chosen as a Cr-free alternative to chromate conversion coatings. Cerium-based conversion coatings were applied to a range of metals, including tinplate, 19 magnesium (AZ91), 20 zinc (Zn), 21,22 and aluminum alloys [23][24][25][26][27][28][29] including AA2024-T3. 30,31 The accelerated cerium-based coatings described herein have been previously reported to show good corrosion resistance on a range of aluminum alloys.…”