Matthew J. O’Keefe

The Phase Stability of Cerium Species in Aqueous Systems

Cerium compounds have been identified as leading candidates to replace hexavalent chrome as conversion coatings on aluminum alloys to improve corrosion resistance. Cerium also shows promise for use as an inhibiting pigment in paint systems. The cerium conversion coatings can be deposited using either spontaneous or nonspontaneous electrolytic processes. In both cases the protective cerium oxide film forms by a precipitation mechanism that is very dependent on electrochemical potential and pH. The oxidation state and phase of the condensed cerium has been shown to be an important aspect of the corrosion protection properties provided by the film. Because of the strong influence of the solution chemistry and operating parameters on film performance, a basic knowledge of the system stability is essential. Toward this end, a revised E-pH diagram was developed for the Ce-H 2 O system. The Ce-H 2 O-HClO 4 system was chosen as an example system in which hydroxy ions are the only significant complexing species for the Ce ions. A stability diagram was constructed using more recent thermodynamic data on cerium species not available for the original diagram published in Pourbaixs atlas. Significant differences were noted between the previously published Ce-H 2 O diagram and the one presented here. Precipitation tests were carried out to verify the trends indicated in the new diagram. The importance of the updated E-pH diagram in understanding the formation processes of cerium conversion coatings are discussed.

The Influence of Galvanic Current on Cerium‐Based Conversion Coatings on Mg, AI, and Galvanized Steel Couples

The influence of galvanic current on cerium-based conversion coatings (CeCCs) for magnesium (AZ91, AZ31), aluminum (6016), and electro-galvanized steel (EGS) couples has been studied using zero resistance ammeter (ZRA) measurements in prohesion solution. The galvanic current measured between magnesium-aluminum, magnesium-galvanized steel, and aluminum-galvanized steel couples correlated with significant changes in coating morphology and deposition rate. The ZRA galvanic currents (mA) were 0.02 for 6016-EGS, 0.38 for AZ91-EGS, 0.72 for AZ91-6016, 1.08 for AZ31-EGS, and 1.08 for AZ31-6016 couples. The corrosion performance of the coated couples was evaluated by ASTM Bl 17 neutral salt spray testing. Cerium conversion coated couples performed better in salt spray testing compared to uncoated couples. The correlation of galvanic current, cerium deposition, and corrosion performance will be discussed.

Effect of Thickness on the Morphology and Corrosion Behavior of Cerium‐Based Conversion Coatings on AZ31B Magnesium Alloy

Cerium-based conversion coatings (CeCCs) were deposited onto AZ31B magnesium alloy substrates using a spontaneous reaction of CeCl3, H2O2 and gelatin in a water-based solution. The coating thickness was adjusted by controlling the immersion time in the deposition solution. Prior to deposition, the AZ31B substrates were treated using an acid pickling in nitric acid and then an alkaline cleaning in sodium metasilicate pentahydrate. After deposition, the coated samples were immersed in a phosphate bath that converted cerium oxide/hydroxide into cerium phosphate. Electrochemical impedance spectroscopy, potentiodynamic polarization and neutral salt spray testing studies indicated that ~100 nm thick CeCC had better corrosion performance than ~400 nm coatings. Characterization of the CeCCs by transmission electron microscopy (TEM) revealed a three layer structure with different compositions.

Corrosion and Adhesion Properties of Cerium‐Based Conversion Coatings on Magnesium Alloys

Corrosion resistance and adhesion properties of cerium-based conversion coatings (CeCCs) on Al/Zn and Al/Mn magnesium alloys have been investigated. CeCCs were deposited by immersing magnesium panels in a cerium ion containing solution for 2 minutes. Corrosion resistance and adhesion strength of cerium coated magnesium panels were evaluated by electrochemical and pull tab testing methods, respectively. The CeCCs had very good adhesion to and significantly improved the corrosion resistance of magnesium alloys. Electrochemical test result showed that there was a measurable difference in corrosion resistance between the different magnesium alloys. Magnesium alloys with higher aluminum content had better corrosion resistance than alloys with lower Al content. The correlation of cerium deposition conditions, corrosion resistance, and adhesion strength are discussed.

Corrosion Behavior of Cerium-Based Conversion Coatings on Magnesium Alloys Exposed to Ambient Conditions

Exposure of CeCCs on AZ31B and AZ91D Mg alloys to ambient sunlight, temperature, and humidity was done to determine the effect on corrosion resistance. It was found that the CeCCs changed from pale yellow to almost translucent after 24 hours of sunlight exposure. The effect of the solar electromagnetic radiation on the morphological, chemical and optical properties of these coatings was investigated using SEM and UV-Vis characterization techniques. In addition, the corrosion performance of CeCCs before and after ambient exposure was studied by ASTM B117 neutral spray testing and electrochemical polarization measurements. In general, the changes in appearance did not adversely affect the corrosion performance of the coatings.