The Influence of Sm Content on the Surface Morphology and Corrosion Behavior of Zn-Co-Sm Composite Coatings

Abstract

The influence of samarium, as an additional alloying element, on the morphology and corrosion performance of the Zn-Co-Sm alloy electrodeposited coatings, was investigated by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDS) and electrochemical impedance spectroscopy (EIS) measurements. The Zn-Co-Sm coatings were electrodeposited from the aqueous solution containing Sm(NO3)3, ZnCl2, and CoCl2 as the metal ion source. The percentage of Sm in the coating may be very finely tuned by setting electrodeposition parameters, including cathodic current density, glycine concentration in the electroplating solution, and the solution temperature. The coatings with Sm content from 0.5 to 18.5 wt.% were produced. Since low deposition current densities (10–50 mA cm−2) were applied, the samples obtained were of good adhesion and compact. The presence of Sm2O3 inclusion was verified by XRD as the Sm2O3 crystalline phase. Samarium is incorporated in the coatings through the mechanism of oxide/hydroxide formation during the electroreduction of Zn and Co. Corrosion tests in NaCl solution show that the presence of Sm significantly increases the polarization resistance for the corrosion process of Zn-Co-Sm coatings (one order of magnitude, i.e., from ~500 Ω cm2 measured without Sm to 2000–3000 Ω cm2 with 12 wt.% Sm), giving evidence of the self-healing action that is provided by Sm particles in the coatings. This effect is more pronounced in the case when the coatings contain a higher Sm percentage.