Influence of RE element on Ni–P coelectrodeposition process

In this paper, the effects of rare earths (RE) added in bath on the chemical composition, the microstructure, the polarization curves and the corrosion rate of Ni–P coatings have been investigated by energy dispersion spectra (EDS), auger electron spectra (AES), X-ray diffraction (XRD), transmission electron microscopy (TEM), potentiodynamic scan and immersion test. The results of immersion tests show that the corrosion resistance of the Ni–P coatings is greatly improved by the addition of RE, which is attributed to the formation of an amorphous phase in the coatings. This formation of the amorphous phase is from the effect of RE ions on the coelectrodeposition process of Ni and P. RE ions make the cathodic polarization to increase obviously. But RE ions themselves do not codeposite with Ni and P. And they have no effect on the P content of the coatings. According to the results of potentiodynamic scans, EDS, AES and the data of the corresponding standard electrode potential it can be deduced that RE ions can change the structure of the double-layers of cathodic and disturb the depositing behavior of Ni ions, thus promote the formation of amorphous phase.