Morphology and Anti-Corrosive Performance of Cr(III) Passivated Zn–Fe Alloy Coating on NdFeB Substrate

Abstract

In this study, low-iron Zn–Fe alloy coatings and pure Zn coatings, with or without trivalent chromium passivation treatment, were electrodeposited onto a sintered NdFeB magnet from a weak acid chloride bath. The surface morphology and structure of the coatings were then examined using the X-ray diffraction, a scanning electron microscope and 3D white-light interfering surface analysis. Meanwhile, the electrodeposition behavior and anti-corrosive properties of the coatings were investigated using cyclic voltammetry, potentiodynamic polarization, electrochemical impedance spectroscopy, and natural salt spray tests. The results indicate that a passivated Zn–Fe alloy coating with a 0.9 wt.% Fe content provided much better corrosion resistance than a pure Zn coating and could provide both anodic protection and physical barrier function in the NdFeB substrates. The Fe element in Zn–Fe alloy coating was predominantly in solid solution in η-phase and small amounts in elemental form, which was beneficial to acquire a compact coating and passivation film. Finally, the passivated Zn–Fe alloy coating withstood 210 h against a neutral 3.5 wt.% NaCl salt spray without any white rust, which was 3–4 times longer than the pure Zn coating.

Effect of In doping on the evolution of microstructure, magnetic properties and corrosion resistance of NdFeB magnets

“…[17,18] To prevent corrosion, it is necessary to mitigate the potential difference between the grain boundary phase and the matrix phase to improve the corrosion resistance of a magnet. [19,20] Therefore, exploring how to efficiently improve the magnetic properties and corrosion resistance of NdFeB magnets is an important research topic.…”

“…[10][11][12][13][14]21,22] Furthermore, enhancing both magnetic properties and corrosion resistance is a challenge, and one of the properties may deteriorate. [18,20,23] Currently, the addition of microelements during the melting process has been widely used to optimize magnetic properties. [10,24] For example, Al and Ga have been added, resulting in the formation of continuous and thick grain boundaries, thereby increasing the coercivity via the demagnetization coupling strengthening mechanism.…”