| Since 1984,sintered Nd Fe B has been rapidly applied in the fields of instruments,motors,computers,and so on because of its ultra-high magnetic properties and high-cost performance.However,due to the porous and multiphase structure of the Nd Fe B magnet,the corrosion resistance is very poor,which is prone to corrosion in practical application,reduces the stability and reliability of magnetic devices,and seriously affects its service life,which limits the application environment and field of Nd Fe B magnet devices.This paper improves its corrosion resistance by means of surface protection.Chromium-free Zn-Al coating has the advantages of ultra-high corrosion resistance,no hydrogen brittleness,no pollution,and high-temperature resistance.On the basis of chromium-free Zn-Al coating,a Zn-Al coating formula suitable for sintered Nd Fe B magnet is developed in this paper.The Silane coupling agent is selected as a binder and Zn flake,Al flake powder,and other raw materials.ZnAl coating is formed on the surface of sintered Nd Fe B magnet by spraying.The morphology,chemical composition,and formation mechanism of the coating were tested and analyzed by scanning electron microscope(SEM),energy dispersive spectrometer(EDS),X-ray diffraction(XRD),intelligent Fourier transform infrared spectrometer(FTIR),thermogravimetric analysis and differential thermal comprehensive thermal analyzer(TG-DTA).The corrosion resistance mechanism of the coating and its influence on the magnetic properties of the magnet were studied.The following experimental results were obtained:Zn-Al coating is composed of 27.8wt% flake zinc powder,5.6wt% flake aluminum powder,5.0wt% OP-10,8.9wt% PEG-400,46.7wt% KH-560 hydrolysate,and 2.8wt% ammonium molybdate.At the same time,the sintering and curing process parameters of the coating are optimized,and the best process of preheating at 100℃ for 10 min and curing at 300℃ for 30 min is obtained.The thickness of the coating is about 27μm,and the neutral salt spray resistance test time of the coating is more than 1000 h.The corrosion morphology and corrosion products of Zn-Al coating were analyzed by means of an electrochemical test and ammonium nitrate accelerated test,and the corrosion resistance mechanism was discussed.The coating has the cathodic protection function and physical shielding function of a sacrificial anode for the Nd Fe B matrix.The coating corrosion process can be divided into three stages.In the first stage,due to the electrochemical inert layer composed of oxide and silane,the cup electrolyte is destroyed and the metal sheet is activated by corrosive ions;In the second stage,the physical shielding layer is formed due to the joint action of the corrosion products of the coating and the Zn/Al sheet;In the third stage,with the increase of corrosion products,corrosion holes appear in the coating and the coating fails.Based on the study of Zn-Al coating,adding different contents of Ti,Si,and Cu powder,it is found that the comprehensive performance of Zn-Al coating with 10%Ti content is the best,and its corrosion current density is one order of magnitude lower than that of Zn-Al coating containing Si and Cu and the original samples of Zn-Al coating.Based on the study of Zn-Al coating,doped with different contents of Ti,Si,and Cu powder,it is found that the comprehensive performance of Zn-Al coating containing 10% Ti is the best,and its corrosion current density is much lower than that of Zn-Al coating containing Si and Cu and the original samples of Zn-Al coating.Finally,the effect of the coating on the magnetic properties of the substrate was investigated.The magnetic properties of Zn-Al coated samples are close to those after heat treatment,and the squareness decreases slightly.The preparation process of the coating has no significant effect on the magnetic properties of the magnet. |
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