Application and Prospect of rare earth elements in surface treatment technology

Rare earth (RayeEavth (RE) is including scandium (SC) and yttrium (y) and lanthanum (LA) and cerium (CE), praseodymium (PR), neodymium (nd), promethium (PM), samarium (SM), europium (EU), gadolinium (GD), terbium (TB) and dysprosium (Dy), Ho (HO), erbium (ER), thulium (TM), ytterbium (YD) and Lu (Lu) a total of 17 kinds of metal elements. The atomic structure of rare earth elements with common characteristics, outer layer 2 electrons and outer have eight electrons and valence generally positive valence 3. Therefore chemical properties are similar, such as soluble in dilute sulfuric acid to form stable coordination compounds and slightly soluble in water of oxalate, carbonate, sulfate, phosphate, fluoride and hydroxide. Rare earth elements generally have: high corrosion resistance, high wear resistance, high magnetic, superconducting, high catalytic activity and high hydrogen storage properties, etc.. Is an indispensable functional material of modern science and technology. Rare earth research is the need for the development of science and technology in China, and the application of rare earth has become one of the hot topics in the research of science and technology workers.

1, electroplating

1. 1 single metal plating

In the middle of 1980s, the rare earth element was introduced into chromium plating process, which has experienced the development of a single rare earth (such as Ce, La) additive to the multi rare earth to optimize the combination type of additive process. Early by Jiangsu Changshu City Environmental Protection Bureau Huey Kuo Chen invention developed CS type multi mixed rare earth additives for chromium plating is most prominent and achieve the innovation of tradition chrome plating craft, changed the traditional process "three high and one more poor" (i.e., the operation of high temperature, high cell voltage, high current density, chromium mist from escaping, low power consumption, plating solution stability is poor), to enhance the performance of plating solution, covering capacity 60 ~ 85%, improve the cathodic current efficiency of 15 ~ 20%, the hardness of coating increase 30% ~ 60%, chromic anhydride saving more than 50%. At the beginning of 1990s, the general low chromate plating process application of rare earth additives in national electroplating enterprises. Significant economic benefits.

In addition to chromium plating, in the single metal plating, such as zinc plating, copper plating electrolyte to add rare earth additive can make the coating grain density, corrosion resistance is improved. The study found that in the four elements of rare earth Sm, Er, Yb and Ce, the role of Ce is the most prominent. The corrosion resistance of the coating can be improved with the addition of 0.1~0.2g/L cerium oxide in the bright zinc plating bath 20%~35%. The addition of cerium compound in the tin plating solution can prevent the occurrence of "tin plague" and ensure the reliability of the electronic device in the process of improving the ability of the coating to be bright and delicate.

In recent years, the research process of trivalent chromium plating instead of six chromium chromium plating is speeding up. The performance of rare earth to improve the performance of trivalent chromium plating bath is also reflected. In the sulfate trivalent chromium plating solution, the deposition rate of the coating is improved and the corrosion resistance is better when the lanthanum carbonate 0.4g/L or cerium nitrate 0.1g/L or the mixed rare earth 0.3g/L is added.

2 / 1 alloy plating

Adding yttrium salt can change the preferred orientation and texture coefficient of the coating, and the grain size becomes smaller, and the corrosion resistance of the coating is improved. When the mass concentration of yttrium salt is 0.6g/L, the coating is uniform and compact, the crystal growth pattern is laminar, the dimension of the passive current density decreases, the polarization resistance increases, and the corrosion resistance of Zn Fe alloy coating is best.

A small amount of rare earth compound is added to the base of the sulfate type nickel cobalt alloy plating, the nickel cobalt alloy coating with cobalt content of >40% can be obtained, and the deposition process is "abnormal". Due to the characteristic adsorption of rare earth compounds on the cathode surface, cathodic polarization of electrodeposition process is reduced. In KOH solution, the alloy coating is used as an electrolytic cathode, the hydrogen evolution potential in the high current density region is lower than that of the iron electrode, and the over potential of the Ni-Co alloy electrode is reduced by 200mV, while the Ni-Co-RE electrode is reduced by 250mV. It is found that the addition of rare earth elements has a high catalytic activity for the hydrogen evolution reaction.

2, composite plating

Composite electroplating is a new surface treatment technology in recent years. The amount of rare earth compounds added composite plating solution, promotes the insoluble solid particles and metal ions co deposition and improve the particle mass fraction in the composite coatings, to adjust and change the composite coating of the physical, chemical and mechanical properties provide more possibilities.

Some studies have indicated that the addition of rare earth oxide La2O3 nanoparticles in the nickel based composite plating bath can refine the grains and improve the oxidation resistance of the coatings greatly. This is due to the addition of rare earth elements to inhibit the growth of NiO grains. The Cr-SiC ER- composite coating was obtained by adding rare earth compound as accelerator in Cr-SiC composite plating solution, and its wear resistance was more than 60%.

Tang Hongke et al. Study on the effect of lanthanum chloride on Ni-Co-PTEF composite electroplating coating and microstructure of. Results show that in the Ni Co alloy plating solution adding 0.8g/LLaCl3, is conducive to the coating grain refinement, compact structure, uniform distribution of PDEF, with high deposition rate and microhardness, friction coefficient is the smallest, lubrication performance improvement.

The (Ni-W) -SiC, (Ni-W-P) -SiC () and (B Ni-W-) -SiC and other composite coatings were prepared by electrodeposition. After heat treatment, the hardness of the coating can be increased up to 1200HV. In order to further improve the wear resistance and corrosion resistance of the composite coating, the rare earth compound can be added in the plating solution to form a (RE-Ni-W-P) -SiC composite coating. Because of the strong adsorption ability of the rare earth element, the SiC particles are deposited with the alloy composition under the action of electric field. Where RE can be a rare earth element of chloride, oxide or sulfate. (RE-Ni-W-P) the hardness of -SiC alloy coating is higher than that of hard chromium coating.