The Process Of Electroplating With Cu On The Surface Of Mg-Li Alloy

Mg-Li alloy is a potential applied material as its low density. Its poor corrosion and wearresistance hinder the expansive applications. The methods of surface treatment of Mg-Li alloyare relatively few, such as conversion film, chemical plating and electroplating, and so on.The coatings that are formed by conversion and chemical plating own some disadvantages,taking the exaples that the coating is weak and adhesion between coating and substrate is notvery firm. Usually, the electroplating progress mainly contains pre-plating copper, thenelectroplating with another metal and electroplating with chromium. Both the two methodscan get the coating with good property. However, the cyanide and Cr6+used in the twomethods are seriously harmful to human health and environment.This dissertation focuses on electroplating with pyrophosphate on the surface of Mg-Lialloy. Through the characterization methods, such as SEM, XRD, EDS and electrochemicaltests, and so on, the optimum process parameters are concluded, and the best heat treatmentprocess after electroplating is discussed. The results are as following:1. According to orthogonal experiments and Tafel polarization curves, the optimumprocess parameters of chemical zincing are affirmed as follows: concentration of main salt(zinc sulfate) is40g/L, solution temperature is60℃and zincing time is5min, respectively.Among the three factors, zincing time is the most influential one and the influence ofconcentration of main salt on zincing is the weakest one. The surface of sample appears bluegray. It can be obtained that the protrusions in the straight line shape in the micro analysis,and these protrusions in the straight line shape are almost parallel to each other. This zincingcoating distributes uniformly and compactly, and the thickness is uniform.2. A uniform and dense Cu coating with many nodular structures is obtained on thesurface of Mg-Li alloy at the optimum electroplating condition, without the phenomena ofburr, hemp dot, charing and peeling. No obvious holes and cracks appear between coating andsubstrate, the coating adheres with the substrate firmly, and its thickness is more than10μm.Pure copper is accumulated with a FCC structure on the surface of coating. Besides, thecorrosion resistance and hardness are improved after electroplating with copper at theoptimum electroplating condition. The optimum process parameters of electroplating withcopper are affirmed as follows: concentration of main salt is50g/L, current density is2.0A/dm~2, solution temperature is40℃and electroplating time is30min, respectively. Among the four factors, solution temperature is the most influential one.3. When sample after electroplating with copper is heat treated under200℃for2h, thesurface of sample is composed of globular structures that pack compactly and distributeuniformly without gap. Meanwhile, the sizes of globular structures are average and small,about several microns. The corresponding sample owns the lowest corrosion current densityand the most positive corrosion potential, a slowest evolved hydrogen rate. Besides, thecorrosion resistance is improved, compared with the sample electroplated with copper withoutheat treatment. This heat treatment can provide more useful protection to Mg-Li alloy.