Magnesium and its alloys have many excellent physical and mechanical properties, high specific strength and stiffness, easy cutting, easy to cast, good shock absorption and can withstand greater shock and vibration loads.
And good electrical and thermal conductivity,
magnetic shielding performance, is an ideal modern structural materials, is now widely used in automobiles, machinery manufacturing, aerospace, electronics, telecommunications, military, optical instruments and computer manufacturing. For magnesium alloy used in different occasions, often need to change the surface state in order to improve corrosion resistance, abrasion resistance, weldability, decorative properties. There are many processes in magnesium and magnesium alloy coating formed on the surface, Magnesia including electroplating, electroless plating, conversion coating, anodizing, hydride film, organic coating, vapor deposition layer. One of the most simple and effective way is by electrochemical methods in the matrix layer coated on the required performance of the metal or alloy, that is, electroplating and electroless plating. This approach in these two magnesium and magnesium alloy on the problems faced by the application, process, various pre-treatment method used for the coating and the development of a brief overview of the status quo.Magnesium is a difficult direct plating or chemical plating metal, even in the atmospheric environment, the magnesium alloy surface will quickly form a layer of inert oxide film, the combination of impact strength of the coating, or plating during plating must be removed This layer of oxide film. As the oxide film to generate faster, so we must find a suitable pre-treatment method to the magnesium alloy surface layer can prevent the formation of oxide film formation, but also in the electroplating or chemical plating easy to remove the film. Magnesium alloys have high chemical reactivity, making we must ensure that the plating or chemical plating, the plating metal cation reduction should take place, because the magnesium cation with the bath replacement reaction to form the rapid replacement of loose layer affect the coating adhesion. Magnesium also reacts violently with most acids, dissolve quickly in acid medium. Therefore, we magnesium alloy plating or chemical plating process should maximize the use of neutral or alkaline bath, so not only can reduce the erosion of magnesium alloy matrix can also be extended bath life.As the electrode potential of magnesium is very low, as - 2. 34 V (relative to the standard hydrogen electrode), prone to galvanic corrosion in the electrolyte in contact with other metals, easy formation of corrosion micro-cell, resulting in magnesium alloy surface occurred rapidly eclipse. Therefore, plating or chemical plating, Neodymium fluoride the coating formed on magnesium alloy to be non-porous, not only will not effectively prevent corrosion, but will accelerate the corrosion of magnesium alloys. Especially for dip zinc, nickel and other direct chemical pre-treatment, the underlying need to ensure the formation of non-porous. For the magnesium alloy on the Cu / Ni / Cr plating, has been suggested that the thickness of the coating should be at least 50 ¼m, in order to ensure that no hole for outdoor applications.Plating on magnesium alloy formed by coating or plating the quality also depends on the type of magnesium alloy. For different magnesium alloys, because of the different elemental composition and surface condition, pre-treatment should take a different approach, there are a lot of magnesium alloy intermetallic compounds, such as MgxAly presence of intermetallic phases, leading to uneven distribution of surface potential, an increase of the difficulty of electroplating and electroless plating. Electroplating and electroless plating bath containing a common drawback is that heavy metals affect the recycling of magnesium alloys, increasing the difficulty and cost recovery.As the electrode potential of magnesium alloys is very low, high electrochemical activity, is difficult to direct plating or chemical plating. Direct plating or chemical plating, the metal magnesium cation with the bath replacement reaction, deposition of the coating loose, porous, adhesion is poor, and will affect the stability of the bath, to shorten the life of the bath, plating when the impact is more serious. So for magnesium alloys in order to obtain the desired coating, the most important thing is the appropriate pre-treatment process. Source:http://www.mhcocm.com
