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|Fabrication & Finishing|
Protective Coatings for Magnesium
There are various reasons for which a protective coating would be applied to magnesium. These applications allow for increased corrosion and wear resistance, better solder ability, increased electrical conductivity and improved aesthetics.
Magnesium can be coated in an assortment of ways depending on the type of alloy used, the desired qualities of the finished material and the application in which it will be used. Magnesium can be coated by electrochemical plating in which the part is covered with a metal that possess the desired characteristics of the end material. The metal is present as a salt in solution and is reduced into its metallic form onto the part’s surface. Electroplating uses an external source of electrons for reduction in this process whereas in electroless or chemical plating, electrons for reduction are supplied either by the substrate itself or by a chemical reducing agent in solution. Both methods of electrochemical plating have been shown to be useful in improving corrosion resistance in interior or mild exterior environments and in computer and electronics applications. Plating also improves the solder ability of the magnesium parts, creates stable electronic contacts, improves electrical conductivity, thermal conductivity and diffusion, optical reflectance, fatigue strength and the overall lifetime of the plated alloys.
Conversion coatings create a superficial layer of substrate chemically bonded to the magnesium surface. The coating may be metal oxides, phosphates, chromates or different compounds and are produced through chemical or electrochemical treatments on the surface. These layers prevent against corrosion of the magnesium by insulating it from the outside environment.
Chromate conversions provide several beneficial properties when used as a pretreatment for a final sealing or as a post-treatment in the case of a plated surface. These benefits include improved corrosion resistance, better paint and adhesive bonding and an aesthetic finish.
Another type of conversion coating used on a magnesium alloy was formulated from a solution of food additive and an organic acid. Once the surface is coated, it is degreased and submerged in a solution including sodium benzoate, sodium glucosate and organic acid. This coating serves as a good base for painting the magnesium. In addition, a patented process which includes acid pickling of the metal prior to the conversion coating has been shown to demonstrate good paint adhesion and corrosion resistance.
Aside from plating and coating, magnesium can also be anodized to improve its surface properties. Anodizing is an electrolytic process that results in a stable, thick oxide layer that forms on the metal’s surface. This film aids in adhesion of paint as well as allows for successful dyeing of the metal. Immediately after anodizing the metal, the introduction of organic dyes or inorganic pigments allows for the coloring of the surface film. Anodized magnesium can be easily colored as well as provide a good base for painting. There are a number of existing patents for anodizing procedures that give the metal a protective coating.
Magnesium is anodized through the use of an aqueous bath that is alkali rich. A direct current is applied during the process and is either incompletely reversed in polarity or turned off which allows for the formation of magnesium compounds on the surface which have been shown to have good corrosion resistance when tested with a salt spray. The addition of various other elements into an aqueous bath has also been shown to have positive results in improving surface properties such as providing superior aesthetic appearances, and comparatively better corrosion and abrasion resistance.
Coatings Produced in Gas Phase
Metallic and organic coatings can be produced on magnesium in the gas phase. A trade off to the typically high capital costs of gas phase processing is the minimal negative impact they exude on the environment. Thermal spray coatings are formed through this type of processing. The material to be used as the coating is heated to near its melting point, or just above, inside of a torch or gun. This material forms droplets which are accelerated onto the magnesium substrate through a gas stream. There are several different forms of thermal spray coating processing that boast numerous advantages. Thermal spray coating allows for the creation of a coating made from nearly any material that does not decompose when melted. The substrate experiences minimal increase in temperature and coatings can be removed or repaired without changing the original part’s dimensions or properties.
As with any process, thermal spray coating does have some disadvantages. Faces that are parallel or near parallel to the spray angle cannot be coated well due to the line of sight nature of the process. Sealing is also required for the creation of a smooth finish on the part.
Chemical vapor deposition (CVD) is another form of coating produced in the gas phase. CVD uses a chemical reaction from the gas phase to deposit a solid material onto the heated substrate. This process allows for materials to be deposited below melting temperature and results in good adhesion properties. Unlike the thermal spray coatings, chemical vapor deposition can be used on surfaces that are not in the line of sight as well as apply thick coating. Substrates must be thermally stable at temperatures above 600°C in order to be coated with CVD. In addition, the chemical processes used in this method can be toxic in nature and produce solid toxic byproducts which require costly waste disposal.
There are additional processes that can be used for finishing magnesium parts depending on application needs. The variety of coating methods available for use on magnesium adds further benefit to metal’s use in many applications.