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Self-Healing, Chromate-free Conversion Coating for Magnesium Alloys


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Magnesium and its alloys are increasingly being used in a wide range of applications, from aircraft to motor vehicles to consumer electronics because of the low density, high specific strength and stiffness [1-5]. However, since magnesium exhibits high chemical and electrochemical activity, its alloys suffer from poor corrosion resistance, often leading to structural or mechanical failure. Magnesium corrodes rapidly when exposed to salt water, moisture or acidic liquids or gases. Many methods have been used to protect magnesium alloys by inhibiting corrosion or slowing down the reaction mechanisms, including use of conversion coatings, anodizing, electroplating and addition of corrosion inhibitors.

Magnesium is being increasingly used in various industries, including aerospace, automotive, construction, computers, medical, consumer electronics and the military. World production of magnesium currently totals about 400,000 tons per year. The low density of magnesium (33% lighter than aluminum), and high strength-to-weight ratio (second only to titanium) make it one of the lightest and strongest metals available. Magnesium alloys also have high fluidity and a low volumetric specific heat compared to aluminum and zinc, allowing complex geometries to be formed by casting. A high degree of castability makes magnesium a good candidate for lightweight applications, such as fuel-saving vehicles and mobile computing, resulting in a smaller carbon footprint.

Despite its abundance, low density, high strength-to-weight ratio, and castability, manufacturers need to address the poor corrosion resistance of magnesium under certain environmental and operating conditions. Magnesium alloys tend to corrode rapidly when exposed to salt water, moisture, or acidic liquids or gases. Protective coatings applied to the surface of magnesium parts can slow the rate of corrosion and maintain structural integrity and appearance. Over the years, the most widely used conversion coatings for magnesium alloys have been based on hexavalent chromium, also known as hex-chrome or chromate. A chromate coating has the special ability to “self-heal,” i.e., repair itself if scratched or damaged, thereby providing active corrosion protection of the underlying metal. However, the use of hex-chrome has been drastically curtailed because of its carcinogenic nature. This paper describes a viable alternative self-healing conversion coating that is chromate free.
 

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