We shall discuss what are the refractory metals and as well as discuss five primary elements in this article.
Five Primary Elements, Tungsten, Molybdenum, and Others
Tungsten was discovered in 1781 by the Swedish chemist Carl Wilhelm Scheele. Tungsten literally means “heavy stone”. Tungsten, the most abundant refractory metal, also has the highest melting point at 3410 °C (6170 °F). This melting point is twice that of iron and ten times that of lead. Tungsten is also one of the densest metals.
Molybdenum
Like tungsten, Karl Wilhelm Schiele was instrumental in the discovery of molybdenum, which also occurred in the late 18th century. However, a 96% pure metallic version of the element was not developed and used commercially until the late 19th century. Molybdenum is currently mainly used as an alloying element for steel in various production processes.
Tantalum
Another discovery made by Swedish chemists was tantalum, which was first discovered in 1802 by Anders Gustav Ekberg. This element is very rare – about 15 times more abundant than gold. Tantalum is very insoluble and is one of the most corrosion resistant materials are known to man. It is also chemically inert, so it can be used in the manufacture of laboratory equipment. Tantalum is also used as a platinum substitute on occasion.
We will not fear
Niobium is commonly found together with tantalum and has many similar properties and properties. The main difference between the two is that tantalum weighs about twice as much. The first known process for the separation of niobium and tantalum occurred in Europe in the mid-19th century. Its initial use was as an alloy with steel, and it remains its primary use today.
Rhenium was first discovered in 1925. The stone is not found alone – rhenium ore must be mined from other ores such as platinum, tantalite, and molybdenite. In terms of melting point, only tungsten and carbon surpass rhenium, and it also has a very high density.
Titanium
Although not one of the five main refractory metals, titanium has many common refractory metal properties, such as a high melting point (3035°F) and excellent corrosion resistance.
Plating With Refractory and Other Exotic Metals
Plating is a process used to create a protective coating on an underlying metal surface, called a substrate, through a technique called electrodeposition.
In the traditional electroplating process, a metal piece or object is placed in an aqueous electrolyte solution containing dissolved metal ions that form the coating.
An electric current is then applied, giving the ions a positive charge, while the substrate is negatively charged. This leads to the deposition of ions on the surface of the substrate.
Plating can be used for various purposes. The main function is to make the substrate more resistant to the effects of corrosion. It also increases the conductivity of the object and increases its thermal resistance.
Plating can even make pieces more aesthetically pleasing, such as plating gold, silver, or other precious metals on dull metal surfaces.
Refractory metal plating differs from traditional plating processes in that it uses a non-aqueous medium, usually molten salt. Electrolysis of molten salt can produce compounds consisting of refractory metals. Plating coatings can contain pure metals or alloys or combinations of various refractory metals.
Molten salt electrolysis involves chemical reactions with electron transfer. This process is carried out in electrochemical cells that allow electrical energy to extract refractory metals as a further chemical action from compounds.
During electrolysis, current flows from the anode through the electrolyte to the cathode. Then the cathode material is separated by refractory metal.
Over the years, other special metal plating processes have been developed. Now, we will take a closer look at the process of plating refractory metals with titanium, tungsten, and molybdenum.
Plating with Titanium
Like most refractory metals, titanium plating cannot be achieved with conventional water baths. It also cannot be plated alone. An efficient titanium plating process involves the use of an alloy of titanium and nitrogen to form titanium nitride (TiN), which, unlike conventional plating, is deposited by physical or chemical vapor deposition. Although the TiN coating is very thin, it is very hard and very wear resistant.
Because of the thinness, it is also simple to maintain desired tolerances during plating.
In addition to being very resistant to wear, the titanium nitride coating is also very good in terms of biocompatibility.
Medical device manufacturers often use TiN coatings to minimize wear on sliding parts and assemblies and to maintain the sharp edges of surgical instruments.
Other uses include increasing the life of cutting tools and machine tools. The TiN coating has an attractive golden color, making it suitable for applications where aesthetics is important.
Plating with Tungsten
Tungsten also cannot be plated alone. However, plating other ferrous metals, especially nickel, with tungsten is possible. Nickel-tungsten-phosphorus alloys can be deposited by electroless plating. Unlike electroplating, electroless plating does not require the introduction of an electric current. Instead, precipitation occurs through a chemical reaction.
While there are many electroless nickel alloys, the nickel-phosphorus combination is the most used industrially.
The use of nickel-phosphorus-nickel alloys in electroless plating is relatively new, but initial results are promising. The ni-P-W chemical deposition process requires creating baths consisting of nickel, tungsten salts, and various stabilizers as well as buffers and complexing agents.
While phosphorus increases the hardness of the coating, even small amounts of tungsten can significantly increase this property as well as its corrosion resistance. The increase in hardness ultimately increases the wear resistance of the substrate.
Plating with Molybdenum
Molybdenum (Mo) plating can be done by alloying it with other metals. Chromium-molybdenum alloys protect the substrate from wear and increase its corrosion resistance.
Molybdenum content is very low and is usually around one percent. However, pulse plating processes have been developed that can efficiently deposit chromium-MOBA with up to three times the molybdenum content, while still producing crack-free deposits. In pulse plating, the direct current enters the plating bath in short pulses instead of being maintained continuously.
Hardness values up to 900 KHM can be achieved with pulse plating compared to conventional DC plating, an increase of nearly 20%. In addition, the use of low pulse frequencies increases the concentration of molybdenum in the sediment.
In Conclusion
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