Molybdenum is among a sub group of elements that are known as refractory metals. These types of metals provide numerous metal finishing benefits. Refractory metals are relatively rare and possess their own unique challenges when combined with processes such as electroplating, passivation and anodizing, amongst others.
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Molybdenum benefits from an extremely high melting point of 2623° C (4753.4° F). There are only five other known natural elements with a higher melting point. Additionally, Molybdenum ranks the lowest of all engineering materials regarding its coefficient for thermal expansion, and surpasses only a few elements in terms of overall thermal conductivity. Molybdenum contains numerous beneficial characteristics that makes it useful for a variety of unique applications, some of these features include:
Strong Thermal Shock Resistance
Thermal shock is demonstrated through the rapid expansion of certain parts of an object caused by concentrated heat. Molybdenum provides extraordinary resistance to this potentially damaging condition.
High Melting Point
Molybdenum’s high melting point makes it tremendously valuable when processing molten metals and minerals.
Exceptional Electrical Conductivity
Molybdenum can enable non-conductive materials the ability to conduct electricity, which makes it highly useful in the production of electrical and electronic products and components.
Beneficial Heat Resistance
Sources of heat can be transferred to an area enclosed with molybdenum in electronic and mechanical devices due to its high melting point.
Superior Strength & Hardness
Due to its overall strength and toughness molybdenum was first used during World War 1 for tank armor plating.
Molybdenum is a highly dense metal that is used for both large and small applications that require added weight.
Remarkable Corrosion Resistance
Corrosion is often the culprit of a products early demise. Molybdenum plating can significantly slow down the onset of rust.
Molybdenum can withstand aggressive amounts of heat without expanding or softening. This makes it an extremely valuable resource for a variety of high-heat industrial applications. Additionally, molybdenum’s low density and stable price often make it a better option over tungsten in certain applications. Some of the industries that regularly use molybdenum, include:
Molybdenum is extremely tough, hard, and weldable, with elevated temperature strength. These traits make it a common additive in cast iron and steel manufacturing applications. When used with nickel-based alloys, molybdenum also boosts corrosion resistance and creep deformation due to high-temperatures. Molybdenum’s unique blend of properties makes it a popular choice for specialized manufacturing processes that are not suitable for the use of traditional or precious metals.
Molybdenum Aqueous Plating Solutions
Electroplating molybdenum from a water-based plating bath can often be a challenging task due to its high level of reactivity with other metals. Molybdenum also tends to develop large, complex molecule oxides, which makes it difficult to achieve proper adhesion.
However, the following technique has been proven effective when electroplating molybdenum in an aqueous solution for iron, nickel or carbon substrates. The process involves the formulation of an electrolyte bath containing 100 grams of potassium fluoride dihydrate, 10 cc of 48-percent aqueous hydrofluoric acid, five grams of grams of molybdic anhydride and 50 to 130 cc of water. The electric current is introduced for a period of approximately five minutes at a density ranging from 0.25 to 0.35 ampere per square centimeter of outside surface. The bath temperature should be between 300° C and 500° C, as a higher temperature is likely to produce darker deposits and will negatively impact coating adhesion.
If done correctly, this method will produce a level, bright metallic finish that is impervious to strong alkalis, hydrochloric acid, and sulfuric acid. It will also provide increased protection against corrosion and increase the substrate’s visual appearance. It can also be used with various molybdenum alloys containing nickel, iron or cobalt.
Alternative Aqueous Molybdenum Plating Solutions
There are a few alternative aqueous methods for plating molybdenum, these include:
- Vacuum Deposition – This dry process is a surface engineering treatment that is used to deposit layers of material onto a substrate. Vacuum-deposited metal coatings can be used to either enhance or serve as a substitute for electroplated finishes. However, vacuum deposition requires a substantially large equipment investment.
- Non-Aqueous Media – Fused salt electrolysis is a non-aqueous method that involves a combination of a chemical reaction and an electron transfer using a reactor known as an electrochemical cell. Within this process electric currents pass from the anode through the electrolyte to the cathode. The elemental components of the deposit are then dissolved in molten salt and are removed via the passage of an electrical current. The cathode element is then treated with the molybdenum compound.
- Ionic Fluids – When an ionic charge is delocalized it results in a reduction of lattice energy, which eventually forms ionic fluids. Ionic fluids offer similar advantages to aqueous solutions, with the added benefit of a higher melting point. However, it can be enormously problematic to attain the perfect operating conditions when using these materials. Subsequently, using ionic fluids is only effective for covering a few types of metal substrates. The coating is often very thin, which can often provide insufficient protection. In some cases, it is possible to attain coating success by using an ionic material that melts at a lower temperature.
It is important to point out that most of the above-mentioned molybdenum plating techniques have yet to be established for extensive commercial applications. Thus far, the use of molybdenum electroless plating has been limited to laboratory settings in research and development facilities in government facilities, colleges and universities.
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