Cobalt based Alloys – Benefits, Chemical Composition and more

About Cobalt

Cobalt (Co) is a metal with the atomic number 27 and is ferromagnetic, similar to nickel and iron. It has a density of 8.8g/cm³, comparable to nickel but heavier than iron. One of cobalt’s key properties is its high melting point of 1493°C, making it particularly suited for elevated temperature applications.

Cobalt exhibits two crystalline structures:

• Hexagonal Close-Packed (hcp) at room temperature.
• Face-Centered Cubic (fcc) at 450°C and higher.

The fcc structure is more ductile, enhancing cobalt’s flexibility and making it a valuable material in environments where components need to withstand high stress and temperature fluctuations.

Cobalt is also recognized for its magnetic properties, which play an important role in applications that require magnetic materials, particularly in the electronics and energy sectors. It is also known for its use in cobalt blue pigments, but its primary industrial use lies in metalworking as a strengthening alloy for steel and iron, particularly for improving wear resistance and corrosion resistance.

Benefits of Cobalt-Based Alloys

Cobalt-based alloys offer several mechanical advantages:

• Excellent corrosion resistance, particularly in harsh environments involving acids, sulfides, and high temperatures.
• Wear resistance, making these alloys ideal for parts that experience significant friction or mechanical abrasion.
• Thermal fatigue resistance, ensuring components maintain their structural integrity even under cyclical thermal loading in high-temperature environments.
• Excellent strength, especially at high temperatures, which allows these alloys to be used in critical components in industries like aerospace and power generation.

These properties make cobalt-based alloys a superior choice for components that must maintain durability and performance under extreme conditions.

Cobalt-Based vs. Nickel-Based Alloys: Which Is Better?

While both cobalt- and nickel-based alloys share similarities in heat resistance, corrosion resistance, and mechanical strength, they are often selected based on the specific requirements of the application:

• Cobalt-based alloys offer superior performance at elevated temperatures, with better thermal fatigue resistance and resistance to sulfidation. They are commonly used in environments like gas turbines and aerospace components.
• Nickel-based alloys are more cost-effective and widely used in high-temperature environments where conditions are less extreme. Nickel alloys are typically selected in industries that prioritize cost-efficiency without sacrificing performance.

In high-stress environments, cobalt-based alloys are often the better choice, as they offer better performance at extremely high temperatures compared to nickel-based alloys.

Chemical Composition of Cobalt-Based Alloys

Cobalt-based alloys typically have a base composition of cobalt and chromium, with chromium providing additional strength and corrosion resistance. Tungsten and molybdenum may be added to further enhance the alloy’s strength. Nickel is also a common component in these alloys, contributing to increased temperature resistance.

This unique combination of elements ensures that cobalt-based alloys excel in environments where high wear, extreme temperatures, and corrosion are present.

Applications of Cobalt-Based Alloys

Cobalt-based alloys are used in applications that require outstanding mechanical properties, particularly in extreme conditions. Their exceptional corrosion resistance, high-temperature strength, and wear resistance make them indispensable across several industries:

• Aerospace: Cobalt alloys are ideal for gas turbine blades, where components must endure elevated temperatures and corrosive environments. The ability to resist high thermal stresses and oxidation makes them suitable for turbine engines and jet propulsion systems.
• Oil and Gas: Cobalt alloys are widely used in downhole tools, valves, and other equipment that must operate in highly corrosive environments, such as those exposed to CO₂ injection wells or sour gas. Their resistance to sulfidation and high-temperature strength make them indispensable for these harsh conditions.
• Power Plant Technology: Cobalt-based alloys are often employed in power generation due to their ability to resist heat and wear under high temperatures. They are essential for components like boilers and turbine parts, which experience both abrasion and oxidation.
• Medical: Cobalt-based alloys are frequently used in implants and prosthetics due to their biocompatibility, wear resistance, and magnetic properties. Common applications include hip replacements, dental implants, and bone plates, where the materials must maintain integrity and resist corrosion over time.
• Mechanical Engineering: Components such as bearings, pumps, and valves made from cobalt alloys are vital in mechanical systems where both wear resistance and high-temperature performance are crucial.

Hardening and Wear Resistance with BoroCoat®

Although cobalt alloys are naturally strong, further hardening is often required for parts operating under high stress or exposed to significant wear. BorTec’s BoroCoat® diffusion coatings enhance both hardness and wear resistance, ensuring that components made from cobalt alloys maintain their integrity even in the most demanding conditions.

The BoroCoat® process offers a uniform diffusion layer that enhances not just surface hardness but also corrosion resistance, extending the lifespan of components and reducing maintenance costs. This process can also be applied selectively to parts, treating only the most critical areas and further optimizing performance.