Tungsten cemented carbide balls use carbides (such as WC, TiC) as hard phase and metal binders (such as Co, Ni) as bonding phase. Their corrosion resistance mechanism comes from the chemical inertness of the hard phase, the optimization of the corrosion resistance of the bonding phase and the synergistic inhibition.
I. Chemical Inertness of the Hard Phase: Corrosion Barrier
WC particles possess extremely high chemical stability and virtually no reaction in most corrosive environments (such as acidic, neutral, and weakly alkaline media). They form a dense physical barrier, effectively preventing the penetration of corrosive media into the material.
II. Optimizing the Corrosion Resistance of the Binder Phase: From Defects to Protective Layer
Traditional cobalt-based binders (Co) are a weakness in cemented carbide corrosion resistance. They readily react with acids, bases, and coolant additives, leading to surface cobalt leaching and microstructural damage. Modern processes optimize the binder phase through the following methods:
1. Cobalt-Based Binder Replacement:
Nickel-Based (Ni): Offers significantly better corrosion resistance than cobalt in acidic environments.
Nickel-Aluminum-Based (Ni?Al): Excellent performance in high-temperature oxidizing environments by forming a dense protective layer of aluminum oxide (Al?O?).
2. Cobalt Content Control:
Reducing the cobalt content (e.g., from 10% to 6%) can reduce the proportion of corrosion-sensitive phases. At the same time, increasing material density through grain refinement further hinders the penetration of corrosive media.
III. Synergistic Inhibition: Complementarity Between the Hard Phase and the Binder Phase
The synergistic effect of the hard phase and the binder phase can significantly improve overall corrosion resistance:
1. Electrochemical Synergy:
In WC-Co alloys, cobalt has a lower corrosion potential than WC and corrodes preferentially. However, when the cobalt phase completely dissolves, the WC phase loses its cathodic protection, potentially accelerating corrosion. By optimizing the binder phase composition (such as adding Cr and Al to form a solid solution), the corrosion resistance of the cobalt phase can be improved, its dissolution rate can be slowed, and the protection period of the WC phase can be extended.
2. Microstructure Optimization:
Uniformly distributed fine carbide particles can reduce the continuity of the binder phase and lower the corrosion current density.
IV. Surface Modification Technology: Active Defense Layer
Corrosion resistance can be further enhanced through surface treatment:
1. Physical Vapor Deposition (PVD):
Deposition of hard coatings such as TiN and CrN can significantly reduce the corrosion rate.
2. Chemical Vapor Deposition (CVD):
Forming Al?O? or SiC coatings suitable for high-temperature oxidizing environments.
3. Nitriding:
Producing a nitride layer (such as Co?N and WN) on the material surface improves corrosion resistance.
