The phase of dental amalgam alloy that is most prone to corrosion during clinical service is the Gamma-2 (Sn-Hg) phase.

Dental amalgam is composed of several phases formed after the setting reaction between the silver-tin alloy powder and mercury.
These phases include:
- Gamma (Ag-Sn) phase: This is the original alloy phase responsible for the strength of amalgam.
- Gamma-1 (Ag-Hg) phase: This phase forms after mercury reacts with silver, providing strength and corrosion resistance.
- Gamma-2 (Sn-Hg) phase: This phase results from the reaction between tin and mercury.
- Copper phase: Present in high-copper amalgams to improve properties such as strength and corrosion resistance.

- Among these, the Gamma-2 (Sn-Hg) phase is the least corrosion-resistant and is most prone to degradation in the oral environment.
- This susceptibility to corrosion leads to a breakdown of the amalgam and can result in marginal breakdown, discoloration, and the release of corrosion products.
- The presence of high copper content alloys reduces or eliminates the Gamma-2 phase, which significantly improves the longevity and biocompatibility of dental amalgam restorations.

Key Points:
- Gamma-2 phase (Sn-Hg) is highly prone to corrosion.
- Corrosion of this phase weakens the restoration.
- High copper amalgams reduce Gamma-2 phase formation.
- Improving corrosion resistance enhances clinical longevity of amalgam.

Reference: Sturdevant's Art and Science of Operative Dentistry, 7th Edition, Chapter on Dental Amalgam, Pages 150-155