Which phase of set dental amalgam is the weakest and most prone to corrosion?

Correct answer: C. The gamma-2 (tin-mercury) phase

• A. The eta phase (Cu₆Sn₅) forms in HIGH-copper amalgams and is precisely what makes them superior — it is stable and corrosion-resistant.
• B. The gamma phase is the original silver-tin alloy particle and is the strongest phase present.
• C. CORRECT. The GAMMA-2 phase (Sn₇Hg) is the villain of conventional low-copper amalgam. It is the weakest phase, the most corrosion-prone, and its corrosion products drive CREEP — the slow plastic flow under load that produces marginal breakdown, ditching and eventual fracture. Corrosion also liberates free mercury.
• D. The gamma-1 phase is the principal matrix binding the set mass and is considerably more corrosion-resistant than gamma-2.

Note: This is the entire rationale for high-copper amalgam. Copper preferentially binds tin to form the eta phase, so gamma-2 never forms. The result: less creep, less marginal breakdown, better corrosion resistance and greater strength.