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Dealloying of gold-nickel braze alloy in nitrogen tetroxide
Gold-Nickel braze alloy is not compatible with nitrogen tetroxide, N2O4. This incompatibility manifests itself by dealloying the nickel resulting in a gold sponge and the formation of nickel nitrates, Ni(N03)2. The miscibility gap on the gold-nickel phase diagram means that any alloy composition between 8 and 98 atomic percent nickel when slowly cooled will form a two phase alloy system. Rapid cooling or quenching from a temperature above the miscibility gap for any alloy composition will form a supersaturated single phase alloy. The thermodynamics of the gold-nickel system predicts a two phase alloy because the free-energy of gold-gold, and nickel-nickel bonds are lower than the gold-nickel bond when the system is slowly cooled. When the system is rapidly cooled the nucleation and growth of the two phase alloy is retarded and a single phase alloy is formed. A corrosion test was conducted using the gold-nickel braze alloy cooled at different rates which formed both a single phase alloy, and a two phase alloy. The corrosion weight loss in N2o4 of the single phase alloy was 0.17%, and for the two phase alloy the corrosion weight loss in N2o4 was 0.74%. The two phase alloy showed five times the percentage weight loss of the single phase alloy. It is, the conclusion of this investigation that the quenched, single phase, gold-nickel alloy has a greater corrosion resistance in the N2o4 environment than the slowly cooled, two phase, goldnickel alloy. This is caused by the nickel lattice, in the two phase alloy system, being severely attacked in the N2o4 environment.