Dating silver plate
Soldering gold-plated parts can be problematic as gold is soluble in solder.
Solder which contains more than 4-5% gold can become brittle. Gold reacts with both tin and lead in their liquid state, forming brittle intermetallics.
When eutectic 63% tin – 37% lead solder is used, no lead-gold compounds are formed, because gold preferentially reacts with tin, forming the disperse in the solder matrix, forming preferential cleavage planes, significantly lowering the mechanical strength and therefore reliability of the resulting solder joints.
If the gold layer does not completely dissolve into the solder, then slow intermetallic reactions can proceed in the solid state as the tin and gold atoms cross-migrate.
Impurities in the nickel layer can prevent the solder from bonding to it. Nickel with more than 8% phosphorus is not solderable.
Electrodeposited nickel may contain nickel hydroxide. An acid bath is required to remove the passivation layer before applying the gold layer; improper cleaning leads to a nickel surface difficult to solder.
Alloys of copper with gold and some silver were employed, known as tumbaga.
The ongoing intermetallic reactions also cause Kirkendall effect, leading to mechanical failure of the joint, similar to the degradation of gold-aluminium bonds known as purple plague.
The layer of nickel provides mechanical backing for the gold layer, improving its wear resistance.