Abstract
The comparison of electrolytic characteristics of gold plating and corrosion resistance of the deposits was studied between cyanide and citrate baths under various conditions.
The following results were obtained.
(1) The optimum conditions for good adhesion of gold in cyanide striking bath were as follows:
KAu(CN)2……more than 3g/l Bath temperature……lower than 50°C
Free KCN…… more than 60g/l Dk……less than 0.75 Amp./dm2
K2CO3…… more than 30g/l Plating time less than 20sec.
(2) The optimum conditions for good brightness, good color tone, and high current density in cyanide bath were as follows:
KAu(CN)2……5g/l pH……12
Free KCN……50g/l Bath temperature……60°C
K2CO3……30g/l Dk……0.1Amp./dm2
K2HPO4……20g/l Violent stirring
The above optimum conditions in citrate bath were as follows:
KAu(CN)2……10g/l pH……4.5
Na citrate……50g/l Bath temperature……60°C
Citric acid……30g/l Dk……0.5Amp./dm2
KH2PO4……140g/l Violent, stirring
Free KCN……1.5g/l
(3) Throwing power of deposits from cyanide bath was superior to that from citrate bath. High power was obtained at 50g/l of free cyanide in cyanide bath and at pH of 4-5 in citrate bath.
(4) Porosity of deposits from citrate bath was far lower than that from cyanide bath. It was increased with the increase of pH or free cyanide.
(5) Corrosion resistance depended upon porosity. Rusting rate of deposits from citrate bath was lower than that from cyanide bath by the results of salt spray and nitric acid fume tests. The results of salt spray tests showed that the rate was 50% or more in the deposits from cyanide bath and 50% or less in those from citrate bath.
(6) Heat resistance did not depend upon the porosity, but depended upon the discoloration due to diffusion of the substrate metal. Gold deposits except for on nickel substrate were discolored by heating at above 300°C.
