Barrier Effect of Nickel on Tarnish of Gold-Plated Copper

OBJECTIVES: Gold-plating is currently used for orthodontic wires and brackets on a limited basis to provide an alternative for patients with known nickel allergies, and in the process of electroforming gold crowns. In an effort to better understand the effect that tarnish has on plated metal substrates, this study had two components: 1) Evaluating the tarnish resistance of gold-plated copper (Group 1) and gold over nickel-plated copper (Group 2) samples to study the influence of nickel as a barrier layer in the tarnish process, and 2) To test for nickel release through a layer of immersion type gold-plating. METHODS: In the first part of the study 30 copper samples were nickel-plated using a conventional electroplating machine. These samples were then gold-plated using a new electrode-less immersion plating technique. In the second part of the study a standard tarnish test was carried out on the 30 samples in each group. This consisted of making baseline colorimetric measurements on the samples, and then subjecting them to tarnish in a 3.0% sodium sulfide solution for 10 weeks. RESULTS: There was no nickel release detected in any of the 30 gold over nickel-plated copper samples tested with the ammoniacal dimethylglyoxime (Merck & Co., Inc, Rahway, NJ). The independent t-test showed a significant difference between the two groups following tarnish (p < 0.0001). The rank sum test also showed a significant difference between the two groups (p < 0.05) for both the examiners with the samples in Group 1 showing a greater degree of tarnish than Group 2. CONCLUSIONS: 1) Nickel intermediate coating provided an adequate barrier plate by improving the tarnish resistance of copper based samples, 2) The plating thickness achieved with immersion type gold-plating (Oromerse SO; Technic Inc ®, Rhode Island) was sufficient to prevent nickel release from underlying copper substrates.