I. Field of the Invention
This invention generally relates to the use of a barrier coating to prevent the migration of copper in basis metal to overplate.
II. Description of the Prior Art
Jewellery products such as fashion watch bracelets, watch cases, imitation jewellery etc. are often made from copper or copper alloy. These products are often coated with fine gold or gold alloy. It is undesirable to deposit gold directly on copper or copper alloy. When gold is in intimate contact with copper or its alloys, a solid solution of gold-copper is formed. Copper will migrate in gold even at room temperatures. When copper atoms reach the gold top surface they react with atmospheric oxygen and tarnish the attractive appearance of these ornamental products.
To overcome this migration problem, a copper diffusion barrier layer is placed between the basis metal and the top gold coating. A number of high melting metals are known to be effective copper diffusion barriers, such as nickel, cobalt, chromium, tungsten, and molybdenum. Various factors have to be considered on the suitability of a metal to act as an effective diffusion barrier. The adhesive properties of gold overplate on the barrier coating, the capacity to reduce the thickness of the gold coating, volatility of the barrier coating on further treatments under high temperature and high vacuum; and its corrosion resisting properties are important elements for consideration of its suitability.
When these jewellery products are in prolonged contact with human skin during use, human perspiration penetrates the gold overplate and may corrode the diffusion barrier underneath to release metal ions as corrosion products. These corrosion products mix with the perspiration and may irritate the skin. The allergic effects of several metals used in plating has been studied and published. These results have been used as one of the references for the selection of diffusion barriers (see for example, xe2x80x9cReinst-Palladium als Ersatz fur Palladium/Nickel. Einsatz. fur Endschichten und als Diffusionssperrexe2x80x9d, K.-P. Beck, Glavanotcchnik, vol. 47, no. 1; 1993; pp.20).
Nickel has been widely used as the barrier coating for engineering applications. Until the implementation of the EEC Directive 76/769/EEC in 1994 controlling the. use of nickel in consumer articles and the liberation of nickel(II) ions, it was also used as the barrier coating for jewellery products intended for prolonged contact with the skin (see, for example, xe2x80x9cControl of nickel emission in jewellery and related itemsxe2x80x9d, R. V. Green and J. F. Sargent, Transactions of the Institute of Metal Finishing, vol. 75, no. 3; 1997; p. B51-52). The above mentioned Directive stipulates that metal objects with the intent for prolonged contact with human skin and made of nickel-containing alloys or coated with nickel-containing substances, should not release nickel(II) in excess of 0.5 xcexcg/cm2/week. The specifications for monitoring the release rate are documented in the standards, EN1811 and EN12471 adopted by the European Committee for Standardization (CEN) in late 1999.
Various barrier coatings have been suggested to replace nickel including chromium and palladium. Palladium is used to overcome the allergy problem but the cost of the metal can be prohibitive. Copper-tin alloy is another alternative but its internal stress properties and relatively high vapor pressure pose a lot of production problems.
The invention provides a barrier layer of cobalt-molybdenum-phosphorus, CoMoP.
The invention further provides a process for electrolytic depositing a cobalt-molybdenum-phosphorus alloy, which possesses excellent barrier property for copper. The use of this alloy as a barrier coating on copper or copper alloy base metal significantly inhibits the diffusion of copper atoms into the gold overplate at moderate temperatures over a long period of time and at high temperatures over a shorter period of time. It is a corrosion resistant alloy which can withstand corrosion on prolonged contact with the human skin.
The presence of cobalt in the alloy contributes to the inhibition of diffusion of copper. The presence of molybdenum in the alloy inhibits the corrosion of cobalt. It forms a passive film on the surface in the presence of an oxidizing agent such as oxygen in air, and protects the alloy.
The oxy salts of molybdenum are a potential replacement for chromium, especially the hexavalent ion function of chromium (see for example, xe2x80x9cMolybdenumxe2x80x94A Corrosion Inhibitorxe2x80x9d, E. Groshart, Metal Finishing, January 1989; p53-54). Molybdenum cannot be deposited in aqueous solution because of its high overpotentials. However, it can be electrolytic and electrolessly deposited with iron groups elements.
The presence of phosphorus favors the formation of amorphous characteristics in the alloy. It reduces grain boundaries and inhibits corrosion. The introduction of phosphorus thus enhances diffusion barrier properties.