A process for plating a coating of a noble metal onto a non-noble metal substrate material, said process comprising the steps of:
- a) preparing a starter aqueous plating solution containing an amount of free ions of a noble metal to be plated out onto said substrate material, said noble metal being selected from the group consisting of silver, gold, platinum, palladium, iridium, rhodium, ruthenium and osmium, such that said amount of free ions of said noble metal is sufficient to plate an amount of an active, non-noble metal substrate material, in the form of a powder, with a coating of from 2 to 60 weight percent, based on the total weiqht of a final noble metal-coated active non-noble metal substrate material product, and such that said amount of free ions of said noble metal plated onto said active non-noble metal substrate material is sufficient to at least provide a coating on the surface of said active non-noble metal substrate material that completely covers the entire surface of said active non-noble metal substrate material, leaving no exposed surface of said active non-noble metal substrate material, said non-noble metal substrate material being selected from the group consisting of copper, nickel, aluminum, titanium, zirconium, vanadium, hafnium, cadmium, niobium, tantalum, molybdenum, tungsten, gallium, indium, thallium, and combinations of a first one of said non-noble metal substrate materials seeded with atoms of a second one of said non-noble metal substrate materials having a greater affinity than that of said first non-noble metal substrate material for said selected noble metal to be plated thereon;
- b) dividing said starter plating solution, prepared in step (a), into a plurality of portions, such that each portion of said starter plating solution contains a percentage amount of from less than about 1% to about 85% by weight, of the total amount of free ions of noble metal to be plated out that are contained in said starter plating solution, the percentage amount that is present in any said portion being the same as or different from the percentage amount present in other of said portions;
- c) preparing a plurality of individual plating solution baths into which said active non-noble metal substrate material is immersible, by selecting a concentration of free ions of noble metal for each said plating solution bath which is to be made from a corresponding one of said plurality of portions of said starter plating solution, prepared in step (b), said concentration of free ions of noble metal for each said individual plating solution bath being in the range of from about 0.3 to about 65 grams of free ions of noble metal per liter of plating solution bath, and making each said individual plating solution bath by adding water to each corresponding one of said plurality of portions of said starter plating solution, to increase the volume thereof, such that each one of said plurality of individual plating solution baths has the concentration of free ions of noble metal, as selected above therefor;
- d) immersing an amount of an active non-noble metal substrate material to be plated into one of said individual plating solution baths, prepared in step (c), which is maintained at a temperature in the range of from about 20.degree. C. to about 100.degree. C., such that the temperature of said individual plating solution bath is inversely related to the oxidation potential of the non-noble metal substrate material being plated, to cause said free ions of noble metal to plate-out onto said substrate material until said one of said individual plating solution baths is depleted of all but a trace amount of said free noble metal ions contained therein, thereby forming an intermediate plated substrate material on which is plated the fraction of free ions of noble metal contained in said one of said individual plating solution baths;
- e) separating said intermediate plated substrate material, prepared in step (d), from the depleted plating solution bath;
- f) rinsing said intermediate plated substrate material, separated in step (e), at least once with a first series of water rinses;
- g) repeating steps (d), (e), and (f) with the rinsed intermediate plated substrate material resulting from each previous sequence of steps (d), (e), and (f), and another one of said individual plating solution baths, until all of said individual plating solution baths prepared according to step (c) have been utilized, the sequence of utilization of said individual plating solution baths being such that when the concentration of free noble-metal ions in at least two of said individual plating solution baths is different, said individual plating solution baths are successively utilized in the order of decreasing concentration of free ions of noble metal therein, and further such that the temperature of each successively utilized individual plating solution bath is at least as high as the temperature of the preceding individual plating solution bath, thereby forming further intermediate plated substrate materials with each repetition of the sequence of steps (d), (e) and (f), such that each successive intermediate plated substrate material is cumulatively plated with the amounts of free ions of noble metal contained in each of the plating solution baths into which the intermediate plated substrate material has been immersed, thereby ultimately forming a final plated substrate material, onto which has been plated the total said amount of free ions of noble metal in said original starter plating solution;
- h) rinsing said final plated substrate material, prepared in step (g), at least once with a second series of rinses, including rinses with water, an acid, and an alcohol;
- i) further rinsing said final plated substrate material, as rinsed according to step (h), at least once with a third series of rinses, including rinses with water, and an alcohol;
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- j) drying said final plated substrate material, as rinsed accordinq to step (i), to produce a final noble metal-coated active non-noble metal substrate material product.