Method for silver plating

The present invention relates to a method of non-cyanide silver electroplating and in particular, to a silver plating method which provides good adherence onto the so-called difficult to plate metals by utilizing an acidic plating bath.

Silver has industrially applicable properties such as good electroconductivity along with good solderability, and is thus widely utilized for manufacturing of electrical parts. Furthermore, its aesthetic appeal has been widely appreciated for decorative purposes. Moreover, owing to its ductility, it can be utilized to prevent seizure of screws or to improve lubricity and wear/abrasion resistance of sliding parts.

To prevent seizure of screws, or to improve lubricity and abrasion/wear resistance of sliding parts, silver plating is often required onto so called difficult-to-plate metallic substrates. Such substrates are prone to form an oxide layer on their surfaces, or to cause displacement deposition, or the combination of the both. As these factors leads to poor adhesion, they are referred to as difficult-to-plate metallic substrates. Such substrates include aluminum (alloys), magnesium (alloys), tin (alloys), stainless-steel, titanium (alloys) and die cast zinc alloys.

As a method for plating onto the difficult-to-plate metallic substrates with good adherence, nickel strike plating is commonly utilized. The method comprises, immediately after removing an oxide layer from a substrate with an acid, forming a very thin nickel layer on the substrate using a nickel plating solution which contains a large amount of chloride, and then forming a desired plated layer thereon.

The method utilizing nickel strike plating has been widely practiced as general procedure with some modifications over the years.

For instance, Japanese unexamined patent publication No. 2005-133169 discloses a method for producing a silver-plated stainless steel strip for moving contacts comprising forming a nickel underlayer onto a stainless steel base metal surface, and then forming a silver- or silver alloy-plated layer thereon,

Furthermore, Japanese unexamined patent publication No. 2002-237312 discloses a method for producing a silver or silver-alloy plated metal separator for solid electrolyte batteries comprising silver- or silver alloy-plating onto a stainless steel having a nickel plated layer as an underlayer.

However, elimination of nickel-strike plating step affords reduction in manufacturing cost. Furthermore, for optimization of the favorable properties of a silver plated layer, the presence of nickel underlayer is sometimes not preferable. For that purpose, Japanese unexamined patent publication No. 2002-121693 discloses a method and bath for silver plating onto stainless steel without nickel strike plating, the method utilizing a plating bath containing halide ion with a pH value of −1.0˜2.0.

It has been known that fair adherence can be achieved without a presence of nickel underlayer by utilizing a silver plating bath which contains high concentration of halide ion, but industrial application of such bath entails some problems. As an iodide bath is commonly utilized as the halide bath, the problems will be hereafter explained specifically in connection with the iodide bath.

(1) With high concentration of halide ion, silver forms complex which dissolves easily whereas with low halogen concentration, it exhibits poor solubility and such property of silver halide is utilized for detection of residual halogen. Since precipitation of silver is generated in a low-concentration iodide bath, use of a high-concentration iodide bath is preferable but doing so may create problems such as corrosion of equipment or wastewater management.

(2) Moreover, in a rinse process, because the plating solution attached on the surface of the object to be plated is diluted with rinse water and the concentration of iodide ion becomes low, precipitation of silver iodide, which is very difficult to remove, is formed on the surface.

(3) Silver-plated layer deposited from an iodide bath exhibits low ductility and it is prone to cracking.

(4) In an iodide bath, iodine is generated through the oxidization of iodide ion on the anode and that has, even if slightly generated, negative effect on the plated layer which causes poor adherence and coarsening. Furthermore, when significant amount of iodine is generated, iodine may be released as a toxic gas. In particular, since the generation of toxic gas becomes significant when an insoluble anode is used, use of the insoluble anode becomes unacceptable.

To optimize the properties of silver-plated layer, there has been a demand for a method of silver plating that does not comprise the step of forming a nickel plated layer between the difficult-to-plate substrate and the silver plated layer. However, there is much to be improved to meet industrial requirements.

Thus the object of the present invention is to provide a method for silver plating that does not comprise the step of forming the nickel-underlayer and that can form a silver-plated layer having sufficient adherence directly on the difficult-to-plate substrates with the use of a halide-free plating bath under a good working environment.

The present inventors have found that a fine silver-plated layer with good adherence can be formed onto the difficult-to-plate substrate by utilizing a silver plating method that can prevent formation of an oxide layer, the method comprising silver plating onto the substrate using an acidic silver plating bath which contains phosphines as a complexing agent and substantially does not contain halide ion, preceded by degreasing the substrate and then removing the oxide layer formed on the substrate surface from the substrate with a strongly acidic solution.

Accordingly, in one aspect, the present invention is a silver plating method onto a substrate on which an oxide layer that inhibits adherence of a plated layer is prone to form, comprising at least the following steps of;

(A) degreasing the substrate,

(B) removing the oxide layer with a strongly acidic solution from the substrate, and(

(C) silver plating onto the substrate, without a step of nickel or nickel alloy strike plating in advance, utilizing a phosphines-containing acidic silver plating bath which essentially does not contain halide ion or cyanide ion.