Cyanide-free silver-plating bath, method for preparing same, corresponding silver-plating method and uses

Abstract

The present invention relates to a cyanide-free silver-plating bath, characterised in that it consists of an aqueous solution, adjusted to a pH of 1.5 to 2 using citric acid, based on silver iodide and potassium iodide, the silver iodide being present in a proportion of 40 to 60 g per litre of aqueous solution, i.e. 18.4 to 27.6 g / l of Ag0 and the potassium iodide being present in a proportion of 425 to 475 g per litre of aqueous solution, excluding any cyanide.

Description

Cyanide-free silver plating bath, its preparation process, corresponding silver plating process and applications

[0001] The present invention relates to the technical field of conventional wet surface treatments, more precisely to the obtaining of electrolytic silver coatings, in other words to the silver plating technique.

[0002] Silver plating is used in technical and decorative applications.

[0003] Examples of technical applications include the silver plating of: aeronautical parts such as bolts and nuts made of stainless steel coated with a layer of nickel, such parts being subjected to high temperatures, above 400°C and up to 650°C, or even 700°C, the silver plating here facilitating disassembly and especially preventing seizure; stainless steel fuel tank caps, the silver plating here having the effect of allowing the caps to be easily removed; and inner and outer bearing rings, bearings made of a material such as bronze (copper + tin), the silver plating here having the effect of providing excellent resistance to friction, and facilitating disassembly.

[0004] Silver plating also generally has the advantage of improving the electrical conductivity of parts, which is sought after in the case of engine regulation systems.

[0005] In decoration, we can cite the silver plating of goldsmith pieces, for example goldsmith tableware (cutlery, dishes, etc.), pieces of jewelry, clocks, etc.

[0006] Current silver plating baths contain a double cyanide medium of silver and potassium in the form KAg(CN)2.

[0007] The dangers of toxicity and mortality of cyanides are well known, and their use is very strictly regulated by environmental constraints. Thus, potassium cyanide (KCN) has a threshold value to be monitored per workstation, which is the average exposure value (AEL) which is 5 mg / m 3 . Also, an accidental mixture of cyanide and acid inevitably leads to the formation of cyanogen gas and / or hydrocyanic acid in the form of HCN, these being fatal for humans by inhalation from a concentration of 300 mg / m 3 of air.

[0008] Since silver is a very electropositive element (+0.800 V / ENH), direct immersion without current in a silver plating bath of any metal part (copper, steel, brass, etc.) results in a phenomenon of displacement of non-adherent silver. To overcome this phenomenon of displacement, it is customary to use a "pre-silver plating" bath with a very low metallic silver content (for example, around 2-3 g / L instead of 50 g / L for the silver plating bath), which allows a thin film of adherent silver to be obtained, and this is followed by direct immersion, without rinsing with water, in the silver plating bath of the part that has undergone pre-silver plating.

[0009] The inventors therefore studied the possibility of preparing a non-cyanide silvering bath and discovered that the combination of silver iodide and potassium iodide makes it possible to prepare a silvering bath which does not have any of the above disadvantages, namely having much less toxicity than cyanide and not requiring a pre-silvering sequence as is the case for conventional cyanide baths.

[0010] The present invention therefore relates to a cyanide-free silver plating bath, characterized in that it consists of an aqueous solution, adjusted to a pH of 1.5 to 2 using citric acid, based on silver iodide and potassium iodide, the silver iodide being present at a rate of 40 to 60 g per liter of aqueous solution, i.e. 18.4 to 27.6 g / L of Ag 0 and potassium iodide being present at a rate of 425 to 475 g per liter of aqueous solution excluding any cyanide.

[0011] The pH can be adjusted using citric acid, monohydrate, then present in the aqueous solution at a rate of 50 to 70 g per liter of aqueous solution.

[0012] The present invention also relates to a process for preparing a silvering bath as defined above, characterized in that the potassium iodide is dissolved in water, in particular deionized water, with stirring, then the silver iodide is gradually added, with stirring, to the solution thus obtained, then the pH is adjusted from 1.5 to 2. Note that silver iodide is not soluble in water.

[0013] The present invention also relates to a method for silvering a part using the silvering bath as defined above or prepared by the method as defined above, characterized in that the part to be coated with silver, which has not previously been subjected to a pre-silvering operation, is immersed in said silvering bath, said part, acting as a cathode, being connected to a current generator, said bath also containing a silver plate, acting as an anode, which is connected to said current generator, and a current is passed through it until the desired silver coating is obtained.

[0014] In the case where the part to be coated with silver is a part made of stainless steel or a super-alloy based on nickel or cobalt, before silvering, it is advantageous to apply a nickel undercoat to the part by electrolytic Wood's nickel plating.

[0015] In the case where the part to be coated with silver is a part made of aluminum or an aluminum alloy, before silvering, it is advantageous to carry out chemical zinc plating of the said part (chemical zincate process).

[0016] In the case where the part to be coated with silver is a part made of copper or copper alloy (bronze, brass, etc.) before silver plating, it is advantageous to apply a nickel undercoat, in particular of the order of 3 to 5 µm thick.

[0017] The present invention finally relates to the use of the silvering bath as defined above or prepared by the process as defined above for the non-cyanide electrolytic silvering of technical parts such as bolts, nuts, tank covers, bearing rings, bearings.

[0018] The present invention also relates to the use of the silvering bath as defined above or prepared by the process as defined above for the non-cyanide electrolytic silvering of substrates made of steel, stainless steel or a nickel-based alloy, such as bolts made of steel or a nickel-based alloy or bearing rings or jewelry or watch parts.

[0019] The following Examples illustrate the present invention without, however, limiting its scope.

[0020] Example 1 of preparation of a silvering bath according to the invention

[0021] 1L of silver plating bath was prepared using the following protocol: 180g of potassium iodide (KI) was dissolved in 400mL of deionized water (i.e. 450g / L) by adding the potassium iodide to the deionized water while stirring continuously; 50g of silver iodide (AgI) was gradually added while stirring continuously, i.e. [Ag°]=23g / L; 60g of citric acid monohydrate (C6H8O) was dissolved separately in an aliquot solution 7. H2O) in 350 mL of deionized water, while stirring continuously; the aqueous solution of citric acid obtained in (3) was gradually added to the AgI + KI solution obtained in (2) while stirring continuously; the volume was gradually topped up to 1 l (sufficient quantity for a volume of 1 liter – QSP=1 l) with deionized water while stirring continuously; the pH of the bath was checked at 1.7, if necessary by adjusting the pH value obtained either by C6H8O 7.H2O, or by sodium carbonate (Na2CO3).

[0022] The composition of the silvering bath obtained is therefore as follows:

[0023] [AgI]=50 g / L, or [Ag 0 ]=23 g / L

[0024] [KI]=450 g / L

[0025] [C6H8O7.H2O]=60 g / L

[0026] Implementation example 2: electrolytic silver plating of stainless steel in a non-cyanide bath using the silver plating bath according to Example 1 Operating mode

[0027] The following sequence of steps was carried out: 1 Surface preparation

[0028] degreasing using a solvent; alkaline chemical degreasing by dipping and rinsing with water; and electrolytic degreasing and rinsing with water. 2 Anodic sulfuric electrolytic pickling

[0029] current density: 20 A / dm 2 ;duration: 2 min 30 s to 2 min 45 s;thorough rinsing with water 3 Wood Pre-Nickel Plating

[0030] current density: 8 A / dm 2 ;duration: 3 to 4 minutesthorough and vigorous rinsing with water for at least three minutes.

[0031] 4 Silver plating in a silver plating bath according to Example 1 current density: 1 A / dm 2 ; deposition speed 0.68 µm / min; duration: 11 min to 21 min for a coating thickness of 7 to 13 µm; careful water rinsing 5 Drying

[0032] with dry, oil-free compressed air.

[0033] Characterization of the silver coating thus obtained

[0034] (a) Adhesion by crosshatch test according to ISO 2409 and ASTM D3399.

[0035] The coating was gridded with combs with teeth spaced 3 mm apart, adhesive tape (7 daN) was then applied and removed abruptly.

[0036] No peeling or flaking was observed, the coating being classified as class 0.

[0037] (b) Thermal shocks according to ISO 4522-2

[0038] The sample was heated at 250°C for 30 min and then abruptly quenched in demineralized water.

[0039] No peeling or flaking was observed.

[0040] (c) Control of thicknesses deposited on screws according to ISO 1463 standard

[0041] Seven measurements were made along the sample on an X500 micrographic section.

[0042] Thread thicknesses between 7 and 13 micrometers were obtained.

Claims

– Cyanide-free silver plating bath, characterized by the fact that it consists of an aqueous solution, adjusted to a pH of 1.5 to 2 using citric acid, based on silver iodide and potassium iodide, the silver iodide being present at a rate of 40 to 60 g per liter of aqueous solution, or 18.4 to 27.6 g / L of Ag 0 and potassium iodide being present at a rate of 425 to 475 g per litre of aqueous solution excluding any cyanide. - Bath according to claim 1, characterized in that the pH is adjusted using citric acid monohydrate, then present in the aqueous solution in a proportion in particular of 50 to 70 g per liter of aqueous solution. – Process for preparing a silvering bath as defined in one of claims 1 and 2, characterized in that the potassium iodide is dissolved in water, in particular deionized water, with stirring, then the silver iodide is gradually added, with stirring, to the solution thus obtained, then the pH is adjusted from 1.5 to 2. – Method for silvering a part using the silvering bath according to one of claims 1 and 2 or prepared by the method according to claim 3, characterized in that the part to be coated with silver, which has not previously been subjected to a pre-silvering operation, is immersed in said silvering bath, said part, acting as a cathode, being connected to a current generator, said bath also containing a silver plate, acting as an anode, which is connected to said current generator, and a current is passed through it until the desired silver coating is obtained. – Silver plating process according to claim 4, in which the part to be coated with silver is a part made of stainless steel or a superalloy based on nickel or cobalt, characterized in that before silver plating, a nickel underlayer is applied to the part by electrolytic Wood nickel plating. – Silvering process according to claim 5, in which the part to be coated with silver is a part made of aluminum or an aluminum alloy, characterized in that before silvering, chemical zinc plating of said part is carried out. – Silver plating process according to claim 4, in which the part to be coated with silver is a part made of copper or copper alloy, characterized in that before silver plating, a nickel underlayer is applied, in particular of the order of 3 to 5 µm thick. – Use of the silvering bath as defined in one of claims 1 and 2 or prepared by the process as defined in one of claims 3 to 7 for the non-cyanide electrolytic silvering of technical parts such as bolts, nuts, tank covers, bearing rings, bearings.