EP0570033B1 - Process for electroplating tin or tin alloy on a metal workpiece - Google Patents

Abstract

Process for electroplating, with high current densities of between 100 and 400 A/dm<2>, preferably between 200 and 300 A/dm<2>, in an electrolysis bath, of tin and/or tin alloys especially of Sn-Pb, Sn-Co and Sn-Ni alloys, on a metal substrate such as a steel strip, according to which an electrolysis bath is employed containing at least one halide, such as chloride, bromide or iodide, of the metal to be deposited on the metal substrate, and stannous and stannic ions, employing an electrolysis bath in which the content of metal ions is maintained between 60 and 150 g/l and is preferably of the order of 115 to 125 g/l in the case of tin plating and of the order of 135 to 145 g/l for the deposition of tin alloys, and in that the electrolyte is circulated at a velocity, relative to the metal substrate, of between 2 and 9 m/s and preferably between 3 and 6 m/s.

Description

The present invention relates to a method of electroplating at high current densities as defined in the preamble of claim 1.

Document EP-A-0357839 describes such a tinning process in an electrolysis bath based on stannic chloride and stannous chloride. This process has the drawback of releasing harmful gases into the atmosphere.

In the extract from "Chemical Abstracts" Vol. 88, No. 2, January 9, 1972, Columbus Ohio U.S. Abstract No. 13625z, SHINDO, "rapid dissolution of tin in a tin electroplating bath" p. 423 and JP, A, 7789536 (Nippon Steel) 27/7/1977, mention is made of such a process, which can however only be applied on the basis of current density.

compris entre 0 et 1.The process according to the invention is characterized in that an electrolysis bath is used containing a metal bromide in a ratio $${\mathit{\text{R}}}_{\text{2}}\text{=}\frac{\mathit{\text{Br}}}{{\mathit{\text{B}}}_{\mathit{\text{r}}}\text{+}\mathit{\text{I}}\text{+}\mathit{\text{Cl}}}$$

between 0 and 1.

More particularly, in the case where the aforementioned support is formed from a steel strip, the latter acts as a cathode which is displaced opposite an insoluble anode at a distance of between 0.75 and 1.5 cm while circulating the electrolyte between this cathode and the anode in the form of a substantially turbulent flow.

According to a preferred embodiment of the invention, the electrolysis bath is regenerated by bringing it into contact with granules of the metal to be deposited, so as to thus constantly maintain a bath sufficiently rich in metal ions to deposit on the aforementioned support.

Other details and particularities of the invention will emerge from the description given below by way of nonlimiting example of some particular embodiments of the invention.

In general, the method according to the invention consists in carrying out an electrolytic deposition of tin and / or tin alloys, preferably Sn-Pb, Sn-Co or Sn-Ni alloys on a support. metallic and in particular on one or two sides of a steel strip which moves in an electrolysis bath, in which the metal halide is formed by bromide, opposite an insoluble anode and which preferably makes cathode office while the electrolyte is circulated between this cathode and the anode at a sufficiently high speed to obtain a substantially turbulent flow.

This speed is generally between 1 and 9 m / s and preferably between 3 and 6 m / s, an ideal speed being of the order of 5 m / s.

It is also a high current density electrolysis, this density advantageously being between 100 and 400 A / dm ² and preferably between 200 and 300 A / dm ² .

In addition, preference is given to the use of an electrolysis cell in which the distribution of cathodic current over the part of the steel strip moving in the electrolysis zone takes place in such a uniform manner as possible in order to thus creating a high diffusion limit current density which is substantially equal at each location in this part of the strip.

Furthermore, preference is also given to an electrolysis bath in which the metal halide is formed by bromide.

The high current density implies relatively high concentrations of metal cations. Thus, satisfactory results have been obtained by using, according to the invention, an electrolytic bath, the metal ion content of which is maintained between 60 and 150 g / l, preferably around 115 to 125 g / l for tinning, and around 135 to 145 g / l for depositing tin alloys.

In the case of the deposit of tin alloy, the content of the other metal or metals of this alloy is generally maintained between 10 and 80 g / l.

Particularly for an Sn-Pb alloy, the lead content of the electrolysis bath is advantageously maintained between 20 and 30 g / l, and is preferably of the order of 25 g / l.

For an Sn-Co alloy, the cobalt content of the electrolysis bath is advantageously between 45 and 55 g / l and is preferably of the order of 50 g / l.

It has, in fact, been observed that if the total content of metal ions is less than 60 g / l, the cathodic yield risks decreasing too strongly and the deposits obtained could be pulverulent at high current densities.

If, on the contrary, a metal ion concentration greater than 150 g / l is maintained, the losses due to the entrainment of the electrolyte by the steel strip to be covered may be too great.

It has been found that excellent results are obtained, in particular as regards the cathodic yields and the structure of the deposits at current densities of 100 to 400 A / dm ² and more particularly between 200 and 300 A / dm ² , if one combines either high contents of dissolved metal cations in the bath and low speeds of circulation of the electrolyte, or low contents of dissolved metal cations in the bath and high speeds of circulation of the electrolyte. It is therefore necessary to combine, between the limits given above, the upper values of the contents of metal cations with the lower values of the relative speeds of circulation of the electrolyte, and vice versa.

These various possible combinations are defined by the value of the product Cv, ^0.8 , with C the concentration of dissolved metal ions in the bath (in g / L) and v the relative speed between the electrolyte and the metal support (in m / s). For current densities between 200 and 300 A / dm ² , the value of this product is preferably equal to 350.

The ratio R ₁ = Sn ⁴⁺ / (Sn ⁴⁺ + Sn ²⁺ ) in the electrolysis bath is advantageously maintained between 0.1 and 0.95. The exact value of this ratio is chosen according to the nature of the criterion that we want to optimize. Thus the specific energy consumption is minimal for R = 0.1.

With regard to the pH of the electrolysis bath, to avoid hydrolysis of the metal ions, the pH is advantageously maintained between -0.2 and 0.3 and is preferably of the order of 0.

At pH values too close to -0.2 the risks of seeing the cathodic deposition yields drop are not negligible. On the other hand, at pH values too close to 0.3 and for tin contents approaching 120 g / l, the risks of precipitation are not negligible.

The temperature of the electrolysis bath is advantageously maintained between 30 and 50 ° C. and is preferably of the order of 40 ° C.

At temperatures above 50 ° C, the activity of stannic tin on electrodeposited metallic tin is such that the electrodeposition yields decrease. This effect can be compensated for provided that cathodic protections, known per se, are placed at the exit of the cell.

At temperatures below 30 ° C, the conductivity of the electrolyte could become too low, which would increase the voltage between the anode and the cathode and therefore the specific energy consumption.

• les sels métalliques doivent être fortement solubles
• la réaction d'oxydation de l'étain stanneux à l'anode de la cellule d'étamage se produit avec un mauvais rendement (de l'ordre de 5 %). Pour pallier cet inconvénient, le contre-ion choisi dit pouvoir s'oxyder aisément à l'anode et son produit d'oxydation doit pouvoir oxyder très rapidement l'étain stanneux en étain stannique.

The counterion accompanying the cations in the electrolysis bath must meet two independent criteria, namely:

• metal salts must be highly soluble
• the oxidation reaction of stannous tin at the anode of the tinning cell occurs with poor efficiency (of the order of 5%). To overcome this drawback, the chosen counterion is said to be able to oxidize easily at the anode and its oxidation product must be able to oxidize stannous tin to stannic tin very quickly.

The bromide, chloride and iodide ions meet this double criterion.

avec Y = Br et/ou Cl et/ou I.The reaction scheme becomes:

with Y = Br and / or Cl and / or I.

As already indicated above, according to the invention, bromide is preferably used which makes it possible to avoid the release of harmful gases into the atmosphere.

For a halide mixture of molarity advantageously between 3M and 5M and for a current density between 50 and 400 A / dm ² , the yield of reaction 1 is 100%.

The concentration of halide Y ^- in the bath is preferably of the order of 4M.

The ratio R2 = Br / (Br + I + Cl) is between 0 and 1.

This ratio is preferably of the order of 1 if it is desired to avoid any risk of release of chlorine and precipitation of stannous iodide.

It is preferably of the order of 0 if it is desired to avoid any risk of corrosion and destruction of the anodes which are preferably made of Ti / RuO2 or Ti / IrO ₂ .

The fact of using such an anode makes it possible to generate the bromine Br2 at high current density with yields of the order of 100%.

The anodic potential of this electrode, during its polarization in the tinning cell, should preferably be maintained between 1.5 and 2.5 volts relative to the normal hydrogen electrode (ENH) and is preferably of the order of 2 volts.

It has been observed that if the anode potential is less than 1.5 volts relative to the above-mentioned normal hydrogen electrode, the large current densities which are generally not attained. required for the tinning process according to the invention at high current density.

If the anode potential is greater than 2 volts compared to this normal hydrogen electrode, the titanium of the anode may corrode after a few minutes in the case where Y ^- = Br ^- .

The invention also relates to a particular process for regenerating a tin plating bath and tin alloys, in particular Sn-Pb, Sn-Co and Sn-Ni alloys, rich in stannic ions by the bringing the electrolysis bath in which this electrodeposition takes place into contact with tin, lead, cobalt and nickel granules of moderate specific surfaces.

In this regard, it has been found that economically and industrially valid results have been obtained with granules having a specific surface of between 5 and 0.25 m ² / kg of metal. The stannic ions consumed during the dissolution of the above granules are continuously regenerated at the anode of the electrodeposition cell and thus make it possible to maintain the concentration of these ions in the electrolysis bath, during electrodeposition, substantially at level as defined above.

The regeneration reaction is carried out in a regeneration reactor known per se according to the following reaction scheme: $$\text{Sn4 + + Sn → 2 Sn2 +}$$

The tinning bath is therefore a mixture of Sn4 + and Sn2 + ions, i.e. a mixture of the regeneration reagent Sn4 + and the regeneration product Sn2 +.

In the case of alloy deposition, granules of Pb and / or Ni and / or Co are added to the regeneration reactor.

avec X = Co, Ni, Pb.The dissolution of these elements is carried out according to the reaction scheme: $$\text{Sn4 + + X → Sn2 + + X2 +}$$

with X = Co, Ni, Pb.

à la cathode $${\text{Z}}_{\text{1}}\text{(Sn4+ + 4e → Sn)}$$

$${\text{Z}}_{\text{2}}\text{(2 Sn2+ + 4e → 2 Sn)}$$

Z₁ représente la fraction du courant cathodique servant à réduire l'étain stannique en étain métallique et Z₂ la fraction de courant cathodique servant à réduire l'étain stanneux en étain stannique.The main reactions in the tinning cell are: at the anode $$\text{2 Sn2 + → 2 Sn4 + + 4th-}$$

at the cathode $${\text{Z}}_{\text{1}}\text{(Sn4 + + 4th → Sn)}$$

$${\text{Z}}_{\text{2}}\text{(2 Sn2 + + 4th → 2 Sn)}$$

Z ₁ represents the fraction of the cathodic current used to reduce stannic tin to metallic tin and Z ₂ the fraction of cathodic current used to reduce stannous tin to stannic tin.

$${\text{Z}}_{\text{2}}\text{(2 Sn2+ + 4e → 2 Sn)}$$

$${\text{Z}}_{\text{3}}\text{(2 x2+ + 4e →2 X)}$$

avec Z₁ + Z₂ + Z₃ = 1.In the case of alloy deposition, the cathodic reactions are: $${\text{Z}}_{\text{1}}\text{(Sn4 + + 4th → Sn)}$$

$${\text{Z}}_{\text{2}}\text{(2 Sn2 + + 4th → 2 Sn)}$$

$${\text{Z}}_{\text{3}}\text{(2 x2 + + 4th → 2 X)}$$

with Z ₁ + Z ₂ + Z ₃ = 1.

The overall cathodic reaction (reaction 6 + reaction 7 or reaction 6 + reaction 7 + reaction 8) added to the anodic reaction (reaction 5) perfectly balances the regeneration reaction (reaction 4a or reaction 4a + reaction 4b), which allows the system consisting of the tinning cell and the regeneration reactor to operate without the addition of external reagents other than tin, nickel, cobalt and lead, all in metallic form.

As already indicated above, the concentration in the electrolyte of metal ions Sn4 + + Sn2 + + X2 + is maintained between 60 and 150 g / l and is preferably of the order of 120 g / l for tinning and 140 g / l for depositing alloys.

Regarding the ratio R ₁ = Sn4 + / (Sn4 + + Sn2 +), the regeneration speed is maximum for R ₁ = 0.95.

More particularly, in the context of optimizing the regeneration speed of the tin, it has been found that the ideal value of R is of the order of 0.92.

Furthermore, it has also been found that if the temperature of the electrolysis bath is below 30 ° C. the speed of regeneration generally becomes low.

Below are now given examples of concrete embodiments of the method according to the invention.

Example 1

This is an example of tinning in which the electrolysis bath consisted of a solution of 0.9 M in stannic bromide and 0.1 M in stannous bromide.

The electrolyte was prepared by the action of hydrobromic acid at 47% P.A. and hydrogen peroxide at 30% P.A. on granules of pure metallic tin.

During tinning, the Br2 electrogeneration yield at 300 A / dm ² and 40 ° C was 100% on the Ti / RuO ₂ anode.

The conductivity of the electrolytic bath was of the order of 0.4 S / cm and the voltage U A / C across the terminals of the cell was of the order of 10 volts.

The specific energy consumption was around 9 KWhr / kg.

The regeneration rate was of the order of 27.5 A / dm ² at 40 ° C and 55 A / dm ² at 60 ° C (respectively 600 and 1200 mg / cm ² .hr.).

For spherical metallic tin granules with a diameter d = 25 mm, the volume of the regeneration reactor was 200 l at 40 ° C and 110 l at 60 ° C for an hourly consumption of tin of the order from 220 kg.

Example 2

In this example, the electrolytes were prepared by dissolving metal halides to be deposited in an Sn4 + / Sn2 + electrolyte prepared as in Example 1.

The regeneration reaction was carried out by the action of the electrolyte thus prepared on granules of tin, lead, nickel and cobalt.

Finally, below is given a table in which are grouped the ideal conditions of deposition and regeneration according to the invention. Board Setting Field "Ideal" tinning "Ideal" Sn / Pb alloy "Ideal" Sn / Ni alloy "Ideal" Sn / Co alloy Flow velocity relative to the web (m / s) 1 - 9 5 5 5 5 Temperature (° C) 30 - 50 40 40 40 40 Sn4 + (g / L) 10 - 140 110 60 40 60 Sn2 + (g / L) 10 - 140 10 55 50 55 X2 + (g / L) [*] 0 - 80 0 25 50 25 Sn2 + + Sn4 + + X2 + (g / L) [*] 60 - 150 120 140 140 140 Sn4 + / (Sn4 + + Sn2 +) 0.1-0.95 0.92 0.52 0.44 0.52 pH -0.2 - 0.3 0 0 0 0 Halide Y- (mole / L) [**] 3 - 5 4 4 4 4 Br / (Br + Cl + I) 0 - 1 1 1 1 1 Current density (A / dm ² ) 50 - 400 200 - 300 200 - 300 200 - 300 200 - 300 Sn deposited (%) 40 - 100 100 60 65 70 Regeneration rate at 40 ° C (mg / hr.cm ² ) 100 - 1200 600 120 740 800 [*] X = Pb, Ni, Co [**] Y = Br, Cl, I.

From the above, it follows that the process according to the invention makes it possible to use an acid electrolytic bath very rich in stannous and stannic ions free of any organic inhibitor.

In the case of the deposit of tin alloys, a fraction of the total tin is replaced in the electrolysis bath by Co2 + and / or Ni2 + and / or Pb2 + ions.

The bromide Br-, chloride Cl- and iodide I-ions allow the easy solubilization in acid medium of the cations Sn2 +, Sn4 +, Co2 +, Ni2 + and Pb2 +.

The use of the Br-, Cl- and I- ions in the context of the process according to the invention as an insoluble anode makes regeneration of the electrolysis bath easy.

The amphoterization reaction between stannic tin and metallic tin in the form of granules makes it possible to regenerate the bath at moderate temperature and specific surface of liquid / solid contact.

The oxidation reaction of cobalt, lead and nickel with stannic tin allows the bath to be recharged with Co2 +, Pb2 + and Ni2 + ions with the same advantages.

Advantageously, this regeneration scheme makes it possible to couple the electrodeposition unit with a regeneration unit of moderate size.

Moreover, the use of insoluble anodes Ti / RuO 2 or Ti / IrO ₂ minimizes the release of oxygen.

It is understood that the invention is not limited to the particular embodiments, described above, of the method according to the invention (more particularly the examples given above) but that other variants can be envisaged without depart from the scope of the present invention.

Thus, the process according to the invention relates, on the one hand, to a process for the electrodeposition of tin or a tin alloy on a metal support with high current density by making use of an anode insoluble in an electrolysis bath containing as metal salt essentially bromide of the metal to be deposited and preferably only bromide, and, on the other hand, a process for the regeneration of an electrolysis bath containing a metal halide or metals to be deposited, these halides can be chlorides, bromides, and iodides.

In some cases, these two methods can be combined or used separately. Thus, the use of the bromide-based electrolyte does not necessarily have to be combined with a regeneration step, although the combination of an electroplating with such a step constitutes a preferred embodiment of the invention.

Claims (14)

1. Process for electroplating tin and/or tin alloys, in particular Sn-Pb, Sn-Co, Sn-Ni alloys, on a metal substrate, such as a steel strip, at high current densities of between 100 and 400 A/dm², preferably between 200 and 300 A/dm², in an electrolysis bath, according to which an electrolysis bath containing at least one halide of the metal to be deposited on the metal substrate, stannous ions and stannic ions is used, the metal-ion content of which bath is maintained between 60 and 150 g/l and is preferably about 115 to 125 g/l for tin plating and about 135 to 145 g/l for depositing tin alloys, this electrolyte being made to flow at a speed relative to the metal substrate of between 2 and 9 m/s and preferably between 3 and 6 m/s, characterized in that an electrolysis bath containing a bromide of the metal in a ratio $${\text{R}}_{\text{2}}\text{=}\frac{\text{BR}}{\text{Br + I + Cl}}$$

   

   of between 0 and 1 is used.
2. Process according to Claim 1, characterized in that, if the aforementioned substrate is formed by a steel strip, the latter acts as the cathode which is moved past an insoluble anode lying at a distance of between 0.75 cm and 1.5 cm away and in that the electrolyte is made to flow between the cathode and the anode in the form of a substantially turbulent flow.
3. Process according to Claim 1 or 2, characterized in that, in the case of tin-alloy deposition, the content of the other metal or metals of this alloy is maintained between 10 and 80 g/l.
4. Process according to Claim 3, characterized in that, in respect of an Sn-Pb alloy, the lead content of the electrolysis bath is maintained between 20 and 30 g/l and is preferably about 25 g/l.
5. Process according to Claim 3, characterized in that, in respect of an Sn-Co alloy, the cobalt content of the electrolysis bath is between 45 and 55 g/l, and is preferably about 50 g/l.
6. Process according to any one of Claims 1 to 5, characterized in that the ratio $${\text{R}}_{\text{1}}\text{=}\frac{{\text{Sn4}}^{\text{+}}}{{\text{Sn}}^{\text{4+}}{\text{+ Sn}}^{\text{2+}}}$$

   

   is maintained between 0.1 and 0.95.
7. Process according to any one of Claims 1 to 6, characterized in that the pH of the electrolyte is maintained between -0.2 and 0.3, and is preferably about 0.
8. Process according to any one of Claims 1 to 7, characterized in that the temperature of the electrolyte is maintained between 30 and 50°C.
9. Process according to any one of Claims 1 to 8, characterized in that a halide mixture having a molarity of between 3 M and 5 M is used.
10. Process according to Claim 9, characterized in that a ratio $${\text{R}}_{\text{2}}\text{=}\frac{\text{BR}}{\text{Br + I + Cl}}$$

    

    of between 0 and 1 is maintained in the electrolysis bath.
11. Process according to any one of Claims 1 to 10, characterized in that an insoluble anode preferably of the Ti/RuO₂ or Ti/IrO₂ type is used.
12. Process according to any one of Claims 1 to 11, characterized in that the potential of the anode immersed in the electrolysis bath is maintained between 1.5 and 2.5 volts relative to the standard hydrogen electrode (SHE), and preferably about 2 volts.
13. Process according to any one of Claims 1 to 12, characterized in that the electrolysis bath is regenerated by bringing the latter into contact with metallic granules of the metal to be deposited.
14. Process according to Claim 13, characterized in that granules with a moderate specific surface area preferably of between 0.25 and 5 m²/kg are used.