DE69314990T2 - Process for the electrodeposition of tin or tin alloys 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 for galvanic deposition at high current densities, as defined in the preamble of claim 1.

The publication EP-A-0 357 839 describes such a process for tinning in an electrolysis bath based on tin(IV) chloride and tin(II) chloride. This process has the disadvantage that harmful gases are released into the atmosphere.

In the excerpt from "Chemical Abstracts", Volume 88, No. 2, dated January 9, 1972, Columbus Ohio U.S. Abstract No. 13625z, SHINDO, "rapid dissolution of tin in a tin electroplating bath", page 423 and in JP-A-7789536 (Nippon Steel) dated July 27, 1977, such a process is mentioned, which, however, can only be used at low current density.

The process according to the invention is characterized by the fact that an electrolysis bath is used which contains a bromide of the metal in a ratio

which lies between 0 and 1.

In particular, in the case where the above-mentioned piece is formed from a steel strip, the latter serves as a cathode which is placed at a distance of between 0.75 and 1.5 cm from an insoluble anode, while the electrolyte is allowed to circulate between this cathode and the anode in the form of a clearly turbulent flow.

According to a preferred embodiment of the invention, the electrolysis bath is regenerated by bringing the latter into contact with grains of the metal to be deposited, so as to permanently maintain a bath having a sufficient content of metal ions to be deposited on the above-mentioned workpiece.

Further details and characteristics of the invention will become apparent from the description of several particular embodiments of the invention given below by way of non-limiting example.

Generally speaking, the process 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 metal workpiece, in particular on one or two surfaces of a steel strip, which moves opposite an insoluble anode in an electrolytic bath in which the halide of the metal is formed by a bromide and which preferably serves as a cathode, while the electrolyte is circulated between this cathode and the anode at a sufficiently high speed to obtain a clearly turbulent flow.

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

It is also an electrolysis at high current density, which is advantageously between 100 and 400 A/dm² and preferably between 200 and 300 A/dm².

Furthermore, preference is given to the use of an electrolysis cell in which the distribution of the cathode current over the region of the steel strip moving in the electrolysis zone is carried out in as uniform a manner as possible in order to create a high and essentially equal diffusion limit current density at every point in this region of the strip.

In addition, an electrolysis bath in which the halide of the metal is formed by bromide is also preferred.

The high current density requires comparatively high concentrations of metal cations. Thus, satisfactory results were obtained by using an electrolytic bath according to the invention whose content of metal ions is kept between 60 and 150 g/l, preferably in the order of 115 to 125 g/l for tinning and in the order of 135 to 145 g/l for the deposition of tin alloys.

In the case of depositing a tin alloy, the content of the other metal(s) of this alloy is generally kept between 10 and 80 g/l.

In particular, for a Sn-Pb alloy, the lead content of the electrolysis bath is advantageously kept between 20 and 30 g/l, preferably around 25 g/l.

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

It has in fact been found that when the total content of metal ions is less than 60 g/l, there is a risk that the cathode yield will decrease significantly and that the deposits obtained may be powdery at high current densities.

On the other hand, if a concentration of metal ions is maintained that is greater than 150 g/l, there is a risk that the losses due to the entrainment of the electrolyte by the steel strip to be covered will become too great.

It has been found that excellent results, in particular in terms of cathode yields and the structure of the deposits, are obtained at current densities of 100 to 400 A/dm², in particular between 200 and 300 A/dm², by combining either high contents of metal cations dissolved in the bath and low circulation rates of the electrolyte, or low contents of metal cations dissolved in the bath and high circulation rates of the electrolyte. Consequently, within the limits indicated above, the higher values of the contents of metal cations must be combined with the lower values of the relative circulation rates of the electrolyte, and vice versa.

These different possible combinations are defined by the value of the product C v0.8, where C is the concentration of metal ions dissolved in the bath (in g/l) and v is the relative speed between the electrolyte and the metal workpiece (in m/s). For current densities between 200 and 300 A/dm², the value of this product is preferably equal to 350.

The ratio R₁ = Sn⁴⁺/(Sn⁴⁺ + Sn²⁺) in the electrolysis bath is advantageously kept between 0.1 and 0.95. The exact value of this ratio is chosen according to the type of criterion to be optimized. Thus, the specific energy consumption is minimal for R = 0.1.

As regards the pH of the electrolysis bath, in order to avoid hydrolysis of the metal ions, the pH is advantageously kept between -0.2 and 0.3 and is preferably approximately 0.

At pH values too close to -0.2 there is a non-negligible risk that the cathode yields of the deposition will fall. On the other hand, at pH values too close to 0.3 and for tin contents approaching 120 g/l there is a non-negligible risk of precipitation.

The temperature of the electrolysis bath is advantageously maintained between 30 and 50ºC and is preferably about 40ºC.

At temperatures above 50ºC, the effect of tin (IV) on the electrolytically deposited metallic tin is such that the yields of the electrodeposition are reduced. This effect can be compensated for by placing a cathodic protection, known per se, at the output of the cell.

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

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

- the metallic salts should be very soluble;

- the oxidation reaction of tin(II) at the anode of the tinning cell takes place with a poor yield (of the order of 5%). To overcome this disadvantage, the counterion should be easily oxidized at the anode and its oxidation product should be able to oxidize the tin(II) very quickly to tin(IV).

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

The reaction scheme becomes:

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

As already stated above, according to the invention, it is preferable to use bromide, which makes it possible to avoid the release of harmful gases into the atmosphere.

For a halide mixture with a molarity advantageously between 3M and 5M and for a current density which is between 50 and 400 A/dm², the yield of reaction 1 is 100 %.

The concentration of halide Y⊃min; in the bath is preferably about 4M.

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

This ratio is preferably about 1 if it is desirable to avoid any risk of chlorine release and precipitation of stannous iodide.

It is preferably about 0 if it is desired to avoid any risk of corrosion and destruction of the anodes, which are preferably made of Ti/Ru0₂ or Ti/Ir0₂.

The fact that such an anode is used allows the bromine Br2 to be produced at high current densities with yields of the order of 100%.

The anode potential of this electrode should preferably be kept between 1.5 and 2.5 volts, relative to the hydrogen standard electrode (NHE), during its polarization in the tinning cell and is preferably approximately 2 volts.

It has been found that when the anode potential is below 1.5 volts, relative to the above-mentioned hydrogen standard electrode, the high current densities required for the high current density tinning process of the invention are generally not achieved.

If the anode potential is above 2 volts, referred to this hydrogen standard electrode, there is a risk that the titanium of the anode will corrode after a few minutes in the case where Y = Br.

The invention also relates to a particular process for regenerating a bath for the electrodeposition of tin and tin alloys, in particular the alloys Sn-Pb, Sn-Co and Sn-Ni rich in tin(IV) ions, by bringing the electrolytic bath in which this electrodeposition takes place into contact with grains of tin, lead, cobalt and nickel with moderate specific surface areas.

In this regard, it has been found that economically and commercially viable results have been obtained with granules having a specific surface area of between 5 and 0.25 m²/kg of metal. The tin(IV) ions consumed during the dissolution of the above-mentioned granules are continuously regenerated at the anode of the electrodeposition cell, thus making it possible to maintain the concentration of these ions in the electrolytic bath during electrodeposition substantially at the level defined above.

The regeneration reaction takes place in a known regeneration reactor according to the following reaction scheme: Sn4+ + Sn T 2 Sn2+ Reaction 4a

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 case of alloy deposition, grains of Pb and/or Ni and/or Co are added in the regeneration reactor.

The solution of these elements occurs according to the reaction scheme:

Sn4+ + X T Sn2+ + X2+ Reaction 4b

with X = Co, Ni, Pb.

The main reactions in the tinning cell are:

at the anode 2 Sn2+ T2 Sn4+ + 4e- reaction 5

at the cathode Z�1;(Sn4+ + 4e T Sn) Reaction 6

Z2 (2 Sn2+ + 4e T2 Sn) Reaction 7

Z�1 represents the fraction of the cathode current that serves to reduce the tin(IV) to metallic tin, and Z�2 represents the fraction of the cathode current that serves to reduce the tin(II) to metallic tin.

In the case of alloy deposition, the cathode reactions are:

Z�1;(Sn4+ + 4e T Sn) Reaction 6

Z2 (2 Sn2+ + 4e T 2 Sn) Reaction 7

Z3 (2 X2+ + 4e T2 X) Reaction 8,

with Z&sub1; + Z&sub2; + Z&sub3; = 1.

The total cathode reaction (reaction 6 + reaction 7 or reaction 6 + reaction 7 + reaction 8) added to the anode reaction (reaction 5) is in complete equilibrium with the regeneration reaction (reaction 4a or reaction 4a + reaction 4b), which allows the system consisting of the tinning cell and the regeneration reactor to function without the addition of any other external reagents other than tin, nickel, cobalt and lead, each in metallic form.

As already indicated above, the concentration of metal ions Sn4+ + Sn2+ + X2+ in the electrolyte is kept between 60 and 150 g/l and is preferably about 120 g/l for tinning and about 140 g/l for alloy deposition.

Regarding the ratio R₁ = Sn4+ / (Sn4+ + Sn2+), the regeneration rate is maximum for R₁ = 0.95.

In particular, during an optimization of the tin regeneration rate, it was found that the ideal value for R is approximately 0.92.

In addition, it has also been found that when the temperature of the electrolysis bath is below 30ºC, the regeneration rate generally becomes low.

In the following, concrete embodiments of the inventive method are given.

example 1

This is an example of a tinning process in which the electrolysis bath was formed from a solution of 0.9M tin(IV) bromide and 0.1M tin(II) bromide.

The electrolyte was prepared by the action of hydrobromic acid at 47 atomic percent and hydrogen peroxide at 30 atomic percent on grains of pure metallic tin.

During tinning, the electrogeneration yield of Br at 300 A/dm2 and 40ºC was 100% at the Ti/Ru02 anode.

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

The specific energy consumption was approximately 9 kWh/kg.

The regeneration rate was approximately 27.5 A/dm² at 40ºC and 55 A/dm² at 60ºC (or 600 and 1200 mg/cm² h).

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

Example 2

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

The regeneration reaction took place through the action of the electrolyte thus produced on grains of tin, lead, nickel and cobalt.

Finally, a table is given below summarizing the ideal conditions for deposition and regeneration according to the invention. Table

[*] X = Pb, Ni, Co [**] Y = Br, Cl, I.

From the foregoing it follows that the process according to the invention makes it possible to use an acidic electrolytic bath which is very rich in tin(II) and tin(IV) ions and free from any organic inhibitor.

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

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

The use of the ions Br-, Cl- and I- in the context of the inventive process with an insoluble anode makes the regeneration of the electrolysis bath easy.

The amphoteric reaction between tin(IV) and metallic tin in the form of grains allows the regeneration of the bath at a moderate temperature and a moderate specific liquid/solid contact area.

The oxidation reaction of cobalt, lead and nickel by tin(IV) allows the bath to be refilled with Co2+, Pb2+ and Ni2+ ions with the same advantages.

Advantageously, this regeneration scheme allows the electrodeposition unit to be coupled with a regeneration unit of moderate size.

In addition, the use of insoluble Ti/Ru02 or Ti/Ir02 anodes minimizes the release of oxygen.

It is understood that the invention is not limited to the particular embodiments of the method according to the invention described above (in particular to the examples given above), but that further modifications are conceivable without departing from the scope of the present invention.

It is thus the case that the method according to the invention relates, on the one hand, to a method for the galvanic deposition of tin or a tin alloy on a metal workpiece at high current density, using an insoluble anode in an electrolysis bath which contains, as a metal salt, essentially a bromide of the metal to be deposited and preferably exclusively bromide, and, on the other hand, to a method for the regeneration of an electrolysis bath which contains a halide of the metal or metals to be deposited, where these halides can be chlorides, bromides and iodides.

In certain cases, these two processes can be combined or used separately. Consequently, the use of the bromide-based electrolyte does not necessarily have to be combined with a regeneration step, although the combination of electrodeposition with such a step represents a preferred embodiment of the invention.

Claims (14)

1. Process for the galvanic deposition of tin and/or tin alloys, in particular Sn-Pb, Sn-Co or Sn-Ni alloys, on a metal workpiece, for example a steel strip, at high current densities between 100 and 400 A/dm², preferably between 200 and 300 A/dm², in an electrolysis bath, in which an electrolysis bath is used which contains at least one halide of the metal to be deposited on the metal workpiece and tin(II) ions and tin(IV) ions and whose content of metal ions is kept between 60 and 150 g/l, preferably approximately from 115 to 125 g/l for tinning and approximately from 135 to 145 g/l for the deposition of tin alloys, this electrolyte at a relative speed between 2 and 9 m/s, preferably between 3 and 6 m/s, in relation to the metal workpiece, characterized in that an electrolysis bath is used which contains a bromide of the metal in a ratio which lies between 0 and 1. 2. Process according to claim 1, characterized in that, in the case where the above-mentioned piece is made of a steel strip, the latter serves as a cathode which is placed at a distance of between 0.75 cm and 1.5 cm from an insoluble anode, and in that the electrolyte is caused to circulate between the cathode and the anode in the form of a substantially turbulent flow. 3. Process according to one of claims 1 or 2, characterized in that, in the case of depositing a tin alloy, the content of the other metal(s) of this alloy is kept between 10 and 80 g/l. 4. Process according to claim 3, characterized in that, for a Sn-Pb alloy, the lead content of the electrolysis bath is kept between 20 and 30 g/l, preferably at approximately 25 g/l. 5. Process according to claim 3, characterized in that, for a Sn-Co alloy, the cobalt content of the electrolysis bath is between 45 and 55 g/l, preferably approximately 50 g/l. 6. Method according to one of claims 1 to 5, characterized in that the ratio is kept between 0.1 and 0.95. 7. Process according to one of claims 1 to 6, characterized in that the pH of the electrolyte is kept between -0.2 and 0.3, preferably approximately. 8. Process according to one of claims 1 to 7, characterized in that the temperature of the electrolyte is kept between 30 and 50°C. 9. Process according to one of claims 1 to 8, characterized in that a halide mixture with a molarity between 3 M and 5 M is used. 10. A method according to claim 9, characterized in that in the electrolysis bath a ratio which lies between 0 and 1, is maintained. 11. Process according to one of claims 1 to 10, characterized in that an insoluble anode, preferably of the type Ti/Ru0₂ or Ti/Ir0₂, is used. 12. Method according to one of claims 1 to 11, characterized in that the potential of the anode immersed in the electrolysis bath is kept between 1.5 and 2.5 volts, preferably at about 2 volts, relative to the hydrogen standard electrode (NHE). 13. Process according to one of claims 1 to 12, characterized in that the electrolysis bath is regenerated by bringing the latter into contact with metal granules of the metal to be deposited. 14. Process according to claim 13, characterized in that granules with a moderate specific surface area, preferably between 0.25 and 5 m²/kg, are used.