POLYCHROMATE CHROMIUM-PLATING PROCESS AND BATH

Abstract: A chromium-plating bath containing polychromates and sulphates undergoes the addition of sulphonamides and/or sulphonimides to produce bright deposits and to inhibit anode corrosion, and the addition of cations of Na, Mg and ammonium to increase the covering power.

Description: "POLYCHROMATE CHROMIUM- PLATING PROCESS AND BATH" Technical Field This invention relates to a process and a bath for chromium- plat ing by electrodeposition from a polychromate- based solution. Background Art Polychromate-based chromium- plat ing baths have been known since 1933. They are usually prepared by mixing chromium anhydride and sodium hydroxide in solution. A characteristic of polychromate-based chromium-plating baths is that they use high current densities and maintain high cathode efficiencies - 30% or more. As well as this advantageous characteristic, polychromate- based chromium plating baths have a number of disadvantages which have limited their use on an industrial scale. The deposit obtained is dull and has low covering power; it is difficult to prepare the polychromate-based chromium-plating baths on an industrial scale and, usually, the temperature of the bath must not exceed 20°C. Chromic-acid based chromium-plating baths are normally used to give brighter deposits, but these give low cathode efficiencies . Disclosure of the Invention It is the object of the present invention to solve the aforementioned problems and to provide polychromate-based baths with high covering-power which produce penetrating and bright deposits, are easily used on an industrial scale and have high cathode efficiencies . This object is achieved by the present invention, which relates to a process for chromium-plating by electrodeposition of chromium from a bath comprising polychromates and sulphates, characterized by effecting the said electrodeposition in the presence of sulphonamides of formula R-S0 2 Y, where R is an alkyl chain and Y is an amine group, and in which the ratio of carbon atoms in the R chain and nitrogen atoms in the amino group Y is < 12, or of the corresponding sulphonimides. The invention also relates to a chromate bath for the realization of the above process. It was further discovered, surprisingly, that sulphonamides, or the corresponding sulphonimides, added to a chromium bath comprising polychromates and sulphates had an unexpected protective effect on the anode of the same. The use of sulphonamides or sulphonimides to protect the anode of a chromium bath is therefore a further object of the invention. According to a preferential embodiment of the invention, the sulphonamide used has the general formula R-SC -NR'R", where R is a linear alkyl chain C1-C12, R' and R" are, together or separately, hydrogen, -NH 2 , -NHOH, -CONH 2 or an aliphatic or heterocyclic substituent. According to another aspect of the invention Na, Mg or ammonium salts of alkyl sulphonic acids of the general formula R-SO3-, where the ratio between the sulphur atoms and the carbon atoms in the R chain is greater or equal to 1/4, (S:C ≥ 1:4), or corresponding salts of sulfamic acid, are used to increase the covering power of the bath and to give extensive substantially uniform micro-cracking in the presence of selenium. Further objects of the invention are therefore the use of salts of alkyl sulphonic acids according to Claim 12 and the use of salts of sulfamic acid according to Claim 13. The invention has numerous advantages over the present state of the art. The use of sulphonamides or sulphonimides gives bright deposits; the use of polychromates in the salt form results in stable baths even if bath temperature is changed; the sulphonamides protect the anodes from corrosion; the use of Na, Mg or NH 4 + cations (obtained from their corresponding salts of the alkyl sulphonic acids or sulfamic acid) increases the covering power and gives extensive substantially uniform micro-cracking in the presence of selenium. Best Mode for Carrying Out the Invention Preferred polychromates are tri- and tetra-chromates as well as higher homologues. These compounds are obtained by treating their corresponding di-chromates with nitric acid or chromic anhydride. The following is a list (which is illustrative but not limiting) of polychromates according to the invention: Compound Method of production (NH 4 ) 2 Cr 3 O 10 (NH 4 ) 2 Cr 2 0 7 + HNO 3 K 2 Cr 3°10 K 2 Cr 2°7 + HN0 3 MgCr 4 0 13 MgC0 3 + Cr0 3 (NH 4 ) 2 Cr 4 0 13 (NH 4 ) 2 Cr 2 0 7 + HN0 3 K 2 Cr 4 0 13 K 2 Cr 3 O 10 + HN0 3 SrCr 3 0 10 SrCr 2 0 7 + Cr0 3 Some of these compounds were obtained in the presence of chromium borate as initiator. The preferred concentration of such compound in the bath was between 4 and 40 g/litre. These polychromates, in aqueous solution with a preferred concentration of between 200 and 400 g/1, give a bath which is unaffected by changes in temperature and which maintains a high cathode efficiency even with relatively high temperatures. The preferred operating temperature is between 40 and 70°C and more preferably, between 45 and 60°C. The ratio of polychromate concentration to sulphate concentration is preferably between 20 and 250. The deposits obtained from these baths are dull and have low covering power. It was surprising to find that bright deposits can be obtained if sulphonamides of the type R- S0 3 -NH 2 (where R is a linear alkyl chain C1-C12) or the corresponding sulphonimides are added to the bath. When sulphonamides or the corresponding sulphonimides are added to the polychromate bath, the crystalline structure of the chromium deposit is modified from large-grain hexagonal (dull deposit) to fine-grain body-centered cubic structure (bright deposit) . Short-chain alkyl sulphonic acids have been used as additives in some of the known electroplating processes to increase the cathode efficiency of the bath (in chromic acid based baths) as described, for instance, in EP-A- 0196053. However, the alkyl sulphonic acids have the disadvantage of causing electrochemical corrosion of anodes in Pb/Sn and in other lead-based alloys. This corrosion occurs both in the chromic acid based chromium baths and in polychromate based baths as in the present invention, though in the latter it is in a reduced form due to the less acid pH of the same. As described above, it has been discovered that the presence of sulphonamides or sulphonimides of the type already described confer high resistance to corrosion of the anode by the alkyl sulphonic acids in the chromium plating baths. The sulphonamides and the sulphonimides have the same function in protecting the anode even in traditional baths based on chromic acid and sulphuric acid. The use of sulphonamides and sulphonimides in such traditional baths has been found to have other advantages, in particular, to lead to higher cathode efficiency and brighter deposits. A further object of the invention is thus the use of sulphonamides or sulphonimides according to Claim 14. The sulphonimides preferably used in the present invention are generally prepared by the Schotten and Baumann reaction of chlorides of C1-C12 alkyl sulphonic acids: R-S0 2 C1 + NH 3 + NaOH -> R-S0 2 NH 2 + NaCl + H 2 0 Ammonium compounds such as hydroxylamine, hydrazine, urea, aliphatic or heterocyclic amines may be used instead of ammonia. The aliphatic amines are preferably primary amines with aliphatic C1-C12 chain. The sulphonamides of interest for the present invention are those mentioned above with the general formula R-S0 2 - NR'R", where R is a C1-C12 alkyl chain and R' and R" are, together or separately, hydrogen, -NH 2 , -NHOH, -CONH 2 or an aliphatic or heterocyclic substituent. Preferred concentrations of sulphonamides are such as to give a ratio of the concentration (by weight) of polychromate to the concentration (by weight) of sulphonamide of between 20 and 450. It has been found that the covering power of the chromium deposits obtained from a chromium bath is improved by the addition of Na + , Mg and ammonium ions. Such ions are present (either alone or in combination) in total concentrations of from 2 to 30 g/1 in addition to any such cations that are possibly present in the polychromates used. These cations are preferably added to the bath in the form of contra-ions of the organic anions of alkyl sulphonic acids R-S0 3 H, where R is a C1-C4 alkyl group with possible substitution in the 2 position by -OH or -NH2. Preferred salts are Na, Mg or ammonium salts of CH 3 S0 3 " ; CH 3 CH 2 S0 3 " ; CH 3 CH 2 CH 2 S0 3 " ; CH 3 CH(OH)CH 2 S0 3 " ; NH 2 CH 2 CH 2 S0 3 " . The corresponding salts of sulphammic acid, i.e. NH 2 S0 3 Na; (NH 2 S0 3 ) 2 Mg; NH 2 S0 3 NH 4 can also be used. These compounds are chosen as sources of cations because they do change neither the internal equilibrium of the polychromate nor the polychromate/sulphate ratio, which would change if the sulphate of the chosen cation was used. The preferred concentration of the alkylsulphonic salts are thos e whi ch give s a rati o of polychromate concentration (by weight) to alkyl sulphonic acids concentration (by weight) between 25 and 450. A further advantage of using the cations referred to above is that they produce very extensive micro-cracking in the presence of selenium even in areas of reduced current density. The process according to the invention provides, having prepared a bath giving the characteristics listed above, for operation at a current density of between 11 and 230 A/dm and preferably between 30 and 100 A/dm . In conclusion, the bath according to the present invention produces a bright, adhering and non-iridescent deposit on a metal base, free of grey or rough deposit at low current-density with a cathode efficiency of at least 25% measured at a current density of 50 A/dm 2 and a chromate bath temperature of 52°C. The invention will now be further described with reference to examples which are illustrative but not limiting. Example 1. A chromate bath was prepared , based on tri - and tetra- chromates with the following initial composition : Polychromate 380 g/1 H 2 S0 4 3 . 5 g/1 A Standard Hull Cell was used to produce chromium-plating; the cathode efficiency was 25%, with a dull deposit. Example 2. Sulphonamides were added to the bath of Example 1 in a series of steps to give from 2 g/1 to 12 g/1. The sulphonamides tested had R= linear alkyl with 1, 4, 6, 10 and 12 carbon atoms . The cathode efficiency was still 25%. The deposit was bright in all concentrations of the example. Example 3 Sodium cation bound to alkyl sulphonic acid as a salt was added to the bath of Example 2 in increasing quantities. The cathode used in this bath was a 1.5 X 15 cm "V"-shaped panel. The chromium plating was carried out at a current of 10 A. The distance of the plating from the base of the indentation was the following: Sodium Alkyl Sulphonate (g/1) Non-plated (mm) 0 8.5 2 4.0 4 3.0 10 2.0 20 0.0

Claims: CLAIMS 1. A process for chromium-plating by electrodeposition of chromium from a bath comprising polychromates and sulphates, characterized by effecting the said electrodeposition in the presence of sulphonamides of formula R-S0 2 Y, where R is an alkyl chain and Y is an amine group, and in which the ratio of carbon atoms in the R chain and nitrogen atoms in the amino group is less or equal to twelve (C/N 12) . 2. A process according to claim 1, characterized by operating in the presence of Mg, Na or ammonium ions or a mixture thereof. 3. A process according to claim 2, characterized by adding at least a part of the said ions to the said chromium- plating bath in the 'form of the corresponding salts of alkyl sulphonic acids of formula R-S0 3 ~ , where R is an alkyl chain and where the ratio between the sulphur atoms and the carbon atoms of the R chain is greater or equal to 1/4, (S:C ≥ 1:4), or salts of sulphammic acid or salts of C2-C4 alkyl sulphonic acids substituted in position 2. 4. A process according to any of the claims 1 to 3 , characterized by operating in the presence of sulphonamides with formula R-S0 2 -NR'R", where R is a linear C1-C12 alkyl group and R' and R" are, together or separately, hydrogen, -NH 2 , -NHOH, -CONH 2 or an aliphatic or heterocyclic substituent, or in the presence of the corresponding sulphonimides. 5. A process according to claim 3 or 4, wherein the concentration of polychromates is within the range from 200 to 400 g/1. 6. A process according to any previous claim, wherein by the ratio between the concentration of the said polychromates and the concentration of the sulphonamides is within the range from 20 to 450. 7. A process according to claim 3 and claim 5 or 6, wherein by the ratio between the concentration of the said polychromates and the concentration of the salts of Na, Mg and ammonium is within the range from 20 to 450. 8. A process according to any of the preceding claims, wherein the bath temperature is within the range of 45 to 65°C. 9. A chromium-plating bath for the electrodeposition of chromium from a solution of polychromates, characterized by having a composition according to any of the claims 1 to 8. 10. The use of sulphonamides of formula R-S0 2 -Y, where R is an alkyl chain, Y is an amino group and in which the ratio of carbon atoms in the R chain and nitrogen atoms in the amino group Y is ≤ 12 (C/N≤12) , or of the corresponding sulphonimides in a chromium-plating bath comprising polychromates and sulphates to obtain bright chromium deposits. 11. The use of sulphonamides according to Claim 10, or o f the corresponding sulphonimides, in a chromium-plating bath comprising polychromates and sulphates to protect the anode of the said bath. 12. The use of alkyl sulphonic salts of formula (R(R')S0 3 ) n X where R is a C1-C4 alkyl group, R' is H, NH 2 or OH in position 2 of the R group, X is a cation selected from Na, Mg or NH 4 + and where n = 2 when X is Mg but n = 1 otherwise, to increase the covering power in a chromium- plating bath comprising polychromates and sulphates . 13. The use of salts of Na, Mg or NH 4 of sulphamic acid, or a mixture thereof, to increase covering power in a chromium-plating bath comprising polychromates and sulphates . 14. The use of sulphonamides or sulphonimides in a chromium-plating bath comprising chromic acid and sulphuric acid to improve the brightness of chromium deposit and the cathode efficiency, and to protect the anode of the said bath.

Inventors: FREDIANI LIDO, MERELLO GIOVANNI, FREDIANI, LIDO, MERELLO, GIOVANNI
Applicants: FREDIANI LIDO, MERELLO GIOVANNI, STOPPANI LUIGI SPA, FREDIANI, LIDO, LUIGI STOPPANI S.P.A., MERELLO, GIOVANNI
Application year: 1996
Main classification: C25D3
IPC codes: C25D3