Electrodeposition of chromium

Abstract

Chromium containing deposits free of cracks and having good hardness are plated by the use of a novel aqueous chromium (III) electrolyte. The electrolyte includes an aquo chromium (III)-thiocyanato complex as a source of chromium (III) cations.

What is claimed is: 
     
       1. A composition of matter for use in the electrodeposition of .Iadd.bright .Iaddend.chromium containing deposits consisting essentially of an aqueous solution of aquo chromium (III) thiocyanato .[.complex.]. .Iadd.complexes .Iaddend.as a source of trivalent chromium, the concentration of .[.the.]. Cr (III) .Iadd.in the solution .Iaddend.being in the range of 0.03 to 0.5 M.Iadd., and the thiocyanate concentration in the solution being in the range of 0.05 to 1.0 M, wherein the concentration of thiocyanate is higher than the concentration of Cr (III), and wherein the aquo chromium (III) thiocyanato complexes are an equilibrium mixture of complexes of the formula   (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)     where n is a range of whole numbers from 1 through 6.Iaddend..  .[.2. The composition of claim 1 wherein the aquo chromium (III) thiocyanato complex is an equilibrium mixture of complexes having the formula     (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)     
     
     
       where n is a whole number from 1 through 6..]. 3. The composition of claim .[.2.]. .Iadd.1 or 26 .Iaddend.in which the complexes are prepared by the 
     
     
        process of reacting thiocyanate anion with chromium perchlorate. 4. The composition of matter of claim 1 .Iadd.or 26 .Iaddend.for plating cobalt-chromium containing alloys in which cobalt cations are present in 
     
     
        said aqueous plating solution. 5. The composition of matter of claim 1 .Iadd.or 26 .Iaddend.for plating nickel-chromium containing alloys in which nickel cations are present in said aqueous plating solution.  .[.6. The composition of claim 1 in which the thiocyanate concentration is in 
     
     
        the range of 0.05 to 1.0 M..]. 7. The composition of claim .[.6.]. .Iadd.1 or 26 .Iaddend.in which the pH of the Aqueous solution is in the range of about 2 to 3.5 .[.and.]..Iadd., .Iaddend.the solution is saturated with boric acid. .Iadd., and the thiocyanate-to-Cr (III) concentrations are in 
     
     
        about a 2-to-1 ratio.Iaddend.. 8. A method of electrodepositing .Iadd.bright .Iaddend.chromium containing material on a cathode, which comprises passing an electroplating current between an .Iadd.insoluble .Iaddend.anode and said cathode in a Plating solution consisting essentially of an aqueous solution of .[.an.]. aquo chromium (III) thiocyanato .[.complex.]. .Iadd.complexes .Iaddend.as a source of trivalent chromium.Iadd., said solution being prepared by means of time and temperature equilibration in which the thiocyanate is substituted into the chromium complexes, and in which chromium (III) concentration in the solution is in the range of 0.03 to 0.5 M and the thiocyanate concentration in the solution is in the range of 0.05 to 1.0 M, the concentration of thiocyanate being higher than the concentration of 
     
     
        chromium (III).Iaddend.. 9. The method of claim 8 .Iadd.or 28 .Iaddend.wherein the aquo chromium (III) thiocyanato complex is an equilibrium mixture of complexes having the formula   (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)     where n is a .Iadd.range of .Iaddend.whole .[.number.]. .Iadd.numbers   
     
     
        .Iaddend.from 1 through 6. 10. The method of claim .[.9.]. .Iadd.8 or 28 .Iaddend.in which complexes are .Iadd.an equilibrium mixture of complexes having the formula   (H.sub.2 O).sub.6-n Cr(SCN).sub.n.sup.(+3-n)     where n is a range of whole numbers from 1 through 6, .Iaddend.prepared by   
     
     
        the process of reacting thiocyanate anion with chromium perchlorate. 11. The method of claim 8 .Iadd.or 28 .Iaddend.for plating cobalt-chromium containing alloys in which cobalt cations are present in said aqueous 
     
     
        plating solution. 12. The method of claim 8 .Iadd.or 28 .Iaddend.for plating nickel-chromium alloys in which nickel cations are present in said aqueous plating solution.  .[.13. The method of claim 8 in which the chromium (III) concentration is in the range of 0.03 to 0.5 M and in which 
     
     
        the thiocyanate concentrations is in the range of 0.05 to 1.0 M..]. 14. The method of claim .[.13.]. .Iadd.8 or 28 .Iaddend.in which the pH of the aqueous solution is in the range of about 2 to 3.5 and the solution is 
     
     
        saturated with boric acid. 15. The method of claim .[.14.]. .Iadd.8 or 26 .Iaddend.in which .Iadd.the pH of the aqueous solution is in the range of about 2 to 3.5, the solution is saturated with boric acid, .Iaddend.the electroplating current density is in the range of about 20 to 120 mA/cm 2  of the cathode surface.Iadd., the thiocyanate-to-chromium (III) concentration ratio is about 2-to-1, and the deposited chromium containing material is of about a Vicker Hardness number 700.Iaddend.. 
     
     
            The method of claim 8 .Iadd.or 26 .Iaddend.in which the electroplating current density is in the range of about 20 to 120 mA/cm 2  of the 
     
     
        cathode surface. 17. The method of claim 8 .Iadd.or 28 .Iaddend.in which the anode is immersed in an anolyte and said anode and anolyte are 
     
     
        separated from the plating solution by an ion exchange barrier. 18. The process of claim .[.17.]. .Iadd.8 or 26 in which the anode is immersed in an anolyte and said anode and anolyte are separated from the plating solution by an ion exchange barrier, and .Iaddend.in which the anolyte is 
     
     
        sodium perchlorate. 19. The method of preparing a plating solution for electroplating of .Iadd.bright .Iaddend.chromium comprising the step of reacting a thiocyanate anion with chromium perchlorate in the presence of boric acid in an aqueous solution having a pH of 1 to 2 to form an aquo chromium III thiocyanato complex of the formula (H 2  O) 6-n  Cr(SCN) n .sup.(+3-n), .Iadd.in .Iaddend.which n is a .Iadd.range of .Iaddend.whole .[.number.]. .Iadd.numbers .Iaddend.from one through six.Iadd., wherein the chromium in the perchlorate solution is hydrated in the form of Cr1H 2  O) 6   +3 , and wherein the cyanato complex is formed by the progressive replacement of the H 2  O groups, said replacement being produced by heating said solution for one hour at 80° C., the thiocyanate concentration being in the range of about 0.05 to 1.0 M, and the chromium III concentration being in the range of about 0.03 to 0.5 M, and the concentration of thiocyanate being higher 
     
     
        than the concentration of chromium III.Iaddend. . 20. The invention as defined in claim 19 wherein the source of thiocyanate anion is sodium thiocyanate.Iadd., and wherein the concentration ratio of thiocyanate to chromium III is about 2 to 1.Iaddend..  .[.21. The invention as defined in claim 19 wherein the chromium in the perchlorate solution is hydrated in the form of Cr(H 2  O 6 ) -3 ..]. .[.22. The invention as defined in claim 21 wherein the cyanato complex is formed by the progressive replacement of the H 2  O groups..]. .[.23. The invention as defined in claim 19 wherein the reaction takes place at an elevated temperature..]. 
     
     
          . The invention as defined in claim 19 wherein the reaction solution contains sodium hydroxide to control the pH.  .Iadd.25. The composition of claim 1 wherein said equilibrium is produced by heating said composition of matter for one hour at 80° C. .Iaddend. .Iadd.26. The method of claim 8 wherein said equilibration is produced by heating said solution for one hour at 80° C. .Iaddend.