Continuous trivalent chromium plating method

Abstract

This invention belongs to the field of chemistry and metallurgy and it is specifically related to a process for the electrolytic and electrophoretic production of electrochemical coatings with a bath from trivalent decorative chromium solutions on a metal or plastic substrate with trivalent chromium ions, sulfate ions, graphite anodes, hexavalent chromium control and false cathode purifications in a continuous industrial operation.

1 . Process of trivalent chromate plating for decorative coating on a metal substrate or plastic substrate with a thickness coating of 0.3 μm to 2 μm, in which process is a continuous bath containing organic and inorganic chromium complexes from Cr+3 ions taken from salts preferably from Cr 2 (SO 4 ) 3  with SO 4   −2  ions and graphite anodes with an anode-cathode relation of 2:1 resistant to cracking by current density, with control of Cr+6 production with reducers generating sodium sulfate and removal by crystallization of such sodium sulfate generated in the process; and with purification of false cathode for control of parasite metals that have weak complexes in the bath such as Ni, Fe, Cu and Zn for stability in coating color, electrolytic yield and compliance with resistance to corrosion and penetration specifications. 
     
     
         2 . Process of trivalent chromate plating as claimed in  claim 1  in which it starts with the preparation of a trivalent chromium solution with graphite anodes and in the process of coating obtaining a hexavalent chromium controlled by reducers that convert the excess of hexavalent chromium in trivalent chromium is produced releasing sodium sulfate which is removed in the crystallization stage by bath cooling. 
     
     
         3 . Process of trivalent chromate plating as claimed in  claim 1  in which the graphite anode must be without pores and put into acid-resistant fabric bags avoiding the pass to the solution of eroded particles of anode in a continuous filtration process at a speed of 4 to 6 times the bath volume per hour to control carbon particles in the bath an protect coating quality. 
     
     
         4 . Process of trivalent chromate plating as claimed in  claim 1  in which density of applied current must be between 4 dm2 to 12 A/dm2 and preferably between 5 A/dm2 to 8 A/dm2. 
     
     
         5 . Process of trivalent chromate plating as claimed in  claim 1  in which Cr+3 ions concentration is between 10 g/l to 30 g/l, preferably between 15-g/l to 25 g/l. 
     
     
         6 . Process of trivalent chromate plating as claimed in  claim 1  in which the source of products forming complexes in trivalent chromium comes from the HCOONa/HCOONH 4 /NaCH 3 COO/NH 3 CH 3 COO organic acid salts; lactate ions, C 3 H 5 O 3   − , Oxalate ions (COO) 2   −2 , malate C 4 H 6 O 5 , glycine NH 2 CH 2 COOH individually or mixed with C+3 in a concentration between 50/l to 150 g/l, preferably between 70 g/l to 120 g/l. 
     
     
         7 . Process of trivalent chromate plating as claimed in  claim 1  in which the bath is added as a Na 2 SO 4  conducer salt between 30 g/l to 60 g/l, preferably between 40 g/l to 50 g/l only to form the bath and includes salts containing Na+, K+ and (NH) 4   +  ions. 
     
     
         8 . Process of trivalent chromate plating as claimed in  claim 1  in which chromium complex discomposes by the electric current and deposits Cr0 on the cathode and releases SO 4   −2  anions which combines with sodium ions present in the solution forming Na 2 SO 4  and also during the mechanism of reduction from Cr+6 to Cr+3 and increase of acidity is produced by the formation of H 2 SO 4 , where control of acidity is made with NaOH, Na 2 CO 3  neutralizing the solution with the Na 2 SO 4  formation. 
     
     
         9 . Process of trivalent chromate plating as claimed in  claim 1  in which pH is controlled between 3, 4 to 4.0, the superficial tension of the bath must be between 30 dynes to 70 dynes and process temperature must be between 40° C. to 60° C., preferably between 45° C. to 50° C. due to the constant reduction of Cr+6 to Cr+3 in the bath concentrates the Na 2 SO 4  and a temperature reduction may cause the crystallization of the concentrated salt to Na 2 SO 4 . 
     
     
         10 . Process of trivalent chromate plating as claimed in  claim 1  in which the adding of reducers is made controlled by the consumption of amperes-hours procuring maintaining the Cr+6 between 0 ppm to 40 ppm, preferably between 0 ppm 20 ppm, range in which there is no side effects in the coating quality.