Chromium plating process and bath

Claim 1

AN IMPROVED METHOD OF ELECTRODEPOSITING CHROMIUM ON A METAL ARTICLE BY MEANS OF AN AQUEOUS CRO3-CONTAINING, CATALYST-CONTAINING, CHROMIUM PLATING BATH, SAID METHOD BEING CHARACTERIZED BY HAVING A CRO3/CATALYST RATIO IN SAID BATH WHICH HAS A SUBSTANTIALY REDUCED TENDENCY TO CHANGE WITH CHANGES IN CRO3 AND CATALYST CONCENTRATIONS AND TEMPERATURES, WHICH COMPRISES ESSENTIALLY: PASSING CURRENT FROM AN ANODE TO SAID ARTICLE AS A CATHODE IMMERSED IN SAID BATH AT A TEMPERATURE OF 90 TO 140*F., SAID BATH CONTAINING A TOTAL OF DISSOLVED CRO3 OF ABOUT 150 TO ABOUT 380 G./1., STRONTIUM SULFATE AND POTASSIUM SILICOFLUORIDE AS CATALYST-SUPPLYING COMPOUNDS EACH IN AN AMOUNT SUFFICIENT TO SATURATE SAID BATH WITH DISSOLVED CATALYTIC SULFATE AND SILICOFLUORIDE, RESPECTIVELY, AND TO PROVIDE THEREIN AN UNDISSOLVED RESIDUE OF EACH COMPOUND, AND POTASSIUM DICHROMATE AS A NON-CATALYTIC, SILICOFLUO RIDE-SUPPRESSING, CRO3- AND POTASSIUM-SUPPLYING COMPOUND, SAID POTASSIUM DICHROMATE BEING PRESENT IN AN AMOUNT IN EXCESS OF ITS SOLUBILITY IN THE BATH AND PROVIDING: (1) AN UNDISSOLVED RESIDUE THEREOF IN SAID BATH, (2) A PORTION OF SAID TOTAL DISSOLVED CRO3 EQUAL TO ABOUT 30 TO ABOUT 77 BY WEIGHT OF SAID TOTAL, AND (3) AN AMOUNT OF DISSOLVED POTASSIUM TO SUPPRESS THE CONCENTRATION OF THE DISSOLVED SILICOFLUORIDE, THE CONCENTRATION OF SAID PORTION OF DISSOLVED CRO3 BEING SELF-REGULATED OWING TO THE PRESENCE OF SAID DISSOLVED RESIDUE OF POTASSIUM DICHROMATE, SAID SUPPRESSED SILICOFLUORIDE CONCENTRATION BEING SELFREGULATE OWING TO THE PRESENCE OF SAID UNDISSOLVED RESIDUE OF POTASSIUM SILICOFLUORIDE, SAID DISSOLVED POTASSIUM CONCENTRATION BEING SELF-REGULATED BY VIRTUE OF THE PRESENCE OF SAID UNDISSOLVED RESIDUES OF POTASSIUM SILICOFLUORIDE AND POTASSIUM DICHROMATE, AND SAID DISSOLVED SULFATE CONCENTRATION BEING SELF-REGULATED OWING TO THE PRESENCE OF SAID UNDISSOLVED RESIDUE OF STRONTIUM SULFATE, 70 TO 170 G./1. OF SAID TOTAL DISSOLVED CRO3 BEING SUPPLIED AS CHROMIC ANHYDRIDE, THE RELATIONSHIP BETWEEN SAID TEMPERATURE, SAID CHROMIC ANHYDRIDE, SAID TOTAL AND SAID PORTION OF DISSOLVED CRO3 BEING VARIABLE IN THE MANNER DEFINED BY THE AREA EFGHE OF THE GRAPH SHOWN IN THE ACCOMPANYING DRAWING, AND THE RATIO OF TOTAL DISSOLVED CRO3 TO THE SUM OF DISSOLVED SULFATE PLUS 1/2 THE DISSOLVED SILICOFLUORIDE BEING IN THE RANGE OF 115:1 TO 140:1.