Method of electroplating an adherent chromium electrodeposit on a chromium substrate
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electroplating of chromium, and, more particularly, it is concerned with an improved process of forming an adherent chromium electrodeposit on a chromium substrate. 
     2. Description of the Prior Art 
     Normally, electrodeposited chromium does not adhere well to a chromium underlayer, and for this reason electrolysis must not be interrupted during the plating process. If the article needs to be removed from the solution for any reason, the plating process must be restarted in a special way. Fink, in U.S. Pat. No. 1,942,356, describes the only known method of obtaining an adherent chromium electrodeposit on such a chromium substrate. The method disclosed by Fink comprises warming the chromium part to the bath temperature and then applying voltage slowly until the plating current is reached. Subsequently, Weiner and Walmsley included a mild anodic electrochemical etching step in the Fink process, as described in Chromium Plating, Finishing Publications Ltd., Teddington, England (1980) p. 147-8. 
     The Fink process works well for the conventional sulfate, or a mixed catalyst chromium electroplating bath, and for the 25% efficiency bath described by Chesin and Newby in U.S. Pat. No. 4,588,481. 
     Unfortunately, however, it is not satisfactory for the high energy efficiency (HEEF) baths, such as described by H. Chessin in U.S. Pat. No. 4,472,249, where the plating efficiency is 40% or higher. 
     Accordingly, an object of the present invention is to provide an improved process of electro-depositing adherent chromium onto a chromium substrate. 
     Another object herein is to provide such a process which can be used with the HEEF-40% efficiency chromium baths. 
     Still another object is to provide a method which can be applied after an interruption in the plating chromium process. 
     A particular object of this invention is to provide an activation treatment for chromium substrate in a solution which can be added to the chromium plating bath without deleterious effects. 
     SUMMARY OF INVENTION 
     What is described herein is a method of electroplating an adherent chromium deposit on a chromium substrate. The process is characterized by chemically oxidizing the chromium substrate before starting the electrodeposition of chromium thereon. A suitable chemical oxidizing agent is hydrogen ion, which can be furnished by a dilute acid solution. In this invention, the acid is characterized by being destroyed in the plating bath, if dragged therein. Suitable acids include oxalic and citric acids. The process is applicable to any chromium electroplating bath, including high energy efficient chromium baths, such as HEEF-40 baths, which are presently in commercial use. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention can be understood most clearly by comparison with the prior art method, as described below. Accordingly, in the prior art method of Weiner and Walmsley, the chromium substrate is treated after a electroplating process has been interrupted where less than the desired amount of chromium deposit has been formed. The process steps of the prior art, and what happens during each such process step, are summarized below in Table I. 
     
                       TABLE I                                                     
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PRIOR ART                                                                 
Step            What Happens During Process Step                          
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(1) Electrochemically etch                                                
                    Cr → Cr.sup.+3 (no film                        
    chromium substrate in                                                 
                    formation)                                            
    plating bath.                                                         
(2) Plating is initiated at                                               
                    2H.sup.+ → H.sub.2 ; Cr.sup.+6 →        
    less than the Cr                                                      
                    Cr.sup.+3, (the Cr.sup.+3 may                         
    deposition voltage.                                                   
                    form a film)                                          
(3) Voltage is raised to                                                  
                    Adherent Cr is deposited                              
    allow full plating                                                    
                    from conventional, mixed                              
    current. Continue                                                     
                    catalyst and HEEF-25%                                 
    chromium electro-                                                     
                    Cr baths only. (does not                              
    plating.        work for HEEF-40% baths                               
                    of U.S. Pat. No. 4,472,249)                           
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     In the present process, as described in copending application Ser. No. 908,434, filed Sept. 17, 1986, and assigned to the same assignee as herein, the oxide film on the chromium substrate, if present, is removed, chemically or electrolytically, and the substrate is chemically oxidized by hydrogen ion, whereupon a film is formed which is substantially free of Cr +6 . There is no requirement for the slow application of voltage, although it may be used. The present method is usable with all known hexavalent chrome baths, including the commercial HEEF-40% baths which are based on U.S. Pat. No. 4,472,249. 
     In this invention, the hydrogen ion in the activation step of chemically oxidizing the chromium substrate is furnished by a dilute acid which can be destroyed in the plating bath. Suitably, such acids are hydroxy or oxy-substituted carboxylates which are capable of being destroyed in the bath by being oxidized by chromic acid to form a gaseous product, for example, CO 2 . The Cr +3  reduction product then is reoxidized in the bath to Cr +6  at the anode. Accordingly, the use of such oxidizable acids avoids the necessity of water rinsing the activated chromium substrate to prevent drag-in of deleterious chemicals into the plating bath. 
     Suitable acids thus include those acids which provide the necessary hydrogen ion for chemical oxidation of chromium and are themselves oxidized by chromic acid, preferably to form a gaseous product. Representative acids for use herein include oxidizable carboxylic acids, e.g. hydroxy, amino and oxycarboxylic acids, as for example, oxalic, citric, tartaric, glutaric and formic acid. The present invention will be illustrated by reference to Table II. 
     
                       TABLE II                                                    
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PRESENT INVENTION                                                         
Process Step    What Happens During Process Step                          
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(1) Remove oxide film on                                                  
                    2H.sup.+ → H.sub.2                             
    chromium substrate and                                                
                    Cr → Cr.sup.+3                                 
    initiate chemical                                                     
                    A gray/green/black film                               
    oxidation in defined                                                  
                    form on the Cr surface                                
    acid, as evidenced                                                    
    by H.sub.2 evolution (if                                              
    necessary, briefly                                                    
    apply cathodic current                                                
    to initiate H.sub.2 evolu-                                            
    ation film on Cr sub-                                                 
    strate, e.g. 1 min.                                                   
    at 1 asi). Continue                                                   
    H.sub.2 evolution with-                                               
    out current for about                                                 
    2-3 minutes.                                                          
(2) Plating is commenced in                                               
                    Adherent Cr is deposited                              
    any Cr.sup.+6 bath.                                                   
                    (applicable to HEEF-40%                               
                    baths); Acid is oxidized                              
                    by chromic acid to form                               
                    a gaseous product, and                                
                    Cr.sup.+3, which is reoxided                          
                    to Cr.sup.+6 at the anode.                            
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     The invention will be further illustrated by the accompanying examples. 
     EXAMPLE 1 
     A chromium substrate having an oxide thereon was placed in a solution of 15% by weight oxalic acid at room temperature and made electrically cathodic, whereupon the oxide film was removed and evolution of H 2  commenced. The current was then turned off and H 2  evolution was allowed to continue for 3 min. A gray/black film formed on the chromium substrate. The substrate then was plated with chromium from a commercial M&amp;T Chemicals, Inc. HEEF-40% chromium bath, based on U.S. Pat. No. 4,472,249, at 5 asi for 30 min. Adhesion of the chromium deposit was excellent. 
     EXAMPLE 2 
     The process of Example 1 was repeated using solutions of citric acid or tartaric acid in place of oxalic acid at tempratures ranging from room temperature to 60° C.; and excellent adhesion of the chromium deposit on the treated chromium substrate was obtained in each instance. 
     EXAMPLE 3 
     The process of Example 1 was repeated using a chromium substrate which had been plated with chromium several months previously and had a thick oxide coating thereon. This substrate first was made anodic in a 100 g/l NaOH solution for 3 minutes at 3 asi and water rinsed before following the steps of Example 1. Excellent adhesion of the chromium deposit was obtained. 
     Similar results were attained when 10% H 2  SO 4  or 250 g/l CrO 3  solution were substituted for the 100 g/l NaOH electrolyte in the first step above. In fact, the acid bath of Examples 1 and 2 can be used for this step. 
     EXAMPLE 4 
     The processes of Examples 1-3 were repeated using (a) a conventional chromium plating bath, and (b) a HEEF-25% bath (U.S. Pat. No. 4,588,481), with similar excellent adhesion of chromium on the chromium substrate. 
     While the invention has been described with respect to certain embodiments thereof, it will be understood that changes and modifications may be made which are within the skill of the art. Accordingly, it is intended to be bound only by the following claims, in which: