Abstract:
Residual chlorides are removed from electrochemically etched aluminum foil by coupling the foil to an inert electrode having a low hydrogen overvoltage while contacting the foil with nitric acid.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the post-treatment of aluminum capacitor foil that has been electrochemically etched in a chloride-containing solution. More specifically, it relates to contacting the etched foil with nitric acid while the foil is galvanically coupled to an inert electrode having a low hydrogen overvoltage. 
     Nitric acid has been used in the prior art to open up the fine etch structure of aluminum and to remove some chlorides. However, residual chlorides have remained in the fine tubes or tunnels, particularly with highly etched foil for high voltage capacitor use. It is well-known that chloride contamination causes aluminum electrolytic capacitor failure. 
     SUMMARY OF THE INVENTION 
     It is a feature of this invention to remove residual chlorides from electrochemically etched aluminum foil, and particularly those residual chlorides entrapped in the fine etch structure of high-voltage capacitor foil. 
     The foregoing and related features of this invention are accomplished by contacting etched rinsed foil with nitric acid while the foil is connected to an electrode that is inert to nitric acid and which has a low hydrogen overvoltage so as to form a galvanic couple to improve the rate of pore opening and the removal of entrapped chlorides. 
     Among the desirable inert electrode materials for this invention are platinum, platinized materials, stainless steel, and graphite. Copper has been used in prior art processes, but unfortunately has the tendency to go into solution and plate out on the aluminum foil surface. Care must be taken with platinized materials to insure that the platinum coating is thick enough so that the underlying material is not exposed to attack by nitric acid. When using graphite, electrode selection should take into account the type of binder used to insure that it is not leached by the nitric acid. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following examples, data are presented for various nitric acid concentrations with the electrode coupled via a cable or bus bar and uncoupled (no connection other than the electrolyte). In some of the examples, aluminum nitrate was added to more closely simulate a commercial bath in which aluminum ion concentration would build-up. The initial chloride level on the etched foil before nitric acid post-treatment varied, but the worst foil to be treated analyzed 10 μg/cm 2  chloride. No chloride could be detected in foil treated in accordance with this invention. Weight loss measures not only chloride removal but also pore opening, which also aids in chloride removal. 
     EXAMPLE 1 
     In this example, high voltage foil was treated with various concentrations of nitric acid alone and when the foil was coupled with a platinized electrode. 
     
                       TABLE 1______________________________________Nitric acid    Treating Bath       Weight loss, %concentration    Time     Temperature                        Not Coupled                                 Coupled______________________________________1.0M     1.5 min  90° C.                        4.26     6.971.0M     3.0 min  90° C.                        10.14    12.570.6M     1.5 min  99° C.                        5.3      7.22.0M     1.5 min  99° C.                        10.3     16.33.0M     1.5 min  99° C.                        13.0     17.9______________________________________ 
    
     EXAMPLE 2 
     In these runs, aluminum nitrate was added to the bath to simulate the bath conditions during continuous use. A platinum sheet electrode and one of 2.5μ Pt electroplated on titanium were used. 
     
         ______________________________________Reaction   Bath       Aluminum  Nitric                              Weight loss %time, sec   Temperature              ion conc. Acid  Pt    Pt/Ti______________________________________60      98° C.              1.0M      1.0M  17.5  16.6120     98° C.              1.0M      1.0M  32.2  32.260      98° C.              2.0M      1.0M  11.9  12.9120     98° C.              2.0M      1.0M  22.8  23.090      99° C.              0.6M      2.0M  27.6  --______________________________________ 
    
     EXAMPLE 3 
     In this Example, the inert electrode coupled to the foil was a 302 stainless steel one. The solution was 1.0 M nitric acid containing 2.0 M aluminum ions. Typical results are shown below. The temperature of the bath was 98° C. 
     
         ______________________________________Reaction          WeightTime, sec.        loss, %______________________________________30                3.860                7.590                11.4120               15.2______________________________________ 
    
     EXAMPLE 4 
     In this Example, results using a graphite electrode are compared with those using platinum and platinum on titanium electrodes. The solution was 1.0 M nitric acid containing 2.0 M aluminum ions. The temperature of the bath was 98° C., and reaction time was 2.0 min. 
     
         ______________________________________Electrode     Weight loss, %______________________________________Pt            22.8Pt/Ti         23.0graphite      24.0______________________________________ 
    
     EXAMPLE 5 
     Because the nitric acid bath developed a yellow color during use with the graphite electrode in the previous example, a different graphite electrode that did not contain a resin binder was evaluated at the same conditions as were used in Example 4 except that the same solution was used throughout with make-up nitric acid being added. A total of 16 foil samples were treated. Results are presented below and compared with those using a platinum electrode. 
     
         ______________________________________Foil samples  Weight loss, %______________________________________#1, #2        24.5#15, #16      19.5Pt electrode  22.8______________________________________ 
    
     Foil samples treated by the process of the present invention were subjected to electron probe microanalysis and no chloride could be detected. A total of 10 points on both sides of the foil were measured. Untreated foil samples varied in chloride concentration with the worst, or &#34;dirty foil&#34;, averaging 10 μg/cm 2  chloride.