Abstract:
A method for color-anodizing (hereinafter referred merely to &#34;coloring&#34;) aluminum and aluminum alloys in a short period of time. The method comprises adding sulfosalicylic acid or sulfophthalic acid and along with sulfuric acid to a solution, of which the main ingredient is citric acid, succinic acid, to form an electrolyte and effecting anodic oxidation on surfaces of aluminum and aluminum alloys with the electrolyte to form a colored oxide coating.

Description:
This invention relates to an improved method for coloring and electrolyzing aluminum and aluminum alloys. 
     The method according to the present invention is capable of reducing the time required for coloring aluminum and aluminum alloys and the quantity of electric current required for electrolysis process, over that required by the prior art. 
     BACKGROUND OF THE INVENTION 
     In conventional methods for coloring and electrolyzing aluminum and aluminum alloys, electrolytic time requires, for example, more than 30 minutes, and in addition, an energy quantity generally, more than 1.5 Ampere/dm 2 , which results in a high cost for the finished aluminum and aluminum alloy. 
     It is therefore an object of this invention to improve conventional methods as noted above and to provide a method for coloring aluminum and aluminum alloys, which can cut electrolytic time and minimize the energized quantity required for electrolysis. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention provides a method for coloring aluminum and aluminum alloys which comprises adding sulfosalicylic acid and/or sulfophthalic acid along with sulfuric acid to a solution, of which the main ingredient is citric acid, succinic acid, to form an electrolyte; and effecting anodic oxidation on surfaces of aluminum and aluminum alloys with the electrolyte to form a colored oxide coating in a shorter period of time than is required by the prior arts. 
     Although a combination of citric acid, succinic acid, sulfocitric acid or sulfosuccinic acid (each being a treating agent as a main ingredient) and sulfosalicylic acid or sulfophthalic acid (each being a treating agent to be added), as noted above, may variously be considered as the case may be, there exists no difference in function of the coloring reaction in any case. 
     In accordance with the present method, the sulfosalicylic acid or sulfophthalic acid (treating agent to be added) serves to facilitate coloring of the anodic oxidation coating of aluminum and aluminum alloys, to cut electrolytic time and to minimize the energy quantity that is required. 
     DETAILED DESCRIPTION 
     The present invention will become more apparent from the following description. 
     As described above, the electrolyte employed in the method of the present invention may variously be combined as follows: 
     Citric acid, sulfosalicylic acid, and sulfuric acid; 
     Citric acid, sulfophthalic acid, and sulfuric acid; 
     Succinic acid, sulfosalicylic acid, and sulfuric acid; 
     Succinic acid, sulfophthalic acid, and sulfuric acid; 
     The detail of coloring reaction of electrolyte according to the respective combinations is given in the examples later described. 
     Various particulars used in the present method are given as follows: 
     
         ______________________________________Concentration of solution, of whichmain ingredient is citric acid orsuccinic acid        5 - 100 g/lit.Quantity of addition where sulfo-salicylic acid or sulfophthalicacid is added as an additive                1 -  30 g/lit.Quantity of sulfuric acid                0.1 - 5.0 ml/lit.Liquid temperature of electrolyte                10 - 40°CDensity of electric current                0.5 - 2.0 A/dm.sup.2                In case of DC0.5 - 2.0 A/dm.sup.2 D.C. plus 0.1 - 2.0 A/dm.sup.2 A.C.______________________________________ 
    
     The method of the present invention was carried out in accordance with the above-described particulars, and as a result, it has been proved that a standard energized quantity which was enough to accomplish this process was 0.5 - 1.0 A/dm 2  and the electrolytic time was 5 - 15 minutes to insure a color tone similar to that of conventional colored products. 
     In conventional methods for electrolyzing and coloring, merely one row of workpieces which are anode could be treated in the intermediary of cathodes, whereas in the method of the present invention, even two rows of workpieces could be treated between cathodes without creating substantial irregularities of color tone. 
     From the foregoing, it will be appreciated that the method of the present invention may increase the efficiency of production and decrease the cost of production. 
     Examples of the present invention will now be described in the following: 
    
    
     EXAMPLE 1 
     
         ______________________________________Electrolyte compositionCitric acid        50 g/lit.Sulfuric acid      0.5 ml/lit.Sulfosalicylic acid              20 g/lit.Liquid temperature 20°C ± 2°CElectrolytic time  10 minutesDensity of electric current              0.5 A/dm.sup.2 - 1.5 A/dm.sup.2 DCMaterials usedAluminum alloy 63S in accordance with theJapanese Industrial Standard is composed of:(Aluminum alloy 6063 in America)Copper             Less than 0.1 %Silicon            0.2 - 0.6 %Iron               Less than 0.35 %Manganese          Less than 0.1 %Magnesium          0.45 - 0.09%Zinc               0.1 %Bromine            Less than 0.1 %Titanium           Less than 0.1 %Aluminum           The remainder______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         ______________________________________                             THICKNESSDENSITY OF   FINAL      COLOR     OFELECTRIC CURRENT        VOLTAGE    TONE      COATING______________________________________0.50 A/dm.sup.2        60 V       Amber     2 micron0.75 A/dm.sup.2        71 V       Amber     3 micron1.00 A/dm.sup.2        76 V       Amber     4 micron1.50 A/dm.sup.2        85 V       Somewhat  6 micron                   dark amber______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________Electrolyte composition Same as Example (1)Liquid Temperature     12°C - 40°CDensity of electric current                  1.0 A/dm.sup.2 DCElectrolytic time      10 minutes______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         __________________________________________________________________________   FLUCTUATION OFLIQUID TEMP.   LIQUID TEMP.             FINAL VOLTAGE                       COLOR TONE__________________________________________________________________________12°C   12  -  13°C             86 V      Pale amber15°C   14.5 - 15.1°C             85 V      Pale amber20°C   19.5 - 20.5°C             76 V      Medium                       amber30°C   29.0 - 31.0°C             71 V      Somewhat                       dark amber40°C   39.5 - 40.5°C             60 V      Dark amber__________________________________________________________________________ 
    
     As shown in this example (2), coloring can satisfactorily be achieved even at a high level of liquid temperature i.e. much higher than a standard value of electrolyte temperature generally in use such as 40° C above. 
     EXAMPLE 3 
     
         ______________________________________Electrolyte composition                 Same as Example (1)______________________________________Liquid temperature    20°C ± 2°CDensity of electric current                 1.0 A/dm.sup.2 DCElectrolytic time     5 - 40 minutes______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         ______________________________________ELECTROLYTIC      FINAL                 THICKNESSTIME       VOLTAGE   COLOR TONE  OF COATING______________________________________ 5 minutes  55V      Pale amber  2 micron10 minutes  76 V     Medium amber                            4 micron20 minutes 102 V     Dark amber  8 micron30 minutes 120 V     Bronze      12 micron40 minutes Over 125 V                Somewhat black                            16 micron______________________________________ 
    
     In order to obtain a dark color darker than the dark amber in accordance with the prior art methods, a thickness of coating of more than 25 micron and an electrolytic time more than 40 minutes is required. In accordance with the present method, the dark color darker than the dark amber is produced within 20 minutes. 
     EXAMPLE 4 
     
         ______________________________________Electrolyte compositionCitric acid, 50 g/lit.Sulfuric acid, 0.5 ml/lit.Sulfosalicylic acid, 1.0 g/lit. - 30 g/lit.Liquid temperature    20°C ± 2°CElectrolytic time     10 minutesDensity of electriccurrent               1 A/dm.sup.2 DC______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         ______________________________________QUANTITY OFADDITION OFSULFOSALICYLIC        FINALACID         VOLTAGE     COLOR TONE______________________________________1.0   g/lit.     73 V        Dark amber5.0   g/lit.     74 V        Somewhat dark amber10.0  g/lit.     75 V        Medium amber20.0  g/lit.     76 V        Medium amber30.0  g/lit.     78 V        Medium amber______________________________________ 
    
     When sulfosalicylic acid is added in a quantity of less than 0.5 g/lit., a chapping pattern appears, and where it is added in a quantity of less than 0.2 g/lit., coloring is only partially produced and a pattern of the grain of the wood appears. 
     EXAMPLE 5 
     
         ______________________________________Electrolyte compositionCitric acid         5 g/lit. - 100 g/lit.Sulfuric acid       0.5 ml/lit.Sulfosalicyclic acid               20 g/lit.Liquid temperature  20°C ± 2°CElectrolytic time   10 minutesDensity of electric current               1 A/dm.sup.2 DC______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         ______________________________________QUANTITY OFADDITION OF    FINALCITRIC ACID    VOLTAGE     COLOR TONE______________________________________ 5 g/lit.      70 V        Medium amber10 g/lit.      70 V        &#34;30 g/lit.      75 V        &#34;50 g/lit.      76 V        &#34;60 g/lit.      80 V        &#34;______________________________________ 
    
     Where citric acid is added in a quantity in the range of from 5 g/lit. to 20 g/lit., a somewhat rough pattern appears, but practical use is not affected thereby. Most stabilized coloring may be obtained by the addition of citric acid in the range of from 40 g/lit. to 60 g/lit. with the electrolytic time extended, and in this case, the workpiece can be colored evenly. 
     EXAMPLE 6 
     
         ______________________________________Electrolyte compositionCitric acid          50 g/lit.Sulfuric acid        0.3 - 0.6 m/lit.Sulfosalicylic acid  20 g/lit.Liquid temperatue    20°C ± 2°CElectrolytic time    10 minutesDensity of electric current                1 A/dm.sup.2 DC______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         ______________________________________QUANTITY OFADDITION OF    FINALCITRIC ACID    VOLTAGE     COLOR TONE______________________________________0.3 ml/lit.    100 V       Yellowish amber0.4 ml/lit.    85 V        Amber0.5 ml/lit.    76 V        Amber0.6 ml/lit.    60 V        Darkish amber______________________________________ 
    
     EXAMPLE 7 
     
         ______________________________________Electrolyte compositionSuccinic acid     30 g/lit.Sulfuric acid     0.18 ml/lit.Sulfosalicylic acid             10 g/lit.Liquid temperature             20°C + 2°CDensity of electric current             1.0 A/dm.sup.2 DCElectrolytic time 2.5 minutes to 15 minutes______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         ______________________________________ELECTROLYTIC   FINALTIME           VOLTAGE     COLOR TONE______________________________________2.5 minutes     90 V       Pale indigo amber5 minutes      102 V       Medium indigo amber10 minutes     115 V       Amber15 minutes     132 V       Amber______________________________________ 
    
     EXAMPLE 8 
     
         ______________________________________Electrolyte composition Citric acid      30 g/lit. Sulfuric acid    0.18 ml/lit. Sulfophthalic acid             10 g/lit.Liquid temperature             20°C ± 2°CDensity of electric current             1.0 A/dm.sup.2 DCElectrolytic time 2.5 minutes - 15 minutes______________________________________ 
    
     The test results obtained by the aforementioned particulars are as follows: 
     
         ______________________________________ELECTROLYTIC   FINALTIME           VOLTAGE     COLOR TONE______________________________________2.5 minutes     93 V       Pale indigo amber5 minutes      105 V       Medium indigo amber10 minutes     117 V       Amber15 minutes     135 V       Amber______________________________________