Cathodic deposition of oxide coatings
BACKGROUND OF THE INVENTION 
     The importance of titanium, in the manufacture of air frame and space-craft structures, for example, because of its superior heat resistance, high strength, and relatively light-weight, is now well established. Along with the use of titanium in sheet form, forgings, and stampings, titanium is a promising candidate for wide use in high-strength, lightweight laminated structures and composite assemblies fabricated by weld-bonding, diffusion bonding, and adhesive bonding. 
     It is also well recognized in the metal processing field that it is difficult to bond titanium to other materials, as in sandwich or composite structures, because organic adhesives do not bond well to titanium. Thus a number of special processes for preparing the surface of titanium for adhesive bonding heretofore have been proposed, which processes, however, tend to be relatively complex, time-consuming, and expensive. Time and expense are important considerations, and especially so in the mass production of aircraft, spacecraft, and automobiles, for example. 
     DESCRIPTION OF THE PRIOR ART 
     The following patents are cited herein as the most pertinent prior art of which the applicant is aware: 
     
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Number       Name           Date                                          
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Group I                                                                   
2,275,223    Hardoen        Mar. 1942                                     
2,733,199    Wick           Jan. 1956                                     
3,446,717    Farquhar et al May 1969                                      
3,574,069    Roberts        Apr. 1971                                     
4.007,099    Wu             Feb. 1977                                     
Group II                                                                  
2,825,682    Missel et al   Mar. 1958                                     
3,640,778    Winfree et al  Feb. 1972                                     
3,959,091    Moji et al     May 1976                                      
3,989,876    Moji et al     Nov. 1976                                     
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     The Group I patents all relate to the cathodic coating of metal surfaces. For example U.S. Pat. No. 2,275,223 (Hardoen) discloses the application of an artificial magnetite coating onto iron or steel by suspending the iron or steel in an electrolytic solution as the cathode. 
     Wick (U.S. Pat. No. 2,733,199) teaches the cathodic deposition of a hydrated oxide of chromium on a metal surface such as iron, steel, or zinc. 
     Farquhar et al (U.S. Pat. No. 3,446,717) discloses the cathodic treatment of metals to form a protective chromate coating thereon. 
     U.S. Pat. No. 3,574,066 (Roberts et al) discloses a process for cathodically depositing a thin film of chromium on a steel strip. 
     Wu (U.S. Pat. No. 4,007,099) discloses a method for the cathodic production of micropores on a decorative metal plate wherein the metal base to be treated is used as the cathode. 
     The Group II patents all relate to processes for coating titanium. 
     Missel et al (U.S. Pat. No. 2,825,682) for example, discloses a process for coating the surfaces of titanium base alloys containing about 5% chromium and about 3% aluminum with a metallic film as a preparatory step for subsequent electroplating. 
     Winfree et al (U.S. Pat. No. 3,640,778) teaches a process for providing titanium alloys with a chemically bonded high-temperature resistant coating, which coating subsequently requires heat treatment at a temperature of from 650°-950° F. 
     U.S. Pat. Nos. 3,959,091 and 3,989,876, both of Moji et al, relate to a process and article, respectively, in which porous, adhesion-promoting oxide coatings are produced on titanium by anodizing in an aqueous solution containing fluoride ions and one or more oxidizing electrolytes at current densities ranging from 0.25 to 5 amp./ft 2 . 
     While it is possible that more pertinent art exists, the applicant&#39;s search is believed to have been conducted with conscientious effort to locate and evaluate the most pertinent prior art available at the time, but the above prior art statement is not to be construed as a representation that no better prior art exists. 
     In view of the prior art, it is an object of my present invention to provide a process for depositing a durable, tenaceous oxide coating on titanium, which coating is ideally suitable for adhesive bonding. 
     It is further object of my invention to provide a simple, inexpensive, and rapid process for depositing a durable, tenaceous oxide coating on the surface of titanium or titanium alloys. 
     Other objects and advantages gained with my invention will be readily seen by those skilled in the metal processing arts. 
     SUMMARY OF THE INVENTION 
     A durable, tenaceous oxide layer is deposited on the surface of titanium or titanium alloys by suspending the titanium or titanium alloy part, to be processed, as the cathode in a solution containing isoproponol and a metal salt selected from the group including aluminum nitrate, nickelous nitrate, cobalt nitrate, and cupric nitrate, and electrolyzing said part at current densities ranging from 0.02 amp./in. 2  to 0.5 amp./in. 2 , for processing times ranging from 5 to 60 seconds. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the primary object of my invention is to provide a rapid, inexpensive process for depositing oxide coatings on titanium and titanium alloys, I have discovered that my process also can be carried out with surprisingly good results on aluminum and aluminum alloys, stainless steel, AZ 31 magnesium, and graphite composites. 
     One particular advantage in the process of my invention is that satisfactory oxide coatings are deposited very rapidly. Normal processing times can be reduced to 5 to 20 seconds at ambient temperature, although any temperature up to the boiling point of the solvent can be used. Longer processing times will of course produce coatings of greater thickness. 
     As will be adduced in the following examples, varying concentrations of the metal salt can be employed. Although all of the examples employ a solution composed of a metal salt dissolved in isoproponol, I have found that any of the metal salts dissolved in an aqueous solution will produce a satisfactory oxide coating, although, as taught in the examples, I have used isoproponol as the preferred solvent. 
     The oxide layer produced by the process of my invention exhibits a number of features of particular importance in the mass production of laminates and composite structures which are finding a much higher rate of application in aircraft, spacecraft, and automobiles. Among the more promising advantages are lower processing times hence lower costs, and the relatively superiority of the oxide layer due to its tenaceous character, compatibility i.e., chemical neutrality, with other materials including adhesives. Generally, oxides of titanium tend to be unstable; aluminum oxides, on the other hand, are not. Other oxides, such as those produced by cobalt nitrate, cupric nitrate, nickelous nitrate also are found to be compatible with other metals and adhesives. 
     In view of the advantage cited above, the process of my invention should be equally applicable to weld bonding wherein satisfactory spot welds that penetrate through the oxide and adhesive layers are required. 
    
    
     The following are examples carrying out the process of my invention for depositing an oxide coating or layer on the surface of a metal part. 
     EXAMPLE I 
     Specimens of Ti.6Al.- 4V. alloy were pre-processed by (1) degreasing the specimens in methyl ethyl ketone, (2) cleaning the specimens in a 10% solution of hydrofluoric acid, (3) rinsing the specimens in water, and (4) finally drying the part. The specimens were than suspended as the cathode in a solution containing 8 grams of aluminum per liter of isoproponol and electrolyzed at a current density of 0.07 amp./in. 2  for 10-15 seconds. The resultant oxide layers produced by this process were porous and uniform with a thickness ranging from 700-1000 A. 
     EXAMPLE II 
     A Ti.Al.-4V specimen was preprocessed by the steps (1)-(4) described in Example I. The specimen was then suspended as the cathode in a solution of 20 grams of cupric nitrate per liter of isoproponol and electrolyzed at a current density of 0.1 amp./in. 2  for 30 seconds. A SEM microphotograph showed the resultant copper oxide layer to be approximately 1500A in thickness. 
     EXAMPLE III 
     A Ti.Al.-4V. specimen was preprocessed by the steps (1)-(4) described in Example I, and subsequently suspended as the cathode in a solution containing 20 grams of cobalt nitrate per liter of isoproponol, and electrolyzed at a current density of 0.1 amp./in. 2  for 30 seconds. An SEM microphotograph showed the resultant cobalt oxide layer to be approximately 1100A in thickness. 
     EXAMPLE IV 
     A Ti.Al.-4V. specimen was preprocessed with steps (1)-(4) described in the previous Examples, and suspended in a solution containing 20 grams of nickelous nitrate per liter isoproponol, and electrolyzed at a current density of 0.1 amp./in. 2  for 30 seconds. A SEM microphotograph showed the resultant layer of nickel oxide to be approximately 1600A in thickness. 
     EXAMPLE V 
     Specimens of AZ magnesium alloy were preprocessed with the steps (1)-(4) described in the previous Example, and subsequently suspended as the cathode in a solution containing 25 grams per liter of isoproponol and electrolyzed at a current density of 0.5 amp./in. 2  for 5 seconds. The specimens exhibited improved corrosion resistance in a salt spray environment. 
     EXAMPLE VI 
     Specimens of 7075 aluminum alloy were preprocessed by (1) vapor degreasing, (2) alkaline cleaning, (3) rinsing, (4) deoxidizing, (5) rinsing, and (6) drying. Subsequently the specimens were suspended as the cathode in a solution containing 0.3 gram of aluminum per liter of isoproponol, and electrolyzed at a current density of 0.02 amp./in. 2  for 45 and 60 seconds. 
     The specimens were sprayed with BR 127 adhesive primer. BR 127 is a proprietary product of 3 M Company and bonded with FM 73 adhesive. FM 73 also is a proprietary product of 3 M Company. Table I shows the results of lap shear tests with the Boeing phosphoric anodize and the FPL pretreatments as controls. 
     EXAMPLE VII 
     Ti.Al-4V. alloy wedge test specimens were preprocessed using steps (1)-(4) described in Example I. 
     The specimens were then suspended as the cathode in a solution containing 25 grams of aluminum nitrate per liter of isoproponol and electrolyzed at a current density of 0.4 amp./in. 2 . The specimens were then bonded with FM 400 adhesive, both unprimed and primed with BR 400 adhesive primer. FM 400 is a proprietary product of 3 M Company and BR 400 is a proprietary product of 3 M Company. Table II shows the average crack growths observed when the bonded specimens were exposed to 120° F condensing humidity with the modified phosphate fluoride etch as a control. 
     EXAMPLE VIII 
     Titanium 6Al.4V. lap shear specimens were preprocessed in accordance with steps (1)-(4) described in Example I, and the suspended as the cathodes in a solution containing 2 grams of aluminum nitrate per liter of isoproponol, and electrolyzed at a current density of .15 amp./in. 2  for times of 30, 45, and 60 seconds. 
     The specimens were then sprayed with BR 127 adhesive primer and bonded with FM 300k adhesive. BR 127 is a proprietary product of 3 M Company and FM 300k is a proprietary product of 3 M Company. Table III shows the results of the lap shear tests using the Turco 5578 process as a control. 
     EXAMPLE IX 
     Specimens of AM 355 stainless steel were vapor degreased and alkaline cleaned, and the suspended as cathodes in a solution containing 2 grams of aluminum nitrate per liter of isoproponol, and electrolyzed for times of 30, 45, and 60 seconds at a current density of .10 amp./in. 2 . 
     The parts were then sprayed with BR 127 adhesive primer and bonded with FM 300k adhesive. Table IV shows the results of the T-peel tests. Turco 5578 control was used. 
     EXAMPLE X 
     The surfaces of a specimen of epoxy-graphite composite was abraded with sandpaper and cleaned with methyl ethyl ketone. The specimen was then suspended as the cathode in a solution containing 25 grams of aluminum nitrate per liter of isoproponol and electrolyzed at a current density of 0.3 amp./in. 2  for 10 seconds. A tenaceous coating of oxide approximately 1000A in thickness was observed on the surface of the specimen. 
     
                       Table I                                                     
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Standard Lap Shear Values for                                             
7075-T6 Aluminum Alloy                                                    
                   Ultimate strength                                      
                                Mode of failure                           
Process Process time                                                      
                   PSI          % Cohesive                                
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Cathodic                                                                  
Oxide   45 sec.    5675         100                                       
Cathodic                                                                  
Oxide   60 sec.    5710         100                                       
FPL Etch                                                                  
(control)                                                                 
        12 min.    5420         100                                       
Phosphoric                                                                
Anodize 30 min.    5290         100                                       
(control)                                                                 
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                       Table II                                                    
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Wedge Test Results for 6Al, 4V Titanium Alloy                             
Unprimed                                                                  
               Average Crack   Failure                                    
Process time   Growth (inches) mode                                       
Process seconds    1 hr.   24 hr.                                         
                                 72 hr.                                   
                                       % cohesive                         
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Cathodic                                                                  
Oxide   10         0.11    0.22  0.27  95-100                             
Cathodic                                                                  
Oxide   15         0.21    0.23  0.33  95-100                             
Phosphate-                                                                
Fluoride           0.25    1.2         10-20                              
BR400 Primed                                                              
Cathodic                                                                  
Oxide   10         0.27    0.30  0.38  90-100                             
Cathodic                                                                  
Oxide   15         0.27    0.30  0.39  90-100                             
Phosphate-                                                                
Fluoride           0.25    1.1         10-20                              
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                       Table III                                                   
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Standard Lap-Shear Values for                                             
6A1 4V Titanium Alloy                                                     
                   Ultimate strength                                      
                                Mode of failure                           
Process Process time                                                      
                   PSI          % Cohesive                                
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Cathodic                                                                  
Oxide   30 sec.    5950         100                                       
Cathodic                                                                  
Oxide   45 sec.    5685         100                                       
Cathodic                                                                  
Oxide   60 sec.    5750         100                                       
Turco 5578                                                                
control  5 min.    6650         100                                       
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                       Table IV                                                    
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T-Peel Test Values for AM355 Stainless Steel                              
                    peel values Mode of failure                           
Process Process time                                                      
                    (inch pounds)                                         
                                % Cohesive                                
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Cathodic                                                                  
Oxide   30 sec.     12.5        60                                        
Cathodic                                                                  
Oxide   45 sec.     14.5        70                                        
Cathodic                                                                  
Oxide   60 sec.     10.7        30                                        
Turco 5578                                                                
control  5 min.     15.0        70                                        
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