Abstract:
The formation of insoluble oxide precipitates during the electrodepositionf cobalt and cobalt-base alloys from an acidic aqueous cobalt sulfate electrolyte in the presence of an insoluble anode is suppressed by adding the salt or oxide of a metal selected from the group consisting of vanadium, titanium, and molybdenum. While the additive participates in the electrochemical activity occurring at both the anode and the cathode, there is no codeposition thereof with the electroplated metal or metals.

Description:
The invention described herein may be manufactured, used, and licensed by or for the Government for Governmental purposes without the payment to us of any royalties thereon. 
    
    
     The present invention is a continuation-in-part of our co-pending application, Ser. No. 736,901, filed on Oct. 29, 1976 and now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the electroplating of cobalt and cobalt base alloys on metal substrates and is more particularly directed to an improved electrolyte therefor which will suppress or inhibit the formation of undesirable precipitates in the presence of an insoluble or passive anode. 
     Certain critical areas of the interior bore surfaces of gun tubes are customarily coated with a layer of chromium plating to protect against the erosion and wear encountered during the firing of projectiles therethrough. Recent investigation of other plating materials has demonstrated that even greater protection is afforded by a coating of cobalt or an alloy of cobalt and iron. However, while chromium can be successfully electroplated by techniques which utilize an insoluble or passive anode, such is not the situation in regard to the deposition of cobalt and iron since the particular chemistry involved produces insoluble oxide precipitates which not only interfere with the efficiency of the plating process but are also codeposited with the desired metals to the detriment of the mechanical and physical properties thereof. Although these difficulties can be partially avoided by the employment of an appropriate soluble anode within the gun tube, the continuous dimensional change in the diameter thereof during the depletion of the plating metal therefrom complicates the desired attainment of a deposit of uniform thickness. Consequently, the electroplating of cobalt and cobalt base alloys on the interior bore surfaces of gun tubes has heretofore been limited to those sizes in which the bore diameters thereof are 20mm or larger. 
     It is therefore an object of the present invention to provide an improved method for electroplating a layer of cobalt or an alloy thereof on the interior bore surfaces of metal tubes at ambient temperatures to provide a corrosion-resistant ductill coating thereon. Ambient temperatures are normally room temperatures in the vicinity of 20° C. 
     It is another object of this invention to provide an electroplating method, as aforesaid, which will produce cobalt and cobalt-iron deposits of uniform thickness even in gun tubes with bore diameters of less than 20mm in size. 
     A further object of this invention is to provide an electroplating method, as aforesaid, wherein the electrolyte therefor includes an additive capable of inhibiting or suppressing the formation of the undesirable cobalt and iron oxides heretofore produced in the presence of an insoluble or passive anode. 
     Still another object of this invention lies in the provision of an electrolyte, as aforesaid, wherein the additive participates in the electrochemical activity occurring at both the anode and the cathode and yet is not codeposited with the electroplated metals. 
     These and other objects will become apparent from the following detailed description and claims. 
     SUMMARY OF THE INVENTION 
     It has been found that the foregoing objects can best be achieved by carrying out the electroplating from a 1-molar concentration of aqueous cobalt sulfate electrolyte maintained at a nominal pH of 1.0 to which has been added the salt or oxide of a metal selected from the group consisting of vanadium, titanium, and molybdenum. When a solution of this type is electrolyzed in the presence of an insoluble anode, such as platinum, the preferred additive of vanadium pentoxide (V 2  O 5 ) forms ions which interact with the trivalent cobalt ions to effect the reduction thereof to the bivalent ions which are, in turn, readily reducible to elemental metal at the cathode. Because of its position in the electromotive series, vanadium cannot be electrodeposited from an acid media. Consequently, the reduction of the vanadium ions at the cathode is counterbalanced by the subsequent oxidation thereof at the anode together with the continuing interaction between the vanadium and the trivalent cobalt ions. Such simultaneous electrochemical activity maintains a balance of the additive ions in the various oxidation states thereof during the cyclic electroplating process. Thus, because these additive metals are not consumed or codeposited, they can be considered as catalysts. In the event a cobalt-iron alloy is to be plated, the vanadium ions will not only prevent the formation of ferric oxide (Fe 2  O 3 ) in the anode region but also serve to reduce the quantity of the ferric ions in the vicinity of the cathode to thereby prevent the codeposition of ferric hydroxide (Fe(OH) 3 ) which would otherwise increase the plating stress and lead to a relatively brittle deposit. 
    
    
     DESCRIPTION OF THE INVENTION 
     In order to achieve maximum catalytic action from the foregoing metallic additives, the cobalt is deposited from an aqueous cobalt sulfate solution of 1-molar concentration to which sufficient sulfuric acid (H 2  SO 4 ) is added to bring the pH down to a nominal value of 1.0. The solution is prepared by dissolving 280 grams/liter of cobalt sulfate (CoSo 4 .7H 2  0) in water and also includes boric acid (H 3  BO 3 ) in a concentration of about 30 grams/liter to stabilize the pH at the desired value. However, satisfactory plating is also achieved with an electrolyte wherein the concentration of the cobalt sulfate ranges from 250-450 grams/liter, the boric acid concentration rises as high as 40 grams/liter, and the sulfuric acid addition provides a pH of up to 2.0 When iron is to be deposited along with the cobalt, the former is added to the electrolyte in the corresponding sulfate form in the particular concentration which will produce the desired alloy ratio between the cobalt and the iron. The relationship between the concentration of iron in the electrolyte and the percentage of iron desired in the plating deposit can be readily calculated by the formula % Fe deposit = 0.416 C where C is the concentration of the iron salt in grams/liter. In order to provide a satisfactory deposit, the alloy should possess an iron content of from 0.83-12.5% by weight and such range is provided when from 2-30 grams of ferrous sulfate (FeSo 4 .7 H 2  O) is added to each liter of cobalt sulfate solution. But where a deposit of maximum ductility is desired, the iron therein is preferably limited to 9-11% by weight to provide a face centered cubic structure at all temperatures up to the melting point of the alloyed metal. Such limitation is achieved when the electrolyte is formulated from the following composition: 
     
         ______________________________________CoSO.sub.4 ·7 H.sub.2 O          383 grams/literFeSO.sub.4 ·7 H.sub.2 O          21-26 grams/literH.sub.3 BO.sub.3          30 grams/literH.sub.2 SO.sub.4          sufficient to provide a pH of 1.0______________________________________ 
    
     It has been found that the best deposition of both cobalt and iron from these electrolytes is achieved with an insoluble anode fabricated of platinum and various substrate metals dimensioned to provide a 2:1 cathode to anode ratio. It was also found desirable to plate at temperatures from about 20° C. to 60° C. at a current density ranging from 2-10 amps/sq. decimeter. However, increasing the minimum electrolysis temperature to 40° C. will improve the plating efficiency as well as the optimum physical properties of the end product. 
     In the past, the electroplating of cobalt and cobaltiron alloys on the interior bore surfaces of gun tubes has generally been limited to those of larger diameter (20mm or greater) due to the dimensional changes incurred during the rapid and frequently erratic depletion of the soluble anode. When electric current is passed through a conventional cobalt sulfate electrolyte, Co 2+  ions are produced which are eventually reduced to form a deposit of pure cobalt at the cathode. However, when the soluble anode was replaced by an insoluble anode in an effort to improve the uniformity of the cobalt plating within a gun tube having a bore diameter less than 20mm, the resulting plating was highly unsatisfactory both as to appearance and ductility. It was found that the presence of the insoluble anode in the electrolyte resulted in the excessive discharge of oxygen at the anode according to the reaction 2H 2  O→4H +  + O 2  + 4e -  and this together with the presence of Co 3+  ions lead to the formation of the black cobalt oxide (Co 2  O 3 ) particles which codeposit with the cobalt to the detriment of the desired ductility thereof. 
     However, when a salt or oxide of such multivalent metals as vanadium, titanium, and molybdenum is added to the electrolyte of the present invention, the electrolysis products thereof interact with the Co 3+  ions to produce the soluble Co 2+  ions which are readily reduced to the desired Co at the cathode. While titanium and molybdenum additives are equally effective because they do not codeposit in the particular electrolytes involved in the deposition of cobalt, vanadium pentoxide (V 2  O 5 ) is preferred in view of the lower cost and ready availability thereof. Upon the addition of from 2.3-4.3 grams/liter to the electrolytic solution, the V 2  O 5  dissolves to form VO 2   +  ions in accordance with the reaction V 2  O 5  + 2H +  →2VO 2   +  + H 2  O. During electrolysis, the majority of the VO 2   +  ions are reduced to V 2+ , V 3+  and VO 2+  at the cathode in accordance with the particular applied potential and solution pH. These lower oxidation state ions interact with the Co 3+  ions to form the more stable Co 2+  ions in accordance with the reactions V 3+  + Co 3+  →Co.sup. 2+ + VO 2+  and V 2+  + Co 3+  →Co 2+   + V 3+ . As shown in Table I below, a concentration of 2.8 grams of V 2  O 5  per liter of electrolyte is required to completely suppress the formation of Co 2  O 3  particles. However, because the buildup of the Co 3+  ions and Co 2  O 3  is dependent on the length of time of the electrolysis process, a concentration of 4.3 grams per liter is actually required to completely suppress any formation of the undesirable Co 2  O 3 . 
     
                       TABLE______________________________________Concentration     Concentration                 Concentration                             Duration ofof V.sub.2 O.sub.5 in     of Co.sup.3+ ions                 of Co.sub.2 O.sub.3 in                             electolysisgrams/liter     in moles/liter                 grams/liter in hours______________________________________0         1.20 × 10.sup.-3                 0.07        0.50         5.50 × 10.sup.-3                 0.44        5.51.0       1.60 × 10.sup.-3                 0.08        3.52.3       0.56 × 10.sup.-3                 trace       .52.3       1.04 × 10.sup.-3                 0.025       5.52.8        .90 × 10.sup.-3                 .00         3.54.3       0           .00         8.0______________________________________ 
    
     While Table I clearly shows the suppressive effect of various concentrations of V 2  O 5  on the formation of Co 3+  ions and the consequent Co 2  O 3  particles, the additive is equally effective in suppressing the formation of Fe 2  O 3  which can form at the anode, during cobalt-iron alloy plating. However, unlike the pure cobalt electrolyte, the addition of V 2  O 5   to the electrolyte containing iron will oxidize the Fe 2+  ions to the ferric state where it is stablized by the vanadium ions. Both the formation of Fe 2  O 3  in the anolyte region and Fe(OH) 3  in the catholyte region are suppressed by the reductive activity of the vanadium ions thereby resulting in electrodeposits of improved appearance and ductility at a lower temperature than previously required. 
     The 2.3-4.3 grams/liter of vanadium pentoxide shown in Table I is the equivalent of 1.3-2.4 grams/liter of vanadium ions. It should be understood that titanium and molybdenum are equally operable additives in the same ionic concentration. 
     The foregoing disclosure and description of this invention is illustrative only. Various changes may be made within the scope of the appended claims without departing from the spirit of the invention.