US 3104221 A
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R. H. HILL SELF-CIRCULATING SOLUTION ANODE FOR CHROMIUM PLATING VESSELS Filed Feb. 17. 1960 INVENTOR. Balletti H Hill kf/f. @am ma Sept. 17, 1963 United States Patent O 3,104,221 SELF-CIRCULATING SQLUTION ANODE FR CHROMIUM PLATING VESSELS Robert H. Hill, Watervliet, NY., assignor to the United States of America as represented by the Secretary of the Army Filed Feb. 17, 1960, Ser. No. 9,403 2 Claims. (Cl. 20d-237) (Granted under Title 35, US. Code (1952), sec. 266) 'Ilhe invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.
This invention relates to apparatus `for felectroplating chromium lonto the interior surfaces of hollow articles and is particularly directed to means `tor obtaining a plate -of substantially uniform thickness throughout even though one end of the article being plated is completely closed.
It has been found that the wear resistance 'of gun barrels can be considerably increased by plating the bore surface thereof with a corrosion resisting metal such as chromium. While suitable plating methods and apparatus have been `developed as a result :of suspending the barrels in a vertical position, difficulty has been experienced in obtaining a plate of substantially uniform thickness along the entire length Iof the barrel. 'lhis dicul-ty is particularly evident where, as in the case of lguns such as mortars, one end of the barrel `or tube is completely closed.
For one thing, `the passage of electric current through the plating solution `or electrolyte generates gaseous products which appreciably impede `the required ionization thereof. Since the gases generated in the lower region of the mortar tube rise to ymerge with the gases generated in the upper region thereof, there is a progressive upward increase in the `gas density of the electrolyte. In view of the closed end structure of the mortar tube, the replenishment :of the electrolyte must be accomplished from the upper end thereof. However, the rising gases block the downward circulation of the electrolyte to such an extent that the replacement of the exhausted solution becomes increasingly :difficult toward the lower regions of the mortar tube and is nonexistent at the lower extremity thereof. This condition continues until the weight .fof the solution in the upper end 4of the tube becomes great enough to overcome the force lof the rising ygases and sink toward the lower end of the tube. Thus, as the replenished electrolyte is subsequently exhausted, the repetition of the foregoing cycle will set up an .oscillation in the circulation of the electrolyte which contributes to the unevenness of the chromium deposit along the length of the tube and produces a complete absence zof deposit in the lower extremity thereof. rlThis unevenness of chromium deposit is, `of course, primarily produced by the :fact that the current-carrying capacity of the electrolyte is reduced in proportion to the quantity fof the gases present at any particular point therein thereby resulting in a ythinner coating lof chromium being produced at the upper regions of the tube than at the lower regions thereof. This uneven :distribution of the chromium plating is even further enhanced by the fact that the upward flow of Agases is accompanied by a corresponding increase in the concentration of the electrolyte in the upper regions of the tube thereby reducing the thickness of chromium deposit at the lower regions thereof. Prior art attempts to avoid this uneven plating by increasing the circulation of the electrolyte within the tube being plated have invariably required complicated and time-consuming procedures as well as elaborate and expensive apparatus.
Accordingly, it is an `object lof this invention to improve the conventional anode construction for electroplatin-g the interior surfaces of relatively long tubular Patented Sept. 17, 1963 ICC articles through the incorporation lof self-contained means for promoting the circulation of the electrolyte therethrough.
A more specific y.object of this invention is to provide an improved anode construction capable of plating a substantially uniform layer of chromium yonto the interior surface of a tubular member, such as a .mortar tube, which is completely closed at one end.
'llhe specific nature :of the invention as well as other objects and advantages thereof will cleanly appear from a description of a preferred embodiment as shown in the accompanying drawings in which:
FIG. 1 is a schematic representation of the anode lof this invention in position to plate the interior surface lof a mlortar tube with a coating of chromium; and
FIG. 2 is a similar view of a modified anode which can be .used where an open inlet port would be undesirable.
As shown in FIG. 1, the article to be interiorly plated with a layer of corrosion-resistant metal, such as chromium, may be a mortar tube 10 having a closed end as shown at 11. "llube 10 -is completely immersed in a tank 12 containing the ydesired electrolytic solution 13 and is arranged to be supported therein in a vertical position by suitable means (not shown).
The anode 14 `of the present invention is formed as a hollow rod .of 4greater length than the tube lll to be plated and is centrally positioned therein with the lower end thereof adjacent the bottom of tube 1Gy but spaced sulficiently therefrom to permit an unrestricted ilow lof electrolyte :out of the lower end of the anode 14.
During the plating operation, the llow `of electrical current from anode 14 to cathode 10, as shown by connections 24 and 25, causes considerable quantities of .gas to be evolved lthroughout the length of tube 10 and being lighter than the electrolyte 13, these gases rise to the top in progressively increasing quantities as best indicated at 1S. Since appreciable quantities of the solution itself are carried upwardly by these. rising gases, the solution density at the lower regions of tube 10 is corresponding-ly reduced thereby resulting in an funeven deposit of chrormium. Y
ln `order lto replace this loss :of electrolyte at the closed end of tube 10, an lopenin-g is provided in the upper end of `anode 14 above `the upper end of the article undergoing plating but belowthe level of the electrolyte. Such opening may be of any size or conguration but is preferably formed by a plurality of radially ydisposed ports 17 whose axes intersect at a common point. Thus, as the density `of the electrolyte at the lower regions of tube 10 is decreased, normal ygravity will cause the more concentrated and heavier solution to move downwardly in anode 14 and be simultaneously replaced by fresh solution entering through ports 17. This flow sets up a continuous circulation Aof electrolyte 13 down through anode 14 and up through tube lll, as indicated by the arrows in FIGS. l and 2, which has been found to be completely successful in removing heavy concentrations of gases in the areas to be plated and at the same time continuously introducing a fresh supply of undiluted electrolyte. 'Inasmuch as the circulation :of the electrolyte is dependent upon the upward movement .of the gases, it is readily apparent that any change in the amount of electrical current will cause a corresponding increase :or decrease in the rate of gas evolution and, consequently, automatically control the quantity of the undiluted electrolyte being conducted tothe lower regions of tube 10.
In some instances the formation of the plating gases occurs so rapidly that the concentration thereof in the vicinity of ports 17 actually interferes with the ow of electrolyte therethrough to the detriment of the desired uniform plating along the length of tube 10. This difficulty has been resolved by providing the upper end of anode 14 with a closed piping system 13 having the free end thereof immersed into the electrolyte 13 as shown in FIG. 2.
This piping system is formed by eliminating the inlet ports 17 and continuing the upper end 0f anode 14 with a horizontally extending L-shaped pipe 19 whose free end 20 is arranged to project into the electrolytic solution 13 at a location remote from the area containing the plating gases. As the plating action is initiated, the resulting downward gravity movement of the electrolyte in anode 14 produces the continuing circulation indicated by the arrows in FIG. 2. 'Ihe opposite end of pipe 19 is provided with an extension 21 containing a valve 22. Air entrapped in pipe 19 is removed through vent 23 in pipe 19, when both ends of anode 14 are lowered into the electrolyte 13. Additional fresh plating solution may be supplied to the interior of hollow anode 14 through valve 22 in extension 21. Once the circulation of electrolyte 13 has been initiated, Vent 23 and valve 22 are closed. Subsequent opening of valve 22 will, of course, stop the circulation of Ithe electrolyte and halt the deposit of chromium onto the interior surfaces of tube 10. If desired, valve 22 may also be employed to control the rate of electrolyte circulation in accordance with the extent to which such valve is partially opened.
Thus, lthere is here provided a simple yet ecient anode which ensures a relatively uniform plating of chromium along the entire interior of an elongated hollow member without the complex and costly prior art pumping devices required to maintain a constant flow of electrolyte along the surfaces to be plated. Moreover, the anode of this invention is so eicient that the required constant circulation of the electrolyte can be readily maintained even though one end of the hollow member being plated is completely closed.
Although a particular embodiment of the invention has been described in detail herein, -it is evident that many variations may be devised within the spirit and scope thereof and the following claims are intended to include such variations.
1. Apparatus for electroplating a substantially uniform layer of corrosion resisting metal on the interior surface of an elongated closed-end tube vertically immersed as a cathode in an electrolytic plating solution:
a tank for holding said solution, said tank being of a depth greater than the length of said tube;
an anode vertically positioned in said tank and connected to a source of electric current, said anode comprising a hollow rod of uniform internal and external diameter and open at both ends thereof, the external diameter of said rod being substantially less than the internal diameter of said tube so as to provide an annular ow passage therebetween for the ow of undiluted plating solution from the lower end of said rod in replacement of the less concentrated gas-filled solution surrounding said rod;
a hollow extension communicating with the upper end of said rod and adapted to project laterally therefrom, said extension terminating in a downwardly directed open end adapted to communicate with said plating solution at an area remote from the concentration of plating by-product gases about the upper end of said tube being electroplated;
closable venting means in said extension for removing air initially entrapped therein, so as to start flow of concentrated plating solution through said extension and thence downwardly through the interior of said hollow rod in counter-current with the upward flow of diluted spent plating solution along the exterior of said hollow rod;
and means for connecting said tube cathodically to said source of electric current.
2. Apparatus according to claim 1, including means for introducing fresh plating solution from an external source into said hollow rod, said last-named means comprising a closable inlet connected to the interior of said hollow rod near the upper end of said anode.
References Cited in the file of this patent UNITED STATES PATENTS 884,522 Priestman Apr. 14, 1908 1,066,909 Jones July 8, 1913 1,872,290 Hitner Aug. 16, 1932 FOREIGN PATENTS 959,776 Germany Mar. 14, 1957 962,564 Germany Apr. 25, 1957