US 2397583 A
Description (OCR text may contain errors)
April 2, 1946. E. M. wlsE ETAL ELECTROFORMED ARTICLE Original Filed Feb. 26, 1940 .fand/V0 M M55, vp PHY/wvo E 14A/55 0. a @b5 Oe F8 7 Cw m z Z 2 @weeg Fw" Patented Apr. 2,1946- Q p ELEcrnoFonMEn ARTICLE Edmund Merriman Wise, Westfield, and Raymond Francis Vines, Plainfield, N. J., assignors to The International Nickel Company, Inc., New York, N. Y., a corporation of Delaware I Original application February 26, 1940, Serial No. 320,850. Divided and this application November 30, 1942, Serial No. 467,320
The present invention relates to electroformed articles and, more particularly, to a process for electroforming articles including mirrors and searchlight reflectors, especially reflectors having high reflectivity and resistant to severe corrosive conditions.
The prior art electroforming processes for producing mirrors and searchlight reflectors have certain inherent disadvantages which hinder or prevent the deposition of a sufficiently thick, corrosion resisting precious metal electroplate to insure freedom from porosity and thus to obtain the required degree of protection. For example, if a suillcient thickness of platinum metal plate is deposited to obtain adequate corrosion resists ance, the'refiecting surface is rough and dull and must be polished to raise the reflectivity to the required minimum. This is a costly operation, removes some precious metal and destroys the contour of the refiecting vsurface and in some cases, as in reproducing gratings, is iinpossible. Another disadvantage of the prior art processes is the inability inherent in them to produce satisfactory rhodium surfaced palladium plated reflectors owning to the fact that when rhodium is depositedl upon palladium the codeposited hydrogen causes puckering and roughening of the underlying palladium plate, thus impairing the smoothness of the reflecting surface.
Furthermore, where a highly polished base metal or silver reflecting surface is to be pro tected by e1ectro-depositing a non-tarnishing noble metal upon it, difficulty is encountered in securing adherence to the very smooth surface and the plating baths must he restricted to those of the low metal ion type in an effort to reduce local galvanic effects which would roughen the base metal and impair the adherence and continuity of the noble metal deposit.
In prior efforts to secure better corrosion resistance various metals such as gold, palladium and the like have been deposited upon the base metal and then the final reecting surface, generally rhodium, was applied by Aelectrodeposition but even such composite coatings have proved inadequate for marine use particularly in submarines, so recourse has been made to reflectors of highly corrosion resistant high chromium con- N tent alloys which are not only difficult to forge, grind and polish and hence are very expensive, but possess inferior reflectivity. Glass -reflectors are unsuitable for this application because of their fragility and vulnerability to even small projectiles. l
From the foregoing it will be readily appreciated that the art has failed t provide a process whereby mirrors may be electro-formed to have a precious metal surface of such thickness as to provide the corrosion resistance necessary to meet the rigorous requirements of marine use 5 vand at the same time provide the eflicient reflecting surface necessary for such service. We have discovered that reflecting surfaces may be electro-formed to have a precious metal or platil num metal surface which meets the most rigorous requirements.
It is an object of the present invention to provide electroforrned articles having a platinum metal surface.
It is another object of the present invention to provide electroformed articles having a precious metal reflecting surface capable of withstanding normal corrosive conditions for longer periods than prior art reflectors without appreciable reduction of the reflectivity thereof.
It is Sa further object, of the present invention to provide electroformed articles having a precious metal reflecting surface having a higher reiiectivity in the unpolished state than prior electroformed reflecting surfaces.
The present invention also contemplates the provision. of electroformed articles having preclous metal or platinum metal reflecting surfaces having high corrosion resistance sumcient to meet the requirements of marine use and having with alloys of high chromium content.
Other objects and advantages will become apparent from the following description taken in conjunction with the .drawing in which Figs. 1-10/ areillustrative in a schematic manner of the steps in a process employing the principles of the present invention.
Broadly speaking, electrodeposits tend to become progressively rougher with increase in 4@ thickness. Difficulties also arise in the prior art processes due to galvanic effects occurring when a more noble metal is deposited onl a less noble one. It is also to be noted that the adherence of successive layers of electrodeposited metals improves with increasing roughness of the surface to which the deposit is applied.
In the present invention all of the above factors are in optimum relation because the most noble metal of the final structure, is normally de- 09 posited rst and upon the smoothest surface;
while the subsequent plates of lower nobility are deposited upon progressively rougher surfaces.
By this procedure a wide variety of plating baths are suitable and the thickness of the highly corrosion resistant layer or layers is limited only by cost and not by loss of smoothness. peeling or other limitations. On the other hand, in the prior art a refiectivity coeicient higher than is .possible process the major corrosion resisting plate was applied to a very smooth, less noble surface and the final reflecting surface tended to become progressively rougher with the increase in the thickness of the protective plate and in some cases, particularly where palladium Was used for the protective plate, the surface would suffer further roughening when the final reflective rhodium plate was applied, this aggravated roughness being due to hydrogen absorbed by the Palladium during rhodium plating.
From the foregoing it will be recognized that a reflector produced in accordance with the principles of the present invention can readily be distinguished from prior art electroformed precious metal reflectors. When a prior art electroformed reflector having an unbuffed precious metal reecting surface and corrosion resistant layer is examined microscopically in section, it will be found 'that the unbuied reflecting surface of the precious metal layer is rougher than the interface betweentheprecious metalreecting-surface-andcorrosion-resistant layer and the backing layer of non-noble metal. In distinctcontrast, the unbuied reflecting surface of the precious metal reflecting-surface-and-corrosion-resistant layer is smoother than the interface between the precious metal layer and the backing layer of non-noble metal of a reiiector produced in accordance with the principles of the present invention.
In the prior art processes, a glass or other matrix having a contour the reverse of that desired in the nal product was employed. In contrast to previous practice the preferred embodiment of our invention employs a glass or metal matrix possessing the contour desired in the final product. This matrix of the desired contour, if of glass, is carefully cleaned. and coated with a conducting layer such as silver deposited by the Brashear or other wet process, by vacuum sputtering or distillation, or by other convenient means and is then plated with copper from a standard copper sulphate bath (it may be remarked that cyanide baths appear less suitable as they lare apt to damage the silver surface) to a thickness suflicient toavoid deformation when thev electro-formed shape is removed from the matrix. After stripping the electroformed shape from the matrix the copper surface is protected with a stop-off and the surface which had been in contact with Vthe glass matrix is plated withA a thin layer of rhodium followed by a thicker layer of the 'major corrosion resisting metal, preferably platinum, palladium deposited from the complex nitrite or similar' baths or from .newly devised high metal ion high speed plating baths or gold depositedfrom a double cyanide bath. The corrosion'resistant surface is then vbacked up by base metal deposits, generallyv copper and nickel in one or more layers, to provide the strength reoperation. Final plating with rhodium cannot be done where palladium is used for the principal corrosion resisting surface as the hydrogen deposited along with the rhodium will cause slight puckering of the palladium thus impairing the micro flatness of the surface. Y
Metal matrices can also be used in which case the deposition of the initial conducting layer of silver or other metal may be dispensed with but generally the surface of the matrix will require surface treatment by one of the processes known to the art to prevent an excessively good bond developing on electroplating with copper or nickel. In the absence of such special treatments 'it is diicult or impossible to remove the preliminary electroformed shape without damaging it or the matrix. Further steps in the process are the same as described for a glass matrix.
It has not been proved feasible to produce mirrors by depositing rhodium directly upon a metal matrix and then to deposit palladium or platinum followed by nickel and copper due to the diiilculty of removing the composite from the metal matrix due to the excessively good bond developed between the rhodium and the metal matrix. However, this difliculty can be avoided by applying a thin nickel plate, preferably bright nickel to the matrix, the latter having been treated to prevent the formation of an excessively good bond. This thin nickel plate is then coated with rhodium, platinum, palladium or gold followed by a heavy nickel and copper plate. The composite is then stripped from the metal matrix and the thin nickel plate is then stripped olf electrolytically or chemically as described.
In order that those skilled in the art may have a better understanding of the principles of the present invention for the preparation of electroformed articles having high 4reflectivity without polishing, the preparation of a reflector which has given satisfactory results both from the standpoint of corrosion resistance and the standpoint of reflectivity will be described in conjunction with Figs. 1 to 10. l
A matrix 2| (Fig. 1) having the contour desired in the' finished reiector is provided with a silver coating 22 on the reecting or concave surface (Fig. 2). A copper layer 23 is then deposited yupon the silver coating to provide an article with sufficient strength for handling in subsequent treatments (Fig. 3).
concavel surface of the copper is then protected quired in` the nal product. The stop-off is then' removed from the preliminary electroform and an acid resistant stop-off is appliedv to the last dev posited base metal surface. .The preliminary elec-v tro-form is then removed by treatment with nitric acid or by electrolyzing it as anode in dilute sulphuric acid. Whenthe stripping has been completed the rhodium reiiecting surface will be exposed and will possess exactly the contour of the original matrix. A
If platinum is used for the principal corrosion resisting layer and where its slightly lower reectivity is suitable, the preliminary rhodium plate may be dispensed with or the thin rhodium plate may be appliedto the platinum as a nal `5 and 6).
with lacquer, wax or other stop-01T 24, and the composite removed from the-matrix (Fig. 4). Rhodium 25 which provides the ultimate reflecting surface is then deposited upon the convex silver surface 22 and backed up with a corrosion resistant layer of palladium or platinum 26 (Figs. Nickel or copper or both 2l are then deposited upon the corrosion resistant layer 26 tof provide the strength vandrigidity required in the finished product (Fig.v 7.). After protecting the last deposited strengthening layer' or layers 21 with wax, lacquer or other stop-o3 28- (Fig. 8)
theV stop-off layer24 is removed (Fig. 9) and the initial transitory silver and copper layers 22 and 23 then areremoved with nitric acid or by electrolysis exposing the rhodium reflecting surface and thus completing the article (Fig. i0).
Plane-mirrors can be made in a manner similar to the foregoing. For example, optically fiat glass is silvered, copper plated on'` the silvered surface and the two layers are then removed from the glass. After fstopping off the copper surface, rhodium, platinum or palladium or coml The exposed v binations thereof are deposited on the silver surface and the platinum metal layer or layers are stiii'ened by a backing of heavy copper or nickel plate. The initial stop-off layer is removed and the second heavy copper or nickel layer protected by a stop-off layer. The initial transitory 1ayers of silver and copper are removed electrolytically in sulfuric acid after protecting the copper or nickel backing layer with a stop-olf, which is a material resistant to the agent or agents employed to remove the initial silver and copper layers.
The preparation of the mold for the deposition of the initial silverlayer may be carried out in any suitable manner but We prefer to polish a glass surface with suspension of chalk or other suitable agents and cotton, applying plenty of pressure to the cotton. Care must be exercised not to contaminate the glass surface by contact with greasy objects such as the hands of the operator. By the use of rubber gloves this difficulty is obviated. After polishing the glass mold is rinsed with water, immersed in a cleaning solution, such as 50% by volume nitric acid, and swabbed. It is rinsed again with Water without removal therefrom. At this point we have found that, in accordance with accepted methods of preparing glass surfaces for the deposition of silver mirrors, it is advisable to swab the polished glass surface with a stannous chloride solution of say about 15% concentration. The polished glass is again rinsed with Water without removal therefrom and finally rinsed with distilled water and allowed to stand just covered with water until ready for silvering. We have found that the inclusion of the step of treating the polished glass surface with stannous chloride solution improves the adherence of the silver film applied later.
A silver film is applied to the polished and cleaned glass surface in the usual manner employing a suitable silver solution such as that of Brashear or variations thereof. The silvered glass mold is then ready for the deposition of successive films as described hereinbefore.
We have found it advisable when depositing copper upon the silver film at room temperature to employ a high starting current of the order of about 50 amperes per square foot. We have also encountered Adifficulties when attempting to deposit palladium, platinum, rhodium or nickel or copper (from a cyanide copper solution) upon the silver lm because there has been a tendency for the silver film to ake.
We have' found that reflector surfaces comprising a film of platinum about x10-4 inches thick gives satisfactory service while a' reflector surface plated with 5X 10-4 inches of platinum and then nished with an electrodeposit of.
rhodium 2X 10-6 inches thick gives even higher reflectivity and very satisfactory service. However, we prefer to employ palladium about 5x10*4 inches to about 2X 10-3 inches thick as the corrosion resisting layer which is surfaced with a film of rhodium about 2 106 inches to about 5 1O5 inches thick to provide the maximum reflectivity coupled with minimum cost.
Electroformed reflectors produced by means of the present process and having a palladium layer about 5X 10-4 inches thick, the exposed surface of which provides the reflecting surface of the reflectors, have been subjected to a severe salt spray test under conditions which have resulted in the complete failure of prior art electroformed precious metal reflectors. The electroformed reflectors produced in accordance with the principles of the present invention under test have withstood the rigors of this severe salt spray test with unimpaired reflectivity and without corrosion. Similar electroformed articles but having aplatinum layer about 5 10-4 inchesy thick in place of the palladium layer have likewise been tested under the same conditions and have also been found satisfactory, whereas prior art electroformed precious metal reflectors have failed completely.
Furthermore, an electroformed reector produced in accordance with prior art methods was subjected to an atmospheric corrosion test together with a reflector produced by the process of l v' the present invention. The site of the test was a shore front about 250 yards from the beach with the prevailing winds from the south and southwest. The test reflectors faced south at an angle of 30 from the horizontal. The test reilector'prepared in accordance with prior art electroforming methods and designed as A hereinafter consisted of a., reflecting surface layer of rhodium about 1 10-5 inches thick, a nickel undercoat about 2 103 inches thick and the remainder copper. The simultaneously tested reflector electroformed in accordance with the principles of the present invention and designated as B hereinafter consisted of a reflecting surface of rhodium 1 10*5 inches thick, a palladium undercoat 2 104 inches thick and the remainder copper. At the end of about four months exposure to sea air carrying a considerable amount of spray, test specimen A showed a myriad of tiny pits in many of which a greenish corrosion product could be detected, On the other hand, the surface of test specimen B was entirely free from pits. The'foregoing comparative ltest of a prior art electroformed precious metal reflector and a reflector made in accordance with the principles of the present invention clearly proves the complete superiority of the present electroformed precious metal reflector. The foregoing test also clearly proves that, whereas prior art electroformed precious metal reflectors are unacceptable .for use under severe marine conditions, the present electroformed refiectors are acceptable.
In producing reiiectors in accordance with the principles of the present invention the following baths have been used under the conditions set forth to plate nickel, copper, platinum, palladium and rhodium.
Copper plating (for protective layer and for Agitated C. D, at room temp. 5 0 amp/ft2 Nickel plating Grams per liter Nisoi 33o Nich 16.5 H3BO3 30 4 c. c. ofi-i202 (3%) per liter Agitated C D. at 130 C. 50 amp/ft.2 pH (preferred) 2.2.
Palladium plating (employing porous cell) Add NHlOI-I and boil for some time; evaporation made up with 5% (by vol.) ammonia solution. Agitated Temp. 90 C., C. D. 'l amps/ft.
Rhodium plating Bakers solution: c. c./1iter N0. 219 20 H2SO4 (conc.) 20
Temp. 40 C., C. D. 5 and 40 amps/ft.2
The present application is a division of our copending U. S. application Serial No. 320,850 led February 26, 1940, now Patent No. 2,305,050 issued December 15, 1942.
Although the present invention has been described in conjunction with certain preferred embodiments thereof, it is to be understood that variations and modifications thereof can be made as those skilled in the art will readily understand. Thus the expression precious metal or platinum metal includes gold, platinum, palladium, rhodium and iridium. Similarly, for copper and nickel may be substituted cobalt, or less desirably, iron and iron-nickel alloys. Such variations and modifications are -to be considered within the purview of the specication and the scope of the claims.
Furthermore, those skilled in the art know of the diflculties encountered particularly in making dat reectors. A method by which these difficulties can be overcome involves the use of a preliminary electroform of such thinness that it does not have the structural strength requisite for use as an electroform per se. This modification of the present invention involves preparing a matrix having the desired optical properties. The matrix may be of any'suitable material, preferably of glass having an optically at surface of the matrix. The choice of the metal of this very thin film is dependent upon the adherence of the metal of the very thin film to the mirror surface of the matrix. The adherence should be suiicient that the thin lm doesnot fracture or curlor blister but still insufficient to markedly increase the diiculties of separating the thin fllm from the mirror surface of the matrix after the previous metal reflector h'as been produced. Gold, nickel and similar metals may be employed as the metal of the thin film. Gold may be sputtered upon the mirror surface of the matrix or nickel may be deposited from nickel carbonyl. Each method of producing the very thin film has given satisfactory results. After the mirror surface of the matrix has been covered with the very thin lm of metal, a very thin but heavier layer of non-noble metal, for example, copper is electrodeposited upon the exposed surface of the very thin nlm first deposited. Thereafter the precious metal layer or layers of the reector is built-up upon the exposed surface ofthe electrodeposited non-noble metal in the manner set forth hereinbefore and a supporting layer of non-noble metal electro-deposited upon the exposed surface of the precious metal layer as described hereinbefore.
When the electro-deposition of the non-noble metal structural layer has been completed, the precious metal reflector with its structural layer of non-noble metal and the very thin preliminary electroform are separated from the matrix in any suitable manner, for example, by heating the composite article sufficiently to cause a separation between the rst layer of metal deposited upon the mirror surface of the matrix and the matrix. However, the temperature to which the composite article is raised should not be high enough to cause any appreciable warping of the reflector surface of the precious metal refiector. After the precious metal reflector and the preliminary electroform have been separated from the matrix, the exposed surface of the non-noble structural layer of the precious metal reflector is protected by a stop-off layer`of lacquer or wax or the like and the preliminary electroform dissolved chemically or electrochemically to expose the precious metal reflector surface of the electroformed reflector. In the unbuffed state, the reiiector surface thus obtained is far smoother than the interface between the metal of the refiector surface and the precious metal of the corrosion resistant layer or the interface between the precious metal and the non-noble metal structural layer.
1. As a new article of manufacture, a parabolic multi-layer composite metal reiiector comprising a transitory layer of copper-silver laminae in interfacial contact formed upon and lifted from a glass matrix, said transitory layer having a concave copper outer surface and a convex silver inner surface, an electrodeposited rhodium layer about 2 10-6 inches to about 5 X 10-5 inches thick onsaid convex silver surface, a corrosionresistant layer of electrodeposited palladium about 5 X 10-4 inches to about 2 X l03 inches thick on the convex surface of said rhodium layer, and at least one supporting layer of electrodeposited non-noble metal selected from the group consisting of copper, cobalt, nickel, iron and ironnickel alloys in sufficient thickness to protect the aforesaid precious metal layer from deformation on the convex surface of said palladium layer, the said transitory layer serving'as a matrix and being adapted for separation from the remaining layers whereby the concave rhodium surface is vexposed as a reflecting surface.
2. As a new article of manufacture, a parabolic multi-layer composite metal reflector comprising a transitory layer of copper-silver laminae in interfacial contact formed upon and lifted from a matrix, said transitory layer having a concave copper outer surface and a convex silver inner surface, a layer of electrodeposited rhodium on said convex silver surface, an electrodeposited palladium layer considerably thicker than said rhodium layer on the convex surface thereof, and a layer of electrodeposited non-noble metal of structural strength supporting said precious metal layers on the convex surface of said palladium layer, said transitory layer serving as a matrix and being adapted for separation from the remaining layers whereby .the concave rhodium surface is exposed as a reflecting surface.
3. As a new article of manufacture, a multilayer metal composite reflector comprising a 4transitory layer of copper-silver laminae in interfacialcontact, said transitory layer having a concave copper outer surface and a convex silver inner surface, an electro-deposited rhodium layer on said convex surface, an electrodeposited layer of a noble metal selected from the group consisting of platinum and palladium as the convex surface of said rhodium layer, and an electrodeposited layer of 'a non-noble metal selected from the group consisting of nickel and copper on the convex surface of said noble-metal-group metal, the said transitory layer serving as a matrix and being adapted for separation from the remaining layers whereby the rhodium layer is exposed as an optically correct reflecting surface.
4. As a new article of manufacture, a multilayer metal composite reflector comprising a transitory layer of copper-silver laminae in interfacial contact, said transitory layer having a concave copper outer surface and a convex silver inner surface, an electrodeposited rhodium layer on said convex silver surface, an electro-deposited layer of a platinum metal-other-than-rhodium on the opposite face of said silver lamina, an electrodeposited layer of a platinum metal-otherthan-rhodium on the opposite face of said rhodium'layer, and an electrodeposited layer of non-noble metal on the opposite face of said piatinum metal-other-than-rhodium layer, the said transitory layer serving as a silver-surfaced ma- 20 trix and being adapted for separation from the