ADHERENCE O F ELECTROLYTIC METAL DEPOSITS
BY C. F. BURGESS A N D CARL H A M B U E C H E N
A study of the conditions which determine the adherence of one metal electrolytically deposited upon another is an interesting and promising field of research from the scientific standpoint, as well as of importance from the technical standpoint. I t is not the purpose of this paper to present instructions as to how adherent deposits may be attained, but rather to call attention to some of the problems which are worthy of scientific investigation and the solution of which will have a bearing upon practical applications, The value of an electro-deposited metal is determined by its degree of adherence to the surface upon which it is deposited, and upon the physical conditions or structures that the molecules assume during the process of deposition. T h e adherence is governed by various factors, some of which are well understood and others of which offer promising lines for research. The metal surface which is to receive the deposit should have, in the greatest degree, that property which Wesley has defined as next to godliness T h e surface should be clean ; not only apparently, but actually free from all foreign materials. Surfaces which may pass inspection of the eye, may not meet the tests of the electroplating tank, and appearances alone are here, as in many other cases, frequently deceptive. Everything on the metal surface, whether it be visible oxide, grease and scale, or invisible and transparent layers of oil, hydroxides, or other chemical compounds, and even films of gas, may be considered as dirt in accordance with that definition T h e cleaning which defines it as " matter in the wrong place of the surface is an operation requiring the utmost care of the operator, and the work thereby involved constitutes the largest item of expense in the operation of an electroplating plant. T h e scientist may be inclined to regard this problem of ((
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cleaning as belonging to the kitchen and work-shop rather than to the scientific laboratory, but further progress, for which there is much room, requires more attention than has previously been given this matter. T h e methods of preparing the surface may be, for convenience, divided into the mechanical, physical, chemical and electrolytic methods. T h e mechanical means include grinding, scouring, and the use of abrasives and sand blast. T h e physical methods include burning, and dissolving by means of. solvents, such as gasoline, alcohol, and the like. T h e chemical methods make use of the acids, alkalies, cyanides, and various other materials which act che‘mically upon the materials to be removed, making them sduble or changing the nature of them in such a manner that they may be readily removed by mechanical means. Disadvantages which are attendant upon chemical methods in common use are due to the deleterious action of the acids upon the metal to be plated, to the penetration of the acid into the pores of the metal with the consequent difficulty of complete removal, and to the liberation of hydrogen and other gases which may be absorbed by the metal. T h e acid dip which is almost invariably used where steel is to be plated upon, in some instances affects the physical property of the steel to such an extent that certain grades of steel sheet and wire cannot be plated. T h e finest grades of steel wire are so seriously affected by the chemical treatment, commonly deemed necessary in preparation for plating, that its strength and elasticity are greatly reduced. To just what this rotting ” is due is perhaps not definitely known. It cannot be ascribed entirely to the removal of a thin layer of the surface metal, or to the penetration of the acid into the metal, and the absorption of hydrogen is assumed by some to be the cause of the deterioration. A simple experiment to show the harmful action of hydrogen may be performed as follows: A piece of flexible steel piano wire is immersed in a dilute solution of sulphuric acid, and after the liberation of hydrogen has proceeded for a few ((
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minutes, the steel may be found to have become so brittle that it cannot be bent without breaking, whereas previously, it could have been bent at a sharp angle. If the ends of a piece of steel wire about one foot long, are held in such a position that the wire assumes a semi-circular form, after the liberation of gas has proceeded for a few minutes, the wire will become so brittle that it will snap in two even before any relatively large amount of metal has been removed by the dissolving action of the acid. These experiments show the marked influence of gases which may be absorbed by a metal. Iron, which has absorbed hydrogen, may be restored to its original condition by an annealing process. Experiments which we have performed, show that such iron may be partially annealed electrolytically by using the iron as an anode in a solution from which oxygen is liberated upon it. Electrolytic methods of preparing the surface consist in the utilization of various phenomena which may take place in the electrolytic cell and, in some cases, with decided advantage over other methods. The oxidation or corroding action at the anode may be utilized in dissolving certain substances from the metal surface constituting the anode without the disadvantage of a simultaneous evolution of hydrogen. The articles to be prepared may also, under other conditions, be used as the cathode, and the reducing action there taking place may be utilized in the reduction of reducible oxides and similar compounds. An investigation which might be of interest would be the determination of what compounds are capable of electrolytic reduction, and the rapidity and efficiency of the operation. The oxides of copper and lead may be reduced very quickly. Certain oxides of iron may be reduced after a long subjection to the cathode action, and other oxides seem to undergo little or no reducing action. The determination of the relative amounts of hydrogen absorbed by a metal in contact with an acid and at the cathode, might also be of interest. I n addition to the chemical effects of the electric current, the physical action of the liberated gases may also play a useful
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part in the removal of materials from the surface preparatory to plating. If a metal upon which there is a layer of varnish; paint, or enamel, be used as cathode in a solution from which hydrogen is liberated, the hydrogen bubbles will gradually work their way under the layer of material and loosen it up to such an extent that it may readily he removed. An experiment which we performed to show this effect was the following: A piece of bicycle tubing was coated with enamel and subjected to a thorough baking process. By using it as the cathode in a dilute solution of sodium hydroxide, the coating of enamel was almost completely removed in about an hour’s time. After the surface has been cleaned by one of the foregoing methods, it is necessary to placeit in the plating solution without allowing it to be exposed to the air for any considerable length of time. With some metals an instantaneous exposure to the atmosphere causes oxidation to take place, thus necessitating very rapid handling. This is especially so with aluminum. Other metals, such as copper, may be exposed for some time without harming the surface. A peculiarity, the reason for which does not seem to have been explained, is that a metal, by contact with the plating electrolyte itself, may change the character of its surface to such an extent that a coating subsequently deposited will not adhere firmly to it. We have observed during an investigation on the electro-deposition of iron, that if the iron cathodes are allowed to remain for a few hours in the solution without the flow of current, a subsequent deposition of iron will take place, but the layer thus deposited may be readily separated from the previous layer. This gives rise to a laminated structure in the metal cathodes and a cross section of the cathode will show the times at which the current has been interrupted. This same phenomenon has previously been pointed out in the electrolytic separation of nickel? Aside from the proper preparation of the surface, the degree of adherence of one metal upon another depends upon a cextain r elation between the metals themselves. It is claimed by some,
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that the adherence is due to an actual alloying of the twometals at their surface of contact, while others hold the view that the cohesion of the metal particles in intimats contact is sufficient to account for the adherence of the coating. It is quite likely that each of these views is correct to a certain extent. It is a matter of common observation that certain combinations of metals present conditions for better adherence than do others, and on the supposition of a true alloying, this fact would be explained by the chemical affinity existing between different metals. That iron and nickel do not readily form alloys is cited as an explanation of the difficulty which has been encountered in producing an adherent nickel coating upon iron. This does not seem entirely warranted, however, inasmuch as almost all of the nickel plating in this country is done directly upon iron without the intervention of a layer of copper which was at one time supposed to be necessary for the best results. This better adherence of nickel has been accomplished by a more careful preparation of the iron surface, and a study of the alloying of the metals during electrolytic deposition is made difficult on accoiint of this question of preparation of the surface. If the adherence depends solely upon the alloying property, it would naturally be expected that it would become better after considerable length of time, inasmuch as the process of alloying proceeds indefinitely. It is a matter of common observation, however, that many deposited coatings become non-adherent after a lapse of time. A striking experiment to illustrate the fact that one metal may be absorbed by another metal, under suitable conditions, may be performed by depositing a thin layer of gold upon a zinc surface. The gold will be absorbed by the zinc to such an extent that in a few weeks time the gold will have entirely disappeared from view. The alloying effect of metals was also pointed out by Dr. Haber in the Proceedings of the American Electrochemical Society, Vol. II., pages 189-196, and similar instances were also cited by Drs. Carhart and Kahlenberg and Mr. Reed in a discussion of the same paper. . There is little doubt, however, that a considerable degree of
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adherence may be produced without any alloying effect whatever, and to produce such adherence, requires an intimate contact of a large number of particles of metal. If the surface to receive the deposit is absolutely stnooth, it is evident that a snialler number of particles of the deposited metal can cohere with it than if the surface is somewhat roughened. This explains the reason for the common practice of dipping aluminum and certain other metals which have previously been polished, into an acid or other chemical bath to give a rough or dull surface, and thereby virtually increasing the amount of surface upon which the deposition takes place. T o obtain the best results, requires that every particle of this surface thus exposed shall receive a particle of the deposited metal and to attain this condition in the highest degree, the character of the electrolyte itself plays an important part. I t is a well-known fact that a metal deposited from one solution may be more adherent than the same metal deposited from another solution, although the exact cause for such difference is not thoroughly understood. While the chemical nature of the solution may be the influencing factor, its physical properties undoubtedly play a certain part. Certain observations which we have made, tend to show that phenomena such as the viscosity, surface tension, and capillarity, exert an influence, I n some cases the addition of a small amount of alcohol or other similar chemical inert agent may materially improve the quality of adherence. The readiness with which E solution will (‘wet’’the surface to be plated upon determines, to a considerable degree, the adherence. If there is little attraction between the metal and the solution, the liquid will not work its way into the cavities and interstices of microscopic size on the surface, and it is only the more elevated points that actually make contact with the solution. The result is that it is only such elevated points that receive a deposit, and the intervening spaces are bridged over by the metal cpating without actually making contact with the entire underlying metal surface. The natural result will be that,
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while we may have a continuous metal layer deposited, such layer does not make intimate metal contact with the entire metal surface receiving the deposit. Investigations, such as have been carried out by Herzen (Archives des Sciences, p. 232, 1902), on the surface tension of liquids, and the relation of such property to the quality of adherence might lead to some interesting results and conclusions, and indicate principles to be followed in the composition of plating solutions for attaining the most satisfactory results. Laboratory of Applied Electrochemistry, University of Wisconsin, 1903.
