INDUSTRIAL A N D ENGINEERING CHEMISTRY
Electroplating’ By William Blum BUREAU OF STANDARDS, WASHINGTON, D. C.
T IS more than a coincidence that the most rapid progress in electroplating has been made during the past twentyfive years, a period that also maIks the growth of the automobile industry. Formerly most electroplating was carried out in small plants, upon parts that were plated chiefly for ornament and with no specification to be met. The platers had by long experience learned to accomplish their work with only occasional difficulties, and when these arose the consequent delays were more annoying than serious. Today a great part of the plating of the country is being conducted in relatively large plants, many of which are associated directly or indirectly with the automobile industry. In such plants production and quality are so strongly emphasized that it is no longer possible to achieve success by rule-of-thumb methods. Many progressive platers, through the American Electroplaters’ Society and its classes, have acquired a working knowledge of the chemistry of their operations, and have learned to make the tests and analyses required for the control of plating baths. I n addition, they have frequently cooperated with the works chemists in developing improvements and economies in their processes. Out of these associations has grown a demand for research on plating, a demand that is being actively met in university, industrial, and governmental laboratories. It is, then, no reflection upon the electroplaters to suggest that the automobile has perhaps contributed as much to the progress of the electroplating industry as electroplating has to the automobile industry. Although in this paper emphasis will be laid upon the contributions of chemistry to electroplating, it should be clearly recognized that the industrial progress made in this field has been due largely to the cooperation of progressive platers who have contributed from their large store of practical experience. Future success in this direction will depend principally upon the extent to which chemists and platers will work together harmoniously and understandingly and upon the support that they receive from their employers. Applications of Plating to Automobiles
If the average chemist were asked about electroplating on automobiles, he would a t once think of the exposed nickel plating, and might on second thought recall the fact that the lamp reflectors are silver-plated. Many of the most important applications of plating in this industry are, however, Before discussing on concealed or inconspicuous parts. the progress and possibilities in this field, it might be well to summarize briefly some of these uses of plating on automobiles, and especially the reasons for such uses. The metal most extensively applied by plating is nickel, which contributes so much to the initial appearance of the car. Nickel owes its use to the fact that it resists tarnish better than most of the common metals. This is because of the passivity which it acquires on exposure. It does, however, tarnish in time and sometimes acquires a yellowish tint. Even more important than this tarnish is the fact that the coatings are not impervious, and may therefore permit, or even accelerate, the corrosion of the underlying steel that is exposed through pores in the coating. The greatest single need in nickel plating today is a means of producing more nearly impervious thin coatings. 1
Published by permission of the Director, National Bureau of Stand-
Copper plating is applied extensively, even though it may not be visible on a finished car. The most important use is for protection of steel against carburization in the casehardening process. I n this method the parts such as gears, cams, and shafts are generally copper-plated all over from a copper cyanide solution. The copper is then ground off of those areas to be case-hardened-& method that is usually simpler than to protect such areas against the copper plating. Copper plating is also extensively applied as a coating preliminary to nickel plating or as a deposit intermediate between two layers of nickel. Experience has shown that by the use of relatively thick copper coatings in this way the protective value of nickel plating can be materially improved. Recently there has been much interest in the “electroforming” of thin sheet copper to be used for making gaskets or for surfaces in air-cooled radiators. Such processes, while promising, still involve sufficient mechanical difficulties to make their commercial success a t least problematical. Zinc plating is especially valuable where protection of steel against corrosion is more important than appearance. Consequently, we find it is regularly used on tire rims and on many parts of the chassis. Some of the largest plating installations are for zinc plating, which is conducted in the acid baths and the cyanide baths; of which the latter have better “throwing power.” Cadmium plating has recently received much consideration, though it is not yet in extensive use on automobiles. Like zinc, cadmium by its electrolytic action protects small areas of exposed iron against corrosion. Because the cadmium itself is less rapidly attacked than zinc, a cadmium coating is likely to furnish protection longer than a zinc coating of equal thickness. Conclusive data are not yet available, however, as to whether the advantages of cadmium over zinc offset its increased cost. Thus far cadmium has found its most obvious applications on small, accurately dimensioned parts, where a very thin coating is an advantage. Silver pluting has been used chiefly on reflectors, where its high initial reflectivity (about 90 per cent) is especially desirable. The ease with which it tarnishes, especially from sulfur fumes, renders it far from perfect for this purpose. Occasionally silver is used on the inside trim of cars, a purely ornamental application. Lead plating has been occasionally applied to storagebattery fittings, because of its resistance to attack by sulfuric acid. I t is usually deposited from fluosilicate or fluoborate baths. Iron deposition has been sometimes used, especially abroad, for bringing worn or under-dimensioned parts, such as axles, up to the required size. (Nickel plating has also been used for this purpose.) Obviously iron plating has no value for protection or appearance. Chromium, the modern sensation in the plating field, has received its full share of attention by the automobile industry, even though it is not yet in extensive use. The outstanding properties of chromium that warrant its present notoriety are its extreme hardness and its resistance to tarnish and to many chemical reagents. Its hardness has led to its extensive use on gages, which when plated with, for example, 0.00.5 mm. (0.0002 inch) of chromium yield greater service than any other gages thus far tested. I t is a comparatively easy matter to strip off this thin chromium coating when it begins to show wear, and to replate and relap it to exact dimensions. Such
INDUSTRIAL AND ENGINEERING CHEMISTRY
a procedure represents not only an appreciable saving, but also an increased assurance of accurate dimensions on the parts tested. The hardness of chromium may lead to its adoption on various parts as a substitute for case-hardening. Most of such possible applications are, however, still in the experimental stage. The marked resistance of chromium plating to tarnishing under most conditions has led t o its adoption on exposed parts of cars, such as the radiator shell, bumper bar, headlights, reflectors, and windshield frames. The more general adoption of chromium plating for such purposes is largely dependent upon the ability to circumvent by experience and ingenuity the intrinsically poor “throwing power” in chromium plating. Chromium coatings, especially if thin, are usually not impervious. Chromium does not furnish protection against corrosion of steel that may be exposed through pores in the coating, as the chromium is passive and exerts no such electrolytic action as do zinc and cadmium. I n most cases when chromium is applied to steel that is to withstand atmospheric exposure, it is preceded by copper and nickel plating. I n such applications the chromium is especially useful for its tarnish resistance, though it does add appreciably to the protection against corrosion furnished by the copper and nickel. Contributions of Chemistry
Vol. 19, No. 10
work whether the bath needs more acid, alkali, or metal salts, and recourse must be had to systematic chemical analyses. Works chemists have contributed very effectively by the development and application of simple methods of analysis of plating baths. I n such work it is important to realize that from the industrial standpoint no one is interested to know the composition of the solution per se. What the plater and superintendent want to know is whether the composition is correct, and if not, what must be done to correct it. The analysis is simply a means to an end. The chemist should therefore cooperate with the plater in determining the best range of composition. I n general, such a range is not critical, and frequent analyses with an accuracy of, for example, 2 per cent (of a given constituent) are generally as useful as analyses of higher accuracy. I n many cases the greatest service the chemist can render is to teach the plating foreman to make the necessary titrations and p H measurements, and to depend upon the chemist merely to check his standards and results occasionally. Such a course increases the interest and pride of the plater in his work and leaves the chemist more free t o investigate possible improvements in the plating processes. I n recent years the chemist has also rendered valuable service in checking the composition and purity of plating materials such as salts and anodes. This work has led to specifications for these materials, and has thus improved the Of the numerous kinds of plating that have been used quality of plating, without in most cases causing any increase on automobiles, chromium plating is the only one for which in the price of the plating supplies. the operating conditions are directly dependent on the reSPECIFICATION AND TESTING OF FINISHED PRODUCTSsults of chemical study. Even though most of the actual Even when plating is conducted primarily for appearance, solutions used for plating other metals were also discovered this appearance is no safe criterion of its quality, as it gives by chemists, the conditions for their commercial operation no necessary indication of how long it may be maintained have been developed chiefly from the experience of platers. under given conditions of service. If, in addition, protection Most of the plating formulas in use today-such as for nickel, against corrosion is essential, some definition of quality is copper, and silver-are not essentially different from those required. One of the most important problems of plating on employed twenty-five or even fifty years ago. We may well automobiles is the adequate specification of the quality of ask, then, “What has the chemist contributed to the plating the coatings, a problem which is still far from solved. industry during this period?” Such a specification may depend upon (a) the definition of The answer in brief is that he has made it possible to obtain, plating conditions, such as solution composition, temperature, especially on a large scale, more uniformly satisfactory results current density, and time of plating; or (b) a statement of the than were previously secured, and he has pointed the way to thickness and distribution of the coating upon the finished valuable, even though minor, improvements in various proc- articles. The first is a process specification; the second, a esses. The contributions of chemists to plating have or will product specification. The former is applicable, a t least come about principally in five directions: (1) analysis of in some degree, in the plant where the products are actually solutions, anodes, and salts; (2) the specification and testing made or plated. But if, as is now the condition in the autoof finished products; (3) the design of plating equipment; mobile industry, many of the plated parts are furnished by (4) research on principles of deposition; and (5) the de- other manufacturers, or are plated in job shops, it is obvious velopment of new plating processes. These activities will that some means of testing the finished products is necessary. It is just here that there is a great need for further study. be considered briefly. ANALYSIS OF PLATIXG SOLUTIONS ASD MATERIALS- The determination of the average thickness of coating, as Solutions used in plating undergo changes in composition computed from the weight of metal per piece or per unit area, through (a) replacement by water of the mechanical losses involves troublesome analytical procedures, and then may give of solution adhering to the plated articles, or carried off no true index of quality. Metallographic measurements of in spray; (b) differences in anode and cathode efficiency; the thickness of the coating a t selected points are even more (c) chemical changes produced by secondary electrode re- expensive and time-consuming, and may be misleading. actions, by hydrolysis, or by reaction with the oxygen or Accelerated corrosion tests and porosity tests are useful, but carbon dioxide of the air; and ( d ) accumulation of im- it is often difficult to reproduce or interpret the results. purities derived from the anodes, from the water used t o re- The whole field of testing metal coatings deserves a n plenish the evaporation losses, or from alkali or acid adhering extensive investigation. DESIGXOF PLATING EQUIPMEST-StTiCtly speaking, this to the articles from cleaning and pickling processes preceding the plating. The relative magnitude of these factors varies is an engineering problem, but as it does, or should, involve greatly and the total resultant of their effects may be small chemical engineering, it may properly be considered as an in plating baths that are used to only a moderate extent. application of chemistry “Plating engineering” is an almost But if such a bath is operated with a mechanical conveyor, new, but very important, phase of plating, especially when, a t maximum output, for 16 or even 24 hours a day, changes as in the automobile industry, it is necessary to have maxiin composition may occur in 1 or 2 days equal to those mum production with uniform quality. Much progress has produced in a month in a still tank in moderate use. It is, been made, especially along mechanical lines, by the manuthen, no longer feasible t o judge from the appearance of the facturers of plating equipment in the development of auto-
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matic plating machines and plating barrels. There is, however, still a great need for an engineering analysis of the various factors involved in the plating process, such as optimum bath composition, temperature, current density, electrode spacing, and rate of motion-all from the s i andpoint of quality, efficiency, and cost. Until more reliable data are available on such points as these, the design of equipment for producing a given output with a specified quality and thickness of plating will be largely empirical. RESEARCH O N PRIKCIPLES O F DEPosITIoN-The most important need of the plating industry, if it is to progress as much in the next twenty-five years as in the last, is a better understanding of the principles upon which plating is based. There is no reason to believe that chemists associated with plating will make any greater progress from purely empirical studies, than might be made by the platers themselves, though the chemists would probably progress more rapidly. The chemist engaged in research in this field will find ample opportunity for the application of all the modern views and methods of physics, physical chemistry, electrochemistry, analytical chemistry, and metallography. The progress that has been made in recent years in the study and application to plating of such subjects as electrode potentials and polarization, hydrogen-ion concentration, conductivity, throwing power, and crystal structure, while encouraging, serves more as a reminder of the large unexplored portions of this field, than as a justification for relaxation of effort. These studies have in many cases developed or adapted tools for research in this field, and have led to some useful conclusions and generalizations. As a result, the study of any specific problem in plating is less empirical than it was in former years. But we are still far from having a science of electrodeposition, much less applying it to industry. Until most of the effects of such variables as metal-ion and hydrogen-ion concentration, addition agents, temperature and current density, upon the cathode polarization, cathode efficiency, throwing power, and structure of deposits can be predicted, a t least qualitatively, for the various metals that are or may be used in plating, we will still be dependent upon recipes and rule-of-thumb methods. Experience indicates that most rapid progress can be made in this field by detached investigators, who, while in close contact with the practice and needs of plating, are not responsible for the control of existing processes. It is doubtful whether any single automobile manufacturer would a t present feel justified in maintaining a laboratory for research on the principles of plating. Instead, his employees can much better devote their energies to the more extensive application of knowledge now available. Joint efforts of the automobile industry, for example, in cooperation with existing organizations such as the American Electroplaters’ Society and the Bureau of Standards, would no doubt lead to greater progress in the study of the fundamentals of plating than can be achieved efficiently by the individual manufacturers. K E W METHODS OF PL.4TING-In the automobile industry, as in others, there are no doubt many unsolved problems of tarnish, corrosion, and abrasion, which cannot be, or have not been, met by the use of the metal coatings that are now generally available. The existence of such needs is illustrated by the enthusiasm with which chromium plating has been hailed in this and other fields. Regardless of whether or not these “great expectations” for chromium plating are realized, it is evident that the study of the possibilities of depositing this and still less common metals, such as molybdenum, tungsten, and vanadium, deserves careful consideration. Lessons derived from the experience with chromium may be applicable t o other metals. It is significant that, although chromium was deposited
as early as 1856 by Guether from chromic acid baths that were similar to those used today, it has taken seventy years to bring chromium plating to a state where its commercial use can even be considered. It is still too esrly t o evaluate the individual work in recent years of the msny chemistsincluding Carveth, Sargent, Fink, Schwarz, Phillips, Humphreys, Haring, and others-whose efforts hare brought chromium plating to its present status. But it is safe t o say that collectively these investigators have shown that chromium plating is a process which can be successfullyapplied only when the favorable operating conditions are more carefully controlled than has been customary in other plating practice. I n consequence chromium plating will make new demands on platers, and to the extent that they meet these demands the platers will also, even unconsciously, pay greater attention to the conditions in their other plating processes and thus improve and standardize them. Another lesson from chromium plating is that the methods which, on general principles, appear least promising may be found with a given state of knowledge to be most practicable. From general considerations one would hardly advise for metal deposition the use of a compound containing the metal in its highest valence, much less one that is a strong oxidizing agent, such as chromic acid. And yet this is the principal constituent of chromium-plating baths. However, n high price (electrically) is paid for the low electrochemical equivalent of the chromium in chromic acid, and the low efficiency with which it is deposited. This suggests the study of other baths containing chromium salts, in the hope that some of their present disadvantages may be overcome. There is still room for extensive investigations of chromium plating, which may yield results that will revolutionize the present practice. Among the recent surprises in plating is the electrodeposition of rubber. Even though this may hardly be considered as an example of electroplating in the ordinary sense, it is of interest because it may even serve as a substitute for metal plating-e. g., on storage-battery fittings or other surfaces to be protected against corrosion. The production of sheet rubber or tubes by electrodeposition may be considered as an example of electroforming. Whether rubber deposition will actually be applied to automobiles remains to be seen. Conclusions
The foregoing discussion has emphasized, not so much the accomplishments of chemists in the plating field, as the need and possibilities for far more extensive contributions of chemistry. What has been accomplished has been done by a comparatively small number of chemists. of whom a still smaller number have received much support or encouragement from the automobile or other industries in which plating plays an important part. I n the automobile industry, as in many others, plating has until recently been regarded as an incidental factor, and comparatively little technical attention has been given to it. Much of the plating may not be necessary to the successful running of the car. It does, however, contribute to its initial and subsequent appearance and t o its useful life. The plating that is associated with the hardness of moving parts is a very vital factor in the durability of the car. Metal finishes, like the body design, are so much matters of style that it is unsafe to predict future practice. It is reasmably certain, however, that some of the many kinds of plating will always find application on automobiles. The quality of the plating and its suitability for meeting various needs will depend very largely upon the work of chemists, especially of those who are able and willing to cooperate with the platers. We may then look forward to the time when our knowledge of plating is less superficial than the plsting itself!