Faraday’s Law Applied to Cleaning
This paper is a re-publication of the the 2nd William Blum Lecture, presented at the 47th AES Annual Convention in Los Angeles by Dr. A. Kenneth Graham, 1959 AES Scientific Achievement Award recipient. While Faraday's Law had long been applied to electrodeposition processes, Dr. Graham took it a step further, considering its usage in the electrocleaning of various metal substrates in terms of electrical charge passed during the cleaning operation.
Dr. A. Kenneth Graham
Recipient of the 1959 AES Scientific Achievement Award
Editor’s Note: Originally published as A.K. Graham, Annual Technical Proceedings of the American Electroplater’s Society, 47, 41-44 (1960), this article is a re-publication of the 2nd William Blum Lecture, presented at the 47th AES Annual Convention in Los Angeles, California, on July 25, 1960. A printable PDF version is available by clicking HERE.
Introduction and background
“What is Faraday's Law?" That was the only question asked me by the plating foreman of the Welsbach Company in 1921 when the superintendent introduced me and inquired if there might be a job available for me. At the time, I had my BS degree in chemical engineering and two years’ experience in industry, but recognized the practical value of working over the tanks under an experienced plater. The superintendent agreed to give me the opportunity to apply for such a job without disclosing my background. I got the job by answering the question - "What is Faraday's Law?"
All of you appreciate that Faraday's two Laws are the basis of all plating and, in fact, of the entire Electrochemical Industry. In the years since the last war, we have learned that they can be applied with great advantage to cleaning as well as plating. It is about their application to cleaning that I wish to speak today.
Cleaning of the common basis metals for present-day decorative and protective plating applications has been developed to the point where the conscientious plater can apply the available information and do an acceptable job. However, the present-day plating applications in other areas, the so-called engineering applications, are presenting an ever increasing array of cleaning and plating problems. One industry requires electrodeposited gold, silver or alloy coating on beryllium copper, bronze or brass tape or wire. Coatings of buffed nickel over buffed nickel with or without a final chromium coating have been used to overcome cavitation failure of diesel cylinder liners and to obtain improved corrosion resistance in various applications. Many varieties of stainless steel alloys must be adherently plated with various metals to meet special service requirements. The extremely passive materials, such as Stellite, Inconel, Hastalloy, and Carballoy sometimes require adherent plated coatings for some applications. Sometimes the more exotic metals, such as niobium, titanium, zirconium, uranium, beryllium and molybdenum, require adherent plated coatings for various reasons. It is reasonable to assume that such problems will increase in the future.
Such engineering plating applications will only be successful if one is able to develop cleaning cycles that will permit one to deposit adherent metal coatings. To accomplish this one must remove oxide or other surface films and then keep the basis metal surface activated until the plated coating can be applied. The chemistry of some of the basis metals involved render this extremely difficult. There is much yet to be learned about these matters.
The conventional approach to solving such a problem is to first remove organic soils by suitable degreasing means, including an electrocleaning treatment. Whether the latter should be anodic or cathodic is not always clear. In any event, the commonly recommended conditions for a proprietary cleaner such as concentration, temperature, current density and time, are usually employed. Some acid treatment with or without the use of current is then chosen to etch and activate the surface and the commonly recommended conditions of bath composition, concentration, temperature, time and current, if any, are precisely followed. In some cases, if allowable, the Wood's type of nickel chloride strike is finally used under the conditions recommended. If these procedures then do not give the desired results, we really do have a problem.
Under such circumstances, I have found it most helpful to simply apply Faraday's Laws to the electrolytic treatments. I therefore trust you will bear with me if I briefly discuss these Laws in an elementary fashion as applied to cleaning.
According to Faraday's first Law, the amount of chemical change produced by an electric current flowing through an electrolyte is proportional to the quantity of electricity. The quantity of electricity is the product of the current flowing times the time. The unit of quantity, a coulomb, is one ampere flowing for one second. Ten amperes flowing for six seconds will cause the same chemical change as one ampere for sixty seconds, both being the same quantity of electricity, 60 coulombs or one ampere minute. In either case, the same amount of hydrogen will be liberated at a metal surface in cathodic cleaning. If one doubles the quantity of electricity, the amount of hydrogen liberated also will be doubled. Whatever benefit may be derived from the liberation of hydrogen in cathodic cleaning may therefore be varied by applying this concept.
According to Faraday's second Law, the amounts of different substances liberated by a given quantity of electricity are proportional to their chemical equivalent weights. Stated another way, 96,500 coulombs or one Faraday of electricity will liberate one equivalent weight or one gram of hydrogen at the cathode and one equivalent weight or eight grams of oxygen at the anode in alkaline electrocleaning. Therefore, the effect of the oxygen liberated in anodic cleaning, whether beneficial or otherwise, can also be varied quantitatively by applying this Law.
According to Avogadro, a gram molecular weight of any gas occupies the same volume at the same temperature and pressure. Since the equivalent or combining weight of hydrogen is one-half its molecular weight, but that of oxygen is only one-fourth its molecular weight, then the volume of hydrogen liberated at the cathode in alkaline electrocleaning is twice the volume of oxygen liberated at the anode. Thus this well-known volume relationship follows directly from Faraday's Laws. Also, the greater volume of hydrogen liberated at the cathode led to the preferred use of cathodic electrocleaning in the early days.
In many cases the scrubbing action of the volume of gas liberated at the surface of the metal in alkaline electrocleaning is of secondary importance to the nature of related chemical reactions and this depends upon the reactions of hydrogen and oxygen at the electrode surfaces. In our opinion, the resulting adhesion of the electrodeposited metal coating is the most important factor, both as an indication of a properly cleaned surface and as a means of insuring quality of the plated coating. Experience has shown that good adhesion is favored by anodic cleaning of the common ferrous metals. Cathodic cleaning is usually preferred for nickel. Copper or zinc can be cleaned either way for good adhesion, but anodic cleaning is most commonly used to avoid deposition of films. Lead is cathodically cleaned to avoid etching and staining.
The cleaning of many metals prior to plating as practiced today frequently involves various pretreatments in combination with both electrolyte alkaline and acid steps. These electrolytic treatments are usually limited to not over two minutes and often to one minute or less. The tank sizes and conveyor chain speed frequently determines this. Also, the current density is either limited to that obtainable at the voltage of the current source available or is purposely restricted because of the sensitivity of a particular metal surface with respect to etching or staining. This is especially true for decorative bright-plated finishes. Thus the quantity of electricity, the product of the time multiplied by the amperes flowing, is thereby limited and the application of Faraday's Laws, in any real sense, has been disregarded as far as cleaning is concerned.
Of course this is not so with plating. One Faraday or 96,500 coulombs of electricity will deposit one chemical equivalent or combining weight of any metal at 100 per cent cathode efficiency. We routinely refer to the Table of Electrochemical Equivalents and Related Data to find the ampere minutes required to deposit any metal to a coating thickness of one mil per square foot. From this we determine the plating time required at a given current density to deposit any thickness of metal or vice versa. We still design and control our plating operations to obtain a given plating time at a controlled average current density to obtain a controlled average weight of metal coating. We also make allowance for the efficiency of the plating process and for variations in current and metal distribution with the design of the part being plated. All this is strictly in accordance with Faraday's Laws.
We also know that Faraday's Laws apply to the performance of soluble anodes in plating and that the metal plated out at the cathode is substantially all supplied by metal dissolving at the anode in properly controlled processes.
To apply Faraday's Laws to cleaning is much more difficult and one might ask, "Why bother?" It is more difficult because we are obliged to remove so many different types of soil, and the term soil is used here in the broadest sense. The surface chemistry of the basis metal itself cannot be defined. As many of you well know who chromium plate nickel, the surface chemistry of a nickel coating immediately after plating is different than one that has just been buffed and both will be different after exposure to air for 24 hours. The surface chemistry also varies with each metal and its metallurgical history. One therefore cannot determine the chemical equivalent weight of the combination of soils, oxides and metal surface films and relate it to a given quantity of electricity in cleaning. Our only recourse is to apply the quantitative concepts of Faraday blindly. Increasing the current density and/or the time will quantitatively increase the hydrogen and oxygen liberated and the chemical reactions resulting at both anode and cathode in electrocleaning treatments, even though the reactions are undefined. By so doing we can accomplish results in cleaning and plating that can be guaranteed and not left to chance. This is especially true with respect to the so-called engineering applications, many of which require plating upon the more difficult or unusual basis metals.
Mr. F. W. Stockton, formerly of the Standard Steel Spring Co., was the first, to our knowledge, to emphasize the importance of Faraday's Laws in electrocleaning. He observed that cathodic cleaning of steel prior to nickel plating gave very poor adhesion, compared to anodic cleaning. He then showed, if cathodic cleaning was first used, the adverse effect of this treatment on adhesion could be overcome by following with anodic cleaning, using at least the same quantity of ampere minutes per square foot and preferably more.
We have extended this application of Faraday's Laws by increasing time and/or current density of both the electrolytic alkaline and acid treatments in developing cleaning cycles for specific plating applications. Each cycle so developed must be shown by test to meet the required specifications, especially the adhesion, before being used in production. Then by controlling the production cycle steps, as established by this procedure, the quality of the plated product can be assured. A few illustrations of how this has been applied may be of interest.
Plating chemical equipment
The late Carl Heussner used 18 cleaning steps including rinses for the first atomic energy program (the Manhattan Project) in the preparation of steel equipment for nickel plating. He naturally included every favorable treatment step that had been recommended in the literature in order not only to meet the corrosion and adhesion tests that were specified, but in the hope that the quality of nickel plating so produced would prove satisfactory for the intended service. Fortunately the plated nickel coatings performed successfully. Otherwise solid wrought nickel would have been required and this program alone would have consumed the available output of nickel in America for two years.1 Of course, no such quantity was available, so if plating had failed, we might not have had the atom bomb.
The equipment programs that followed the war were no longer on a crash basis and money was no longer being spent on an emergency basis. It was important therefore to limit the number of preplating steps to a minimum in order to reduce plant investments. Fortunately we had the time to investigate this. By applying the quantitative concept of Faraday in developing the cleaning cycle we were able to meet the nickel plating specification for both the adhesion and hot water porosity rating with a cycle of only four steps, two of which were rinses.2 (See Table 1.)
We knew that anodic alkaline and anodic sulfuric acid treatments favored the adhesion of nickel to mild steel. We did not know what quantity of electricity in these treatments was required to meet the specifications or, in fact, whether some further cycle variations would be required. We increased the quantity of current stepwise in both the anodic alkaline and acid treatments and ultimately found that with a minimum of 300 A-min/ft2 in both treatments we got perfect adhesion. The non-silicated proprietary cleaner that had originally been specified was operated at a concentration of 10-12 oz/gal and a temperature of 190-200°F. The anodic acid 50 per cent by volume sulfuric acid was operated at a temperature not in excess of 85°F. A minimum current density of 50 A-min/ft2 and six minutes treatment time was used in both treatments.
Table 1 - Cleaning cycles for nickel plating.*