Historical Articles

April, 1953 issue of Plating

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Radioactive Tracers Track Metal Cleaning Effectiveness

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J.W. Hensley, Manager, Nucleonics Laboratory Research and Development Division, Wyandotte Chemicals Corporation, Wyandotte, Michigan

INTRODUCTION
Cleaning operations in the metal finishing industry become more and more complex as the variety of substances to be removed increases. Because of this, greater emphasis is being placed on research in the metal cleaning field. Such research, often involving reactions at surfaces or solid-liquid interfaces, is difficult because of the number of variables involved. Negligible quantities of materials concentrated on a surface or at an interface, can have an effect out of proportion to the quantity of material involved. An example occurs in electroplating where trace contaminants on the base metals may have a pronounced effect on results.

To measure such small quantities of materials, often immeasurable or even undetectable by conventional test methods, a new sensitive technique, for quantitative measurement of trace quantities of materials is of considerable interest. Radioactive tracers provide a technique capable of extreme sensitivity and quantitative measurements of trace quantities without the ]imitations and complications usually inherent in highly sensitive, quantitative methods. For several years Wyandotte Chemicals Corporation has been working with radioactive tracers in various fields of detergency research.

Tracers are used to advantage in several ways. Cleaner components, normally alkalies and surfactants, are tagged by synthesizing with radioactive atoms and used to determine filming characteristics and adsorption on metal surfaces. Tagged solutions are used to measure the rate at which a cleaner component is rinsed from metal surfaces.

Metals are tagged (by bombarding in an atomic pile) and used to study corrosion, attack by cleaners, and the action of inhibitors in cleaners. Another application is in tests for evaluation of cleaning effectiveness using soils tagged with radioactive atoms. Here the radioactivity, which is proportional to the amount of tagged material present, is used to measure the removal of soil from a metal surface.

In devising a cleaning test using tracer methods, prime considerations are the choice of soil and the method for incorporating the radioactive tag. To have a valid tagged soil, radioactive atoms must be incorporated in the molecules of the soil component to be studied. In the case of organic materials, the desired compounds must generally be synthesized with the incorporation of radioactive atoms.

Soils encountered in metal cleaning operations are almost infinite. The most suitable application of tracers seemed to be in connection with processes in which the last traces of soil must be removed for optimum results, as where metals are cleaned prior to electroplating. Soils most commonly encountered are materials applied to the metal in previous operations, such as buffing compounds and cutting oils. These soils frequently consist of combinations of hydrocarbons, including mineral oils and waxes, fatty acids and derivatives, tallow, vegetable oils, etc., and particulate matter, such as abrasives, metal, and carbon particles.

Of these, fatty acids generally are considered troublesome. There have been no satisfactory quantitative methods for studying removal of residual films of fatty acids. Frequently present in buffing compounds and polishing materials, their affinity for metals provides desirable characteristics, but also makes them difficult to remove. Fatty acid films may also be deposited on metals from preliminary soak cleaners containing soap. Such films must be removed by subsequent cleaning operations.

Radioactive fatty acids, synthesized with carbon-14, may be purchased from several sources, with authorization from the Atomic Energy Commission. Tagged stearic acid was chosen for first investigation. Soil types other than fatty acids are being investigated, but the bulk of the work so far has been done with tagged stearic acid, and this discussion will be limited to that soil.

Fig. 1. A measured volume of radioactive stearic acid soil solution is spread over a metal disk automatically. Solvent is evaporated, leaving a solid fatty acid film

TAGGED STEARIC ACID SOIL USED
In most of the work, tagged stearic acid has been used alone as the soil. Some trials have been male with combinations of the stearic acid with various types of oil, in order to determine whether the presence of oil would have any pronounced effect on the removal of stearic acid in the cleaning process. Under the conditions investigated, removal characteristics of the stearic acid appear to be essentially the same whether used alone or combined with mineral oil. The metal cleaning test procedure using tagged stearic acid soil has been described in detail.1.

Metal disks, 1.5 inch in diameter, after preliminary cleaning with solvent, are machine abraded to provide a clean surface with a reproducible finish. The tagged stearic acid soil is applied as a solution by the apparatus shown in Fig. 1.

Five microliters of solution are spread in a thin, continuous film on the disk. After the solvent is evaporated, a thin film of stearic acid remains which is essentially uniform and highly reproducible. Radioactivity on the metal surface is measured with Geiger counting equipment shown in Fig. 2. This initial measurement or “count” on the soiled piece is merely a check, as a constant, known amount of stearic acid is applied. Initial count from one disk to another is essentially constant. The piece is then cleaned by a standardized procedure, rinsed, dried, and the final count taken.

Since radioactivity, stated as counts per minute, is proportional to quantity of stearic acid on the surface, the final count provides a relative evaluation of cleanliness, without converting counts per minute into weight of soil.
It should be appreciated, however, that counts per minute for a given quantity of radioactive material depends on location of activity with respect to the Geiger tube, Geiger tube characteristics and distribution of activity on the sample surface. Several counts on the same kind of radioactive substance are directly comparable only when taken under identical conditions.

Although simple in principle, this test method is so sensitive that slight variations in procedure can produce large variations in results. All variables must be standardized and carefully controlled. Especially critical is the surface finish of the metal to which the soil is applied. This factor has given more difficulty than any other in the development of this test method. It has not yet been possible to obtain an entirely constant surface condition. Frequent checks with standard cleaning solutions selected as references are necessary.

Fig. 2. Measurement of radioactivity. Soiled steel disk is placed on sample support, which slides under the windo of a Geiger tube, inside a lead shield

Several variables involved in removal of this fatty acid type soil from steel and from brass by simple immersion cleaning and by electrocleaning procedures have been studied. More than 2500 tests have been made with individual cleaner components and compounded cleaners. Effects of concentration, cleaning time, current density and direction have been determined. Effects of surface finish and surface treatments on soil removal, and the removal of the soil during the rinsing and acid dipping processes have been determined.

Fig. 3 shows the rate of removal of the stearic acid in electrocleaning steel anodically in a sodium metasilicate solution as compared with a compounded electrocleaner solution. The rate of soil removal by the compounded cleaner during the first minute is appreciably more rapid than by sodium metasilicate. After 8 minutes cleaning, the compounded cleaner left one fifth as much residual soil as the sodium metasilicate. The metasilicate (anhydrous) and the compounded cleaner were used at concentrations of 8 oz/gal, both having approximately 3 per cent titratable NaO in solution, with pH values between 13 and 14.

Fig. 4 illustrates a type of study carried out with all the common alkalies, and with many compounded cleaners. Here, the effect of concentration was determined with constant cleaning time and current density. Base metal was brass. There is a pronounced rise in residual soil (or decrease in cleaning efficiency) as the soda ash concentration is increased above a certain optimum value.

Fig. 3. Comparative rates of removal of tagged stearic acid soil from steel disks by anodic electrocleaning in a sodium metasilicate solution and in a solution of a compounded cleaner	Fig. 4. Effect of concentration on removal of tagged soil from brass by anodic electrocleaning in a soda ash solution and in a solution of a compounded cleaner. Cleaning time, 1 minute, at 25 amp/sq ft

Similar results are obtained with other commonly used alkalies, with a rise in residual soil at concentrations above 2 to 4 per cent titratable Naao in solution. The effect is found in cleaning steel or brass, and is more pronounced in cathodic than in anodic electrocleaning. It is even more pronounced in simple immersion cleaning than in electrocleaning. In contrast, the compound cleaner shows no decline in cleaning effectiveness at concentrations up to 16 oz/gal.

Another way in which the radioactive tracer method can be used is shown in Fig. 5. Radioautographs of the tagged stearic acid soil on steel disks have been obtained by placing the surfaces of the disks in contact with no-screen X-ray film and exposing for several days. In Fig. 5 A, a soiled disk was electrocleaned in concentrated soda ash solution for a short period, leaving a high residual soil of about 600 counts per minute. Soil removal is non-uniform, leaving a spotty appearance. The autograph of stearic acid adsorbed on the steel surface is shown in Fig. 5 B. In this case, a clean steel disk, polished in the usual way, was immersed in a tagged soap solution (sodium stearate plus excess caustic) for 1/2 minute at 90° F, then rinsed thoroughly. The stearic acid (or stearate) has adsorbed in a non-uniform manner, indicating variations in the nature of the steel surface, even though the polished surface was considered uniform.

CONCLUSIONS
The tests have been used primarily to supply basic research data and are not intended, at the present stage of development, to provide a method for routine comparison and evaluation of commercial cleaners for practical use. Field performance of a metal cleaner depends on a number of factors, aside from completeness of removal of a specific soil, which must be taken into account in making an evaluation. The method does, however, provide a sensitive and versatile means for evaluating cleaning effectiveness. Any laboratory test method must be correlated with practical cleaning results under plant conditions before it can be considered reliable.

Absolute residual soil left by a given cleaner, as well as differences between cleaners, will depend OD surface finish and other conditions selected for the laboratory test method. Significance of such results either on an absolute or relative basis must be determined by correlation with other types of laboratory tests and plant trials.
At present, tests are being made in which pieces carrying known amounts of soil, as determined by the tracer method, are electroplated, with - subsequent measurement of the quality and adherence of the plate. This work will provide information on what constitutes a “clean’? surface in a practical sense.

REFERENCE
1. J. W. Hensley, H. A. Skinner and H. R. Suter, “A Metal Cleaning Test Using Radioactive Stearic Acid as Soil”, Special Technical Publications No. 115, pp. 132, American Society for Testing Materials, 1952.