US 2711484 A
Description (OCR text may contain errors)
' atent 2,?ll id.
7 Patented dune El, 1955 ice FILJPARA'EIGN 8F RABKGACTWE @BJECTS No Drawing. Application January 26, 1953, Serial N 333,338
7 Claims. (Cl. 2583.6)
This invention relates to a method for making solid objects radioactive by diflusion of a radioactive isotope. In one of its more specific embodiments it involves depositing or electroplatin a radioactive isotope on the surface of a lead containing bearing or other engine part, whereupon the plated part is heated under controlled conditions so as to speed up diffusion of the radioactive isotope into the part to an appropriate depth. Finally, the original surface may be restored by stripping the electroplated film. in this manner it is possible to determine wear, abrasive or corrosive or both, of a radioactive part by measuring the radioactiv' y of the crankcase oil or other fluid medium within which the part operates.
Some time ago a very useful technique of studying engine wear was developed based on the use of radioactive metal surfaces such as piston rings. Unlike the conventional weight loss method, the radioactive method for determining wear does not require disassembling the engine at the end of each test, but permits following the extent of wear continuously while the engine is in operation.
A continuous analytical technique would be similarly valuable, for instance, in the study of lead bearing corrosion in internal combustion engines, which corrosion is often taken as a measure of the oxidation stability of the oil used. Previously in such studies it was usually necessary to determine the bearing corrosion by dismantling the engine and actually determining the weight loss of the bearing. Obviously, it would have been useless to determine such corrosion by chemical analysis for lead in the lubricating oil, since the oil normally contained lead not only due to corrosion of the bearing, but also due to the tetraethyl lead present in the gasoline.
On the other hand, no successful adaptation of the radioactive technique has been proposed for this and many other problems. This has been largely due to the fact that it has been customary to make objects radio active by neutron bombardment in an atomic pile, which, however, is practical only in certain specific cases. For instance, lead objects cannot be treated in this way, since neutron bombardment does not make lead radioactive. Secondly, this type of activation is not feasible where the object to be activated is too large to be placed in an atomic 1 'fhirdly, neutron bombardment is not suited for making only a surface such as a cylinder wall radioactive, since it generally results in making the entire irradiated part radioactive, producing an unnecessarily high total radioactivity. This may often cause an appreciable health hazard, and generally increases the background radiation to a level which makes it impractical to determine the amount of radioactive debris in the oil, except after removal of the oil from the engine.
In instances where neutron irradiation proved impracticable, either because of size or chemical nature of the part to be studied, it has also been proposed to fabricate the entire part out of a radioactive isotope. However, this would be rather uneconomical in view of the relatively large quantity of radioactive material required, and
would call for special fabrication of the test pieces since the attendant radioactivity would make it unsafe to handle the radioactive material on the regular assembly line. This would be especially true where high radioactivity is desired in a particular portion or surface of the part studied, since such high specific activity could be obtained only by raising the total radioactivity to a hygienically as well as economically prohibitive level.
it is an object of the present invention to make solid objects radioactive without the need for neutron irradiation of the entire test piece and without the need of fabricating it entirely from radioactive material. Another object is to make test pieces having high radioactivity in specific portions, without high total activity. A more specific object is to make lead containing surface layers radioactive. These and other objects as well as the nature and scope of the invention will become more clearly apparent from the followng description and appended claims.
it has now been Cll"-COVI'-Cl that surfaces of machine parts such as copper-lead bearings can be made radioactive by diffusion. In particular it has been discovered that lead containing surfaces can readily be made radioactive to a defin'te depth by diffusion when in contact with radium D, which is a radioactive lead isotope whose half life of 22 years is suitable for wear or corrosion studies.
An especially suitable method for doing this involves electroplating the lead containing surface or" the test piece with a thin film of radium D, or with a homogeneous mixture or alloy of radium D and ordinary lead. The use of such radium-lead 1 Xtures is preferable both because it reduces the cost of the process and because it causes a desirably moderate concentration gradient of the radioactive isotope in the body of the electroplated part. For instance, it may be advantageous to use an electroplating bath containing some 1 to lfiil parts of radium D per million parts of lead, and deposit from it a leadradium film about 0.5 to 5 mm. thick. In order to avoid depletion of the active film, it is desirable that it contain at least five times as much radium D as will be absorbed by the test piece. However, metal films containing substantially higher or lower concentrations of the radio active isotope may be used, and deposited in thicker or thinner layers than those stated, depending on the depth of the surface layer to be studied, the rate of diffusion of the radioactive isotope, and so on.
instead of electroplating, it may likewise be possible to deposit the desired radioactive film by dipping the test piece in a molten bath of the radioactive metal or metal mixture. However, in such a case it is important to keep the clipping temperature and time such as to avoid destructive melting of the specific structure of the surface being plated.
After deposition of the radioactive film the plated test piece is preferably heated to speed up the diffusion of the radioactive isotope into the main body of the part. For instance, in the case of diffusing radium D into lead, the test piece may be heated for about 0.1 or 1 to 10 or more days at a temperature of about 200 to 400 C., preferably at about 280 to 320 (3., and especially at about 300 0., again depending on the depth of the surface layer which is of interest, on the concentration of the radioactive isotope in the film, on the melting point of the plated metal or alloy and so on. Temperature has of course a very pronounced effect on the rate of diffusion, the latter being perhaps 200 times as slow at 200 C. as at 300 C. Likewise, heating temperatures above the melting point of the metal are preferably avoided so as to avoid disturbing the uniformity of the 0 electroplated film. However, in the case of very thin plated films, temperatures somewhat above the melting point of the metal, e. g., above 327 C. in the case of lead, may be tolerated without causing flow of the electroplated film. Relatively short diffusion periods at high temperature, and relatively high original radium D concentrations are generally practical when surfaces are to be studied to a very shallow depth only, since both these factors tend to give a rather abrupt change in concentration of the radioactive isotope near the surface being investigated.
Upon cooling to room temperature the diffusion is virtually halted and the approximate depth distribution of radioactive material in the test piece can be calculated using equations derived from the laws of diffusion, or it can be determined experimentally on a reference piece prepared under identical conditions.
Next it is generally desirable to restore the original surface, such as the copper-lead bearing surface, by stripping the depositcd surface film. This can be done by electrostripping, that is, by reversing the electroplating procedure, or in some cases the deposited film may be removed by exact machining, or by dissolving it in mixtures of nitric acid, mixtures of acetic acid and hydrogen peroxide, and the like. Finally, the radioactive bearing is installed and its wear or corrosion determined while the engine is in operation. Knowing the depth distribution of the radioactive material, wear or corrosion of the test bearing, and hence also the breakdown of the surrounding oil, can be followed by continuous or periodic determination of the radioactivity of the oil. This can be done either directly in situ, for instance in the crankcase of the engine tested, by installing therein a counter such as a scintillation tube or a Geiger-Muller counter. Counting directly in the crankcase has the important advantage of permitting a truly continuous and instantaneous observation of oil breakdown or bearing corrosion while the engine is in operation. No similar in situ determination of radioactivity would be possible if the entire part were made out of a homogeneously radioactive material. In the latter case the background count emanating from the radioactive part itself would be too high at such close range to permit accurate determination of the radioactivity of the debris in the oil. Alternatively, however, even with the present invention it may be advantageous to minimize the background count while determining the radioactivity of the oil. This can be achieved by draining all or an aliquot portion of the h oil from the engine and determining its radioactivity at some distance from the radioactive source. It is generally desirable to run the test at a bearing temperature at least about 50 C. lower than the temperature used in the preparation of the test piece, or to run for a relatively short time, so as to avoid any extensive redistribution of radioactivity by additional diffusion during the test.
The concentration of diffused material at depth x (cm.) in a solid is related to the original concentration 60 on the original surface (x) and to the time of diffusion 13 (sec.). For instance, according to Wahl & Bonner, Radioactivity Applied to Chemistry, p. 510, I. Wiley (1951), the concentration 0 at depth x may be calculated from the equation wherein D is the diffusion constant in cm. /sec., and
erf(y), mathematically known as the error function," is defined as the integral f e de V 0 A specific illustration of the invention is given in the following example, it being understood that all ratios or concentrations of materials are always expressed herein on a weight basis, unless stated otherwise.
Li. Example A steel backed bearing having a fine-structure matrix of 30% lead and copper and a bearing surface of about 30 cm. is coated with a stop-off material such as a masking lacquer or paraffin wax except on the bearing surface to be plated. The partially coated bearing is placed in an electroplating cell containing, for instance, a conventional lead fluoborate bath, except that some of the lead is present in the form of the radium D isotope, e. g. in a concentration of about 10 parts per million. The plating may be carried out substantially as described in 1950 Metal Finishing Guidebook, Finishing Publications, N. Y. (1950), pp. 256-257. However, other known lead electroplating baths and procedures may be similarly used.
After depositing about 25 g. of the lead-radium D mixture as a uniform film about 0.7 mm. thick on the surface of the bearing, the plating is stopped, the bearing removed from the bath, and washed with water. The electroplated bearing, containing radium D in an amount equal to about 0.20 millicurie, is then heated in an electric furnace for about 4 days at 305 C. (diffusion constant 13:2.5 1C- and allowed to cool to room temperature. By this time the radioactive isotope will have diffused until its concentration at a depth of 0.125 mm. is about one-sixth as great as at the original bearing surface, or about 1.6 parts per million. This concentration is adequate to permit detecting differences as small as 15 mg. in lead weight loss, equivalent to 50 mg. of total bearing weight loss, at a depth of 0.125 mm., assuming that the available atomic disintegration counter gives satisfactorily reproducible results by having a count of 2 atomic distintegration counts per second per cubic centimeter of the oil present in the engine. For a total oil charge this corresponds to a total of 7.6 10 counts/sec./cc., or about 0.0002 millicurie. Pure radium D has an activity of about 8.6 curies per gram, or 129 millicuries per 15 mg. Since the concentration of radium D in the lead portion of the bearing matrix is about 1.6x 10- at the depth of about 0.125 mm., it is apparent that a lead weight loss of 15 mg. is just sufficient to give the activity of 0.0002 millicurie, which is required for proper accuracy in the system used. Of course, if less efficient atomic disintegration counters are used, it may be advisable to diffuse a greater concentration of radioactive material into the test piece.
Returning to the actual test procedure, the plated heat-treated bearing is next immersed in a solution containing volume per cent of glacial acetic acid and 5 volume per cent of 30% hydrogen peroxide, to remove the 25 g. of electro-deposited lead, and the stripped bearing surface is mechanically polished to restore its original physical condition.
The bearing containing radioactive lead diffused therein is finally installed in the engine in which the oxidation stability or bearing corrosivity of a lubricating oil, or the effectveness of corrosion inhibiting additives, is to be measured. The test may be carried out either on a stationary test engine or in a regular operating engine in the field. A typical test run may last 36 hours, though substantially shorter or longer runs are equally feasible.
As previously indicated earlier herein, corrosion of the bearing may be measured in terms of radioactivity of the motor oil either directly in the engine by installing a Geiger counter in the crankcase, or by periodically draining all or a portion of the oil from the engine for measurement outside. Of course, in order to use the radioactivity of the oil as a measure of bearing corrosion in any series of comparative tests, it is desirable that the procedure used for making the test piece radioactive be identical, particularly with respect to time and temperature level of the radioactive diffusion step. Where it is desired to use the radioactivity of the engine oil as a quantitative measure of bearing weight loss, an empirical reference curve relating to the two may be established.
For instance, a reference bearing made radioactive by diffusion as described above may be machined down in very thin layers so as to permit correlating actual weight loss with the corresponding radioactivity of the metal shavings. In other instances, a satisfactory correlation between weight loss and radioactivity of the debris resulting from wear or corrosion of the bearing to a given depth may be established by calculation as indicated in the foregoing example.
It will be apparent from the general description arr: from the specific example of the invention, that numerous variations and modifications are possible without departing from the broad scope hereof. For instance, instead of causing diffusion into the test piece from a solid radioactive film deposited from the surface, in cases where a surface is to be studied only to an extremely small depth, such as cm. or less, it may be practical to cause sufiicient diffusion of the radioactive material by immersing the piece in a hot aqueous solution containing a high concentration of a salt of the radioactive isotope, e. g. radium D nitrate or acetate. Or the solution may contain the radioactive isotope salt in admixture with its inactive isotope, so as to minimize abrupt concentration gradients in the surface layer of the test piece. If desired, the diffusion may be speeded up by operating under pressure so as to permit raising the temperature of the radioactive bath to as high as 170 C., or even higher when special pressure equipment is available.
For purposes of clarity, it will be understood that the generic expressions stratum of a radioactive isotope cr radioactive stratum will be used herein to describe a solid film of a certain weight and thickness containing a radioactive isotope and deposited on the test piece for purposes of diffusion, as well as a liquid radioactive bath in which the test piece may be immersed during the desired difiusion. Likewise, it will be understood that when the radioactive stratum and the test piece are separated from each other after the diffusion step, the radioactive stratum, while weighing the same amount as originally, will contain less than the original concentration of the radioactive isotope, whereas the test pieces, while also weighing the same as originally, will have a higher radioactivity due to the diffusion of the active isotope into the test piece in exchange for diffusion of the inactive isotope from the test piece into the surrounding radioactive stratum.
Furthermore, while most of the foregoing discussion has been specifically directed to the activation of lead, because the latter is particularly unsuited for activation by more conventional methods, it will be understood th t the principle of the present invention can be advantageously applied to other materials. That is, the present method broadly involves coating or surrounding the surface of a test piece with a solid layer, or a liquid bath, which contains radioactive material of a nature either chemically identical with, or otherwise soluble in a suitable component of the part to be tested. Thereafter the radioactive material is allowed to diffuse into the test piece to a desired depth, preferably at a temperature at least 50 or 100 C. above the temperature at which the radioactive piece is intended to operate. After cooling, the original surface of the test piece may be restored by stripping or the like, polished if necessary, and installed in the system in which wear or corrosion of the surface is to be studied.
For instance, surfaces containing iron, copper, bisrnuth, cadmium, nickel, chromium, phosphorus, silver, or any number of other elements may be investigated by means of the present invention by difiusing into the test piece a radioactive isotope of an element contained in the test piece. Alternatively, instead of using an isotope of the element in question, it may be similarly feasible to use a radioactive isotope of an element capable of forming a solid solution with the material contained in the surface to be studied. Thus one may diffuse radioactive carbon into an iron-containing surface, or radioactive mercury into a gold or platinum surface, or radioactive antimony into a tin surface, and so on. Likewise, instead of applying the invention to elements, in some instances it may be profitable to use oxides or salts of radioactive isotopes in the study of glasses, crystals, and similar materials containing a corresponding inactive substance.
Furthermore, while the invention has been described primarily with reference to wear tests, it may be similarly used wherever a relatively small total radioactivity is to be imparted to a thin specific layer of an object, without making the entire object radioactive. For example, it may be used to activate the surface of batching pigs for use in pipe lines, in which case the progress of the activated pig in the line may be detected by a Geiger counter located outside of the line.
The scope of the present invention is particularly pointed out in the appended claims.
1. A process for making a specific layer of a solid object radioactive which comprises contacting the object with a stratum of a radioactive material containing a radioactive isotope capable of diffusing into a component constituting the solid object, heating the object in contact with the radioactive stratum at a temperature below the melting points of the solid object and the radioactive stratum until the isotope diffuses into the object to a suitable depth, and separating the activated object and the radioactive stratum from each other.
2. A process according to claim 1 wherein the object contains lead, and the radioactive stratum contains radium D.
3. A process for making a lead containing surface layer of a metal part radioactive which comprises electroplating a lead film up to about 5 mm. thick and containing about 1O to 10 of radium D on top of the said surface layer, heating the electroplated part at a temperature between about 200 and 327 C. for about 0.1 to 10 days, and stripping the plated lead film from the metal part.
4. A process according to claim 3 wherein the object is a bearing having a surface layer composed of about 30% lead and copper.
5. A process for determining the amount of wear of a lead containing metal surface which comprises electroplating a thin film of a mixture of lead and about 10 of radium D on said lead containing surface, heating said electroplated surface at a temperature of about 300 C. for a period of about 1 to 10 days, stripping the surface plating from the resulting activated surface, polishing the stripped activated surface to restore its original surface characteristics, subjecting the activated surface to corrosion in contact with a fluid capable of carrying radioactive wear debris, and measuring the radioactivity of said fluid.
6. A process according to claim 5 wherein the radioactivity of said fluid is determined in situ.
7. A process according to claim 5 wherein the fluid is oil and wherein portions of said oil are withdrawn from contact with the activated surface and the radioactivity of the withdrawn oil is determined.
References Cited in the file of this patent UNITED STATES PATENTS 2,254,170 Dillon Aug. 26, 1941 2,339,545 Dillon Jan. 18, 1944 2,367,949 Langer Jan. 23, 1945 2,468,905 Warren, Jr. May 3, 1949