US3037886A - Metal cleaning - Google Patents

Description

3,037,886 METAL CLEANING John W. Ryznar, La Grange, 111., assignor to Nalco Chemical Company, a corporation of Delaware No Drawing. Filed Apr. 11, 1957, Ser. No. 652,096 1 Claim. (Cl. 134-4) i in defective articles that are not satisfactory as items of commerce and industry.

There are several methods of cleaning metals which are used in one form or another throughout the industrial world. The first is solvent cleaning in which the metal part to be cleaned is placed in a bath of solvent and is allowed to soak for a period of time suificient to remove the contaminants found on the metal. Such solvents as high boiling naphthas, trichloro ethylene, ethylene dichloride, xylene and toluene are illustrative of the types used in this process.

The second cleaning technique employs a hydrocarbon solvent emulsified in water. In using such a cleanse-r it is usually the practice to spray the emulsion onto the surface to be treated or, alternatively, to place the solvent on the metal and flow water over the treated surface.

Another popular solvent cleaning technique is vapor degreasing. In this process the part to be cleaned is dipped into a container of boiling solvent to remove the heavier coatings of contaminants. :It is then removed and held above the container where the hot vapors remove vestiges of dirt and deposits. In this latter stage the action of the process is dependent upon the hot solvent vapors condensing upon the surface sought to be cleaned.

While in many instances solvent cleaning processes are satisfactory they suffer from several disadvantages. Organic solvents used in metal cleaning operations are in some instances toxic and care must be exercised in their use. When elevated temperatures are employed there is always the danger to personnel of scalding or burning. The great disadvantage, however, resides in the fact that they do not completely remove residual, thin films of oil which often contaminate metal surfaces even after careful solvent cleaning.

To remove the last traces of dirt and oil films it is usual to follow solvent cleaning by alkali cleaning in the form of a soaking process. Solutions of such cleansers as caustic soda, caustic potash, sodium silicates, sodium orthophosphate, sodium carbonates, molecularly dehydrated polyphosphates, organic emulsifiers, and synthetic wetting agents compose the majority of ingredients found in most alkali cleaners. They are used as hot solutions, e.g. 1602Jl0 F. with the metals to be cleaned being in contact with these solutions for periods of time ranging from a few minutes to several hours. When ferrous metals are cleaned in such hot soak processes they are often oxidized and hence rendered unsuitable for further finishing.

It would be a valuable contribution to the art if a non-toxic, non-volatile metal cleaner were aavilable which would rapidly clean metal surfaces of dirt, grease and the like. It would also be beneficial if a metal cleaner were available which would completely and rapidly remove thin films of oil from metal surfaces. It therefore becomes an object of the invention to provide a new method of cleaning metal surfaces.

ice

Another object is to provide a cleaner which will effectively and rapidly remove thin oil films from metal surfaces.

A further object is to furnish a non-toxic, non-volatile metal cleaning process.

Still another object is to provide a metal cleaning process that may be used in conjunction with other metal cleaning processes. Other objects will appear hereinafter.

In accordance with the invention, it has been found that carbonaceous deposits maybe removed from-metal surfaces by cleaning said surfaces with colloidal silica. The expression carbonaceous deposits is a generic expression intended to cover such materials as oils, greases, bufiing compounds, fatty substances and other materials which commonly contaminate metal surfaces. The colloidal silicas are beneficial in removing all types of dirt and grease. They are particularly well suited for rapidly removing residual oil films from metal parts.

When the colloidal silicas are used to treat oi-l films they apparently act as an absorbent and remove the oil by capillary action. If this theory is correct, then it at once becomes apparent the amount of colloidal silica used must be of a quantity sufficient to absorb all the dirt and oil present. Due to the high specific surface areas and large pore volumes only small amounts are required to treat relatively large quantities of contaminants. This theory is presented as an aid in explaining the operation of the processes involved but is not intended as a limitation of the invention.

The colloidal silicas may be used either alone or in combination with any of the well known cleaning processes now in practice. Thus, they may be used as adjuncts to solvent cleaning, emulsion cleaning, vapor degreasing and alkali cleaning. They are most beneficial in removing thin residual oil films which are often times not removed by conventional cleaning methods.

When metals are to be electroplated they must be completely clean and free of dirt. To test the cleanliness of metals it is the practice of the art to treat them with water and see if a uniform film forms. Any break in the film indicates the presence of oil. Treatment of oil contaminated metals with colloidal silica for periods of time as short as a few seconds produce surfaces which are uniformly wet by water.

In general, the colloidal silicas, preferably employed in the practice of the invention fall into four classes:

(A) The silica sols.

(B) The hydrolyzed organic esters of silicic acid.

(C) The fine silicas.

(D) The synthetic silica-alumina zeolites.

(A) THE SILICA SOLS Silicic acid sols are generally believed to be a polymeric derivative of monomeric silicic acid which arbitrarily may be assigned the formula:

As this material is produced it is capable of polymerizing to form various sized polymers containing the monomer unit OSi-O OH in the mechanism is considered linear. It is known, however, if the reaction carries to completion a gel is formed which evidences a three dimensional cross-linked network formed from the starting silicic acid molecule. The polymerization mechanism thus described may begin immediately upon the formation of monomeric silicic acid and proceeds to form particles or masses of colloidalsize within a very short time. When colloidal dimensions have been formed in sols preferred for use in this invention, the cross-linking or gelation of the sol may be inhibited by adjusting the pH of the system with acid or alkali to between 2.5 to 4.8 and 8.5 to 10.0, respectively, or adding a small amount of alkali metal. Under these conditions, the sol will retain its colloidal dimensions for long periods of time. When the silicic acid reaches colloidal size and is stabilized, the particle size present is about'l millimicron and possibly 2 to 3 millirnicrons in diameter. The exact size will depend on concentration, presence or absence of electrolytes, pH and temperature.

The silicic acid sols preferably employed in the present invention are those containing at least 3.0% Si and most preferably 7% to 48% SiO and have a pH sufiicient to stabilize said sol against gelation. Silicic acid sols may be prepared by any number of well known methods, many of which are summarized in Bechtold et al., U.S. Patent No. 2,574,902, and the present invention contemplates the use of any of such sols. Silica sols capable of use in the present invention may be conveniently prepared by the techniques described in Bird, U.S. Patent No. 2,244,- 325, although the well known methodof hydrolyzing sodium silicate with strong mineral acids with the subsequent removal of excess salt is equally suitable.

' about 10% SiO The Bird patent shows that alkali metal silicate solutrons may be contacted with a cation exchange resin in silica content thereof, several fold. While the Bird patent f shows generally the method of concentrating silica sols, there are now several methods available which produce sols having a relatively high silica concentration in the form of discrete non-agglomerated particles. Such methods are shown in Bechtold et al., U.S. Patent No. 2,-.

574,902; Brage et a1., U.S. Patent No. 2,680,721; and Parma et al., U.S. Patent No. 2,601,235.

In concentrating colloidal silicic acid sols, the usual methods increase the size of the discrete particles present in the sol. As the general rule the more concentrated the sol, the larger will be the particle size of the silica present. In a freshly prepared batch of stabilized 3.5% by weight SiO sol prepared by the Bird method the particle size is believed to be about 1 to 3 millhnicrons in diameter. If such asol is concentrated to say about 30% SiO by using the techniques of Bechtold et al., U.S. No. 2,574,902, the average particle size will vary from 15 to 130 millimicrons'in diameter. 7

As can be seen from the previous discussion a wide variety of colloidal silica in the form of aqueous sols are readily capable of being prepared. The invention will be further illustrated by the following examples in which the proportions are given by weight unless otherwise indicated. All of the colloidal silicacompositions described in these examples are suitable for the practice of the inven tion.

Example I Commercial sodium silicate was diluted with Chicago tap water to produce a sodium silicate solution having present therein about 4.5% SiO The weight ratio of Na ozsiO was about 123.2, with a specific gravity of about 1.050. This diluted sodium silicate was passed through a column of a hydrogen form sulfonated polystyrene divinylbenzene copolymer cation exchanger of the type disclosed in U.S. No. 2,366,007. The efiluent contained about 3.5% SiO had a pH of 3.5 and a conductivity of about 400 to 800 micromhos. To this silicic acid sol eiiiuent was added an amount of 26 Baum ammonium hydroxide suflicient to adjust the pH of the acid sol to about 9.0.

Example 11 The procedure used in Example I was the same except that the solution of the starting sodium silicate contained The finished sol had an SiO concentration of about 7% and a specific gravity of about 1.050. Ammonium hydroxide was added to the sol so that the final pH was about 10.5.

Example III A portion of the sol of Example I was placed in an evaporating kettle and heated until ammonia and steam vapors began to come oif. At this point a small amount of permanent alkali (KOH) was added and fresh ammonia stabilized sol was added to maintain the evaporating volume constant. Throughout the process the pH was never allowed to go below 8.5. This was accomplished by continually adding gaseous ammonia during 2 the heating process. The evaporation was continued with constant checks being maintained to keep the pH always above 8.5, and was continued until specific gravity of the sol had reached 1.20 at 68 F. When this specific gravity had been obtained an amount of potassium hydroxide was added to give the finished sol a pH of 9.0. This sol had an Si0 concentration of 30%.

Example IV Another sol was produced by using the method shown in Example III. In this instance, however, the concentration process was continued until the sol had an Si0 concentration of about 48% In general, the silicic acid sols are preferably used in 7 their more concentrated form. Sols containing about 30% SiO have given superior results, yet sols having an SiO content as low'was 3% have shown effectiveness.

These aqueous sols are sensitive to low temperatures but they may be treated by two methods whereby the effects of lower temperatures are avoided. The first method is to incorporate therewith an antifreeze such as methyl alcohol or ethylene glycol. This can be easily done by adjusting the pH of the sol to about 2.5. The second method is to treat the sol with an alkyl amine so that if the sol is frozen it may be redispersed in a colloidal state. This latter mode of treatment is shown in Horning, U.S. Patent No. 2,- 601,291.

In order to facilitate application of the sols to afford a more even distribution over the surface of the metal to be cleaned, it is helpful to add thereto about 0.1% to 2% by weight of a compatible Wetting agent. The only requirement of such agent is that it be compatible with the sol insofar as it is soluble and does not cause precipitation or gelation. Anionic synthetic detergents of the alkali metal alkyl (e.g., ootyl or nonyl) benzene sulfonic acid type are admirably suited for the purpose of the invention. Any compatible anionic or non-ionic wetting agent can be employed, for example, those listed in the article Synthetic Detergents-To Date, II, by John W. McCutcheon, appearing in Soap and Sanitary Chemicals, July, August, September and October issues, 1952. Cationic wetting agents tend to form gels or precipitates when added to these sols and should be avoided (B) THE HYDROLYZED ORGANIC ESTERS OF SILICIC ACID Another source of colloidal silica for use in the practice of the invention are the hydrolyzed organic esters of silicic acid. These esters are usually prepared by the reaction of an organic compound having a free hydroxyl group with SiCl lowing equation where R is a hydrocarbon group.

The organic hydroxyl containing compounds are usually low molecular weight acyclic aliphatic alcohols having not more than 6 carbon atoms and aromatic hydroxyl compounds such as phenol. If the condensation reaction above is carried to completion under substantially anhydrous conditions the tetra substituted alkoxy or aroxy silane will usually result. If, however, the esterification is incomplete or water is present, a condensed ester of the general formula (RO )Si--O(Si-) -Si(RO will result where n is a small whole number and R is a hydrocarbon radical. Examples of several commercially available ethyl silicates are shown in Table I below.

This reaction is shown by the fol- TABLE I Tetraethyl Condensed Ethyl ortho ethyl (40% SiOr) silicate silicate silicate Molecular weight"- 208.30 SrgeciQfg goravity at 0.9356 0.9323 1.0508. Boiling rahge, 760 Below 160 0. Below 100 0. [BF 80 0.

mm. hg. maximum; 5% maximinimum;

below 170 0. mum; below 110 0. 95% minibelow 190 5% maximum. O. 92% minlmum,

mum. 40% minimum. Agaiclable silica as 28% minimum 28% minimum. 40% minimum.

1 a. Freezing point, (L 77 -84 90.0. Average weight 7.78 7.78 8.82.

pler gallon at C. Viscosity at 20 0., 0.6 0.72 3.9.

cps. Flash point 135 90 90.

(Cleveland open cup) F. Refractive index 1.3832 1.3838 1.3965.

at 20 0. up.

1 98% tetraethyl silicate.

In addition to those silicates shown above, others that may be used are methyl silicate, butyl silicate, amyl silicate, phenyl silicate, benzoyl silicate, hexamethoxydisiloxane and hexaethoxydisiloxane.

All the silicic acid esters shown above have the property of hydrolyzing in water, or water alcohol mixtures at rather uniformly controllable rates. A small amount (0.03 to 0.1%) of a strong mineral acid such as hydrochloric will accelerate the hydrolysis reaction. When less than the equivalent amount of water is used in an alcohol solution of these silicates they can be stored for long periods of time without gelation. Examples of hydrolyzed ethyl silicate solutions are given in the publication Ethyl Silicates, January 4, 1954, by Carbide and Carbon Chemical Company. While some of the compounds shown above are esters of polymeric silicic acid they are herein considered to be simply esters of silicic acid.

(C) THE FINE SILICAS The finely divided silicas having a surface area of at least square meters per gram usually have an ultimate particle diameter not greater than about 100 millimicrons. In general, the diameter of the silica particles will be within the range from about 1 to about 100 millimicrons. While the specific surface area of the silica particles is preferably at least 25 square meters per gram, it usually will not exceed 1000 square meters per gram, and a preferred range of specific surface area is from about 25 m. g. to about 400 m. g.

A number of different types of silicas can be prepared by Well known methods and many of these are available commercially. The following examples are given to illustrate various types of silicas which can be employed for the purpose of the invention.

1) A silica, in the form of very small discrete particles having a gel structure within the particles, prepared by reacting sodium silicate and an acid at a pH below 3.0 to give a silica sol, polymerizing the silicic acid in the sol sufilciently to make the sol viscous, mixing an organic hydrogen donor bonding agent, such as tertiary butyl alcohol, with the sol, dissolving salt in the mixture whereby a phase separation occurs giving a hydrogen bonder phase containing the silicic acid and an aqueous brine phase, polymerizing the silicic acid further in the bonder phase until hydrated silica is precipitated in the form of discrete particles, separating the precipitated silica from the mother liquor and washing free of salt.

(2) A hydrated amorphous silica powder consisting of super colloidal aggregates of ultimate units of from 10. to 50 mil-limicrons in diameter described in Chemical Engineering, 54, 177 (1947), having a specific surface area of about 240 square meters per gram and a bulk density of about 0.064 gram per cc. at 3 pounds per square inch gauge.

(3) An amorphous silica aerogel having a specific surface area of about 160 m. /g., as determined by nitrogen adsorption, and a bulk density of about 0.087 gram per cc. at 3 pounds per square inch gauge (Santocel C).

(4) An amorphous silica powder consisting of super colloidal aggregates of ultimate units having an average diameter of about 25 millimicrons, a surface area of about m. /g., and containing a small amount of calcium (1% to 2% by weight), known as Hi-Sil.

(5) An amorphous silica powder consisting of supercolloidal aggregates having a surface area of about 200 m. /g., sold under the name K-3.

(6) A silicate treated with heavy metal salts or hydrous heavy metal oxides to form heavy metal silicates which are water insoluble and usually amorphous as determined by X-ray determination, as, for example, a pre cipitated hydrated calcium silicate having a molar ratio of SiO /CaO equal to about 3.25, containing aggregates of ultimate particles of the order of 30 to 50 millimicrons in diameter, described in Chemical and Engineering News, 24, 3147 (1946), and marketed as Silene EF. This product is a calcium silicate having the following analysis:

CaO 19.0% by weight.

SiO 67.0% by weight.

Loss on ignition 14.0% by weight.

pH in water suspension 10.1.

Specific gravity 2.10.

Bulk density 15-16 pounds per cubic foot. Refractive index 1.475.

The metal ion in the aforementioned silicas can be an ion other than calcium, as, for example, barium, strontium, magnesium, zinc, cadmium, lead, tin, iron, cobalt and nickel.

(7) Colloidal silicas obtained by hydrolysis of silicon tetrachloride.

(8) Estersils obtained by the esterfiication of an amorphous or crystalline silica as described in Iler, US. Patent No. 2,657,149. The estersils employed for the purpose of the invention are preferably obtained by esterifying any of the silicas described under (1) to (7), inclusive, so as to produce a super colloidal substrate coated with OR groups, the substrate having a surface of silica and having a specific surface area of from 25 to 900 square meters per gram, there being at least 100 OR groups per 100 square millimicrons of substrate surface area, and R being a hydrocarbon radical of from 2 to 18 carbon atoms wherein the carbon attached to oxygen is also attached to hydrogen. For the purpose of the present invention, the estersils employed are hydrophobic and can be both hydrophobic and organophilic.

The following table gives specific examples of silicas which can be employed for the purpose of the invention.

In the foregoing table silica A is a calcium silicate'containing about 19% CaO' and 67% SiO Silicas B, C, D, E and F are precipitated silicas. Silicas G and H are incompletely surface esterified silicas of the type described in US. Patent No. 2,657,149. Silica K i a silica derived from a silica gel. Silicas I and L are fine silica aerogels.

Silica J is a commercial silica prepared by the hydrolysis of silicon tetrachloride.

In using the fine silicas it is desirable to use a liquid slurry or pasty suspension of these materials by using a hydrophilic liquid such as water or a lower water soluble monohydric alcohol such as methanol, ethanol or isopropanol. Such suspensions should contain at least 2% by weight, and preferably to 50% by weight of the fine silicas.

When the estersils are used they may be conveniently suspended in a chlorinated hydrocarbon solvent such as carbon tetrachloride, perchlorocthylene and the like, which in some instances enhance the valuable cleaning properties of the surface modified fine silicas. The estersils may be suspended in the chlorinated organic solvent at a concentration of at least 3% and preferably 5% to 30% by weight. The particular solvent chosen should not have a viscosity at 26 C. greater than 35 centipoises and preferably not greater than 8.5 centipoises. The products thus produced are usually in the form of pastes or jellies.

(D) THE SYNTHETIC SILICA-ALUMINA ZEOLITES Zeolites contain combined water, sodium oxide (Na O) alumina (A1 0 and silica (Slo in various ratios. Usually the Na O:Al O molecular ratio is about 1:1, but the alumina-silica ratio may vary considerably ranging from about 1:2 to as high a's 1:15. The sols and gels of zeolites may be prepared by mixing together in various proportions solutions of sodium silicate, such as diluted commercial water glass solutions, sodium aluminate solutions and a strong acid such as sulfuric acid. By varying the ratios of the solutions, the pH and the Al O :SiO ratio can be controlled. One method for making zeolite gels is described in considerable detail in Bruce, US. Patent No. 1,906,202.

The following examples are supplied as illustrations of methods for preparing sols and gels in various ratios and at various pHs. All parts are by weight unless otherwise designated. v

Three solutions designated A, B and C were prepared as follows, Solution A was made by diluting 36.8 parts of commercial water glass solution with 314 parts of water. The commercial water glass solution had a Na O:SiO ratio of 1:322 at 37.5% SiO concentration. Solution B was made by dissolving 6.4 g. of sodium aluminate in water and diluting the solution to 94.4 cc. Solution C was sulfuric acid.

8 Example V A mixture of 35 cc. of solution A and 9.4 cc. of solution B, to which was added 3.2 cc. of solution C, gave a sol which quickly gelled and had a pH of 10.7. The Al O :SiO ratio Was 1:8.

Example VI .A mixture of 35 cc. of solution A and 9.4 cc. of solution B, to which was added 10 cc. of solution C, gave a solution which gelled after five days and had a pH of 3. The Al O :SiO ratio was 1:8.

Example VII A mixture of 18.2 cc. of solution A and 10 cc. of solution B gave a quickly gelling sol which had a pH of about 11. The Al O :SiO' was 1:4.

To illustrate several typical compositions that are useful in the practice of the invention the following are presented by way of example:

Composition A: Percent Silica sol, Example III 60.3 Isopropanol 39.4 Phosphoric acid 85% 0.3

This product had a pH of 3.7.

Composition B:

' Silica sol, Example III.

Composition C v Percent Silica s01, Example III 99 Dodecyl benzene sodium sulfonate 1 Composition Dz.

Silica sol, Example III.

Composition E:

Ethyl silicate (40% SiO hydrolyzed in ethanol and water to 16% SiO content.

Composition F:

Silica sol, Example IV, diluted to 7% SiO Composition G:

To the silica sol of Example III was added 39.3% by weight of isopropanol to form a slurry. The slurry was divided into two portions. One portion was passed through a homogenizer and the other placed in a blender and violently agitated. In each instance the product was a white, easily spreadable material similar in texture to library paste.

Composition H:

30 cc. of a 50% by volume water-isopropan'ol solution mixed with 5 grams Silica F.

Composition I:

20 cc. of a 50% by volume water-isopropanol solution mixed with 2 grams Silica J.

Composition J:

375 cc. isopropanol 375 cc. water 200 grams Silica B Composition K:

20 grams of Chicago tap water 8 grams Silica J Composition L: Percent Silica G -4 20 Tetrachloro ethylene Composition M:

Silica G 15 In practicing the invention it is preferable to use the colloidal silicas in the form of a liquid or a paste. Good results are afforded when the liquid carrier for the silicas contains a lower monohydric alcohol. Experimental observations have shown these water-alcohol silica containing products to have the ability to more uniformly coat metal surfaces contaminated by oil or grease. When aqueous liquids containing colloidal silica are used the cleaning action is beneficial but the surface contact is not as good as that obtained with the water-alcohol products.

To achieve maximum cleaning effects in using aqueous colloidal silica compositions it is usually necessary to allow the treated surfaces to dry. Although the drying is not critical to achieve water wettable surfaces it may become critical in those cases where an absolutely clean, dirt free surface is desired. In a preferred embodiment the metal to be cleaned is first treated with an aqueous suspension of colloidal silica dried and then rinsed with a clean polar liquid such as water, acetone or the like to remove the last traces of contaminant and silica residue.

To clean metal parts preparatory to coating or treating operations such as painting it is not necessary to use a drying step or an elaborate rinse process. It is only necessary to treat the surface with the colloidal silica. When excessive amounts are used a rinse step may be necessary to wash oi the excess silica which, after drying, appears as a white film.

The amount of colloidal silica used to clean contaminated metal parts is not critical but as a general rule the greater the amount of carbonaceous deposits, the greater Will be the amount of silica necessary and the longer will be the drying time. Laboratory tests have shown colloidal silica, as SiO will absorb 2% of its weight of oil and grease to produce an absolutely clean surface. Lesser amounts may be used, however, when only a water wettable surface is desired and removal of all oil from the microscopic spaces in the metal is not essential.

In use, the metal to be treated may be conveniently dipped in a container filled with an aqueous and/ or wateralcohol suspension of colloidal silica and allowed to air dry with any residue of silica being rinsed oll with Water. Another method is to dip the metal part into a container of colloidal silica and then immediately into a rinse bath or solution. Alternatively, the colloidal silica may be sprayed, brushed, coated or wiped on the metal surface to be cleaned with good results being obtained in each case. It is to be understood the invention is not intended to be limited to any'particular mode of application.

The manner in which the colloidal silica is used as a cleanser will be governed by the particular operation involved. In some instances it may be desirable to first clean the part with a solvent and then finish the process with a colloidal silica treatment. The amount of deposits on the metal surface, their nature, the cleaning cycles used in a particular operation, the type of metal, etc. will make the choice of colloidal silica as well as its mode of application dependent on the particular circumstances of each cleaning situation. Routine investigation will enable one skilled in the art to effectively use a colloidal silical cleaner to its greatest advantage.

EVALUATION OF THE INVENTION To evaluate the cleaning efiiciency of the various colloidal silicas a test was devised which very accurately gave an indicia of their effectiveness. In its simplest form, a steel specimen was treated with an oil containing a radioactive compound and the radioactivity, (beta-rays), measured using a 211- proportional gas flow counter. The contaminated specimen was then treated with a colloidal silica cleaner. After treatment the specimen was again inspected with the counter to determine the decrease in radioactivity. From these measurements the percent of oil removal could be calculated. A detailed description of this test is given in the following examples:

10 Example VIII Ml. from Mg. of oil Counts dilution min.

From the above figures a formula was derived to ob tain the thickness of the oil layers in all future experiments. Thickness of oil in mm.=1.198 10-' Initial c.p.m. (counts per minute).

In order to prevent interference with accurate counting due to edge effects, only the central area of the slugs were counted. A counting shield was used to define the area. This consisted of a piece of stainless steel large enough to cover the slugs and having a hole in the center with the area of 1.54 cm. Thisshield was used in all counts so that the same area was counted in all tests.

A one inch square steel slug that had been highly polished was treated with one small drop of C labeled oil and it was spread evenly over the surface with a piece of absorbent tissue. (In all subsequent tests this method of spreading the oil was used.) The slug, covered with the stainless steel shield, was placed in a 21r counter and an initial count was taken over a ten minute period. Two drops of composition A were placed on the slug and allowed to air dry. When dry the'slug and shield were placed in the 211' counter and another ten minute count was taken. The slug Was then brushed lightly with a piece of paper to remove the flaky deposit of composition A. The slug and shield were placed in the 21r counter and a third ten minute count was taken.

The loss in the final count is assumed to be due to adsorpton of the oil on the dried silica.

The drop in count after applying composition A is due to absorption of the beta radiation by the film.

Intlal Thickness of O.p.m. after O.p.m. after Percent oil c.p.m. oil, mm. treatment treatment removed removal 3, 553 4.2(5 10' 1, 677 8 99. 8 5, 669 6.8)(10- 2, 654 19 99. 7 6, 859 8.23Xl0- 3, 380 99 98. 6 7, 965 9.55Xl0' 4, 019 1 218 97. 3 5 0 100.0

1 First treatment. 2 Second treatment.

Example IX Steel slugs were treated by allowing composition A to flow over the surface. The excess was allowed to drain off and the film allowed to dry. Ten minute counts were taken in the 21r counter before treatment, after treatment, and after treatment removal.

Thickness of oil, O.p.m. after C.p.m. after Percent Initial c.p.m. mm. treatment treatment oil removed removal 1, 474 l. 77Xl0' 843 2 99. 9 2, 419 2. 9X10- 1, 406 35 98. 6 7, 623 9.15X10- '4, 274 2, 568 66. 3

1 1 Example X A series of steel slugs treated with C labeled oil were treated with two drops of composition A. At varying time intervals after treatment the slugs were dipped in a beaker of distilled water to wash off the treatment.

The last two results were obtained by spraying composition A on the test slug, allowing the treatment to dry, then rinsing under distilled water tap.

Counts were taken on the 21r counter before and after treatment.

A steel slug treated with C labeledoil was wet with a few drops of water. Composition A was sprayed over.

the wet surface and allowed to stand for several minutes without drying completely. The treatment was then rinsed off under the distilled water tap.

Counts on the 211' counter were taken before and after treatment.

O.p.m. Percent Initial Oil thickness, after oil rec.p.m. mm. treatment moval removal 3, 316 4.0)(- l 983 I 70.0

Example XII Steel slugs treated with C labeled oil were treated with two to three drops of composition H. The treatment was allowed to dry then brushed ott lightly with a piece of paper.

Counts were taken before-and after removal of treatment.

l O.p.m. C.p.nr. Perl'nitial Oil thickness, after after cent 0.13.111. mm. treatment treatment 011 removal removal 6, 697 8. OXlCP 3, 101 26 99. 6

From these tests it becomes apparent that drying of the colloidal silica improves oil removal and also if repeated recleanings are made a cleaner surface is produced.

Example XIII F. and the time of the immersion one-half hour, which were in accordance with the manufacturers instructions. The specimen was removed from the cleaning bath with clean wire holders and placed in aweak NaCN solution to prevent corrosion. The specimen was then rinsed in distilled water and spun dry. Another specimen was dipped into composition A which was at room temperature for a few seconds, rinsed with distilled water and spun dry. Both cleaning methods produced uniform, water Wet films that covered the entire surface of the specimen.

Example XIV In another series of tests, steel specimens contaminated with oil and dirt were treated with compositions B-Q, alowed to dry and were then rinsed with distilled water. In each case a uniform, water wet film was produced on the metal surface thus cleaned.

From the above it is apparent that a fast, simple and safe method for cleaning metal surfaces has been demonstrated. The colloidal silica cleaners are safe, commercially available products that produce a rapid cleaning action heretofore not obtainable with commercial alkaline cleansers. To compare the speed of the colloidal silica cleansers of this invention to those available to the prior art reference is made to the publication, Soluble Silicates, vol. II, James G. Vail, Reinhold (1952), pp. 29-30. There it is shown such materials as 2% solutions of To demonstrate the superiority of the compositions of this invention as opposed to commercial alkaline cleansers a direct comparison of cleaning efliciency was conducted.

Two small flatsteel. specimens were polished with N0. 2 emery, paper. One specimen was cleaned by soaking in an 8 oz. per gal. solution of a commercial alkaline phosphate cleaner. The temperature of the solution was 180 periods of time varying from 2.2 minutes to minutes to clean oil contaminated steel specimens. By comparison Examples XIV and XV show the colloidal silicas require only a few seconds to remove oil contamination. Both test procedures used the water wettability of the specimen as the'norrn of cleanliness.

- The expression cleaning as used herein is meant'to include the removal of contamination by any physical or mechanical means such as dipping, scrubbing, rinsing, spraying or the like and is not intended to cover any particular mode or means of dirt removal.

I claim; V

A process for removing oil films on metal surfaces which comprises applying over a metal surface having an oil film thereon a coating of a colloidal silica sol substantially free of alumina andcontaining atleast 3% SiO drying said coating and thereafter rinsing olf said coating from said surface with a polar washing liquid.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Her: The Colloid Chemistry of Silica and Silicates, Cornell University Press, Ithaca, N.Y., 1955, page 186, (Copy in Div. 64,)