US3567676A - Corrosion preventive coating consisting of binder,metal pigments and fluoride activators - Google Patents

Abstract

A COATING AND METHOD FOR REDUCING THE SUSCEPTIBILITY OF METAL ALLOYS INCLUDING TITANIUM ALLOYS, ALUMINUM ALLOYS, HIGH STRENGTH AND STAINLESS STEELS TO STRESS CORROSION. THE COATING COMPRISES THREE COMPONENTS: (1) A METALLIC PIGMENT OF EITHER ZINC OR ALUMINUM; (2) AN ACTIVATOR WHICH, IN THE CASE OF AN ALUMINUM PIGMENT, MAY BE EITHER AN IONIZABLE FLUORIDE (SODIUM FLUORIDE, CALCIUM FLUORIDE, LITHIUM FLUORIDE OR MAGNESIUM FLUORIDE) OR AN ACID SALT (POTASSIUM ACID SULFATE, SODIUM ACID SULFATE OR SODIUM DIACID PHOSPHATE); AND IN THE CASE OF EITHER AN ALUMINUM OR A ZINC PIGMENT, THE ACTIVATOR IS A METALLIC HYDROXIDE (BARIUM HYDROXIDE, CALCIUM HYDROXIDE, CESIUM HYDROXIDE, LITHIUM HYDROXIDE, MAGNESIUM HYDROXIDE, POTASSIUM HYDROXIDE, RUBIDIUM HYDROXIDE, SODIUM HYDROXIDE, OR STRONTIUM HYDROXIDE); AND (3) A BINDER OF CHLORINATED RUBBER, POLYSTYRENE, POLYVINYL CHLORIDE, ALKYD RESINS, PHENOLIC RESINS, ACRYLIC RESINS, OR SILICONE RESINS.

Description

"United States Patent 01 iice 3,567,676 Patented Mar. 2, 1971 U.S. Cl. 260-37 3 Claims ABSTRACT OF THE DISCLOSURE A coating and method for reducing the susceptibility of metal alloys including titanium alloys, aluminum alloys, high strength and stainless steels to stress corrosion. The coating comprises three components: (1) a metallic pigment of either zinc or aluminum; (2) an activator which, in the case of an aluminum pigment, may be either an ionizable fluoride (sodium fluoride, calcium fluoride, lithium fluoride or magnesium fluoride) or an acid salt (potassium acid sulfate, sodium acid sulfate or sodium diacid phosphate); and in the case of either an aluminum or a zinc pigment, the activator is a metallic hydroxide (barium hydroxide, calcium hydroxide, cesium hydroxide, lithium hydroxide, magnesium hydroxide, potassium hydroxide, rubidium hydroxide, sodium hydroxide, or strontium hydroxide); and (3) a binder of chlorinated rubber, polystyrene, polyvinyl chloride, alkyd resins, phenolic resins, acrylic resins, or silicone resins.

BACKGROUND OF THE INVENTION This invention relates to protective coating comprising a pigment, an activator, and a binder for protecting metals and alloys from corrosion. While the protective coating of this invention may be used on a wide range of metals and alloys, its ability to protect titanium metal and titanium alloys from corrosion is particularly significant. Titanium has outstanding physical and chemical properties, including high yield strength and high ultimate strength per unit weight at elevated temperatures which make its use in structural parts Where light weight is required very attractive. Titanium has found considerable widespread application in the aerospace industry.

However, attractive as titanium is for applications requiring high strength, low weight materials, the use of titanium in structural members exposed to corrosive environment has been somewhat limited due to its susceptibility to stress corrosion cracking. There are generally two types of corrosion phenomena against which a metal has to be protected: (1) simple corrosion and (2) stress corrosion. Simple corrosion refers to the relatively slow and uniform loss of material from the surface of a metal. In its more familiar occurrence, this loss of surface material is due to the oxidation of the surface layer resulting in a film of metal oxide or, more generally, a change of the surface material from its normal valence state to one that is more positive. Stress corrosion refers to the tendency of certain metal alloys (including aluminum alloys, high strength steel, stainless steels, and titanium alloys) to crack when highly stressed in a corrosive environment. Unlike simple corrosion, stress corrosion occurs along fine cracks or other nucleating points and proceeds deeply into the metal structure, reducing its total overall strength and its strength in certain directions. Compared to simple corrosion, stress corrosion is very unpredictable in occurrence and the rapidity of cracking can be amazingly high. These characteristics, of course, make stress corrosion of much greater concern than simple corrosion to struc tural designers.

There is presently no uniformly accepted mechanism for stress corrosion. However, it has been demonstrated that a relatively high stress is required; thus metals having a high yield strength are particularly subject to stress corrosion cracking. Further, the presence of limited areas or paths in the alloy which are more susceptible to corrosion and cracking than the remainder of the metal appears to promote susceptibility to stress corrosion cracking. Finally, for stress corrosion to occur, a corrosive environment (such as an electrolyte which will cause galvanic corrosion) must be present. For a further discussion of the mechanism of stress corrosion cracking in steels and other metals see Uhlig, Corrosion and Corrosion Control 117 (1963).

Certain metals which are resistant to simple corrosion are susceptible to stress corrosion cracking. Titanium, for example, is very resistant to simple corrosion, but may be very prone to stress corrosion cracking. The same is true of stainless steels, and to a considerably less degree of aluminum and high strength steel. These metals exhibit a property referred to as passivity, and the following discussion of the behavior of titanium serves to illustrate the manner in which passivity may promote stress corrosion cracking.

Titanium is quite active chemically and when a fresh surface of the metal is exposed in an atmosphere containing oxygen, an oxide layer will form on the surface in a matter of a few seconds. For several hours thereafter, the oxidation of the surface layer will continue to proceed until an equilibrium is established in approximately 24 hours. If the surface oxide layer remains undisturbed, the underlying layers of titanium metal will be accorded some protection from further corrosion. This oxide layer on the surface of titanium metal is relatively inactive chemically and titanium exhibiting this condition is said to be in a passive state while titanium metal without such a surface oxide layer is chemically active and is said to be in an active state. In the case of stress corrosion cracking, there is a deficiency of oxygen in the base of the crack where the new surface is formed. Consequently, no oxidation occurs at that point and the titanium at the base of the crack remains in the active state.

If the titanium metal exhibiting these conditions is in a corrosive environment, i.e., immersed in an electrolyte, all of the elements of a galvanic cell are present and the ensuing electrochemical reactions between the active and passive titanium areas in the presence of the electrolyte will promote rapid corrosion of the titanium structure. At the anodic active titanium areas at the base of the crack, titanium ions will dissolve into the electrolyte leaving an excess of electrons on the surface of the metal according to the following reaction:

These excess electrons migrate through the bulk metal to cathodic passive areas of titanium metal surrounding the crack base promoting the other half of the galvanic cell reaction in which oxygen is absorbed into the electrolyte to produce an excess of hydroxide ions according to the following reaction:

The standard oxidation potential of the anodic cell reaction referred to the standard hydrogen electrode is 1.63 volts. The standard oxidation potential of the cathodic reaction mentioned above is 0.4 volt. (Other cathodic reactions, such as the reduction of hydrogen ions, can also occur depending on the particular chemical environment present.)

The net result of these reactions is the continual dissolution of titanium metal from the base of the crack into the electrolyte solution until the critical stress intensity factor of the member is reached at which time catastrophic failure occurs. Such failure can be surprisingly rapid. A cracked specimen of titanium metal that can withstand a given loading indefinitely in a non-corrosive environment will fail in a matter of seconds under an identical load when immersed in a salt water solution.

Of the many methods that have been used to protect a metal from corrosion, perhaps the oldest is the introduction of a barrier coating onto the surface of the metal to isolate it from its corrosive environment. The most well-known such barrier coating is paint, but other coatings such as varnishes, resins, sealants, and electroplated inert or noble metals have also been used. All barrier coating protective measures suffer from the fault that they are only effective as long as the coating remains intact and continuous. Discontinuities are extremely difficult to avoid in the application of a barrier coating and are practically impossible to avoid during the service life of a structural element. In the latter case, discontinuities in the barrier coating may be caused by abrasion of the coating or by impact upon the coating or other mechanical damage. In addition, it is essentially impossible to avoid the presence of cracks due to fatiguing of the metallic structure during its useful service life. In order to overcome some of these obstacles to the effectiveness of barrier coating, the coatings are often applied with excessive thicknesses in order to insure complete coverage during the application and they may be renewed from time to time during the service life of the structure. However, the application of excessive thicknesses of the barrier coating incurs a serious weight penalty and since many structural elements of, say, an airplane, are not accessible for renewal of barrier coatings during their service life, barrier coatings are generally considered impractical and not a complete solution to the corrosion problem, particularly the stress corrosion problem.

Another method of providing corrosion protection is by introducing chemical inhibitors into the environment in which the structural element exists. While this approach may have some degree of success in the case of static structures, such a method cannot be seriously considered for dynamic systems such as an airplane. However, it should be pointed out that to overcome the problem of using chemical inhibitors in dynamic systems, certain soluble inhibitors have been developed which slowly impart a corrosive inhibitor into the dynamic environment of the structure. However, no commercial coating of this type has been demonstrated for protecting titanium and titanium alloys.

Yet another method used for protecting materials susceptible to corrosion has been the use of a galvanic coating. Where the structural member consists of iron or an iron alloy, for example, such coatings are ordinarily compounded with powdered metallic pigments such as zinc and aluminum, both of which stand higher than iron in the electromotive series. These pigments, when incorporated into a suitably permeable vehicle, will afford a certain degree of galvanic protection. Various galvanic coatings have been prepared in which powdered metals or metallic materials are incorporated in suitable vehicles. For instance, materials such as chlorinated rubber, polystyrene, polyvinyl chloride, alkyd resins, phenolic resins, and acrylic resins are conventionally dissolved in suitable solvents such as ethanol, methyl-ethyl ketone, or the like, to form a liquid vehicle. The preparation of such vehicles is common practice in the paint and varnish technology. A comparatively large proportion of the powdered metallic pigment such as powdered zinc, is thoroughly mixed into the vehicle and applied as a paint upon the surface to be protected. Similarly, galvanic coatings are known which consist of silicates with a powdered metallic pigment incorporated therein. Such coatings have been used to a limited extent to provide protection against stress corrosion of high strength steels. However, for titanium, par icul ly, which when in the active state stands high in the electromotive series, corrosion protection by use of prior art galvanic coating, has proved to be ineffective. The galvanic coating, when in a corroding environment, does not produce an adequately high potential to protect the underlying titanium metal.

SUMMARY OF THE INVENTION In their simplest form, the protective coatings of this invention comprise three components:

(1) A metallic pigment selected to be sacrificial to the substrate upon which the coating is applied. Where the substrate comprises titanium, iron, cobalt, or nickel, the pigment may be either zinc, aluminum, or mixtures thereof.

(2) An activator selected to increase the cell potential between the corrosion preventive coating and the metal substrate so that the coating becomes sacrificial to the substrate. In the case of an aluminum pigment, the activator may be either anionizable fluoride or an acid salt. The ionizable fluoride may be selected from the group consisting of sodium fluoride, calcium fluoride, lithium fluoride and magnesium fluoride. The typical acid salts are potassium acid sulfate, sodium acid sulfate and sodium diacid phosphate. A metallic hydroxide activator selected from the group consisting of the hydroxides of barium, calcium, cesium, lithium, magnesium, potassium, rubidium, sodium, and strontium may be used with either an aluminum or a zinc pigment (3) A binder providing adhesion and abrasion resistance to the corrosion protective coating. The binder component may be selected from the art normally practiced in the paint and varnish technology and may comprise chlorinated rubber, polystyrene, polyvinyl chloride, alkyd resins, phenolic resins, acrylic resins, or silicone resins.

While the corrosion protective coating of this invention will be useful for protecting a wide range of alloys including stainless steel, its efiicient inhibiting qualities are best seen when applied to titanium and titanium alloys. The cracks that inevitably form in the surfaces of a structural member made from titanium provide opportunities for further extension of the crack through corrosive mechanisms when an electrolyte or other corrosive environment is present. With the application of the protective coating of this invention, the electrochemical reactions which would otherwise occur between the activated titanium at the base of the crack and the surrounding passivated titanium surfaces is inhibited and the electrochemical reactions are constrained to operate between the activated metallic pigment of the coating and the passivated titanium metal. Thus, further growth and extension of the crack are prevented and the danger of catastrophic failure due to the crack reaching such proportion that critical stress intensity levels are reached is greatly reduced. While the protective coating of this invention does provide many of the benefits inherent in the barrier coatings used in the prior art, it should be noted that the protection offered by this coating does not depend upon the maintenance of an intact and continuous barrier. In addition, these coatings provide protection for metals that are highly active, such as titanium, which could not be previously protected by either the chemical inhibitor or by the simple metal-pigmented galvanic type of protective coatings.

It is, therefore, an object of this invention to provide a protective coating for protecting metals from corrosion.

Another object of this invention is to provide a protective coating for reducing the susceptibility of titanium to stress corrosion.

Another object of this invention is to provide a corrosion resistant protective coating that is highly adherent to the surface to which it is applied and abrasion resistant.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The protective mechanism of the coatings of this invention requires the incorporation of an activator into a metal-rich coating in order to increase the electrical potential between the metal-rich coating and the underlying substrate to a critical value that prevents corrosion of the substrate. This activator alters the electrochemical reactions which normally occur between the passive and active state of the metallic substrate so that the coating becomes sacrificial to the passive state of the substrate and prevents chemical activity of the active substrate material.

6 tential referred to the standard hydrogen electrode, i.e., less than 1.63 volts, the presence of non-normal conditions, such as an excess of activator ions, will insure that the protective reactions will occur to the exclusion to the corrosion of the titanium structure.

By the operation of these protective reactions, the active titanium surface essentially becomes passive since it is cathodic in relation to the protective coating which acts as an anode. Thus, instead of the active titanium dissolving into the electrolyte, the metallic pigment of the coating dissolves and the passive titanium remains protected by its oxide surface layer.

Representative protective reactions and their half cell The metallic element of the pigment of this invention potentials for selected protective coatings Of this invenmay be either aluminum or zinc or combination of both. tion are as follows:

Anotlic pigment Zn A1 Al A1. Activator OH- (from Sr(OI-I) OH (from Ca(OH)z) F- (from NaF) H (from NaHSO). Anode half cell Zn+4OH- ZI1O2+2HzO+261. A1+4OH A O 2+ 2 e' +6 1 o AI T Anode half cell potential, volts +1.216- +2. 5 +2.07 +1.67.

1 Standard H electrode.

Where an aluminum pigment is used, the activator may be either an ionizable fluoride or an acid salt. Ionizable fluorides that have been found to be particularly useful in the practice of this invention includes fluorides of sodium, calcium, lithium, and magnesium. Typical acid salts to be used as an activator are potassium acid sulfate, sodium acid sulfate and sodium diacid phosphate. A metallic hydroxide activator may be used for both aluminum and zinc metallic pigments. Suitable metallic hydroxides for this application include the hydroxides of barium, calcium,

cesium, lithium, magnesium, potassium, rubidium, sodium,

and strontium. The metallic pigment and activator are mixed with a suitable vehicle or binder which will enable the coating to adhere to the metallic substrate and to withstand abrasion. The composition of the binder component is not critical to the practice of this invention and resins, acrylic resins and silicone resins which are con- 4 ventionally dissolved in suitable solvents such as ethanol, methyl-ethyl ketone, or the like, to form a liquid vehicle. Examples of such components used to make coatings of this invention are silicone resin binders designated as XR-6-216O and DC 808; and an amine catalyst used to cure the binder designated XR62163; all of which are made and sold by the Dow Corning Corporation.

In addition to the foregoing three components, i.e., the metallic pigment, the activator, and the binder, other neutral components may be added to the corrosion preventive coating in order to impart to it other desired qualities. For example, a filler may be added to the coating in order to reduce the cost of fabrication or a diluent may be added in order to reduce the viscosity of the coating so that it may be applied by spraying.

With the use of the protective coatings of this invention, the electrochemical reactions that normally lead to failure of a structure by stress corrosion are replaced by other reactions in which the coating is sacrificial to the metal of the structure, thus leaving the structure intact. In particular, the activator component leeches out and dissolves into the corrosive electrolytic medium surrounding the structure producing a surplus of activator ions which readily combine with the metallic pigment component of the coating. This reaction has a half cell potential, in most instances, greater than that for the reactions between the active and passive titanium that would normally occur in the absence of the protective coating. In those instances where the protective reactions have a half cell potential less than that for the active-passive titanium half cell po- It has been found that the percentages of the three principal ingredients of the protective coating of this invention can vary over relatively wide ranges depending upon the physical characteristics as well as the protective characteristics desired. However, in order for the coating to exhibit sufiicient adhesion to the surface upon which it is applied to insure its structural integrity, the final film should contain at least 20% by volume of binder. A smaller concentration of binder results in a coating of poor adhesion with a powdery texture that is not resistant to abrasion. Of course, increasing the volume percentage of binder results in a tougher film but at the higher binder concentrations, the efiectiveness of the electrochemical protection provided by the coating decreases. Analysis of finished coatings shows that the pigment and activator particles settle to and cencentrate at the bottom of coating adjacent to the surface of the article being coated leaving a binder layer relatively deficient in pigment and activator exposed to the electrolyte. The binder then prevents the dissolution of activator into the electrolyte and also prevents the exposure of the sacrificial pigment particles. Coatings exhibiting this condition will not afford any substantial chemical inhibition of the corrosive processes and are generally referred to as nonselfactivating coatings. Activation can be instituted by lightly abrading the binder rich surface of the coating to expose the underlying pigment and activator particles. Good resistance to corrosion will then be obtained.

The degree to which the pigment and activator are overburdened by the binder can be controlled not only by the concentration of binder used, but also by the particulate nature of the pigment. If the metallic pigment particles are irregular in shape, and of similar sizes, the packing or settling of the pigment will be much less and the pigment particles will be more evenly distributed through the coating resulting in a protective coating that is self activating. Conversely, pigments containing particles in a gradation of sizes will tend to pack more tightly than those contain ing particles of only one size, and metallic pigment particles in the form of thin plate-like structures will pack tightly at the base of the coating resulting in a heavy overburden of binder.

The concentration of activator in the coating is dictated by the requirement that sutficient activator ions must be maintained in the electrolyte surrounding the coated structure in order to afford the desired electrochemical protection. For long term protection, a relatively large concentration of activator will be required if the activator is relatively soluble. For less soluble activators, the activator ions will leech out of the coating more slowly and the same term of protection can be obtained with a lesser concentration. Also, the dynamics of the corrosive medium will influence the concentration and desired solubility of the activator component. Where the protected structure is subjected to a moving electrolyte, such as a ship, a considerably greater supply of activator ions will have to be made available by either adjusting the concentration or solubility, or both, of the activator component of the coating. For relatively static environments, it has been found that an activator component of at least 9l0 volume percentage is required where an ionizable fluoride is used as the activator.

The concentration of metallic pigment in the protective coating does not appear to be dependent upon factors as critical as those controlling the binder and activator concentrations, except that higher pigment concentrations insure that the coating will be self-activating, as discussed above. Of course, other ingredients may be added to the coating, such as fillers and solvents, for producing certain desired physical characteristics in the final coating or for accommodating specific application methods. In general, as is common in the paint and varnish technologies, variations in the pigment content, solvent content, and the type of cure employed will affect the application properties and environmental resistance properties of the activated coating. Not only must the desired half cell potential and the need for surface abrasion resistance be considered in the formulation of these activated coatings, but consideration must also be given to the method of coating application desired, the type of environment to which the coating will be exposed, and the condition of the metal surface to which the coating will be applied.

While extensive different formulations of the protective coatings of this invention are possible, the following compositions have been found to be particularly effective for protecting the titanium alloy Ti-SAl-lMo-lV from stress corrosion cracking while subjected to a load while immersed in a 3.5% NaCl salt water solution:

EXAMPLE 1 Percent Percent by weight volume (formu- (final Constituent lation) coating) Aluminum metal powder 52. 3 55 Sodium Fluoride (NaF) 8.2 9

Dow Corning resin solution (X R-G-Zlfi 30. 6 36 Xylene 8. l)

EXAMPLE 2 Percent Percent by weight volume (formu- (final Constituent lation) coating) Aluminum metal powder 54. 8 5f) Sodium fluoride (NaF). 2. 2

Calcium fluoride (CaFz) 8. 4 8

Dow Corning resin solution (DC 808) 25. 51

Xylene 9. d

EXAMPLE 3 Percent Percent by weight volume Constituent lation) coating) Aluminum metal powder 37. 4 38 Sodium fluoride (NaF) 17. 6 17 Dow Corning resin solution (X 11-6-2160) 39. 8 45 Xylene 5. 2

EXAMPLE 4 Percent Percent by weight volume (formu- (final Constituent lation) coating) Aluminum metal powder 50. 59

Calcium hydroxide (Ca(OH) 7.0 10

Dow Corning resin solution (DC 808). 23. 4 31 Xylene 19. 1

EXAMPLE 5 Percent Percent by In the fabrication of a corrosion protective coating of this invention, the constituents are first weighed to the desired percentages, after which they are combined and agitated to form a homogeneous mixture in a ball mill, blender or other suitable device. The formulated coating is then applied to the member for which corrosion protection is desired. The method of application of the coating is not critical and depending upon the viscosity and flow characteristics of the particular composition, it may be applied by dipping, brushing, or spraying. After the protective coating has been applied, it is cured at either ambient temperature or at elevated temperature by baking. When a more rapid cure at ambient temperature is desired, a catalyst may be added to the resin.

Extensive tests have been conducted with specimens cracks were introduced into the metal specimens which had been previously coated with the protective coating of this invention. To insure that the barrier effect of these coatings was not effective during the test, the coating was completely removed along the fatigue crack. Specimens which were painted, cracked, and prepared in the manner described above experienced no reduction in load carrying capacity when immersed in a 3.5% sodium chloride salt water bath.

In other tests, two groups of titanium specimens were fatigue cracked. The specimens of the first group were coated with the corrosion preventive coating of this invention and the specimens of the second group were left uncoated. Specimens of both groups were placed in a 5% salt solution fog environment. The uncoated titanium specimens catastrophically fractured when a load was applied to them while the coated titanium specimens sustained an identical load for prolonged periods of time up to several weeks without evidence of failure due to stress corrosion cracking. The ability of these titanium alloy specimens to withstand the corrosive environment is attributable to the effect of the activator and metallic pigments in the coatings of this invention, which increase the cell potential between the coating and the underlying titanium metal to a level that inhibits the electrochemical reaction that would otherwise occur between the passive and active areas of the titanium. Thus, there is no further increase in crack depth caused by electrochemical action and stress corrosion cracking is prevented. This increase in cell potential is accomplished through the action of the activator ions which slowly leech out of the coating, react with the metallic pigment and protect the titanium metal by sacrificial action.

The corrosion protection provided by the coating of this invention will permit the use of titanium alloy as well as ferrous alloys, including stainless steel, in severe salt water environments which are known to be productive of catastrophic failure due to stress corrosion. The use of titanium alloys will now be greatly extended to exposed parts of aircraft designed for operation over oceans. Of course,

the advantages to be gained from the application of this coating in the marine technologies is obvious.

However, even though the benefits derived from the use of the protective coating of this invention are most striking when the protected material is used in an extremely corrosiv-e environment, it should be realized that these coatings also offer considerable advantage in less rigorous applications. Because corrosive elements are found in all but the most carefully controlled environments, the design of structural members must always take into consideration simple corrosion and stress corrosion factors. Where extremely light structures are required, the penalty imposed by providing suflicient material to maintain the capability of enduring the anticipated stresses is considerable. With the use of the coatings of this invention, however, the allowances made for withstanding environment factors can be reduced. Because of the simplicity with which the coatings are fabricated and applied, they can be readily employed in shipyards, factory assembly lines, and other areas where titanium, ferrous metals, and parts of related materials are normally fabricated.

We claim:

1. A composition of matter for use as a corrosion inhibiting protective coating upon a metallic substrate comprised substantially as follows:

(a) at least 38 percent by volume of a particulate aluminum pigment component for forming metallic ions in the presence of an electrolyte;

(b) at least 9 percent by volume of an ionizable fluoride 10 activator component selected from the group consisting of sodium fluoride, calcium fluoride, lithium fluoride, magnesium fluoride, and combinations thereof, for reacting with the metallic ions, and

(c) at least 20 percent by volume of a binder component for adhering the coating to the substrate.

2. The composition as claimed in claim 1 wherein the percentage by volume of the particulate aluminum pigment component is 38 to and the selected ionizable fluoride component of sodium fluoride is 9 to 17.

3. The composition as claimed in claim 1 wherein the percentage by volume of the particulate aluminum pigment component is approximately 59; and the selected ionizable fluoride component is sodium fluoride, approximately 2, and calcium fluoride, approximately 8.

References Cited UNITED STATES PATENTS 3,336,163 8/1967 Wolfe, II 136120 3,380,858 4/1968 Champaneria et a1. l486.2 3,395,027 7/1968 Klotz 106-1 MORRIS LIEBMAN, Primary Examiner R. H. ZAITLEN, Assistant Examiner US. Cl. X.R.