US3285788A - Anhydrous chromic acid solution and process of treating metal therewith - Google Patents

Description

United States Patent 3,285,788 ANHYDROUS CHROMIC ACID SOLUTION AND PROCESS OF TREATING METAL THEREWITH Rudolf Eldo Svadlenak, Lewiston, N.Y., assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Filed June 14, 1963, Ser. No. 287,774

Claims.v (Cl. 148-62) This application is a continuation-in-part of my copending application Serial No. 212,712, filed July 26, 1962.

This invention relates to a new and improved metal treatment solution and process for the treatment of metals therewith. More particularly, this invention relates to an anhydrous chromic acid metal treating solution and a process of treating aluminum, aluminum alloy, iron, steel and zinc articles to impart thereto a higher resistance to corrosion and an improved bonding of paints, lacquer and the like thereto.

The treatment of metal surfaces with chromic acid formulations in aqueous solution is known in the art. Such processes have not been satisfactory since they involve numerous steps and have not imparted a satisfactory corrosion resistance not satisfactory paint adhesion to the metal surfaces. Moreover, aqueous chromic acid solution treatments required a drying operation prior to painting.

It is an object of this invention to provide a new and improved anhydrous chromic acid metal treating solution.

It is another object to provide a new and improved process for the treatment of aluminum, aluminum alloy,

iron, steel and zinc structures or articles to impart thereto improved corrosion resistance and paint adhesion.

Other objects of the invention will appear hereinafter.

The objects of this invention may be accomplished, in general, by the provision of a metal treatment solution comprising a chlorinated hydrocarbon solvent containing dissolved therein chromic acid, a zinc compound taken from the group consisting'of zinc fluoride, zinc oxide and a tertiary alcohol soluble in the chlorinated hydrocarbon and containing 4 to carbon atoms, preferably tertiary butanol.

An aluminum, aluminum alloy, iron, steel or zinc structure, particularly iron, galvanized iron or aluminum strip, may be treated by applying to such structure, for example by immersion, painting, spraying or the like a coating of the treating solution, and then subjecting the coated surface to ultraviolet light, heat or a reducing gas.

By the term chromic acid as used herein is meant chromic anhydride, chromium trioxide, CrO or the hypothetical substance H CrO chromic acid are expressed in terms of the equivalent anhydride (CrO By the term aluminum alloy is means one of the wellknown series of alloys containing not less than about 90% aluminum and minor amounts of alloying ingredients such as Cu, Mn, Mg, Si, Zn or Cr. For example, Alloy 2021 is composed of 4.5% Cu, 0.6% Mn, 1.5% Mg and the balance Al. Alloy 6061 is composed of 0.25% Cu, 0.6% Si, 1% Mn, 0.25% Cr and the balance Al. Alloy 7075 is composed of 1.6% Cu, 2.5% Mg, 5.6% Zn, 0.3% Cr and the balance Al. Alloy 3003 is composed of 1.2% Mn and the balance aluminum.

By the terms aluminum, aluminum alloy, iron, steel, or

zinc structure is meant structures composed entirely of examples of such solvents suitable for use in the preparation of the chromic acid solutions are trichlorethylene,

Reference to quantities of through the vapor zone of the solvent.

3,285,788 Patented Nov. 15, 1966 perchlorethylene, carbon tetrachloride, chloroform, methylchloroform methylene chloride and mixtures thereof. Depending upon the mode of operation, the chromic acid is added to the solvent in an amount of between about 0.02% by weight up to the saturation point thereof or even in excess of the saturation point. The chromic acid is dissolved in the solvent by the presence in the solution of tertiary butanol, or other tertiary alcohol, soluble in the solvent, containing 4 to 20 carbon atoms, as a solubilizing agent. The tertiary alcohol should be present in an amount of at least 2% by weight and it may be present in an amount up to 15% or higher, the only limit being the objectionable flammability of the solution if present in too great an amount. The greater the quantity of tertiary alcohol present, the greater the quantity of CrO that will dissolve in the solution since CrO appears to react with the alcohol as follows:

Of the tertiary alcohols, tertiary butanol appears to be oustanding as a solubilizing agent being operative for extended periods of time. Other tertiary alcohols, soluble in chlorinated hydrocarbon solvents, that may be used include tertiary amyl alcohol; 1,1-dimethyl 2,2-dichloropropanol; 1,1-2,2-tetramethyl propanol and triphenyl carbinol.

Zinc fluoride or zinc oxide are added as stabilizers. For example, chromic acid in solution in trichloroethylene containing between about 0.002% up to its saturation point at reflux of ZnF or ZnO, will be 50% reduced after about 30 hours with the solvent at reflux temperature. A similar chromic acid solution containing no zinc fluoride or zinc oxide will lose 50% of the chromic acid in only 15 hours.

The preferred process of treating the metals is'carried outby the following steps:

First, it is preferred, although not essential, to brush the metal with a wire brush or sand blast or otherwise abrade the surface thereof.

The metal, aluminum, aluminum alloy, iron, steel or zinc is then degreased by a conventional degreasing process with trichlorethylene or perchlorethylene, i.e., by immersing the same in trichlorethylene or perchlorethylene liquid or vapor, or both, until the metal is clean. This, however, is purely a cleaning step-and if the metal is clean can be omitted.

The metal is then immersed in the anydrous chromic acid treating solution. Although the treating solution may be at any temperature between room temperature and the boiling point of the solvent, it is preferred for economical reasons to operate the treating step at reflux temperature of the solution. The solution is heated to its boiling point and the vapors are condensed by cooling coils surrounding the top inner surface of the treating vessel. Trichlorethylene, methylene chloride, perchlorethylene and mixtures thereof are therefore eminently suitable as the base solvent in the solution.

The metal structure is contacted with the treating solution until a uniform coating is obtained, for example, between about 0.1 second and 5 minutes. Under some circumstances it may be desirable to coat the metal with the treating solution repeatedly. When the metal is removed from an immersion in the treating solution operating under reflux conditions, it will necessarily pass In order to prevent washing off the coating formed on the metal in the liquid solvent solution, it should be passed through the vapor zone quickly to prevent undue condensation of the vapor on the metal, or the vapor zone may be reduced to minimize condensation of vapor on the metal.

The chrominum oxide coating formed on the metal is a uniform thin powdery layer and does not at this point withstand extensive handling. The coated material is therefore subjected to a treatment with ultraviolet light, heat or a reducing gas. Any source of ultraviolet light, i.e., emitting rays in the range of 1800 A. to 3900 A. can be used, for example, an ultraviolet light-emitting sun lamp. The coated surface should remain in the presence of the ultraviolet light for a period of 1 to 300 seconds depending upon the source of the ultraviolet light and the spacing of the light source from the surface. Thirty seconds exposure at a spacing of 1 inch with a Hanovia type SH bulb sold by Englehard Hanovia, .Inc., Newark, NJ., will produce the desired setting of the coating to permit handling.

The chrominum oxide coating may be set by subjecting the coated structure to heat, for example in an oven having a temperature between 100 and 600 C. At a temperature of -100 C. the coated stnucture may have to be heated for a period as long as 15 to 20 minutes to obtain the necessary set, whereas at an oven temperature of 600 C. the CrO coating will set in 5 to seconds. The heating of the panels may be carried out in any desired manner such as by direct flame, oven, electric heat lamp or the like.

The CrO coating may also be set by treating the same with a reducing gas for a period of 5 seconds to 10 minutes depending upon the particular reducing gas used and the temperature thereof. Gases such as hydrogen, hydrogen sulfide, formaldehyde, sulfur dioxide and hydrogen peroxide-may be used for this purpose.

An aluminum panel having a chromate film, produced by treating the same in a solution comprising trichlorethylene containing 0.35% CrO 0.007% ZnF and 5% tertiary butanol maintaining at reflux temperature for a period of 10 seconds, was positioned above petri dishes containing, in one case hydrogen peroxide and in another formaldehyde. On removing the panels, the metal was rinsed with water and an insoluble tbrown stain was found on the portions of the panels in contact with the hydrogen peroxide and formaldehyde vapors. After one week in a 5% salt fog cabinet held at 9 4 F. no corrosion was found in the area of the brown stain. The other portions of the panels were corroded and pitted.

The exposure of the chromium oxide coated surface to ultraviolet light, heat or a reducing .gas as above described furictions to fix the chrominum oxide film on the metal surface causing it to become substantially integral with the surface. As an example, two sets of samples of an aluminum panel were treated by first brushing the panels with a wire brush to uniformly abrade the surface and then degreasing the same with trichlorethylene, then treating the same in a solution comprising trichlorethylene containing 0.35% CrO 0.007% ZnF and 5% tertiary butanol maintained at reflux temperature for a period of 10 seconds. One set of samples was rinsed with water before exposure to heat or ultraviolet light and one set was first exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds from a spacing of 1 inch before rinsing with water, while a third set was first exposed to heating in an oven having a temperature of 250 C. for a peniod of one minute. All samples were then exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of this time the samples which had not been first exposed to ultraviolet light were corroded and pitted, whereas the samples that had first been exposed to ultraviolet light or to heat showed no signs of corrosion. Similar good results were obtained with the above-described chromic acid solution and heat, reducing gas and ultraviolet light treatment on panels of aluminum alloys 2021, 6061 and 7075, and zinc.

The chromate treatment of this invention may be used to great advantage as an after treatment on aluminum, aluminum alloys, iron, steel or zinc which have first been treated with a phosphatizing composition. For example, a number of galvanized iron panels were phosphatize-d by submersing then in a solution of trichlorethylene containing 5% amyl alcohol and 0.2% phosphoric acid having a temperature of 87 C. for a period of 60 seconds.

A like number of galvanized iron panels were similarly phosphatized and then chromatized by submersion in a solution comprising trichlorethylene containing 0.35 CrO 0.007% ZnF and 5% tertiary butanol, maintained at reflux temperature, for a period of 10 seconds. All of the panels were then painted, and baked to a pencil hardness of HB to F with an alkyd baking enamel of the composition set forth below. In the series of panels which were phosphatized and painted, the paint film had an impact resistance of 16 inch pounds, whereas in the panels which were phosphatized, chromatized and painted, the paint film had an impact resistance of inch pounds.

The following examples illustrate the comparative paint adhesion obtained by the indicated treatment of a series of alumnium panels which were subsequently sprayed and baked to a pencil hardness of HB to F with an alkyd baking enamel having approximately the following formulation:

Example I Several panels of aluminum and aluminum alloys 2021, 6061 and 7075, having a thickness of 25 mils, were degreased by a conventional trichloroethylene degreasing apparatus to clean the same. These panels were painted with alkyd resin baking enamel .and baked at 250 C. for 3 minutes to give a paint film of about 1 mil thickness.

These panels were then subjected to bending at a 90 angle with a bend radius to thickness ratio of 018 and also to an impact of 22 inch pounds. The paint surfaces of all panels were badly cracked and shattered.

Example II A similar series of aluminum and aluminum alloy panels were subjected to brushing with a wire brush until well abraded and degreased with trichlorethylene and then painted and tested in the same manner as in Example I.

The paint surfaces of all panels were still badly cracked but not as severely as those in Example I.

Example III A third series of aluminum and aluminum alloy panels 7 were degreased with trichlorethylene and then placed for 10 seconds in a solution containing trichlorethylene, 0.35 CrO 0.007% ZnF and 5% tertiary butanol which was maintained at reflux temperature, and finally placed for 30 seconds directly beneath a watt type SH Hanovia ultraviolet bulb with a spacing of about 1 inch.

These panels were then painted and tested the same as in Example I. There was comparatively little paint cracking as a result of the tests on any of the panels.

Example IV 1 A fourth series of aluminum and aluminum alloy panels were degreased, chromated, painted and subjected to ultraviolet light, and then tested as in Example III excorroded and pitted.

trichlorethylene.

5 cept that they were abraded with a steel brush before degreasing. The paint on these panel-s withstood the tests with very little cracking or other damage. These panels had superior paint bonding than similarly painted and tested panels that were pretreated by the best previously known methods of surface treatment of aluminum panels.

Example V A degreased sample of mild steel was immersed 10 seconds in a refluxing trichlorethylene bath containing 0.35% CrO 0.007% ZnF and 5% tertiary butanol. The sample was removed and exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds from a distance of 1 inch. The panel was exposed for 10 minutes in a salt fog cabinet. No corrosion was observed on this panel in contrast to severe corrosion on a degreased but nontreated control panel.

Example VI The use of zinc xide.Degreased aluminum panels (3003 H14) were treated for 10 seconds in a refluxing solution containing 0.35% CrO t-BuOH and trichlorethylene which was saturated with zinc oxide. The panels were exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds at a distance of 1 inch. The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of this time the panels were found to be in excellent condition whereas untreated controls were corroded and pitted.

Example VII The use of methylene chI0ride.-Degreased aluminum panels (3003 H14) were treated for seconds in a refluxing solution containing 0.40% CrO 5.46% t-BuOH, 0.007% ZnF and methylene chloride. The panels were exposed to ultraviolet light from a Hanovia type SH bulb 'for 30 seconds at a distance of 1 inch. The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of the time the panels were found to be in excellent condition-whereas untreated controls were Example VIII The use of perchl0rethylene.Degreased aluminum panels 3003 H14) were treated for 10 seconds in a solution containing 0.31% CrO 4.65% tertiary butanol and 0.002% zinc fluoride and perchlorethylene. The panels were exposed to ultraviolet light from a Hanovia type SH 1 bulb for 30 seconds at a distance of 1 inch. .The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the end of the time the panels were found to be in excellent condition whereas untreated controls were corroded and pitted.

Example Dr Operation of the process at room temperature-Degreased aluminum panels were treated for'l0 seconds at room temperature in a solution containing 5.0% tertiary butanol, 0.4% CrO 0.002% zinc fluoride and 94.6% The solvent was allowed to volatilize at room temperature and then the panels were exposed to ultraviolet light from a Hanovia type SH bulb for 30 seconds at a distance of 1 inch. The samples were exposed for one week in a 5% salt fog cabinet held at 94 F. At the. end of the time the panels were found to be in excellent condition whereas untreated controls were corroded and pitted. v

. Example X A series of steel panels were chromatized by immersing the same in a solution of trichlorethylene containing 0.3%

CrO 5% tertiary butanol and 0.007% zinc fluoride having a temperature of reflux, for a period of 10 seconds. One of these panels, after chromatizing was given no further treatment; the remaining panels were heat-treated for various times in an oven having a temperature. of 250 C.;

The panels were all painted and baked to a pencil hardness of HB to F with an alkyd "baking enamel of the formulation above set forth. The resulting painted panels were subjected to bend tests with a bend radius to panel thickness ratio of about 0.8 with the following tabulated results:

takes place after heat treatment for 1 minute at an oven temperature of 250 .C. Long treatment times or higher temperatures improve the resistance of painted chromate coated panels to cracking when subjected to bending tests.

Example XI Galvanized screen was dipped into the chromatizing bath of Example X for 10 seconds and then exposed for one minute to either a 250 C. oven or to exposure for one minute directly beneath a 140 watt type SH Hanovia ultraviolet light bulb with a spacing of about 1 inch.

After one week in a 5% salt fog cabinet, these screen samples were entirely free from corrosion or pitting whereas similar untreated control samples had become covered with red rust.

Example XII Galvanized steel panels, treated by the process of Example XI,-were painted with an alkyd resin baking paint of the formulation above described, were exposed to a 5% salt fog cabinet and compared with untreated galvanized steel panels. The untreated panels failed by blistering and peeling in 3 days whereas the treated panels required 30days for similar failure to take place.

The following examples are given to show the improvernent in resistance to the salt spray test of painted steel panels by treating phosphated steel panels with a chromatizing treatment in accordance with this invention prior to painting. 7 v

In the examples, where reference is made to vapor degrea-sing, the steel panels are subjected to degreasing in trichlorethylene vapors at reflux temperature for a peri- Y od of about one minute after which they are withdrawn and the vapor allowed to flash off.

In the zincphosphatizing operation referred to in the examples, the panels are immersed for a period of about .90 seconds in'a phosphatizing solution having a temperature of 87 C. and containing about 94.3% trichlorethylene, 5% pentanol-l, 0.2% dinitrotoluene and 0.3% phosphoric acid (85%) containing 6 grams zinc oxide per ml. phosphoric acid. In the phosphatizing operation .as referred to in the examples the zinc oxide is omitted from the phosphatizing bath above described. The chromatizing as referred to, unless otherwise stated, was carried out by dipping the panels, for a period of about 5 seconds, in a chromatizing solution, having a temperature of about 25 C., and consisting of:

Percent by weight Trichlorethylene 94.17

Tertiary butyl alcohol 5.5 Chromic acid 0.35 Zinc fluoride .002

The painting of the panels was carried out with an alkyd resin baking enamel having the approximate formulation hereinabove described and were baked at a temperature of 300 F. for ten minutes, to obtain a dry pain-t film approximately 0.001 inch thick.

'8 The'pane'ls were then subjected to a 5% salt spray test with the results tabulated below.

I TABLE I Dry Salt- Sp. Gr. Dip MgJft. Percent Temp., Paint Spray Example No. Tri-Phos. Titer Time Coating CrO C. 'Iri- Mg./ft. Film Hrs. to

at; ml. in See. Wt. by Wt. Cr Sol. (010 Thiok- A, Total 83 C. (phos ness in Layback mils YTTT 0.9 840 XIV- 1 .1 816 XV 1.3 864 XVI 1 .309 5.9 r 90 1.0 408 XVIL- 1.309 5.9 90 0.8 572 XVIII 1 .308 11 .6 90 1 .0 400 XIX--- 1 .308 11.6 90 1.0 572 Control, uncoated ste 1.0 24

Excluding Drip Line.

Example XIII Example XIV Steel panels were trichlorethylene vapor degreased for one minute, then dipped in the tri-chromatizing solution for three to five seconds and withdrawn at a medium rate of speed to obtain a chromate coating weight of 14.0 rmg./ sq. ft. The C10 content of the tri-chromatizing solution had previously been increased to 0.575% by weight, film fixed as in Example XIII. These panels were then painted as above described.

Example XV Steel panels were trichlorethylene vapor degreased, then followed by a five-second dip in the tri-chromatizing solution, containing 0.575% CrO by weight, and withdrawn at a rapid rate to obtain a heavy chromate film of 16.5 mg./sq. ft. This film was heated at 300 F. (149 C.) for twenty minutes, then allowed to cool for painting, after which they were painted as above described.

Example XVI Steel panels were vapor degreased and zinc phosphatized to obtain a mixed zinc-iron phosphate coating of 180 mg./ sq. ft. and then painted as above described.

Example XVII Steel panels were vapor degreased and zinc phosphatized to obtain a mixed zinc-iron phosphate coating of 180 mg./ sq. ft. after which they were chromatized to obtain a chromate film of 2.4 mg./sq. ft. The chromate coating was fixed by heating at 300 F. for 10 minutes.

After cooling the panels were painted as abovev described.

Example XVIII Steel panels were vapor degreased and phosphatized to obtain 205 mg./ sq. ft. of iron phosphate coating, after which they were painted as described.

Example XIX Steel panels were vapor degreased and phosphatized as in Example XVIII after which they were chromatized to obtain a chromate film of 2.4 mg./sq. ft., plus a heatfixing cycle of 300 F. (149 C.) for ten minutes, and allowed to 'cool for painting. The panels we're then painted as described.

The chromatizing process of the present invention functions to lay down dry uniform films of chromium trioxide of any desired thickness on metal surfaces.

Throughout the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.

Since it is obvious that many changes and modifications can be made in the above-described details Without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I-cl-aim:

1. A metal treating solution consisting essentially of a chlorinated hydrocarbon containing 0.02% up to the saturation point of chromic acid, from about 2% to about 15% of tertiary alcohol, soluble in the chlorinated hydrocarbon, containing 4 to 20 carbon atoms and between approximately 0.002% to the saturation point of a zinc compound taken from the group consisting of zinc fluoride and zinc oxide.

2. A metal treating solution as claimed in claim 1 in which the tertiary alcohol is tertiary butyl alcohol.

3. A metal treating solution consisting essentially of trichlorethylene containing 0.02% up to the saturation point of chromic acid, from about 2% to about 15 of tertiary alcohol, soluble in trichlorethylene, containing 4 to 20 carbon atoms and between approximately 0.002% to the saturation point of a Zinc compound taken from the group consisting of zinc fluoride and zinc oxide.

4. A metal treating solution consisting essentially of perchlorethylene containing 0.02% up to the saturation point of chromic acid, from about 2% to about 15% of tertiary alcohols, soluble in perchlorethy'lene, containing 4 to 20 carbon atoms and between approximately 0.002% to the saturation point of a zinc compound taken from the group consisting of Zinc fluoride and zinc oxide.

5. A metal treating solution as claimed in claim 3 in which the tertiary alcohol is tertiary butyl alcohol.

6. The process of treating a metal to increase its resistance to corrosion which comprises applying to the metal a solution consisting essentially of a chlorinated hydrocarbon containing 0.02% up to the saturation point of chromic acid, from about 2% to about 15% of tertiary alcohol, soluble in the chlorinated hydrocarbon, cont-aining 4 to 20 carbon atoms and between approximately 0.002% to the'saturation point of a zinc compound taken from the group consisting of zinc fluoride and zinc oxide.

7. The process of treating a metal to increase its resistance to corrosion which comprises applying to the metal a solutionconsisting essentially of a chlorinated hydrocarbon containing 0.02% to saturation of chromic acid, from about 2% to about 15% of tertiary alcohol,

soluble in the chlorinated hydrocarbon, containing 4 to 20 carbon atoms and between approximately 0.002% to saturation of a zinc compound taken from the group consisting of Zinc fluoride, and Zinc oxide, and subjecting the resulting coated metal to irradiation with ultraviolet light for a period of 1 to 300 seconds.

8. The process of treating a metal to increase its resistance to corrosion which comprises applying to the metal a solution consisting essentially of a chlorinated hydrocarbon containing 0.02% up to the saturation point of chromic acid, from about 2% to about 15% of tertiary alcohol, soluble in the chlorinated hydrocarbon, containing 4 to 20 carbon atoms and between approximately 0.002% to the saturation point of a zinc compound taken from the group consisting of zinc fluoride and zinc oxide, and subjecting the resulting coated metal to heating at a temperature of 100 C. to 600 C. for a period of 5 secends to 15 minutes.

9. The process of treating a metal to increase its resistance to corrosion Which comprises applying to the metal a solution consisting essentially of a chlorinated hydrocarbon containing 0.02% up to the saturation point of chromic acid, from about 2% to about 15 of tertiary alcohol, soluble in the chlorinated hydrocarbon, containing 4 to 20 carbon atoms and between approximately 0.002% to the saturation point of a zinc compound taken from the group consisting of zinc fluoride and Zinc oxide, and subjecting the resulting coated metal to a reducing gas for a period of 5 seconds to minutes.

10. The process of claim 6 in which the chlorinated hydrocarbon is trichl-orethylene.

References Cited by the Examiner UNITED STATES PATENTS 1,194,899 7 7/1916 Strippel 11793.3 X 1,542,539 6/1925 Wright 11793.3 2,114,151 4/1938 Romig 148--6.2 2,762,732 9/1956 Somers 148-6.2 2,927,046 3/ 1960 Andrade.

2,992,146 7/1961 Low 1486.15 3,100,728 8/1963 Vullo et a1. 1486.15

FOREIGN PATENTS 891,390 3/ 1962 Great Britain.

ALFRED L. LEAVITT, Primary Examiner.

RICHARD D. NEVIUS, Examiner. J. R. BATTEN, 111., Assistant Examiner.

Claims (1)

1. A METAL TREATING SOLUTION CONSISTING ESSENTIALLY OF A CHLORINATED HYDROCARBON CONTAINING 0.02% UP TO THE SATURATION POINT OF CHROMIC ACID, FROM ABOUT 2% TO ABOUT 15% OF TERTIARY ALCOHOL, SOLUBLE IN THE CHLORINATED HYDROCARBON, CONTAINING 4 TO 20 CARBON ATOMS AND BETWEEN APPROXIMATELY 0.002% TO THE SATURATION POINT OF A ZINC COMPOUND TAKEN FROM THE GROUP CONSISTING OF ZINC FLUORIDE AND ZINC OXIDE.