US2115005A - Electrochemical treatment of metal - Google Patents

Description

Patented Apr. 26,, 193% r orrics ELECTROCHEMICAL EATMENT F DIETAL Samuel J. Blaut, Mount Vernon, and Harold M. Lang, New York. N. Y.

No Drawing. Application October 15, 1036,

Serial No. 105,796 g 19 Claims.

This invention relates to novel methods of electrochemically removing scale from stainless steel and other metals and producing special desirable finished surfaces, including soft satin, burnished and etched surfaces, as a result of the treatment or treatments.

When base metals are heated in air for the purpose of rendering them soft after previous working, a skin of oxide forms on the surface,

which is customarily removed by a so-called pickle comprising solutions of one or more of the common mineral acids, nitric, hydrochloric, sulphuric, phosphoric and hydrofluoric acids. In some cases, electrolytic pickling is resorted to, but dissolving the scale chemically is most common. Steel, copper and brass, and nickel silver, for example, can readily be pickled chemically in a hot solution containing approximately ten per cent sulphuric acid by volume.

Stainless steel, however, forms a very hard, strong scale that resists ordinary acid treatment, and is acted on slowly by certain mixtures of mineral acids. Sandblasting is a common prior art method of scale removal for stainless steel. The heretofore most generally used process for commercial stainless steel pickling is described by C. C. Snyder in The Book of Stainless Steels, pages 150 to 152, (2nd ed. 1935), published by The American Society For Metals, Cleveland,

Pickling operations on the stainless alloys begin in the rolling mill when the slab or billet is ready for the sheet or bar mill. Mill scale produced prior to that operation is generally removed by a 10 to 15% solution of sulphuric acid B.) at to 150 F. Sometimes it is necessary to add a little hydrochloric acid to loosen in the tank, and it is necessary to put inhibitors into the bath, to slow down'the acid attack on the bare metal.

On the other hand, when one of the high chromium alloys is placed in sulphuric acid, the

metal is considerably less soluble than the scale. Oxide must therefore be removed by actual dissolution in the acid, a slow process; in effect, it is a process of rotting the scale so that it can be rubbed off fairly easily by high pressure water and abrasive wipers.

In some rollingmills, the bulk of the oxide on semi-finished products is first removed by sandblasting, and the surface then cleaned up with the pickle noted above to bring out defects which must be ground out. This; of course, is only possible when a subsequent forming or roL- ling operation are able to produce a satisfactorily smooth surface.

Cold working all the alloys hardens them fairly rapidly. Rather than overwork and break the article, annealing is necessary. Box annealing produces a scale that is ordinarily even more difficult to remove than the rolling-mill scale. The usual pickling solution for sheet and strip is ten to fifteen per cent sulphuric acid (50 B.) plus five to twenty-five per cent commercial hydrochloric acid (both by volume) at to 160 F. Since these very active pickling baths over-etch easily, the job is not completed.- The scale is rotted, the sheet removed, the sludge scrubbed on, and a milder solution used.

Finish pickling solutions may be (1) hydrochloric followed by nitric, (2) hydrochloric and nitric followed by nitric, (3) nitric with hydrofluoric, or (4) nitric, hydrofluoric and hydrochloric. For clean gray surface pickling, a mixture of 10% nitric by volume carrying 2% hydrofluoric acid is very satisfactory. The operating temperature should be kept low, that is, between 130 and F. Agitation is not necessary. Where a brilliant metallic lustre is desirable, the nitric-hydrofluoric mixture should carry about 1% HCl. The hydrochloric acid mixture is very prone to over-etching, and must be handled by a very skillful pickler, to avoid damage to the sheet.

Time varies with the condition and type of scale, the kind of acid used and its effective concentration, but should require no longer than 20 to 30 minutes for hot-rolled material. Coldrolled strip is covered with a much lighter scale, and may be cleaned in a pickle bath, (usually continuous) in a much shorter time.

"hegardiess of the solution used, it is essential to wash very clean, and finish with a dip in nitric acid 20 to 30% by volume, at 130 F. to produce a passivated surface. This passivating dip requires from 15 to 30 minutes.

Accordingly, it is noted that commercial and large scale pickling operations resided in chemigeneously.

In addition, no commercial etching process for stainless steel, produced a white or bright finish directly.

We-have discovered that stainless steel may be effectively descaled by employing the metal as anode in an electrolytic treatment at current densities of the order of 75 to 200 amperes per square foot and using fairly strong sulphuric acid solution as the electrolyte. The scale is removed very uniformly, resulting in a smooth, very white, satin finish, with but slight attack on the metal itself.

In the following, the percentages of acid composition of the electrolyte, will in all cases refer to percentages by volume of commercial concentrated acid. The electrolytic treatment to be described in further detail hereinafter is in all cases to be understood to mean anodic treatment of the metal.

We have found that an electrolytic bath of over 50% sulphuric acid descales the steel at a relatively slow rate. For example, a light scale formed by brief open annealing or soldering of 18-8 stainless steel cold-rolled strip requires three to ten minutes to descale. Heavier scale is removed in a correspondingly longer time. Dilute sulphuric acid baths act faster, but attack and dissolve the surface of the steel too rapidly and unevenly.

The use of the stronger sulphuric acid bath, then, although its action is retarded, has a desirable eifect, in that a partial passivation of the metal occurs, which seems to account for the very uniform surface after descaling. k

In other words, although the scale may not be uniform in thickness and may not dissolve or loosen uniformly, those spots of metal which may have been exposed where the scale first came off, remain almost passive, so that when'descaling is finally complete, the metal surface has a uniform appearance, and but little metal will have been removed. Likewise, the usual bad efiect of spots of dirt and grease produced by handling either before or after annealing, is eliminated or minimized.

Now, we have also found that the descaling action of the stronger sulphuric acid baths may be speeded up by the addition of a halogen acid. Hydrochloric acid increases the descaling speed, but it is so active on steel that the passivating effect of sulphuric acid is overcome, and severe etching and a gray surface results.

Hydrofluoric acid, on the other hand, added to the stronger sulphuric acid bath in any proportion from about 0.25% up, brings about a decided increase in thespeed of descaling without increase in the required current density, and without destroying the passivating action of the original bath. Moreover, the finish on the stainless steel is a smoother, whiter satin finish than with sulphuric acid alone, and much brighter than the dead grayof standard pickle finishes.

Thus we have overcome the slow action of the simple sulphuric acid baths and at the same time retained the partial passivation of the metal sur- Articles of face, the condition favorable to a uniform, smooth surface after descaling. We consider that the very white surface produced by the stronger sulphuric acid baths may be ascribed at least partly, to the formation of a small amount of p'ersulphuric acid at the anode; where conditions favora greater yield of persulphuric acid, a different effect is produced, which will be discussed hereinafter. We believe that hydrofluoric acid, which has a whitening effect when used alone, has a complex action in this bath, partly as a slight depolarizer aiding the action of sulphuric and persulphuric acids, partly asa catalyst increasing slightly the yield of persulphuric acid, and partly its own direct electrochemical attack on scale and metal.

The amount of hydrofluoric acid necessary to speed the action of descaling and increase the whiteness of the surface, is not critical, and the accelerating action is produced in most cases using 2 to 15% hydrofluoric acid.

Medium strong baths, that is, containing between 30 and 50% sulphuric acid plus some hydrofluoric acid, remove the scale somewhat less evenly and attack the metal more, unless the current density is maintained too low for effective descaling; thus the eifect of uneven scale thickness, grease spots, etc., is more pronounced. A satin finish is produced at current densities below 200 amperes per square foot.

In the first example, using the more concentrated acid baths, descaling of 18-8 stainless steel is accomplished in a period of from afew seconds up to three or four minutes depending upon the condition of the anode metal and the thickness of the scale. The anode action is ac companied by the liberation of oxygen and ozone and dissolving or peeling of the oxide scale with formation of a colored solution, which appears to adhere to the anode surface.

We have found it possible to descale the following metals or alloys in accordance with our present invention: all commercial grades of stainless steel and nickel-chromium alloys; iron and steel; high-speed steel, copper, brass and all grades of nickel silver; nickel and Monel metal.

We have found that the following meta-ls and alloys may be treated by our process to produce a satisfactory, clean, white, satin finish, either as a result of the descaling treatment or, as is obvious, upon a metal which may have been previously pickled or cleaned or polished before being subjected to our electrolytic bath: 17% chromium stainless steel; 18-8 stainless steel; nichrome; 15% nickel silver; 18% nickel silver andhigher nickel grades; nickel and Monel metal.

Although we have herein tabulated the particular metals and alloys which we have satisfactorily treated, it is to be understood that other metals and alloys of corresponding characteristics may be similarly treated by our process, which is notlimited specifically to these examples. Copper, brass, 5 and 10% nickel silver, become passive after descaling, with no further change in the resultant clean and fairly bright surface. With metals which do not become entirely passive after descaling, we obtain a clean white satin finish in the case of metals such as those mentioned above, while in the case of alloys which are actively attacked by the bath chemically, without current,

Summarizing our invention in relation to the removal of oxide scale and the production of a satin finish on metals and alloys:

1. Baths containing 50% and more sulphuric acid, plus hydrofluoric acid preferably over 2%, operated at current densities between about 75 and 200 .amperes per square foot of anode surface, remove the scale most uniformly, with negligible attack on the metal, and with a resultant smooth, very white satin finish. Baths of similar sulphuric acid content not containing hydrofiuoric acid, work more slowly and the finish is not quite so white.

2. Middle strength baths, that is, baths containing between 30 and 50% sulphuric acid, plus hydrofluoric acid, remove the scale less uniformly and attack the metal more, but produce a white satin finish at current densities between 75 and 200 amperes per square foot.

3. Baths weaker than about 30% sulphuric acid remove the scale rapidly but etch the metal severely at all practicable current densities, and leave arather dark gray surface.

In this disclosure, we shall arbitrarily choose three ranges of the sulphuric acid content of the bath, namely:

1.50% and over as the strong bath;

2. 30 to 50% as the medium strength bath;

and

3. Below 30% as the weaker bath. It is to be understood, however, that the change in-the electrochemicaleflects is gradual in passing from one range to the next, and these separate arbitrary classifications are made for convenience and simplicity of disclosure and not for expressing any critical limits in the results.

We have discovered that a surface having a highly polished appearance may be produced electrolytically. This surface which we call a burnished surface herein, will result if the process is carried out at current densities suiliciently high to overcome or reduce partial passivation, and under conditions which coincide with those favorable to the greatest yield of persulphurlc acid, H2S20s. Such conditions are the following:

1. A strong sulphuric acid bath containing preferably over 55% sulphuric acid and containing hydrofluoric acid, operating at current densities over 500 amperes per square foot of anode.

2. A strong sulphuric acid bath containing preferably over 55% sulphuric acid and containing hydrofluoric acid and hydrogen peroxide, operating at current densities over 300 amperes per square foot.

3. Medium strength sulphuric acid baths containing between 30 and 50% sulphuric acid, plus hydrofluoric acid, operating at current densities over 200 amperes per square foot. In these baths, the addition of hydrogen peroxide has no effect.

The burnished finish does not appear us ing baths weaker than about 30% sulphuric acid, at least within the practical limits of current density. In the second condition above described, hydrogen peroxide permits the concentrated sulphuric acid to produce the "burnished" effect at a materially reduced current density as compared to the first condition. It may be noted also, that hydrogen peroxide is one of the products of anodic oxidation in the medium strength. baths, along with persulphuric acid. The presence of hydrofluoric acid also aids in the production of persulphuric acid. The degree of burnishing and the rapidity at which it is pro..

duced, are dependent, we believe, on the amount of persulphuric acid formed at the anode and on the current density, which must be at least sufilcient to overcomeor reduce partial passivation.

By varying the hydrofluoric acid content of the burnishing" bath, maintaining sulphuric acid at 40%, we found a progressive increase in luster from zero to about 20% hydrofluoric acid 'content, at the same current density, and a progressive increase in speed of burnishing. At 250 to 270 amperes per square foot, 18-8 stainless steel began to show a really brilliant luster at a content of about 10% hydrofluoric acid; a moderately brilliant luster was obtained with 5%, and a decided increase in whiteness but no luster with 0.25%. A satin finish was obtained between zero and 2%. The greatest brilliance was found at about 20% hydrofluoric acid, further increases showing no improvement in brilliance.

Hydrofluoric acid alone, without sulphuric acid, gave a very white satin finish.

We have succeeded in producing a "burnished finish on the following metals and alloys, either on a surface resulting from our descaling treatment, or upon a surface previously cleaned or prepared by our invention or otherwise; 17% chromium and 18-8 stainless steel; nichrome: l5 and 18% nickel silver; 30% nickel silver (burnishes to a brass color) nickel and Monel metal. The alloys which can most readily be "burnished and which show the highest luster are, in the order given, nickel and Monel metal, nickel silvers containing 18% and more nickel, nichrome, and nickel bearing stainless steel. The first two, in fact, burnish easily in the stronger sulphuric acid baths of Class 1. This would indicate a selective action on nickel in the burnishingf treatments. 10% nickel silver cannot be satin finished or burnished by the treatment herein described for the other metals. Stainless steel treated byour invention does not appear to lose any of its stainless or corrosion-resistant properties.

The process should not be carried out at high temperature since the irritating and corrosive spray and fumes from the acid baths make it desirable to keep the bath cool, for example, by using water-cooled cathodes, and secondly, the unstable nature of persulphuric acid at higher temperatures may make it less effective.

Tanks and cathodes moisture from the air. The addition of less than 3% nitric acid to the bath is also advantageous for this purpose, in greatly reducing the gassing at both electrodes.

Preferable cooling means are water-cooledcathodes made of any of the resistant metals, and

protected from acid and electrolytic action at and above the liquid level.

The apparatus may be designed for continuous treatment of strip steel, etc., the tanks then be- Copper, brass, 5 anding provided with suitable rolls and contacts for carrying and conducting current to the strip.

The cathode area'does not appear to be critical, but for efiective cooling (if water-cooled cathodes are used) and for prevention of excessive spray at the cathode by rapid liberation of hydrogen, it is advantageous to use a large cathode area. If a metal tank be used, the tank itself may be used as the cathode. Electrical connection to the cathode is made in a well-known manner. In some cases it is advantageous to insert a porous partition to keep the cathode gas away from the anode. A solid partition extending a short distance vertically above and below the'surface of the bath, would also be sufficient to limit the movement of the hydrogen bubbles. In either case, an oil layer on the cathode compartment thus formed, without oil on the anode compartment, would eliminate practically all spraying and most of the fuming without getting oil on the metal.

Anode conductors The anode, consisting of the metal to be treated, may be placed in conductor racks, or suspended from conductor bars, or contact may be made to a continuous strip by contact rolls or otherwise, in any desired manner. -Stainless steel of high chromium content, or some other metal or alloy which does not become passive in the bath, is preferred for anode contacts, but copper or brass may be used, provided the contact surfaces are held together with a certain amount of pressure. In general, because of the high current employed, the contact surfaces should be large.

In the treatment of large sheets, if the current required to treat the entire sheet at the necessary current density should be in excess ,of the available generator capacity or contact capacity it is possible to treat part of the sheet at a time by masking the remainder (with an acid-resistant coating .as is customarily done in the etching process) thus carrying on the treatment in successive limited areas until the entire sheet has been treated.

Operating conditions Since we have found that the most favorable conditions for descaling and producing a white satin finish, occur when a fairly strong sulphuric applied to surfaces prepared otherwise than by our invention, such as surfaces previously sandblasted, pickled, etched, scratch-brushed, polished or cold-rolled. With proper adjustment of current density and time of treatment, any concentration of sulphuric acid above about 30% may be selected to p oduce either the satin finish or the burnished e ect, with or without the addition of hydrofluoric acid. Our selection of baths A and B, and our loose division of sulphuric acid concentrations into two useful ranges, is based on practical considerations, and represents the most favorable operating conditions, but does not define critical limits beyond which the desired results cannot be obtained.

Operation of the descaling and satin finish process The descaling and white satin finish operations are carried on in a bath containing from 50 to approximately, of commercial concentrated sulphuric acid, and 2 to 15%, more or less, of commercial concentrated hydrofluoric acid, The preferred composition, designated herein as bath A, is: 65% sulphuric acid and 10% hydrofluoric acid.

This stage descales the metal, and if the white satin finish is desired, or if a smooth surface suit able forbufilng, is desired, the operations perform this service.

The current density preferably applied ranges from 75 to 200 amperes per square foot. A voltage in excess of 3.5 volts is required in the bath A; under usual operating conditions with 18-8 stainless steel, 4.0 volts is sumcient to produce a. current density of 75 or more amperes per square foot, on 18-8 steel.

The anode action is accompanied by liberation of oxygen and ozone, and dissolving or peeling of the oxide scale, with formation of a colored solution, which appears to adhere to the anode surface. Electrode gassing in this bath it not very severe, and spraying from this cause is not sufficient to create operating difiiculty; it can be practically eliminated by the addition of about 1% nitric acid to the bath which also produces a smoother finish.

Descaling takes place in most cases, in a period of from a few seconds up to 3 or 4 minutes, depending on the nature of the anode material and the thickness of the scale. Cold-rolled 18-8 stainless steel strip, annealed at 1800 F. is completely descaled in seconds or less. 17% chromium stainless steel seems to require about twice as long as other grades to descale.

The process frequently removes the scale completely, leaving a uniform, lively white satin finish.

In some cases, a soft sludge adheres, which is easily removed by rinsing or light wiping.

is the final step. i

In the case of stainless steel, the attack on the metallic surface is very slight, only the scale being dissolved if the action is not prolonged. The bath thus retains its active acid strength a long time, and the strength may drop considerably without detriment to its working qualities. When the bath finally becomes too low in acid for effective use, it may be used to replenish bath B hereinafter described.

There is generally no metallic deposit on the cathode, or a slight deposit which redissolves in the bath.,

Operation of the burnished process The "burnished effect is produced by anode treatment in a bath containing from 30 to 50% sulphuric acid, and 2 to 20%, more or less, hydrofluoric acid. Our preferred composition, designated herein as bath B, is: 40% sulphuric acid and 10% hydrofluoric acid.

This bath may be used both for descaling and for producing the burnished finish in one treatment, if desired. However, better results on stainless steel are obtained by first descaling in bath A, and then treating in bath B.

A current density of 250 amperes per square foot or more is used. The minimum initial voltage used on 18-8 stainless steel, for example, is about 3.5 volts, a gradual increase of voltage up to about 4.0 volts being required.

The anode action is accompanied by liberation of oxygen and ozone, and dissolving of the metal surface, with formation of a colored solution which appears to adhere to the anode surface.

[ greater the amount of pitting. A satisfactory current density can be arrived at, however, which will give a very good surface with practically no pitting. In the case of l88 stainless steel, this condition is found at about 250 amperes per [6 square foot. By deliberately permitting pitting to take place,-strikingly beautiful effects may be obtained. Agitation somewhat retards pitting. The procedure may be modified with certain metals; nickel, for example, is best descaled in :0 bath B and best bumished in bath A.

A thorough washing completes the process.

There is a gradual consumption of the acid in this bath B, and a gradual lessening of its burnishing power as it becomes loaded with 35 metal. This can be compensated for to some extent by increase in current density and by periodic additions of fresh acid. The spent acid may finally be used for ordinary pickling solutions for other steels. A

Operation of the etching process We have successfully applied our invention for the purpose of producing decorative effects by electrolytic etching of stainless steel, and it can 35 be applied to certain other metals, such as the metals referred to hereinabove as susceptible to our treatment.

Various types of procedure known in the etching art are possible with our process, and we cite lo the following as one example.

We first subject the steel to an anode treatment in bath A, producing a clean, uniform satin finish. We then imprint or draw or paint a design on the surface, using an acid-resisting medium such as wax or paint, and subject the steel to a second anodic treatment in bath B, thereby producing a burnished effect, and, if desired, a pitted effect, on the exposed metal. The contrast between the satin finish and the lustrous m finish is a very eifective one. Intermediate tones may be produced by manipulation such as masking, or dodging with a small, movable cathode as will be understood by those skilled in the art.

, Additional contrast in relief may be effectively produced by longer etching time. Photographic or offset printing methods may be used to place the design upon the steel.

Although we have described preferred prol0 cedures for carrying out our invention, modifications may be practiced and will be evident to those skilled in the art; accordingly we do not intend to be limited except as set forth in the following claims.

I We claim:

1. The method of removing from chromium alloy steel the mill scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel ro to electrolytic action as anode in a bath comprising concentrated sulphuric acid, the sulphuric acid being in suflicient quantity to develop on the surface of the chromium alloy steel a partial passivity to prevent any substantial attack on the F5 descaled chromium alloy steel by the acid bath,

said method being operable with the acid bath at a temperature as high as 140 F.

2. The method of removing from stainless steel the mill scale formed during the commercial production and fabrication thereof comprising subjecting the stainless steel to electrolytic action as anode in a bath comprising concentrated sulphuric acid, the sulphuric acid being in sumcient quantity to develop on the surface of the stainless steel a partial passivity to prevent any substantial attack on the descaled stainless steel by the acid bath.

3. The method of removing from chromium nickel alloys the mill scale formed during the commercial production andfabrication thereof comprising subjecting the chromium nickelalloys to electrolytic action as anode in a bath comprising concentrated sulphuric acid, the sulphuric acid being in suflicient quantity to develop on the surface of the chromium nickel alloys a partial passivity to prevent any substantial attack on the descaled stainless alloys by the acid bath.

4. The method of removing from chromium alloys the mill scale formed during the commercial production and fabrication thereof comprising subjecting chromium alloys to electrolytic action as anode in a bath comprising concentrated sulphuric acid the sulphuric acid being in sumcient quantity to develop on the surface of the chromium alloys a partial passivity to prevent any substantial attack on the descaled chromium alloys by the acid bath.

5. The method of removing from chromium alloy steel the mill scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel to electrolytic action as anode in a bath comprising concentrated sulphuric acid containing on the order of or more of sulphuric acid by volume.

6. The method of removing from chromium nickel alloys the mill scale formed during the commercial production and fabrication thereof comprising subjecting the chromium nickel alloys to electrolytic action as anode in a bath comprising concentrated sulphuric acid containing on the order of 80% or more of sulphuric acid by volume.

7. The method of removing from chromium alloy steel the mill scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel to electrolytic action as anode in a bath comprising concentrated sulphuric acid containing about 50 to 80% sulphuric acid by volume.

8. The method of removing from chromium al- 10y steel the scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel to electrolytic action as anode in a bath comprising concentrated sulphuric acid and hydrofluoric acid.

9. The method of removing from chromium alloy steel the scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel to electrolytic action as an'ode in a bath comprising concentrated sulphuric acid and hydrofluoric acid on the order of 2 to 20% by volume.

10. The method of removing from chromium alloy steel the mill scale formed during the com- I prising subjecting the chromium alloy steel to electrolytic action as anode in a bath comprising concentrated sulphur acid and hydrofluoric acid at a current density of about '15 to 200 amperes per square foot of anode surface.

12. The method of removing from chromium alloy steel the scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel to electrolytic action as anode in abath comprising about 50 to 80% sulphuric acid by volume and hydrofluoric acid. I

13. The method of removing from chromium alloy steel the scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel to electrolytic action as anode in an electrolyte comprising hydrofluoric acid.

14. The method of removing from chromium alloy steel the scale formed during the commercial production and fabrication thereof comprising subjecting the chromium alloy steel to electrolytic action as anode in an electrolyte comconcentrated prising on the order of! to 20% hydrofluoric acid.

15. A stainless steel electrolytic pickling bath 20% hydrofluoric acid.

18. A stainless steel electrolytic pickling bath comprising a bath containing about 30% or more of concentrated sulphuric acid and on the order of 2 to 20% hydrofluoric acid by volume.

17. The method of producing a luster on chromium alloy steel comprising subjecting the chromium alloy steel as anode to electrolytic action ina bath comprising on the order of 30 to 50% concentrated sulphuric acid by volume and hydrofluoric acid.

18. The method of producing a luster on chromium alloy steel comprising subjecting the chromium alloy steel as anode to electrolytic action in a bath containing on the order of 30 to 50% concentrated sulphuric acid by volume and on the order of 2 to 20% hydrofluoric acid by volume.

19. The method of producing controlled finishes on chromium alloy steel which comprises subjecting the chromium alloy steel at a predetermined current density as anode in a bath comprising sulphuric acid and hydrofluoric acid. 1 Y

SAMUEL J. BLAUT. HAROLD M. LANG.