US2641559A - Method of cleaning and descaling ferrous articles and a bath therefor - Google Patents

Description

a predominant portion of the bath has the following composition:

Per cent Sodium nitrate (NaNOs) 40 to 99.9 Sodium nitrite (NaNOz) to 60 Sodium peroxide (Na-202) 0.1 to 50 the percentages given being by weight.

While it is preferred that the bath consist at least substantially wholly, if not whollyof the above composition, our inventionalso contemplates fused baths, and the use of rsuch baths, in which there is also present, in minor amounts not affecting the action cf the bath, other fused salts essentially neutral to the bath and the metal and oxide of the metal being cleaned.

All the compounds used in this bath may be of the ordinary commercial or technical grades, the

only requirement being that the nitrate or nitrate-nitrite mixture used, preferably sodium nitrate, or a mixture o f sodium nitrate and nitrite, must be dry and free of organic matter, and the alkali metal higher oxide, preferably sodium peroxide, relatively free of moisture, since either organic matter or moisture in or on any materials added to the bath will cause it to foam badly. The market prices of sodium nitrate and sodium nitrite are about the same as or slightly less than the market price of sodium hydroxide, and the cost per ton of sodium peroxide is generally near that of metallic sodium, so it is apparent that the bath of our invention can be operated to clean and descale steel at a cost per day no greater than the cost of operating the sodium hydroxide-sodium hydride bath. In addition, our bath requires no ammonia, operates at a lower temperature than the hydroxide baths, and cleans at a faster rate, so that the cost per ton of steel cleaned may be considerably reduced by the process of our invention.

Since the bath isV extremely active, another requirement is that the pot or container in which the sodium nitrate is fused be made of materials that will resist chemical attack. The bath will cause a pot made of ordinary grades of carbon steel, as well as many high alloy steels, to fail within a few months to a year. Since the metals that show greatest resistance to the action of the bath are pure nickel, and the nickel alloys known as Monel metal and Inconel, a pot made of a low carbon steel'clad with one of these metals on the inside is to be strongly recommended, especially since these metals are readily bonded to plain steel andthe nickel or nickel alloy layer used not be more than one-twentieth of the thickness of the clad steel plate of which the pot is made. A pot made of such clad steel plates properly welded together and uniformly heated will last indefinitely.

The pot may be heated by any of the conventional methods in use, but the preferred method is by circulating hot gases, such as the products of combustion of a gaseousuor liquid fuel, exteriorly about the wallsv of the vessel, through iiues properly insulated to avoid loss of heat to` trically or with gas-fired radiant tubes, the heating elements mustv be madeof materials to resist the action of the bath, preferably of nickel or ofV one of the nickel alloys mentioned. The

article to be treated and leave sufficient space surrounding it so thatV the article may be completely immersed.

To start a bath of the preferred composition Which is given only by Way of example, the pot is thoroughly cleaned and dried, the requisite amount of sodium nitrate or the sodium nitratesodium nitrite mixture to be used is added and heat is applied gradually until the salt or mixture is melted and the fused bath has reached a Y temperature of 650 F., the minimum operating highest temperature to Which steels of the stainthe surrounding air. If the pot isA heated elecless and corrosion and heat resistant types may be heated after the nal heat treatment is 900 F. As a final treatment these steels, or such as contain more than 0.02% carbon, are heated to high temperature (1950 F. to 2200 F.) and quenched to hold carbides in solution, and subsequent heating to a temperature above 900 F. starts a precipitation of the carbides which reduces the corrosion resistance of the steel. Within the range 650 F. to 850 F., the activity of the bath and consumption of sodium peroxide increase as the temperature of the bath is raised, and above 800 F. the decomposition of the nitrate increases rapidly as the temperature is raised.

After the initial charge of the bath has been melted, more of the salt or salt mixture is added to fill the pot to a level that will permit complete immersion of the article or articles to be cleaned and descaled Without raising the liquid level above a line 3 to 4 inches from the top. The temperature of the bath is then raised to the desired Working,temperaturebetween 650 F. and 800 F., and a predetermined amount of sodium peroxide is added sucient to give the concentration desired, usually 0.5% to 1.0% of the weight of the bath for batch type operation, unless it is desired to increase the activity of the bath, when peroxide equivalent to 3% of the Weight of the bath may be added. High activity is desirable to reduce the time of immersion as when several lots of articles are to be cleaned in Vrapid succession, or when the scale to be removed is heavy or of a type difcult to remove. To save peroxide and thus reduce the cost, it is desirable to maintain the concentration of peroxide somewhat below 1% when the bath must stand idle for varying periods between treatments. y

Assuming that the articles to be cleaned are stainless steel sheets, the operations to effect `a complete cleaning and descaling of their surfaces are recited as follows: The sheets to be treated are set up edgewise upon a rack made of Monel metal or Inconel, and each sheet in the rack is prevented from making contact with the sheets adjacent to it, preferably, by means of` grooves in the support bars of the rack and in a cross bar extending across the top. Alternately, the sheets may be kept from contacting each other at the top byinserting short round separator bars between adjacent sheets. Since pot may be of any dimensions desired and Should f the effect of immersing cold sheets into the liquid be built to accommodate the largest size of bath is to Ycause the salts to solidifyupon the cold surfaces, the rack of sheets is preferably` preheated to the temperature of the bath to reduce the time of immersion required. The

@Bahasa I racks;` usedare; of-f conventional .desigrrxsuspended fromia'. cross.4 :bar 'org crossgbars atztheftopwhich beu attached. to, they hookg., and. chain; ofi an overheadgelectric. crane Ispanning the. lbuilding inv which; all,.thetreating.= equipment ishoused.v After. thecrack.; of sheets.4 hasl been hea-ted, iti is; transferredvby-'thef crane. lowered into` the; fusedl bath until the sheets; are; immersed; andthe crossabar.- or.i bars rest uponfthe top ofthe pot. The sheetsearezpermittedfto remain in the Vfbath,y preferablytozsave.time, no longer than-necessa1ryy toifdissolveg the scale, although; longer immersion does f not harmthe; sheets, since the base 1 metal; of theflatter isgattacked but slightly byythe bath; The; exact; time required; to dissolve the scale Varies; from; several seconds. to:three' to four.l minutes.. depending vupon. `the temperaturev ofthe sheets when immersed, therthickness Vandgcharf-v acter ofi the scalef to bey removed; the;y condition of; the surface, of; thef sheets,v whether; pitted or;

smooth.. Athe typegf steel being-treated; the conditnns of :the previous iheat treatment, and-to a certain-,extent thetemperature of the bath and the; concentrationqof: sodium peroxide., For ex.-A

amplertoclean A.cold--rolled lmunie-annealed sheets: ofi 18-82'. chromium-nickel steels immersed coldr requiresL-only'itwo minutes, whereas three to fourv minutes arerequired to clean cold-rolled boxannealed .sheets .of vstraight 12 'to 14% :chromium steelimmersedcold into the bath maintained ata temperature of '650 F. for both steels. Fork reasons to bevexplained dater, concentrations of sodium'peroxidevarying from 0.1%\to.1%' have little effectupon the time required to clean. either of'y these; types` of. steel. As previously stated hereinthe-time required-to clean can be reduced somewhatby preheating the sheets to the temperature-offthebath, and theremoval of scale is `also. accelerated somewhat bymoving the rack of Ysheets-.backandforth inthe bathor by proi ducingcirculation' ofthe bath between the sheets:

as with. a submerged propeller. Therefore,

some experience in operation of abathand knowledge-of theqtype'of .steel and the treatment. it hasreceivedare necessary to permit an oper.- ator to determine. in ,advance the exact time-of.

immersion.

At the end of the immersion period-therack of sheets is raised out of and held over the bath.

forashort period to permit the moltensalts toA drain from them, when the rack of sheets islowered into awater bath to cool themand re move soluble salts adhering to them. This treat..- ment generally leaves the surfaces of the sheets. coated Witha thin red film of ferrie. hydroxide,

which is removedy by immersing the sheets in` hot (140 F. or higher) dilute aqueous solution of sulphuric. acid.. As this treatment generallyv leaves the surfaces darkened with a smut, the.

rack of sheets is next dipped into a hot (120 F. or higher) dilute (2% or higher) aqueous solution ofnitric acid, containing .05% or more of hydrouoric acid. Thistreatment usually leaves;

the surfaces-of the sheets with abright metallic` luster, the brightness and smoothness of which.:

are largely determinedby` treatmentsprior to immersion in the cleaning bath. Followingthis;A treatment the sheets are rinsed bydippingthemA To` resume discussionv of theffused;` bath, .its

most. emcient .operationre11. -1ires;I observance; of

quired and somef is carriedout of they-bath: with.

each- 101; or sheet treateaiuiabeaertoadathis g reagent; in small 1 lots; jpst. before .immersion of;

each alot. oi( sheets :than tto. add i a largequantity; at; therA beginning-g off. any;- one period of fthe; day.- Underfsuch" @renditions-,1 it g is well. toz` devise/ a quick; chem-icalg test, for sodiumperoxide; take samples :from-.ithefbath @at frequenti intervals and makevadditionsfof; peroxide` as indicated by. the. resultsf.of(these;-ftests;f

Raising;z thetemperature ofthe bathf above. 65.03 increases l its activityfbut, also increases: thel c onsum-ptio n-` of sodium. peroxide, particularlyg; f if and; the-f temperature; is, raised. abova '7305 G., becausel the :v peroxide acts-to oxidizea or reoxidize; sodium l. nitrite;T added to. or formed.. infr thef bath; Sincefthis actionf isf. relatively,l slow-, adding?, theperoxideT just before. sheetsare .inimersed. in .the bath. helps to.- reduce the loss.X of, peroxide.from thiscause. Y

The salts; carried. out of the 5 bath with. each. lot.-4 of sheets.; treated'.- is.r replaced at intervals f by: small .addi-tions of.. sodiumfn-itrate., After; a. bath i has-been-.in usefor a -time, the4 products formed; bytreactionsfof. the peroxide or. nitrate .and per-.- oxide: with the,v scale.. are f precipitated. and. settle., to;y theloottom',v so that. iteventual1y becomes.. necessary to,r remove.. this, sedimenm, This. remoya] `may be( madein .different Ways, -but a .most convenientmeans fconsistsrof. shallow.. rectangur-V lar1 pansmade voiinickel, zlVIonel-metal .or Inconel,' setin so` as,td.completely cover thebottomlof. thepotrand providedwithelarge lugs.along the. sidesr so ...thatfthey may belifted .byf acrane. from the bath. as.` they@ become... filled With-z sludge,y emptiedandmeplaced.

Thev treatments f following; immersion, in. the? fused .bath maybe ,variedfroinfthose given ,above.

` Eorf example4 the. red,coating. leftupon. the. iirstl immersion in water maybe removed'mechanicah ly, byvr passing the sheets throughr a scrubbing.; machine. Also,. after the. fused salts..y have drained oif, the. sheets may be .cooledinthe air..

' or, with amant. blastandthen immersed finto. a.` bath-:off sulphuric acid; to which. a little hydroluoric acidis added. In this treatment thesulf p huric.r acid. reacts with.l theA sodium nitrate to form sodium. sulphate, andmitric acid and with "anyY adhering. spdilumjchrol'nate. formed in'. the

fused lbath Ctogiv .Sodiumsulphateand lchromic, acid;l andj this single acidytreatment followed by.

rinsing-' in.` boiling? hot water may.y be all. that is required to give thenish desired.v In .thetreatf mentof sheets made from .certain .grades of vsteelv byA4 certain processes,n such ias lil-8 chromium.-

nickel steels.` muilie` annealed; the. dip in sulpliurid acid may beomitted, andthe, red to redldish brovvn.deposit?` left from, the water Wash..

rriayv bie removed'iby a singledipy in a nitrc or.

nitric.livdrofluorcacidibath. y

Regarding choice between thestraightsodi.-

um Initrate fb'ase. bath', andthe. nitrate-nitrite -base bath.. v,it wouldappar vthat the. latter is ...desirable I at lthe s,tart` only. in special cases. requiring. a. low,

temperaturev working range. between 3.6503 and...-- 7 00?. E. for .the following-reasons: l Y

The. mening l poi-nt. of.. sodium init-nata isi givm at-iel". 'EL and thatgof' sodium;r nitrltegass 725i but a mixture of the twomelts at a lower temperature than either alone, a fused mixture made of equal parts of sodium nitrate and sodium nitrite forming :an eutectic that melts at -about 482 F. Other mixtures containing from 30% to 50% sodium nitrate With the fbalance of sodium nitrite melts at temperatures between 440.a F.- and 450 F. With any of these mixtures heated to the minimum operating temperature of 650 F. the consumption of sodium peroxide appears to be somewhat less than in the straight sodiumnitrate base bath operating at the same temperature, but the chemical activity of the former bath appears to be slightly greater than that of the latter for the same concentration vof sodium peroxide. Aside from these advantages, there is seldom any reason for starting with the nitratenitrite bath, because the first cost of the nitrite is usually three to four times that'of the nitrate, and as will be shown later, the straight nitrate bath may be operated to form nitrite if desired. From the foregoing description of the invention, it is obvious that it may be applied to the cleaning and descaling of numerous articles besides sheets, made of other ferrous metals as well as of the high chromium steels cited as examples of steels most dicult to descale. Furthermore, various modifications may be made in both the bath `and the method without exceeding the scope of lthe invention. Thus, instead of Sodium nitrate or a mixture of sodium nitrate and nitrite, the sodium salt or salts being preferred, as explained, because of their low cost, a nitrate or a mixture of a nitrate and a nitrite of the other alkali metals, potassium, lithium, rubidium, and ceasium may be substituted in whole or in part and used in conjunction with either sodium peroxide or one of the higher oxides of the other alkali metals, of Which the oxides of potassium represented by the formulas KzOz, K2O3, and K2O4 are representative. The particular alkali metal nitrate or nitrates, mixture of nitrate and nitrite or mixture of nitrates and nitrites, and higher oxide or oxides of an alkali metal chosen for the bath depend upon the melting point of the bath desired and the stability required of the bath. Ordinarily, to obtain baths which have melting points well below 900 F. and which are quite stable, it is preferred to employ the sodium nitrate or mixture of sodium nitrate and nitrite and sodium higher oxide, the corresponding potassium compounds, or mixtures of such sodium and potassium compounds. I

It is possible to substitute other strong oxidizing agents, such as potassium or sodium Vchlorate or perchlorate, or potassium or sodium permanganate, for the higher oxide or oxides of the alkali metals in the fused bath of the invention. The use of such other oxidizing agents is ordinarily to be avoided, however, since in all instances their cost is greater than that of either the sodium or potassium higher oxides such as the peroxide. The action of such other oxidizing lagents is in most instances such as to form a substance insoluble in the bath, thus adding to the amount of sludge formed. Many of such other oxidizing agents are poisonous, or at least are hazardous to handle. In the case of the chlorates and perchlorates, the introduction of chlorine into the bathby them causes foaming of the bath, which, of course, is undesirable. As a more serious disadvantage, the chlorine has a deleterious effect upon the steel to be cleaned and the container. In all instances the higher oxides of the alkali metals have been found to form a vfused bath,

with 'the other recited components, which is not excelled even initially in cleaning Aaction by baths in which such other oxidizing agents, that is, the chlorates, perchlorates, and permanganates have been substituted for such higher oxides of the alkali metals. In addition, the bath of the invention is stable, free from hazard in the handling ofthe ingredients, and cheap.

Although it is to be understood that the method of cleaning and descaling ferrous articles and the bath therefore of the present invention are not to be defined, restricted, or confined to theoretical discussion, the following theory of operation which is supported by experimental evidence, is given as a means of distinguishing the present invention from others closely related to it and of aiding inexperienced operators to a better understanding of the process.A Y

To understand the action of the bath, it is necessary to know the nature and composition of the scale to be removed. The scales formed on the surfaces of steels of the class and type under consideration from exposure to the usual heating furnace atmospheres are complex in character and vary in composition from the outer surface of the scale to the surface of the metal beneath. At the outer surfaces of scales thus formed on the steels of the straight chromium types are found intermingled the oxides of iron and chromium, FeO, FeaOi, and lCrzOa, With the probability that some of the FeO is combined with some of the Cr2O3 as chromite, FeO'CrzOa, and with SiO2 vas iron silicate,if the steel contains much silicon. In any case,`the oxide of chromium, CrzOs, makes up a major portion of this outer surface layer of scale. On steels of the chromium-nickel types, nickel oxide, NiO, will also be found in this outer surface layer intermingled with the other oxides. Beneath the outer surface layer the oxides exist mainly as FeO and CrzOs, and if the steel contains nickel it is present as the metal, not the oxide, while the scale 4adja- -cent 4to the metallic surface consists of the lower oxides of iron, FeO, and chromium, CrO, intermingled with metallic iron, or metallic iron and metallic nickel, bonding the scale tightly to theA In the fused baths of our invention, however,

these scales and any carbonaceous or organic matter present are rapidly attacked, because the baths, whether of the nitrate-base Vor the Vnitratenitrite-base type, are intensely oxidizing and weakly basic in action. Fused baths of either sodium nitrate alone or any mixture of sodium nitrate and sodium nitrite in which the former is a major constituent are capable of strong oxidizing action, and the addition of sodium peroxide intensifies this action and, at the same time, makes the bath basic. I-Iow the action of the two is controlled in the practice of the present invention to remove the scale without appreciable attack upon the base metal of the article being descaled is exp-lained as follows:

As previously explained, the scale as a whole on all types of the corrosion and heat resistant steels maybe looked upon as an intimate mixture of the oxides of iron, chromium, and nickel with minute bodies of metallic iron and metallic Awhich-"it oxidizes to ferrie oxide, FezOa, thus:

Toward tliem'etals, thezznitrite smay vv"act Eas van oxidizing agent except :chromiumlbutzthef nitrate eiects,surfacefoxidationof all lby reactions represented asffol-lows:

Sodium peroxide, however, is capable 'ff'oxidizing not only the'metalsand` oxides to the'same stage asA sodium nitrate, but'also ofj'oxidizing 'chromium oxide to chromic lanhydride, Ywith which it `or the vsodium-oxide vformed in thereaction combines to yform-sdclium chromatewor sodium dichromate,'as represented by the following reactions:

VvThe-'excess vsodium xidelforrned in' reaction-5, or other reactions of sodiumperoxide, combines with ferrie oxide to form sodium ferrite, thus:

Considering the reactions that may take place when sodium peroxide is added to the nitrate or nitrate-nitrite bath, we nd that sodium peroxide may react alone to oxidize the chromic oxide v and form either sodium chromate or sodium dichromate and sodium oxide, or react in combination with sodium nitrate to oxidize the chromic oxide and form sodium chromate and sodium nitrite; and that sodium oxide may react in combination with sodium nitrate to form sodium dichromate and sodium nitrite, which reactions are represented as follows:

Also, sodium peroxide may react directly with ferrous oxide to form sodium ferrite, thus:

and in the bath described herein it oxidizes sodium nitrite slowly, thus: 14) NaNOz -i-NazOz- NaNOa -l-NazO A thermodynamical study of these reactions indicates that reactions 9 and 11 are most likely to occur and a mere inspection of all shows that (a) only reactions 5, 9, and 10 give sodium oxide to combine with ferric oxide or chromic oxides, as in reactions 7, 8, and 12; (b) to accomplish the descaling by reactions 5, 6, 9, 10 and 13 requires that the bath contain at the time of immersing the steel an amount of sodium peroxide suflicient to react with the scale, probably a large excess, if means of circulating the fused salts are lacking; and (c) the bathis more or less selfadjusting in that, in the absence of suflicient s0- "dium'per'oxideito'reactwith-iall the scale, as in reactioris'S-'and 1Y0, thefs'odi-umnitrate making up a major portion of the bath at all times may become' active to assist in the descaling action Yorfnitrate-nitrite bath of thepresent invention diiers from all others,-fand-alsofholdsann-advantage-in that, due-toitschem-ical-activitypit -feffects descaling in a much shorter time than any other'bath. It'alsoll'lasfan advantage over certain baths I'lknown heretofore iin lthat fit can lbe ,operated in safety and withoutf`liazardto f the workmen. ifNeitherlthe .reagents lnor `.the 'prod- 4uets'f fthe reactions are 'classed las 'poisonsfand the A:only l:special fp'recauti'ons -it lis necessary ato observein"operatingithelbath isto protect lthe hands from contact 'fwithlthe 'is'odium peroxide 'and `lavoid introducing iwate'r, .carb'onaceous for organic matter into the bath, ar'iyfofrwhichwill -causeitftowfoam ,-r-Arspe'cial advantage 'fiperatingthebathwith 'adninimum of fsodiumiperoxi'd'eisithat themetallic surface beneath the scale is attacked but slightly. The sodium nitrate or nitrate-nitritebase bath not only serves as a diluent of the peroxide, but also reacts with the surface metal, as represented by reactions 2, 3, and 4, to form a coatingv on the surface that acts to break contact of the liquid bath with the metal.

Among other advantages of the bath of our invention is the low temperature at which it may be operated and the fact that all the chemicals carried out of the bath on the surfaces treated, except the oxides of iron and nickel, dissolve rapidly in Water to form a harmless solution that may be discharged into most streams with beneficial rather than deleterious effects upon either animal or vegetable life. An appreciable portion of the sodium chromate and ferrite formed dissolves in the bath where they accumulate until the bath becomes saturated with them, when they 'are precipitated. The drag-out and additions, however, tend to prevent such a build-up, with the result that sludge accumulates slowly.

The sodium ferrite carried outof the bath on the surfaces of articles treated reacts With water to form sodium hydroxide and ferrie oxide, in accordance with the following reaction:

The ferric oxide remains on the surface vwith I the nickel oxide and may be removed in part with high pressure water sprays, or in whole by scrubbing or by dipping in hot dilute sulphuric acid followed, if necessary, by a dip in hot dilute nitric hydrofluoric acid, as hereinbefore explained.

Having thus fully disclosed and described the cleaning and descaling method and bath there-l for of our invention, we claim as new the following.

We claim:

l. The process of cleaning ferrous articles which comprises contacting the articles with a fused mixture relatively free of moisture and -11 organic matter, consisting essentially of the following composition given in percentages by weight:

. Per cent Alkali metal nitrate 40 to 99.9 Alkali metal nitrite to 60 i Alkali metal oxide (containing at least 2 atoms of oxygen per molecule) 0.1 to 5.0

Y Per cent Alkali metal nitrate 40 to 99.9 Alkali metal oxide (containing at least 2 atoms of oxygen per molecule) 0.1 to 5.0

3. The process defined by claim 2 characterized by maintaining said bath at a temperature between 650 and 800 F.

4. The process defined by claim 2 characterized by withdrawing the article from the fused bath and washing its surfaces with water; contacting the washed surfaces with dilute acids to remove any coating remaining thereon; exposing the surfaces to Water to remove the acid, and finally drying the article.

5. The process defined by claim 1 characterized by withdrawing the articles from the fused bath, washing their surfaces to remove the coating 12 adhering thereto; and nally scrubbing the surfaces with water and drying the articles.

6. A cleaning bath for an oxide-coated ferrous metal article which consists essentially of a fused mixture relatively free of moisture andv organic matter, having substantially the following composition in percentages by weight:

Perl cent Alkali metal nitrate to 99.9 Alkali metal nitrite 0 to 60 Alkali metal oxide (containing at least l 2 atoms of 'oxygen per molecule) 0.1 to 5.0

MATTI H. PAKKALA.

' FREEMAN J. PHILLIPS.

References cited in the fue ofthis patent UNITED STATES PATENTS Number Name Date 1,714,879 Lang May 28, 1929 2,244,526 MacKay June 3, 1941 2,311,099 rTainton Feb. 16, 1943 2,395,694 Spence Feb. 26, 1946 2,538,702 Noble et al Jan. 16,1951

FOREIGN PATENTS Number Country Date 466,661 Great Britain May 26, 1937 OTHER REFERENCES Condensed Chemical Dictionary-Reinhold Pub. Co., N. Y., 3rd Edition (1942) p. 588 (Copy in Scientific Library).

Claims (2)

1. THE PROCESS OF CLEANING FERROUS ARTICLES WHICH COMPRISES CONTACTING THE ARTICLES WITH A FUSED MIXTURE RELATIVELY FREE OF MOISTURE AND ORGANIC MATTER, CONSISTING ESSENTIALLY OF THE FOLLOWING COMPOSITION GIVEN IN PERCENTAGES BY WEIGHT:

6. A CLEANING BATH FOR AN OXIDE-COATED FERROUS METAL ARTICLE WHICH CONSISTS ESSENTIALLY OF A FUSED MIXTURE RELATIVELY FREE OF MOISTURE AND ORGANIC MATTER, HAVING SUBSTANTIALLY THE FOLLOWING COMPOSITION IN PERCENTAGES BY WEIGHT: