US2761765A - Composition and method of inhibiting the corrosion of ferrous equipment used in the regeneration and boiling of alkali metal solutions - Google Patents

Description

Sept. 4, 1956 T. A. MATTHEWS 2,751,755 COMPOSITION AND METHOD OF INHIBITING THE CORROSION OF FERROUS EQUIPMENT USED IN THE REGENERATION AND BOILING OF ALKALI METAL SOLUTIONS Filed Oct. 28, 1952 4 Sheets-Sheet 1 w'r. LOSS, Mg/Gm TIME, HOURS OOI HOUR

FLANGE mo/v FIG. I

INVENTOR.

By THOMAS A. MATTHEWS ATTORNEY p 1956 T. A. MATTHEWS n 2,

COMPOSITION AND METHOD OF INHIBITING THE CORROSION OF FERROUS EQUIPMENT USED IN THE REGENERATION AND BOILING OF ALKALI METAL SOLUTIONS Filed Oct. 28, 1952 4 Sheets-Sheet 2 WT. Loss, Mg/Cm TIME, HOURS o 5 N u a 01 m 0 8 8 8 8 8 HOURS FIG. 2

- INVENTOR.

BY THOMAS A. MATTHEWS ATTORNEY p 1956 T. A. MATTHEWS n 2,761,755

COMPOSITION AND METHOD OF INHIBITING THE CORROSION OF FERROUS EQUIPMENT USED IN THE REGENERATION AND BOILING OF ALKALI, METAL SOLUTIONS Filed Oct. 28, 1952 4 Sheets-Sheet 3 WT. LOSS, Mg/Gm V8.

' 0.8 TIME, HOURS Mg/(Jm 0 5 N 01 4s 8 as o 8 8 8 o 8 HOURS CARPENTER 20 FIG-3 INVENTQR.

BY THOMAS AMATTHEWS ATTORNEY I Sept. 4, 1956 T. A. MATTHEWS ll COMPOSITION AND MET 2,761 ,765 HOD OF INHIBITING THE CORROSION OF FERROUS EQUIPMENT USED IN THE REGENERATION AND BOILING OF ALKALI METAL SOLUTIONS Filed Oct. 28, 1952 4 Sheets-Sheet 4 WT. Loss, Mg/Cm I vs. Q9 TIME, HOURS K o 6 N x/ e O E o 5 E /3 04 0.2 I 9 /4 m o/ vv g 7 l .a I 5/ 2, 0 a I 1 /2\ HOURS AVERAGE 01- Mfr/:2, ZVGO/VEL a NICKEL INVENTOR BY THOMAS A. MATTHEWS ATTORNEY F ALI METAL SOLUTIONS Thomas A. Matthews II, Crystal Lake, lik, assignorto The Pure Oil Company, Chicago, 111., a corporation. of Uhio Application October 28, 1952, Serial No. 317,277

8 Claims. c1. 23-484) This invention relates to a method and a novel inhibitor for reducing the corrosion of ferrous metal equip ment used in the processing and handling of solutions containing active percentages of caustic alkali. Specifically the invention comprises a method of inhibiting the corrosion of ferrous equipment used in the regeneration and boiling of alkali solutions containing large and active amounts of alkali, which alkali solutions may or may not contain solubility promoters and oxidation catalysts for the sweetening of hydrocarbon oils or ingredients added to facilitate the removal of sulfur compounds from hydrocarbon oils and gasoline.

Strong alkali solutions, particularly aqueous or alcoholic sodium and potassium hydroxide solutions having per cent or more by weight of active alkali present are known to be corrosive to metal parts, especially ferrous metal parts of equipment used to handle the alkali solutions. The extent of corrosive action of the alkali depends upon its concentration, the temperature and pressure of the system, and the rate of flow or impinging action of the solution on the metal surfaces involved. In the treatment of hydrocarbon oils and distillates for the purpose of removing sulfur compounds or transforming lobnoxious sulfur compounds into non-odorous form, it is necessary to regenerate the alkali solutions. This re generation is accomplished by heating, steam stripping, or air blowing conducted at temperatures above 60 F. and as high as or higher than the boiling point of the alkali solution. During such treatment, the alkali solution is generally circulated through a packed tower or through a bubble tower, and portions thereof recycled to insure adequate regeneration. This necessitates the use of piping and pumps to convey the heated alkali solutions. Severe corrosion of the pumping equipment and metal conduits has been experiencedin such operations. The used alkali solutions from such operations contain various sulfur compounds including mercaptans, mercaptides, and disulfides which tend to increase the corrosiveness of the solutions. In addition, certain amounts of dissolved oxygen, various solubility promoters, oxidation catalysts, along with bits of extraneous matter, may be present which increase the corrosiveness and add to the difficulties attendant to attaining a regenerated alkali which is clear and clean, free of sedimentary materials, and readily reused. The boiling and steam stripping to which these alkali solutions are subjected during regeneration presents an atmosphere which is especially corrosive to ferrous metal surfaces and results in many instances in the formation of precipitates which render it colored and often containing sediments making it'unfit for contacting hydrocarbons.

To increase the life of the pumps, conduits, and other handling equipment subjected to the above conditions, it has been the practice to fabricate these pieces of equipmerit from corrosionresistant stainless steel alloys at considerable expense. Even certain stainless steel alloys are attacked by hot caustic under extreme conditions and item the use of a noble metal alloy promotes the severe States Patent 2 corrosion of adjacent parts made of base metals. During the course of investigating the rate of corrosion of ferrous metals in contact with boiling strong caustic alkali solutions, the observation was made that the rate of corrosion increases rapidly during the first few hours of contact and then levels oif at a much reduced rate with the passage "of time. A study of this phenomenon was made by applicant in order to find an inexpensive means for overcoming the corrosion problem.

One explanation for the decreased rate of corrosion over extended periods of time is the formation of a protective film over the metal surface which stops or slows up further attack by the alkali. In an effort to find conditions which demonstrate the true rate of corrosion of these ferrous metals in !order to study the actual effective.

ness of experimental oxidation and corrosion inhibitors, experiments were conducted using an abrasive in the form of ferric oxide in the alkali solutions under test on the theory that the abrasive would remove the partially )rotec tive metal film that seemed to be forming. It was found that, contary to these expectations, the iron oxide acted in an opposite manner and produced a marked reduction in the corrosion rate. The degree of reduction in corrosion ratewas further found to be a maximum with certain optimum concentrations of the iron oxide present in the alkali solution.

Accordingly, it is a fundamental object of the present invention to provide a method of preventing the corrosion which takes place in the boiling of active alkali solutions.

A second object of the invention is to pnovide a method of preventing corrosion of equipment used in the regeneration of alkali sweetening solutions or alkali desulfurizing solutions which have been used-in the treatment of hydrocarbons.

A further object of the invention is to provide a method of inhibiting the corrosion occurring during the regeneration of alkali solutions and maintaining the solutions in clear, clean, usable condition.

Another object is to provide a composition for use in mitigating corrosion of ferrous metal surfaces by alkali solutions.

Other objects and advantages of the invention will appear in part or be obvious from the following description.

The invention, accordingly, comprises the discovery that the corrosive action .of alkali solutions on ferrous metal surfaces may be mitigated by the presence of small but sufficient amounts of red iron oxide (FezOs) and/or the so-called alkali metal ferrites, sodium, potassium, and lithium feirites, which may be designated by the formula NazFezOi. The invention has as its embodiments, the process for inhibiting the corrosion of ferrous equipment used in handling alkali solutions, including such processes as the regeneration of alkali used in treating hydrocarbon oils; and the product therefor comprising a composition characterized by its utility in mitigating this type of cor- 1'0S1011.

The process includes the several steps to be described and the relation of one or more of such steps to each of the others thereof, which will be exemplified in the method hereinafter described and defined in the claims.

In the drawings,

Figure 1 is a graph of weight loss of flange iron samples in contact with hot caustic solution plotted as ordinates against the time in hours plotted as abscissas.

Figure 2 is a graph of weight loss vs. time in hours as determined for stainless steel 304.

Figure 3 is a graph of weight loss vs. time in hours as determined for Carpenter 20, a stainless steel alloy.

Figure 4 represents a graph of the average weight losses i plotted as abscissas. f

, Typical examples jalkalisolut'ions may cause corrosion problems as atomic-1 imsflfisnd newbie may b o er o e b e: seem the present invention are thosejproeesses employi g causti I scrubbing: to remove Ihydrogen sulfide and the, majority of the more} solu Mercapsoll process.

potassium hydroxide'or may v with hot active alkali solutions. from'whatever source, it

j areusedthree Mercapsol process din - about 3 per cent of organic catalyticjagents, such as; hydro qui'none, naphthoquinoneg wood tars, catechol, aromatic quinone forr ning' compounds, an,thraquinone, nd deriv hich, have. been .found useful as accelerators. 1

li solutionsm'ayalso contain; thiophenoljie compounds,

unsaturated organic aci s, reactionproducts alkali and natural or synthetic shellac, been; acidic 'pe roleuni oxidation products, alkali metal s'alts of high ?boil;ngtar; 1

acid oils,.and alkali metal phenol f ana mia, vaeca gum,

r a htlmbof er; aictersfsmpies in c'ontactw hfhot%c}austic 1 j solution plotted: as ord nates against-the time hours i of Imdust rial. assess; wherein. hot

I I I lkai sweetening of drocarbons includes generally an aqueous solvent which; may, or. may not eontain an alcohol, having 55 to 50. per 'cent of: caustic alkali 1 1 usually in% the form-of sodium; hydroxide and depending 1 thiocresols, alkali metal soaps of naphthenic acids: and of will be: demonstratedfbyrefierence to alkali solutions which 1 v The type of solution .1 1

having a tight fitting: Tygon tubingigasket'was equipped 1 japan the type ot treating: process; contemplated, up to "of orros'ion under: conditons of, noagitatiommild agitae 1 I I I ties; and extremeagitation. i For the: latter purpose, the 1 1 i apparatus was equipped with a PM motor drivng f I s eering shaft through an idlerpulley, which shaft susaj I pended a disc, type. turbin'e inimmersion contact withthei 1 1 at tives :of these quinones' and; quinoneeforming? compounds f I i The solution may also contain other; ingredients 1 i which have the effect of improving the effieiency of the operations for the regeneration of used alkali solutions therefrom are those described in United States Patents extraction. Typical of the processes employing this typeof solution in hydrocarbontreatment and also including- 2,292,636 of August 11, 1942, to Lawrence M. Henderson 1 and George W. Ayers, Jr.;. 2,297,621 of September 29, 1942, to Lawrence M. Henderson and George W. Ayers, .lr.; and 2,314,919 of March 30, 1943, to Donald C. Bond, George W. Ayers, In, and Lawrence M. Henderson.

It is known in the prior art to use many different inorganic salts as inhibitors against various corrosive atmospheres in various systems. United States Patent 2.297,666 of September 29, 1942, to Aaron Wachter, employs sodium nitrite in oil pipe lines. United States Patent 2,153,952 of April 11, 1939, to Alfred L. Bayes, em- 1 ploys sodium nitrate in anti-freeze solutions and Patent 2,135,160 dated November 1, 1938, to Herman Beekhuis, discloses sodium dichrornate to inhibit the corrosiveness of ammonium nitrate systems. The use of red iron oxide as a paint pigment is well established and numerous metal protective pigments comprising metal oxides have been used as constituents of primary coats applied to metallic surfaces for the purpose of inhibiting or passivating various kinds of corrosive atmospheres. Lead and zinc pigments, for example, are known for their inhibitive nature.

iron oxides are generally known as neutral pigments. However, it has been observed that strong caustic solutions will actually remove red iron oxide paint and many other protective coatings from metal surfaces.

The state of the art previously outlined emphasizes the fact that the problem of the inhibition of corrosion is one of empirical phenomenon and each particular situation or environment creates its own peculiar corrosion problem requiring specific and exacting solutions. Each problem of the inhibition of corrosion in a given environment, therefore, calls for a process which is designed specifically to meet that problem. A given corrosion Z and Louis R. :Mazurk, .it is pointed out thatsodiurn.phos I 3 Worse than uselessin preventing; the corrosion. of ferrous;

. metal solutions. That patent is. directed to the use. of i alkali metal 'nitrites to reduce; the corrosionf of. boiling. l caustic alkali solutions on the. ferrous vessels. i The present inventionand: its applicab'lity to 1 solution, known as: Mercapsol solution, maintained 5 at: about 230; boiling tem eraturelg for from o hours to astrnuch as. 12.35 hours contact time. I I I v t i I The Mercapsol solution {used in these tests contained I 1 f 19.3 per cent sodium hydroxide, Z 10., per cent; cresols, 15;

. 41 inhibitor may be very useful in; a certain. application but wh w ul I I I I I I f United States Patent'2,5:56, 38T I oflnne 12, 1951.10 GeorgeEW. Ayers; Erskine 1E; Harton,

pirate; and sodium gchromate, which} are very well known I 1 inorganic salt corrosion} inhibitors, were found g to be tmrnent. used in the regeneration and bojilingofalkali I the; inh' I of corrosion-of ferrous materials by causticsolutions I ontaining various. extraneous materials; is best under- 1 tali solution; for} rotating samples therein. Various inples of ferrous metals iand; ferrous metal alloys were;

subected to contact with a typical corrosive: caustic alkali? stood and demonstrated by 5 reference: to. the following I I I amples and tab 1 s;, I I ,I I I I I 1 1 :A .five gallon stainless; steel 1pai per centnapht-henic acids, and 55.7 per cent water. The. I l A Baum gravity was maintained at approximately 255 by daily addition of water, which was lost at about 0.3 Weight per cent per day. During the runs, the temperature remained between 220 and 230 F. Four removable bafiles were used to prevent vortexing during mixing. Teflon plastic washers were used to suspend the samples and insulate the bolted samples from the turbine. The Mercapsol solution had no efiect on this plastic after weeks of immersion.

Each sample before testing was first rubbed with steel wool and emery cloth, then pickled in hot hydrochloric acid solution, rinsed with water and acetone, dried, and weighed. After immersion for about hours, the samples were removed, washed with water and acetone, and dried. During this operation they were rubbed briskly with a cloth to remove any reasonably loose scale. Any adherent scale remaining was included in the weight determinations made. The Weight of each sample was taken and the samples were immersed for further contact with the alkali. This procedure provided data relating weight loss with time. All samples were given this same treatment regardless of the degree of agitation which was applied during the tests.

The condition of no agitation represents one wherein the samples were hung in a static solution. The condition of mild agitation wherein the samples are hung in a solution surrounding a mixing turbine and convection currents are allowed to mildly agitate the solution. Conditions of more extreme agitation are represented by the amount of travel in lineal feet per second of the samples when fastened to the mixing turbine.

The following table sets forth. an analysis of the samples of ferrous materials used in the experiments.

F. with varying degrees of agitation, it is seen al immersion of stainless steels 410, 316, 316

of ferrous metal alloys in contact with Mercapsol solution at 230 that the tot TABLE I (rolled plate) 91 5 9660202 LILLLHWLOJLO Chemical analysis of steels and nickel alloys ClSiMn S 29 60 5 5 5406730012 QQLQQLQQQMLQQO 0 00 0 0 0 Q0 0 &

Type of Steel Flange iron.

NickelWrou M 6, where chosen Some of the metal samples selected, as, for example, Carpenter and alloy steel 304 and 31 2 for their extreme resistance to this type of caustic cor- 0 fro to 0.00069 for these metals with the protective film and o initial rate of corrosion before film formation was a typical ferrous metal to corrosion resistance The results of these experiments lowing table.

Rate of corrosion of ferrous metals in IPY 10 rosion and a comparison is made to show the relative effectiveness of the corrosion inhibitor of the vention in bringing flange iron,

alloy which is readily corroded comparable to that of the more resistant and more expensive a lloy steels.

are set forth in the fol 1 Tert.-octyl mercaptan. 2 Tert-butyl mercaptan. Tert.-hexyl mercaptan (under N1 atm.).

All determinations were made in an atmosphere of larger (0 to 0.0178 IPY) with the time for film formation air except run No. 14 wherein an atmosphere of nitrogen being 70 to 300 hours.

The samples all lost more weight immediately after immersion in the hot Mercapsol soluwas used. During the last 378 hours of run No. 11 with flange iron and all of runs 12, 13, and 14, two flange iron tion than after passivation by the formation of a protective samples were tested, one insulated from the stainless film. The film formed was a black scale, not steel disc turbine and designated flange iron #1, and the readily attacked by strong acids and difficult to remove other not insulated and designated flan lllium-G is the only alloy devoid of such film. For these determinations, it vation and the rate of corrosion after passivation are dependent on th ge iron #2, giving by rubbing with an emery cloth. a direct comparison in corrosion rates under otherwise This caused no apparent change in can be concluded that the time of passi 5 degree of agitation during the tests. Conditions of mild agitation (runs 2 and 5) gave more consistent results while rotation of the samples at a constant s feet per second through the Merc gave the most consistent data.

evidenced from the sample in the In run No. 12, from the 305 hour time 6 to 428 hour time, the following sulfur compounds were found by analysis to be present: 0.6 weight volume per cent of sulfate and 0.1 weight volume per cent of S203. In run No. 13, iron sulfide (FeS) was present from 351 hours to 501 hours in 0.4 weight per cent concentration. though giving accelerated rates A complete fresh charge of Mercapsol solution was not equivalent to the rates of corrosion experienced in certain pipe fittings and other equipment made of flan iron in a Mercapsol unit.

added at 334 hours at run No. 11. No make-up inhibitor was added or needed during the determinations.

Referring to Table II, runs 1 through 6, having as Consequently, in a further study of the corrosion osion problem in which additional determinations were made their objective the determination of the rate of corr 7 to ascertain the effect of ferric oxide alone and the addition of other corrosive sulfur compounds, the unusual inhibiting effect of ferric oxide was discovered. The efficacy of this substance as a corrosion inhibitor is illustrated by runs 8 through 14, inclusive, in Table II. Herein it is seen that the corrosion rates are reduced to about one-twelfth that of the flange iron in straight Mercapsol solution. Although the concentration of ferric oxide was varied from 0.04 to 0.6 weight per cent and the relative corrosivity of the caustic increased by addition of mercaptan, no conditions were found which disturbed the inhibiting action of the ferric oxide at optimum. concentrations of inhibitor. It is to be observed that with a concentration of ferric oxide of around 0.18 weight per cent, the maximum reduction in corrosion rate amounting to about one-sixteenth that of the blank is obtained.

From these experiments, it is seen that small quantities of inhibitors of the present invention may be used to accomplish the desired result. Since ferric oxide is soluble in alkali solutions with the formation of alkali metal ferrites, the sodium salt being representative, Na2Fe2O4, it is apparent that the inhibiting action may be attributed to either or both of these materials. No added results are obtained by using quantities of ferric oxide in excess of the range of optimum concentrations. The presence of small amounts of suspended particles of ferric oxide in the alkali solution is not deleterious, except that the use of such quantities of ferric oxides or alkali metal ferrites as to form a sludge which may foul the conduits or pumps in the system is to be avoided. The concentration of the ferric oxide either as such or in the form of th alkali metal ferrite may, therefore, be as low as 0.04 weight per cent, and it is immaterial whether quantities are used in excess of the solubility of the ferric 3 oxide in the alkali. For maximum protection under most conditions it is contemplated that from 0.04 to 0.6 weight per cent of ferric oxide, for example, may be used, although the preferred range is about 0.1 to 0.2 weight per cent to obtain the most efficient inhibition.

The optimum concentration of 0.18 weight per cent of inhibitor as used in accordance with this invention serves to bring the rate of corrosion of flange iron down to that of the expensive stainless steels. In addition, the inhibitor of this invention successfully reduces the rate of corrosion of such alloys as that represented by 304 and 410. The conclusions herein drawn are vertified by reference to the graphs, Figures 1, 2, 3, and 4.

Electron diffraction analyses were conducted to determine the composition of the scale that was formed on the metal surfaces. These analyses showed that the scale was composed predominantly of ferrosic oxide (F6304) with strong possibilities of some ferrous oxide (FeO). No evidence was found of ferric oxide or sodium ferrites in the scale. This serves to confirm in part the conclusion that the action of the ferric oxide and alkali metal ferrite inhibitors of the present invention is one of true inhibition or negative catalysis and not a mere blanketing of the metal from the influence of the alkali through some type of film formation.

In the graphs, Figures 1, 2, 3, and 4, wherein weight loss in milligrams per square centimeter of sample surface is plotted against time and hours, the various curve numbers shown correspond to the run numbers set forth in Table II. For example, runs No. 7 and 8 of Table ll, which are conducted for the various test samples with no inhibitor present, are represented by curves 7 and 8 in each graph for the different alloy samples, and run No. 9, conducted with 0.60 weight per cent of ferric oxide, present, is represented by curve 9 in each graph for the indicated alloy, etc. And, likewise, run No. 14,

conducted with 0.18 weight per cent of ferric oxide and 1.0 weight per cent of tertiary hexyl mercaptan in an atmosphere of nitrogen, is represented by curve 14. In Figure 1, it will be noted that curves 11, 12, and 13 are shown in duplicate. The left-hand curve of each pair indicates the results of those determinations conducted after removal of the insulating washer from between the sample and the rotating disc turbine. The curves graphically represent the data of Table II and point up the unusual inhibiting effect of ferric oxide in transposing the resistive of flange iron into that approaching the more resistive stainless steel and nickel alloys.

An exemplary operation wherein caustic solutions con taining corrosive sulfur compounds are subjected to conditions conducive to the type of corrosion just illustrated by Table II is the regeneration of spent caustic containing about 12 per cent by weight of free alkali and about 0.25 weight per cent of rnercaptan sulfur. In such a regeneration system, the spent caustic is withdrawn from storage at a temperature of to 100 F. at a rate of 20 to 30 barrels per hour and is heated to about 220 to 230 F. by indirect heat exchange with steam for entry into an upper section of a regeneration tower operating at about 4 pounds per square inch. Steam at 400 F. is fed to a reboiler operating in connection with the tower to maintain temperatures at about 240 to 245 F. therein. The approximate temperatures at the top and bottom of the tower are maintained at 212 and 250 F., respectively. Hot regenerated, concentrated caustic containing about 0.06 weight per cent of mercaptan sulfur leaves the reboiler, passes in indirect heat exchange with incoming spent caustic, where it drops to a temperature of about 110 F., and passes back to the hydrocarbon treating tower. In such operations, the addition of ferric oxide to the system in accordance with this invention will greatly reduce corrosion.

Regeneration by blowing the spent caustic with an oxygen-containing gas, wherein temperatures of F. and higher are encountered and hot caustic must be circulated by pumps and conduits, comprises another process wherein the present invention may be practiced to reduce corrosion.

The present invention may also be practiced by using separate compositions comprising the inhibitor dissolved or suspended in a suitable vehicle, which compositions may be added to the corrosive alkali metal hydroxide solutions to be inhibited. Suitable vehicles for this purpose may comprise a solvent or suspending medium, as water, oil, aqueous alkali, and portions of the corrosive alkali solution, as the Mercapsol solution. Such soluticns of ferric oxide and/or alkali metal ferrites in a vehicle may be prepared in concentrations designed to attain, when added to the corrosive alkali, sufficient inhibitor therein to function in mitigating corrosion.

The rate of corrosion of ferrous metals by alkali solutions increases almost directly with increase in temperature and is most severe at temperatures of 250 F. or higher. However, the invention is not to be limited by any temperature range since caustic solutions exhibit corrosivity at low temperatures, that is, 60 to F., depending on the rate of flow of the caustic through the equipment. In ordinary circulatory systems where caustic solutions are transported from tanks to treating towers and recycling is practiced, the rate of flow of the solutions within the tanks heating coils, heat exchangers, pumps, and conduits will range from 0 feet per second to 20 to 25 feet per second or higher. The volume rate of feed of caustic solutions through a given system may be as high as 50 to 100 barrels per hour depending on the size of conduit used and the capacity of the system. The degree of turbulence and impingement of caustic against the metal surfaces will vary throughout the system depending on the flow characteristics of the conduits. Since the inhibitors of the present invention are operative under static conditions and shown to be more effective where the degree of turbulence and corrosion are higher, no upper limit need be set on the rate of flow.

The invention is not to be limited by any theories expressed or implied herein. It may be postulated, however, that several influences or theoretical reactions may be operating singly or in concert to produce the results observed. One contributing cause of the corrosion may be small amounts of oxygen present in the alkali solutions, as evidenced by the reduction in corrosion found in test 14 (Table II) conducted in an atmosphere of nitrogen. Some nickel alloys seem to increase in their rate of corrosion upon the exclusion of oxygen which would suggest the presence of a protective film of nickel oxide as a reactant. The corrosion may be caused by a galvanic action which is upset by the presence of the novel inhibitor'of the present invention due to a change in the nature of the scale formation. The influence of hydrogen embrittlement in the presence of sulfides in solution must not be overlooked. The ferric oxide may act to screen the hydrogen ions from the metal or even prevent their formation.

It has been stated that the solution of ferric oxide in alkali produces alkali metal ferrites, and therefore alkali metal ferrites are included as inhibitors herein. Certain alkali metal ferrites decompose in water and are existent in stable form, mainly in strong alkali metal hydroxide solutions, i. e., concentration of alkali of 40 per cent or more. If decomposition of a ferrite solution takes place, there is formed hydrated ferric oxide or ferric hydroxide. Consequently, the invention is not limited to the use of ferric oxide and/or alkali metal ferrites per se, and includes operable concentrations of hydrated ferric oxide which may form from such decomposition and also is directed to those solutions of alkali metal ferrites which are soluble in concentrated alkali Without decomposition under the conditions imposing in the system being treated.

What is claimed is:

1. In the method of regenerating spent aqueous alkali solutions which have been usedto extract sulfur compounds from hydrocarbon oils without substantial corrosion of ferrous equipment in contact with said solutions, said alkali solutions containing about to about 50% by weight of caustic alkali and containing phenolic organic material, but being substantially free of alkali metal sulfides, the step comprising, incorporating about 0.04 to 0.6% by weight of an inhibiting material of the group consisting of ferric oxide and alkali metal ferrites in said alkali solution throughout said regeneration as a corrosion inhibitor.

2. The method in accordance with claim 1 in which the ferrous metal surface being protected is flange iron 0.42 manganese, and small percentages of sulfur and phosphorus.

3. In the method of regenerating spent aqueous alkali solutions used to extract sulfur compounds from hydrocarbon oils wherein said solutions are subjected to heating temperatures ranging from about 212 to 250 F. in contact with ferrous metal surfaces under conditions conducive to corrosion of said surfaces, the improvement comprising maintaining about 0.04 to 0.6 percent by weight of an inhibiting material of the group consisting of ferric oxide and alkali metal ferrite in said alkali solution throughout said regeneration to prevent corrosion of said metal surfaces.

4. The method in accordance with claim 3 in which the inhibiting material is ferric oxide in a concentration of about 0.18 percent by weight and said ferrous metal surface is flange iron.

5. The method in accordance with claim 3 in which the alkali metal ferrite is sodium ferrite.

6. The method in accordance with claim 3 in which the alkali metal ferrite is potassium ferrite.

7. The method in accordance with'claim 3 in which the alkali metal ferrite is lithium ferrite.

8. The method in accordance with claim 3 in which the alkali solution contains about 19.3 percent by weight of sodium hydroxide, 10 percent cresols, 15 percent naphthenic acids, and 55.7 percent water.

References Cited in the file of this patent UNITED STATES PATENTS 1,883,211 Wilson Oct. 18, 1932 2,132,585 Spittle Oct. 11, 1938 2,415,798 Pye et a1 Feb. 11, 1947 2,556,387 Ayers June 12, 1951 OTHER REFERENCES ment, published in Chem. and Metallurgical Chem.

Sept. 1944, page 125.

Claims (1)

1. IN THE METHOD OF REGENERATING SPENT AQUEOUS ALKALI SOLUTIONS WHICH HAVE BEEN USED TO EXTRACT SULFUR COMPOUNDS FROM HYDROCARBON OILS WITHOUT SUBSTANTIAL CORROSION OF FERROUS EQUIPMENT IN CONTACT WITH SAID SOLUTIONS, SAID ALKALI SOLUTIONS CONTAINING ABOUT 5% TO ABOUT 50% BY WEIGHT OF CAUSTIC ALKALI AND CONTAINING PHENOLIC ORGANIC MATERIAL, BUT BEING SUBSTANTIALLY FREE OF ALKALI METAL SULFIDES, THE STEP COMPRISING, INCORPORATING ABOUT 0.04 TO 0.6% BY WEIGHT OF AN INHIBITING MATERIAL OF THE GROUP CONSISTING OF FERRIC OXIDE AND ALKALI METAL FERRITES IN SAID ALKALI SOLUTION THROUGHOUT SAID REGENERATION AS A CORROSION INHIBITOR.

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