US3146208A

US3146208A - Corrosion inhibition

Info

Publication number: US3146208A
Application number: US79183A
Authority: US
Inventors: Fisher Arthur Orman
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1960-12-29
Filing date: 1960-12-29
Publication date: 1964-08-25
Anticipated expiration: 1981-08-25

Description

3,146,208 CORROSION INHIBITION Arthur ()rman Fisher, Richmond Heights, Mo, assignor to Monsanto Company, a corporation of Delaware No Drawing. Filed Dec. 29, 1960, Ser. No. 79,183 15 Claims. (Cl. 252-147) This invention relates to novel compositions and methods useful for inhibiting the corrosion of metals by acids. More specifically, this invention relates to methods for inhibiting acidic corrosion of metals, which methods utilize dialkylphosphorothioic acids and their salts and to novel compositions containing dialkylphosphorothioic acids and/ or their salts, plus certain ingredients that dramatically enhance the abilities of the dialkylphosphorothioic acids (and salts) to inhibit the acidic corrosion of metals.

The above, as Well as other objects of the invention, are achieved through the use of one or more 0,0-dialkyl phosphoro mono-(or di-)thioic acids and/or certain salts thereof, either with or without the additional presence (in the acidic composition being inhibited) of either (1) an inorganic iodide or bromide, (2) a quaternary ammonium compound, or (3) a combination of (l) and (2), above.

For the sake of simplicity and convenience, these 0,0- dialkyl phosphoro monoand di-thioic acids will hereafter be referred to as thioic acids.

The thioic acids which can be used in the practice of this invention are those which have the formula:

wherein R and R are either like or unlike, branchedchain or straight-chain saturated hydrocarbyl radicals having from 4 to 18 carbon atoms per radical or cyclohexyl radicals, and A and B are either (oxygen) or S (sulfur) radicals, and at least one of A or B is a sulfur radical. For consistently superior results when practicing this invention, R and R are saturated aliphatic hydrocarbyl radicals having between 4 and 10 carbon atoms per radical, and preferably between 4 and 6 carbon atoms per radical. For example, in the embodiments of the invention which are particularly preferred, R and R can be butyl, amyl, hexyl, isobutyl, tertiary butyl, isoamyl, and the like. Additionally, optimum inhibitory results can be attained when both R and R are like (that is, they are essentially identical). Apparently the only exception to the above statement that R and R should be aliphatic is when either R or R (or both R and R is cyclohexyl, such as, for example, in the compounds 0,0-dicyclohexyl phosphorodithioic acid, O-cyclohexyl, O-butyl phosphorodithioic acid, 0,0-dicyclohexyl phosphorothioic acid, etc. The thioic acids contemplated by this invention are well known in the art, and have been prepared in several ways, as, for example, by reaction of the appropriate alcohol or alcohols with P 8 according to the procedure of Kosolapoff, or by disproportionation of the neutral trialkylthioor dithiophosphate esters which, in turn, can be prepared by reacting phosphorus sulfachloride with the alcohol in the presence of pyridine or with the appropriate alcoholates. Ordinarily the salts of the thioic acids can be prepared by simple neutralization by an appropriate base. However, the ammonium and amine salts can probably be prepared more inexpensively by the reaction of a dialkylphosphite with sulfur 1 Kosolapoft, G. M. Organophosphorus Compounds, pp. 236-7, John Wiley & Sons, NY. (1950).

"ice

and the appropriate amine according to the procedure of Jonas, US. 2,647,140.

Although any of the metallic or amine salts of the thioic acids described above can be utilized to inhibit the acidic corrosion of metals contemplated by this invention, there are some rather large variations in the over-all effectiveness within this general class. Examples of useful metallic salts are cadmium, cuprous, cupric, ferric, mercuric, aluminum, vanadium, nickel and chromium salts. Alkaline-earth metal salts such as calcium, magnesium and barium can also be used, as can the alkali metal salts such as sodium, potassium, lithium, rubidium and ammonium. Of these, the alkaline-earth metal salts and alkali metal salts are generally more effective than are the other metallic salts. However, the most effective class of the salts of the thioic acids described above are the organic aliphatic primary, secondary and tertiary amine salts, the amine portion of which can be represented structurally as:

wherein R is an aliphatic hydrocarbyl radical having between 0 and 25 carbon atoms, and preferably between 4 and 18 carbon atoms, in its chain (and can be either saturated or unsaturated, branchedor straight-chain without any apparent detrimental effect upon the performance of the compound as an inhibitor of metallic corrosion caused by acids), while R and R are saturated aliphatic hydrocarbyl radicals having between 0 and 6 carbon atoms, and preferably between 0 and 3 carbon atoms, in their chains.

Besides making possible more effective corrosion inhibition than the corresponding metal salts of the thioic acids contemplated by this invention, the amine salts make possible the utilization of some thioic acids that ordinarily are ineffective in the inhibition of acidic corrosion. For example, when 0,0-diethyl phosphorodithioic acid is utilized in its acid form or as one of its metallic salts at 0.1 weight percent concentration as the only inhibitor component in phosphoric acid, it does not noticeably inhibit corrosion. However, when this same thioic acid is utilized in the form of one of the appropriate amine salts (such as, for example, the dodecylamine or octadecenylamine salts), under otherwise similar circumstances, the degree of corrosion inhibition is remarkably high. Therefore, when the appropriate amine salts (described above) of the thioic acids are utilized to inhibit corrosion as contemplated by this invention, the class of effective thioic acids becomes somewhat broader than in those corresponding instances Where the thioic acids are utilized in their acid form or as one of their metallic salts (or even as their ammonium salts). For example, when the appropriate amine salts of the thioic acids are utilized in the practice of this invention, the thioic acids which can be used effectively (as the appropriate amine salts) are those represented by the structural formula:

where R, and R are either like or unlike branched-chain or straight-chain hydrocarbyl radicals having from 1 to 18, and preferably between 1 and 10, carbon atoms per radical, and A and B are either oxygen or sulfur radicals (at least one of A or B being a sulfur radical). For optimum results, R and R should have between 2 and 6 carbon atoms per hydrocarbyl radical. Thus, it can be seen that, in the application of thioic acids to inhibit the acidic corrosion of metals, their application as one of the amine salts described above (rather than as the acids themselves or as one 'of their metallic salts), makes possible the utilization of the 0,0-dimethyl-, 0,0-diethyl-, O-ethyl, O-methyl-phosphoro monoand di-thioic acids, whereas, otherwise, these particular compounds (in the absence of these amines) do not function in the desired manner at all.

One particularly surprising aspect of the embodiment of this invention involving the use of the above-described amine salts of thioic acids is that for some unexplained reason the inhibitory capacity of the amine salt, per se, is substantially greater than is the Case when the thioic acid and the amine are added to the acidic compositions separately. This is surprising because in the acidic medium (particularly a liquid system which contains some water, as do the great majority of the acidic compositions that'are inhibited by this invention) one would normally expect an amine salt to ionize and thereby have the same effect as if the acid and amine were added separately to the acidic composition. Therefore, it must be concluded that the amine salts of this invention should preferably be preformed or formulated as the amine salt, per se, before it is added to, or combined with, the acid or the acidic composition, the metallic corrosion of which is to be inhibited. The preparation of such amine salts as those contemplated herein is a simple matter, well Within the skill of anyone normally skilled in the art. Most simply (in addition to the procedure described heretofore), approximately equal-molar amounts of the desired thioic acid and amine are intermixed in any suitable conventional manner, thus yielding the appropriate amine salt. The evolution of heat during the aforesaid mixing or blending operation evidences that the reaction is proceeding. The presence of a small amount of water in the mixture of thioic acid and amine usually speeds up the salt formation reaction. In the preparation of the amine salt (whatever procedure has been utilized in its preparation), one ordinarily neednt be too careful that exactly equal-molar quantities of amine and acid are mixed, because the presence of excess amine or excess thioic acid in the amine salt composition when it is added to the acidic composition being inhibited has been found to have no noticeably detrimental effect whatever on the beneficial performance of the appropriate thioic acid-amine salts as corrosion inhibitors.

The amounts of the thioic acids and their metallic and amine salts which are used to inhibit the acidic corrosion of metals according to the practice of this invention are dependent to some extent upon the acid being inhibited, its concentration, and the metal to be protected, as Well as whether or not one is utilizing one of the amine salts of these thioic acids (as compared with utilizing one of the acids per se or one of their metallic salts). As a general rule, the thioic acids and their salts will be used at effective concentrations ranging between about 0.002 to about 4 weight percent, and preferably between about 0.005 and about 1 weight percent of the acidic compositions being inhibited. Generally, for a given level of protection of the metals somewhat smaller amounts of the inhibitor are necessary when the amine salts are utilized. Again, generally, it is well known that higher levels of inhibitor are required to inhibit the corrosion of the more dilute acidic compositions (such as 20% H P H 80 10% HCl, etc.) than would be required to inhibit corrosion caused by the more concentrated acids (i.e., 75% H PO 65% H 80 37% HCI, etc.).

It has been found that the inhibitory benefits of this invention will accrue at most concentrations of the acids contemplated and at most temperatures. Of course, limitations that are logical and will ordinarily be applied by those skilled in the art are to be observed. For example, since two of the three basic ingredients in the inhibitor compositions of this invention are organic in nature, conditions that are ordinarily destructive to organic compounds (i.e., extremely high temperatures, combined with very high acid concentrations) are generally to be avoided in the practice of this invention.

The actual blending of the appropriate inhibitor with the acidic composition which is to be inhibited can be accomplished by any of a number of conventional procedures. The only requisite in such a blending operation is that the inhibitor be well distributed through the acidic composition and dissolved therein. Usually simply stirring (with nominal agitation) the mixture of acid plus inhibitor for 510 minutes is sufiicient to assure adequate protection.

EXAMPLE I In order to demonstrate the effectiveness of the thioic acids as corrosion inhibitors, the following comparisons were made:

The corrosion resistance of S.A.E. type 1010 mild steel to 75% phosphoric acid at F., was observed and measured after the steel was exposed to the acid both without and with some of the thioic acids and thioic acid salts contemplated by this invention. Data for these measurements are listed in Table 1, below. Corrosion in mils per year was calculated from weight lost by pieces of the metal after 72 hours of exposure to the 75% H PO containing, for example, 0.1% by weight of 0,0- bis(1,3-dimethylbutyl)phosphorodithioic acid and some of its salts. Similarly other thioic acids and their salts were tested, the data from the inhibited acid compositions being tabulated below. Note that, as a rule, the metallic and ammonium salts are inferior to their respective acids (possibly because of the dilution effect of the cation), while the amine salts are generally superior to their corresponding acids in this test.

Table 1 CORROSION IN 75% HaPOs AT 120 F.

Corrosion in 72 hours Inhibitor: (m.p.y.)

Control (no inhibitor) 3095 (a) Dithioic acids- 0,0 bis(1,3-dimethylbutyl)phosphorodithioic acid 10.1 0,0 (2-ethylhexyl)isopropyl phosphorodithioic acid 400 0,0 bis(Z-ethylhexyl)phosphorodithioic acid 45 0,0-bis(dodecyl)phosphorodithioic acid 800 0,0-amylbutyl phosphorodithioic acid 70 0,0-bis(amyl)phosphorodithioic acid 34 0,0-bis(decyl)phosphorodithioic acid 18.5 0,0-bis(isoamyl)phosphorodithioic acid- 57.5 0,0 bis(hexadecyl)phosphorodithioic cid 450 (b) Monothioic acids 0,0 bis(1,3 dimethylbutyl)phosphorothioic acid 8.7 0,0 bis(2 ethylhexyl)phosphorothioic acid 37 0,0 (2-ethylhexyl)isopropyl phosphorothioic acid 320 0,0-bis(hexyl)phosphorothioic acid 14.3 0,0-bis(decyl)phosphorothioic acid 32.7 0,0-bis(hexadecyl)phosphorothioic acid 775 0,0-isoamyl butyl phosphorothioic acid 68 (c) Metallic salts and ammonium Potassium 0,0 bis(dioctyl)phosphorodithioate 7.8 Zinc 0,0 bis(1,3 dimethylbutyl)phosphorodithioate 7.7 Zinc 0,0 bis(tridecyl)phosphorodithioate 46 Sodium 0,0 bis(1,3-dirnethylbutyl)phosphorothioate 73.5

1 36 hours.

T able 1-Continued Corrosion in 72 hours InhibitorContinued (m.p.y.)

(0) Metallic salts and ammoniumContinued Magnesium O-O bis(1,3 dimethylbutyl) phosphorodithioate Magnesium 0,0 bis(1,3 dimethylbutyl) phosphorothioate Ammonium 0,0 bis(1,3-dimethylbutyl) phosphorodithioate Ammonium 0,0 bis(l,3-dimethylbutyl) phosphorothioate Amine salts 0,0 bis(l,3 dimethylbutyl)phosphorodithioic acid, monooctadecenylamine salt 0,0 bis(l,3 dimethylbutyl)phosphorodithioic acid, monoclodecylamine salt 0,0 bis(1,3 dimethylbutyl)phosphorodithioic acid, methyloctadecylamine salt 0,0 bis(1,3 dimethylbutyl)phosphorodithioic acid, dimethyl octadecylamine salt 0,0 bis(l,3 dimethylbutyl)phosphorothioic acid, monooctadecenylamine salt 0,0 bis(1,3 dimethylbutyl)phosph0rothioic acid, monooctadecylamine salt 0,0 bis(2 ethylhexyl)phosphorothioic acid, mono-ndodecylamine salt 0,0 diethyl phosphorodithioic acid, octadecylamine salt 0,0 dipropyl phosphorodithioic acid, octadecenylamine salt 0,0 dipropyl phosphorothioic acid, octadecenylamine salt 0,0-bis(amyl)phosphorodithioic acid, N-

ethyloctylamine salt 0,0 bis(isoamyl)phosphorothioic acid, N,N-diethyl dodecylamine salt 0,0 diisopropyl phosphorodithioic acid,

octadecyl amine salt 37 It has also been found that the remarkable corrosion inhibition of the thioic acids and the salts of thioic acids of this invention can be even further enhanced by the addition to the inhibited acidic compositions of (1) an inorganic iodide or bromide salt and/ or (2) a quarternary ammonium compound.

Stated broadly, in terms of the over-all effectiveness of the corrosion inhibiting compositions of this invention, the thioic acids and their metallic and ammonium salts of this invention are effective inhibitors, but are usually not quite as effective as certain of their amine salts. Thus, the best performers of the class, which includes both thioic acids and thioic acid salts, are the amine salts. If one Wishes to further enhance the efiectiveness of these thioic acids and thioic acid salts, it has now been found that he can add a small amount of either an inorganic iodide or bromide salt or a quaternary ammonium compound. If he desires to achieve optimum results, he can use a small amount of both the inorganic iodide or bromide salt and the quaternary ammonium compound.

Although certain of the quaternary ammonium compounds have heretofore been known to possess some corrosion inhibiting properties, the excellent, extremely high degrees of protection from acidic corrosion which is afforded the metals which are ordinarily corroded by acids, according to this invention, is surprising because it is apparently due to an unexpected two and three-component synergism or beneficial interaction with the thioic acids or thioic acid salts. This interaction or synergism could not have been foretold from the actions in the acidic systems of any of the individual ingredients which, when combined, make up the binary and trinary inhibitor compositions of this invention. Example II, below, demonstrates the unexpectedly beneficial interaction described above with one of the preferred compositions of this invention. While only one composition is illustrated in the following example, it will be understood that the effects demonstrated therein can be observed with any of the twoor three-component compositions contemplated by this invention.

EXAMPLE II The compounds listed below were dissolved in 75% H 1 0, at the various (Weight percent) concentrations described. This resulting inhibited acidic compositions were then warmed to and held at F. Mild steel, SAE type 1010, was then exposed to these solutions for 72 hours. The corrosion of the acid containing these compositions or compounds was compared with that of uninhibited 75% H PO under similar conditions by determining the weight lost during the exposure by pieces of the steel, calculated in mils per year (m.p.y.) corrosion.

Table 2 SYNERGISTIC CORROSION INHIBITION- Inhibitor: 72 hour corrosion (m.p.y.) Control (no inhibitor) 3,095 0.2% Kl 16.4 0.05 Kl 14.1 0.01% Kl 25.8 0.005% KI 77.1 0.1 quaternary A 2 4.5 0.05% quaternary A 2 23.0 0.01% quaternary A 2 47.5 0.2% thioic acid D 3 8.5 0.1% thioic acid D 3 10.1 0.05% thioic acid D 1,000

0.01% thioic acid D 16,000

0.1% thioic acid D --{-0.005% KI 4.0 0.005% thioic acid D +0.005% KI 9.3 0.05 thioic acid D +0.05% quaternary 1 In 75% H3PO4 at 120 F.

Dodecenyl trimethyl ammonium chloride.

3 0,0-bis(l,3 dimethy1buty1) phosphorodithioic acid.

Almost any material can be present in the acidic compositions which are inhibited by the compositions and processes-of this invention without any noticeably deleterious effect on their excellent over-all eifectiveness in inhibiting the acidic corrosion of metals. For example, the corrosion of phosphoric acid containing practically any of the impurities often found therein (such as polyphosphoric acid, ammonium phosphate, halide, sulfate, etc.) as Well as any of the usual acidic compositions containing phosphoric acid (such as fertilizer compositions, and other phosphatic solutions, etc.) can be readily inhibited by practicing this invention. The metallic corrosion caused by solutions of other nonoxidizing acids, such as hydrochloric, acetic, benzoic, butyn'c, maleic, oxalic, phosphorus, propionic, pyrophosphoric, tartaric, chloroacetic, alum, and the like, either with or without most impurities, as well as in the presence of almost any other chemical compound, can be similarly inhibited by practicing the invention. There is only one notable exception to this very Wide applicability of the present invention. That is that this invention is ineffective in the presence of more than about 1 weight percent of an oxidizing acid such as, for example, nitric, perchloric, bromic, hypobromous, iodic, hypochlorous, etc. An oxidizing acid is Well known to be an acid which can be very readily reduced by organic materials in an acidic medium.

The metals which are protected from acidic corrosion by the compositions of this invention are all of those which are normally corroded by nonoxidizing acids, and which are nongraphitic, that is, do not contain an appre ciable amount of free carbon in the graphite form. For example, nickel, ferruginous metals (except for cast iron) such as cast steel, Wrought iron, mild steels, certain of the various so-called stainless steels which are corroded by some nonoxidizing acids (e.g., type 316 stainless steel is attacked by hydrochloric acid under some conditions), Monel metal, the Hastelloys (containing nickel, molybdenum, sometimes chromium, etc.) and the like, are generally protected from acidic corrosion by the compositions of this invention. The reason that cast iron is not protected is believed to be that the presence of graphite flakelets act as focal points for localized cathodic corrosion. These focal points of corrosion apparently cause areas adjacent to them to corrode more readily and thus defeat the effect of the inhibitor composition. Apparently, in the absence of graphitic flakelets, as in steels, wrought iron, etc., the inhibitor can prevent the formation of most of the focal points of corrosion which ordinarily would have been formed in the absence of the inhibitor composition.

Although the class of iodide and bromide salts generally is applicable with respect to the present invention, there are rather marked variations in effectiveness within the general class. Examples of typically useful salts are cadmium, zinc, cupric, aluminum, and mercuric salts. Alkaline-earth metal salts such as calcium, magnesium, and barium can also be used. Alkali metal salts such as sodium, potassium, lithium and ammonium are useful as well. The iodides generally are somewhat more effective than are the corresponding bromides. Particularly preferred iodides are potassium iodide and magnesium iodide.

The amounts of inorganic iodides or bromides, which are used to enhance the inhibitory effectiveness of the thioic acids and their salts according to the practice of this invention, are, as a general rule, within the range of from about 0.0005 weight percent to about 1.0 weight percent of the acid composition, the corrosion of which is being inhibited. These weight percentages are calculated on the basis of the iodide or bromide ion portion of the particular salt utilized. For best results, these inorganic iodides and bromides are used at concentrations from about 0.002 weight percent to about 0.2 weight percent of the said acid composition. Generally, considering only the corrosion inhibiting composition (i.e., the combination of thioic acid and/ or thioic acid salt plus the inorganic iodide or bromide salt), the thioic acid (or salt) will be present in by far the greater amount, the weight ratio of thioic acid (or salt) to the inorganic iodide or bromide salt being as high as about 2011 or even higher. However, effective synergism between these components can be observed at weight ratios as low as 1:1, with the effective ratios between about 3:1 and about :1 being preferred.

Although any quaternary ammonium compound that is soluble in the acid system which is being inhibited in the extent of at least about 0.02 weight percent can be used in the practice of this invention, the tetraalkyl ammonium halides, sulfates, acetates, etc., which are particularly preferred, are represented structurally as:

wherein R is an aliphatic hydrocarbyl radical having between 6 and 30 carbon atoms, and preferably between 8 and 20 carbon atoms; R is hydrogen or a saturated aliphatic hydrocarbyl radical having between 1 and 16 carbon atoms, and preferably between 1 and 13 carbon atoms; R is hydrogen or a saturated aliphatic hydrocarbyl radical having between 1 and 15 carbon atoms, and preferably between 1 and 8 carbon atoms; and R is hydrogen or a saturated aliphatic lower hydrocarbyl radical having between 1 and 4 carbon atoms, such as methyl-, ethyl-, propyl-, isopropyl-, butyl-, and isobutyl-; and X is an anion' derived from a nonoxidizing acid, such as, for example,

a halogen (i.e., chloride, bromide, iodide), a mineral acid (i.e., phosphate, sulfate, etc.), acetate, and the like. These compounds perform equally well Whether R is saturated or unsaturated, branchedor straight-chained. Note that R can be described more simply as a saturated aliphatic hydrocarbyl radical having the empirical formula (C I-1 +H, where n is a whole number from 0 to 16 and preferably from 0 to 13. R can be described similarly as a saturated aliphatic hydrocarbyl radical having the empirical formula (C I-I +H, where b is a whole number from 0 to 15, and preferably from 0 to 8. Similarly, R, can be described as a saturated, aliphatic lower hydrocarbyl radical having the empirical formula (C H )+H, where k is a whole number from 0 to 4. For example, especially fine performance in the inhibition of the corrosion of cold rolled steel by phosphoric acid was observed when the only quaternary ammonium compound in the inhibiting composition was didodecyl dimethyl ammonium chloride. Other examples of the preferred quaternary ammonium compounds are trioctylmethyl ammonium chloride, dodecyltrimethyl ammonium chloride, dodecyltrimethyl ammonium acetate, dodecyl ammonium acetate, didodecyldimethyl ammonium iodide, dodecylbenzyldimethyl ammonium chloride, hexadecylbenzyldimethyl ammonium iodide, hexadecylbenzyldimethyl ammonium sulfate, tridodecylmethyl ammonium chloride, cetyltrimethyl ammonium stearate, cetyltrimethyl ammonium chloride, cetyltrimethyl ammonium bromide, cetylethyldimethyl ammonium chloride, cetylethyldimethyl ammonium bromide, cetylbenzyldimethyl ammonium chloride, stearyltrimethyl ammonium sulfate, docosaneethyldimethyl ammonium iodide, octadecenyldodecenyldimethyl ammonium iodide, and the like.

The actual amounts of the above-described quaternary ammonium compounds which can be used to enhance the inhibitory effectiveness of the thioic acids and thioic acid salts, according to this invention, are dependent, to some extent, upon both the acid being inhibited and the metal to be protected. For noticeably improved results, the concentration of the quaternary ammonium compound (based upon the total weight of the acidic composition being inhibited) can be varied between about 0.002 to about 4 weight percent, and preferably between about 0.005 and about 1 weight percent. For best results, the concentration of the quaternary ammonium compound will vary from about /2 to about 1% times the concentration of the thioic acid or thioic acid salt in the inhibited acid composition.

Since quite often the ingredients for the enhanced inhibitor compositions contemplated by this invention (i.e., the particular thioic acid or thioic acid salt plus the inorganic iodide or bromide and/or the quaternary compound) are more readily available in bulk at one location, while the acids to be inhibited will probably be manufactured and/or utilized at one or more different locations, it would usually be desirable to prepare the actual inhibitor composition (which will be subsequently used to inhibit the acid) at the location close to the source of the inhibitors raw materials, then ship the inhibitor composition as a concentrate ready for use to any location at which there is a need for such extremely effective corrosion-inhibiting compositions. Such ready for use inhibitor concentrates are easy to use, would be either readily available at, or easily shipped to, any location, and are consequently preferred embodiments of this invention.

Since inorganic iodide or bromide salts are usually not soluble in the thioic acids, the thioic acid salts or the quaternary ammonium compounds contemplated by this invention, uniform blends of the inhibitor concentrates described above, which will maintain their uniformity for any extended period of time, cannot readily be attained by simply mixing the individual ingredients together. As a result, some difficulty is encountered when concentrates containing the inorganic iodide or bromide salts are utilized (due to the settling out of the inorganic iodide or bromide salts). Similarly, certain mixtures of the thioic acids plus some of the quaternary ammonium compounds contemplated by this invention do not readily form uniform blends that maintain their uniformity for an extended period of time. It has been found, however, that these difficulties can be precluded if these preferred inhibitor concentrates are combined with between about 10 weight percent and about 90 weight percent of the type of acid whose corrosion is eventually to be inhibited by the inhibitor composition being prepared. The resulting blends are homogeneous and will not separate into their component ingredients under normal conditions of storage and handling. Therefore, the homogeneous blends of up to about 90 (or even higher if desired) weight percent of the acid to be inhibited with the following classes of inhibitor concentrates constitute particularly preferred embodiments of this invention:

(1) A thioic acid and/or a thioic acid salt;

(2) A thioic acid plus an inorganic iodide or bromide;

(3) A thioic acid plus a quaternary ammonium compound;

(4) A thioic acid plus an inorganic iodide or bromide plus a quaternary ammonium compound;

(5) A metallic, ammonium, or amine salt of a thioic acid plus an inorganic iodide or bromide;

(6) A metallic, ammonium, or amine salt of a thioic acid plus a quaternary ammonium compound;

(7) A metallic, ammonium, or amine salt of a thioic acid plus an inorganic iodide or bromide plus a quaternary ammonium compound.

These particularly preferred homogeneous concentrated inhibitor blends can ordinarily be prepared simply by physically mixing the individual ingredients in any desired order by any of the usual conventional blending techniques. It should be noted that the 90 weight percent figure is offered, above, merely as a guide to define an area which is believed to be merely a practical limitation, from a shipping cost standpoint, upon the amount of acid which will ordinarily be utilized for the preparation of the homogeneous concentrate. This figure should not be construed as a strict functional limitation, since, clearly, homogeneous inhibitor concentrates within the scope of this invention can be prepared which contain considerably higher proportions of acid than those suggested above.

Generally, in these particularly preferred embodiments (the homogeneous concentrates) the ratios of iodide or bromide salt to thioic acid and/or quaternary ammonium compound will be the same as those which will be utilized for the inhibition of the acidic corrosion, described hereinbefore. In addition, the concentration of the acid in the homogeneous inhibitor concentrates can vary considerably. Generally, however, at least about 10 weight percent, and usually less than about 75 weight percent (and preferably between about 20 and about 60 weight percent) of the acid to be inhibited, calculated on the basis of the free acid itself, will be utilized. Water can be present in the concentrated homogeneous inhibitor systems to any desired extend (and will sometimes be used to facilitate the solubilization of the inorganic iodide or bromide salt in the concentrate), although concentrate compositions containing as little Water as possible are usually more desirable, because, compositions containing only a very small amount of water (or other diluents described below) can be used to inhibit the corrosion caused by highly concentrated acids without unduly contributing to their dilution. Materials other than acids and water can also be used to convert the ordinarily nonhomogeneous concentrated inhibitor composition of this invention into the homogeneous state-usually without any adverse effect whatever upon the over-all ultimate performance of the inhibitor composition. For example, a lower alcohol, such as methyl, ethyl, isopropyl, and the like, with water, can be blended with the basic classes of inhibitor ingredients with which this invention is concerned at concentrations of up to about 30 weight percent (of alcohol) of the total homogeneous inhibitor concentrate. Usually more than 50 weight percent of the alcohol-Water mixtures utilized in this manner is water. Acetone can also be used in a manner similar to the lower alcohols, as can methyl ethyl ketone, diethylketone, methyl, isopropyl ketone, and the like, also.

In spite of the fact that the above-described inhibitor concentrates considerably simplify the task of supplying high quality corrosion inhibitors to those who need them at various locations, times, etc., it is even more desirable that an inhibitor concentrate be available which can inhibit acidic corrosion of the nongraphitic metals in as many different acids as possible. With this goal in mind, it was found that the presence of a very small amount of phosphoric acid in almost any other acid is practically innocuous in almost every respect, while the presence of almost any of the other nonoxidizing acids is usually somewhat less widely desirable. Therefore, it is with phosphoric acid that another particularly preferred, widely applicable, embodiment of this invention is prepared. This particularly preferred homogeneous inhibitor concentrate should generally contain between about 20 and about 80, and preferably between about 35 and about 65 weight percent of phosphoric acid, usually less than about 30 weight percent of water, sometimes a small amount of a lower alcohol or ketone, and the remainder one of the preferred inhibitor compositions (i.e., one of the six synergistic inhibitor compositions described above as preferred embodiments of this invention).

A few examples of the particularly preferred homogeneous inhibitor concentrates, which can be prepared as follows, are given below. (All parts are by weight unless otherwise specified.)

(1) Thioic acid+inorganic iodide or bromide:

(a) To 50 parts of 30% H add 45 parts of 0,0-bis(1,3-dimethylbutyl) phosphorodithioic acid plus 5 parts of potassium iodide. Mix well (usually for about 5 minutes) until the resulting blend becomes a homogeneous liquid. Add a small quantity of ethyl alcohol or acetone to lower the viscosity if desired.

(b) To 60 parts of 75% H PO add 36 parts of 0,0-bis(dodecyl) phosphorodithioic acid and 4 parts of magnesium iodide. Stir the resulting blend until it forms a clear, homogeneous liquid. Add up to 20 parts of isopropyl alcohol to increase the rate of formation of the homogeneous system.

(c) To 30 parts of 20% P101, add 60 parts of 0,0-

bis(2-ethylhexyl) phosphorothioic acid plus 5 parts of sodium bromide plus 5 parts of methyl alcohol. Stir well.

(2) Thioic acid+quaternary ammonium compound:

(a) To 30 parts of 75% H PO add 38 parts of 0,0-bis(1,3-dimethylbutyl) phosphorodithioic acid plus 32 parts of octadecenyl trimethyl ammonium chloride. Stir well until a homogenous liquid is formed. Add additional acid, water, or a lower alcohol if desired in order to decrease the viscosity of the resulting homogeneous concentrate.

(b) To 50 parts of 45% H 50 add 50 parts of 0,0-isoamyl butyl phosphorothioic acid plus 40 parts of cetyl ethyl dimethyl ammonium bromide. Mix well until the resulting blend becomes clear and homogeneous.

(3) Thioic acid+inorganic iodide or bromide+quaternary ammonium compound:

(a) To 50 parts of H PO add 25 parts of 0,0-bis 1,3 -dimethylbutyl) phosphorodithioic acid plus 5 parts of potassium iodide plus 20 parts of hexadecyl benzyl dimethyl ammonium chloride. Stir well, and add water and/or a lower alcohol, such as, for example, ethyl alcohol, to reduce viscosity and speed up the KI dissolution, as desired.

(b) To 60 parts of 35% I-ICl, add 17 parts of 0,0-

bis(2-ethylhexyl) phosphorothioic acid plus 4 parts of cupric iodide, plus 19 parts of dodecyl trimethyl ammonium acetate. Stir well.

() To 40 parts of 60% H PO add 40 parts of 0,0-bis amyl phosphorodithioic acid plus 5 parts of potassium bromide plus 30 parts of stearyl trimethyl ammonium sulfate. Stir Well. Thin with a small amount of butyl alcohol if desired.

(d) To 50 parts of 30% H 30 add parts of 0,0-decylamyl phosphorodithioic acid plus 3 parts of potassium iodide plus 10 parts of cetyltrimethyl ammonium bromide. Stir well.

(4) Metallic, ammonium or amine salt of thioic acid rt-inorganic iodide or bromide:

(a) To 50 parts of 75% H PO add 25 parts of potassium 0,0-bis(dioctyl)phosphorodithioate plus 5 parts of potassium iodide. Stir until the blend become homogeneous. Add Water to speed up this blending process if desired.

(b) To 45 parts of 50% H PO add parts of zinc 0,0-bis( 1,3-dimethylbutyl) phosphorothioate plus 3 parts of magnesium bromide. Stir Well.

(c) To 40 parts of 40% H 80 add 30 parts of ammonium 0,0-bis( 1,3-dimethy1butyl) phosphorothioate plus 4 parts of sodium iodide. Stir well.

(d) To 50 parts of 75% H PO add 30 parts of the monooctadecenylamine salt of 0,0-bis(l,3-dimethylbutyl) phosphorodithioic acid, plus 7 parts of potassium iodide. Stir well. Add water and/or methyl alcohol to thin if'desired.

(e) To 60 parts of I101, add 15 parts of the monododecylamine salt of 0,0-bis(2-ethylhexyl) phosphorothioic acid plus 3 parts of magnesium iodide. Stir well.

(f) To 40 parts of 75 H PO add 30 parts of the monooctadecylamine salt of 0,0-diethyl phosphorodithioic acid plus 10 parts of potassium bromide. Add water to speed up the dissolution of the KBr if desired.

(g) To 50 parts of 75 H PO add 30 parts of the N-ethyl octadecylamine salt of 0,0-bis(amyl)- phosphorothionic acid plus 5 parts of potassium iodide. Stir until homogeneous.

(h) To 50 parts of 75% H PO add 30 parts of the N,N-(diethyl) dodecenylamine salt of 0,0-bis(isoamyl)phosphorodithioic acid plus 3 parts of potassium iodide. Stir well. Add isopropyl alcohol to speed up dissolution, if desired.

(5) Metallic, ammonium, or amine salt of thioic acid +quaternary ammonium compound:

(a) To 50 parts of 75% H PO add parts of potassium 0,0-bis(dihexyl)phosphorodithioate plus 20 parts octadecenyltrimethyl ammonium chloride. Stir well.

(12) To 70 parts of H 80 add 20 parts of ferric 0,0-bis(2-ethylhexyl) phosphorothioate plus 25 parts of didodecyldimethyl ammonium bromide. Stir well. Add a lower alcohol to this if desired.

(c) To 50 parts of 25% HCl, add 20 parts of ammonium 0,0-bis(n-octyl)phosphorodithioate plus 12 parts of cetyl benzyl dimethyl ammonium acetate. Stir well. I

(d) To 30 parts of 775%. H PO add 30 parts of the monooctadecenylamine salt of 0,0-bis(1,3-dimethylbutyl) phosphorodithioate plus 30 parts of octadecyl trimethyl ammonium chloride. Stir Well. Add more H PO water or isopropyl alcohol to thin, if desired.

(6) Metallic, ammonium, or amine salt of thioic acid +inorganic iodide or bromide-l-quaternary ammonium compound:

(a) To the blend described in 4(a), above, add 15 12 parts of hexadecyl octyl dimethyl ammonium bromide. Stir well.

(b) To the blend described in 4(b), above, add 20 parts of cetyltrimethyl ammonium iodide. Stir well.

(0) To the blend described in 4(0), above, add 20 parts of docosane' ethyldimethyl ammonium chloride. Stir well. Add a small amount of a lower alcohol to speed dissolution, if desired.

(d) To the blend described in 4(d), above, 'add 30 parts of octadecenyl trimethyl ammonium chloride. Stir well.

(8) To the blend described in 5 (b), above, add 2 parts of mercuric iodide. Stir well.

The amounts of such homogeneous inhibitor concentrates that are added to the acid or acidic composition (the corrosion by which is to be inhibited), Will depend upon the actual concentration of the inhibitor ingredients contained in the concentrated compositions. However, since the ratios of the various inhibitor ingredients in the concentrates will almost invariably be the same as those which are utilized to inhibit the corrosion, the correct amounts of the concentrate to be used can readily be calculated from the amount of inorganic iodide or bromide salt contained in the concentrated composition to be utilized, if one of these salts is present in the concentrate. Otherwise the concentration of the quaternary ammonium compound in the inhibitor concentrate can be used in a similar fashion. For example, the amount of a concentrate containing 5 weight percent of magnesium iodide (based upon the iodide ion content) which can be utilized can preferably be varied between about 0.04 and about 10 weight percent of the inhibited acid composition, while the amount of a concentrate containing 10 weight percent of potassium iodide which can be utilized will vary between about 0.02 and about 5 weight percent of the inhibited acid composition.

For the utilization of the aforesaid inhibitor concentrate to inhibit corrosion, they are simply blended or stirred into the appropriate acid by conventional means until they are dissolved and well dispersed through the acid.

Various materials such as foam depressors, foaming agents, inorganic and organic diluents and bases, etc. can be added to the acids being inhibited according to this invention -in any of the conventional procedures, or even with the inhibitor materials themselves, Without any apparently detrimental effect Whatever on the surprisingly beneficial corrosion inhibition of metals by the compounds and processes of this invention. The application of the particularly preferred homogeneous inhibitor concentrates contemplated by this invention (such as those illustrated immediately above) to acidic systems will be demonstrated by some of the following examples.

EXAMPLE III Into 50 gallons of 85% H PO is stirred 0.2 gallon of a homogeneous concentrate composed of 60 parts of 75% H PO 36 parts of 0,0-bis(dodecyl)phosphorodithioic acid, 4 parts of MgI, and 10 parts of isopropyl alcohol. Stirring is continued for about 5-10 minutes or until the inhibitor concentrate is thoroughly dissolved in the acid. The resulting inhibited acid is stored in SAE 1010 mild steel drums under ambient conditions for 60 days Without any deleterious corrosion of the drum. A similar drum containing uninhibited 85% H 1 0; corrodes severely in this same period of time.

EXAMPLE IV Into 50 gallons of 93% H 50 is stirred 0.4 gallon of a homogeneous concentrate containing 50 parts of 30% H 50 45 parts of 0,0-bis(1,3-dimethylbutyl) phosphorodithioic acid, 5 parts of KI, and 10 parts of ethyl alcohol. The stirring is continued for about 10 minutes. The resulting inhibited H is stored in SAE 1010 mild steel drums under ambient conditions for 60 days without severe corrosion of the drums. A similar drum containing uninhibited 93% H 80 corrodes severely when stored similarly.

EXAMPLE V Into 100 gallons of 15% by weight alum solution is stirred 0.05 gallon of the octadecenylamine salt of 0,0- bis(l,3-dimethylbutyl) phosphorodithioic acid until the thioic acid salt is completely dissolved. Mild steel, exposed to this solution at 120 F. for 72 hours, corrodes at a rate of only 3 mils per year, while mild steep exposed to an uninhibited 15% alum solution corrodes at more than 3 times this rate.

EXAMPLE VI Into 50 gallons of 50% H 80 is stirred 0.03 weight percent of 0,0-bis(2-ethylhexyl) phosphorothioic acid, 0.005 weight percent of sodium bromide, and 0.04 weight percent of oleylethyldimethyl ammonium chloride (weight percentages are based upon the weight of 50 gallons of 50% H SO until these ingredients are dissolved. The resulting inhibited acid is then stored in SAE type 1010 mild steel drums under ambient conditions for 60 days with only negligible attack on the drums metal by the acid. Uninhibited 50% H 80 stored similarly, attacks the metal severely.

EXAMPLE VII Into 100 gallons of 50% H 80 is stirred /2 gallon of a homogeneous concentrate composed of 50 parts of 75% H PO 25 parts of 0,0-bis(1,3-dimethylbutyl) phosphorodithioic acid, 20 parts of hexadecylbenzyldimethyl ammonium chloride, 10 parts of water, and 10 parts of ethyl alcohol. Stirring is continued for 10 minutes to assure homogeneity, after which the inhibited 50% H 50 is stored in SAE type 1010 mild steel drums at ambient conditions for 60 days, without any noticeable attack on the steel by the inhibited acid.

EXAMPLE VIII Into 100 gallons of 85% H PO is stirred 0.25 gallon of a homogeneous inhibitor concentrate which was prepared by blending together 50 parts of 75% H PO 20 parts of water, 30 parts of the monooctadecenylamine salt of 0,0-bis(1,3-dimethylbutyl) phosphorodithioic acid, 30 parts of octadecenyl trimethyl ammonium chlo ride, and 7 parts of potassium iodide. Stirring is continued for 5 minutes. The resulting inhibited acid is stored in SAE type 1020 mild steel drums for 120 days under ambient conditions without noticeably corroding the drums. Uninhibited 85% H PO stored similarly, corrodes the SAE type 1020 mild steel drums severely.

EXAMPLE IX The homogeneous inhibitor concentrate described in Example VIII is dissolved at the rate of 0.25 gallon of concentrate per 100 gallons of l010 (N-P-K) and 4-168 (N-P-K) liquid fertilizer solutions. Upon exposure to these inhibited solutions for 72 hours at 120 F., SAE type 1010 mild steel corrodes at the rates of and 3 mils per year respectively. If the solutions had been uninhibited, the corrosion rates would have been 79 and 54 mils per year, respectively (other conditions being the same).

What is claimed is:

1. A method of inhibiting the natural tendency of a nonoxidizing aqueous acidic composition to corrode nongraphitic metal, which method comprises dissolving in said acid between about 0.002 and about 4 weight percent of an amine salt of a dialkylphosphorothioic acid, said salt being one of an amine having the structure:

wherein R is an aliphatic hydrocarbyl radical having wherein R and R are saturated hydrocarbyl radicals having from 1 to 18 carbon atoms per radical, and A and Y B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulfur radical, and contacting said metal with the resulting solution.

2. A method of inhibiting the natural tendency of a monoxidizing aqueous acidic composition to corrode nongraphitic metal, which method comprises dissolving in said acid between about 0.001 and about 4 weight percent of a material selected from the group consisting of dialkylphosphorothioic acids and salts thereof, and between about 0.0005 and about 1 weight percent of an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acids having the structure:

wherein R and R are hydrocarbyl radicals selected from the group consisting of cyclohexyl radicals and saturated aliphatic radicals having between 4 and 10 carbon atoms per radical, and A and B are divalent radicals selected from the group consisting of oxygen and sulfur and at least one of A and B is a sulfur radical, and contacting said metal with the resulting solution.

3. A method of inhibiting the natural tendency of a monoxidizing aqueous acidic composition to corrode nongraphitic metal, which method comprises dissolving in said acid between about 0.005 and about 1 weight percent of a material selected from the group consisting of dialkylphosphorothioic acids and salts thereof, and a tetra alkyl quaternary ammonium compound, said quaternary ammonium compound being present in the resulting composition to the extent of at least 0.02 weight percent and being soluble in said composition to the extent of which it is present therein, and said dialkylphosphorothioic acids having the structure:

wherein R and R are saturated aliphatic hydrocarbyl radicals having between 4 and 6 carbon atoms per radical, and A and B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulfur radical, and contacting said metal with the resulting solution.

4. A method of inhibiting the natural tendency of a nonoxidizing aqueous acidic composition to corrode nongraphitic metal, which method comprises dissolving in said acid between about 0.002 and about 4 Weight percent of a material selected from the group consisting of dialkylphosphorothioic acids and salts thereof, between about 0.002 and about 4 weight percent of a quaternary ammonium compound, and between about 0.0005 and about 1 weight percent of an inorganic salt selected from the group consisting of iodides and bromides, and contacting said metal with the resulting solution; said dialkylphosphorothioic acids having the structure:

R2 3) R1O1|BH A wherein R and R are saturated aliphatic hydrocarbyl radicals having from 4 to 18 carbon atoms per radical and A and B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulphur radical, and said quaternary ammonium compound has the structure:

wherein R is a hydrocarbyl radical having between 6 and 30 carbon atoms; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C H +H, where n is a whole number from 1 to 16; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C I-I +H, where b is a Whole number from 1 to 15; R is a saturated aliphatic lower hydrocarbyl radical having the empirical formula (C H +H, where k is a Whole number from 1 to 4; and X is an anion derived from a monoxidizing acid.

5. The method of claim 4 wherein said dialkylphosphorothioic acid is 0,0-bis(1,3-dimethylbutyl) phosphorodithioic acid, said quaternary ammonium compound is octadecenyl trimethyl ammonium chloride, and said inorganic salt is potassium iodide.

6. A corrosion inhibited nonoxidizing aqueous acid composition comprising said acid, between about 0.002 and about 4 weight percent of a dialkylphosphorothioic acid and between about 0.0005 and about 1 weight percent of an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acid having the structure:

wherein R and R are hydrocarbyl radicals selected from the group consisting of cyclohexyl radicals and saturated aliphatic radicals having from 4 to 10 carbon atoms per radical and A and B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulfur radical.

7. A corrosion inhibited nonoxidizing aqueous acid composition comprising said acid, between about 0.005 and about 1 weight percent of a dialkylphosphorothioic acid and between about 0.005 and about 1 weight percent of a quaternary ammonium compound, said dialkylphosphorothioic acid having the structure:

wherein R and R are hydrocarbyl radicals selected from the group consisting of cyclohexyl radicals and saturated aliphatic radicals having from 4 to 10 carbon atoms per radical and A and B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulfur radical, and said quaternary ammonium compound having the structure:

R2 X Rr-ITT-Rt s wherein R is a hydrocarbyl radical having between 6 and 30 carbon atoms; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C H +H, where n is a whole number from 1 to 16; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C H +H, where b is a whole number from 1 to 15; R is a saturated aliphatic lower hydrocarbyl radical having the empirical formula (C H +H, where k is a whole number from 1 to 4; and X is an anion derived from a nonoxidizing acid.

8. A corrosion inhibited nonoxidizing aqueous acid composition comprising said acid, between about 0.005-

and about 1 weight percent. of a dialkylphosphorothioic acid, between about 0.005 and about 1 weight percent of a quaternary ammonium compound, and between about 0.002 and about 0.2 weight percent of an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acid having the structure:

wherein R is a hydrocarbyl radical having between 8 and 20 carbon atoms; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C I-1 +H, Where n is a whole number from 1 to 13; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C I-I )+H, where b is a whole number from 1 to 8; R is a saturated aliphatic lower hydrocarbyl radical having the empirical formula (C H +H, where k is a whole number from 1 to 4; and X is a halogen anion.

9. An acid composition as in claim 8 wherein said inorganic salt is an alkali metal iodide.

10. An acid composition as in claim 8 wherein said inorganic salt is an alkaline earth metal iodide.

11. A composition suitable for inhibiting the corrosion of metal by a nonoxidizing acid which composition contains a dialkylphosphorothioic acid and an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acid having the structure:

wherein R and R are hydrocarbyl radicals selected from the group consisting of cyclohexyl radicals and saturated aliphatic radicals having from 4 to 18 carbon atoms per radical and A and B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulfur radical, the weight ratio of said dialkylphosphorothioic acid to said inorganic salt in said composition being between about 1: 1 and about 20: 1.

12. A composition suitable for inhibiting the corrosion of metal by a nonoxidizing acid which composition contains a dialkylphosphorothioic acid, a quaternary ammonium compound, and an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acid having the structure:

wherein R and R are hydrocarbyl radicals selected from the group consisting of cyclohexyl radicals and saturated aliphatic radicals having from 4 to carbon atoms per radical and A and B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulfur radical, and said quaternary ammonium compound having the structure:

wherein R is a hydrocarbyl radical having between 6 and 30 carbon atoms; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C H +H, where n is a Whole number from 1 to 16; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C H +H, where b is a whole number from 1 to R is a saturated aliphatic lower hydrocarbyl radical having the empirical formula (C H +H, where k is a whole number from 1 to 4; and X is an anion derived from a nonoxidizing acid, the weight ratio of said dialkylphoshorothioic acid to said inorganic salt in said composition being between about 3:1 and about 10:1, and the weight ratio of said quaternary ammonium compound to said dialkylphosphorothioic acid in said composition being between about 1:2 and about 4:3.

13. A homogeneous liquid inhibitor concentrate composition suitable for inhibiting the corrosion of metal by a nonoxidizing acid, said composition comprising between about 10 and about 90 weight percent of said acid, and at least about 5 weight percent of a combination of a material selected from the group consisting of dialkylphosphorothioic acids and salts thereof, and an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acids having the structure:

wherein R and R are hydrocarbyl radicals selected from the group consisting of cyclohexyl radicals and saturated aliphatic radicals having from 4 to 18 carbon atoms per radical and A and B are divalent radicals selected from the group consisting of oxygen and sulfur, and at least one of A and B is a sulfur radical.

14. A homogeneous liquid inhibitor concentrate composition suitable for inhibiting the corrosion of metal by a nonoxidizing acid, said composition comprising between about 20 and 60 weight percent of said acid, and at least about 5 weight percent of a combination of a dialkylphosphorothioic acid, a quaternary ammonium compound, and an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acid having the structure:

wherein R is a hydrocarbyl radical having between 6 and 30 carbon atoms; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C H )+H, where n is a whole number from 1 to 16; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C l-I );+I-I, where b is a whole number from 1 to 15; R is a saturated aliphatic lower hydrocarbyl radical having the empirical formula (C H ).+H, where k is a whole number from 1 to 4; and X is an anion derived from a nonoxidizing acid.

15. A homogeneous liquid inhibitor concentrate composition suitable for inhibiting the corrosion of metal by a nonoxidizing acid, said composition comprising between about 20 and about 60 weight percent of phosphoric acid, and at least about 5 weight percent of a combination of a dialkylphosphorothioic acid, a quaternary ammonium compound, and an inorganic salt selected from the group consisting of iodides and bromides, said dialkylphosphorothioic acid having the structure:

wherein R is a hydrocarbyl radical having between 8 and 20 carbon atoms; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (c H H-H, where n is a whole number from 1 to 13; R is a saturated aliphatic hydrocarbyl radical having the empirical formula (C I-I +H, where b is a whole number from 1 to 8; R is a saturated aliphatic lower hydrocarbyl radical having the empirical formula (C H +H, where k is a whole number from 1 to 4; and X is a halogen anion, the weight ratio of said dialkylphosphorothioic acid to said inorganic salt in said composition being between about 1:1 and about 20:1, and the weight ratio of said quaternary ammonium compound to said dialkyl- Zphosphorothioic acid being between about 1:2 and about References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Corrosion, vol. 6, No. 10, October 1950, pages 346.

'"monoxidizing",

STATES PATENT orrlca EERTIFICATE @F CQRREC'HO N v Patent No. a 146, 20a August 25, 1964 Arthur Orman Fisher] certified that error appears in the above numbered pat- It is hereby that the said Letters Patent should read as ent requiring correction and corrected below.

Column 14, lines 19 and 42 and column l5 line 28, for

each occurrence read nonoxidizing column 17, lines 45 to 50 the formula should appear as shown.

below instead of as in the patent:

62 f R -O-lT-B H Signed and sealed this 22nd day of December 1964.,

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J BRENNER Attesting Officer Commiesioner of Patents '"monoxidizing", each occurrence,

STATES PATENT OFFICE CERTIFICATE OF CORRECTION I Patent No. 3,146,208 q- 25, 1964' Arthur Orman Fisher is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1.4. lines 19 and 42, and column 15 line 28 for read nonoxidizing 5-;

column 17, lines 45 to 50, the formula should appear as shown below instead of as in the patent: r 7

I -P-B-H v i Signed and sealed this 22nd day of December 1964.,

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents a