INDUSTRIAL AND ENGINEERING CHEMISTRY
2592
of the Departnieiit of Geology, Brighain Young U-niversity for samples. The assistance of Dale King is also acknowledged.
Vol. 46,No. 12
(6) Xelson, H. D., Snow, R. D., and Keys, D. R . , IND. EXG. CHEII., 25, 1355 (1933). (7) Quispel, A , , Harmsen, G. W.,and Otzcn, D., Plant and Soil,
4, 54 (1952). w. A., and Ratcliff, IT-. c., h D . E N G .
(8) SdYig,
LITERATURE CITED
CIIEM.,
14, 125
(1 922).. - - - - I
Science, 106, 253 (1947). (1) Coliner, A. R., and Hinkle, SI. ( 2 ) navis, D. B,, "Biological Oxid of sulfide lIinera h , " Masters thesis, Brighain Young University, 1953. (3) [lodge, W.W., IND. ESG. CHEX.,29, 1048 (1937). (4) Leathen, W. W..Braleu, 9. -L, and McIntyre, L. D.. A ~ p 1 . Microbial.. 1, 01, 65 (1953).
copper
( 5 ) Leathen. W. W., McIntyre, L. D., and Araley, S. -4.,Science,
114, 280 (1931).
(9) Temple, K. L., and Colmer, ..1.I?., .I. Racte~-ioZ.,62, 605 (1951). Temple, K. L., and Delchamps, I?. W.,A p p l . M i c ~ o b i o l . ,1,
(10)
255 (1953).
( I 1)
Ursenback, W. O., "Factom Iiiflueiicing the Woist Oxidation of Iron Pyrites," Masters thesis, Riighain Young University,
(12)
Wilson. I).G., "Ytudie Pyrites," Masters the
1949. n the Biological Oxidation of Iron Rrigham Young University, 1952.
RI:CEIVEDfor review Ootobcr 2 7 , l P 3 l .
.&CCEPTBD AUgllSt 9 0 , 1'154
IIAI-R7-4RDR. BAICER Surface Chemistry Branch, Yaval Research Laboratory, Washington 25, D . C .
HE amine salts of weak inorganic acids and amine-organic acid complexes have been found effective as volatile rust inhibitors. The use of volatile rust inhibitors has simplified the packaging, storage, and shipment of military equipment and replacement parts. This form of protection has permitted the immediate use of packaged items without the tedious procedures normally associated with the reinoval of oil or grease prescrva,tives before stored or packaged equipment is placed in service. The savings in man-hours on reactivation of equipment are greater in many cases than the initial cost of packaging. Cornplicat,ed met,allic assemblies, such as electronic equipment,, may be protcctcd by this method. BACKGROUKD
During nwtinie research on the developmeiit of a nonflammable aqueous hydraulic fluid (6, 11), need arose for an inhibitor that would prevent the rusting of ferrous metals in the vapor spaces of hydraulic systems. Volatile compounds had been used for many years to control rusting in steam and condensate lines of heating systems. Because ammonia ( 3 4 ) , ethylenediamine, inorpholine (IW), and c~~clohesylamine ( I S ) prevent or greatly retard such corrosion, these and several other amine-type coinpounds were evaluated in static closed systems, but with only moderate success. I n September 1944, while the search for a satisfactory vapor inhibitor for aqueous hydraulic fluids v a s in progress, a small coded sample of a new vapor-phase inhibitor was received from an industrial source and a t once investigated as a possible approach to the hydraulic problem. It xyas found to be a promising material for this use and was identified as diisopropylammonium nitrite ( 3 6 ) . Because of the urgency of the naval application, this and several other amine nitrites werc synthesized by previously known methods ( 5 0 ) and by n e v procedures developed for this problem (45,46). Borne of these compounds were found to giw adequate vapor-phase rudt inhibition in aqueous nonflam~nablehydraulic fluids and are iiow in use in commercial formulations of these fluids (6, 1 1 ) . During World War I1 it became necessary to protect military equipment and replacement parts against rusting during storagc under all climatic conditions. The existing industrial packaging procedures were found inadequak, and i t was necessary to develop new and special packaging techniques. Many of these new packaging procedures were expensive and time-consuming, and considerable trouble and expense were required to remove such preservatives before the equipment was placed in service.
This problem was serious in coiiibat areas, especially when weapons were being prepared for subzero use, because even trace:: of the viscous preservative oils or greases prevented lox temperature operation. The use of desiccants proved inadequate in many paelcaging problcms because it)wa.8 impossible to obtain and maintain packaging materials with sufficiently low vapor transmission rates. The vapor-phase inhibition obtainable vAth the amine nitr,itc? opened a completely new avenue for att>aclr on packaging 1)rohlenis. h specific problem presented by fhe Savj- Bureau 01' Ordnance early in 1945 called for inhihiting the rusting of the interior of rocket bodies during the time belveen manufactuic in the United St,atesand the subsequent fiiling and use in the l'avific n-ar theater. The previous pracbicci of slushing the interior of the rocket body wit,h Grade 21 10 petroleum oil soon after fabricat,ion had proved inadequate in preventing rust \Then water condensed in the interior of the rocket body. The oil vias also very difficult to remove because of the construction of the rocket body. Studies were made of the effectiveiiess of the follo~vingmethods of inhibiting rusting ( 2 ) : (a)slushing the interior of the rocket body with Grade 2110 petroleum oil, ( 0 ) slushing the interior surfaces of the rocket body with Grade 2110 oil to \yhich an effcctive rust inhibitor had been added; (c) slushing tlie interior of the rocket with a typical comrncrcinl Eolvent cutback rust-iwcveiitive fluid; ( d ) the use of a desiccant, silica gel, under conditions which siinulated the breathing of a closed (but not scaled) container subjected to t,eniperature cycling in a humid atmo;.phere; and ( e ) the use inside the rocket body of a paper tul)e which had previously been impregnated with diisopropylain-, moniuni nitrite. Only rockets treated by methods ( b ) , ( e ) , or ( e ) Kere fourid to be adequately protected against rusting during storage under humid conditions. Methods ( b ) and ( e ) entailcd tedious fieldcleaning operations, whereas the rocket i ~ o d yprotected bj- the impregnated paper tube was ready for use when opened. -1 specification mas therefore written for the procurement of the impregnated paper tubes (43). Work by two independent research groups (36, .45, 46, :i4) established that dicyclohexylaniinoriiuiii nilrite was less volatile and not so soluble in w a k r as the diisopropylarnniotiium nityitc and was in consequence a more suit,ahle paper impregnaiit. Boon after Il'orld War I1 ended a Saval Research report was made of the physical properties of these amine nitrites ( 2 ) and their effects on a wide variety of nonferrous metals. It was shown that these vapor rust inhibitors could be used in solving marry
December 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
of the packaging problems then being encountered. However, applications to electrical equipment were limited because of the formation of undesirable coatings on some nonferrous metals. Soon after the issuance of a coordinated military specification for volatile inhibitors (47), materials other than amine nitrites were qualified and are now in use. Since the end of World War I1 vapor rust inhibitors have been extensively tested both in the laboratory and in the field. Recent publications have discussed the use of these materials for inhibiting the rusting of aircraft engines and repair parts (1, 3 l ) , ordnance material such a3 machine guns, gun barrels, and spare parts (10, 16), instruments, and machinery (9, 14). More recently trade journals have carried articles discussing the application of vapor rust inhibitors to the packaging of military and domestic equipment (15, 35). Some automobile manufacturers are using vapor rust inhibitors extensively for the packaging of automotive replacement parts. Since those early preliminary studies, many products similar in composition have been investigated (B9, 36-4$, 44). Wachter, Skei, and Stillman have recently discussed the properties and use of dicyclohexylammonium nitrite in corrosion-preventive packaging (64). Schwoegler and Hutter (32) have shown that rust inhibition can be obtained by using sodium, potassium, or ammonium nitrite and an organic amide such as urea or acetamide. Vernon (51-53) has described the use of sodium benzoate as a contact rust inhibitor and mentioned the possible uses of esters of benzoic acid a s vapor rust inhibitors. Later Stroud and Vernon (43) reported on the vapor rust-inhibiting properties of a series of amine carbonates. The vapor rust-inhibiting properties of nitrothiophenes neutralized with amines, guanidines, and metal carbonates were described by Wachter and Moore ($9). Kamlet ($3) discussed the protective properties of volatile amines in preventing the tarnishing of silver. The use of ammonia in the control of vapor zone rusting in storage tanks has been described ( 1 7 ) . Lieber ( 2 6 ) has shown that fibrous packaging materials impregnated with alkylolamines inhibit the rusting of ferrous metals around which they are packed. The inhibition of rusting in steam systems by the use of amines, alkylolamines, and amineacid coniplexes is further described by Jacoby (21) and Lane and Thompson ( 2 4 ) . PHYSICAL PROPERTIES AND IMETHODS O F PREPARATION
The solubility of the amine nitrites and the amine-organic acid complexes in water ranges upward from a few per cent, depending upon the structure of the organic component. The vapor pressures also range from a few microns of mercury a t 100' F.upward. In general, these substances show a decreasing solubility in alcohols with an increase in molecular weight of either the inhibitor or the solvent. The compositions of lower molecular weight are very sparingly soluble in hydrocarbon solvents and oils; their solubilities become greater with an increase in molecular weight. However, if the molecular weight becomes too high, the vapor-inhibition properties are reduced because of insufficient vapor pressure. Most of the compositions exhibit greater solubilities in mixtures of water and alcohols than in either solvent alone. The higher molecular weight amine nitritcs are white crystalline solids, while those of lower molecular weights are deliquescent semicrystalline materials. In general, the amine-organic acid complexes range from crystalline solids to waxlike semisolid materials. The complexes of lower molecular weight are deliquescent. The amine salts of inorganic acids can be prepared and isolated in high punty (45, 46), while the amine-organic acid complexes can be prepared by dissolving the acid in methanol and then adding a methanol solution of the amine a t room or slightly elevated temperatures. The solvent may be removed under reduced pressure and the compound recrystallized from an appropriate solvent.
2593
TABLEI. INHIBITION OF RUSTING OF STEELPROVIDED B Y ORQANIC ACID VAPORSIN THE PRESENCE OF WATERVAPORAT 150" F.
(Duration of test, 168 hours) % of Surface N o t Rusted Impregnant for Kraft Paper, 0 25 50 75 2 Grama pcr Square Foot
Sone -
1
Oxalic acid Malonic acid Succinic acid Maleic acid Fumaric acid Malic acid Glutaric acid Adipic acid Azelaic acid Sebacic acid
Cyclohexylcarboxylic acid Cinnamic acid Camphoric acid Camphorsulfonic acid Ethylphosphonic acid Butylphosphonic acid Hexylphosphonic acid Phthalic acid 1-Naphthoic acid 2-Naphthoic acid a Specimen attacked cheinically, degree of rust greater t h a n unprotected specimen.
EVALUATION AND T E S T PROCEDURES
The method to test the rust inhibition was relatively simple.
A known concentrated solution of each chemical in either alcohol or an alcohol-water mixture was employed t o impregnate a 30-pound neutral kraft pa er, so that approximately 2 grams of the compound was retainejin each square foot of the paper after the alcohol and water evaporated. When the chemical coinposition was liquid a t room temperature, the chemical alone mas used to impregnate the paper. One piece of paper (3 X 3 inches) was placed in each test tube (1.5 X 14 em.). The specimen of cold-rolled steel (SAE 1020), 3 X '/z X l/8 inch, was carefully finished and cleaned by methods previously described ( 7 ) . ,4 single specimen was suspended by a sill: thread in each test tube 1 inch from the bottom of the tube and inside the cylinder of impregnated paper. The test tube was stoppered and stored a t 75" F. for about 20 hours, then opened briefly (approximately 5 seconds) while 0.25 ml. of distilled water was introduced onto the bottom through a long hypodermic needle. The assembly was stored in a perpendicular position a t 150" F. for 1 Feek, after which the specimens were rated by estimating the percentage of the total surface which was not rusted. The degree of protection was then recorded as one of five arbitrarily chosen values-0, 25, 50, 75, or 100% protected. A compound was not considered t o have significant inhibiting properties if its rating on this scale was less than 50% protected; nearly all of the practically useful compounds received ratings of 100% protected under the conditions of the test. Many acids, amines, and esters were studied by this test method. Before the inore complex compositions were investigated, a study was made of the effect of p H of the aqueous solution on the vapor inhibition of the solute. A large number of amine-inorganic acid salts and amine-organic acid complexes were prepared in alcohol-water mixtures with p H values of 6, 8, and 10. Unsatisfactory rust inhibitors resulted at pH 6.0 in all cases. At
INDUSTRIAL AND ENGINEERING CHEMISTRY
2594
TABLE 11. INHIBITION OF RUSTINGOF STEEL PROVIDED BY AMINE VAPORSIN PRESENCE OF WATERVAPORAT 150" 1". (Duration of test, 168 Hours) yo of Surface K o t Rusted Impregnant for Kraft Paper, 2 Grams per Square Foot 0 25 50 75 Ammonium hydroxi Ethylamine Isopropylaniine Butylamine Isobutylamine Amylamine Isoamylamine Hexylamine Octylaniine Decylamine Dodecylainine Dietiiylaniinc Diisopropylamine Dibutylamine Diamylainine Tripropylamine Triisohutylaiiiine
de
H
101
I
i\Ionoethanolamine Diethanolamine Triethanolamine Morplioline Cyclohexylamine Dicyclohexylamine Phenylamine Bennylamine %Naphthylamine Diphenylamine Triphenylamine Phenylenediamine Phenylhydrazine
acetamide were weak inhibitors. Table 1V also presents some examples of mixtures of nonvolatile substances whose interaction in the presencc of water results in the formation of effective vapor rust inhibitors. Xeithcr urea nor sodium nitrite i E an effective vapor rust inhibitor when used alone, but \\-hen a mixture of the two is uqed efficient inhibition is obtained. The mixed acids obtained when hydrocarbons are oxidized can be used vith dicyclohexylamine to obtain efficient vapor rust inhibitors. A commercial grade of nitromethane combined with cyclohexylamine also yields an efficient vapor rust inhibitor. Some inhibition was obtained with esters, although in all cases 50% or more of the steel surface was rusted. Hence, some degree of vapor inhibition of ferrous surfaces can be expected from suitable members of the following classes of compounds: aliphatic or aromatic acids; primary, secondary, or tertiary amines; amine salts of weak inorganic acids; amineorganic acid coniplexes; alkylolainines as well acj their salts with weak inorganic acids and their complexes with organic acids; hydroxy-substituted aromatic acids and acid anhydrides; miscellaneous compounds such as nitromethane to nitropropane, morpholine, camphor, carbamides, and ammonium hydroxide; and any combination of cornpounds which on reaction yields any of the compounds mentioned above.
W I I
u None
pH 8 the greatest vapor-phase inhibition Tas manifested, while a t p H 10 inhibition was definitely decreased. Therefore, all of the amine-inorganic acid salts and the amine-organic acid complexes reported in this paper were tested a t pH values In the vicinity of 8.0 as measured in alcohol-water mixtures with Hydrion ( A ) paper or a pII meter. The results of these tests are reported in Tables I to IV. Only B few compounds of any homologous series are reported, as these suffice to show the trend of the series. I n Table I it is shown that a wide variety of organic acid vapors will provide substantial although not complete protection against rusting. The significant exceptions are the acids of very lorn molecular weight, which are well known to promote rusting of moist steel surfaces, and those of a molecular weight great enough to have negligible vapor pressures under the test conditions. The dicarboxylic acids become ineffective a t a slightly kower molecular weight than do the monobasic acids. The amines (Table 11)perform like the acids, except that the low molecular weight amines and ammonia also reduce the rate of rusting. I n the case of low molecular weight amines the inhibition results from the alkaline pH which they impait t o Lhe water condensed on the metal ( 5 5 ) . The noninhibiting amines are either notably weaker as bases or so high in molecuBar weight as to be relatively nonvolatile. In no instance did a single acid or amine give complete prodection to the steel specimen. HoR-ever, by preparing amineorganic acid complexes and amine salts of inorganic acids, many combinations were found which would completely inhibit the rusting of the steel sample in the test described (see Table 111) These materials range from substances which are completely water-soluble to those which are water-insoluble but oil-soluble. Metal salts of the acids were found t o be ineffective vapor rust inhibitors. This would be expected, as the compounds are not volatile under the test conditions. However, many of these Dare excellent contact inhibitors. From the miscellaneous compositions shom-n in Table IV, I t 8s seen that nitropropane is a better inhibitor than the lower molecular weight members of that series. Compounds of higher molecular weight were not available for test. 130th urea and
Vol. 46, No. 12
DISCUSSION
Organic acids of low molecular weight react with steel surfaces to form water-soluble salts. As long as the salts formed can be
TABLEI11 IXHIBITION O F RUSTING O F STEELPROVIDED BY VAPORSARISIYGFROX AVIYE-ACIDCOMPLEXES A N D S~ L T SIW PRESENCE O F \VATER T'APOR AT 150" F (Duration of test, 168 houra) yo of Surface Not Rusted 0 25 50 7 5 100
Iinpiegnant for I i r a f t Paper, 2 Grams per Square r o o t
1
Arnylamnionium benzoate Isopropylammonium benzoate Dibutylammonium benzoate Diisopropylammonium benzoate Diisoainylammonium benzoate Cyclohexylanimonium benzoate Dicyclohexylammonium benzoate Phenylliydrazineammoniuin benzoate R.TnnoPtliannlnmmonium ..-~ ........ ~ . - benzoate ~ Diethanolammoniuin benzoate Triethanolammonium benzoate Isoproyylammoniuni p-isopropylbenzoate Cycloliexylammoniuin p-isopropylbenzoate Ethylniorpholine benzoate 2-Naplithylai~imoniuinbenzoate Pyridine benzoate
I
I
~
Diisopropylammonium acetate Diisopropylammonium naphthaleneacetate Diisopropylammonium 2-ethylbutyrate Diisopropyiainmonium 1-naphthoate Diisopropylammonium 2-napiithoate Diisopropylammonium phenolate Diisopropylammonium azelate Isoamylainnioniuin salicylate Cy clohexylammonium 2-ethylhexanoate Cyclohexylammonium cyolohexylcarboxylate Dicyclohexylammonium 2-ethylhexanoate Diisopropylammonium butylphosphonate Dicyclohexylammonium ethylphosphonate Dicyclohexylammonium ethglsulfonate
U
l
Isopropylammonium nitrite Cyclohexylammonium nitrite Diisopropylammonium nitrite Diisobutylainnionium nitrite Dicyclohexylammonium nitrite Triethylammonium nitrite I3icyclohexylanimonium carbonate Dicyclohexylammonium chromate Dicycloliexylaminoniuin phosphite Uicyclohexylammonium sulfite Dicyclohexylammoniuin borate Dicyclohexylammonium nitrate Diisohutylaminonium nitrate Diisobutylammonium sulfate Diisohutylammonium phosphate
!
l
I
l
l
i
l
l
l
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1954
partially dissolved by water in the system, reaction continues and accelerated attack results. This is the case for formic, acetic, and propionic acids. The higher molecular weight fatty acids form compounds with iron which are not as readily soluble in water. Some vapor inhibition against attack by water is obtained with fatty acids having more than five carbon atoms per molecule, as long as the acid is able to volatilize and adsorb on the metal surface to form a film which helps to block the corrosive action of water. If the acids are not volatile a t the test temperature, rusting of the same magnitude as for the blank specimen is obtained. Complete inhibition is not obtained by using carboxylic acid inhibitors, because those that possess suitable vapor pressures under the test conditions are too water-soluble and too low in molecular weight to adsorb as an effectively impermeable film on ferrous surfaces Higher molecular weight compounds, essential to adequate inhibition (3, 67,are not sufficiently volatile. Lauric acid, for example, inhibits rusting effectively when adsorbed from solution and gives a monolayer on platinum having a maximum contact angle of 75" with water ( 4 ) However, this compound does not provide detectable vapor phase inhibition. Caproic acid, which can adsorb as a monolayer on platinum to give a maximum contact angle of 53" with water ( 4 ) , is sufficiently volatile but provides only mediocre protection to iron, whether it is adsorbed from the vapor phase or from solution (6). Vapor-Transfer Mechanism. It is surprising that such substances as dicyclohexylammonium nitrite and dicyclohexylammonium benzoate (molecular weights 228 and 303, respectively) are sufficiently volatile to provide excellent vapor rust inhibition a t 150" F. Lauric or stearic acid (molecular weights 200 and 284, respectively) provides negligible vapor-phase protcction under the same conditions, although the films they form by adsorption from solution are excellent rust inhibitors. Dicyclohexylammonium nitrite and benzoate, as salts which behave as strong electrolytes in aqueous solution (50), might be expected to be even less volatile than most other organic compounds of comparable molecular weight. I t is significant that all of the most effective vapor rust inhibitors listed in Table I to IV are the products of reaction of a volatile weak base with a volatile weak acid. Such substances, although ionized in aqueous solution, undergo a substantial hydrolysis, the extent of which is almost independent of concentration. In the case of the amine nitrites the net result of the reaction may be formulated as RzNHz+
+ NOz-- G RzKH + HNOz
and in the case of the amine carboxylates as
RzNHzf
+ R'COO-
e RzNH
+ R'COOH
For the most effective inhibitors listed in Tables I to IV, the amine and acid so produced have substantial volatilities a t 150°F. The products of hydrolysis may be expected to vaporize separately and to recombine during deposition on exposed metal surfaces in rather exact analogy to the sublimation of ammonium chloride (at much higher temperatures), for which the separate diffusion of the ammonia and hydrogen chloride gases has been demonstrated experimentally. This process provides for the mass vapor transfer of an effective inhibitor a t a much lower temperature than would be otherwise possible. In addition to their susceptibility to hydrolysis by water, the amine carboxylates are unstable in anhydrous systems ( 2 3 ) . The compound formed between a tertiary amine and a carboxylic acid, for example, has been shown to be extensively dissociated in a hydrocarbon solvent a t 40' F. The dry amine salts of other weak acids also dissociate readily into the component amine and acid. One may therefore expect the same general mechanism of volatilization by dissociated components to operate in the presence or absence of water. These considerations do not rule out the possibility of some direct volatilization of the
2595
inhibitor compound as such, but they make the assumption of such direct evaporation unnecessary. In further support of the mechanism of vapor-phase transfer suggested, it is noted that amine salts such as dicyclohexylammonium nitrate or diisobutylammonium sulfate, which are not extensively hydrolyzed by water, do not give significant vaporphase inhibition. The same is true for slightly hydrolyzed alkali metal salts such as sodium nitrite or sodium benzoate, although the latter are excellent rust inhibitors when their solutions are in direct contact with the metal surface.
TABLEIV. INHIBITION OF RUSTING OF STEEL PROVIDED BY VAPORSFROM MISCELLANEOUS COMPOSITIONS IN THE PRESENCE OF WATERVAPORAT 150' F. (Duration of t e s t , 168 hours) % ' of Surface Not Rusted' 0 25 50 75 100
Impreanant for Kraft Paper. 2 grams per Square Foot
2
Urea Acetamide Sodium nitrite and urea Dioyclohexylamine Cyclohexylamine
-
€34 I I 1
Nitromethane Nitroethane Nitropropane Nitronaphthalene
HIII
+ oxidized hydrocarbons
4-nitromethane
Camphor Phenol
This explanation of the vapor-phase transfer of rust inhibitore is significant because i t points out the directions in which the search for new inhibitors should be extended, and sets limits outside of which useful products are unlikely to be found. It has, in addition, important implications concerning the condition8 under which effective inhibition is possible. Effect of Experimental Variables. CONCENTRATION AND SOLUBILITY.I n aqueous systems the amount of inhibition obtained above the liquid level in the vapor state depends upon the solubility as well as the concentration of the additive in the solvent. If a particular additive is more soluble in one solvent than another, increased concentrations in the better solvent will be required to give comparable inhibition in the vapor state above the solutions. For example, in a solution consisting of 49.25% by weight of ethylene glycol and 49.25% by weight of distilled water, 1.5% by weight of dicyclohexylammonium nitrite gives complete protection in the vapor above the liquid a t 100" F., while in water alone 3.0% by weight is required to give complete protection (5, 11). The effectiveness of the other inhibitors depends similarly upon their respective solubilities and their vapor pressures. Most of the vapor rust inhibitors when added to water inhibit the rusting of steel in contact with the water as well as that in the vapor phase above the water. For example, 0.1% by weight of dicyclohexylamine nitrite dissolved in water inhibits the rusting of steel plate immersed in the water for 30 days a t 100" F. Sodium benzoate and sodium nitrite, although not vapor corrosion inhibitors, are efficient liquid-phase inhibitors for steel. Of the amine-acid complexes, 0.2% by weight of cyclohexylammonium benzoate in water inhibits the rusting of steel plate completely immersed in water for 30 days a t 185' F. Others show comparable inhibition. Higher concentrations give reserve inhibition and thus provide added insurance against rusting. Compositions exhibit their maximum inhibiting properties in the vapor phase when equilibrium is maintained among the solid inhibitor, the vapor in the air, and the inhibitor on the surfaces
2596
INDUSTRIAL AND ENGINEERING CHEMISTRY
inside the package. In order to ensure this condition a reserve of inhibitor must be present to compensate for that lost by leaks and by "breathing"; thue the rate of loss by air leakage or breathing must be small as compared with the rate a t which the inhibitor is renewed by vaporization, It has been shown (54) t h a t by using more effective outer barriers for packages-Le., material with loxer vapor transmission rates-the protect,ion life of a package can be greatly increased. HYDROGEN IONCOA-CENTRATIO~ (pH). If the transfer of inhibitor through vapor spaces occurs primarily by means of the volatile products of hydrolytic or thermal dissociation, it follows that aqueous solutions of ihc inhibitor will provide adequate prot,ect,iononly when the pH of the solution is such that both the amine and the acid are present in significant amounts. The pH range in which carbox)-late ions and free carboxylic acid can coexist in aqueous solution has been discussed by M e r k e ~ and Zisman ( 2 7 ) , n-ho also treated the case for coexistence of alkyl-substituted ammoniuin ions and the corresponding amines. Their results suggest that, appreciable volatilization of both components of an amine carbo. te o r an amine nitrit,e will be probable only in the approximate pH range 5.5 to 8.5. The exact range d l depend upon the relative st,rength and the relative volatility of the acid and amine involved. On the acid side of the neutral point the useful inhibiting range is further limited h>the fact that when the volat,ile acid constituent>is present in the vapor in excess it will dissolve in the condensate on the met,al surface tQ give it an even lorver pH, which in the case of nitrous acid or short-chain organic acids may actually accelerate rusting. Experiments show that, the pH of the aqueous solution must be 7 or above to avoid the enhanced acidity of the condensate above the solution. The results of indicator experiments with various solutions in dicyclohexylammonium nitrite and of diisopropylammonium benzoate are summarized in Table 5:, which shorn that the pH produced by such inhibitors in the condensate forming on exposed steel surfaces depends strongly on the pII of the saturated inhibitor solution from which the protecting vapors originate. The pH of the inhibitor preparat,ion, therefore, affects not only the initial volatility of the inhibitor but also the effect,ivenessof its operation after i t reaches the metal surface, At a pH above 8.5 an acid of the strength of carboxylic acids is present almost exclusively as the nonvolatile anion and any inhibition not,ed from an amine carboxylate in this range is attributable to the volatile amine, which has definite although inadequate rust-inhibiting properties in its own right.
Vol. 46,No. 12
detinitely on the alkaline side. If an inorganic buffer is used, the alkalinity should be controlled a t a p H of about 7.5. However, an excess of the amine can be used to maintain the alkalinity at a p H between 7.0 arid 9.0 without danger of destroying the amine nitrite. As noted earlier, the amine-organic acid complexes showed optimum inhibition a t a p N of 8.0 d= 0.5. XATURE OF ADSORBEDFILMS. Even more important for the efficacyof vapor rust, inhibitors than the pH effects just discussed is the nature of the adsorbed film formed a t the st,eel-water interface. The adsorption of amines and of fatty acids from a q u c o u ~ solution onto metal surfaces has been established by other investigations (4, 38). I\Iet,al suriaces exposed to vapors from vapor rust inhibitors in closed containers also gave evidence of having been covered by a hydrophobic adsorbed layer. The contact angle of distilled water on such surfaces increased with the time of exposure before the addition and measurement of the drop. The major rise in contact angle occurred during the first 2 hours of exposure, and equilibration was complete for dicyclohexylammonium nitrite aft,cr 24 hours. Acids such as benzoic or caproic also deposited hydrophobic films on metal above their aqueous solut,ions (4),and the contact angles so obtained were comparable with those for films deposited directly from the water solution or from the melt,. For the acids of higher riiolecular weight,: the angles were greater than for dicyclohexylainmonium nitrite, although the latter was a much better inhibitor of rusting. The greatly increased protection against rusting that results when both acid and amine w e present is noteworthy and deserves further study. The dat,a available do not make clear whether the gain is a result of cooperative action between adsorbed acid and amine to give a more firmly held monolayer ihan could be formed by either alone. It is pertinent that Rarkins and Florence (20) showed that the mixed film when adsorbed a t the water-air interface as highly condensed. It is also conceivable that the mixed film serves as a buffer to hold t,he pH a t the interface in the optimum range for rust resistance. Another possibility is that the amine and acid contribute t o rust inhihition by different but additive mechanisms (18, IO). I t has been suggested (8, 6,4) that the rust protection afforded by amine nitrites, like that exercised by sodium nitrite in water solution, depends primarily upon the passivating action of the nitrit,e ion on the steel. It is probable that the nitrite ion makeP some such contribution, but the closely comparable effectiveness of the amine carboxylates as vapor inhihitors demonstrates that the contribution of the nitrite ion to the inhibitory process is not essential in vapor rust inhibition. TEAIPERBTURE. The performance of inhibitors at high temTABLE V. p H OF WATERIN EQUILIBRIUM WITH VAPORABOVE peratures depends upon their stability. Cert,ain of t.he amineAQGEOCS SOLUTIONS OF VAPORRUSTIXHIBITORS AS COMPARED organic acid complexes such as dicyclohexylammonium 2-ethylWITH pH OB LATTER PHASE hexanoate are more stable than the amine nitrites and have (Tests a t 77' F.)a demonstrated effective vapor rust inhibition in laboratory tests Concentration, Solution, Condensate, for 30 days a t 200" F. Still other compositions are efficient rust Inhibitor Wt. % PH P E-1 inhihitors even in steam a t temperatures in excess of 212" F. Diisopropylammonium benzoate 5.0 7.0 7.0 Diisopropylammonium benzoate (24,28).Although the antine nitrites are one of the hetter classes + excess benzoic acid 6.0 5.0 4.0 of inhibit,ors, t,hey have a general tendency to decompose slowly Diisopropylanimoniuin benzoate + excess diisopropylamine 6.0 8.0 10.0 a t elevated temperatures into products inferior in inhibition; 7.0 10.0 >10.0 Dicyolohexylammonium nitrite L O 7.3 7 . 0 i- hence, they have not been found practical a t temperatures greater Dicyclohexylammonium nitrite + excess NaNOa and HCl. 6.0 4.5 3.0-4.0 than170"F. Dicyclohexylammonium nit,rite All of the vapor rust, inhibitors require a period of time for + exoem dicyclohexylaniine 6.0 8.0 >9,0 7.0 8.0" >9.0 the establishment of vaporization and adsorption equilibria a pH estimated from indicator colors. inside a package before their inhibiting properties are iully effective. This period can be sliortened by utilizing compoaitioris wit,h higher vapor pressures, but the composition must not be The secondary alkyl ammonium nitrites react in the presence so volatile as to vaporize completely and be lost during exposure of small traces of inorganic acid to give the nitrosamine, which is a t the higher storage temperatures. It is better to utilize a not an efficient vapor rust inhibitor ( 4 6 ) . This limits their use to composition possessing somewhat lower vapor pressures and to acid-free systems or indicates the ube of an alkaline buffer. On activate the inhibitor in a given package by subjecting the the other hand, in the presence of excess alkali the free amine package t,o a warming cycle immediately after sealing. After and an alkali nitrite are formed. Thus for satisfactory operation the vapors from the inhibitors have dcp )sited over the steel suran effective buffer must be used to maintain the pH at a level
December 1954
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
faces of the package, the temperature can be reduced to any lower level without danger of rusting so long aa the package is adequately sealed. LEACHING.Because of the low molecular weight of some of these inhibitors, prolonged contact with bulk water will result in solution of the protective coating. Desorption because of the solubility of the inhibitor in water is the cause of the limited vapor rust inhibition observed under conditions of extreme humidity. No combination of compounds was found that possessed a vapor pressure suitable for vapor rust inhibition that was not a t the same time partially soluble in water. Consequently, in any application where liquid condensate is continuously removed from a surface, the inhibitor will be slowly depleted and finally reduced to a concentration too low to be effective. USE WITH SILICAGEL. Silica gel as a desiccant has been used for many years in packaging operations to maintain low relative humidity within the package. It has not only served to prevent rusting of steel and water-promoted corrosion of nonferrous parts, but has prevented mildew or fungus attack on insulation, electronic equipment, and protective coatings. Since silica gels are usually somewhat acidic, they are normally somewhat detrimental to vapor-phase rust inhibitors. This difficulty can be overcome by incorporating sufficient alkaline reserve to compensate for the acidity and by including a quantity of vapor rust inhibitor sufficient to maintain an inhibitor-saturated vapor in the presence of the adsorbent. By using silica gel along with vapor rust inhibitors many complicated instruments made from ferrous, nonferrous, and nonmetallic materials may be adequately protected and kept dry whether packaged for storage or shipment. Moreover, such equipment may be put into operation with a minimum of labor after unpackaging. Some of the inhibitor is adsorbed by the silica gel, but tests (3) show that this does not destroy the drying properties of the gel. It is probable that water can compete with the inhibitor for adsorption sites in the silica gel, so that as water vapor penetrates the package some of the adsorbed inhibitor is actually displaced from the silica gel. FACTORS CONTROLLING APPLICABILITY
REACTIONS WITH NONFERROUS METALS. The effect of the amine nitrites on nonferrous metals has been extensively investigated (W). Since that study numerous field tests conducted by other laboratories have substantiated these data. Briefly, the detrimental effects of the amine nitrites on nonferrous metals decreases with increase in molecular weight of the inhibitor. Thus diisopropylammonium nitrite is corrosive to copper, Dow metal (magnesium), cadmium plate, lead, soft solder, zinc plate, brass, and bronze; the dicyclohexylammonium nitrite is detrimental to zinc plate, magnesium, cadmium plate, and lead or to alloys of other metals containing more than 15% of zinc, lead, cadmium, or magnesium. The corrosive effect of these compounds on nonferrous metals decreases with a decrease in relative humidity and with a decrease in temperature. Dicyclohexylammonium nitrite has been used satisfactorily on assemblies containing zinc, lead, magnesium, or cadmium when the nonferrous metal parts were coated with a plastic or organic protective coating prior to packaging. The corrosive effect of dicyclohexylammonium nitrite on copper, aluminum, nickel plate, silver plate, and the bronzes is less than that of the lower molecular weight amine nitrites, and also much less than that of most amineorganic acid complexes. Other vapor rust inhibitors have been investigated far less extensively than the amine nitrites. However, laboratory tests show them to. possess similar detrimental corrosive effects on nonferrous metals. A11 of them should be used on multiple-metal assemblies with caution and with special attention to the protection of the nonferrous metals present. COMPATIBILITY WITH NONMETALS. The effects of the vapor from dicyclohexylammonium nitrite on nonmetallic materials
2597
have been reported by Wachter, Skei, and Stillman (&), who found that many materials were unaffected; however, rubber hydrochloride films showed marked deterioration and mild effects were observed on one formulation of Koroseal, on Tygon, and on a cellulose nitrate film. If only the compositions showing 100% inhibition in the tables are considered, their vapors should not be detrimental t o nonmetals such as fabrics, wood, leathers, papers, inks, and cork. However, the alkalinity of these compositions would be expected to affect nonmetals displaying acidic properties. The solvent action of many of the compositions might soften some of the less resistant plastics, protective coatings, rubbers, and adhesives. More work needs to be done before these compositions can be used safely on multiple-component systems without special preservative treatment of some nonferrous metals and some nonmetallic components. TOXICITY. Dicyclohexylammonium nitrite has been found to be similar to sodium nitrite in toxicity and as a health hazard (28). Diisopropylammonium nitrite was found to cause no changes in blood pressure, pulse rate, or respiration in an anesthetized dog when the vapors were inhaled (8). No cumulative toxic effects could be detected nor were local irritant effects on the skin observed from exposure to either compound ( 2 , M ) . T h e vapors given off by these compositions should, in general, b e considered toxic and be handled accordingly. The working space should be well ventilated and the hands should be washed thoroughly before eating. Laboratory personnel working with these compositions and exercising normal laboratory techniques and the above precautions have experienced no harmful effects. PACKAGING PROCEDURES
Volatile rust inhibitors are currently being marketed as the solid crystalline chemical and also as compounds coated on or impregnated into 30- to 60-pound kraft paper or chipboard. Other applications are to kraft paper backed with waterproof adhesive, kraft laminated with asphalt to a second sheet of kraft, kraft coated with wax on one side, kraft laminated to another sheet of kraft with wax, kraft laminated to glassine with wax, kraft embossed, and kraft laminated to polyethylene or aluminum foil. In order to ensure protection of the part before packaging Kith vapor rust inhibitors and also after removal from the package, the item should be given a complete coating of a light preservative lubricating oil prior to packaging. Coatings of this type supplement the preservative action of the vapor rust inhibitor. When volatile rust inhibitor packaging techniques are used, the vapors must have free access to all surfaces of the parts to be protected. If access is denied, supplemental preservation is necessary. I t is suggested that all items requiring preservation be no more than 1 foot from the vapor rust-inhibiting material. The face of special waterproof paper which is coated with vapor rust inhibitor should always be placed toward the item to be protected. In order to have a reserve of vapor rust inhibitor and thus compensate for unavoidable vapor leakage, a t least 2 square feet of paper should be employed for each cubic foot of space inside a package. The inhibitor content of the inhibited paper referred to in this discussion is a t least 2 grams of vapor rust inhibitor per square foot of paper. The type of outside barrier needed depends upon the required life of the package. Indoor shelf storage for periods up to one year requires only a good grade of wax paper overwrap, but extended storage of from 3 to 5 years requires sealed metal containers or bags made of barrier material (48). Assemblies which possess critical working surfaces of nonferrous metals, as well as ferrous metal parts, require additional preservation on the nonferrous metal surfaces in order to protect, the critical working area from attack by the vapor rust-inhibitar chemicals. Nonferrous components which do not comprise critical working surfaces of an assembly can usually be packaged
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INDUSTRIAL AND ENGINEERING CHEMISTRY
without supplemental preservation of the nonferrous metal parts, as very light attack on their surfaces by the vapor rust-inhibitor
,chemicals would not impair their subsequent, efficiency of operation. Here again it is necessary to be careful of the type of inhibitor ueed because the various chemical compositions differ in their corrosive effect on nonferrous metals such as copper, nickel, silver, aluminum, and the bronzes. The suppleniental protection can be furnished by either a plastic or waxlike sprag, Tesulting in a film impervious to the vapor rust-inhibitor vapors. (These films should be removed before the equipment is placed in service.) Thus either complex assemblies such as radios, radar, other electronic equipment’, fire control equipment, typemiters, calculators, guns, rockets, machine tools, hydraulic pumps, aeronautical and automotive engines, watches, and chronometers, or Eimple equipment like tools, empty drums or cans, apare parts, and pipe, can easily be packaged and preserved for many years by utilizing vapor rust inhibitors. Optical equipment should not be packaged with such inhibitors unless some measure has been talrcn to prevent the vapors from condensing on the optical surfaces. Halogenated solvents such as carbon tetrachloride, chloroform, trichloroet,hyIene, and perchloroethylene should not be used for cleaning items prior to packaging with vapor rust inhibitors, as the acid usually produced by hydrolysis of the halogenated solvent interferes with the rust inhibition. Most of the commercial nonflammable solvents are of this general type. Nonhalogenated solvents of the Stoddard type can be used without danger of corrosive aftereffects. However, these solverits are flammable and should be handled with proper precautions. As the potential users of vapor rust inhibitors gain confidence in the merits of vapor rust inhibition and as storage data are accumulated to prove these merits, such inhibitors will be adopted for use in many other packaging problems. If the indicated procedures are followed, instruments and equipment can be packaged and preserved safely for many years. Equipment and parts packaged with vapor rust inhibitors are ready for use immediately when unpacked. The saving in man-hours on reactivation of equipment is greater in many cases than the initial oost of packaging the equipment. Thus, for many applications i t is more economical than the old methods of preservation and packaging. ‘ICKNOWLEUGhIEZlT
The author acknowledges with pleasure many stimulating discussions on various aspects of rust inhibition with W.A. Zisman and C. R. Singleterry, whose suggestions have contributed largely both to the practical testing and to the theory of such inhibitors. LITERATURE CITED
Aviation A g e , 16, 32 (1951). Baker, H. It., “Properties, Saval Uses and Effect on Nonferrous Metals of Vapor Phase Inhibitors VPI 220 and 260,” AVRL R ~ p t P-3047 . (January 1947). Baker, H. R., Jones, D. T., and Zisman, W. A,, IND.EKG. C H E M . , 137 ~ ~ ,(1949). Baker, H. R., Shafrin, E. G., and Zismari, W. A , , J . Phys. Chem.,
56,406 (1952). Bakcr, H. R., Spcssard, D. R., W’olfe, J. K., and Zisman, W.A, U. S. Patent 2,602,780 (July 8, 1952). Baker, H. R.. and Zisman, W. d.,“Antirust Additives for Lubricating, Power Transmission, and Protective Oils,” iV‘RL Rept. P-2474 (February 1945). Baker, H. R., and Zkman, W. A,, IND.EHG.CHEM.,40, 2338 (1948). Baker, H. R., and Zisman, TV. A, Lubrication Eng., 7, 117 (1951). Bannister, €1. L., Research, 5 , 424 (1952). Black. A. R., and Wachter, A., Ordnance, 37, 1052 (1953). 43,884 (1951). Brophy, J. E., and associates, 1x1). ENG.CHRM., Cox, K. L., U. S. Patent 1,903,287 (1933).
Vol. 46, No. 12
Dreyfus, M. E., Heating and Ventilating, 39, 31 (1942). Fisher, E. ITr., InduStTy, 16, ?io. 1, 17-18, 50, 52, 54, 56 (October 1950). Foster, G., I r o n A g e , 167,99 (1951). Foster, G., Ordnance, 35,155 (1950). Gardner, F. T., Clothier, A. T., and Coryell, P., Corrosion, 6 , 58 (1950). Hackerman, If.,and Cook, E. L., J . Electrochem. Soc., 97, 1 (1950). Ilackerman, S . ,and Sudbury, J. D., I b i d . , 97, 109 (1950). Harkins, W. D., and Florence, R. T., J . Chem. Plays., 6, 847 (1935). Jacoby, A. L., U. S. Patents 2,580,923; 2,580,924 (Jan. 1, 1952). Kamlet, J., Ibid., 2,475,186 (July 5 , 1949). Kaufman, S., and Singleterry, C. R., J . Phys. Chem., 56, 604 (1952). Lane, R . W., and Thompson, IT. H., U.S. Patent 2,582,138 (Jan. 8, 1952). Lieber, Eugene, Ihid., 2,512,949 (June 27, 1950). XcOmie, W.A., and Anderson, H. H., Uniu. Calif. Pubs. Pharmacol., 2,231 (1949). Ilerker, R. L., and Zienian, W.A,, J . Phys. Chem., 56, 399 (1952). Llilyavskava, 1‘. O., U.S.S.R. Patent 69,947 (Dec. 31, 1937). Moore. R. AI., and Wachter, A., U. S.Patent 2,592,451 (April 8 , 1952). Ray, P. C., and Rakshit, J. K . , J . Ckern. Soc., 101, 612 (1812). Reiniger, S. W., Aviation W e e k , 55, 48 (1951). Schwoegler, E. J., and Hutter, C. -I.,U. S. Patent 2,521,311 (Sept. 5, 1950). Shafrin, E. G., and Zisman, W. il., J . Colloid Chem., 4, 571 (1949).
Speller, F. S . ,Proc. Xatl. District Hearing Assoc., 24, 203 (1933). Steel, 128,100 (1951). Stillman, S.,and Wachter, A, U. S. Patent 2,419,327 (April 22,
.
1947) Ihid., 2,432,839 (Dee. 16, 1947). Ibid., 2,432,840 (Doc. 16, 1947). Ihid., 2,449,962 (Sept. 21, 1948). I b i d . , 2,484,395 (Oct. 11,1949). Ibid., 2,563,764 ( l u g . 7, 1951). Ibid.,2,577,219 (Dec.4, 1951). Stroud, E. G., and Vernon, W. H. .J., J . -4ppZ. C h e m . , 2, 166
(1952). Sussex. A. G., d ustralasian E n g r . , 1947, 68- 75 (Kovember), Temple, I
