670
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21. 670-672
Aluminum-I ,I, 1-Trichloroethane. Reactions and Inhibition Wesley L. Archer Inorganic Chemicals Department, Dow Chemical U.S.A., Midland, Michigan 48640
The reaction sequence between a chlorinated solvent, 1, 1,l-trichloroethane, and aluminum is explained and a proposed reaction mechanism is given. The addition of a proper inhibitor to the solvent stops the vigorous solvent-metal interaction and allows solvent use in aluminum metal cleaning applications. Twenty-two compounds were evaluated as inhibitors, at reflux temperatures, of the reaction of aluminum with l , l ,1-trichloroethane. Minimum concentrations needed to inhibit the reaction are tabulated. Certain structure-activity relationships of the tested inhibitors are examined. I t is suggested that the inhibitor competes with the solvent for the aluminum chloride produced at micro corrosion sites. Successful inhibition involves complexing of the chemisorbed aluminum chloride product with electronegative groups of the inhibitor. The resultant complex is insoluble in the solvent and acts as a plug or cover over the original reaction site.
Introduction Chlorinated solvents such as l,l,l-trichloroethane and trichloroethylene are used extensively in industrial vapor degreasing metal cleaning applications. Inhibitor systems are used to prevent reactions of the solvent with metals such as aluminum. Proper solvent stabilization allows the use of these solvents in large-scale aluminum cleaning applications, e.g., the aerospace industry. The purpose of this paper is to examine how the solvent attacks a metal such as aluminum and how the inhibitors prevent this interaction. The proposed mechanism of solvent-metal interaction and passivation has several similarities to metal-water interactions. However, the chlorinated solvent-metal interaction is not electrochemical in nature, i.e., having definite anodic-cathodic sites. The corrosion attack is instead a direct chemical attack on the metal by the solvent to yield a metal chloride salt and a dimer of the organic reactant. Twenty-two compounds were evaluated a t reflux temperatures as inhibitors of the reaction of aluminum with l,l,l-trichloroethane. This research on the structure-activity relationships of these inhibitors suggests that inhibition of the aluminum-l,l,l-trichloroethanereaction involves complexing of the chemisorbed aluminum chloride reaction product with electronegative groups of the inhibitor. Experimental Section Aluminum Stability Tests. In these tests, 5 mL of inhibitor-containing solvent was refluxed with 0.5 g of 16-32 mesh pure aluminum pellets in open reaction tubes. The tubes, 33 cm long and 1cm o.d., were placed in an oil bath that was held at 76 “C for l,l,l-trichloroethane reflux tests. The upper portions of the reaction tubes extended through a water-cooled aluminum block that acted as a condenser. If no reaction (as evidenced by darkening of solvent and hydrogen chloride evolution) was observed in 24 h, the inhibitor was considered to be active at the test concentration. By observing the effect of different concentrations, a Minimum Active Concentration (MAC) (expressed as mol/L) needed to inhibit the aluminum reaction was determined for the various inhibitors. Uninhibited, a t reflux temperatures, the l,l,l-trichloroethane-aluminum reaction begins wthin 3-5 min. Historical Background I. Metal Reaction. Aluminum is a very reactive metal that may be corroded by a variety of chlorinated hydro0196-4321/82/1221-0670$01.25/0
carbons. Stern and Uhlig (1952,1953) have systematically investigated the reaction of aluminum with carbon tetrachloride. They found that a definite induction period exists before carbon tetrachloride corrodes an aluminum coupon. The corrosion rate after this induction period is very high (approximately 40 000 mg/decimeter/day) and remains constant until the aluminum is consumed. These workers concluded that the halocarbon-aluminum reaction is a direct chemical attack and not electrochemical in nature. Minford and co-workers (1959) identified hexachloroethane and aluminum chloride as the two reaction products, e.g., in eq 1. 2A1 + 6 CCl, 3C2C16+ 2AlC13 (1) Work from our laboratory (Archer and Harter, 1978) identified two new products, perchloroethylene and hexachlorobutadiene, in the aluminum-carbon tetrachloride reaction. Excess water promotes corrosion at the solvent-water interface because the metal chloride reaction product can be easily dissolved from the metal surface into the water phase. Several other uninhibited chlorinated solvents will completely react with aluminum, these include 1,2-dichloroethane, 1,2-dichloropropane, 1,l-dichloroethane, l,l,l-trichloroethane, and 1,1,2-trichloroethane(Archer and Simpson, 1977). Addition of a variety of selected organic inhibitors will render each of these solvents noncorrosive toward aluminum and allow the solvent’s use in many applications. Uninhibited l,l,l-trichloroethane reacts immediately with aluminum when the metal surface in contact with the solvent is scratched. The resultant “bleeding” red color emitting from the scratch area is the solvent complex with the aluminum chloride corrosion product. The l,l,l-trichloroethane solvent reaction with aluminum affords aluminum chloride and the saturated dimer, (CH3CC12CC12CH3). The saturated dimer can in turn react with the metal to give the unsaturated cis and trans dimers (CH3CC1=CC1CH3). Iron or tin also react with l,l,l-trichloroethane to give similar reaction products although the corrosion rates are much slower than with aluminum (Archer, 1967). The vigorous nature of the aluminum reaction is caused by the subsequent dehydrochlorination (loss of HC1) of CH3CC13 by the aluminum chloride. Proper stabilization of the solvent will prevent all aluminum attack. 11. l,l,l-Trichloroethane. The following reaction sequences are proposed as an explanation of the aluminum reaction in the l,l,l-trichloroethane.
-
1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 671
Unoccupied valence sites on the aluminum oxide surface will normally have adsorbed hydroxyl ions or water molecules, The adsorbed oxygen containing species can, however, be displaced by the l,l,l-trichloroethane (CH,CCl,) as in eq 2. A1203:OH2 CH3CC13 A1203:C1-CC12CH3+ HzO (2) excess Ionization of a chlorine-carbon bond would give the species shown in eq 3.
-
+
CI
\ A1203:CI-C-CH3 / CI
-
c I\
CI
A1203:
-\ C I +C-CH3. / CI
CI,
Loss of a proton from the dichloroethane carbonium ion gives vinylidene chloride (CH2=CC12),a product that has been identified. It has been demonstrated in our laboratories that refluxing uninhibited l,l,l-trichloroethane with high surface area y-alumina causes dehydrochlorination of the solvent to give vinylidene chloride and hydrogen chloride. The amorphous surface oxide on aluminum is the y form. A reaction sequence first suggested by Foroulis and Thubrikar (1975) can be used to explain the chloride ion induced breakdown of aluminum oxide films. The adsorbed chloride ion can facilitate degradation of the oxide structure by formation of a basic aluminum hydroxychloride salt product with the lattice cation A1(OH)zf as shown in eq 4. Al(OH), * Al(OH)2+ OHAl(OH)2+
+ C1-
-
+
Al(OH)&l
(4)
The resultant soluble aluminum hydroxychloride salt, Al(OH),Cl, can then be removed from the oxide structure by complexing with the solvent. The vacant aluminum orbital is satisfied by donation of an electron from a chlorine atom in the solvent to the aluminum atom. An example of this aluminum atom complexing is seen when the addition of aluminum chloride (a dimer) to l,l,l-trichloroethane gives a soluble red complex. Displacement of one of the molecules of aluminum chloride from the dimer by a solvent molecule gives the colored complex. The reaction sequence between aluminum and l,l,ltrichloroethane is shown in eq 5 and 6
Homolytic cleavage of the carbon-chlorine bond and electron donation from the aluminum to chlorine atom gives identified products of aluminum chloride and product I, 2,2,3,3-tetrachlorobutane(Archer and Simpson, 1977). The aluminum chloride, a Lewis acid, can then degrade the solvent as shown in eq 7. CI
C13CCH3
+
AICl3
\C=C
/H
I
\H
CI
+
HCI
+
PIC13
(7)
The aluminum chloride product which is not consumed in the reaction affords autocatalytic degradation of the solvent. The addition of properly selected organic inhibitor will prevent the aluminum-induced breakdown of the chlorinated solvent. 111. Solvent Inhibitors. Proprietary formulations of chlorinated solvents can contain three different types of inhibitors: antioxidants, acid acceptors, and metal stabilizers. Antioxidants are normally amine or phenolic type compounds added to oxidation-prone solvents such as trichloroethylene or perchloroethylene. Acid acceptors can be epoxy type compounds added to neutralize or react with small amounts of hydrogen chloride that are normally formed during many chlorinated solvent applications. The metal stabilizers are Lewis base type compounds that deactivate metal surfaces and remove or complex trace amounts of the metal chloride salts that might form during use. This paper discusses the protection mechanisms of the l,l,l-trichloroethane metal inhibitors. The relationships between structure and inhibition activity of a number of aluminum-methylene chloride/ toluene inhibitors has been reported by the author (Archer, 1979). Discussion of Results Ability to react with or complex the metal halide, e.g., aluminum chloride, is a common property shared by most of the metal reaction inhibitors. Some of these inhibitors have also been shown to terminate the transient free radical species by hydrogen donation, e.g. [CCl,] + RH R. + CHC1, (Archer and Harter, 1978). Neither the simple removal of the metal halide by complex formation nor termination of the free radical species is the primary form of reaction inhibition. Detailed laboratory investigation has shown that the following interpretation best explains the mode of metal-l,l,l-trichloroethaneinhibition. In the reaction between aluminum metal and l,l,l-trichloroethane, the aluminum chloride produced at the metal surface is normally removed from the surface by solvent interaction. The inhibitor competes with the solvent for this aluminum chloride. The arena of reaction and inhibition involves competition between the solvent and the metal stabilizer for the reactive, electron deficient, sites on the metal or metal oxide surface, i.e. active A1 site + CH3CC1, reaction active A1 site inhibitor Al-inhibitor complex
-
+
-
For inhibition to occur, the inhibitor has to compete successfully for the active sites and chemiadsorbed aluminum chloride and convert the aluminum chloride into an insoluble “deposit plug” on the metal surface. Substantiation of this concept of an insoluble deposit has been furnished by the following study. Long term contact (210 h) of an electropolished 99.99% pure aluminum coupon with a commercial vapor degreasing grade of boiling l,l,l-trichloroethane (bp 165 O F ) did not produce any visual or chemical evidence of a metal reaction. However, microscopic examination (500x1of the mirror-finished surface revealed a slow accumulation of solvent-insoluble deposits a t micro reaction sites on the metal surface. The first deposit appeared in 22.5 h. Examination of the surface debris with an electron probe showed high carbon and chlorine in the insoluble deposits formed at the micro reaction sites. Thus the solvent during prolonged contact may afford micro reaction areas at defects on the aluminum surface, but in well-stabilized solvent the reaction site will be quickly sealed over by the insoluble aluminum chlorideinhibitor complex. The insoluble aluminum chloride
672
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
Table I. Coniparison of 1,l,l-Trichloroethane-Aluminum Inhibitors example no. 1 2 3 4 6 6 P7
8
9 10
11 12 13 14 15'
test compound pyridine pentamethylene sulfide tetrahydropyran acetonitrile malononitrile dioxane 1,2-dimethoxyethane 2 , 3 and 4-pyridinecarboxaldehydes o- and p-nitrobenzaldehydes 2,3-butanedione 2,5-hexanedione benzyl fluoride benzotrifluoride 2,3 and 4-cyanopyridines 0 - and p-nitrobenzonitriles 3-aminoquinoline
MAC. a mol/L 0.4 0.8 1.7 0.13 0.03 0.07 0.10 0.02-0.04 0.04-0.05 0.05 0.02 0.017 0.50 0.04-0.05 0.02-0.07
0.03
a MAC = minimum active concentration of inhibitor needed t o stop l,l,l-trichloroethane-aluminumreaction for 24 h at reflux. Smaller MAC values indicate more active inhibitor
complex serves as an effective barrier or repair of the oxide film separating the metal from the solvent. This study shows that certain features must be present in the inhibitor structure to ensure suitable inhibition. First, there must be one, and preferably two, electronegative functional groups in the inhibitor structure. The greater activity of a nitrogen functional group over a sulfide linkage, which is better than an ether linkage, is in accordance with the known ability of these to complex with Lewis acids. Likewise, cyclic structures are generally more active than their straight-chain analogues. Structures commonly cited as inhibitors include ether linkages, sulfide linkages, carbonyl groups, nitriles, amines, and alcohols. A comparison of some l,l,l-trichloroethane-aluminum inhibitors is shown in Table I in terms of the minimum active concentration (MAC) of inhibitor necessary to inhibit the aluminum reaction for 24 h. A low value expressed in moles/liter concentration is desirable. Examples 1-3 demonstrate that nitrogen is more active than sulfur which is more active than oxygen. A difunctional inhibitor such as dioxane (no. 6) with an MAC of 0.07 is more active than the monofunctional analogue tetrahydropyran (no. 3) at an MAC of 1.7. The cyclic dioxane structure (MAC = 0.07) is also more active than the open chain diether, 1,Qdimethoxyethane (no. 7 ) with an MAC of 0.10. See also examples 4 and 5 for mono- and dinitrile type inhibitors. Inserting an electronegative grouping such as an aldehyde into the pyridine ring (no. 8) or an amine group in a quinoline ring (no. 15) gives a difunctional inhibitor and good inhibition. Nitrile groupings often give good inhibition, e.g., pyridine nitrile derivatives (no. 131, nitro-
benzonitriles (no. 14) or a dinitrile like malononitrile (no. 5). Diketone structures, e.g., no. 10 and 11, also afford suitable stabilization of aluminum-solvent systems. The activity of the fluoro derivatives benzylfluoride or benzotrifluoride (no. 12), may not be due to a Lewis base-aluminum chloride interaction but to reaction of the fluorine atom with the metal chloride to give an insoluble aluminum fluoride. The insoluble fluoride salt would seal off the micro reaction sites. Eiseman (1963) has suggested that a reaction formed fluoride film on aluminum may function like the normally protective aluminum oxide film in preventing the halohydrocarbons reactions. Previous work in this laboratory (Archer, 1979), has shown that the better l,l,l-trichloroethane inhibitors are not the best structures for inhibiting methylene chloride/toluene aluminum reactions. For example, the effectiveness of methylene chloride-toluene solvent inhibitors is: (1)0 > S > N; (2) RX > XRX; a complete reverse of inhibitor activity shown in l,l,l-trichloroethane solvent. The solubility of the resultant metal chloride-inhibitor complex, which varies among solvents, helps to determine inhibitor performance in any particular solvent. The more active l,l,l-trichloroethane type inhibitor-aluminum chloride complex may be sufficiently soluble in methylene chloride/ toluene mixtures to initiate a Friedel-Crafts reaction. Conclusions All of the solvent inhibitor types except the fluoro derivatives are electronegative in nature and can be classed as Lewis bases. A Lewis base is a structure with a donor atom(s) that has available electrons capable of complexing by electron donation into an electron deficient atom such as aluminum. The electron deficient (Lewis acid) sites on the aluminum or oxide surface are the reaction sites for a chlorinated solvent. The inhibitor must therefore "cover up" or neutralize these active sites in order to prevent the solvent reaction. In turn, any metal chloride produced is removed from possible solvent interaction by the formation of an insoluble complex with the inhibitor. Any effective inhibitor will extend the induction period of the chlorinated solvent-metal reaction for an infinite time. An extension of the induction period is equivalent to a minimum of reaction and thus corrosion inhibition. Literature Cited Archer, W. L.; Harter, M. K. Corrosion 1978,35(5),159. Archer, W. L.; Simpson, E. L. Ind. Eng. Chem. Prod. R e s . Dev. 1977, 16, 158 Archer, W. L. Ind. Eng. Chem. Prod. Res. D e v . 1979, 18,131. Archer, W. L. Aerosol Age Aug 1987, 16-18. Elseman, B. J., Jr. ASHRAEJ. May lBS3,63. Foroulis, 2 . A.; Thubrikar, M. J. J . Electrochem. SOC.1975, 122(10), 1296. Minford, J. D.;Brown. M. H.; Brown, R. H. J . Electrochem. SOC.1059, 706, 185-1 9 1, Stern, M.; Uhlig, H . H . J . Electrochem. SOC.1952,99, 381, 389. Stern, M.; Uhlig, H. H, J . Electrochem. SOC.1953. 100, 543.
Received for review March 29, 1982 Accepted June 28, 1982
