Naphthalene dicarboxylic acid salts as corrosion inhibitors
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to corrosion-inhibited aqueous solutions and particularly relates to corrosion-inhibited antifreeze compositions containing naphthalene dicarboxylic acid salts useful as coolants such as in the cooling system of an internal combustion engine. 
     2. Other Inhibitors Known in the Art 
     It is well known to use dicarboxylic acids as corrosion inhibitors in aqueous systems. For example, Jones in U.S. Pat. No. 2,726,215 teaches that dicarboxylic acids; namely sebacic acid and azelaic acid, and their alkali and alkali metal salts are useful corrosion inhibitors in aqueous solutions. The use of a mixture of sodium sebacate (sodium salt of sebacic acid) and benzotriazole was disclosed as a useful corrosion inhibitor in engine coolants by G. Butler, et al. in &#34;Inhibitor Formulations for Engine Coolants,&#34; British Corrosion Journal, Vol. 12, No. 3, 1977, pp 171-174. 
     U.S. Pat. No. 3,931,029 to Dutton, et al. teaches the use of certain unsaturated cycloalkylene dicarboxylic acids as antifreeze additives to inhibit corrosion of contacted solder alloys. Corrosion inhibitors containing an imidazoline derivative, a carboxylic acid or its metal salt and/or a phosphate are used for steel in brackish or acidic water according to Chemical Abstracts, Vol. 99, paragraph 126713x, 1983, which describes Japanese Kokai 58-84,981. U.S. Pat. No. 4,382,008 reveals a corrosion-inhibited antifreeze containing a triazole, an alkali metal borate, an alkali metal benzoate, an alkali metal silicate and an alkali metal salt of a C 7  to C 13  dibasic organic acid. The use of sodium sebacate as a corrosion inhibitor in phosphate-based antifreezes is further seen in the Derwent Abstract of Week E14 for French Certificate of Utility No. 2,489,355 to Perrot. 
     The Derwent Abstract of Week K18 for European Pat. No. 77,767-B teaches the use of water-soluble salts of dicarboxylic acids having at least three carbon atoms as antifreeze corrosion inhibitors. These acids are malonic, succinic, glutaric and adipic acids along with smaller proportions of C 8  and/or C 10  dicarboxylic acids. A mixture of a siloxane-silicate copolymer with an azole was found effective in reducing the corrosion of high-lead solder and aluminum in aqueous liquids according to U.S. Pat. No. 4,402,847 to Wilson, et al. U.S. Pat. No. 4,414,126 also to Wilson involves the use of alkali metal mercaptobenzothiazoles as effective inhibitors for the corrosion of high lead solder in aqueous systems. 
     U.S. Pat. No. 4,454,050 to Bertell teaches the use of the alkali metal salts of naphthalene dicarboxylic acids to prevent sticking and staining of castings when in contact with a dye. 
     U.S. application Ser. No. 06/672,736 filed Nov. 19, 1984, discusses other dicarboxylic acid containing corrosion inhibiting compositions. 
     SUMMARY OF THE INVENTION 
     The invention concerns a corrosion inhibited antifreeze composition having a water soluble liquid alcohol freezing point depressant and an effective amount of an inhibitor. The inhibitor is a selected dibasic acid salt of naphthalene dicarboxylic acid and optionally a triazole. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     It has been surprisingly discovered that a corrosion inhibiting composition having a dicarboxylic acid component and optionally a triazole component can be made from plentiful and inexpensive raw materials. These corrosion inhibitors are particularly effective in antifreeze formulations for automobile cooling systems, when used in an effective amount of about 0.1 to 5.0 wt %. 
     The antifreeze formulations most commonly used include mixtures of water and water-soluble liquid alcohol freezing point depressants such as glycols and glycol ethers. The glycols and glycol ethers which can be employed as major components in the present composition include glycols such as ethylene glycol, diethylene glycol, propylene glycol, and dipropylene glycol, and glycol monoethers such as the methyl, ethyl, propyl and butyl ether of ethylene glycol, diethylene glycol, propylene glycol and dipropylene glycol. Also useful are glycol diethers such as methyl and ethyl diethers of ethylene glycol, diethylene glycol and dipropylene glycol. Ethylene glycol is particularly preferred as the major antifreeze component. 
     Naphthalene dicarboxylic acid is known e.g. U.S. Pat. No. 4,454,050. It has been found surprisingly that the 2,6 and 1,5 naphthalene dicarboxylic acid salts are effective corrosion inhibitors. Whereas, the 2,3 and 1,2 salts are not effective (e.g. Examples 4 and 5) because they form metal chelates, thereby promoting degradation of the metal surface. 
     The alkali metal salt is formed by reacting the dicarboxylic acid with alkali metal hydroxide (e.g. sodium, potassium or lithium hydroxide) in water in equivalent proportions to neutralize the carboxylic acid groups. The amine salt is formed by reacting the dicarboxylic acid with a tertiary amine; preferably triethanolamine, in water in equivalent proportions to neutralize the carboxylic acid groups. A pH of 7 to 8 indicates that all of the carboxyl groups have been neutralized and that neither free acid nor excessive free base remains to cause corrosion, odor or handling difficulties. The salt is incorporated in the antifreeze concentrate at a final pH of 6.5 to 9.0. 
     It has been found that the identified dicarboxylic acid salts are effective in inhibiting corrosion in antifreeze compositions when incorporated in an amount of 2 to 5 wt % and optionally with a triazole in an amount of 0.1 to 1.0 wt %, e.g. Example 1. Alkali metal mercaptothiazoles and alkali metal tolyltriazoles and alkali metal benzotriazoles may be used, e.g. sodium mercaptobenzothiazole. Benzotriazole and tolyltriazole is especially preferred. 
     Additionally other conventional corrosion inhibiting compounds may be incorporated and the amount of naphthalene dicarboxylic diacid salt reduced to about 0.1 wt % to 1.0 wt %. These conventional corrosion inhibiting compounds include alkali metal silicates (e.g. sodium metasilicate, potassium metasilicate, lithium metasilicate and preferably sodium metasilicatepentahydrate); amines; nitrites; nitrates, chromates, borates and phosphates. Other corrosion inhibitors and additives which may be used are benzoates (alkali metal benzoates), molybdates (alkali metal molybdates), various antifoaming agents and dyes. Stabilizers may also be added; such as polysiloxane stabilizers to prevent alkali metal silicate-caused gellation problems. 
     When used in an amount of 0.1 to 1.0 wt % in conjunction with conventional corrosion inhibitors, the dicarboxylic acid salts are cast iron corrosion inhibitors. 
    
    
     The invention will be further illustrated by the following examples which are not intended to limit, but rather to illuminate the invention. 
     EXAMPLE 1 
     A blend of 2,6 naphthalene dicarboxylic acid, dipotassium salt (4.2 wt %) and tolyltriazole (0.2 wt %) in ethylene glycol was prepared and tested in the Ford Aluminum Pitting Potential Test with a potential of -361 mV vs SCE (spec, min. -400); passing. 
     EXAMPLE 2 
     The blend of Example 1 was tested in the ASTM D-1384 Glassware Corrosion Test with passing results. 
     
         ______________________________________
ASTM D-1384 Glassware Corrosion Test, Mod., 25%
Weight Loss mg/coupon
       Cu  Solder    Brass   Steel  Fe   Al
______________________________________
          2     1         1     0     -11  11
Spec, max
         10    30        10    10      10  30
______________________________________
 
    
     EXAMPLE 3 
     The blend of Example 1 was tested in the ASTM D-4340 Aluminum Heating Rejecting Surface Test with a weight loss of 0.34 mg/cm 2  /wk (ASTM spec., max 1.0); passing. 
     EXAMPLE 4 
     A blend of 2,3-naphthalene dicarboxylic acid, disodium salt (4.2 wt %) and tolyltriazole (0.2 wt %) in ethylene glycol was prepared and tested in the ASTM D-1384 Glassware Corrosion Test with failing results. 
     
         ______________________________________
ASTM D-1384 Glassware Corrosion test, 25%
Weight Loss mg/coupon
       Cu  Solder    Brass   Steel  Iron Al
______________________________________
          3     0         3     0     168   0
Spec, max
         10    30        10    10      10  30
______________________________________
 
    
     EXAMPLE 5 
     The blend of Example 4 was tested in the Ford Aluminum Pitting Potential Test with a potential of -490 mV vs SCE (spec, min. -400); failing. 
     Examples 4 and 5 demonstrate the corrosive nature of certain isomers. This surprising result restricts the use of naphthalene dicarboxylic acid salts as corrosion inhibitors to the 2,6 and 1,5 isomers. 
     EXAMPLE 6 
     A blend prepared as in Example 1 was tested in the ASTM D-1384 Glassware Corrosion Test to duplicate Example 2 with passing results. 
     
         ______________________________________
ASTM D-1384 Glassware Corrosion Test, 25%
Weight Loss mg/coupon
       Cu  Solder    Brass   Steel  Iron Al
______________________________________
          1     4         2     1      0   14
Spec, max
         10    30        10    10     10   30
______________________________________
 
    
     EXAMPLE 7 
     A blend was prepared as in Example 1 but without tolyltriazole. The blend was tested in the ASTM D-1384 Glassware Corrosion Test with passing results. 
     
         ______________________________________
ASTM D-1384 Glassware Corrosion Test, 25%
Weight Loss mg/coupon
          5     4         6     2      2   11
Spec, max
         10    30        10    10     10   30
______________________________________
 
    
     These results are comparable to those obtained in Examples 1, where tolyltriazole was added to the formation. Triazoles are added to prevent copper and brass corrosion. The formulations of this invention provide good protection for these metals without the use of triazoles which are generally very expensive. 
     The examples are for illustrative purposes and not meant to limit the invention in any way. Variations and modifications of the above are possible without exceeding the scope of the present invention. For example, the corrosion inhibited composition descibed herein is a concentrate. This concentrate is diluted with 40 to 60 vol % water; preferably 50 vol % water when used in automotive cooling systems. In practice, this dilution is not monitored and the diluted compositions of the present invention are designed to be noncorrosive and effective up to a water concentration of 75 vol % or more. 
     It is understood that water dilutions are equivalent to the concentrated compositions set forth in the following claims: