Abstract:
A corrosion inhibiting coating composition for metals comprising a liquid polymer composition which contains a polymer which is capable of being cured to a solid elastomer such as polysulfide, polyurethane, polythioether or polyether and a water soluble ammonium salt of phosphoric acid, orthophosphorous acid or hypophosphorous acid.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a composition and method for inhibiting the corrosion of metal parts by aqueous salt solutions present in the environment (e.g. aqueous NaCl solutions). The method includes coating said metal parts with a corrosion inhibitive liquid polymer composition and curing same to a solid elastomer. The non-toxic corrosion inhibitive polymer composition includes a liquid polysulfide, polyurethane, polythioether or polyether having incorporated therein a metal corrosion inhibitive effective amount of at least one ammonium salt of ortho- or hypophosphorous acid or phosphoric acid (ortho-, meta- or hypophosphoric acid). 
     The present invention is particularly well suited for inhibiting the corrosion of at least two metal parts which have a joint or space therebetween formed by the opposed mating surfaces of said metal parts which are secured together. In an attempt to prevent the corrosion of metal parts, the joint or space formed by the interface between the metal parts (particularly aluminum and/or aluminum alloy) is oftentimes filled with a liquid polymer cured to an elastomeric solid which helps to prevent aqueous salt solutions, as well as oxygen, from coming into contact with the mating surfaces of the parts which are joined. The problem of corrosion of aluminum containing metal parts (including aluminum alloys) is even worse in the case of aircraft and ships since the metal parts making up the aircraft and/or ship are oftentimes made of a number of metals including aluminum and/or aluminum alloys. When dissimilar metals are joined, corrosion is a serious problem when the space or joint formed by the interface between the mating surfaces of the parts is exposed to aqueous salt solutions. This is particularly true when aluminum (including aluminum alloys) is joined or secured to a dissimilar metal. For example, in the case of aircraft, aluminum and/or aluminum alloy panels are often secured together with cadmium, nickel, stainless steel or titanium rivets which cause severe corrosion problems when the spaces or joints between such rivets and panels are exposed to aqueous salt solutions particularly in the presence of oxygen. The same is equally true of ships which have aluminum superstructures joined to steel hulls. 
     In the past, exclusion of aqueous salt solutions, electrical insulation and sacrificial anodes between dissimilar metals have been the primary means employed to control corrosion of the metal parts. The large stresses and movements of the structures of both aircraft and ships have made the use of elastomeric sealants the preferred material to both exclude aqueous salt solutions and accommodate structural movements. In practice, however, many interfaces of metal structures sealed with elastomers become permanently contaminated with aqueous salt solutions which seriously attack and weaken structural components. In addressing this problem, U.S. Pat. No. 3,841,896 added to polysulfide elastomers water soluble alkaline earth chromates. Such chromates were very beneficial in producing corrosion inhibitive elastomeric polysulfides but the use of chromates has been severely restricted because of toxicity and disposal problems. Therefore it is desideratum in the art to produce corrosion inhibitive elastomeric polymers which are corrosion inhibitive but do not contain chromates or other toxic compounds. 
     Although, as noted, soluble chromates have been found beneficial, I have also found that they offer very limited protection in preventing aluminum alloy attack, particularly when coupled with dissimilar metals such as carbon fiber reinforced panels or titanium metal which is a common metal used as a fastener in the aircraft industry. 
     SUMMARY OF THE INVENTION 
     The present composition and methods are predicated upon the surprising discovery that when a liquid polymer curable to a solid elastomer has incorporated therein one or more of the ammonium salts of (ortho or hypophosphorous acid) or phosphoric acid (ortho -, pyro- or hypophosphoric acid) the resulting composition offers excellent protection against corrosion attack when coated on metal parts. They are particularly effective against crevice corrosion attack of aluminum (including aluminum alloys) secured to or in close proximity to other metals such as cadmium, titanium and stainless steel. 
     The method of the present invention is particularly useful in preventing corrosion attack of aluminum metals by applying said composition to the surface of th metal and it is even more useful in protecting the interface of aluminum and dissimilar metals joined together by filling the space between the interface with the liquid polymer composition of the present invention to form an elastomeric sealant between the surfaces of the two dissimilar metal parts. 
     The foregoing is particularly surprising in view of the fact that U.S. Pat. No. 4,212,793 discloses that various alkaline salts of phosphoric acid and phosphorous acids, when added to a poly (arylene sulfide) resin prevents corrosion to the mold used in molding the resin, said corrosion being due to contact of the mold with sulfur dioxide. We have found that the water soluble salts mentioned in this patent (sodium hypophosphite and sodium triorthophosphate) do not have any significant effect when incorporated into elastomers, in preventing corrosion of metals such as aluminum and/or aluminum alloys due to exposure to aqueous salt solutions. 
     Ammonium salts that have been found to be particularly effective when incorporated into liquid polymers curable to elastomeric solids in corrosive inhibitive effective amounts are the ammonium salts of orthophosphoric acid and hypophosphorous acid. The preferred ammonium salts are ammonium hypophosphite and ammonium dihydrogen phosphate, including mixtures thereof. It is presently believed that incorporation of these ammonium salts in a liquid polymer which, when cured to an elastomeric solid when in contact with the metal part or parts, will alleviate pitting and corrosion, particularly crevice corrosion, on such metal parts. The present invention is very useful in preventing such pitting and corrosion of aluminum (including aluminum alloys) surfaces, even when such surfaces are secured or coupled together by a fastener such as a rivet made of a dissimilar metal such as titanium. 
     The presently preferred liquid polymers are polysulfides, polyurethanes, polythioethers, and polyethers and particularly preferred liquid polymers are those which are mercaptan terminated. 
     The present invention is particularly beneficial in those elastomeric mercaptan terminated polymers which are cured by using an alkaline oxidation catalyst. As is known in the art, most mercaptan terminated polymers are cured with an oxidation catalyst which is alkaline, either per se or by the addition of an alkaline material such as sodium hydroxide. In order to affect a cure of mercaptan terminated polymers using most oxidation catalysts the cure must be effected in an alkaline environment, i.e. the pH must be greater than 7. 
     Oxidation catalysts useful in curing the mercaptan terminated polymers of the present invention include organic and inorganic peroxides (e.g. calcium peroxide) and oxides such as manganese dioxide. In the case of manganese dioxide, a slight amount (from 0.5 to 3 weight percent) of sodium hydroxide is added in order to make the catalyst effective. It is particularly surprising that the ammonium salts of the present invention achieve such excellent results because it would be expected that the sodium hydroxide present in the manganese dioxide catalyst would convert the ammonium salts to the corresponding sodium salts, which, as noted above, have been shown to be relatively ineffective in reducing corrosion of metal parts when exposed to aqueous salt solutions. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The addition of the ammonium salts of the present invention to a liquid polymer curable to an elastomeric solid is believed to provide corrosion resistance to metal parts coated or sealed with such liquid polymer compositions. More specifically, the addition of the ammonium salts of the present invention to said liquid polymers eliminates the pitting and erratic dissolution of aluminum or aluminum alloy parts coupled with cadmium plated steel, stainless steel or titanium fasteners, when such parts and fasteners are coated and/or sealed with the liquid polymer compositions of the present invention and such compositions are cured to a solid elastomer. 
     As noted hereinbefore, the particularly preferred polymers are polysulfides, polyethers, polythioethers and polyurethanes, particularly those which are mercaptan terminated and cured with an alkaline oxidation catalyst such as manganese dioxide, calcium peroxide, etc. 
     By &#34;polysulfides&#34; we mean polymers having disulfide linkages, a number of which are commercially available under the name Thiokol polysulfides, such as those disclosed in U.S. Pat. No. 2,466,963. Other polysulfide polymers useful in the present invention are disclosed in U.S. Pat. Nos. 4,623,711 and 4,609,762. Both of these patents also disclose mercaptan terminated polysulfides. Polyurethane polymers useful in the present invention are well known in the art and are specifically disclosed in U.S. Pat. No. 3,923,748 which also discloses mercaptan terminated polyurethanes. 
     Similarly, polythioether polymers are also well known in the art and are, for example, disclosed in U.S. Pat. No. 4,366,307. Mercaptan terminated polythioethers are also disclosed in this patent. 
     Polyethers useful in the present invention are also well known and are, for example, disclosed in U.S. Pat. No. 4,366,307, which also discloses mercaptan terminated polyethers. 
     The amount of ammonium salts to be added to the elastomeric polymers of the present invention may vary. For example, in general the amount of ammonium salts added to the elastomeric polymer is between about weight % and about 20 weight %, (based on the weight of the liquid polymer), with the preferred amounts being between about 3 weight % and about 14 weight %. 
     Insofar as corrosion resistance is concerned, a number of methods have been used to measure this phenomenon against metals. One of the commonest used in the coatings industry involves the use of an aerated salt spray wherein coated, scribed panels are exposed to the environment for long periods and examined for corrosion, undercutting, blistering, etc. This test may not be representative of many critical conditions found in metal structures. We have devised a test which simulates a joint between the surfaces of two dissimilar materials and allows the entrance of the environment, permanently, into the interface, under conditions not unlike those experienced by structures in marine environments where collection of salt and water in joints is essentially irreversible. The driving potential of the coupled metals is also an important factor in increasing the corrosive attack by chemically reducing oxygen and water to form sodium hydroxide in close proximity to the aluminum surface rather than being washed away in salt spray. The nature of the observed corrosion parallels closely that found in the filed. The specific test used by us is as follows: 
     Two inch by five inch panels of untreated aluminum alloy, 7075-ST-6 are coated with five 0.02&#34;×1/2&#34;/2&#34; strips of sealant, each strip separated from the adjacent strip by a 1/2&#34; band of an uncoated section of the aluminum. A candidate test metal (i.e. cadmium plated steel) of similar dimensions to the aluminum panel is pressed against the sealant coated side of the aluminum and held together by adhesive or masking tape on the ends leaving the 5&#34; sides exposed. (Panels are coated on the back side with an insulating film where electrical measurements are to be made.) This sandwich type assembly is one half immersed in a trough of 3% salt water, edgewise, along its 5&#34; length. 
     The trough is open to the atmosphere but loosely covered to limit water evaporation. To encourage galvanic corrosion, the metal couples are connected with alligator clips to induce corrosive current flow between the dissimilar metals. Salt water and oxygen diffuse into the cavities introduced by the 20 mil thick sealant into the 1/2&#34; spacings. The shorted circuits may be opened at intervals to measure voltage and current flows with sensitive voltammeters or a Wheatstone bridge and finally examined for corrosion and undercutting of sealant on the inside surfaces of the cell sandwich. 
     In order to test various salts as inhibitor the following elastomeric sealant was used wherein the Polysulfide Polymer is manufactured and sold as Thiokol LP-32 by Morton Thiokol Chemical Corporation, Chicago, Ill. LP-32 has the formula HS(RSS) n  RSH wherein R is --C 2  H 4  --O--CH 2  --O--C 2  H 4  -- and the value of n is such that the molecular weight is 4,000. 
     
         ______________________________________ELASTOMERIC SEALANTCompound             Parts By Weight______________________________________Polysulfide Polymer (LP-32)                100Calcium Carbonate (filler)                 50Phenolic Adhesion Promoter (2,4-diallyl                 3phenol)Salt Inhibitor       Variable______________________________________ 
    
     To the above sealant composition was added 7 parts by weight of Manganese Dioxide catalyst having about 1 weight percent of sodium hydroxide, the catalyst being dispersed in eight parts by weight of hydrogenated terphenyl (Monsanto HB-40). 
     
         ______________________________________TABLE COMPARING ANTI-CORROSIVE BEHAVIOR OFPOTENTIAL INHIBITORS In a Polysulfide BaseUsing an Aluminum-Cadmium Couple       Time      Weight %Inhibitor   Immersed  Inhibitor Results______________________________________None        3 days    --        Severe pitting                           of aluminumNone        21 days   --        Severe pitting.                           Heavy corrosion                           products between                           sealant strips                           and under                           sealant.                           Adhesion loss.                           Steel rusting                           under cadmium                           plate.Calcium     7 days    5         Severe pittingmolybdate                       and corrosion of                           aluminum. Loss                           of adhesion.Sodium molybdate       7 days    5         Extremely severe                           corrosion of                           aluminum.Magnesium   21 days   5         Aluminum alloychromate                        and cadmium                           shiny,                           unchanged.Ammonium    21 days   3         Cadmium andhypophosphite                   aluminum alloy                           shiny.Ammonium    21 days   3         Metalsdihydrogen                      unchanged.phosphateSodium Hypo 7 days    5         Severe pittingphosphite                       and corrosion of                           aluminumSodium phosphate       3 days    5         Extremely severe                           attack of                           aluminumAmmonium    42 days   4         Appearancehypophosphite +                 unchangedammoniumdihydrogenphosphateAmmonium    42 days   14        Some darkeninghypophosphite                   and evidence of                           corrosion______________________________________ 
    
     In addition to the galvanic-crevice corrosion cells employing aluminum alloy-cadmium couple, several other metals were coupled with the same aluminum alloy coated with strips of inhibited and uninhibited Thiokol polysulfide sealant with the following results: 
     
         ______________________________________TABLE SHOWING EFFECT OF VARIOUS INHIBITORSIN A POLYSULFIDE BASE ON CORROSION OFALUMINUM ALLOY 70-75 ST COUPLED WITHVARIOUS AIRCRAFT CONSTRUCTION MATERIALS   Days/   %              VisualInhibitor   Time    Conc.   Couple Results______________________________________None     7      --      Al--Ti Severe destruction of                          aluminum sealant.                          Blistered.None     7              Al--C  Very Severe destruction                          of aluminum. Adhesion                          loss.None     7              Al-    Worse attack than with                   stainless                          titanium - Sealant                          largely destroyedMgCrO.sub.4   21      5       Al--Ti Aluminum attack but                          less than                          without inhibitorMgCrO.sub.4   21       5%     Al-stain-                          Little or no improve-                   less steel                          ment over no inhibitorMgCrO.sub.4    7      5       Al-    No benefit over no                   carbon inhibitor. Very severe                          aluminum lossNH.sub.4 H.sub.2 PO.sub.2   21      3       Al--Ti No change. Metals still                          shiny Sealant retains                          adhesionNH.sub.4 H.sub.2 PO.sub.2   21      3       Al-stain-                          Slight darkening of                   less steel                          aluminum Adhesion OKNH.sub.4 H.sub.2 PO.sub.2    7      7       Al-    Mild corrosion of metal                   carbonNH.sub.4 H.sub.2 PO.sub.2   21      3       Al--Ti Very slight discoloration                          visible.NH.sub.4 H.sub.2 PO.sub.2   +21      11/2   Al-stain-                          No change. MetalsNH.sub.4 H.sub.2 PO.sub.2            11/2   less steel                          shiny.______________________________________ 
    
     While the aluminum usually shows no visual localized attack, in order to have a more quantitive evaluation of overall metal corrosion, the corrosion cells were opened at intervals and the current flow measured with a high impedance meter with the following results: 
     
         ______________________________________TABLE GIVING OBSERVED CURRENT FLOWSOF ALUMINUM-CADMIUM AND ALUMINUM-TITANIUMCELLS WITH TIME USING DIFFERENT INHIBITORSVERSUS NO INHIBITORCURRENT IN MICRO AMPS                           Ammonium                           dihydrogen                           phosphate +                 Magnesium AmmoniumTime       No Inhibitor                 Chromate  Hypophosphite______________________________________Al--CdcoupleInitial 1 day      11         13.0       3.2Average 21 days      14          7.94      3.82Final (21 days)      25          6.1       3.2Al--Ti1 day      55         45.0      26.0Average 21 days      65         33.3      24.0Final (21 days)      75         29.0      25.0______________________________________ 
    
     The amount of magnesium chromate was 5 wt. % and the amount of the mixture is 5 wt. %, said mixture containing equal amounts of ammonium dihydrogen phosphate and ammonium hyposphosphite. 
     CORROSION PROTECTION OF VARIOUS SEALANTS CONTAINING AMMONIUM HYPOPHOSPHITE 
     The benefits in corrosion resistance of the present invention are found in other polymers (utilizing the basic Elastomeric Sealant formula) using ammonium hypophosphite as the inhibitor. 
     
         ______________________________________                 Inhibitor                 PartsPolymer    Curing Agent                 By Wt.     Time Results______________________________________Mercaptan  Manganese  5          28 days-Noterminated dioxide               observable cor-polyurethane*                    rosion. Metal                            shiny.Thiokol LP-32      Magnesium  5          28 days-No      dichromate            observable cor-                            rosion. Metal                            shiny.Mercaptan  Manganese  5          28 days-Noterminated Dioxide               observable cor-polythioether**                  rosion. Metal                            shiny.______________________________________ *The polymer of Example IV of U.S. Pat. No. 3,923,748. **The polymer of Example 13 of U.S. Pat. No. 4,366,307.