MXPA98002808A - Non chrome chromium inhibitors for alumi alloys - Google Patents

Abstract

A non-chromate corrosion inhibiting coating composition for metal surfaces includes at least one inhibitor selected from the group consisting of phosphates, phosphosilicates, silicates, and mixtures thereof, at least one inhibitor selected from the group consisting of of titanates, zinc salts, and mixtures thereof, and a carrier for these inhibitors, the carrier being able to place the inhibitors in proximity with the metal surface. In a preferred embodiment, the coating composition further includes a borate such as boric acid and a succinate. A preferred phosphate includes calcium diacid phosphate, sodium titanium oxide and is a preferred titanate. The zinc salt may include zinc phosphate or cyanamide cyanide.

Description

NON CHROMIUM CORROSION INHIBITORS FOR ALU MINIO ALLOYS This application claims the benefit of US Provisional Applications Nos. 60 / 005,027 and 60 / 005,028 filed on October 10, 1995.

FIELD OF THE INVENTION The invention relates generally to methods and compositions for inhibiting corrosion, and in particular, to methods and compositions for inhibiting corrosion of metals, especially aluminum and other light metal alloys used in aircraft. More specifically, the present invention relates to a non-chromate containing corrosion inhibitor coating composition, capable of protecting a wide variety of metal surfaces.

BACKGROUND OF THE INVENTION The Environmental Protection Agency of U.S. has identified the metal finishing industry as one of the most significant contributors to environmental pollution in the United States and, most likely, throughout the rest of the world. This is because the materials currently used most in the finishing of metals include chromium, cadmium, zinc, lead, copper, nickel, chromates and many volatile pollutants or toxic organics.

The aircraft industry, being one of the largest of the industrial finishing, provides an example of the environmental impact of these processes. In a 1990 report, Tinker Air Forcé Base (Oklahoma) reported producing and treating 1.4 million gallons of industrial wastewater per day, mostly metal finishing. The main pollutants were chromium, nickel, copper, cadmium, lead, zinc, tartrates, EDTA, phosphate and ammonia. It is estimated that the cost of disposing of these wastes is approximately $ 220 per ton, which equals several thousand dollars per day for this single site. The chromate ion, which is an excellent corrosion inhibitor, has been one of the most widely used for almost one hundred years. It is generally used as a pigment in corrosion inhibitor paints, washing primers, sealants and fillers. It is also used in chromate conversion coatings, etching solutions, and in phosphate and anodizing sealing coatings. However, during the past ten years, chromate has been recognized as toxic and carcinogenic, and due to its health risks, it has become highly regulated. With pressure for elimination being exercised by government regulations, the continued use of chromate will always incur increasing economic penalties. Thus, there is an urgent need for non-toxic substitutes, both from an economic and environmental point of view. Currently, the most widely used inhibitors for inactivating aerospace aluminum alloys and other light metals are the zinc and alkaline earth salts of hexavalent chromium. They vary mainly in their degree of solubility in water (in the order Mg > Ca > Sr > Zn) and to a much lesser degree in their pH (Sr > Ca > Mg). The chromate anion is the active species, reliably performing four functions necessary to be described more fully below. In addition, all the hexavalent chromium salts described above enhance the adhesion in many painting and sealing systems. Chroma performs four functions, thereby becoming a desirable inhibitor. These functions include: 1. Quick exit of a carrier matrix, such as paint or sealant. 2. Adsorption of the chromate anion on the pure metal or metal oxide. This alters the space charge distributions at the interface, decreasing the isoelectric point of the protective anodic metal oxide layer that naturally forms in active metals. This repels the chloride attack, and / or displaces the corrosion potential of that metal and / or its pitting potential in the noble direction. 3. In place of the reduction of oxygen at the cathode sites, the reduction of the chromium (VI) anion occurs to form a chromium oxide layer (I I I), Insoluble to acid ("persistent"). This fills the oxide voids on the cathode sites and further blocks the corrosion reaction. 4. Cushion the pH or neutralize the increasing acidity in the metal / electrolyte interface, which comes from the oxidation of metal in the absence of air. Increasing acidity accelerates corrosion exponentially. In addition to these desirable inhibitory functions, the chromate salts have the advantages of: (1) promoting adhesion to the metal / resin interface under a coating or sealant compound; (2) work well on a wide variety of metal and alloy substrates because they inactivate both anodically and cathodically; (3) be relatively neutral in pH; and (4) being strong oxidants only under acidic conditions, and thus do not destroy or react strongly with the resin matrix in which they are placed. The prior art describes a number of non-chromate species, which have some inhibitory abilities. For example, U.S. Patent 5, 126,074 discloses "acid phosphate" anions as exhibiting corrosion inhibiting activity on aluminum. The patent further describes the use in coatings of monoacid alkaline earth phosphates, together with a carbonate of the same alkaline earth metal and an additive of alkali fluorosilicate or fluoroborate or alkali or alkali fluoride. It is said that this combination prevents filiform corrosion in aerospace aluminum alloys. Other references cite the diacid phosphate anion as adsorbing on alumina and decreasing its isoelectric point (IEP) from pH = 9 to pH = 5. Decreasing the I EP of aluminum oxide in an aluminum metal has been shown to increase its resistance to sting Since this species also exhibits buffering capacity, it performs functions 2 and 4. US Patent 2,624, 708 describes carcinogenic mercaptobenzothiazole (MBT) as an inhibitor of aluminum and steel. The sulfur and mercapto groups, which are "soft bases", are known to have a high affinity for noble surfaces and other pure metal surfaces ("soft acids"). They are effective inhibitory structures under acidic conditions where there is no rust present. By itself, this species performs only function 2. U.S. Patents 4,457,790 and 5,125,989 describe the use of vegetable tannin adducts or polyalkenyl phenols for "conversion coating" aluminum. An ion or titanium compound such as fluorotitanic acid, among others, is claimed as a co-reactant. U.S. Patent 5, 129,967 discloses minute catalytic amounts of dihydrohexafluorotitanic acid and hydrofluoric acid used with much greater amounts of dihydrohexafluorochirconic acid and polyacrylic acid. These patents refer to uses in aluminum or aluminum alloys. U.S. Patent 5,314,532 discloses lead, nickel, cobalt and zinc cyanamide pigments as exhibiting corrosion inhibitory effects in thin mirror coatings and silver. It would be expected that the oxide-free copper, pure, would exhibit adsorption characteristics somewhat analogous to silver, especially in an acidic slit environment. As with the silver groups, the availability of electrons in the cyano group acts as a "soft base" on surfaces of "mild acid" of pure metal, performing function 2. The present invention provides a corrosion inhibiting coating composition which plays many, if not all. The same functions as a composition containing chromate, but without the need for the harmful species of chromate. The problem solved by this invention is the elimination of toxic hexavalent chromium salts, which are known to be human carcinogens, as corrosion inhibitors of treatment solutions, coatings, and sealants used in aluminum and other metal alloys. The present invention provides the synergistic combinations of non-chromate inhibitors for aerospace aluminum alloys and other metal surfaces, which can be incorporated into both curable and non-curable sealants and in a curable primary and coating systems. This synergistic combination of inhibitors can also be incorporated into water-containing or water-absorbing fluids that can cause corrosion when used in the vicinity of metal such as antifreeze fluids and coolants. These and other advantages of the present invention will be readily apparent from the description, discussion and examples that follow.

SUMMARY OF THE INVENTION A non-chromate corrosion inhibiting coating composition for metal surfaces is disclosed herein. As used herein, "coating" is used to mean any composition which can cover a substrate, or which can place the inhibitors in proximity with a substrate. The composition comprises at least one inhibitor selected from the group consisting of phosphates, phosphosilicates, silicates and mixtures thereof, at least one inhibitor selected from the group consisting of titanates, zinc salts and mixtures thereof, and a carrier for these inhibitors, the carrier being able to place the inhibitors in proximity with the metal surface.

In particular embodiments, the coating composition of the present invention may further comprise a borate, such as boric acid, and / or a sulfur-containing succinate such as (2-benzothiazolium) succinic acid or amine salts thereof. A preferred phosphate is calcium diacid phosphate. A preferred phosphosilicate is calcium phosphosilicate, strontium zinc. Sodium titanium oxide is a preferred titanate. Zinc phosphate and / or zinc cyanamide are the preferred zinc salts. The carrier comprises a solution or polymer matrix, which adheres well to metal substrates and is capable of placing the inhibitors very close to the metal surface. The coating composition of the present invention may also include auxiliary ingredients such as pigments, rheological agents and other performance additives.

DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to the synergistic combinations of two to six individual corrosion inhibitors that contribute two to five separate functions to the inhibition of corrosion in metals. The composition of the coating is particularly well suited for protecting from metal luzaleations, such as aluminum alloys used in aircraft. In the broadest sense, the present invention comprises at least one inhibitor selected from the group consisting of phosphates, phosphosilicates, silicates and mixtures thereof, at least one inhibitor selected from the group consisting of titanates, zinc salts and mixtures of them, and a carrier for the inhibitors, the carrier capable of placing the inhibitors in proximity with a metal surface. The coating composition of the present invention may further comprise a borate, such as boric acid, and / or a succinate, preferably a succinate containing sulfur. The succinate comprises a compound selected from the group consisting of (2-benzothiazolylthio) succinic acid, the fatty (2-benzothiazolylthio) succinic acid amine salt, and mixtures thereof. In a preferred embodiment, the inhibitors are loaded in the carrier to reach 3-40% by volume in a dry film, more preferably 5-25% by volume in a dry film, and most preferably 10-20% by volume in a film dry In a preferred embodiment, the phosphate comprises a diacid phosphate, most preferably a compound selected from the group consisting of calcium diacid phosphate, potassium diacid phosphate, diacid ammonium phosphate, sodium diacid phosphate and mixtures thereof. The phosphate may also comprise a pyrophosphate, preferably sodium pyrophosphate, or a monoacid phosphate, preferably dipotassium monoacid phosphate. The phosphosilicate of the present invention preferably comprises a compound selected from the group consisting of calcium phosphosilicate, strontium, calcium phosphosilicate, strontium, zinc, and mixtures thereof. The silicate of the present invention preferably comprises an orthosilicate, most preferably tetrasodium orthosilicate.

In a preferred embodiment of the present invention, the titanate comprises titanium oxide, more preferably sodium titanium oxide.

The zinc salt preferably comprises a compound selected from the group consisting of zinc phosphate, zinc cyanamide, and mixtures thereof. When used in the coating composition of the present invention, the boric acid should be loaded in the carrier so that it would occupy up to 10% by volume in a dry film, more preferably 0.3-5.0% by volume in a dry film, and most preferably 0.5-2% by volume on a dry film. Likewise, when an inhibitor is used, calcium diacid phosphate may be added in an amount up to 10% by volume in a dry film, more preferably 0.3-7% by volume and most preferably 0.5-5% by volume in a film dry When the calcium phosphonilicate, strontium or calcium phosphonilicate, strontium, zinc is used as an inhibitor, it can be added in an amount up to 25% by volume in a dry film, more preferably 0.3-10% by volume, and most preferably 1 -5% by volume on a dry film. If (2-benzothiazolylthio) succinic acid or the fatty acid (2-benzothiazolylthio) succinic acid amine salt is used as an inhibitor in the coating composition of the present invention, it can be loaded in the carrier in an amount up to 10% by volume in a dry film, more preferably 0.3-5% by volume, and most preferably 0.5-3% by volume in a dry film. Sodium titanium oxide can be added in an amount up to 10% by volume in a dried film, more preferably 1-10% by volume, and most preferably 3-6% by volume in a dry film. The zinc phosphate or zinc cyanamide, when used in the present invention, can be added in an amount up to 15% by volume in a dry film, more preferably 1-10% by volume in a dry film and most preferably 3- 6% in volume. The carrier of the present invention is any compound, which is capable of adhesion to a metal surface, and is also capable of placing the combination of inhibitors in proximity to the surface of the metal. Suitable carriers include both curable and non-curable sealants, as well as curable primary and coating systems. The inhibitor combination of the present invention can also be incorporated into water-containing or water-absorbing fluids that could cause corrosion when used in the vicinity of the metal such as antifreeze fluids and coolants, which are mainly glycol and water compounds. Other carriers include, but are not limited to, aqueous solutions, amine-cured epoxy coatings, urethane and polyester coatings, sealant dies such as those based on epoxy-cured polythioether polymers, polysulfide sealants cured with MnO2, polysulfide sealant. covered at the non-healing end, and other carriers as are known in the art. Additional materials may be added to the coating composition of the present invention such as pigments, rheological agents, adhesion promoters and other additives, as are known in the art. For example, a hydrophobic, oily additive may be used with some of the inhibitor combinations of the present invention to achieve a more acceptable barrier performance of sealants in water. Additionally, since the ingredients of the inhibitor of this invention do not necessarily enhance adhesion as do oxidant inhibitors such as chromate, a porous conversion coating or anodized layer on the substrate is required in very humid environments for good mechanical adhesion of a coating or metal sealant. As discussed above, chromate has been identified as an excellent inhibitor because it performs the following four necessary functions: 1. Rapid polymer exit and short-term metal inactivation. 2. Inactivation by adsorption of the metal surface and alteration of double layer space loading. 3. Form a layer of insoluble inactivation to water which persists or remains insoluble in neutral environments, alkaline and acids. 4. Inactivation by pH control or acid neutralization at the metal / electrolyte interface. Therefore, in order to replace chromate, any combination of inhibitors must perform at least some, if not all, of these functions in order to operate as a successful corrosion inhibitor. Additionally, the transport by itself and other inhibitors out of a solid, eg, carrier matrix, polymeric onto exposed exposed pure metal areas in a moisture but not ambient submerged in liquid, is a parameter exhibited by one of the inhibitors ( boric acid) of this invention, which is not exhibited by chromate. In general, the first and second inhibitory functions are performed by one or more fast ingredients, moderately soluble in water, but reversibly adsorbents, which readily exit a coating or sealant, to give a rapid, short-term inactivation of a surface of metal. Boric acid appears to have the predominant fast or first response effect outside of a polymer matrix, and appears also to assist in releasing other more permanent, less soluble, inactivators of the polymer. The third function is performed by a slower response combination of two ingredients to form a "persistent" inactivation layer, resistant to acid, insoluble in water. By "persistent" is meant that it remains on a metal surface that was exposed to the solution containing the inhibitor after the solution containing the inhibitor is removed and replaced with a corrosive solution such as aqueous sodium chloride which does not contain inhibitor. It is believed that this layer is formed in the pure metal as in the oxide. The combination of zinc phosphate or zinc cyanamide and (2-benzothiazolylthio) succinic acid or its fatty amine salt, perform this function. The fourth inhibitory function of these systems consists of buffering the pH, or controlling (neutralizing) the acidification of the interface environment in anaerobic or slit conditions. Acid conditions dissolve the protective oxide and do not allow new insoluble oxide to form. In the formulations described in the present invention, diacid phosphates, monoacid phosphates, pyrophosphate, orthosilicate, titanate, phosphosilicates and cyanamide can perform this function. A variety of different quantitative and qualitative test methods has been used to identify and corroborate the inactivation behavior in aircraft aluminum. These include. 1 . Measurement of Galvanic Current (in solutions, coatings and sealants) a. Use an electrically cut titanium cathode for the active metal, usually Al alloy, immersed in aqueous NaCl solution.

Stainless steel, platinum-plated steel with Cd and carbon composite cathodes are also used. b. The current between anode and cathode (Al and Ti or another, respectively) is measured at regular intervals and plotted against time. c. The configuration accelerates the development of acidic slit conditions in a narrow, narrow air gap between the separated, parallel anode and cathode metals. d. Used to quantitatively test functions 1, 2 and / or 4. e. We define good performance as 2-5 times the initial galvanic current reduction (ie, within 24 hours) compared to non-inhibited systems, with no increase over time outside of 5-6 weeks. (The initial currents of uninhibited sealants and coatings are approximately 0.5-1 microamps per square centimeter in pure 2024 alloy with a Ti cathode, with a visual gain in the stream of 2-5 times within 1-2 weeks. approximately 0.2- 0.3 microamps per square centimeter initially, without increase over time.The corresponding values are approximately 3-5 times higher for pure 7075 alloy with a Ti cathode). 2. Electrochemical Impedance Spectroscopy (EIS) (in solutions, coatings and sealants) a. Uses an active metal working electrode (eg, Al alloy), inactivated stainless steel counter electrode, calomel reference electrode and Schlumberger potentiostat, and frequency response analyzer. b. Run in one or more of the following configurations: i. NaCl solution aerated, neutral or pH adjusted, containing inhibitor (s). ii. Active work electrode covered with sealant film or bound coating containing inhibitor, exposed to NaCl solution aereated, open on the side of the upper film. Measures barrier properties or resistance to penetration of corrosive environment (R? Ro) of bonded films, iii. Active working electrode covered with "free film", unbound, containing sealer or coating inhibitor, exposed to aerated aqueous NaCl solution, open on the upper side. In this configuration, when the film is penetrated with liquid, the complete, air-deficient electrolyte / active metal interface area is moistened and therefore dimensionally known to calculate the precise charge transfer resistors (Rct) c. Used quantitatively to test functions 2, 3 and / or 4. d. We define a good electrochemical performance as Rct's greater than 106 ohm-square centimeter, which persists over time. We define good barrier performance as exhibiting essentially high immeasurable Rp? S (greater than 108 ohm-square centimeter), for as long as possible. We judge by making comparisons in equal film thicknesses. Low RPC can compensate by a large degree for high Rct as is the case with chromate. 3. Filiform test (only in coatings) a. Uses brief HCl vapor exposure (usually one hour) to initiate the filiform corrosion conditions (slit under film) acids at the cut edges of the coatings. Exposure to the subsequent 80 +% humidity further propagates thread-like corrosion. b. The relative effectiveness of inhibitors is compared in the same substrates. [Differences in corrosivity of Al substrates (various alloys, Alelad, coated by conversion) usually give more variation for a coating since the differences between inhibitors, including none, in the same coating, on the same substrate.] C. Use to qualitatively test function 4 and combined effects of functions 1 -3. d. Good performance for a covered system or coating is usually defined as the development of a cut edge of filiform no greater than 3-6 millimeters in length, with most less than 3 millimeters, after 1000 or more hours. 4. Salt spray test (in coatings and sealants) a. Use a salt solution (5%) corrosive condenser to cause electrochemical activity of soluble inhibitors to be a factor, along with barrier properties, but the effects can not be determined separately. b. By leaving something of the uncoated area exposed, the ability of the inhibitors to move from the resin matrix to the uncoated or damaged areas and to protect them is determined. c. As in the filiform test, the relative effectiveness of the inhibitors is compared in the same substrates. d. Used to qualitatively test function 1 and combined effects of functions 2-4. and. Good performance is usually defined as formation of non-corrosive bubbles away from the cutting edges and minimal formation of corrosion bubbles or notches in the cutting edges. The judgments are usually based on visual comparisons over time with chromate inhibited and uninhibited controls. 5. pH Range Immersion Test (only in solutions - solutions containing inhibitors, buffered from pH 3-10) a. It was found to be a mechanism for predicting the formation of "persistent" (insoluble) inactivation layer and corrosion resistance of slit acid condition, prolonged exposure. b. Used to test function 3.c. We define good performance as resistance to visual corrosion on flat surfaces and cutting edges of pure metal over a wide range of pH for as long as possible; in more than one alloy, if possible. Judgments are based on visual comparisons over time with chromate inhibited and uninhibited controls.

In order to work, effective inhibitors must exhibit at least function 2, suppression of corrosion current. The functions 1, 3 and 4 increase their effectiveness. In addition, if they perform on one or both of the most common aerospace aluminum substrates (AA2024-T3 or AA7075-T6) (function 5), and if their addition significantly decreases the water resistance / adhesion of a polymer matrix (function 6) is considered. The following table summarizes some of the preferred inhibitor combinations, evaluated according to how many of the six previous functions performed. However, this invention is not limited to the combinations below. These abbreviations have been used to identify the various inhibitors in which: B = boric acid C = calcium diacid phosphate K = dipotassium monoacid phosphate A = diacid ammonium phosphate P = sodium pyrophosphate H = calcium, strontium or phosphosilicate phosphosilicate calcium, strontium, zinc S = tetrasodium orthosilicate I = (2-benzothiazolylthio) succinic acid or the fatty acid (2-benzothiazolylthio) succinic acid amine salt N = sodium titanium oxide Z = zinc phosphate W = zinc cyanamide The present invention will be better illustrated by the following tables of examples. The same abbreviations of inhibitors discussed above have also been used in these tables. A series of compositions were prepared and tested on various light metal alloys. The test samples were prepared according to the tables set forth below, all the components being mixed together in a container, and the various formulations were applied to 2024 and 7075 alloy substrates. The samples were run through a series of tests to evaluate its inhibitory properties of corrosion. These tests have already been discussed, with the results summarized above. In the examples, as in the rest of the specification and claims, all percentages are by volume in a dry film. FORMULATIONS (as% by volume of non-volatile) It will be appreciated from the foregoing, that a non-chromate containing coating composition, which has excellent properties to inhibit corrosion of a variety of metal surfaces, can be prepared from the synergistic combination of two to six individual corrosion inhibitors which they contribute two to five separate functions to the inhibition of metals, such as light metal alloys. The specific components of the composition will depend on the particular applications and factors such as the metal alloy substrate, the particular solution or polymer matrix in which the inhibitors are carried, and the range of exposure conditions in which the material will be seen. in its particular location (for example, interior, exterior, fuel tank, outer layer coating, folds seam, etc., of the airplane) The above discussion and examples are simply to illustrate the particular embodiments of the invention, and not They claim to be limitations on the practice of it. The following claims, including all their equivalents, are what define the scope of the invention.

Claims (18)

1. CLAIMS 1 . A non-chromate corrosion inhibiting coating composition for metal surfaces, said composition comprising: at least one inhibitor selected from the group consisting of phosphates, phosphosilicates, silicates, and mixtures thereof; at least one inhibitor selected from the group consisting of titanates, zinc salts and mixtures thereof; and a carrier for said inhibitors, said carrier being capable of placing said inhibitors in proximity with said metal surface.
2. 2. The coating composition of claim 1 and further comprising a borate.
3. 3. The coating composition of claim 2, wherein said borate comprises boric acid.
4. 4. The coating composition of claim 3, wherein said boric acid comprises up to 10 percent by volume in a dry film.
5. 5. The coating composition of claim 1 and further comprising a succinate.
6. 6. The coating composition of claim 5, wherein said succinate comprises a succinate containing sulfur.
7. The coating composition of claim 6, wherein said succinate comprises a compound selected from the group consisting of (2-benzothiazolylthio) succinic acid, the fatty (2-benzothiazolylthio) succinic acid amine salt, and mixtures thereof. same.
8. 8. The coating composition of claim 7, wherein said succinate comprises up to 10 percent by volume in a dry film.
9. 9. The coating composition of claim 1, wherein said phosphate comprises an acid phosphate.
10. The coating composition of claim 9, wherein said acid phosphate comprises a compound selected from the group consisting of calcium diacid phosphate, diacid ammonium phosphate, sodium diacid phosphate, potassium dihydrogen phosphate, and mixtures thereof. eleven .
11. The coating composition of claim 9, wherein said acid phosphate comprises dipotassium monoacid phosphate.
12. 12. The coating composition of claim 9, wherein said acid phosphate comprises up to 10 percent by volume in a dry film.
13. 13. The coating composition of claim 1, wherein said phosphate comprises a pyrophosphate.
14. 14. The coating composition of claim 13, wherein said pyrophosphate comprises sodium pyrophosphate.
15. The coating composition of claim 1, wherein said phosphosilicate comprises a compound selected from the group consisting of calcium phosphosilicate, strontium; phosphosilicate calcium, strontium, zinc; and mixtures thereof.
16. 16. The coating composition of claim 1, wherein said phosphosilicate comprises up to 25 percent by volume in a dry film.
17. 17. The coating composition of claim 1, wherein said silicate comprises an orthosilicate.
18. 18. The coating composition of claim 17, wherein said orthosilicate comprises tetrasodium orthosilicate.