Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2000-09-20
2003-03-25
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of metal
C428S472000, C428S704000, C106S014050, C106S014110, C106S014120, C106S014210, C106S014440
Reexamination Certificate
active
06537678
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a corrosion inhibiting additive and, more particularly, a corrosion inhibiting additive which is as effective as hexavalent chromium compounds but which do not have the health hazards associated with hexavalent chromium compounds.
Corrosion inhibitive compounds have long been used on, for example, metallic surfaces to inhibit corrosion thereof. U.S. Pat. No. 2,387,528 describes alkaline earth metal chromates containing trivalent as well as hexavalent chromium as additives for metal protective pigments. U.S. Pat. No. 2,430,589 describes protective pigments comprising calcium chromate associated with minor additions of ferric, manganic or chromic oxides. U.S. Pat. No. 2,902,394 describes the use of soluble chromium containing compounds used in aqueous metal treating or rinsing solutions applied to metal surfaces or to the conversion coating onto metal surfaces to improve corrosion resistance. U.S. Pat. No. 3,063,877 describes aqueous solutions for treating metal surfaces to in part improve corrosion resistance which are prepared by partially reducing a dissolved hexavalent chromium compound with formaldehyde. U.S. Pat. No. 3,279,958 describes rinsing of phosphate, chromate and other chemical conversion coatings on metal surfaces with a dilute aqueous acid solution of a chromium-chromate complex followed by a water rinse. The complex is prepared by treating aqueous chromic acid solution with an organic reducing agent to reduce a portion of the hexavalent chromium to the trivalent state.
In the aerospace industry, aluminum alloys achieve their high strength to weight ratio by inclusion of such additional elements as copper, silicon, chromium, manganese, zinc and magnesium. The presence of these elements in high strength aluminum alloys make them more susceptible to corrosion attack than pure aluminum. These high strength aluminum alloys are, therefore, generally protected in service by use of corrosion inhibitive compounds based on hexavalent chromium. These compounds includes barium or strontium chromate particles used as inhibitive pigments and adhesives, paints and primers, chromic acid, which is used to produce a chromium-rich conversion coating, and sodium and potassium dichromate, which are used as sealing compounds for anodized films.
All forms of hexavalent chromium are recognized by the United States National Institute of Environmental Health Sciences as a Group 1 known human carcinogen. Accordingly, the use of corrosion inhibiting compounds which contain forms of hexavalent chromium are subject to stringent regulation and control. It would be very beneficial to eliminate hexavalent chromium as a corrosion inhibiting additive to corrosion inhibitive compounds as described above.
U.S. Pat. No. 5,030,285 describes a substitute corrosion inhibiting pigment for hexavalent chromium compounds. The corrosion inhibiting additive includes a combination ferric phosphate and ferrous phosphate. While the proposed corrosive inhibiting additive does not suffer from the possible health problems associated hexavalent chromium compounds, it has not proved to be particularly effective in inhibiting corrosion, particularly of metals, when used as an additive in inhibitive pigments in adhesives, paints and primers, as a conversion coating.
Naturally, it would be highly desirable to provide corrosion inhibitors which can be used and substituted for hexavalent chromium inhibitors so as to avoid potential health hazards while at the same time provide effective corrosion protection on metal surfaces, particularly, high strength aluminum alloys used in aerospace applications.
Accordingly, it is the principle object of the present invention to provide a corrosion inhibiting additive which is non-carcinogenic.
It is a particular object of the present invention to provide a corrosion inhibiting additive as set forth above which is effective in preventing corrosion attack on metals.
It is a further object of the present invention to provide a corrosion inhibiting additive as set forth above which is particularly effective when applied to high strength aluminum alloys.
It is a still further object of the present invention to provide a corrosion inhibiting additive as set forth above which is effective against both general corrosion and pitting corrosion.
Further objects and advantages of the present invention will appear hereinbelow.
SUMMARY OF THE INVENTION
In accordance with the present invention, the foregoing objections and advantages are readily obtained.
The present invention is drawn to a non-carcinogenic corrosion inhibiting additive comprising an anodic corrosion inhibitor and cathodic corrosion inhibitor. The inhibiting additive comprises a combination of an anodic corrosion inhibitor and a cathodic corrosion inhibitor. The inhibiting additive of the present invention provides protection against both localized pitting corrosion and general corrosion. Suitable additives which are non-carcinogenic and comprise both anodic corrosion inhibitors and cathodic corrosion inhibitors. Particularly suitable additives include cerous molybdate with bismuth vanadate, cerous molybdate with strontium tungstate, cerous phosphate with strontium tungstate, bismuth vanadate with bismuth molybdate and strontium tungstate, and mixtures thereof. Preferred inhibiting additives are compounds of tungsten and cerium. Particularly preferred inhibiting additives are those additives which comprise cerous and tungstate compounds.
The corrosion inhibiting additive of the present invention may be used as an inhibitive additive in adhesives, paints and primers, sealants, epoxies and the like which are thereafter applied to a substrate by mechanical methods known in the art or dissolved in solution and applied to a substrate as a conversion coating.
DETAILED DESCRIPTION
The present invention is drawn to a non-carcinogenic corrosion inhibiting additive and, more particularly, a non-carcinogenic corrosion inhibiting additive which is effective against general corrosion and pitting corrosion.
In accordance with the present invention, the non-carcinogenic corrosion inhibiting additive comprises, in combination, an anodic corrosion inhibitor and a cathodic corrosion inhibitor. By anodic corrosion inhibitor is meant suppression of metal oxidation reactions. By cathodic corrosion inhibitor is meant suppression of reduction reactions. In order to be effective, both the anodic and cathodic corrosion inhibitors should be “strong” corrosion inhibitors. By strong anodic corrosion inhibitor is meant a compound that is soluble in alkaline media, while precipitating as a reduced, insoluble oxide under neutral and acidic reducing conditions, that is, existing as an insoluble oxide below −600 mv vs Ag/AgCl at pH 7, and below −300 mv vs Ag/AgCl at pH 2. By a strong cathodic corrosion inhibitor is meant a compound that is soluble in acidic media, while undergoing a valance change to precipite as an insoluble oxide under neutral and alkaline and moderately oxidizing conditions, that is, existing as an insoluble oxide above −300 mv vs Ag/AgCl at pH 7, and above −900 mv vs Ag/AgCl at pH 12. The corrosion inhibiting additive requires both an anodic corrosion inhibitor and a cathodic corrosion inhibitor in order to be effective against general corrosion and pitting corrosion. General corrosion means uniform dissolution of base metal. By pitting corrosion is meant localized corrosion of base metal resulting in the formation of corrosion pits. The anodic corrosion inhibitor is effective against general corrosion while the cathodic corrosion inhibitor is particularly effective against pitting corrosion.
Suitable cathodic corrosion inhibitors for use in the inhibiting additive of the present invention are the elements of Group IIIB of the Periodic Table (the CAS version). All of the foregoing elements have cathodic corrosion inhibiting characteristics; however, it has been found that cerium, neodymium and praseodymium are “strong” cathodic corrosion inhibitors as defin
Jaworowski Mark
Putnam John William
Bachman & LaPointe P.C.
Jones Deborah
McNeil Jennifer
United Technologies Corporation
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