1. Field of Invention
The present invention relates to metal protection compositions. More particularly, the present invention relates to two-part coating compositions for protecting metals, methods for applying two-part coating composition to metals and metals having the two-part coating composition.
2. Description of the Related Art
Many methods and composition for protecting metal surfaces exposed to air and water, including salt water, are currently in use. Methods for preparing metal surfaces for subsequent uses, such as applications of adhesives are also known. Generally, before a primer or other coating is applied to a metal surface, it is desirable to treat a bare metal surface to prevent corrosion. It is well known that metal surfaces are subject to corrosion and degradation. A known process for treating metal surfaces before the application of primer or other coatings is phosphating or conversion coating, a procedure by which a water-insoluble film of metal phosphate and/or metal oxide is formed on the metal surface by treatment with an acid such phosphoric acid or acid phosphate salts. The conversion coating can also include resins, such as phenolic based resins to improve corrosion resistance. U.S. Pat. No. 5,011,551 relates to a metal conversion coating composition that includes an aliphatic alcohol, phosphoric acid, an alkali nitrate, tannic acid and zinc nitrate. U.S. Pat. No. 4,293,349 relates to a steel surface protective coating composition that includes pyrogallic acid glucoside, phosphoric acid, phosphates of bivalent transition metals such as Zn or Mn, Zn or Mn nitrate, and, optionally, formaldehyde.
Coatings that form by autodeposition are also generally known in the art. An autodeposition composition generally includes an aqueous solution of an acid, an oxidizing agent and a dispersed resin. Immersion of a metallic surface in an autodeposition composition produces what is said to be a self-limiting protective coating on a metal substrate. The general principles and advantages of autodeposition are explained in a multitude of patents assigned to Parker Amchem and/or Henkel (see, for example, U.S. Pat. Nos. 4,414,350; 4,994,521; 5,427,863; 5,061,523 and 5,500,460).
U.S. Pat. No. 5,691,048 includes phosphoric acid in a list for possible acids in an autodepositing composition, but hydrofluoric acid is the preferred acid. This patent also lists hydrogen peroxide, chromic acid, potassium dichromate, nitric acid, sodium nitrate, sodium persulfate, ammonium persulfate, sodium perborate and ferric fluoride as possible oxidizing agents. Hydrogen peroxide and ferric fluoride are preferred.
The use of accelerating agents, including oxidizing and reducing accelerators to accelerate the deposition of the conversion coating is also known. See, e.g., Freeman, Phosphating and Metal Pretreatment, Industrial Press Inc., 1986, pages 19 to 25. The accelerating agents are believed to increase the rate of deposition by reacting with hydrogen formed on the metal surface during attack by acid, thus, preventing polarization of the treated metal. The use of accelerators is also disclosed in e.g., U.S. Pat. Nos. 4,149,909; 5,427,632; and 5,711,996. The oxidizers disclosed in U.S. Pat. No. 5,691,048 are also used as accelerators.
The use of some accelerators in applications that involve the application of a subsequent coating, such as a primer coating can cause blister formation which results in decreased corrosion resistance and cosmetic defects. Also, the use of high levels of accelerators in metal treatment conversion coatings can cause application difficulties with the conversion coating.