Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-09-20
2003-04-22
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S254000, C148S259000, C148S261000, C148S262000, C106S014120, C252S389520, C252S389530
Reexamination Certificate
active
06551417
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to corrosion-resistant coating compositions for metal substrates, processes for making the same, and processes for imparting anti-corrosive properties to metal substrates using such compositions. More particularly, the present invention relates to a tri-cation coating compositions which include disodium glycerophosphate.
BACKGROUND OF RELATED TECHNOLOGY
It is well known to treat metallic surfaces with phosphating solutions or compositions, which, under appropriate conditions, will deposit or form upon the metallic surface a protective phosphate coating. These phosphate coatings protect the underlying metal from corrosion and are desirable as they provide excellent surfaces for the successful application of organic finishes. Such phosphate coatings typically occur as crystalline deposits to which organic finishes will bond and adhere more tenaciously than to bare metal surfaces.
These phosphating compositions can be classified generally into two categories: (a) nickel/zinc phosphate-based conversion treatment solutions used mainly for coating iron and steel articles; and (b) nickel/manganese/zinc (“tri-cation”) phosphate-based conversion treatment solutions, used principally for coating iron, steel, and galvanized or zinc alloy-plated steels. The tri-cation compositions have been found superior for the purposes of paint adhesion, corrosion resistance, and resistance to alkali solubility. Particularly, nickel contributes to increasing the corrosion resistance of the metal surface after a subsequent protective surface coating is applied, while manganese contributes to increasing the alkali resistance necessary for cathodic electrodeposition of paint. Manganese also functions to improve the water resistance of organic surface coatings over the phosphate film on zinc-rich surfaces.
Further, within these two general categories of phosphate coating compositions exist “high-zinc” and “low-zinc” compositions. High-zinc phosphating compositions are typically used in treating wire and tubing and have been found to be unsuitable for use in treating metal substrates prior to the application of paint. High-zinc compositions are known to undesirably hold lubricants on a metal surface treated with such compositions and have crystal sizes which do not permit an acceptable surface for the application of paint. Processes of treating metal substrates involving low-zinc phosphating compositions (those with a zinc ion concentration from about 0.4 to about 2 g/l) have been found to be superior for treating metal surfaces prior to the application of paint. Such processes involving low-zinc compounds require the metal surface to be treated to be activated prior to treatment in order to affect increased crystal formation of the zinc phosphating compositions.
Low-zinc phosphating baths are generally characterized by a ratio by weight of phosphate ions to zinc ions which is greater than 4 and which may assume values of up to 60. Such baths have been found particularly useful for the cathodic electrocoating of car bodies. The use of low-zinc compositions in combination with an activating agent has been shown to result in uniform and continuous coatings which exhibit superior corrosion resistance. Low-zinc coating processes are described, for example, in German Patent Specification No. 2 232 067.
While low-zinc processes, in combination with the typical subsequent electrodeposited painting step, result in a clearly improved corrosion resistance, they are more sensitive to changes in process parameters and to contaminants which are introduced into the phosphating bath with the metal sheets to be coated. As a result, it has been found advantageous to carefully control the activation of the metal surface prior to coating. Particularly, it has proven to be particularly advantageous to carry out the activation in a separate process step, subsequent to cleaning and degreasing of the metal surface. This has been found to be particularly important where the metal substrate is coated in a dip-coat procedure, but is also important in spray coating and combined spraying/dip-coating procedures.
Activation of the metal surface typically occurs by use of a Titanium(IV) compound, such as those disclosed in U.S. Pat. Nos. 2,310,239 and 2,456,947, both to Jernstedt. The activation serves to increase the rate of formation of coating crystal nuclei and, hence, the number of nuclei, in the initial phase of zinc phosphating, which results in refinement of the coating layer. The porosity of the desired zinc phosphate layer is reduced because the coating crystals are closely spaced, resulting in the formation of a uniform and continuous zinc phosphate layer over the entire metal surface. Further, the low surface area weights of the resulting coatings have been proven to be beneficial as primer for paint finishes.
Titanium-based activators, however, are attended by a variety of problems, particularly when used in combination with known tri-cation coating compositions. For example, they are characterized as having short bath lives, which leads to incomplete coating formation and an increase in coating crystal size, resulting in a decrease in refinement of the coating. Further, factors such as water hardness, cleaner, phosphate salt contamination, and pH typically cause the activator to destabilize.
Additional factors are known to affect the crystal size of the coating, such as varying conditions in the coating bath as the metal is being processed. For example, in a tri-cation coating process, an increase or decrease in zinc ions and variations in ortho-phosphate and cation metal ratios can affect crystal size. These variations may result, for example, in an increase in crystal size, marked differences in coating weight between various substrates, and increased porosity of the coating as evidenced by a decrease in neutral salt spray corrosion performance and adhesion. It is particularly important when electropaints are to be applied to maintain consistently uniform phosphate coating weights between substrates in order to assure consistent paint deposition with a uniform film build and satisfactory appearance.
Various attempts have been made in the art to address the above problems, such as the use of complexing agents based on phosphonic acids, see for example, U.S. Pat. No. 4,957,568 to Endres et al. However, these efforts have met with limited success and have failed to address various factors such as pH and bath contaminants. Further, attempts have been made to address the problems of unstable activating agents by varying methods of manufacture and particle size manipulation, but these efforts have also not proved entirely acceptable.
Therefore, there exists a need for improved coating compositions which permit the uniform coating of tri-cation phosphate compositions by stabilizing and improving the bath life of activating agents. There further exists a need for such a composition which permits uniform coating under a wide and varying range of process conditions.
SUMMARY OF THE INVENTION
Accordingly, in one aspect the present invention is directed to tri-cation conversion coating compositions for metal substrates. The compositions include a phosphate component present in amounts of about 8000 to about 30,000 parts per million (ppm), and desirably about 16000 ppm; a silicon component present in amounts of about 50 to about 300 ppm, and desirably about 100 ppm; ions of nickel present in amounts of about 100 to about 1000 ppm, and desirably about 800 ppm; ions of manganese present in amounts of about 100 to about 1000 ppm, and desirably about 800 ppm; ions of zinc present in amounts of about 500 to about 2000 ppm, and desirably about 1000 ppm; and ions of fluoride present in amounts of about 100 to about 1500 ppm, and desirably about 250 ppm.
The compositions also include a glycerophosphate compound, which is desirably disodium glycerophosphate, present in amounts of about 10 to about 500 ppm, and desirably about 65 ppm. The compositions are capable of providing anti-corrosion p
Haberle Bruce V.
Murphy Joseph E.
Rodzewich Edward A.
GE Betz, Inc.
Hoffmann & Baron , LLP
Oltmans Andrew L.
Sheehan John
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