Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
1998-06-08
2001-05-15
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S262000, C252S186420, C252S396000
Reexamination Certificate
active
06231688
ABSTRACT:
DESCRIPTION
1. Field of the Invention
The present invention relates to zinc phosphate-based conversion coating or treatment compositions for application to metals) for example, steels and zinc-plated steels, and to methods for the zinc phosphate-based conversion treatment or coating of metals. More particularly, this invention relates to a zinc phosphate-based conversion treatment composition, often hereinafter called a “bath” for brevity, even when used by some method other than immersion, and method that can uniformly coat metals with a fine, dense zinc phosphate-type conversion coating that contains extremely small conversion crystals and that, based on the presence of said microfine crystals, can improve the adherence of the zinc phosphate-type conversion film to paint films.
2. Description of Related Art
At present, a zinc phosphate-based conversion treatment is executed as a pretreatment on various metals when the metal is to be painted or subjected to cold working. This pretreatment is carried out in the former case in order to improve the post-painting corrosion resistance and the paint film adherence and in the latter case in order to improve lubrication during cold working.
The conversion treatment baths used in zinc phosphate-based conversion treatments are essentially acidic aqueous solutions that contain zinc ions, phosphate ions, and oxidizer. Nitrite salts, chlorate salts, hydrogen peroxide, organic nitro compounds, hydroxylamine, and the like, are usually considered for use as the oxidizer. These oxidizers function to accelerate the conversion reactions and so are generally called conversion accelerators. While a nitrate salt may be present in the conversion treatment bath, nitrate salts do not exhibit an oxidizing function in zinc phosphate-based conversion treatment baths and so are distinct from conversion accelerators.
In the case of the conversion treatment of ferriferous metals, one role of the conversion accelerator in zinc phosphate-based conversion treatment is to oxidize the divalent iron ions eluted into the bath to trivalent iron ions. The conversion reactions are inhibited, for example, by the accumulation of divalent iron ions during the continuous conversion treatment of ferriferous metals, so the role of the conversion accelerator in preventing accumulation of the divalent iron ions is extremely important.
However, the known conversion accelerators are each associated with problems that must be solved. For example, in the case of the nitrite salts, which are at present the most widely used conversion accelerators, these are unstable in the acidic region and are thus consumed by spontaneous decomposition even when no conversion treatment is being run and the bath is merely stored. This requires continual make up of the consumed amount in order to maintain a constant concentration.
Furthermore, as is known some of the nitrite salt is converted to NO
x
during the spontaneous decomposition or the intended oxidation activity, and this NO
x
diffuses into the atmosphere as a pollutant.
In the case of chlorate salt conversion accelerators, chloride ions are produced during conversion treatment as a decomposition product and accumulate in the conversion treatment bath. The corrosion resistance of the metal suffers a drastic decline when even a trace amount of the chloride ions in the conversion treatment bath remains present on the surface of the treated metal. Moreover, although chlorate salts are generally used in combination with another conversion accelerator, such as a nitrite salt, the use of a chlorate salt by itself results in a substantial reduction in the conversion reaction rate.
The use of hydrogen peroxide as a conversion accelerator is associated with problems of stability in the conversion treatment bath, and hydrogen peroxide is readily decomposed by dissolved oxygen in the conversion bath. In addition, hydrogen peroxide has a narrow optimal concentration range in conversion treatment, which makes management of the conversion treatment bath quite difficult. When the dissolved hydrogen peroxide concentration is too high, a poorly adherent powder-like conversion film is deposited on the metal surface.
Problems also occur with the use of nitrogenous organic compounds such as organic nitro compounds (e.g., nitroguanine, sodium meta-nitrobenzene sulfonate, etc.) as a conversion accelerator. For example, in the case of nitroguanine, this compound has a low water solubility and thus cannot be formulated as a concentrate for addition to the conversion treatment bath. Moreover, it has a weak oxidizing activity for divalent iron ions and so provides poor control of the divalent iron ions concentration in the conversion bath. Sodium meta-nitrobenzene sulfonate by itself has a poor conversion activity and must generally be used in combination with another stronger conversion accelerator. Its concentration management also requires large-scale measurement instrumentation, such as an ion chromatograph. In addition, the accumulation of these organic nitro compounds and their decomposition products in the conversion treatment bath causes an increase in the COD of the conversion treatment effluent, which has a negative effect on the environment.
With regard to the use of a hydroxylamine compound as a nitrogenous organic conversion accelerator, such a compound must, for best results, be added to the conversion treatment bath in concentrations of at least 1,000 ppm, which causes a large, uneconomical consumption of the conversion accelerator.
The use of chromic acid and permanganate salts as a conversion accelerator for zinc phosphate-based conversion treatment baths has been investigated (Norio Sato, et al., Boshoku Gijutsu [English title: Corrosion Engineering], Volume 15, No. 5 (1966)). These authors reported that the formation of conversion coatings was not observed at concentrations of 5 or 10 millimoles per liter.
Many of the already known conversion accelerators as described above are nitrogenous compounds. These nitrogenous compounds are refractory to removal by chemical wastewater treatment methods and must be removed by microbiological treatments. However, microbiological treatments have trouble removing high concentrations of nitrogenous compounds and cannot completely remove even low concentrations. Nitrogenous compounds have recently been one factor contributing to the eutrophication of bodies of water and have therefore been targeted for increasingly stringent discharge regulations. These environmental considerations have created demand for the development of a nitrogenous compound-free zinc phosphate-based conversion treatment bath.
At present, zinc phosphate-based conversion treatments and chromate treatments are widely used to provide underpaint coatings for the purpose of improving the post-painting corrosion resistance and paint film adherence of various metals. Metal substrates of iron and composite materials comprising combinations of different materials are primarily subjected to zinc phosphate-based conversion treatments due to the difficulties encountered in the chromate treatment of these types of substrates.
The size of the crystals in the coatings afforded by zinc phosphate-based conversion treatment generally undergo large variations as a function of the treatment conditions. Thick coatings of coarse crystals are satisfactory when the goal is rust prevention or cold working. However, such coatings do not afford a satisfactory paint film adherence when they are subsequently painted, and the zinc phosphate-based conversion films employed as underpaint coatings must in fact be thin films of uniform, fine, and dense film crystals.
Two methods are known for obtaining thin zinc phosphate-type conversion films. One method consists of terminating the film deposition reactions during the course of these reactions by interrupting contact with the conversion bath. This method results in incomplete deposition of the conversion film and thus in incomplete coverage of the substrate metal. As a result, not only can rusting occur on
Ishii Hitoshi
Nagashima Yasuhiko
Harper Stephen D.
Henkel Corporation
Jaeschke Wayne C.
Oltmans Andrew L.
Sheehan John
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