Coating processes – Immersion or partial immersion – Metal base
Patent
1997-06-06
1999-08-03
Utech, Benjamin
Coating processes
Immersion or partial immersion
Metal base
4274432, 148259, 148262, B05D 512
Patent
active
059322926
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to zinc phosphate-based conversion treatment baths which can be applied to a variety of metal substrates, for example, steel, steel sheet, galvanized steel sheet, and the like. More particularly, this invention relates to a zinc phosphate-based conversion bath and to a surface treatment method that are able to form a fine, dense, and uniform conversion coating on metal surfaces and that are also able to induce fine-crystal formation in the conversion coating.
BACKGROUND ART
The execution of a zinc phosphate-based conversion treatment on various metals prior to the coating or plastic working thereof is known at present for the purpose of improving the paint adherence and post-paint corrosion resistance and improving the lubrication during plastic working.
The conversion treatment baths used for zinc phosphate-based conversion treatment are essentially acidic aqueous solutions that contain zinc ions, phosphate ions, and oxidizing agent(s). Nitrite salts, chlorate salts, hydrogen peroxide, organic nitro compounds, hydroxylamine, and the like, are ordinarily considered for this oxidizing agent. These oxidizing agents are typically called conversion "accelerators" because they function to accelerate the conversion reactions. Nitrate salts may be present in conversion baths, but--because, in the concentrations usually present in zinc phosphate-based conversion baths, nitrate ions do not exercise an oxidizing activity sufficient to convert ferrous ions substantially completely to ferric ions--nitrate ions must be distinguished from the conversion accelerators.
One important role of conversion accelerators during the zinc phosphate based conversion treatment of ferriferous metals is to oxidize the divalent iron ions eluting into the conversion bath to trivalent iron ions. For example, the conversion reactions are inhibited when divalent iron ions accumulate in the conversion bath during the continuous conversion treatment of ferriferous metals, and the role of the conversion accelerator in inhibiting this accumulation of divalent iron ions is thus crucial.
However, each of these already known conversion accelerators is associated with problems that must be addressed. For example, in the case of the nitrite salts, which are the most widely used conversion accelerators at the present time, these compounds are unstable in the acid region. As a result, these compounds undergo spontaneous decomposition and are thereby consumed even when conversion treatment is not being run (storage period). The maintenance of a constant or prescribed concentration of these compounds therefore requires continual replenishment to make up for the amount lost to this consumption.
It is also known that as a result of their oxidative activity and spontaneous decomposition these nitrite salts partially convert to NO.sub.x gas, which diffuses into and pollutes the atmosphere.
When chlorate salts are used as conversion accelerators, chloride ions are produced as a decomposition product during conversion treatment and accumulate in the conversion treatment bath. The corrosion resistance of the metal substrate is substantially impaired when even a trace of chloride ions from the conversion treatment bath remains on the surface of the metal workpiece. In addition, chlorate salts are ordinarily used in combination with another conversion accelerator, such as nitrite salts, and when used alone provide only a significantly reduced conversion reaction rate.
Stability in the conversion treatment bath is also a problem for the use of hydrogen peroxide as a conversion accelerator: Hydrogen peroxide is readily decomposed by oxygen dissolved in the conversion bath. In addition, hydrogen peroxide has a narrow optimal concentration range for conversion treatment, which makes it difficult to manage the conversion treatment bath. When the dissolved concentration is too high, a powdery, poorly adherent conversion coating is deposited on the metal surface.
With regard to the use of nitrogenous organic compounds as conv
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Ishii Hitoshi
Nagashima Yasuhiko
Berdo Robert H.
Deo Duy-Vu
Henkel Corporation
Jaeschke Wayne C.
Utech Benjamin
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