Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1997-02-11
1999-05-18
Kastler, Scott
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148251, 148243, C23C22/36
Patent
active
059047852
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for forming a phosphate conversion coating on metal surfaces that include at least some aluminiferous metal surfaces, which for the purposes of the present invention are to be understood as those containing at least 0.5% of aluminum in their surfaces, are generally exemplified by aluminum sheet, steel sheet plated with Zn-Al alloy, aluminum-plated steel sheet, and the like, and are found, for example, on automotive outer body sheet, on colored sheet-metal roofing material, and so forth.
1. Statement of Related Art
Phosphate conversion coating of metals forms on the metal surface a coating, with a chemical composition typified by Zn.sub.3 (PO.sub.4).sub.2 .multidot.n H.sub.2 O, Zn.sub.2 Fe(PO.sub.4).sub.2 .multidot.n H.sub.2 O, and/or Zn.sub.2 Ca(PO.sub.4).sub.2 .multidot.n H.sub.2 O for the example of zinc phosphate treatments. This is brought about by contacting the metal surface, usually is by spraying or dipping, with a phosphate liquid conversion coating composition in order to dissolve the surface of the metal. Phosphate conversion coatings improve the metal's resistance to rusting and its paint adherence.
Zinc phosphate liquid conversion coating compositions contain phosphoric acid, Zn.sup.2+ ions, and water as their essential components and may contain, for example, Ni.sup.2+, Mn.sup.2+, Ca.sup.2+, Fe.sup.2+, H.sup.+, Na.sup.+, K.sup.+, NH.sub.4.sup.+, NO.sub.3.sup.-, NO.sub.2.sup.-, fluoride (which may be simple fluoride and/or complex fluorides such as fluozirocnate, fluosilicate, fluoborate, and the like), SO.sub.4.sup.2-, Cl.sup.-, or the like, as optional components. The listed components represent those in general use for phosphate liquid conversion coating compositions.
Phosphate sludge, i.e., FePO.sub.4 .multidot.n H.sub.2 O, Zn.sub.3 (PO.sub.4).sub.2 .multidot.nH.sub.2 O, etc., is produced during the phosphate conversion coating of metal surfaces. Sludge-eliminating measures such as filtration and sedimentation are therefore ordinarily used since a low sludge content in the liquid conversion coating composition is desirable for forming phosphate conversion coatings.
Zn-Al alloy-plated steel sheet exhibits a better corrosion resistance and heat resistance than ordinary Zn-plated steel sheet, and as a result its applications as a corrosion-resistant material have in recent years been undergoing active development. In particular, steel sheet plated with Zn containing approximately 5% of aluminum is widely used where painting is to be executed on the steel sheet. However, when this type of steel sheet is subjected to phosphate conversion coating, aluminum ions accumulate in the treatment bath. Removal of aluminum ions from the reaction system is desired because aluminum ions are unnecessary and in fact detrimental to phosphate conversion coating.
In order to prevent Al ion accumulation in the liquid conversion coating composition, it is desirable in the conversion coating of aluminiferous metal surfaces to remove Al ions from the reaction system as a solid. For this purpose, when conversion coating at least partially aluminiferous metal surfaces, a sludge such as NaK.sub.2 AlF.sub.6, Na.sub.3 AlF.sub.6, etc., may also be deliberately produced, in addition to the phosphate sludge which arises spontaneously during conversion coating of any ferrous and/or zinciferous surface. For the purposes of the present specification, the term "sludge" designates a mixture of (a) solids, such as the phosphates and aluminum compounds produced by the reactions occurring during phosphate conversion coating of the metal or produced by the addition of NaHF.sub.2, KHF.sub.2, etc., plus (b) small amounts of metal salts not due to the aforesaid reactions, plus (c) atmospheric dust and entrained contamination adhering on the metal. This sludge is therefore typically a mixture of components such as zinc phosphate, iron phosphate, aluminum compounds such as cryolite (Na.sub.3 AlF.sub.6) or elpasolite (NaK.sub.2 AlF.sub.6), entrained
REFERENCES:
Dai-39-kai Fushoku Boshoku Toronkai Gaiyo (Proceedings of the 39th Japan Corrosion Conference) in Foshoku Boshoku Kyokai-shi (Corrosion Engineering) Dec. 1991.
Aizawa Kenji
Okabayashi Yutaka
Henkel Corporation
Jaeschke Wayne C.
Kastler Scott
Robbins Frank E.
Wisdom, Jr. Norvell E.
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