Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Patent
1996-03-06
1998-08-11
Simmons, David A.
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
148262, C23C 2207
Patent
active
057922837
DESCRIPTION:
BRIEF SUMMARY
A nickel-free phosphating process This invention relates to a process for phosphating metal surfaces with aqueous acidic phosphating solutions containing zinc, manganese and phosphate ions and also hydroxylamine in free or complexed form and/or m-nitrobenzenesulfonic acid or water-soluble salts thereof and to their use for pretreating the metal surfaces in preparation for subsequent lacquering, more particularly electrocoating. The process according to the invention may be used for the treatment of surfaces of steel, galvanized or alloy-galvanized steel, aluminium, aluminized or alloy-aluminized steel and, in particular, for the treatment of steel galvanized, preferably electrolytically, on one or both sides.
BACKGROUND OF THE INVENTION
The object of phosphating metals is to produce on the surface of the metals firmly intergrown metal phosphate coatings which, on their own, improve resistance to corrosion and, in combination with lacquers and other organic coatings, contribute towards significantly increasing lacquer adhesion and resistance to creepage on exposure to corrosive influences. Phosphating processes have been known for some time. Low-zinc phosphating processes are particularly suitable for pretreatment before lacquering. The phosphating solutions used in low-zinc phosphating have comparatively low contents of zinc ions, for example of 0.5 to 2 g/l. A key parameter in low-zinc phosphating baths is the ratio by weight of phosphate ions to zinc ions which is normally >8 and may assume values of up to 30.
It has been found that phosphate coatings with distinctly improved corrosion-inhibiting and lacquer adhesion properties can be obtained by using other polyvalent cations in the zinc phosphating baths. For example, low-zinc processes with additions of, for example, 0.5 to 1.5 g/l of manganese ions and, for example, 0.3 to 2.0 g/l of nickel ions are widely used as so-called trication processes for preparing metal surfaces for lacquering, for example for the cathodic electrocoating of car bodies.
RELATED ART
Unfortunately, the high content of nickel ions in the phosphating solutions of trication processes and the high content of nickel and nickel compounds in the phosphate coatings formed give rise to disadvantages insofar as nickel and nickel compounds are classified as critical from the point of view of pollution control and hygiene in the workplace. Accordingly, low-zinc phosphating processes which, without using nickel, lead to phosphate coatings comparable in quality with those obtained by nickel-containing processes have been described to an increasing extent in recent years. The accelerators nitrite and nitrate have also encountered increasing criticism on account of the possible formation of nitrous gases. In addition, it has been found that the phosphating of galvanized steel with nickel-free phosphating baths leads to inadequate protection against corrosion and to inadequate lacquer adhesion if the phosphating baths contain relatively large quantities (>0.5 g/l) of nitrate.
For example, DE-A-39 20 296 describes a nickel-free phosphating process which uses magnesium ions in addition to zinc and manganese ions. In addition to 0.2 to 10 g/l of nitrate ions, the corresponding phosphating baths contain other oxidizing agents acting as accelerators selected from nitrite, chlorate or an organic oxidizing agent.
EP-A-60 716 discloses low-zinc phosphating baths which contain zinc and manganese as essential cations and which may contain nickel as an optional constituent. The necessary accelerator is preferably selected from nitrite, m-nitrobenzenesulfonate or hydrogen peroxide. A dependent claim is directed to the use of 1 to 10 g/l of nitrate; all the Examples mention 4 g/l of nitrate.
EP-A-228 151 also describes phosphating baths containing zinc and manganese as essential cations. The phosphating accelerator is selected from nitrite, nitrate, hydrogen peroxide, m-nitrobenzenesulfonate, m-nitrobenzoate or p-nitrophenol. Dependent claims specify a nitrate content of 5 to around 15 g/l and
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patent: 5207840 (1993-05-01), Riesop et al.
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patent: 5232523 (1993-08-01), Endo et al.
patent: 5268041 (1993-12-01), Gehmecker et al.
EP -564 -286A2 Oct. 1993.
Brands Karl Dieter
Brouwer Jan-Willem
Gottwald Karl-Heinz
Mayer Bernd
Roland Wolf-Achim
Henkel Kommanditgesellschaft auf Aktien
Jaeschke Wayne C.
Koehler Robert R.
Ortiz Daniel S.
Simmons David A.
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