Phosphating process with a copper-containing re-rinsing stage

Details Phosphating process with a copper-containing re-rinsing stage Phosphating process with a copper-containing re-rinsing stage

1997-09-29

2000-07-18

Willis, Prince

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

148260, 148262, 106 1412, C23C 2283

Patent

active

060902248

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to a process for phosphating metal surfaces with aqueous acidic zinc-containing phosphating solutions. To improve protection against corrosion and paint adhesion, the phosphating step is followed by an after-rinse using a solution containing lithium, copper and/or silver ions. The process is suitable as, a pretreatment of the metal surfaces for subsequent painting, more especially by electrocoating. The process may be used for the treatment of surfaces of steel, galvanized or alloy-galvanized steel, aluminum, aluminized or alloy-aluminized steel.


TECHNICAL BACKGROUND AND RELATED ART

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 paint 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 painting. 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 paint 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 painting, for example for the cathodic electrocoating of car bodies.
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 paint 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, selected from nitrite, chlorate or an organic oxidizing agent, acting as accelerators. 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-nitrobenzene sulfonate or hydrogen peroxide. 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-nitrobenzoate or p-nitrophenol.
German Patent Application P 43 41 041.2 describes a process for phosphating metal surfaces with aqueous acidic phosphating solutions containing zinc, manganese and phosphate ions and, as accelerator, m-nitrobenzene sulfonic acid or water-soluble salts thereof, in which the metal surfaces are contacted with a phosphating solution which

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