Nickel-free phosphating process

Details Nickel-free phosphating process Nickel-free phosphating process

1994-09-30

2001-03-06

Jones, Deborah (Department: 1775)

Metal treatment

Process of modifying or maintaining internal physical...

Processes of coating utilizing a reactive composition which...

C148S273000, C148S275000, C428S628000

Reexamination Certificate

active

06197126

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a process for the production of copper-containing nickel-free phosphate coatings on metal surfaces and to the use of the process as a pretreatment of the metal surfaces before lacquering, more particularly before cataphoretic dip lacquering (CDL).
BACKGROUND OF THE INVENTION
The quality of phosphate coatings before cataphoretic dip lacquering (CDL) depends upon a number of parameters, including physical parameters, such as the shape and size of the crystals, their mechanical stability and, in particular, the free metal surface after phosphating, the so-called pore area. Among the chemical parameters, alkali stability during cataphoretic coating, the binding strength of the water of crystallization of the zinc phosphate crystals during stoving of the lacquers and the rehydration capacity are of particular interest.
The weight of the coating can be controlled and, in particular, reduced by using activating agents before phosphating. Active centers from which crystal growth advances are formed on the metal surface by the polymeric titanium phosphates present in the activating agents. On the one hand, this results in smaller and mechanically more stable crystals, on the other hand the pore area is reduced in size which makes it more difficult for corrosive media to attack the lacquer coating in the event of damage.
RELATED ART
In the prior art, it has proved to be of advantage to provide a separate treatment bath in order optimally to influence the quality of the phosphate coating subsequently applied. However, the effective life of the activating baths is limited by carryover from the preceding cleaning baths. In particular, water hardness ions deactivate the polymeric titanium phosphates.
Accordingly, a search has been made for ways of obtaining a dense, substantially nonporous phosphate coating with a low weight per unit area by other methods or of producing such a coating in the phosphating bath itself in addition to activation. Extensive basic works have been carried out to this end. Some of these works were carried out at the Institute for Crystallography of the University of Cologne and resulted in the discovery of a new crystal phase Ba
3
(PO
4
)
2
.H
2
O (Z. für Kristallographie 196, 312-313 (1991)). Although barium phosphate coatings do not contain any zinc, they have a number of positive properties, including in particular high thermal stability. Unfortunately, the coating weights obtainable are not sufficient to afford high protection against corrosion in combination with cataphoretic dip lacquering. Accordingly, barium phosphate coatings occupy an intermediate position between the “thin” iron phosphate coatings (0.3-0.5 g/m
2
) and the “thicker” zinc phosphating coatings (2.0-3.5 g/m
2
).
Aluminium ions reduce the weights of the phosphate coatings to an even greater extent, so that so-called “passivation phenomena”, i.e. disturbances to the formation of zinc phosphate coatings, occur beyond a concentration of only 5 ppm Al
3+
ions in the phosphating bath.
Additions of magnesium ions have also been investigated. The positive effects of these ions on performance were recognized at an early stage (DE-A-39 20 296) and are based on several factors. The high crystal stability on heating is crucial in this case, too. The release of water of crystallization, which weakens the crystal structure and hence the system as a whole, is displaced to higher temperatures with increasing incorporation of magnesium. On the other hand, the crystals become smaller, the phosphate coating becomes denser and the free metal surface after phosphating is minimized by additions of Mg
+2
ions. The reduction in coating weight by magnesium ions is so considerable that other controlling factors which, normally, are also used for reducing coating weight, such as very low zinc concentrations (0.6 g/l Zn
2+
), high concentrations of accelerators, such as sodium nitrite or meta-nitrobenzenesulfonate/Na salts, do not have to be additionally used to produce a weight per unit area of 1.5 to 2.0 g/m
2
.
The influence of Cu
2+
ions has also been investigated. Additions of small quantities of copper ions to phosphating baths have been known for 40 years. Thus, in U.S. Pat. No. 2,293,716, very small quantities of Cu
2+
ions are added as “accelerators” or as “color neutralizers” to improve the whiteness of anodic electrodeposition lacquers. It was found that additions of copper increase the weight of the coating, particularly on steel.
DE-A-40 13 483 describes a process for phosphating metal surfaces in which phosphating solutions substantially free from nickel are used. Zinc, manganese and small contents of copper are mentioned as key bath constituents. In addition, the concentration of Fe(II) is kept below a maximum value by oxygen and/or other equivalent oxidizing agents. The process in question is used in particular for the pretreatment of metal surfaces for subsequent lacquering, more particularly electrode-position lacquering, and for the phosphating of steel, galvanized steel, alloy-galvanized steel, aluminium and alloys thereof.
EP-A-0 186 823 describes strongly acidic phosphating solutions with a pH value of 1.8 to 2.5 which contain 7.5 to 75 g/l of zinc ions, 0.1 to 10 g/l of hydroxylamine and optionally up to 20 g/l of manganese ions and also 5 to 75 g/l of nitrate ions. The solutions tolerate an iron content of up to 25 g/l.
A process for the zinc phosphating of iron-containing surfaces is known from EP-A-0 315 059. The desired morphology of the zinc phosphate crystals is established by the use of hydroxylammonium salts, hydroxylamine complexes and/or hydroxylamine. All the Examples contain nickel in addition to zinc as another layer-forming cation. The toxicological disadvantages of nickel are well-known.
BRIEF SUMMARY OF THE INVENTION
Accordingly, the problem addressed by the present invention was to provide a process for the production of nickel-free phosphate coatings which, despite the absence of nickel, would guarantee very firm lacquer adhesion and excellent corrosion protection on metal surfaces, such as cold-rolled steel, electrogalvanized steel and aluminium.
According to the invention, this problem has been solved by a specially selected phosphating solution which contains hydroxylamine salts, hydroxylamine complexes and/or hydroxylamine in a quantity of 500 to 5,000 ppm hydroxylamine, based on the phosphating solution, as the active component for modifying the crystal morphology (“accelerator”). With phosphating solutions such as these, it is possible to produce copper-containing phosphate coatings with a defined copper content and a defined edge length of the phosphate crystals.
In a first embodiment, therefore, the present invention relates to a process for the production of copper-containing nickel-free phosphate coatings with a copper content of 0.1 to 5% by weight and an edge length of the phosphate crystals of 0.5 to 10 &mgr;m on metal surfaces selected from steel, galvanized steel, alloy-galvanized steel, aluminium and alloys thereof by treatment of the surfaces by spraying, dipping or spraying/dipping with a phosphating solution containing the following components:
zinc ions 0.2 to 2 g/l
copper ions 0.5 to 25 mg/l
phosphate ions 5 to 30 g/l (expressed as P
2
O
5
)
and hydroxylamine salts, hydroxylamine complexes and/or hydroxylamine in a quantity of 500 to 5,000 ppm of hydroxylamine, based on the phosphating solution.
It has been found that, even in the absence of nickel, these phosphating solutions guarantee very firm lacquer adhesion and excellent corrosion protection on the metal surfaces mentioned above without the formation of any patches. The zinc phosphate coatings thus produced are made up of small (0.5 to 10 &mgr;m), compact and densely grown crystals.
DETAILED DESCRIPTION OF THE INVENTION
More particularly, the investigation of phosphating baths containing copper ions has shown that only very small quantities of copper ions are required in the solution to establish the desired copper content o

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