Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-04-06
2002-09-10
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S495000, C204S499000, C204S504000, C204S506000, C148S260000, C148S262000
Reexamination Certificate
active
06447662
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a section from a processing sequence, as is conventional for coating metal surfaces, in particular in automotive construction: phosphating, followed by post-rinsing and cathodic electrocoating. The present invention solves the problem that, on a phosphate layer produced using a low-nickel phosphating solution, low-lead or lead-free cathodic electrocoating lacquers frequently exhibit substantially poorer corrosion protection and lacquer adhesion properties than either cathodically depositable electrocoating lacquers containing lead or alternatively lead-free cathodically depositable electrocoating lacquers on a phosphate layer which was produced using a high-nickel phosphating solution. The process may be used to treat surfaces made from steel, galvanised or alloy-galvanised steel, aluminum, aluminised or alloy-aluminised steel.
BACKGROUND OF THE INVENTION
An object of phosphating metals is to produce on the metal surface strongly adhering metal phosphate layers which in themselves improve corrosion resistance and, in conjunction with lacquers and other organic coatings, contribute towards a substantial increase in lacquer adhesion and resistance to creepage on exposure to corrosion. Such phosphating processes have long been known. Low-zinc phosphating processes, in which the phosphating solutions have relatively low contents of zinc ions of, for example, 0.3 to 3 g/l and in particular of 0.5 to 2 g/l, are in particular suitable for pretreatment prior to lacquer coating.
RELATED ART
It has been found that phosphate layers having distinctly improved corrosion protection and lacquer adhesion properties may be formed by also using other polyvalent cations in the zinc-phosphating baths. For example, low-zinc processes with the addition 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 in the so-called trication process for preparing metal surfaces for lacquer coating, for example for cathodic electrocoating of automotive bodywork. Reference is made by way of example to EP-B-106 459 and EP-B-228 151.
The elevated content of nickel ions in the phosphating solutions of the trication process and of nickel and nickel compounds in the resultant phosphate layers is, however, disadvantageous in that nickel and nickel compounds are classed as critical with regard to environmental protection and occupational hygiene. Accordingly, increasing numbers of low-zinc phosphating processes have recently been described which, without using nickel, give rise to phosphate layers of a similarly high quality to those obtained using the processes involving nickel.
DE-A-39 20 296, for example, describes a phosphating process which dispenses with nickel and uses magnesium ions in addition to zinc and manganese ions. The phosphating baths described herein contain, in addition to 0.2 to 10 g/l of nitrate ions, further oxidising agents which act as accelerators, selected from nitrite, chlorate or an organic oxidising 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. EP-A-228 151 also describes phosphating baths which contain zinc and manganese as the essential cations. The phosphating accelerator is selected from nitrite, nitrate, hydrogen peroxide, m-nitrobenzoate or p-nitrophenol.
DE-A-43 41 041 describes a process for phosphating metal surfaces using aqueous, acidic phosphating solutions which contain zinc, manganese and phosphate ions and, as accelerator, m-nitrobenzenesulfonic acid or water-soluble salts thereof, wherein the metal surfaces are contacted with a phosphating solution which contains no nickel, cobalt, copper, nitrite or halogen oxo-anions and which contains:
0.3 to 2 g/l of Zn(II)
0.3 to 4 g/l of Mn(II)
5 to 40 g/l of phosphate ions
0.2 to 2 g/l of min-nitrobenzenesulfonate and
0.2 to 2 g/l of nitrate ions.
A similar process is described in DE-A-43 30 104, wherein 0.1 to 5 g of hydroxylamine are used as the accelerator instead of the nitrobenzenesulfonate.
Depending upon the composition of the solution used for phosphating, the accelerator used for the phosphating process, the process for applying the phosphating solution onto the metal surfaces and/or also other processing parameters, the phosphate layer on the metal surfaces is not completely sealed. Instead, “pores” of a greater or lesser size amounting to an area of 0.5 to 2% of the phosphated surface remain which must be sealed in a so-called post-rinsing [“post-passivation”] operation in order to leave no point of attack open to corroding influences on the metal surfaces. Post-passivation moreover improves the adhesion of a subsequently applied lacquer.
It has long been known to use solutions containing chromium salts for this purpose. In particular, post-treatment of the surfaces using solutions containing chromium(VI) substantially improves the corrosion resistance of the coatings produced by phosphating. The improvement in corrosion protection is primarily due to the fact that a proportion of the phosphate deposited on the metal surface is converted in a metal(II)/chromium spinel.
A substantial disadvantage of using solutions containing chromium salts is that such solutions are highly toxic. Furthermore, an increased incidence of unwanted blistering is observed when lacquers or other coating materials are subsequently applied.
Many other possibilities for post-passivation of phosphated metal surfaces have accordingly been proposed, such as using zirconium salts (NL patent 71 16 498), cerium salts (EP-A-492 713), polymeric aluminum salts (WO 92/15724), oligo- or poly-phosphoric acid esters of inositol in conjunction with a water-soluble alkali metal or alkaline earth metal salt of these esters (DE-A-24 03 022) or also fluorides of various metals (DE-A-24 28 065).
EP-B-410 497 discloses a post-rinsing solution which contains Al, Zr and fluoride ions, wherein the solution may be regarded either as a mixture of complex fluorides or also as a solution of aluminum hexafluorozirconate. The total quantity of these three ions is in the range from 0.1 to 2.0 g/l.
DE-A-21 00 497 relates to a process for the electrophoretic application of paints onto surfaces containing iron, wherein the object to be achieved is that of applying white or other light colored paints onto surfaces containing iron without discoloration. This object is achieved by rinsing the surfaces, which may previously have been phosphated, using solutions containing copper. Copper concentrations of between 0.1 and 10 g/l are proposed for this post-rinsing solution. DE-A-34 00 339 also describes a post-rinsing solution containing copper for phosphated metal surfaces, wherein copper contents of between 0.01 and 10 g/l are used.
Of the processes mentioned above for post-rinsing phosphate layers, the only ones to have met with success (other than post-rinsing solutions containing chromium) are those in which solutions of complex fluorides of titanium and/or zirconium are used. Organic reactive post-rinsing solutions based on amine-substituted polyvinylphenols are additionally used. In conjunction with a phosphating process involving nickel, these chromium-free post-rinsing solutions fulfill the stringent lacquer adhesion and corrosion protection requirements of, for example, the automotive industry. However, for reasons of environmental protection and occupational hygiene, efforts are being made to introduce phosphating processes in which the use of both nickel and chromium compounds may be dispensed with at all stages of treatment. Nickel-free phosphating processes in conjunction with a chromium-free post-rinsing do not as yet reliably fulfill lacquer adhesion and corrosion protection requirements on all bodywork materials used in the automotive industry. This is particularly the case if, after phosphating and post-rinsing, a cathodically
Brouwer Jan-Willem
Endres Helmut
Kuhm Peter
Schenzle Bernd
Wichelhaus Winfried
Abrams David S.
Harper Stephen D.
Henkel Kommanditgesellschaft auf Aktien
Mayekar Kishor
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