Method for the electrophoretic dip coating of chromatizable metal surfaces

A method is described for the electrophoretic enamelling of chromatizable metal surfaces. Improved adhesion of the enamel is achieved by a chromating pretreatment of the metal surfaces, the chromated metal surfaces being kept wet from the time of their being chromated up to the time of their introduction into the bath for the electrophoretic enamelling.

FIELD OF THE INVENTION 
The invention relates to a method for the electrophoretic dip coating of 
chromatizable metal surface. Chromatizable metal surfaces are, primarily 
the surfaces of the metals zinc, cadmium, aluminum and magnesium, as well 
as of their chromatizable alloys. The method therefore also is suitable 
for the electrophoretic dip coating of iron and steel surfaces, which are 
coated with such metals or their alloys, for enamelling of galvanized iron 
and steel surfaces. 
BACKGROUND OF THE INFORMATION AND PRIOR ART 
Metal surfaces are enamelled not only for decorative purposes--corrosion 
protection frequently is the main reason, especially when iron and steel 
parts are enamelled. Very good protection against corrosion is achieved by 
enamelling iron and steel parts, provided that the enamel forms a gap-free 
coating. If, however, the layer of enamel has pores, holes, cracks or 
similar defects due to damage, ageing processes or defective production of 
the enamelled layer, there is rapid development of corrosion, which starts 
out from these defects, and a cauliflower-like lifting off of the layer of 
enamel, under which the corrosion process has migrated ("cauliflower" 
corrosion and filiform corrosion). This damage due to corrosion is thus 
not limited to the site of the defect in the enamel layer, but spreads 
rapidly. 
Corrosion protection, which is retained even when the protective layer has 
been damaged slightly, can also be achieved by galvanizing. Excellent 
protection against corrosion is achieved by hot galvanizing steel parts 
because of the thick layer of zinc of typically 50 to 200 microns together 
with the iron-zinc alloy as transition layer from the zinc to the surface 
of the steel base material. 
A zinc layer of lesser thickness is adequate when deposited by 
electrogalvanizing. Moreover, electrogalvanized sheet steel does not have 
brittle intermediate layers of iron-zinc alloys. Electrogalvanizing make 
possible the cathodic corrosion protection of the steel as the sole, 
relatively inexpensive method, which leads to the electrochemical 
suppression of the corrosion of the steel by the zinc layer, even in the 
case of craters up to about 0.7 mm wide. 
In view of its slight thickness (averaging 10 microns), the layer of zinc, 
which is applied as sole protection for the steel during the 
electrogalvanizing process, is however a completely inadequate protection 
against corrosion. This is so especially for the glossy, smooth layers. 
These are not even resistant to handling. The corrosion protection, as 
determined by the Salt Spray Test of DIN 50021, is only minimal; the zinc 
layer has corroded away and brown rust has appeared after only about 6 to 
8 hours. 
A distinct improvement in corrosion protection can be achieved by 
chromating or phosphating the zinc surface. Times of 24 hours before the 
appearance of the first white rust in the Salt Spray Test are customary 
for blue chromatings and times of about 200 hours for yellow chromatings. 
Phosphating on electrogalvanized zinc coatings also increases corrosion 
protection. However, these surfaces are rough and, if damaged, do not show 
the self-healing mechanism that is known from chromatings. Phosphatings 
are therefore used only as a wash primer for subsequent enameling. In the 
Salt Spray Test, these layers are corrosion resistant up to about 150 
hours. 
However, these value are valid only for parts with smooth walls and not for 
critical sites, as represented by re-entering angles (such as depressions, 
blind holes, threads). Because of electrical field effects, frequently not 
enough zinc is deposited in these regions to achieve good corrosion 
protection. 
The protective effect of chromating can be increased by re-immersing 
freshly chromated parts in special, aqueous, so-called sealing solutions. 
This improvement is clearly reflected in the Salt Spray Test. In a similar 
manner, in the method disclosed in the German Auslegeschrift 2,046,449 for 
the protection of corrosion endangered parts that are relevant to safety, 
such as brake linings, electrogalvanized metal parts are chromated, rinsed 
and then, while still wet from the rinsing, immersed in a dispersion of 
synthetic materials, an elastic, intimately interlocked composite layer of 
uneven thickness of chromating and synthetic material being obtained, 
which provides good protection against corrosion. 
The German Auslegeschrift 1,521,656 discloses the chromating of zinc and 
zinc alloys. After being rinsed and subsequently air dried, the chromated 
metal surfaces can be provided with a drying paint based on so-called 
drying oils (unsaturated fatty acids), which cross link with absorption of 
oxygen from the air. 
For reasons of protecting the environment, water-dilutable enamel, 
especially electrophoretic enamel, is used for the combination of 
electrogalvanizing and subsequently pore-free enamelling in large plants 
(for example, in the automobile industry). 
Electrophoretic enamelling is particularly suitable, since uniformly thick 
layers result from this method, so that high requirements for the 
dimensional accuracy of the enamelled pats can be fulfilled. In contrast 
to the galvanic deposition of metals, critical hollow spaces, such as 
blind holes or the inner walls of pipes are also enamelled throughout. 
These inaccessible sites usually are well protected against damage by 
external influences. An electrophoretic dip coating of electrogalvanized 
steel parts can therefore lead to a significant improvement in corrosion 
protection. With this combination, however, the adhesion between zinc and 
the organic, built-up enamels is a major problem. It has turned out that, 
under the influence of weathering and/or mechanical stresses, the enamel 
can peel off within a short time. Priming or adhesion promotion therefore 
is required for the application of the enamel. Adhesion promotion is also 
required in every case, where metals with properties similar to those of 
zinc, such as cadmium or aluminum, are to be enamelled. An adhesion 
promoter also is frequently used with steel. 
In the case of base metals such as steel, zinc (as a pressure die casting 
or when applied as a layer, for example, on steel), cadmium (when applied 
as a layer), aluminum (or its alloys) and magnesium (or its alloys), the 
adhesion promotion usually consists of a phosphating. 
However, the phosphating method has some serious disadvantages: 
The whole procedure of adhesion promotion by means of phosphating is rather 
extensive: 
activation (seeding with TiO.sub.2 particles, in order to obtain fine 
grained, uniform phosphated layers) 
phosphating; rinsing 
dipping in chromic acid, rinsing. 
It therefore requires a corresponding investment in equipment and leads to 
long processing times; in addition the chemicals used are expensive. 
The phosphating solution must be filtered constantly, in order to remove 
precipitated, insoluble tertiary phosphate; this would otherwise interfere 
with the phosphating process. 
The concentration of the phosphating solution of 100 to 200 g/L of 
phosphating salt is very high; this requires a considerable expense for 
rinsing after the phosphating. The filtration as well as the rinsing lead 
to a considerable accumulation of phosphate-containing sludge. Because of 
its heavy metal content, this must be disposed of as hazardous waste. 
The analysis of the phosphating solution is expensive and can be automated 
only with difficulty; however, to ensure a constant quality during 
continuous operation, the method should be automated. 
Until now, those skilled in the art had to assume that adhesion promotion 
for electrophoretic enamelling by chromating is not possible. If namely 
metal surfaces, which are provided, for example, with chromated connectors 
such as screws, are enamelled by electrophoretic enamelling, defective 
adhesion or flaking of the enamel is observed, as is described, for 
example, in "Galvanotechnik" 80 (1989), pages 1615-1621 and in 
"Versiegelung und Lackierung von galvanisch verzinkten Oberflaechen" 
(Sealing and Enamelling Electrogalvanized Surfaces), paper presented at 
the conference on Feb. 21 and 22, 1989, of the Deutsche 
Forschungagesellschaft fuer Oberflaechenbehandlung e.V. (DFO--German 
Research Society for Surface Treatment), together with the deutschen 
Gesellschaft fuer Galvano--und Oberflaechentechnik e.V. (DGO--German 
Society for Galvanic and Surface Technology) pages 143-153. 
OBJECT OF THE INVENTION 
It is an object of the invention to provide a method, by means of which 
electrophoretic enamels can be deposited on chromatizable metal surfaces 
with a significantly better adhesion that achievable with methods of the 
state of the art and by means of which the aforementioned disadvantages of 
phosphating to promote adhesion can be avoided. 
SUMMARY OF THE INVENTION 
Pursuant to the invention, this objective is accomplished by a method for 
the electrophoretic enamelling of chromatizable metal surfaces, which is 
characterized in that the metal surfaces are pretreated by chromating and 
the chromated metal surfaces are kept wet from the time of the chromating 
process until they are introduced into the bath for the electrophoretic 
enamelling. Keeping wet is understood to mean that the aqueously wet 
chromated metal surface, obtained after the chromating process, is kept 
under such conditions until the electrophoretic enamelling process, that 
no drying can take place. As used herein, the term "chromatizable metal" 
refers to a metal other than iron and steel, but one that is known as 
being readily chromatizable by a chemical deposition, as contrasted to any 
electrolytic deposition process, such as zinc, cadmium, aluminum, 
magnesium, or chromatizable alloys thereof, also including base metals 
such as iron and steel when coated with a chromatizable metal. 
Surprisingly, it has been ascertained within the scope of the invention 
that chromating layers can be used as adhesion promoters for 
electrophoretic enamels, provided that these layers are kept in the wet 
state after they are prepared until they are electrophoretically 
enamelled. Within the scope of the invention, it has been ascertained that 
freshly prepared, still wet chromating layers have a hydrophilic surface, 
which is suitable for electrophoretic enamelling. This suitability is 
retained if the freshly prepared chromating layers are kept wet or stored 
moist until they are enamelled electrophoretically. However, after the 
chromated surfaces have dried, electrophoretic enamels can be deposited 
only with poor adhesion. It was also observed that, after drying, renewed 
wetting with water of the chromating surface, once it has dried, does not 
lead to any improvement in the adhesion of the coating materials applied 
by electrophoretic enamelling. This is the state of affairs for all 
chromating layers, which are applied by conventional methods, such as 
yellow, blue, transparent and other chromating layers. 
Pursuant to the invention, the metal surface is chromated as adhesive base 
before the electrophoretic enamelling. In contrast to the previously used 
phosphating, such a chromating has a smooth glossy surface. If the 
chromated surface is kept wet pursuant to the invention, the surface has 
such a high surface tension that, upon being wetted with water, a contact 
angle of the order of 0.degree. results at the interface, that is, at the 
edge of the water droplet. If there is drying, this contact angle is 
increased greatly, for example to 20.degree. to 50.degree., and indicates 
poor wettability. 
The qualitatively high-grade deposition of the electrophoretic enamel on 
the chromating layer can be assured owing to the fact that the deposition 
of the enamel takes place immediately after the chromating without any 
intermediate drying. 
The qualitative high-grade deposition of the electrophoretic enamel on the 
chromating layer can furthermore be assured owing to the fact that, until 
the start of the electrophoretic deposition, the deposited enamel is 
constantly kept moist by being sprayed with water or stored in air with a 
very high relative humidity. Spraying with water comes into consideration 
especially when the whole of the surface can be sprayed. The height of the 
relative humidity, which is required to prevent a drying out of the 
surface of the chromating layers, depends on the time period, which must 
be bridged until the start of the deposition of the enamel. A high 
relative humidity is understood to be one, which is required so that no 
water can evaporate from the chromated surface and any drying out is 
prevented. It depends on the time period, which must be bridged up to the 
start of the deposition of the enamel. In general, it can be assumed that 
the relative humidity must be greater than 90%; for prolonged storage, it 
may amount up to 100%. 
On the other hand, storage of the fresh layers under water until the start 
of the electrophoretic enamelling is less preferred, since components of 
the chromating (especially chromate ions) can go into solution and there 
may be so-called bleeding from the chromating layer. 
An improvement in the adhesion of coatings deposited by electrophoretic 
enameling on surfaces of chromatizable metals can be achieved by the 
inventive method. Such metals, are, for example, zinc, cadmium, aluminum, 
magnesium and their chromatizable alloys. The inventive method thus is 
suitable for iron and steel surfaces, such as sheet metal, which has been 
coated with such chromatizable metals as zinc. 
The chromating of the metal surfaces takes place in the usual manner 
familiar to those skilled in the art. Any known chromating method can be 
used, for example, that described by T. W. Jelinek in "Galvanisches 
Verzinken" (Electrogalvanizing), published by Leuze in 1982. 
Chromating preferably is accomplished with a chromating solution, which 
consists only of inorganic components. Such a chromating solution has, for 
example a concentration of 1 to 10 g/L of chromic acid (H.sub.2 CrO.sub.4) 
and particularly of 4 g/L of chromic acid at a pH of 0 to 3 and preferably 
of 2.3 to 2.7. It is advantageous but not essential if the chromating 
solution contains one or several salts of the metal that is to be 
chromated. Examples of such salts are chlorides, nitrates and/or 
fluorides. The concentration of such optionally present salts is, for 
example, of the order of 0.001 to 0.1 moles/L and preferably of the order 
of 0.05 moles/L. The pH of a freshly prepared chromating solution can be 
adjusted, for example with an oxide or hydroxide of the metal to be 
chromated. It can be checked during the operation by measurement with, for 
example, a glass electrode or by conductivity measurement and adjusted 
once more to the desired value by the addition of acid or oxide or 
hydroxide. 
The usual chromatings represents the last layer, the so-called finish for 
chromatizable metals and are optimized for this purpose, that is, they 
offer some protection against corrosion (in the case of zinc: yellow and 
olive chromating) or improve the appearance (in the case of zinc: blue and 
black chromating). Other aspects, such as the effect on the environment, a 
long lifetime, the ability to regenerate, etc., which do not directly 
affect the quality of the chromating layer, are given hardly any attention 
at the present time. 
In the inventive method, on the other hand, chromating is required only to 
promote adhesion. The requirements with respect to appearance and, partly 
also, with respect to corrosion protection are fulfilled from the enamel 
or by tie combination of galvanically deposited zinc layer and enamel 
layer. 
Any conventional chromating method, which guarantees chromating baths that 
have a long lifetime, can be regenerated easily and have a low consumption 
of chemicals, are therefore particularly suitable of the inventive method. 
For example, chromating solutions, which contain only inorganic components, 
are particularly suitable, since they can be regenerated by the method 
known from the German Patent 3,138,503. According to the teachings of the 
German Patent 3,138,503 interfering decomposition products are removed 
from the chromating solutions with the help of ion exchangers, 
electrodialysis, electrolysis or chemical oxidation, the pH or the 
conductivity is measured during the operation, the Cr.sup.6+ and Cr.sup.3+ 
concentrations are determined photometrically and make-up solutions are 
added depending on these analytical values and the flow through the ion 
exchangers or the fractionating, exchange or reaction apparatuses is 
controlled in such a manner, that the composition of the chromating 
solution is kept within a specified range of concentrations. Chromating 
solutions of very low concentration are sufficient to promote the adhesion 
between galvanically deposited zinc and the electrophoretic enamel. 
Because chromating solution is carried out of the chromating bath together 
with the chromated metal parts, the concentration of unwanted 
decomposition products in the chromating solution cannot increase to 
interfering values. A removal of the unwanted decomposition products, as 
provided for in the German Patent 3,138,503 with the help of, for example, 
ion exchangers, can therefore be omitted; it is sufficient to compensate 
for the chromating solution carried out. 
Pursuant to the invention, the known chromating methods, which work without 
hexavalent chromium, that is, in the absence of chromate, can also be 
used. These methods are also familiar to those skilled in the art and are 
described, for example, in the aforementioned book by T. W. Jelinek. 
Admittedly, such chromatings are not very suitable for corrosion 
protection; however, they do promote adhesion well and have the advantage, 
that the baths used are not an environmental hazard, since they do not 
contain any hexavalent chromium. Such baths containing chromium(III) 
salts, such as potassium chromium sulfate; they may contain acids, such as 
nitric acid and salts, such as fluorides, for example, ammonium hydrogen 
fluoride. 
All conventional chromating methods are suitable for the inventive method. 
In the Federal Republic of Germany, chromating methods are standardized 
according to the regulations of DIN 50960, Pat 1. One differentiates 
between colorless chromating, blue chromating, yellow chromating, olive 
chromating and black chromating. These chromating methods are suitable, 
for example, for the inventive method. According to T. W. Jelinek, 
"Galvanishes Verzinken" (Electrogalvanizing), published by Leuze in 1982, 
page 140, layers up to 0.01 microns thick and weighing 0.03 mg/dm.sup.2 
are attained for colorless chromating, layers up to 0.08 microns thick and 
weighing 0.5-5 mg/dm.sup.2 are attained for blue chromating, layers up to 
1 micron thick and weighing 5-20 mg/dm.sup.2 are attained for yellow 
chromating and layers 1.25 microns thick and weighing 20 mg/dm.sup.2 are 
obtained for olive chromating. All of these thicknesses (which relate in 
the given state in each case to the thicknesses of the dry layers) are 
suitable for the inventive method, for which it is sufficient to form the 
colorless or blue chromatings, which normally are used for decorative 
purposes, but not for corrosion protection. 
Rinsing with water to remove excess chromating solution directly after the 
chromating process may be advantageous. Whether such a rinsing process is 
carried out depends on the concentrations of the chemical compounds and 
ions used in the chromating solution, as well as on the way in which the 
method is carried out. In the case of an electrophoretic deposition of 
enamel, the number of ions carried over into the enamel bath should be 
kept as low as possible. If the chromated metal parts are to brought 
without delay into the bath for electrophoretic enameling, a rinsing 
process before the electrophoretic enamelling may be particularly 
advantageous, irrespective of the composition of the chromating solution 
used. On the other had, if the chromated metal parts are kept wet by being 
sprayed with water until they are brought into the bath for 
electrophoretic enamelling, an additional rinsing process can be omitted 
if the composition of the chromating solution is suitable. 
The metal surfaces, pretreated pursuant to the invention by chromating and 
being kept wet, can be coated or enamelled by the conventional 
electrophoretic enamelling. All conventional electrophoretic coating 
materials and enamelling methods, which with those skilled in the art are 
familiar, are suitable. There are no particular limitations with respect 
to the coating materials or electrophoretic enamelling methods that can be 
used. Of the two basic possibilities of electrophoretic enamelling, namely 
anaphoresis and cataphoresis, the latter, that is the cataphoretic 
enamelling is particularly preferred for the inventive method. However, 
anodic deposition (anaphoresis) is also suitable. 
The enamel layers, prepared by the inventive method, are glossy, smooth and 
free of pores and provide excellent protection against corrosion. These 
enamel layers can serve, for example, as primers, which can be processed 
further in a conventional manner with filling enamels and covering 
enamels. 
An example of the treatment of galvanized sheet steel by the inventive 
method is given in the following.

EXAMPLE 
Sheet steel was treated by the following process steps: 
Defatting (with solvent or aqueous alkaline); rinsing 
Pickling (with nitrosulfuric acid or sulfuric acid); rinsing 
Electrolytic defatting; rinsing 
Electrogalvanizing (cyanidic, alkaline or acidic); rinsing 
Brightening (10 seconds; 3 g/L of nitric acid); rinsing may be omitted 
Chromating (1-3 min); rinse; do not dry 
Cataphoretic enamelling with a conventional, commercial, electrophoretic 
enamelling bath, rinsing with water 
Drying, stoving 
For this example, the chromating bath has the following composition: 
1-5 g/L H.sub.2 CrO.sub.4 
2-10 g/L Zn(NO.sub.3).sub.2 
pH approximately 2.5, adjusted with ZnO or NaOH 
The chromating layer is almost transparent and leads to very good adhesion 
of the enamel layer. The enamel layer, is glossy, smooth, flat, free of 
pores and provides good protection against corrosion. 
The chromating solution shows no signs of autodecomposition. Moreover, 
since the dissolution of zinc (and iron at the regions not galvanized) is 
very slight during the chromating process, the decomposition products do 
not accumulate to a concentration that interferes; a purification of the 
solution by means of a cation exchanger thus is unnecessary. For a 
continuous operation, it is advisable to replace the bath components, 
which are carried Out, continuously and to keep the pH constant (by means 
of analysis or fully automatic and continuously as disclosed in German 
patent 3,138,503). 
EXAMPLE OF A CHROMATE-FREE CHROMATING SOLUTION 
Ammonium hydrogen fluoride (NH.sub.4)HF.sub.2 2.0 g/L 
Nitric acid 4.0 
Potassium chromium sulfate KCr(SO.sub.4).sub.2 .times.12H.sub.2 O 3.0 g/L