Note: Descriptions are shown in the official language in which they were submitted.
CA 02428138 2003-05-06
April 29, 2003
PCT/EP01/12866 44. 393 K
Welter Hillebrand GmbH 8 Co. Galvanotechnik
1
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Westerfiaar 56 - 58, 58738 Wickede/Ruhr
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"Psssivation method"
The invention refers to a method to passivate zinc and Zinc alloy layers as
well as cadmium and cadmium alloy I$yen3 and claims the priority of Gem~an
patent application 100 55 215.3, which is referred to for purpose of disclo..
sure.
It is known to galvanically coat metallic surfaces with metals or metal alloys
in order to improve corrosion resistance. Moreover, it is known to further in-
crease the anticorrosive effect of this layer both by its modification and by
additional coating systems. A system with good anticornosive effect is repre-
sented by a galvanically applied zinc-nickel alloy, which is subsequently
chromatized and which can additionally be treated with an organic or inor-
ganic coat. A good c;omosfon resistance is achieved by employing
chrome(VI) for passivating the zinc-nickel layer.
CA 02428138 2003-05-06
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The toxicity of chrome(VI) compounds contrasts their good anticorrosive
qualities. Accordingly, it has been attempted for a longer period time to es-
tablish chrome(VI) free systems providing a sufficient corrosion resistance.
For this goal it is known to use chrome(ill~oontaining passivation solutions
as described in the US 4 171 231, which however - due the oxidants also
present in the solution -- do not lead to chrome(VI) free passivation layers,
since chrome(Ili) is oxidized during the treatment.
The DE 41 35 524 C2, which is incorporated into the present description and
the teaching of which is fully refer-ed to discloses a passivation method,
which achieves improved anti-corrosion values by a chroive{Ill)-containing
passivation solution with an oxalate complexation (Tab. II; Tab. III).
Different
examples of chrome(lll) compounds for passivatlon are given in table IV of
the mentioned document. The good anticorrosive effect of the passivation
method described in this document is based on the use of oxalate as a cony.
plexing agent, which - in contrast to other oomplexing agents - supports the
in~rporation of chrome into the passivation layer.
With this type of passivation method one can already achieve good enticor
rosion results. Moreover, it is known to increase the anticorrosive protection
provided by chrome(ill~based passivation methods by the use of cobalt.
The imprvvemant of anticorrosive protection by the use of cobalt is described
in the WO 97/40208, which is hereby incorporated.
CA 02428138 2003-05-06
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/~s it can also be seen from this document, the experts aim to produce a
conversion layer as compact as possible in order to achieve the layer's de-
sired qualities with respect to resistance tv corrosion and chemicals.
The invention has the object to provide both a passivation method with fur-
ther improved antioornosive protection characteristics and a respective coat-
ing system.
This aim is achieved by a method and a passivation solution according to the
independent daims. Favorable aspects are subject of the dependent claims.
The invention is based on the finding, that fuming away from the efforts of
focussing on a compact conversion layer and thus aiming to produce a po-
rous conversion layer leads to improved final products when the porosity is
used for binding to at least one further layer.
The invention can be realized by using cobalt In a passivafion solution con-
taining chrome(111) and a weaker complexing agent - preferably a di- or tri-
carbonic acid tike oxalic acid.
By using a cobalt concentration greater than 30 gll, preferably a concentra-
tion between 70 gll and 100 gll, in particular about 90 gll (concentrate -
12%) already surprising cornosion protection values of 240 h until onset of
primary corrosion according to DIN 50961 in the salt spray test according to
OIN 50021 SS are achieved in production. Favorably the chrome-cobalt-ratio
is 1,7:2,0 ~ 0,85.
CA 02428138 2003-05-06
-4-
The method is preferably carried out at temperatures up to 55°C and
pH lev-
els of 0,5 to 5,5. Preferably the passivation is Carried out .at pH 4.
Particularly
good results can be achieved with a suffate free solution. This is based on
the idea that the use of sulfate leads to a disturbance of the catalytic
reaction
and interference with the development of an anticorrosive chrome layer.
The passivated layer is re-treated with an organic or inorganic coating which
penetrates into the porous conversion layer. Preferably, a coat of Aquares'~'M
(Enthone-OMI GmbH, Neuss) is applied to the passivation layer. A particu-
larly good corros~n control can be achieved by a double Aquares layer. The
Aquares layer surprisingly is no longer traceable in the REM in some variants
of the invention.
Both the characteristics of corrosion control and resistance to chemicals can
be further improved by means of a Topcoat containing a dry lubricant. Also
this coat according to the invention engages in a special binding to the po-
rous conversion layer. Hereby, the desired friction and flow values can
achieved at the same time.
Surprising properties are displayed by a combination of the dry lubricant
Molykote~ D-7t 00 (Dow Corning) and the zinc-nickel alloy passivated ac-
cording to the invention. in comparison to known coating systems this sys-
tem Is particularly resistant to chemicals and rim dearer. It is thus
especially
suitable for wheel screws. Furthermore a surprising temperature insensitivity
of the combination acxording to the invention has beers stated.
Preferably this dry lubricant layer is applied onto a double Aquares layer.
