CHROMATE-FREE CONVERSION LAYER AND PROCESS FOR PRODUCING THE SAME

COUCHE DE CONVERSION EXEMPTE DE CHROME(VI) ET SON PROCEDE DE PRODUCTION

English Abstract

A chromate-free, chromic and substantially coherent conversion layer based on
zinc or zinc alloys offers a protection against corrosion
from about 100 to 1000 h, as proven by the salt spray test of DIN 50021 SS or
ASTM B 117-73 to first corrosion as described by DIN
50961, chapter 10, even when it does not contain other components such as
silicate, Cer, aluminium and borate. This conversion layer is
clear, transparent, substantially colourless and iridescent, is about 100 nm
to 1000 nm thick, hard, adheres well and is stable against wiping.

French Abstract

Cette couche de conversion sensiblement cohérente à base de zinc ou d'alliages de zinc est exempte de chrome(VI) et contient du chrome (III). Même sans d'autres composants tels que des silicates, du Cer, de l'aluminium et du borate, cette couche offre une protection contre la corrosion de 100 à 1000 h environ, prouvée par le test au brouillard salin selon DIN 50021 SS ou ASTM B 117-73 jusqu'à la première attaque selon DIN 50961, chapitre 10. Cette couche est translucide, sensiblement incolore et irisée, a une épaisseur de 100 nm à 1000 nm environ, est dure, fortement adhésive et résiste à l'essuyage.

Claims

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CLAIMS
1. A chromium(VI)-free, chromium(III)-containing and substantially
coherent conversion layer on zinc or zinc alloys, wherein even in the absence
of silicate, cerium, aluminum and borate it presents a corrosion protection of
about 100 to 1000 h in the salt spray test according to DIN 50021 SS or
ASTM B 117-73 until first attack according to DIN 50961 Chapter 10;
and wherein said layer is clear, transparent, colorless and presents a
greenish, multi-colored iridescence;
said layer has a layer thickness of about 100 nm to 1000 nm; and
said layer is hard, adheres well and is resistant to wiping; and
said layer has across the conversion layer thickness a chromium
content of greater than 1% based upon zinc and chromium, the conversion
layer having an average chromium content of more than about 5%;
said layer has a chromium-rich zone greater than 20% chromium,
based upon zinc and chromium in the conversion layer, of more than 15 nm;
and
wherein said layer has a chromium index greater than 10, wherein the
chromium index is defined as the average chromium content
(chromium/(chromium+zinc)) in the layer greater than 1% Cr, multiplied by the
layer thickness in nm.
2. The conversion layer according to claim 1, wherein for further
enhanced corrosion protection, said layer comprises additional components
selected from the group consisting of: silicate, cerium, aluminum, borate, 1-
to
6-valent metal compounds, anions, polymers, corrosion inhibitors, silicic
acids, surfactants, diols, triols, polyols, organic acids, amines, plastic
dispersions, dyes, color pigments, amino acids, siccatives, dispersing agents,
and mixtures thereof.

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3. The conversion layer according to claim 2 wherein the 1- to 6-valent
metal compounds are selected from the group consisting of: compounds of
Na, Ag, Al, Co, Ni, Fe, Ga, In, lanthanides, Zr, Sc, Ti, V, Mn, Cu, Zn, Y, Nb,
Mo, Hf, Ta, and W.
4. The conversion layer according to claim 2 or 3 wherein the anions are
selected from the group consisting of: halide ions, sulfurous ions, nitrate
ions,
phosphoric ions, carboxylic acid anions and silicon-containing anions.
5. The conversion layer according to claim 4 wherein the halide ions are
chloride ions.
6. The conversion layer according to claim 4 wherein the sulfurous ions
are sulfate ions.
7. The conversion layer according to claim 4 wherein the phosphoric ions
are selected from the group consisting of: phosphate ions, diphosphate ions,
at least one of linear and cyclic oligophosphate ions, at least one of linear
and
cyclic polyphosphate ions, and hydrogen phosphate ions.
8. The conversion layer according to claim 4 wherein the silicon
containing anions are silicate anions.
9. The conversion layer according to any one of claims 2 to 8 wherein the
polymers are organic polymers.
10. The conversion layer according to any one of claims 2 to 9 wherein the
silicic acids are colloidal or disperse silicic acids.
11. The conversion layer according to any one of claims 2 to 10 wherein

-26-
the organic acids are monocarboxylic acids.
12. The conversion layer according to any one of claims 2 to 11 wherein
the color pigments are selected from the group consisting of: carbon black,
chromogenic agents, and metallic chromogenic agents.
13. The conversion layer according to any one of claims 2 to 12 wherein
the amino acids are glycine.
14. The conversion layer according to any one of claims 2 to 13 wherein
the siccatives are cobalt siccatives.
15. The conversion layer according to claim 1, comprising dyes or
color pigments.
16. A method for producing chromium(VI)-free conversion layers affording
at least the corrosion protection of conventional chromium(VI)-containing
yellow chromations, wherein a metallic surface is treated with a solution of
at
least one chromium(III) complex and at least one salt; wherein chromium(III)
is present in a concentration of about 5 to 100 g/l; and a chromium(III)
complex is used having ligand replacement kinetics more rapid than the
fluoride replacement kinetics in chromium(III)-fluorocomplexes.
17. The method of claim 16 wherein the metallic surface is a zinc or zinc
alloy.
18. The method of claim 17 wherein the zinc alloy further comprises iron.
19. The method of claim 16, wherein said metallic surface is treated at a
temperature from 20°C to 100°C.

-27-
20. The method of claim 16, wherein said metallic surface is treated at a
temperature from 20°C to 80°C.
21. The method of claim 16, wherein said metallic surface is treated at a
temperature from 30°C to 60°C.
22. The method of claim 16, wherein said metallic surface is treated at a
temperature from 40°C to 60°C.
23. The method according to any one of claims 16 to 22, wherein the
ligands of the chromium (III) complex are selected from the group consisting
of: dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, oxalic,
malonic, succinic, glutaric, adipic, pimelic, suberic, azealic, sebacic acid,
maleic acid, phthalic acid, terephthalic acid, tartaric acid, citric acid,
malic
acid, ascorbic acid, acetylacetone, urea, urea derivatives, and mixtures
thereof, and the method is performed repeatedly on the surface to be
passivated.
24. The method of claim 23 wherein the chromium (III) complexes further
comprise inorganic anions and water.
25. Use of a chromium(III)-containing solution, wherein chromium(III) is
present in the form of at least one complex having ligand replacement kinetics
more rapid than fluoride replacement kinetics in chromium(III)-
fluorocomplexes; and chromium(III) is present in a concentration of about 5 to
100 g/l, as a passivation bath for surfaces of zinc or zinc alloys, and
wherein
the solution contains chromium (III) as a passivating component.
26. The use of claim 25 wherein the zinc alloy further comprises iron.

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27. The use of claim 25 or 26, wherein the chromium (III) complex is
selected from complexes with chromium (III) and at least one ligand from the
group consisting of: dicarboxylic acids, tricarboxylic acids,
hydroxycarboxylic
acids, oxalic, malonic, succinic, glutaric, adipic, pimelic, suberic, azelaic,
sebacic acid, maleic acid, phthalic acid, terephthalic acid, tartaric acid,
citric
acid, malic acid, ascorbic acid and mixtures thereof.
28. The use of any one of claims 25 to 27 wherein the chromium (III)
complexes further comprise inorganic anions and water.
29. The use of any one of claims 25 to 28, wherein the solution further
comprises additives selected from the group consisting of: sealers,
dewatering fluids, 1- to 6-valent metal compounds, anions, polymers,
corrosion inhibitors, silicic acids, surfactants, diols, triols, polyols,
organic
acids, amines, plastic dispersions, dyes, pigments, amino acids, siccatives,
dispersing agents, and mixtures thereof.
30. The use of claim 29 wherein the 1- to 6-valent metal compounds are
selected from the group consisting of compounds of Na, Ag, Al, Co, Ni, Fe,
Ga, In, lanthanides, Zr, Sc, Ti, V, Mn, Cu, Zn, Y, Nb, Mo, Hf, Ta, and W.
31. The use of claim 29 wherein the anions are selected from the group
consisting of halide ions, sulfurous ions, nitrate ions, phosphoric ions,
carboxylic acid anions and silicon-containing anions.
32. The use of claim 31 wherein the halide ions are chloride ions.
33. The use of claim 31 wherein the sulfurous ions are sulfate ions.

-29-
34. The use of claim 31 wherein the phosphoric ions are selected from the
group consisting of phosphate ions, diphosphate ions, at least one of linear
and cyclic oligophosphate ions, at least one of linear and cyclic
polyphosphate ions, and hydrogen phosphate ions.
35. The use of claim 31 wherein the silicon-containing anions are silicate
anions.
36. The use of claim 29 wherein the polymers are organic polymers.
37. The use of claim 29 wherein the silicic acids are colloidal or disperse
silicic acids.
38. The use of claim 29 wherein the organic acids are monocarboxylic
acids.
39. The use of claim 29 wherein the pigments are selected from the group
consisting of carbon black, chromogenic agents, and metallic
chromogenic agents.
40. The use of claim 29 wherein the amino acids are glycines.
41. The use of claim 29 wherein the siccatives are cobalt siccatives.
42. The use of any one of claims 25 to 41, wherein chromium(III) is
present in a concentration of about 5 g/l to 80 g/l.
43. The use according to claim 42, wherein chromium(III) is present in the
concentration of about 5 g/l to 60 g/l.

-30-
44. The use according to claim 42, wherein chromium(III) is present in the
concentration of about 10 g/l to 30 g/l.
45. The use according to claim 42, wherein chromium(III) is present in the
concentration of about 20 g/l.
46. The use of any one of claims 25 to 45, wherein the solution has a pH
between about 1.5 and 3.
47. The use of any one of claims 25 to 41, wherein the solution contains
about 20 g/l chromium(III) and has a pH of about 2 to 2.5.
48. The use of any one of claims 25 to 47, wherein the solution has a bath
temperature of about 20° to 100°C.
49. The use of claim 48, wherein the bath temperature is about 20° to
80°C.
50. The use of claim 48, wherein the bath temperature of about 30° to
60°C.
51. The use of claim 48, wherein the bath temperature of about 40° to
60°C.
52. The use of any one of claims 25 to 51, for the treatment of objects
wherein the objects are immersed into the chromium(III)-containing solutions
for between 15 and 200 seconds.
53. The use of claim 52, wherein the objects are immersed for between

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15 and 100 seconds.
54. The use of claim 52, wherein the objects are immersed for about 30
seconds.
55. The method of any one of claims 16 to 24 comprising an elevated-
temperature chromate coating method operating at 20° to 100°C
with rinsing
water recycling over at least two cascaded rinsing stages.
56. The method of claim 55, wherein a blue chromation is performed in
one of the rinsing stages.
57. The use of claim 25 , wherein said ligand is selected from the group
consisting of acetylacetone, urea, and urea derivatives.

Description

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-1-
Description
CHROMATE-FREE CONVERSION LAYER
AND PROCESS FOR PRODUCING THE SAME
Metallic materials, in particular iron and steel, are plated with zinc or
cadmium in
order to protect them from corrosive environmental influences: The corrosion
protection of zinc resides in the fact that it is even less precious than the
base
metal and therefore at first exclusively draws the corrosive attack; it acts
as a
sacrificial layer. The base metal of the respective zinc-plated component
remains
unimpaired as long as it is continuously covered with zinc, and the mechanical
functionality remains preserved over longer periods of time than in the case
of
parts not plated with zinc. Thicker zinc layers naturally afford higher
corrosion
protection than thin layers inasmuch as corrosive erosion of thicker layers
simply
takes more time.
The corrosive attack on the zinc layer, in turn, can be greatly delayed by
application of a chromation, or chromate coating, whereby corrosion of the
base
metal is even further postponed than by mere zinc plating. A considerably
better
corrosion protection is afforded by the zinc/chromate layer system is than by
a
mere zinc layer of identical thickness. Moreover by means of chromation the
optical deterioration of a component due to environmental influences is
further
postponed - the corrosion products of zinc, referred to as "white rust",
equally
interfere with the optical appearance of a component.
The advantages of an applied chromation are so important that almost any
galvanically zinc-plated surface is in addition chromate coated as

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well. The prior art knows four chromations named after their
colorations, which are each applied by treating (immersion, spraying,
rolling) a zinc-plated surface with the corresponding aqueous chromate
coating solution. Moreover yellow and green chromations for aluminum
are known which are produced analogously. In any case, these are
variously thick layers of substantially amorphous zinc/chromium oxide
(or aluminum/chromium oxide) with non-stoichiometric compositions, a
certain water content, and inserted foreign ions. These are known and
classified into method groups in accordance with German Industrial
Standard (DIN) 50960, Part 1:
1 ) Colorless and blue chromations, Groups A and B
The blue chromate layer has a thickness bf up to 80 nm, is weakly blue
in its inherent color and presents a golden, reddish, bluish, greenish or
yellow iridescent coloring brought about by refraction of light in
accordance with layer thicknesses. Very thin chromate layers lacking
almost any inherent color are referred to as colorless chromations
(Group A). The chromate coating solution may in either case consist of
hexavalent as well as trivalent chromates and mixtures of both,
moreover conducting salts and mineral acids. There are fluoride-
containing and fluoride-free variants. Application of the chromate
coati;~g solutions is performed at room temperature. The corrosion
protection of unma«ed blue chromations amounts to 10-40 h in the salt
spray cabinet according to DIN 50021 SS until the first appearance of
corrosion products. The minimum requirement for Method Groups A and
B according to DIN 50961 Chapter 10 Table 3 is 8 h for drumware and
16 h for shelfware.
2) Yellow chromations, Group C
The yellow chromate layer has a thickness of approx. 0.25-1 ~~m, a
golden yellow coloring, and frequently a strongly red-green iridescent
coloring. The chromate coating solution substantially consists of
hexavalent chromate, conducting salts and mineral acids dissolved in
water. The yellow coloring is caused by the significant proportion
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(80-220 mg/m2) of hexavalent chromium which is inserted besides the
trivalent chromium produced by reduction in the caurse of the layer
formation reaction. Application of the chromate coating solutions is
performed at room temperature. The corrosion protection of unmarred
yellow chromations Gmounts to 100-200 h in the salt spray cabinet
according to DiN 50021 SS until the first appearance of corrosion
products. The minimum requirement for Method Group C according to
DIN 50961 Chapter 10 Table 3 amounts to 72 h for drumware and 96 h
for shelfware.
3) Olive chromations, Group D
The typical olive~chromate layer has a thickness of up to 1.5 ym and is
opaquely olive green to olive brown. The'chromate coating solution
substantially consists of hexavalent chromate, conducting salts and
mineral acids dissolved in water, in particular phosphates or phosphoric
acid, and may also contain formates. Into the layer considerable
amounts of chromium(VI) (300-400 mg/m2) are inserted. Application of
the chromate coating solutions is performed at room temperature. The
corrosion protection of unmarred olive chromatians amounts to
200-400 h in the salt spray cabinet according to DIN 50021 SS until the
first appearance of corrosion products. The minimum requirement for
Method Group D according to DIN 50961 Chapter 10 Table 3 is 72 h for
drumware and 120 h for shelfware.
4) Black chromations, Group F
The black chromate layer is fundamentally a yellow or olive chromation
having colloidal silver inserted as a pigment. The chromate coating
solutions have about the same composition as yellow or olive
chromations and additionally contain silver ions. With a suitable
composition of the chromate coating solution on zinc alloy layers such
as Zn/Fe, Zn/Ni or Zn/Co, iron, nickel or cobalt oxide will be incarporated
into the chromate layer as a black pigment so that silver is not required
in these cases. Into the chromate layers considerable amounts of
chromium(VI) are inserted, namely between 80 and 400 mg/m2
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depending on whether the basis is a yellow or olive chrornation.
Application of the chromate coating solutions is performed at room
temperature. The corrosion protection of unmarred black chromations on
zinc amounts to 50-150 h in the salt spray cabinet according to DIN
50021 SS until the first appearance of corrosion products. The minimum
requirement for Method Group E according to DIN 50961 Chapter 10
Table 3 is 24 h for drumware and 48 h for shelfware. Black chromations
on zinc alloys are considerably above the specified values.
5) Green chromations for Aluminum, Group E
The green chromation on aluminum (known under the name of aluminum
green) is of a ri~att green and not iridescent. The chromate coating
solution substantially consists of hexavaient chromate, conducting salts
and mineral acids dissolved in water as well as particularly phosphates
and silicofluorides. Contrary to a prevailing opinion ;he formed
chromate/phosphate layer is, as evidenced by iodised starch tests, not
always 100% chromium(VI)-free. The production of aluminum green in
chromate coating solutions exclusively on the basis of chromium(Ili) is
not known.
in accordance with the prior art, thick chromate layers affording high
corrosion protection > 100 h in the salt spray cabinet according to DIN
50021 SS or ASTM B 1 17-73 until the appearance of first corrosion
products according to DIN 50961 (June 1987) Chapter 10, in particular
Chapter 10.2.1.2, in the absence of sealing or any other particular
aftertreatment (DIN 50961, Chapter 9) may only be produced by
treatment with dissolved, markedly toxic chromium(VI) compounds.
Accordingly the chromate layers having the named"requirements to
corrosion protection still retain these markedly toxic and carcinogenic
chromium(VI) compounds, which are, moreover, not entirely immobilised
in the layer. Chromate coating with chromium(VI) compounds is
problematic with respect to workplace safety. Use of zinc-plated
chromations produced with chromium(VI) compounds, such as the
widespread yellow chromations e.g. on screws, constitutes a potential
hazard to the population and increases the general cancer risk.
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US 43 84 902, in particular with Examples 1, 2, 4 and 5, describes
conversion layers which satisfy the requirements in the salt spray test.
in all of the cases, these are cerium-containing layers presenting a
yellowish coloration which is accentuated by the cer ium(IV) ion. The
examples only contain cerium(III), and hydrogen peroxide as an oxidant,
in the bath solution. In the description it is set forth that hydrogen
peroxide in the acidic medium does not represent an oxidant for Ce(III),
however during deposition the pH value nevertheless rises so high at the
surface that a sufficient amount of Ce(IV) may be generated. The
yellowish coloration achieved by this bath composition indeed appears
to indicate that an oxidation has taken place - however, only an
oxidation from'Ce(III) to Ce(IV). Tetravalent cerium is an even more
powerful oxidant than hexavalent chrom>>am, for which reason Ce(iV)
will produce from Cr(III) the Cr(~VI) which is to be avoided. Cr(VI) has a
very strong yellow coloration and is known as an anticorrosion agent.
The layer described in US 43 84 902 is thus not free of hexavalent
chromium.
The layer according to the invention is, however, produced in the
absence of any oxidant and consequently free of hexavalent chromium.
This can in particular be seen from the fact that the layer according to
the invention is not yellow.
Even where the yellow coloration and the enhanced corrosion protection
should be brought about by nothing but Ce(IV), the layer according to
the invention affords the desired corrosion protection even without this
very costly and rare addition.
US 43 59 348 also describes conversion layers which satisfy the above
mentioned requirements in the salt spray test. These, too, in all cases
are cerium-containing layers which present the yellowish coloration
accentuated by the cerium(IV) ion. This document thus does not exceed
US 43 84 902.
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It is therefore an object of the present invention to furnish a chromium(VI)-
free, thick conversion layer having a high chromium content on zinc or zinc
alloys.
For the purposes of the present inventions the applicant coined the term
"chromitation" in order to clearly distinguish the present invention from the
chromations which are customary in the prior art, and in order to make clear
that neither the obtained conversion layer nor the compositions
(concentrates/passivation baths) whereby the coatings according to the
invention are produced contain the toxic chromium(VI), whereas the obtained
corrosion protection nevertheless is superior to that of yellow chromation.
EP 00 34 040 A1 does describe a multiplicity of layers, of the larger group of
which (produced under the standard conditions set forth by Barnes/Ward) the
color is not specified, howE:ver referred to as clear. Two Examples, namely
Nos. 16 and 17, describe a greenish borate-containing layer described as
cloudy-dull to non-transparent.
Example 14 describes a layer affording a corrosion protection of only 4 hours.
The subclaims represent preferred embodiments of the present invention.
Concerning the features of claim 2, the following should be noted:
In glow-discharge spectrometry several elements could not be detected while
others could not be calibrated. Therefore the chromium/(chromium+zinc)
phases were compared to each other. The chromium index is the average
chromium content in % in l:he layer >

CA 02252036 2006-04-10
-7-
1 % Cr multiplied by the layer thickness. The chromium index is proportional
to the chromium quantity on the surface (mg/m2).
In accordance with an aspect of the invention, there is provided a
chromium(VI)-free, chromium(III)-containing and substantially
coherent conversion layer on zinc or zinc alloys, wherein even in the absence
of silicate, cerium, aluminum and borate it presents a corrosion protection of
about 100 to 1000 h in the salt spray test according to DIN 50021 SS or
ASTM B 117-73 until first attack according to DIN 50961 Chapter 10;
and wherein said layer is clear, transparent, colorless and presents a
greenish, multi-colored iridescence;
said layer has a layer thickness of about 100 nm to 1000 nm; and
said layer is hard, adheres well and is resistant to wiping; and
said layer has across the conversion layer thickness a chromium
content of greater than 1 % based upon zinc and chromium, the conversion
layer having an average chromium content of more than about 5%;
said layer has a chromium-rich zone greater than 20% chromium, based
upon zinc and chromium in the conversion layer, of more than 15 nm; and
wherein said layer has a chromium index greater than 10, wherein the
chromium index is defined as the average chromium content
(chromium/(chromium+zinc)) in the layer greater than I% Cr, multiplied by the
layer thickness in nm.
In accordance with your another aspect of the invention, there is provided a
method for producing chromium(VI)-free conversion layers affording
at least the corrosion protection of conventional chromium(VI)-containing
yellow chromations, wherein a metallic surface is treated with a solution of
at
least one chromium(III) complex and at least one salt; wherein chromium(III)
is present in a concentration of about 5 to 100 g/1; and a chromium(III)
complex is used having ligand replacement kinetics more rapid' than the
fluoride replacement kinetics in chromium(III)-fluorocomplexes.

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In accordance with a further aspect of the invention, there is provided use of
a
chromium(III)-containing solution, wherein chromium(III) is present in the
form
of at least one complex having ligand replacement kinetics more rapid than
fluoride replacement kinetics in chromium(III)-fluorocomplexes; and
chromium(III) is present in a concentration of about 5 to 100 g/1, as a
passivation bath for surfaces of zinc or zinc alloys, and wherein the solution
contains chromium (III) as a passivating component.
Further advantages and features of the present invention result from the
description of embodiments and from theoretical reflections which are not
binding on the one hand and were, on the other hand, carried out by the
inventors while having knowledge of the present invention, and by referring to
the drawing, wherein:
Fig. 1 shows a comparison of the present invention with blue and yellow
chromations;
Fig. 2 is a scanning electron microscope image (40,000x) showing a
comparison of the present invention ('chromitation") with blue and yellow
chromations;
Fig. 3 is a color photo showing the band width of the iridescent coloring in
accordance with the present invention on zinc surfaces;
Fig. 4 shows coatings of the prior art in accordance with EP 0 034 040;
Figs. 5 to 36 show depth profile analyses of layers according to the invention
and layers resulting from the conventional blue and yellow chromations,
wherein the depth profile analyses were measured by glow-discharge
~30 spectrometry (spectrometer: JY5000RF); and

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Fig. 37 is a table containing the evaluation of the depth profile analyses of
Figs. 5 to 36.
Example 1
The following experiment was carried out:
Small steel parts were bright-zinc coated electrolytically (approx. 15 Nm)
and,
following galvanisation, singly immersed in a boiling (approx. 100°C),
1 fl aro iPrn iS ~nh~tinn cnntaininw

CA 02252036 1998-10-09
_g_
100 g/1 CrCl3 ~ 6 H20 (trivalent chromium salt)
100 g/1 NaN03
15.75 g/1 NaF
26.5 g/1 citric acid ~ 1 aq
which had previously been adjusted to a pFi value of 2.5 with sodium
hydroxide solution. The immersion lime was 30 s. The parts were then
rinsed with water and dried in air flow. On the parts a greenish, str ongly
iridescent layer had formed which later on turned out to be comprised of
zinc/chromium oxide. In the corrosion test in the salt spray cabinet
according to LZIN 50021 SS it was surprisingly found that the chromate
layer formed presented a spectacular corrosion protection until the
appearance of first corrosion products of 1000-h according to DIN
50961 Chapter 10, in particular Chapter 10.2.1.2.
The novel greenish chromate layer had a layer thickness of approx.
800 nm and was produced by a process not involving ary chromium(Vi)
and could be proven to be chromium(VI)-free.
The production method according to Example 1 for the novel, greenish
chromium(VI)-free chromation is not very economical for conventional
plants due to the relatively high temperature of the process solution.
Further theoretical reflections concerning chromium(VI)-free chromate
coating and further trials finally resulted in economical production
conditions.
Theoretical Reflections Concerning Chromium(Vl)-Free Chromation
Chromate coating of zinc takes place by the formation of a so-called
conversion layer on the zinc surface, i.e. the zinc surface chemically
reacts with the chromate coating solution and is converted into a
chromate layer. The formation of conversion layers is a dynamic
process beyond chemical equilibrium. In order to describe the
underlying processes, one must therefore employ chemical kinetics. By
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the especially established kinetic ;yodel it was possible to obtain
starting points in order to optimise the present invention.
Conversion layer formation in a chromium(III)-based chromate coating
solution may be described by means of two reaction equations:
I Elementary zinc passes into solution due to acid attack:
Zn + 2 H+ ~~ Zn2+ + ~2_
II and precipitates on the zinc surface as zinc chromium oxide together
with chromium(III):
Zn2+ + x Cr~l~l) + y H20 ----------~ -ZnCrxOy + 2y H+
The kinetic model must encompass differential equations for the
concentration developments of Zn2, H+, Cr(III) and for the thickness
growth of the ZnCrO layer. In the reaction rate starting points it was
taken into consideration by inserting the term 7~(1+ ~q m~nCrO)2 that
Reaction I is increasingly slowed down by the growing passive layer.
P1 is a measure for tightness of the layer.
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2+-
dc~n - klxcH+~~+ pl~ZnCrO)2 Reactionl
dt
- kZxcZnz+xcCr(II1) + kgxcH+xtanl~p2xlnZnCrO) Re actionIl
+ kTx(c0 ZnZ+ cZn2+) Mass transfer
- -2klxcH+/(1+ plxmZnCrO)Z ReactionI
d1
+ 2yk2xcZn2+xcCr(II1) - 2yk3xcH+xtant(p2xmZnCrO) Reaction Ii
+ kTx(cD~+- cH+) Mass transfer
dcCraI1)
- - xk2 xcZn2+ xcCr(II~ + xk3xcH+xtant(p2 xm ZnCrO) ReactionIl
d1
+ kTx(cOCr(III)'cCr~I~) Mass transfer
d~ZnCrO - k ~ ~, k xc xtan Wn ) ReactionII
di_' z ZnZ+ Cr~I~ 3 H+ ~p2 ZnCrO
The term tanl(~p, m~I,cro represents the indispensable precondition of
reverse reaction II, namely the presence of ZnCrO. The tank function
provides for a smooth transition from 0 to 1, which may be adjusted
with P2. The differential equation system was resolved numerically by
means of a computer. As a result, the layer thickness developments
and the concentration developments over time were obtained. As
starting values for time t0 = 0 there were employed:
c0,Zn2+ - 0
cp,H + - 10-2 moi/I (pH 2) ,
cO,Cr~lll) - 0,5 mol/I
mO,ZnCrO - 0
In Fig. 1 the layer thickness developments for various values of the rate
constant kj are represented. For good corrosion protection, the passive
layer should have maximum possible thickness and at the same time
compactness.
[FiIe:ANM\SU991984.DOCJ Description, 02.10.98
PCT~DE97i00800, Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
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Fig. 38 (originally Illustration 1 ) shows a computer simulation of the
kinetic model for chromate coating of zinc for various rate constants.
The faster the initial dissolution of zinc (rate constant k~ ) is and the
faster the dissolved zinc precipitates with the chnomium(III) (rate
constant k2), the thicker the chromate layer will become. Layer growth
is strongly favored by the presence of zinc already dissolved in the
bath, which fact resulted from simulations with cp,Zn2+ > 0. A lower
pH value favors dissolution of zinc but also brings about increased
redissolution of the layer.
Based on the model, basically two demands may be established for
producing a maximum possible thickness chromate layer. Reaction I and
forward reaction II must take place as rapidly as possible, the reverse
reaction II must remain slow, In this sense, there result the following
starting points:
Reaction; I
a pH optimisation
b Avoiding carrying over of inhibitors from the zinc bath
c Addition of oxidants for accelerating zinc dissolution
d Acceleration of zinc dissolution by formation of galvanic elements
Forward reaction I!
a The rate constant k2 should be as high as possible. Chromium(III)
complexes generally have slow kinetics. By using suitable ligands it
should be possible to accelerate the reaction rate.
f Upon use of further transition metal rations in the chromate coating
solution there also result i.a. higher rate constants than for Cr(III).
Moreover these transition metal rations may act as catalysts in ligand
replacement on chromium(III).
[FiIe:ANM\SU9919B4.DOC] Description, 02.10.98
PCT/Dt97100800, Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
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Reverse reaction II
g Insertion of poorly redissolvable hydroxides, e.g. nickel, cobalt
and/or copper hydroxide.
Serial investigations were carried out. Starting points a and b are known
to the skilled person. Acceleration of zinc dissolution via points c and d
did also result in thick coatings, however yellowish ones having a
chromiurr~/zinc ratio of 1:4 to 1:3, which only afforded low corrosion
protection. It was found that good corrosion protection values may only
be obtained at chromium/zinc ratios above 1:2.
A higher chrornium/zinc ratio at concurrently thicker chromate layers is
obtained when the rate constant k2 (starting point e) is raised, or the
forward reaction 1l is accelerated. After the inventors of the present
application had realised that hot chromium(III) solutions result in
surprising passive layers, there are the following possibilities in
connection with the inventors' theoretical reflections:
- Raising the temperature of the chromate coating solution and/or of the
partial surface
- Raising the chromium(III) concentration in the process solution
- Acceleration of ligand replacement kinetics at the chromium(III).
Herefor one should know that chromium(III) in aqueous solutions is
essentially present in the form of hexagonal complexes generally having
high kinetic stability, and moreover that ligand replacement is the step
determining the rate in forward reaction II. By the selection of suitable
complex ligands, with which the chromium(III) forms kinetically less
stable complexes, k2 is accordingly increased.
Addition of elements having a catalytic effect on ligand replacement
into the chromate coating solution.
[FiIe:ANM\SU991984 DOC] Description. 02 10 98
PCTlDE97i00800, Chramitierung III
SurTec GmbH

CA 02252036 1998-10-09
-13-
In serial investigations chelate ligands (such as di- and tricarboxylic
acids as well as hydroxydi- and hydroxytricarboxylic acids) as such
forming kinetically less stable complexes with chromium(Ili), whereas
the fluoride complexes are kinetically very stable. When using only such
chelate ligands for complexing the chromium(III) and omitting fluoride in
the passivation solution, excellent results were obtained even at a
treatment temperature of only 60°C, as is shown by Examples 2 and 3.
Fxamaie 2
to
Electrolytically bright-zinc coated (15 ~tm) steel parts were immersed in
an aqueous chromate coating solution containing:
50 g/1 CrCl3 ~ 6 H20 (trivalent chromium salt)
100 g/1 NaN03
3 i ,2 g/1 malonic acid
the pH of which had previously been adjusted to 2.0 with sodium
hydroxide solution. The immersion time was 60 s. Following rinsing and
drying there resulted in the salt spray cabinet according to DIN 50021
SS a corrosion protection of 250 h until first attack according to DIN
50961.
Malonic acid is a ligand enabling more rapid ligand replacement kinetics
at the chromium(III) than the fluoride of Example 1. Good corrosion
protection by far exceeding the minimum requirement of DIN 50961 for
Method Group C (yellow chromation) may thus already be achieved at
60°C.
Example 3
Electrolyticaily bright-zinc coated ( 15 ym) steel parts were immersed in
an aqueous chromate coating solution consisting of:
50 g/1 CrCl3 ~ 6 H20 (trivalent chromium salt)
[FiIe:ANM'.SU991984.DOC] Description, 02.10 98
PCT;DE97~00800. Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
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3 g/1 Co(N03)2
100 g/1 NaN03
31,2 g/1 malonic acid
previously adjusted to pki 2.0 with sodium hydroxide solution.
Immersion time was 60 s. Following rinsing and drying there resulted in
the salt spray cabinet according to DIN 50021 SS a corrosion protection
of 350 h until first attack according to DIN 50961.
Cobalt is an element which was capable, in accordance with the model
concept, of catalysing ligand replacement and moreover reducing
reverse reaction II owing to insertion of kinetically stable oxides into the
chromate layer, so that the chromate layer altogether should become
thicker. In this point, as well, the model concept established for the
present invention is verified under practical conditions. Corrosion
protection could once more clearly be enhanced in comparison with
Example 3 by nothing but the addition of cobalt into the chromate
coating solution.
Novel greenish chromate layers on zinc were produced in analogy with
Example 2 at 40, 60, 80 and 100°C. The layer thicknesses of the
respective chromate layers were determined by RBS ( _
Rutherford-Backscattering) testing. In the Table the corresponding
corrosion protection values in hours of salt spray cabinet according to
DIN 50021 SS until first attack according to DIN 50961 Chapter 10 are
additionally listed.
J / C thickness / nm Corr. Prot.
/ h
40 100 50-60
60 260 220-270
80 400 350 450
100 800 800-1200
Depending on the complex ligands used, which is maionate in Examples
2 and 3, it is partly possible to achieve even considerably higher layer
thicknesses and corrosion protection values. By complex ligands
[FiIe:ANM\SU991984.DOC] Description, 02.10.98
PCT/DE97;00800, Chrornitierung III
SurTec GmbH

CA 02252036 1998-10-09
-15-
containing as the complexing functional group nitrogen, phosphorus or
sulfur, (-NR2, -PR2 wherein R independently is an organic, in particular
aliphatic radical and/or H, and/or -SR, wherein R is an organic, in
particular aliphatic radical or H,), it is possible to even produce the
indicated layer properties within limits at room temperatures.
Example 4
Steel parts electroiytically coated with a zinc/iron alloy (0.4-O.6% iron)
were immersed at 60°C in the following aqueous chromate coating
solution:
50g/1 CrCl3. 6 H 20
100g/1 NaN03 '
31.2 g/1 malonic acid
The solution was beforehand adjusted to pH 2.0 with NaOH. Immersion
time was 60s. Following rinsing and drying a transparent, greenish,
slightly grey, strongly iridescent layer was visible on the zinc/iron. in the
salt spray cabinet in accordance with the above specified DiN and
ASTM standards there resulted a corrosion protection of 360 h until first
attack according tc DIN 50961. .
Example 5
Steel parts electrolytically coated with a zinc/nickel alloy (8-13% nickel)
were immersed at 60°C into the following aqueous chromate coating
solution:
50g/1 CrCl3. 6 H20
100g/1 NaN03
31 .2 g/1 malonic acid
The solution was beforehand adjusted to pH 2.0 with NaOH. Immersion
time was 60s. Following rinsing and drying a transparent, greenish,
[File:A~IM1SU991984.DOC] Description, 02.10.98
PCT/DE97/00800, Chromitierung III
SurTec GmbH

CA 02252036 2002-O1-03
-16-
dark-grey, strongly iridescent layer was visible on the zinc/nickel. In the
spray cabinet in accordance with the above specified D1N and AS~fM
standards there resulted a corrosion protection of 504 h until first attactc
according to DIN 509fi1.
The novel greenish chromium(VI)-free chromate layer accordingly
depending on the production temperature has a thickness of between 100
and 1000 nm, has a weakly green inherent color and a red-green
iridescent coloring. The chromate coating solution consists of trivalent
chromates, moreover of conducting salts and mineral acids. Application of
the chromate coating solutions is generally performed at temperatures
above 40°C. The corrosion protection of unmarred greenish
chromium(VI)-free chromate coatings depending on the production
temperature amounts to 100-1200 h in the salt spray cabinet according to
DIN 50021 SS until the first appearance of corrosion products. Thus the
novel chrornation satisfies the minimum rettuirements to corrosi,~n
protection for Method Groups C and D according to pIN 50961 (Chapter
10, Table 3), i.e. without chromium(Vl) either in production or in the
product_
By the present invention it is for the first time made possible tc provide
chromiurnlVl)-free conversion layers or passive layers on the basis of
chromium(lil), which do, however, furnish the corrosion proteccion of
yellow chrornations customary in the prior art - i.e., of chromium(VI)-
containing passive layers.
This is a singular novelty in the entire galvanisation industry.
Hitherto on a chromium(III) basis only clear to blue layers, referred to as
"flue passivation" in technical circles, were known which are variously
applied practically.

CA 02252036 1998-10-09
-17-
Moreover yellowish-transparent layers with an addition of cerium are
known which are, however, not used practically owing to the very
costly cerium addition and their poor corrosion protection properties.
Moreover powdery-greenish layers are known for which the applicant -
one of the leading enterprises in the field of surface technology - is not
aware of any practical applications.
Even the difference in terms of color of the conversion layers of the
present invention is conspicuous in Fig. 1, wherein three treatment
methods were performed on zinc-plated screws.
The left-hand pile of screws in accordance with the illustration of Fig. 1
was subjected to a classical blue chromafion - as set forth on page 2 of
the description under No. 1.
The right-hand pile of screws on the photograph according to Fig. 1 was
subjected to a conventional yellow chromation in accordance with page
2, No 2 of the present description.
The center pile of screws shows the result of passivation of the screws
by means of the method in accordance with the invention.
This is consequently a greenish-iridescent, transparent conversion layer,
or passive layer.
Moreover the colors represented in Fig. 1 are the true colors, which can
be seen from the fact that a color plate and moreover a grey wedge was
jointly photographed for the purpose of neutral color representation.
As can be seen from the white test field "White" and from the
corresponding field having the density ".00" from the grey wedge, both
test fields are pure white, making evident the neutral filtering and the
resulting realistic color representation.
[FiIe:ANM\SU9919B4.DOC) Description, 02.10.98
PCTlDE97/00800, Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
-18-
In Fig. 2 scanning electron microscope (SEM) images of the conversion
layers of a yellow chromation and of a blue chromation in accordance
with the prior art are shown in comparison with the "chromitation" of
the present invention.
The layer samples are derived from the correspondingly passivated zinc-
plated iron screws shown in Fig. 2, lower half.
The samples treated in accordance with the invention (by
"chromitation") presented a chromium(VI)-free conversion layer having a
thickness of approx. 300 nm. In the photographs of Fig. 2 it should be
considered that the layers were photographed in a viewing angle of
approx. 40°, resulting in foreshortening by approx. cos (40°) =
0.77.
Based on the SEM images of the chromitation layer of the invention it
therefore results that conversion layer thicknesses like in yellow
chromation are obtained, however with the difference that the
conversion layer of the invention does not contain any toxic
chromium(VI).
The color phonograph of Fig. 3 r,~oreover shows the bandwidth of the
iridescent coloring of the passive layer according to the invention under
practical conditions.
It can already be seen in the photographs of Figs. 1 and 3 that the
passive layer according to the invention does not contain any
chromium(VI) ions as it lacks the typically yellow color (cf. right-hand
pile of screws of the color photograph of Enclosure 1 ).
Objects according to the photograph of Figs. 1 and 3 as well as zinc-
plated steel sheets passivated by the method of the invention were
tested in the salt spray cabinet according to DIN50021 SS or ASTM B
1 17-73, respectively, until the occurrence of first corrosion products
according to DiN50961 Chapter 10. Herein it was surprisingly found that
the passive layers of the present invention, and thus the objects
passivated by the present met'~od, fulfilled the corrosion protection of
[Fiie:ANM~SU991984.DOC] Description, 02.10.98
PCT/DE971G0800, Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
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chromium(VI) passivations, i.e. yellow chromations, although not
containing any chromium(VI).
It is worth mentioning that a typical yellow chromation of the prior art
affords resistance for approx. 100 hours of exposure to saltwater in
accordance with the above specified DIN or ASTN standard, whereas
even the tenfold corrosion protection was achieved by the passive layers
of the present invention.
The layers of the present invention as well as the methods for producing
this layer, or the method for passivation of metal surfaces, thus satisfy
the long-standing demand in this technical field for conversion layers
doing without any toxic and carcinogenic chromium(V1) compounds
while nevertheless even presenting and generally even excelling the
corrosion protection of yellow chromations.
EP 00 34 040 A1 does describe a multitude of layers, wherein the
colorations of the larger group thereof (produced under the standard
conditions set forth by Barnes/Ward) are not specified, however which
are referred to as clear. Two examples, i.e. Nos. 16 and 17, describe a
greenish, borate-containing layer referred to as cloudy-dull to non-
transparent.
Example 14 describes a layer affording a corrosion protection of no more
than 4 hours.
In Example 15 of EP 00 34 040, an aluminum-containing layer is
described which attains a corrosion protection of 100 hours. This is
achieved - in comparison with the remaining examples - merely by the
corrosion protection additive aluminum which is lacking in the present
invention. Aluminum-free layers of identical or similar baths do,
however, only present poor corrosion protection. The layer according to
the invention offers significantly higher corrosion protection, namely up
to 1000 h, even without this addition.
(FiIe:ANM\SU9919B4.DOCJ Description, 0210.98
PCTIDE97i00800, Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
-20-
Examples 16 and 17 describe layers affording a corrosion protection of
300 and 200 hours in the salt spray test and thus in the range claimed
by the applicant. Description page 19, line 7 sets forth that layers of
more than 1000 nm are required for good corrosion protection. It is thus
understandable that these layers, without exception moreover produced
from solutions containing boric acid, are described to be cloudy and
rather non-transparent (page 14, line 10). The enhanced corrosion
protection, in accordance with page 15, lines 1-5, is due to the insertion
of borate-containing species.
The layer according to the invention, on the other hand, also offers high
(and even higher) corrosion protection without this addition.
There is, however, another difference that is relevant in terms of patent
law as well as in practical application: namely, the layers described in
Examples 16 and 17 of EP 00 34 040 are soft and come off when wiped
and consequently require some sort of harder;ing process as an
aftertreatment (page 17, lines 12-21 ).
The present layers according to the invention are hard and resistant to
wiping even without a hardening process. Corrosion protection layers
which come off when wiped and which do not adhere to the substrate
are useless for practical application.
In Fig. 4, a photograph is shown as a comparison example. This
photograph represents the result of comparison tests carried out by the
applicant in comparison with EP 00 34 040. In particular the applicant
reproduced the Examples 16 and 17 given in this prior art. Herein steel
sheets were immersed into the solutions described in Examples 16 and
17 of EP 00 34 040 and the respective treatment times were observed.
Fig. 4 shows the layers on the substrate surfaces obtained in
accordance with the prior art, namely from the top to the bottom the
first and second sheets successively treated by immersion.
The photograph of Fig. 4 shows from the left to the right in the top half
of the illustration a cloth whereby the layer produced in accordance with
~FiIe:ANM1SU991984.DOC] Description, 02.10.98
PCTIDE97100800, Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
-21 -
Example 16 - prior art - was wiped, a zinc-plated steel sheet treated in
accordance with Example 16, beside it a zinc-plated steel sheet treated
in accordance with Example 17 - prior art - and on the extreme right also
a cloth whereby the layer of Example 17 was wiped. In the second line
on the left side - - beside the indication of Example 16 and beside it to
the right ( beside the indication of Example 17) a respective zinc-plated
steel sheet coated in accordance with the prior art is shown.
What is visible is a milky, white-greenish powdery coating which already
comes off when wiped with a soft cloth even without application of
particular pressure (see Fig. 4, top half of illustration). The prior art
itself
suggests that this layer is not a compact oxide zinc-/chromium
conversion layer firmly adhering to the substrate sheet but a loosely
overlying coating substantially consisting' of chromium hydroxide. The
pH for this coating must be so high that the precipitation limit for
chromium hydroxides is already exceeded (page 26, line 12 of EP 0034
040). Precipitation of chromium hydroxide is kinetically inihibited and is
favored by immersion of a more or less rough surface. The fact that the
layer formation mechanism has to be a different one from the other
examples may also be seen from the circumstance that with (Example
16 prior art) or without (Example 17) complexing agents more or less
the same result was achieved. In practical reproduction of Examples 16
and 17 of the prior art it was moreover found that the layer became
thicker, softer and more powdery with an increasing number of metal
sheets coated in the solution. In addition, more and more chromium
hydroxide precipitated, whereby the useful life of such a coating solution
is limited to a few hours. The layer according to the invention, on the
other hand, is produced only from suitable "rapid" complexes and
furthermore in a distinctly acidic pH range. The solution is stable over
months, presumably even years.
The measurements underlying Figs. 5 to 36 were performed with a
glow-discharge spectrometer.
The element F and die anions could not be analysed by this method. O,
H, CI and K could not be quantified.
[FiIe:ANM\SU9919B4.DOC] Description, 0210.98
PCTIDE97I00800, Chromitierung III
SurTec GmbH

CA 02252036 1998-10-09
-22-
The fiollowing Table shows the concentration ranges for which
calibration is valid:
Element Concentration min. Concentration max.
in in
C 0.0067 3.48
S 0.0055 0.168
Cr 0.0001 99.99
N i 0.0001 99.99
Co 0.0001 7.00
Zn 0.0001 99.99
Na ' 0.0001 0.0068
N 0.0001 .. 6.90
B 0.0001 0.040
Fe 0.0005 99.91
Sample allocation in Figs. 5 to 36 results from the following Table:
Sample Coating Conditions Measurement
No. point
1 Chromitation 60C, 1 min, pH 2 A
on
Zn (invention)
B
2 60 C, 2min, pH A
2
B
3 60 C, 1 min, pH A
2.5
4 60 C, 1.5min, pH A
2.5
5 60 C, 2min, pH A
2.5
6 100C, 1min, pH 2 A
B
C
D
[F~Ie:ANM1SU991984.DOC] Description, 02.10.98
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SurTec GmbH

CA 02252036 1998-10-09
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7 Chromitation 60C, 1 min, pH 2 A
on
Zn/Fe
B
8 Blue chromation20C, 30s, pH 1.8 A
on Zn
9 Yellow chromation20C, 45s, pH 1.8 A
on Zn
B
Fig. 37 shows a Table containing the evaluations of the depth profile
measurements, which indicates that all of the tchromitation) layers of
the invention have thicknesses exceeding.100 nm.
(FiIe:ANM\SU991984.DOC] Description, 02.10.98
PCT/DE97I00800, Chromitierung III
SurTec GmbH