Field of Technology
[0001] The present invention relates to a processing solution for forming a hexavalent chromium
free, black conversion film on zinc or zinc alloy plating layers, and a method for
forming the hexavalent chromium free, black conversion film on zinc or zinc alloy
plating layers.
Technical Background
[0002] As methods for rust preventing the surface of a metal, there has been known a zinc
or zinc alloy-plating method. However, it is not possible to ensure sufficient corrosion
resistance of the metal by such plating alone. For this reason, there has widely been
adopted, in this industrial field, the treatment with chromic acid containing hexavalent
chromium or the so-called chromate treatment after the plating. Nevertheless, it has
recently been pointed out that the hexavalent chromium may adversely affect the human
body and the environment and there has correspondingly been such a strong and active
trend that the use of hexavalent chromium should be controlled.
[0003] As one of the substituent techniques therefor, the formation of a corrosion resistant
conversion film, in which trivalent chromium is used, has been known. For instance,
Japanese Examined Patent Publication (hereunder referred to as "J.P. KOKOKU") No.
Sho 63-015991 discloses a method, which comprises the step of treating the surface
of a metal with a bath containing a mixture of trivalent chromium and a fluoride,
an organic acid, an inorganic acid and/or a metal salt such as cobalt sulfate. However,
a fluoride is used in this plating bath and therefore, a problem of environmental
pollution would arise. In addition, J.P. KOKOKU No. Hei 03-010714 discloses a method,
which makes use of a plating bath comprising a mixture of trivalent chromium and an
oxidizing agent, an organic acid, an inorganic acid and/or a metal salt such as a
cerium salt. However, this method makes use of an oxidizing agent and cerium and therefore,
the trivalent chromium may possibly be oxidized into hexavalent chromium, during the
processing and/or the storage of the bath.
[0004] Japanese Un-Examined Patent Publication (hereunder referred to as "J.P. KOKAI") No.
Hei 10-183364 discloses a method which comprises the step of treating the surface
of a metal with a bath containing a phosphoric acid, a salt of metal such as Mo, Cr
3+ and Ti, and an oxidizing agent to provide the surface with a hexavalent chromium
free, corrosion resistant conversion film. This method uses a large quantity of the
oxidizing agent and therefore, the trivalent chromium may possibly be oxidized into
a hexavalent chromium, during the processing and/or the storage of the bath.
[0005] J.P. KOKAI No. 2000-54157 discloses a method which comprises the step of chemical
conversion treating the surface of a metal with a bath containing phosphorus, a metal
such as Mo, and trivalent chromium, but no fluoride. However, as a result of our experiments
to confirm the effects produced by the method, acceptable corrosion resistance could
not be obtained.
[0006] Furthermore, J.P. KOKAI No. 2000-509434 discloses a method, which comprises the step
of treating the surface of a metal using a plating bath comprising 5 to 100 g/L of
trivalent chromium and nitrate residues, an organic acid and/or a metal salt such
as a cobalt salt. This method uses, for instance, trivalent chromium in a high concentration
and the plating operation is carried out at a high temperature. Therefore, this method
is advantageous in that it can form a thick film and ensure good corrosion resistance.
However, the method suffers from a problem in that it is difficult to stably form
a dense film and that the method cannot ensure the stable corrosion resistance of
the resulting film. Moreover, the processing bath contains trivalent chromium in a
high concentration and also contains a large amount of an organic acid. This makes
the post-treatment of the waste water difficult and results in the formation of a
vast quantity of sludge after the processing. Although one can recognize that it is
advantageous to use a processing solution free of any hexavalent chromium for ensuring
the environmental protection, the method suffers from a serious problem in that it
may give a new burden to the environment such that the method generates a vast quantity
of waste.
[0007] In addition, there could be obtained only films having colorless or an interference
color appearances. In this connection, with regard to a black conversion film containing
a trivalent chromium on zinc-nickel alloy plating layers (containing 8% or more of
nickel in the layers) and zinc-iron alloy plating layers, U.S. Patent No. 5415702
discloses a method which comprises the step of treating the surface of a metal with
an acidic bath containing a phosphate compound and trivalent chromium. Also, with
regard to a chemical conversion interference color film containing a trivalent chromium
on zinc-nickel alloy plating layers (containing 8% or more of nickel in the layers),
U.S. Patent No. 5407749 discloses a method, which comprises the step of treating the
surface of a metal with an acidic bath similar to that disclosed in U.S. Patent No.
5415702 containing a phosphorus compound, trivalent chromium and oxy-halogen acid
ions.
[0008] However, in many cases, a Ni eutectoid rate in zinc-nickel alloy plating layers actually
produced is less than 8%, and thus it is difficult to obtain a black feature from
a practical standpoint. Furthermore, the black conversion film on zinc-iron alloy
plating layers does not have enough corrosion resistivity.
[0009] Moreover, there have been proposed a method for processing the surface of a metal
with a bath containing trivalent chromium in a low concentration and an organic acid
and a metal salt such as a nickel salt (U.S. Patent No. 4,578,122) and a processing
method, which makes use of a bath containing trivalent chromium in a low concentration
and an organic acid (U.S. Patent No. 5,368,655). However, these methods never ensure
sufficient corrosion resistance of the resulting film as compared with the conventional
hexavalent chromate treatment.
[0010] It is thus an object of the present invention to provide a processing solution used
for forming a hexavalent chromium free, black conversion film, which is applied onto
the surface of zinc or zinc alloy plating layers, and which has corrosion resistance
identical to or higher than that achieved by the conventional hexavalent chromium-containing
conversion film.
[0011] Another object of the present invention is to provide a method for forming such a
hexavalent chromium free, black conversion film.
Summary of The Invention
[0012] The present invention has been completed on the basis of such finding that the foregoing
problems associated with the conventional techniques can effectively be solved by
depositing a zinc or zinc alloy plating layer(s) on a substrate and then subjecting
the plating layer to a trivalent chromate treatment using a processing solution having
a specific composition, i.e. containing a low concentration of nitrate ions, cobalt
ions and nickel ions.
[0013] According to an aspect of the present invention, there is provided a processing solution
for forming a hexavalent chromium free, black conversion film on zinc or zinc alloy
plating layers and the processing solution comprises:
nitrate ions and trivalent chromium in a mole ratio (NO3-/Cr3+) of less than 0.5/1, wherein the trivalent chromium is present in the form of a water-soluble
complex with a chelating agent; and
cobalt ions and / or nickel ions, wherein cobalt ions and / or nickel ions are stably
present in the processing solution without causing any precipitation by forming a
hardly soluble metal salt with the chelating agent; wherein the solution reacts with
zinc when it is brought into contact with the zinc or zinc alloy plating to form a
hexavalent chromium free, black conversion film containing zinc, chromium, cobalt
and / or nickel, and the chelating agent on the plating.
[0014] According to a further aspect of the present invention, there is provided a method
for forming a hexavalent chromium free, black conversion film, which comprises the
step of bringing zinc or zinc alloy plating into contact with the foregoing processing
solution.
[0015] The substrates used in the present invention may be a variety of metals such as iron,
nickel and copper, alloys thereof and metals or alloys such as aluminum, which have
been subjected to zincate treatment and the substrate may have a variety of shapes
such as plate-like, rectangular prism-like, column-like, cylindrical and spherical
shapes.
[0016] The foregoing substrate is plated with zinc or a zinc alloy according to the usual
method. The zinc-plating layer may be deposited on the substrate using either, for
instance, acidic baths such as a sulfuric acid bath, an ammonium chloride bath or
a potassium chloride bath, and alkaline baths such as an alkaline non-cyanide bath
and an alkaline cyanide bath, but an alkaline non-cyanide bath (NZ-98 available from
Dipsol Chemicals Co., Ltd.) is preferable.
[0017] In addition, examples of zinc alloy plating are zinc-iron alloy plating, zinc-nickel
alloy plating having a rate of nickel-co-deposition ranging from 5 to 20% by mass,
zinc-cobalt alloy plating and tin-zinc alloy plating. The thickness of the zinc or
zinc alloy plating to be deposited on the substrate may arbitrarily be selected, but
it is desirably not less than 1 µm and preferably 5 to 25 µm.
[0018] In the present invention, after the zinc or zinc alloy plating is deposited on a
substrate according to the foregoing method, if desired, the plated substrate is water-rinsed
or subjected to nitrate activation processing after being water-rinsed, and then brought
into contact with a processing solution for forming a hexavalent chromium free, black
conversion film according to the present invention. For instance, the zinc or zinc
alloy plating subjected to a dipping treatment using this processing solution.
[0019] In the foregoing processing solution of the present invention, the source of the
trivalent chromium may be any chromium compound containing trivalent chromium, but
preferred examples thereof usable herein are trivalent chromium salts such as chromium
chloride, chromium sulfate, chromium nitrate, chromium phosphate and chromium acetate
or it is also possible to reduce hexavalent chromium such as chromic acid or dichromic
acid into trivalent chromium using a reducing agent. Particularly preferable trivalent
chromium source is chromium phosphate (Cr(H
nPO
4)
(3/(3-n))). The foregoing sources of trivalent chromium may be used alone or in any combination
of at least two of them. The trivalent chromium concentration in the processing solution
is not limited. It is preferably as low as possible from the viewpoint of the easiness
of the wastewater treatment, but it is preferably 0.5 to 10 g/L and most preferably
0.8 to 5 g/L, while taking into account the corrosion resistance. In the present invention,
the use of trivalent chromium in such a low concentration falling within the range
specified above is also quite advantageous from the viewpoint of the wastewater treatment
and the processing cost.
[0020] The processing solution of the present invention comprises nitrate ions in a mole
ratio of nitrate ions to a trivalent chromium (NO
3-/Cr
3+) of less than 0.5/1, and preferably in a range of from 0.02/1 to 0.25/1. The nitrate
ion concentration in the processing solution preferably ranges from 0.1 to 1 g/L.
Examples of the nitrate ion source include nitric acid or a salt thereof.
[0021] Examples of the chelating agent used in the processing solution of the present invention
include a hydroxycarboxylic acid such as tartaric acid and malic acid, a monocarboxylic
acid, a polyvalent carboxylic acid such as a dicarboxylic acid such as oxalic acid,
malonic acid, succinic acid, citric acid and adipic acid, or a tricarboxylic acid,
aminocarboxylic acid such as glysinic acid. Moreover, the chelating agent may be used
alone or in any combination of at least two of these acids or salts thereof (e.g.
salt of sodium, potassium, ammonium or the like). The chelating agent concentration
in the processing solution preferably ranges from 1 to 40 g/L, and more preferably
5 to 35 g/L in total.
[0022] The molar ratio of the chelating agent to the trivalent chromium (the chelating agent
/ Cr
3+) present in the processing solution of the present invention preferably ranges from
0.2 / 1 to 4/1 and more preferably 1/1 to 4 /1.
[0023] The processing solution of the present invention comprises cobalt ions and / or nickel
ions. As the sources of the cobalt ions and / or nickel ions, there may be used any
metallic compounds containing either of these metals. One of these metallic compounds
or any combination of at least two of them may be used, but one or more of each of
metal salt, cobalt and nickel, is preferably used. The metallic salt concentration
in the processing solution preferably ranges from 0.1 to 2 g/L, and more preferably
0.5 to 1.5 g/L in total.
[0024] In addition, the processing solution of the present invention may comprise monovalent
to hexavalent metal ions, for example silicon, iron, titan, zirconium, tungsten, vanadium,
molybdenum, strontium, niobium, tantalum, manganese, calcium, barium, magnesium, aluminum
and the like. Said metal ions may be added alone or in any combination of at least
two of them to the processing solution of the present invention. Moreover, as the
sources of said metal ions, there may be used any metallic compounds containing either
of these metals, but nitrate, sulfate or chloride are preferably used. These metallic
compounds may be used alone or in any combination of at least two of them. The concentration
in the processing solution preferably ranges from 0.05 to 3.0 g/L, and more preferably
0.1 to 2.0 g/L in total.
[0025] The trivalent chromium and a chelating agent such as oxalic acid should be present
in the processing solution in the form of a stable water-soluble complex formed therebetween,
which is supposed to have a structure represented by the following general formula,
while the metal ions such as cobalt ions should stably exist in the solution without
causing any precipitation by forming a hardly soluble metal salt with the chelating
agent.
wherein A represents a chelating agent, and n represents a valence of the chelating
agent.
[0026] For instance, if the foregoing stable complex is not formed in the solution or excess
chelating agents such as oxalic acid ions are present in the processing solution,
metal ions such as cobalt ions react with chelating agents present in the processing
solution in its free state to thus form precipitates of a hardly soluble metal salt.
As a result, the processing solution cannot form any chemical conversion film (coating)
having excellent corrosion resistance.
[0027] In order to obtain an excellent black film, the molar ratio of the chelating agent
to the trivalent chromium (m/l) in the processing solution of the present invention
preferably ranges from 0.2/1 to 4/1.
[0028] In addition, an even more excellent black film can be obtained by adding phosphate
ions to the foregoing processing solution. The sources of phosphate ions include oxyacid
of phosphorus such as phosphoric acid or phosphorous acid and a salt thereof. One
of these sources or any combination of at least two of them may be used. The concentration
of phosphate ions in the processing solution preferably ranges from 0.1 to 50 g/L,
and more preferably 5 to 25 g/L.
[0029] In addition, sulfate ions, halogen ions and/or borate ions may be added to the foregoing
processing solution. Examples of the sources of these ions include sulfuric acid,
hydrochloric acid, boric acid and an inorganic salt thereof and the like. The concentration
of ions of these inorganic acids in the processing solution preferably ranges from
1 to 50 g/L, and more preferably 1 to 20 g/L in total.
[0030] The pH value of the processing solution of the present invention is preferably adjusted
to the range of 0.5 to 4 and more preferably 2 to 3. In this respect, it is possible
to use ions of the foregoing inorganic acids or an alkaline agent such as an alkali
hydroxide or aqueous ammonia in order to adjust the pH value thereof to the range
specified above.
[0031] The rest (balance) of the processing solution used in the present invention, except
for the foregoing components, is water.
[0032] If zinc or zinc alloy plating is brought into contact with the processing solution
according to the present invention, the components of the solution react with zinc
to thus form a hexavalent chromium free, black conversion film on the zinc or zinc
alloy plating, as is presumed below.
[0033] As the method for bringing the zinc or zinc alloy plating into contact with the foregoing
processing solution according to the present invention, it is usual to immerse an
article plated with zinc or zinc alloy in the foregoing processing solution. For instance,
such an article is immersed in the solution maintained at a temperature ranging from
10 to 80°C and more preferably 40 to 60°C for preferably 5 to 600 seconds and more
preferably 30 to 120 seconds.
[0034] In this connection, the subject to be treated may be immersed in a dilute nitric
acid solution in order to activate the surface of the zinc or zinc alloy plating layers,
before it is subjected to the trivalent chromate treatment.
[0035] The conditions and processing operations other than those described above may be
determined or selected in accordance with the conventional hexavalent chromate processing.
[0036] In addition, further improvement of the corrosion resistance of the film can be achieved
by subjecting the trivalent chromate treated film to aging treatment (heat treatment).
In cases where it is applied to zinc-nickel alloy plating layers, particularly good
effects are obtained. The aging treatment is conducted at 100 to 250°C for 10 to 300
minutes, preferably at 150 to 200°C for 10 to 300 minutes, and more preferably at
200°C for 4 hours.
[0037] Moreover, a topcoat film may be applied onto the hexavalent chromium free, black
conversion film and this would permit the further improvement of the corrosion resistance
of the film. In other words, this is a quite effective means for imparting more excellent
corrosion resistance to the film. For instance, the zinc or zinc alloy plating is
first subjected to the foregoing trivalent chromate treatment, followed by washing
the plating with water, subjecting the plating to immersion or electrolyzation in
a topcoating solution and then drying the processed article. Alternatively, the article
is subjected to immersion or electrolyzation in a topcoating solution after the trivalent
chromate treatment and the subsequent drying treatment, and then dried. The term "topcoat"
effectively used herein means not only an inorganic film of, for instance, a silicate
or a phosphoric acid salt, but also an organic film of, for instance, polyethylene,
polyvinyl chloride, polystyrene, polypropylene, methacrylic resin, polycarbonate,
polyamide, polyacetal, fluorine plastic, urea resin, phenolic resin, unsaturated polyester
resin, polyurethane, alkyd resin, epoxy resin or melamine resin.
[0038] The topcoating liquids for forming such an topcoat film usable herein may be, for
instance, DIPCOAT W or CC445 available from Dipsol Chemicals Co., Ltd.. The thickness
of the topcoat film may arbitrarily be selected, but it desirably ranges from 0.1
to 30 µm.
Reaction Mechanism of Film-Formation
[0039] The reaction mechanism of the hexavalent chromium free, black conversion film-formation
according to the present invention can be supposed to be as follows:
(i) A dissolution reaction of Zn, Fe, Ni or the like from a plating film by the action
of hydrogen ions and an oxidizing agent such as nitric acid, a supply of metal ions
such as zinc to the plating film and re-dissolution reaction of deposits;
(ii) Consumption of hydrogen ions and an increase of the pH value at the interface
to be plated subsequent to the dissolution reaction:
(iii) The reduction of the stability of a chelating agent, the formation and deposition
of Cr hydroxide, and the generation and supply of excess oxalic acid, due to the increase
in the pH value:
(iv) Formation and deposition of a hardly soluble metal salt through the reaction
of the excess chelating agent with metal ions in the solution. For example, in a case
where a chelating agent is an oxalic acid and a metal is cobalt:
(v) Formation and deposition of a hardly soluble metal salt through the reaction of
the anions such as phosphoric acid in the solution with metal ions such as zinc, Fe
and Ni dissolved from the plating film into the solution or Ni, Co and / or Fe added
into the solution, and the deposition of another hardly soluble substance produced
when the plating film is dissolved onto the zinc or zinc alloy plating film.
For example, in a case where phosphate ions are added into the solution:
M + n = 3, X, Y: metal ions such as zinc, iron, nickel and / or manganese.
(vi) These reactions are repeated to thus cause the growth of the chemical conversion
film.
[0040] In this connection, it would appear that the black conversion film is a composite
film of (iii), (iv) and (v).
[0041] In the concentration of nitrate ions being within a suitable range (0.1g/L to 1 g/L),
it would appear that the re-dissolution reaction of the hardly soluble metal salt
in (v) is inhibited and the hardly soluble metal salt in (v) is incorporated into
the chemical conversion film, so that the film exhibits a black feature.
Description of the Preferred Embodiments
Examples 1 to 6
[0042] A steel plate, which had been plated with zincates using a NZ-98 solution available
from Dipsol Chemicals Co., Ltd. to form a Zn plating layer having a thickness of 8
µm, was immersed in a trivalent chromate-containing processing solution having a composition
as shown in the following Table 1.
Table 1
Ex. No. |
1 |
2 |
3 |
4 |
5 |
6 |
Cr3+ (g/L) |
4.5 |
4.5 |
4.5 |
4.5 |
4.5 |
2 |
NO3- (g/L) |
0.2 |
0.4 |
0.1 |
0.4 |
0.6 |
0.4 |
Mole ratio of NO3- /Cr3+ |
0.04/1 |
0.07/1 |
0.02/1 |
0.07/1 |
0.11/1 |
0.17/1 |
PO43-(g/L) |
12 |
12 |
0 |
15 |
12 |
12 |
SO42- (g/L) |
15 |
0 |
0 |
2 |
0 |
2 |
Cl- (g/L) |
10 |
10 |
10 |
0 |
15 |
0 |
Oxalic acid (g/L) |
15 |
15 |
7 |
0 |
0 |
0 |
Malonic acid (g/L) |
0 |
0 |
7 |
15 |
12 |
12 |
Succinic acid (g/L) |
0 |
10 |
20 |
0 |
0 |
0 |
Adipic acid (g/L) |
0 |
0 |
0 |
20 |
0 |
0 |
Molar ratio of the chelating agent / Cr3+ |
2.0/1 |
3.0/1 |
3.7/1 |
3.4/1 |
1.4/1 |
1.4/1 |
Co (g/L) |
1 |
1.5 |
0.5 |
1 |
1 |
1 |
Ni (g/L) |
0.1 |
0 |
1.0 |
0 |
0.3 |
0.5 |
Other metal salt |
Si |
Si |
Ti |
Si |
Si |
Si |
pH of Processing Soln. |
2.3 |
2.3 |
2.3 |
2.4 |
2.6 |
2.5 |
Processing Temp. (°C) |
50 |
50 |
60 |
50 |
40 |
30 |
Processing time (sec.) |
30 |
60 |
120 |
60 |
60 |
60 |
[0043] In Table 1, Cr
3+ sources used were CrCl
3 (in Examples 1, 2, 3 and 5), CrPO
4 (in Examples 4 and 6) and Cr(NO
3)
3 (in Example 5). The concentrations of NO
3- were adjusted by Cr(NO
3)
3 (in Example 5) or by adding HNO
3 (in Examples 1, 2 and 3) or NaNO
3 (in Examples 4 and 6). Further, the SO
42- source used was Na
2SO
4 and the PO
43- source used was NaH
2PO
4. The balance of each processing solution was water. The metallic salts used such
as Co and Ni were its sulfate (in Examples 1, 4 and 6) and chloride (in Examples 2,
3 and 5). The Si source used was colloidal silica and the Ti source used was titanium
trichloride. The concentration of metal ions other than Co and Ni was 1g/L. The pH
value of each solution was adjusted using NaOH.
Examples 7 to 10
[0044] A steel plate, which had been plated with alkaline zinc-nickel alloy (Ni%: 5 to 15%)
or zinc-iron alloy (Fe%: 0.3 to 2.0%) in a thickness of 8 µm, was immersed in a trivalent
chromate-containing processing solution having a composition as shown in the following
Table 2.
Table 2
Ex. No. |
7 |
8 |
9 |
10 |
Cr3+ (g/L) |
4.5 |
4.5 |
4.5 |
4.5 |
NO3-(g/L) |
0.6 |
0.4 |
0.2 |
0.4 |
Mole ratio of NO3-/Cr3+ |
0.11/1 |
0.07/1 |
0.04/1 |
0.07/1 |
PO43- (g/L) |
12 |
12 |
12 |
15 |
SO42- (g/L) |
10 |
0 |
15 |
2 |
Cl-(g/L) |
0 |
10 |
0 |
0 |
Oxalic acid (g/L) |
15 |
7 |
15 |
15 |
Malonic acid (g/L) |
0 |
7 |
0 |
0 |
Succinic acid (g/L) |
10 |
0 |
0 |
0 |
Adipic acid (g/L) |
0 |
0 |
0 |
20 |
Molar ratio of the chelating agent / Cr3+ |
3.0/1 |
1.7/1 |
2.0/1 |
3.7/1 |
Co (g/L) |
1 |
1 |
1 |
1 |
Ni (g/L) |
0.3 |
0 |
0.1 |
0 |
Other metal salt |
Si |
Si |
Si |
Si |
pH of Processing Soln. |
2.6 |
2.0 |
2.3 |
2.5 |
Processing Temp. (°C) |
35 |
50 |
50 |
40 |
Processing time (sec.) |
60 |
60 |
60 |
60 |
Plating Eutectoid rate (%) |
Zn-Ni 6.5 |
Zn-Ni 15 |
Zn-Fe 0.5 |
Zn-Fe 2.0 |
[0045] In Table 2, the Cr
3+ sources used were CrCl
3 (in Example 8) and CrPO
4 (in Examples 7, 9 and 10).
[0046] The concentrations of NO
3- were adjusted by adding HNO
3 (in Example 8) or_NaNO
3 (in Examples 7, 9 and 10). Further, the SO
42- source used was Na
2SO
4 and the PO
43- source used was NaH
2PO
4. The balance of each processing solution was water. The metal salts used such as
Co and Ni were its sulfate (in Examples 7 and 9) and chloride (in Example 8). The
Si source used was colloidal silica and the concentration thereof was 1g/L. The pH
value of each solution was adjusted using NaOH.
Examples 11 to 14
[0047] After the trivalent chromate treatment in Examples 1, 8 and 9, the steel plate was
subjected to a topcoating treatment. The conditions for the topcoating treatment used
herein are summarized in the following Table 3.
Table 3
Ex. No. |
11 |
12 |
13 |
14 |
Trivalent chromate treatment |
Example 1 |
Example 8 |
Example 9 |
Example 9 |
Kind of Topcoat |
Silicate type inorganic film |
Silicate type inorganic film |
Polyurethane type organic film |
Methacrylic resin type organic film |
Concn. Of Processing Soln. |
200 mL/L |
200 mL/L |
100 mL/L |
Stock solution was used as such |
Processing Conditions |
45°C - 45 sec |
45°C - 45 sec |
25°C - 60 sec |
25°C - 60 sec |
Name and Origin of Reagent |
CC-445 available from Dipsol Chemicals Co., Ltd. |
CC-445 available from Dipsol Chemicals Co., Ltd. |
SUPERFLEX R3000 available from Dai-ichi Kogyo Seiyaku Co., Ltd. |
DIPCOAT W available from Dipsol Chemicals Co., Ltd. |
Comparative Example 1
[0048] A steel plate, which had been plated with zinc in a thickness of 8 µm, as a comparative
example, was subjected to a hexavalent chromate treatment. The hexavalent chromate
bath used herein was ZB-535A (200 mL/L) and ZB-535B (10 mL/L) available from Dipsol
Chemicals Co., Ltd.
Comparative Example 2
[0049] A steel plate, which had been plated with zinc in a thickness of 8 µm, as a comparative
example, was subjected to a trivalent chromate treatment using a processing solution
having the following composition: 15 g/L (3.3 g/L as expressed in terms of Cr
3+) of Cr(NO
3)
3; 10 g/L of NaNO
3; and 10 g/L of oxalic acid (pH: 2.0). In this respect, the processing was carried
out at 30°C for 40 seconds.
Comparative Example 3
[0050] A steel plate, which had been plated with zinc in a thickness of 8 µm, as a comparative
example, was subjected to a trivalent chromate treatment using a processing solution
having the following composition as disclosed in the example of J.P. KOKAI No. 2000-509434:
50 g/L (9.8 g/L as expressed in terms of Cr
3+) of CrCl
3·6H
2O; 3 g/L (0.6 g/L as expressed in terms of Co) of Co(NO
3)
2; 100 g/L of NaNO
3; and 31.2 g/L of malonic acid (pH: 2.0). In this respect, the processing was carried
out at 30°C for 40 seconds.
Processing Steps:
[0051] In these Examples and Comparative Examples, the details of the processing steps are
as follows:
Plating → Water Rinsing → Activation with Dilute Nitric Acid → Water Rinsing → Trivalent
Chromate Treatment → Water Rinsing → (Topcoating Treatment)1 → Drying2 → (Heat Treatment)3
Note 1: This step was used only when the steel plate was subjected to a topcoating
treatment.
Note 2: The drying step was carried out at a temperature ranging from 60 to 80°C for
10 minutes.
Note 3: When carrying out the test for the corrosion resistance after heating, each
steel plate was treated at 200°C for 2 hours.
Salt Spray Test:
[0052] The zinc plated steel plates obtained in Examples 1 to 14 and Comparative Examples
1 to 3 and each provided thereon with a trivalent chromate film were inspected for
the appearance and subjected to the salt spray test (JIS-Z-2371). The results thus
obtained are summarized in the following Table 4.
[0053] As will be clear from the data listed in Table 4, it is found that the films obtained
in Examples 1 to 10 show the corrosion resistance almost identical or superior to
those observed for the chromate film obtained in Comparative Examples 1 to 3. In addition,
the films of Examples 11 to 14, which were subjected to a topcoating treatment, especially
at the time required for the formation of red rust, show corrosion resistance superior
to that observed for the conventional chromate film.
Table 4:
Results of Salt Spray Test (JIS-Z-2371) |
Ex. No. |
Appearance of Film |
Corrosion Resistance (1) (hr.) |
Corrosion Resistance After Heating (2) (hr.) |
1 |
Black |
120/600 |
240 |
2 |
Black |
72/500 |
240 |
3 |
Black |
72/400 |
120 |
4 |
Black |
96/500 |
240 |
5 |
Black |
120/500 |
240 |
6 |
Black |
120/500 |
240 |
7 |
Black |
120/800 |
240 |
8 |
Black |
120/1500 |
240 |
9 |
Black |
240/1000 |
240 |
10 |
Black |
240/1000 |
240 |
11 |
Black |
240/1000 |
|
12 |
Black |
300/2000 |
|
13 |
Black |
300/1200 |
|
14 |
Black |
300/1200 |
|
1* |
Black |
120/500 |
12 |
2* |
Pale Blue |
24/250 |
24 |
3* |
Interference Color |
72/300 |
48 |
(1) Time (hour) required for the formation of white rust / red rust (5% by mass). |
(2) Time (hour) required for the formation of white rust (5% by mass). |
*: Comparative Example |
Effect of the Invention
[0054] As has been described above in detail, the present invention permits the formation
of a hexavalent chromium free, black conversion film directly on zinc or zinc alloy
plating layers. The plated article obtained according to this method has not only
the corrosion resistance due to the zinc or zinc alloy plating as such, but also the
excellent corrosion resistance due to the presence of the trivalent chromate film.
Moreover, the processing solution used in the present invention comprises trivalent
chromium in a low concentration and therefore, the present invention is quite advantageous
from the viewpoint of the wastewater treatment and production and processing cost.
The film obtained by directly forming trivalent chromate on the plating possesses
not only corrosion resistance, resistance to salt water and after heating resistance
identical or superior to those observed for the conventional hexavalent chromium-containing
film, but also expresses a black feature, and therefore, the film of the present invention
can widely be used in a variety of fields in the future.
1. A processing solution for forming a hexavalent chromium free, black conversion film
on a zinc or zinc alloy plating layer(s), the processing solution comprising:
nitrate ions and trivalent chromium in a mole ratio (NO3-/Cr3+) of less than 0.5/1, wherein the trivalent chromium is present in the form of a water-soluble
complex with a chelating agent; and
cobalt ions and / or nickel ions, wherein cobalt ions and / or nickel ions are stably
present in the processing solution without causing any precipitation by forming a
hardly soluble metal salt with the chelating agent; wherein the solution reacts with
zinc when it is brought into contact with the zinc or zinc alloy plating to form a
hexavalent chromium free, black conversion film containing zinc, chromium, cobalt
and / or nickel, and the chelating agent on the plating layer(s).
2. The processing solution according to claim 1 wherein the trivalent chromium concentration
ranges from 0.5 to 10g/L and a molar ratio of the chelating agent to the trivalent
chromium (the chelating / Cr3+) ranges from 0.2/1 to 4/1.
3. The processing solution according to claim 1 or 2 wherein the trivalent chromium source
is chromium phosphate (Cr(HnPO4)(3/(3-n))).
4. The processing solution according to any of claims 1 to 3 wherein the chelating agent
is one or more members selected from the group consisting of monocarboxylic acids,
dicarboxylic acids, tricarboxylic acids, hydroxycarboxylic acids, aminocarboxylic
acids and salts thereof.
5. The processing solution according to any of claims 1 to 4 wherein the chelating agent
is one or more members selected from the group consisting of oxalic acid, malonic
acid, succinic acid and a salt thereof.
6. The processing solution according to any of claims 1 to 5, which further comprises
one or more metal ions selected from the group consisting of silicon, iron, titan,
zirconium, tungsten, vanadium, molybdenum, strontium, niobium, tantalum, manganese,
calcium, barium, magnesium and aluminum.
7. The processing solution according to any of claims 1 to 6 wherein the nitrate ion
concentration ranges from 0.1 to 1 g/L.
8. The processing solution according to any of claims 1 to 7, which further comprises
phosphate ions.
9. The processing solution according to any of claims 1 to 8, which further comprises
one or more ions selected form the group consisting of a sulfate ion, a halogen ion
and a borate ion.
10. A method for forming a hexavalent chromium free, black conversion film, which comprises
the step of bringing a zinc or zinc alloy plating layer(s) into contact with the processing
solution of any of claims 1 to 9.
11. The method according to claim 10, which comprises the step of subjecting a hexavalent
chromium free, black conversion film to an aging treatment at 100 to 250°C for 30
to 300 minutes.
12. The method according to claim 11 wherein the aging treatment is applied at 200°C for
60 to 300 minutes.
13. A method for rust-proofing zinc and zinc alloy plating, which comprises the step of
subjecting a hexavalent chromium free, black conversion film formed by the method
according to any of claims 10 to 12 to an overcoating treatment.