Black chromate solution

A chemically stable black chromate solution capable of forming an anticorrosive black chromate film on a Zn-Ni alloy plating is provided, the components being easily soluble in the solution. The black chromate solution comprises 1 to 50 g/l of hexavalent chromium; 1 to 50 g/l of sulfate ion; 0.05 to 1.0 g/l of soluble silver; 0.5 to 100 g/l of a sulfonated carboxylic acid selected from the group consisting of sulfo-aromatic carboxylic acids having 7 to 15 carbon atoms, sulfonated aliphatic dicarboxylic acids having 3 to 10 carbon atoms and salts thereof; and a balance of water, the solution having a pH of 0.5 to 2.5.

BACKGROUND OF THE INVENTION 
The present invention relates to a black chromate solution used for forming 
a highly anticorrosive black chromate film on a Zn plate, Zn-plating, Zn 
alloy plating such as Zn-Ni alloy plating, and a method of forming a 
chromate film thereon, especially on a Zn-Ni alloy. 
The treatment of a zinc plating with a chromate is still widely employed 
for the purposes of decoration and anticorrosion in the fields of 
automobiles, domestic electric appliances, etc. 
However, the plating thus formed has defects that the corrosion resistance 
thereof is yet insufficient under severe conditions when it is kept in an 
overheated engine room or on an icy road, when it is exposed to a chemical 
used for removing snow when it snows, or when it is damaged by salty sea 
breezes. Under these conditions, various Zn alloy platings capable of 
forming a plating film having a corrosion resistance far superior to that 
of a zinc plating were investigated and developed. Among them, a high 
corrosion resistance of a Zn-Ni alloy attracts attention mainly in the 
automobile field. As a monotony tint and a high grade are inclined to be 
liked by the users recently, a substance for blacking the Zn-Ni alloy 
plating is demanded. In addition, there is investigated a black chromate 
which does not impair the properties of the Zn-Ni alloy plating and 
further a chemically stable black chromate solution useful when the Ni 
deposition rate is in the practical range of 3 to 12% by weight. The best 
technique comprises adding an aliphatic carboxylic acid to an ordinary 
black chromate solution for treating Zn to form a black chromate film on 
the Zn-Ni alloy plating. 
However, the process wherein formic acid, citric acid or tartaric acid is 
used as the aliphatic carboxylic acid has a problem that the treating 
solution is impractical, since the solution is chemically unstable and the 
ingredients in the solution tend to be decomposed. The process wherein 
succinic acid is used has a problem that the appearance thereof is yet 
insufficient in the blackness and gloss, though the treating solution is 
chemically stable. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to provide a chemically stable 
black chromate solution capable of forming a highly anticorrosive black 
chromate film on a Zn-plating, Zn alloy plating and the like, the 
components being easily soluble in the solution. 
Another object of the present invention is to provide a method of forming a 
black chromate film on a Zn-Ni alloy. 
These and other objects of the present invention will be apparent from the 
following description and examples. 
The present invention has been completed on the basis of a finding that the 
above-described problems can be efficiently solved by using a 
sulfo-aromatic carboxylic acid, its salt, sulfonated aliphatic 
dicarboxylic acid or its salt as the organic acid to be contained in the 
black chromate solution. 
Namely, the present invention provides a black chromate solution which 
comprises hexavalent chromium; sulfate ion; soluble silver; and a 
sulfonated carboxylic acid selected from the group consisting of 
sulfo-aromatic carboxylic acids, sulfonated aliphatic dicarboxylic acids 
and salts thereof; and water. 
The present invention further provides a method of forming a black chromate 
film on a Zn-Ni alloy plating which comprises immersing the Zn-Ni alloy 
plating into a black chromate solution comprising hexavalent chromium; 
sulfate ion; soluble silver; and a sulfonated carboxylic acid selected 
from the group consisting of sulfo-aromatic carboxylic acids, sulfonated 
aliphatic dicarboxylic acids and salts thereof; and water.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Examples of the sulfo-aromatic carboxylic acids usable in the present 
invention include those having 7 to 15 carbon atoms such as sulfobenzoic 
acid, sulfophthalic acid, sulfoterephthalic acid and sulfonated naphthalic 
acid. The sulfonated aliphatic dicarboxylic acids include, for example, 
those having 3 to 10 carbon atoms such as sulfonated succinic acid and 
adipic acid. The salt of them may be any of those soluble in the black 
chromate solution. Examples of the salts include sodium and ammonium 
salts. 
The sulfonated carboxylic acids can be used either singly or in the form of 
a mixture of two or more of them in the present invention. Although the 
amount of the sulfonated carboxylic acid is not particularly limited, it 
is preferable that the concentration thereof in the black chromate 
solution be 0.5 to 100 g/l, more preferably 5 to 40 g/l. 
The black chromate solution of the present invention contains hexavalent 
chromium, sulfate ion (SO.sub.4.sup.2+) and soluble silver. Compounds 
usable as the hexavalent chromium source are, for example, chromic acid, 
salts thereof, bichromic acid, salts thereof and mixtures of two or more 
of them. The compounds for forming the sulfate ion include sulfuric acid, 
salts thereof (such as sodium, potassium and ammonium salts thereof) and 
mixtures of two or more of them. The compounds for forming the soluble 
silver include, for example, silver sulfate. 
The black chromate solution of the present invention can contain these 
components in optional amounts. Preferably, it contains 1 to 50 g/l of 
hexavalent chromium, 1 to 50 g/l of sulfate ion, 0.05 to 1.0 g/l of 
soluble silver and 0.5 to 100 g/l of the sulfonated carboxylic acid. The 
pH of the black chromate solution of the present invention is preferably 
controlled in the range of 0.5 to 2.5. 
The black chromate solution of the present invention contains water as the 
balance. Further, amines such as methylamine, ethylenediamine and 
diethylenetriamine can be incorporated into the solution. 
The black chromate solution of the present invention is effective in 
forming a black chromate film on a Zn-Ni alloy plating having any 
composition. It exhibits an excellent effect particularly when a black 
chromate film is formed on a Zn-Ni alloy plating having a Ni deposition 
rate of not higher than 15% by weight, preferably in the range of 3 to 12% 
by weight. Namely, when the black chromate film is to be conventionally 
formed on the Zn-Ni alloy plating of this composition, the formed film, 
even when it can be obtained, is not uniform and the corrosion resistance 
is seriously impaired, since the pH is rapidly increased at the boundary 
(interface) between them. On the contrary, in the present invention 
wherein the specified sulfonated carboxylic acid is used, the interface 
formed by the chromate film is exposed to a stable pH atmosphere to make 
the stable formation of the film possible. Thus, the high anticorrosive 
function of the Zn-Ni alloy plating is not impaired, and further, more 
excellent anticorrosive effect can be obtained. 
The treatment with the black chromate is usually conducted by immersing the 
substrate to be treated in the black chromate solution of the present 
invention. For example, the substrate is immersed in the solution having a 
temperature of 15 to 50 .degree. C. for 30 to 300 seconds, preferably 30 
to 180 seconds to form a black chromate film having a thickness of 0.5 to 
3 .mu.m, preferably 1 to 2 .mu.m. 
Thus, in a preferred embodiment, the substrate is plated with a Zn-Ni alloy 
(Ni deposition rate: preferably 3 to 12% by weight) (thickness of the 
plating: at least 2 .mu.m, preferably 5 to 20 .mu.m) and then the black 
chromate film is formed by the above-described process. 
By the process of the present invention, the black chromate film can be 
uniformly formed on the Zn-Ni alloy plating. Since the specified 
sulfonated carboxylic acid exhibits a buffer action under chromic 
acid-acidic conditions in a pH range of 1 to 3.5, the uniform black 
chromate film can be formed and, as a result, the corrosion resistance can 
be improved and excellent stability can be obtained. 
Therefore, the products obtained by forming the black chromate film by the 
process of the present invention can be widely used in the fields of 
automobiles, domestic electric appliances, building materials and parts, 
etc. 
The following Examples will further illustrate the present invention. 
EXAMPLE 1 
A steel plate plated with a Zn-Ni alloy (Ni content: 3, 5, 7, 9, 12 or 15% 
by weight) in a thickness of 8 .mu.m was immersed in a chromate solution 
having a composition given below at 25 .degree. C. for 90 seconds. As a 
result, an excellent black chromate film was formed when the Ni content 
was 3, 5, 7, 9 or 12% and, on the contrary, a black chromate film having 
an interference color was obtained when the Ni content was 15%. 
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CrO.sub.3 10 g/l 
Na.sub.2 SO.sub.4 
15 g/l 
AgNO.sub.3 0.8 g/l 
sulfonaphthalic acid 
7 g/l 
water balance 
pH 1.3 (adjusted with NaOH). 
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EXAMPLE 2 
A Zn-Ni plating having a thickness of 6 .mu.m and Ni content of 6% was 
treated with a chromate solution having the following composition at 
20.degree. C. for 3 minutes to obtain a uniform black chromate film: 
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CrO.sub.3 5 g/l 
Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O 
20 g/l 
Na.sub.2 SO.sub.4 
25 g/l 
H.sub.2 SO.sub.4 
2 g/l 
Ag.sub.2 SO.sub.4 
0.5 g/l 
sulfoterephthalic acid 
3 g/l 
water balance 
pH 1.6 (adjusted with NaOH). 
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EXAMPLE 3 
A Zn-Ni plating having a thickness of 10 .mu.m and Ni content of 8% was 
treated with a chromate solution having the following composition at 25 
.degree. C. for 2 minutes to obtain a uniform black chromate film: 
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Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O 
15 g/l 
H.sub.2 SO.sub.4 
5 g/l 
Ag.sub.2 SO.sub.4 
0.5 g/l 
sodium salt of 15 g/l 
sulfonated adipic acid 
water balance 
pH 1.5 (adjusted with NaOH). 
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Comparative Example 1 
A steel plate was plated with Zn to form a plating film having 8 .mu.m 
thickness. The plate was then treated with a black chromate solution 
ZB-547 (a product provided by Dipsol Chemical Co., Ltd.) for Zn plating at 
25 .degree. C. for 1 minute. 
Table 1 shows the results of the corrosion resistance tests of the products 
having the black chromate film obtained in above Examples 1 to 3 and 
Comparative Example 1 (the salt water spray test was conducted according 
to JIS 2371). 
TABLE 1 
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Ni Plating 
Kind of content thickness 
plating (%) (.mu.m) 
Tint 
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Ex. 1 Zn--Ni 3 8 uniform black 
" 5 " " 
" 7 " " 
" 9 " " 
" 12 " " 
" 15 " black having inter- 
ference color 
Ex. 2 Zn--Ni 6 6 uniform black 
Ex. 3 Zn--Ni 8 10 " 
Comp. Ex. 1 
Zn 0 8 " 
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Salt water spray test 
Time taken for white 
Time taken for red 
rust formation (h) 
rust formation (h) 
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Ex. 1 at least 500 at least 2000 
" " 
" " 
" " 
" " 
" " 
Ex. 2 " " 
Ex. 3 " " 
Comp. Ex. 1 
144 600 
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