Hot-dipped galvanized steel sheet having excellent black tarnish resistance and process for producing the same

A zinc- or zinc alloy-hot-dipped, unpainted steel sheet having excellent black tarnish resistance, which has a coating of Ni, Co or a combination thereof deposited in an amount of from 0.1 to 15 mg/m.sup.2 on the zinc or zinc alloy plating layer of the sheet, and optionally a chromate layer on the deposited coating, is provided. The steel sheet is produced by immersing a zinc- or zinc alloy-hot-dipped steel sheet in an aqueous solution containing 1 to 20 g/l of Ni ions, Co ions or a combination thereof to deposit Ni, Co or a combination thereof in an amount of from 0.1 to 15 mg/m.sup.2 on the zinc or zinc alloy plating layer of the sheet, optionally followed by chromating in the usual manner.

BACKGROUND OF THE INVENTION 
The present invention relates to a zinc- or zinc alloy-hot-dipped, 
unpainted steel sheet having excellent black tarnish resistance and to a 
process for producing such steel sheets. 
Recently, with the expanding demands and the diversification of 
applications, there are growing needs for further improvements of 
hot-dipped galvanized steel sheets in performance characteristics of, for 
example, appearance, workability, chemical treatability (treatability 
before painting), and corrosion resistance. For improving the workability 
and chemical treatability, the so-called "zero-spangle treatment" is 
practiced, which treatment is carried out by spraying atomized water 
against the surface of a molten zinc layer after hot dipping and before 
the solidification of the layer, to make fine zinc crystals. Further the 
treated sheet is subjected to skin-pass rolling (usually 1-3% draft) for 
improving the appearance or surface microscopic geometry (surface 
smoothness). 
However, the skin-pass rolling, when followed by chromating for improving 
the corrosion resistance and organic coat adhesion, is often caused the 
trouble of discoloring the plating surface to blue-brown-black 
(hereinafter the discoloring is referred to as black tarnishing) during 
relatively short time storage or during transport. 
For improving the corrosion resistance, techniques of alloying zinc by 
adding Al, Mg, Ti, or a rare earth element (La or Ce) to a molten Zn bath 
have been studied inside and outside this country and some of them were 
already put into practical operation. 
However, when these zinc alloy-hot-dipped steel sheets were chromated, 
black tarnish was observed on the plating surface during storage or 
transport similarly in case of zero-spangle treated galvanized steel 
sheets (skin-pass sheets). 
When these zinc alloy-hot-dipped steel sheets are skin-pass-rolled (draft 
1-3%), black tarnish will appear thereon in shorter time. Also on usual 
hot-dipped galvanized steel sheets (non-skin-pass sheets), black tarnish 
due to the spangle is occasionally observed in spotty form during 
long-term storage, but the black tarnish on the above-mentioned 
zero-spangle treated skin-pass sheets and alloy plated sheets is 
characterized by developing for a very short period of time. In addition, 
the black tarnish is more liable to develop when these sheets are stored 
in stacked form after cutting into pieces or in coiled form than when 
these are left standing in bared form in a room. As regards the effect of 
environments, the black tarnish is liable to develop in high temperature 
and humidity surroundings. 
The present inventors detected a thicker layer (estimated at 
hundreds-thousands .ANG.) of zinc hydroxide-zinc carbonate mixture on the 
plating surface where black tarnish had developed than on the surface 
where black tarnish did not yet develop. From this result and the above 
information, the mechanism of the black tarnish development is presumed as 
follows: on the surface of the zinc alloy-hot-dipped steel sheet, Al or Mg 
added for alloying is present in the form of metal or oxide and forms 
local cells jointly with Zn, thereby causing the electrochemical corrosion 
(reaction of metallic zinc with moisture and chromic acid) to proceed 
abnormally quickly under corrosive environmental conditions mediated by 
the chromate coating layer which inherently contains moisture and is 
liable to absorb moisture. The black appearance of the plating surface may 
be contributed to by the light interference due to the presence of a thin 
zinc hydroxide-zinc carbonate layer on fine depressions and projections 
formed by the corrosion of the Zn layer. 
The reason why the skin-pass-rolled sheet becomes liable to undergo black 
tarnishing is conceivably because strain and fresh Zn surfaces are 
produced in the plating layer by the rolling, which develops more 
chemically active local cells accelerating the corrosion. For the purpose 
of preventing the plating surface from being black tarnished, there has 
been proposed after-treatment with non-chromate chemicals, or zinc 
electroplating. For instance, a method of coating with a water-soluble 
resin has been proposed as the non-chromate after-treatment method. 
However, as can be assumed from the above-mentioned mechanism of black 
tarnish development, a thin resin coating cannot shut out the penetrating 
moisture, thus allowing the black tarnishing. 
Recently, Japanese Patent Application Laid-Open No. 114695/82 discloses a 
process for producing galvanized steel sheets superior in resistance to 
black tarnishing and resistance to intergranular corrosion. This technique 
comprises plating steel sheets with zinc or a zinc alloy by hot dipping, 
and treating the plated sheets with an alkali metal carbonate solution, 
followed by electroplating the treated sheets with zinc. According to this 
patent application, it is said that oxides of Zn, Al, Fe, and Si formed on 
the hot-dipped galvanized surface are completely dissolved by the 
treatment with an alkali metal carbonate solution and this permits the 
black tarnishing and the intergranular corrosion to be prevented even when 
the subsequent zinc electroplating gave a thin film. 
The above-mentioned technique, however, requires two additional operation 
steps of alkali cleaning and zinc electroplating, and thus high investment 
cost for equipment and complicated management of the plating bath being 
inevitable. 
Other examples of prior art include the techniques disclosed in Japanese 
Patent Application Laid-Open No. 152834/77 entitled "Method for 
Pretreatment of Hot-Dip Galvanize Steel Sheet before Painting", Japanese 
Patent Application Laid-Open No. 115624/78 entitled "Method for 
Conditioning Galvanized Steel Sheet Surface", and Japanese Patent 
Publication No. 26434/83 entitled "Method for Zinc Surface Treatment". 
These known techniques, however, deal with only the adhesion of organic 
coating materials and the corrosion resistance of the coating, as problems 
to be solved. These Japanese Patent Application Laid-Open Nos. 152834/77 
and 26434/83 disclose the method of removing the oxide film formed on 
galvanized steel sheets and dissolving the zinc surface at the same time, 
by means of a strong acid solution containing Co, Ni, or Fe metal ions, to 
substitute and deposit the metal. It is widely known that the surfaces of 
galvanized steel sheets when etched with an acid shows a white, gray, or 
black appearance according to spangle patterns. The etching followed by 
the metal substitution-deposition intensifies the black tarnish. The 
Japanese Patent Application Laid-Open No. 115642/78 discloses the method 
of simultaneously conducting the formation of a phosphate coat and the 
removal of the oxide film, dissolution of zinc, and 
substitution-deposition of a metal. This phosphate coat is similar in 
coating weight to those obtained by the usual phosphating, which are well 
known to exhibit a gray or gray-black color. Thus, steel sheets according 
to these three known methods each exhibits a color (gray, brown, or black) 
characteristic of the coat before chromating, in other words, black 
tarnishing of the sheets has already occurred before chromating. The 
subsequent chromating will add a yellow color to the above color, 
resulting in a gray-black-yellow mixed color. However, the three methods 
of the above-mentioned three Japanese documents do not intend to remove a 
color or avoid the black tarnishing since the sheets after chromating are 
painted. 
SUMMARY OF THE INVENTION 
Accordingly, the present inventors made intensive studies on treatment 
methods which will be simple in operation and effective in preventing the 
black tarnishing without requiring any expensive equipment, and as a 
result, have found out a method for the treatment of unpainted hot-dipped 
galvanized steel sheets with an aqueous solution containing Ni ions, Co 
ions or a combination thereof. 
An object of the present invention is to provide unpainted hot-dipped 
galvanized steel sheets having metallic gloss, zinc metal color as such or 
slightly tinged with yellow even when the amount of metal deposited is 
large, and excellent resistance to black tarnishing. 
Another object of the present invention is to provide a process for 
producing such hot-dipped galvanized steel sheets as stated above. 
According to the present invention, a steel sheet plated by hot dipping 
with zinc or with a zinc alloy containing low concentrations of Al and Mg 
is immersed for a short period of time in a solution of Ni ions and/or Co 
ions or sprayed with this solution and, if necessary, is subjected to the 
usual chromating, thereby achieving a superior effect of preventing black 
tarnish.

DETAILED DESCRIPTION OF THE INVENTION 
The conditions of the process in accordance with the present invention are 
as follows: For the solution containing Ni ions, Co ions or a combination 
thereof, there may be used either an acidic bath containing a sulfate, 
chloride, nitrate, acetate or citrate of Ni, Co or a combination thereof, 
or an alkaline bath containing a complexing agent such as ammonia or an 
organic acid together with the above-mentioned Ni- and/or Co-salts. There 
are no particular limitations on the metal ion concentration in the bath 
and on the temperature and pH of the bath. These factors can be selected 
from wide ranges. But, the concentration of Ni ions, Co ions or a 
combination thereof is desirably in the range of 1 to 20 g/l. If the 
concentration is below 1 g/l, then a prolonged period of time for the 
treatment will be required, and if the concentration is higher than 20 
g/l, then the quantity of the metal ions carried away by drag-out will be 
as large as economically unsatisfactory. The bath temperature is suitably 
from 30.degree. to 60.degree. C. If the temperature is below 30.degree. 
C., then a prolonged period of time for the treatment will be required, 
and if the temperature exceeds 60.degree. C., too much energy will be 
required for heating. 
The pH value of the bath is suitably from 1 to 4 in case of the acidic 
bath, and from 11 to 13.5 in case of the alkaline bath. Outside this pH 
range an excessive or deficient amount of Ni, Co or a combination thereof 
will be deposited on the plating surface. According to elaborate 
experiments by the present inventors, the amount of Ni, Co or a 
combination thereof deposited on the plating surface is suitably from 0.1 
to 15 mg/m.sup.2, preferably from 0.3 to 3 mg/m.sup.2. If the deposited 
amount is less than 0.1 mg/m.sup.2, then the black tarnishing will be 
liable to occur during storage under so high temperature and humidity 
conditions as in summer. If the deposited amount is larger than 15 
mg/m.sup.2, the sheet will not only become slightly yellow or brown, 
losing the original metallic appearance, but also the bath will be much 
aged, this being also uneconomical. 
With respect to this matter, the present inventors examined the relation 
between the Co amount deposited and the black tarnishing tendency. Tests 
for the black tarnishing tendency were conducted by using pieces of a 
Zn-Al alloy-hot-dipped steel sheet, treating the pieces, except that some 
of them were used as control pieces, with the Co solution according to the 
invention, subjecting all the pieces to chromating, and exposing portions 
of the Co solution-treated pieces and of the control pieces in a dewing 
state at 49.degree. C.--more than 98% relative humidity for 7 days after 
not giving dews on their surface and the other portions of the pieces in a 
non-dewing state at 70.degree. C.--more than 98% relative humidity for 2 
days after giving dews on their surface, thereby observing the degree of 
black tarnishing on each piece. Results of the observation are shown in 
FIG. 1, wherein the data are plotted with the Co amount deposited for 
abscissa and with the degree of black tarnishing (rating of black tarnish 
hesistance) for ordinate. The meaning of the rating marks are as follows: 
.circleincircle. . . . Not tarnished 
.circle. . . . Very slightly turned gray 
.DELTA. . . . Less than 30% of the whole area turned black 
X . . . 30% or more of the whole area turned black. 
FIG. 1 indicates that the sheet treated with the Co solution according to 
the invention is superior to the untreated sheet in black tarnish 
resistance. 
The period of immersion or spraying for this treatment is suitably chosen 
so that the Co amount deposited becomes from 0.1 to 15 mg/m.sup.2, 
depending upon the above-stated concentration, temperature, and pH. 
According to the present inventors' experiments, the period of time for 
the treatment can be chosen as desired from the wide range of 0.3 to 10 
seconds. 
The mechanism of preventing the black tarnishing according to the invention 
is presumed as follows: When the zero-spangle zinc-galvanized steel sheet 
(skin-pass sheet) or zinc alloy-galvanized steel sheet which is subject to 
black tarnishing is immersed in a Co- and/or Ni-containing solution, the 
Ni and/or Co will be deposited in metallic or oxide form on the active 
sites (electrochemically, anodic sites) of a plated surface, thereby 
depressing the activity. As a result, the progress of the corrosion based 
on local cells will be prevented. 
The treatments with solutions containing Fe, Mn, Sn and/or Cu ions were 
investigated, besides the treatment according to the invention, but with 
the result that the sheets treated with the Sn ion-containing solution and 
with the Cu ion-containing solution, respectively, became more subject to 
black tarnishing though the respective treatments with the Fe 
ion-containing solution and with the Mn ion-containing solution were found 
to be slightly effective. 
Other advantages of the treatment method according to the present invention 
are that the equipment is simple because of the immersion or spray 
treatment, that the management of the treating solution is easy, that the 
treatment is effective with a trace amount of Ni, Co or a combination 
thereof deposited, and, hence, that the treatment can be accomplished at a 
low cost. 
The present invention is illustrated in more detail with reference to the 
following examples. 
EXAMPLE 1 
A zero-spangle galvanized steel sheet produced in a known continuous 
hot-dip galvanizing line was skin-pass rolled, and cut into specimens. 
These specimens, except for those used as controls, were treated according 
to the present invention under various conditions. Then, the treated 
specimens and control specimens were subjected to accelerated corrosion 
tests to examine the black tarnish resistance. 
Conditions of the treatments and tests and the criterion of the black 
tarnish resistance were as follows: 
______________________________________ 
(i) Specimen 
Size: 0.5 .times. 100 .times. 200 mm 
(not chromated) 
Amount of Zinc plating: 
150 g/m.sup.2 
Draft in skin-pass rolling: 
1% 
(ii) Pretreatment (treatment according to the present 
invention) 
(a) Not treated 
(b) Ni treatment: 
NiSO.sub.4.6H.sub.2 O 50 g/l, pH 2, 
(acidic bath) 
at 50.degree. C., immersion for 2 sec. 
(c) Co treatment: 
CoSO.sub.4.7H.sub.2 O 50 g/l, pH 2, at 
50.degree. C., immersion for 0.5 sec. 
(d) Co treatment: 
CoSO.sub.4.7H.sub.2 O 50 g/l, citric 
(alkaline bath) 
acid 15 g/l, NaOH 40 g/l, 
pH 13.2, at 60.degree. C., 
immersion for 2 sec. 
(e) Ni + Co NiSO.sub.4.6H.sub.2 O 25 g/l, CoSO.sub.4.7H.sub.2 O 
treatment 25 g/l, pH 2, at 50.degree. C., 
immersion for 1 sec. 
(f) The same as (b), except for immersion for 10 sec. 
(g) The same as (c), except for immersion for 6 sec. 
(iii) 
Washing: City water 
(iv) Chromating: 
CrO.sub. 3 30 g/l, at 50.degree. C., immersion for 
5 sec. 
(v) Accelerated test for black tarnish 
(1) Sample size: 
100 .times. 100 mm, 5 stacked sheets 
packed with wrapping paper 
(2) Atmosphere and standing time: 
.circle.A 
49.degree. C., 98% RH, 7 days 
.circle.B 
70.degree. C., 98% RH, 3 days 
(3) Criterion of resistance to black tarnishing: 
Rating marks have the same meaning as 
described above. 
______________________________________ 
Results of the tests are shown in Table 2. 
As is seen Table 2, the specimens treated according to the present 
invention have excellent black tarnish resistance. Similarly treated 
sheets stored in coil form for one year did not undergo black tarnishing. 
TABLE 1 
______________________________________ 
Results of accelerated black tarnish tests (Example 1) 
Coating weight 
Evaluation of black 
(one side) tarnish resistance 
Co and/ .circle.A 
Sam- Pretreatment 
or Ni Cr 49.degree. C., 
.circle.B 70.degree. C. 
ple method (mg/m.sup.2) 
(mg/m.sup.2) 
7 days 
3 days 
______________________________________ 
a not treated 
0 9 X X 
b Ni treatment 
2.8 11 .circleincircle. 
.circle. 
c Co treatment 
1.7 8 .circleincircle. 
.circleincircle. 
(acidic bath) 
d Co treatment 
2.1 9 .circleincircle. 
.circleincircle. 
(alkaline 
bath) 
e Ni + Co Ni 1.5 9 .circleincircle. 
.circle. 
treatment Co 0.7 
f Ni treatment 
13 13 .circleincircle. 
.circleincircle. 
g Co treatment 
8.5 11 .circleincircle. 
.circleincircle. 
(acidic bath) 
______________________________________ 
EXAMPLE 2 
A steel sheet was plated with a Zn-Al (4%)-Mg (0.3%) alloy in a known 
continuous hot-dip galvanizing line, and cut into specimens. These 
specimens, except for those used as controls, were treated according to 
the present invention under various conditions. The treated specimens and 
control specimens were subjected to accelerated corrosion tests to examine 
the black tarnish resistance. 
Conditions of the treatments and tests and the criterion of the black 
tarnish resistance were the same as in Example 1, but the specimens were 
as follows: 
______________________________________ 
(i) Specimen 
______________________________________ 
Size: 0.5 .times. 100 .times. 200 mm (not chromated) 
Amount of zinc plating: 
120 g/m.sup.2 
Skin-pass rolling: 
Not rolled 
______________________________________ 
Results of the tests are shown in Table 2. 
Therefrom it proves that all the specimens treated according to the present 
invention have excellent black tarnish resistance. Similarly treated 
sheets stored in coil form for one year did not undergo black tarnishing. 
TABLE 2 
______________________________________ 
Results of accelerated black tarnish tests (Example 2) 
Coating weight 
Evaluation of black 
(one side) tarnish resistance 
Co and/ .circle.A 
Sam- Pretreatment 
or Ni Cr 49.degree. C. 
.circle.B 70.degree. C. 
ple method (mg/m.sup.2) 
(mg/m.sup.2) 
7 days 
3 days 
______________________________________ 
a Not treated 
0 13 X X 
b Ni treatment 
2.2 10 .circleincircle. 
.circle. 
c Co treatment 
2.1 9 .circleincircle. 
.circleincircle. 
(acidic bath) 
d Co treatment 
1.6 11 .circleincircle. 
.circleincircle. 
(alkaline 
bath) 
e Ni + Co Ni 0.8 8 .circleincircle. 
.circleincircle. 
treatment Co 0.5 
f Ni treatment 
12.5 14 .circleincircle. 
.circle. 
g Co treatment 
7.3 14 .circleincircle. 
.circleincircle. 
(acidic bath) 
______________________________________