Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates to a æinc- 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 (herein-
after the discoloring is referred to as black tarnishing~
during relatively short time storage or during transpor~
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1 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 surround-
ings.
The present inventors detected a thicker layer
(estimated at hundreds-thousands A) of zinc hydroxide-zinc
carbonate mixture on the plating surface where black tarnish
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1 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, A1 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 platlng
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
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1 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 paten-t appli-
cation, it is said that oxides of Zn, Al, Fe, and Si formedon 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
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1 with only the adhesion cf organic coating materials and
the corrosion resistance o~ 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. 115642/7~
discloses the method of simultaneously conducting the forma-
tion 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
2~ 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 sub-
sequent 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
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1 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
sub~ected to the usual chromating, thereby achieving a
superior effect of preventing black tarnish.
BRIEF DESCRIPTION OF THE DRAWING
Fig. 1 is a graph showlng the relation between
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1 the amount of Co deposited and the reslstance to black
tarnishing when a steel sheet hot dipped in a Zn-Al alloy
was treated with a Co solution according the invention.
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
10 combination fhereof, 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
15 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/Q. If the concent-
ration is below 1 g/Q, then a prolonged period of time
for the treatment will be required, and if the concentration
20 is higher than 20 g/Q, 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 to 60C. If the temperature is below 30C, then a
prolonged period of time for the treatment will be
25 required, and if the temperature exceeds 60C, too much
energy will be required for heating.
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1 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 al~aline 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 mg1m2, preferably from 0.3 to
3 mg/m2. If the deposited amount is less than 0.1 mg/m2,
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 , 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 49C - 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 70C - more
than 98~ relative humidity for 2 days after giving dews on
their surface, thereby observing the degree of black
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1 tarnishing on each piece. Results of the observation are
show~ 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 hesistance3 for ordinate.
The meaning of the rating marks are as follows:
~ ...... Not tarnished
O ...... Very slightly turned gray
a ..... 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 lS mg/m2, 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
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1 (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 treat~
ment 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 treat-
ment can be accomplished at a low cost.
The present invention is illustrated in more
detail with reference to the following examples.
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Example 1
A ~ero-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
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1 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
crlterion of the black tarnish resistance were as follows:
(i) Specimen
Size: 0.5 x 100 x 200 mm (not chromated)
Amount of Zinc plating: 150 g/m2
Draft in skin-pass rolling: 1%
(ii) Pretreatment (treatment according to the present
invention)
(a) Not treated
~b) Ni treatment: NiSO4-6H2O 50 g/Q, pH 2,
(acidic bath) at 50C, immersion for 2 sec.
15 (c3 Co treatment: CoSO4-7H2O 50 g/Q, pH 2, at
50C, immersion for 0.5 sec.
(d) Co treatment: CoSO4-7H2O 50 g/Q, citric
(alkaline bath) acid 15 g/Q, NaOH 40 g/Q,
pH 13.2, at 60C,
immersion for 2 sec.
(e) Ni + Co treatment: NiSO4-6H2o 25 g/Q, CoSO4 7H2O
25 g/Q, pH 2, at 50C,
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: CrO3 30 g/Q, at 50C, immersion for
5 sec.
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1 (v) Accelerated test for black tarnish
(1) Sample size: 100 x 100 mm, 5 stacked sheets
packed with wrapping paper
(2) Atmosphere and standing time:
~ 49C, 98~ RH, 7 days
~ 70C, 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.
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Table 1 Results of accelerated black
tarnish tests (Example 1)
Coating weight Evaluation of
Pretreatment (one side) black tarnish
Sample method resistance
Co and/ Cr
or Ni ~ 49C, ~ 70C
(mg/m2) (mg/m2) 7 days 3 days
a not treated 0 9 X X
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b Ni treatment 2.8 11 ~ O
c (acidic bath) 1.7 8 _
Co treatment
d bath) 2.1 9
_
Ni + Co Ni 1.5 9 ~ f~
e treatment Co 0.7
f Ni treatment 13 13
(acidic bath) 8.5 11
1 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
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1 in Example 1, but the specimens were as follows:
(i) Specimen
Size: 0.5 x lO0 x 200 mm (not chromated)
Amount of zinc plating: 120 g/m
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
: Sample Pretreatment (one side) black tarnish
method resistance
Co and/ Cr
or Ni ~ 49C ~ 70C
(mg/m2) (mg/m2) 7 days 3 days
a Not treated 0 13~ . . . X
b Ni treatment2.2 10 O O
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c (acidic bath) 2.1
Co -treatment
d (alkaline 1.6 ll
e treatment NCo 0 5 8
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A Ni treatment12.5 14 ~ O
(acidlc bath) 7.3 14
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