Note: Descriptions are shown in the official language in which they were submitted.
2668
,
Electrolytically Preplating Steel with Zinc-Nic~el Prior
to Plating With Zinc-Nickel
Technical Field of the Invention
_
This invention relates to a process for Zn-Ni-
a}loy-plated steel sheets which comprises first pre-plating
steel sheets with Zn-Ni alloy containing a higher amount of
Ni before effecting the principal Zn-Ni-alloy-electroplating
in order to imprc,ve the adhesion of said plated layer.
Background of the Invention
In recent years there has arisen a demand for Zn-
electroplated st~!el sheets with higher corrosion resistance
I especially from cnutomobile industry circles. In compliance
with this demand, improved Zn-Ni type alloy-plating of steel
sheets such as Zn-Ni-alloy plating, Zn-Ni-Co-alloy plating,
etc. has been de~eloped.
Although steel sheets with such Zn-Ni type alloy
electroplating are excellent in corrosion resistance in the
bare state, they are inferior in the susceptibility to the
phosphating (phosphate salt treatment) as the pretreatment
for electrodeposition coating (electrophoretic painting).
That is, uniform dense phosphate salt crystals are not
formed on the plated surface, and therefore adhesion of the
electrodeposited coating is not satisfactory.
~$
~L2~2~6l3
--2--
We previously developed improved Zn-Ni-alloy
electroplated steel sheets, said alloy containing a slight
amount of Ti, Co, etc~ in order to overcome the above-
mentioned problem, which is described in United States
Patent No. 4,610,937. Said process comprises electroplating
steel sheets in an acidic plating bath containing lO - 40
g~l (gram as atom per liter hath) zinc, 15 - 160 g/l nickel,
0.2 - lQ g/l titanium, O.l - 5 g/l cobalt and O.l - 5 g/l
aluminum or 0.2 - ~ g/l magnesium, the pH of which is
1.5 ~ 2.5.
The plated steel sheets obtained by this process
have remarkably improved adhesion of electrodeposited
coatings (electrophoretically painted layer) and are
excellent in corrosion resis1:ance in the bare state, since
the plated layer contains a slight amount of Ti which is
uniformly distribued in the plated layer and represents
nuclei for crystalization of phosphate salts in the
phosphating.
However, the adhesion of the plated layer to the
substrate steel sheet in the above-mentioned plated steel
sheets is not quite satisfactory. The plated layer on which
a thick electrodeposited coating is applied is liable to be
peeled off from the steel substrate by impulsive
deformation, sincs the electrodeposited coating is dense and
hard per se and is firmly bonded to the plated layer so that
the latter suffers strong compressive stress.
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For instance, the plated layer on which an electrodeposited
coating of more than 15 um applied is easily peeled off when
tested by an impulse tester, such as one marketed by "DuPont
Company''0 Thus, the above-mentioned improved Zn-Ni-alloy-
electroplated steel sheet is not quite satis~actory as a
substrate for electrodeposition coatingD
We proceeded with research ~or improving adheslon
of said plated Zn Ni alloy layer to the steel substrate and
have found that the adhesion can be improved by pre-
electroplating substrate steel sheets with a Zn-Ni alloy
using an electroplating bath of a speci~ic composition under
specific conditions before effecting said Zn-Ni
electroplating.
Disclosure of t e Invention
That is, this invention provides a process for
preparing Zn-Ni-alloy plated steel sheets with excellent
adhesion of the plated layer c:omprising ~re-plating a steel
sheet in an acidic plating bath contain:ing 7 - 38 g/l Zn and
41 -88 g/l Ni whereby the concentration ratio (Ni2+)/1(Zn2+)
+ (Ni2+)) is 0.70 - 0085 with an electric current densit~ of
2 - 20 ~/dm2 at a temperature between 55 and 80 C so that
the resulting Zn-Ni pre-plated layer contains 12 - 87 wt ~
Ni; and electroplating said pre-plated steel sheet in an
acidic electroplating bath containing 25 - 33 g/l Zn,
~1 ~ 88 g/l Ni, 0.2 - 10 g/l
,
~2~2~6~
titanium and 0.1 - 5 g/l aluminum or 0.2 - 4 g/l magnesiusT.,
the pH of which is 1.5 - 2.5.
In the improved Zn~Ni-alloy-plating process which
we previously developed, the composition of the
electroplated layer which gives products of most stable
quality essentially consists of 10 - 12 % by weight nickel,
0.005 - 1% by weight titanium, 0.01 - 0.5% by weight cobalt,
0.001 - 2% by weight aluminum and the balance zinc, or 8 -
16% by weight nickel, 0.005 - 1% by weight titanium, 0.05 -
0.5% by weight cobalt~ 0.001 - 1% by weight magnesium and .
the balance zinc.
. In accordance with this invention, the adhesion of
the st.eel substrate and the above-mentioned principal Zn-Ni-
alloy-plated layer is improved by pre-plating the substrate
with another Zn-Ni alloy.
The pre-electroplated layer must contain a higher
amount of Ni than the principal Zn-N.i-alloy-electroplc.ted
layerO According to our study, the pre-plated layer must
contain more than 12 wt % and not more than 87 wt % Ni.
.,~ ., ,
.c5
2~
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1 For the pre plating bath, a chloride bath, a sulfate
bath or a mixed chloride/sulfate bath can be employed.
The bath should contain 7 - 38 g/Q Zn ions and 41 - 88 g/~
Ni ions, whereby the concentration ratio [Ni+2]/([Zn2+] ~
~Ni2 ]) must be 0.70 - 0.85. By employment of such a bath
the Ni content of 12 wt % to 87 wt % is achieved.
The plating bath preferably contains 11 - 34 g/Q Zn and
62 - 72 g/Q Ni, more preferably 15 - 30 g/Q Zn and 85 -
70 g/Q Ni.
When electroplating is effected in a bath contain-
ing Zn2+ and Ni2+, adhesion between the preplated layer
and the steel substrate and adhesion between the pre-
plated layer and the principal plated layer are influenced
by electric current density, and smaller densities give
better adhesion and increase Ni content in the resulting
plated layer. Although it is preferred to carry out
the electroplating with lower current density in order to
improve adhesion, the Ni content in the pre-plated layer
exceeds 87 wt ~ when current density of less than 2 A/dm2
is employed.
In this condition, if crackin~ occurs in the principal
plated layer, corrosion of the principal plated layer is
promoted although it is temporarily protected. This is
probably because the corrosion electrolytic potential of
the pre-plated layer is higher than that of the principal
-- 5
1 plated layer. As the result, adhesion between the principal
plated layer and the electrodeposited coating formed thereon
gradually is deteriorated. Therefore, electrop].ating must be
carried out with a current density of not less than 2 A/dm2.
On the other hand, when current density is .in excess of
20 A/dm2, the Ni content of the pre-plated layer becomes
close to that of the principal plated layer~ This means
that adhesion of the pre-plated layer to the steel sub-
strate is weakened to the same level as that of said
Zn-Ni-alloy plating. Therefore, adhesion is improved by
carrying out the pre-electroplatina with a current
density of between 2 and 20 A/dm2.
If the Ni content of the pre-plated layer is lower
than or of the same level as that of the principal
plated layer, adhesion of the principal plated layer to
the steel substrate is not sufficient, and the pre-plated
layer is preferentially aorroded when cracking occurs in
the principal plated layer and thus the plated layers are
peeled off. Therefore, the Ni content of the pre-plated
layer must be higher than that of the principal plated
layer, that is, higher than 12 wt % and not higher than
87 wt ~ when the pre-plating is carried out under the above-
mentioned currency condition. Thus, the corrosion potential
of the pre-plated layer is higher than the principal plated
layer so that the difference in the corrosion potential
2qS6~3
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1 from that of the principal plated layer is not too large,
and corrosion between the steel substrate and the principal
plated layer is inhibited and good adhesion and endurance
of coatings are brought about. The Ni content of the pre-
plated layer more preferable for inhibiting corrosionbetween the steel substrate and the principal plated layer
is 17 - 42 wt %, that is, higher than that of the principal
plated layer by 5 - 30 wt %.
In order to make the Ni content of the pre-plated
layer more than 12 wt % and not less than 87 wt %, further
the pre-electroplating should preferably be carried out
at a temperature between 55 and 80C. If the concentra-
tion ratio [Ni ~ Zn ~ + ~Ni ~) is less than 0.70 and
the bath temperature is lower than 55C, the Ni content
in the pre-plated layer becomes less than 12 wt ~. If the
ratio is in excess of 0.85, the Ni content of the resulting
plated layer fluctuates and pre-plated layers of uniform
quality cannot be produced. If the bath temperature is
in excess of 80C, there is the disadvantage of requiring a
special material for constructing vessels and plenty of
energy for warming the bath.
In order to achieve the preferred Ni con-tent of 17 -
42 wt ~ in the pre-plated layer as mentioned above, the
electroplating should be conducted in a bath of which
25 said concentration ratio is 0.70 - 0.77 at 65 - 80C or
2~
the ratio is 0O77 - 0.80 at 55 ~ 65C.
The thickness of the pre-plated layer should be not
less than 0.05 umO With pre-plating thinner than this, no
improvement in corrosion resistance is expected. If the
thickness is in excess of 1 um, it will invite cracking in
the pre-plating layer when the sheet is subjected to
deformation, which is undesirable for corrosion
prevention. The.refore the thickness of the pre-plated layer
should be 0.05 ~ 1 um.
The principal plated layer is formed by
elecl:roplating steel sheets with an acidic p:lating bath
containi.ng 25 - 33 g/l ~gram as atom per liter bath) zinc 41
- 88 g/l nickel, 0.2 - 10 g/l titanium, 0.1 - 5 g/l cobalt
and 0.1 - 5 g/l aluminum or 0.2 - 4 g/l magnesium, the pH of
which is 1.5 - 2.5O
As has been explained in the above, this lnvention
improves adhesion of the plated Zn-Ni-alloy layer by
applying a pre-plated layer in which the Ni content is
higher than l:hat of the principal plated layer with
relatively low current density. Although the reason why
electroplating with .such low current density brings about
better adhesion is not quite clearly understood, .it is
surmized that the pre-plated layer of which the Ni content
is higher than that of the principal plated layer causes a
crystall growth different from that of the principal plated
layer which contributes to the better adherence of the
plated layers.
.
9 -
It has been confirmed that adhesion of the plated
layer is not impaired if a slight amount of another metal
such as Co, Cr, Ti Fe, etc. is codeposi-ted in the pre-plated
layer when a pre-plated layer contains more than 12 wt % and
not more than 87 wt % Ni.
D ailed Description of the Embodiments
Examples
Cold-rolled steel sheets 0.8 mm in thickness were
degreased and pickled by the conventional method, and were
pre-plated with a Zn-Ni alloy and thereaf-ter was subjected
to the Zn-Ni-alloy-pla-ting using baths as indica-ted in Table
1.
Cation
The plated sheets were pre-treated wi-th a
commercially available phosphating solution (DT-3030*) and
sub~ected to electrodeposition coating (20 um thick coating
using a cation type electrodeposition solution (Power-Top
U-30**) at 200V for 3 minutes, the coa-ted sheets were
*Trade Mark of Nippon Parkerizing Corporation, for
phosphating solutions
** Trade Mark of Nippon Paint Co. for cation type
electrodeposition solutions
,~ s. .
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1 baked at 180C for 20 minutes. Adhesion between the steel
substrate and the preplated layer and adhesion
bQtween the principal plated layer and the coated film
were checked. The tests were carried out using a Du Pont
impact tester (1 kg weight, 50 cm dropping distance).
An adhesive tape was applied to the deformed part and
the tape was forcibly detached, and peel-off of the
coated layer or the coated layer with plated layer was
observed. The test of adhesion of coating was carried
out with samples as coated (primary adherence test) and
samples after having been soaked in water of 40C for
240 hours (secondary adhesion test). The results are
shown in Table 2 together with compositions of the plated
layers. The evaluation standard is shown in Table 3.
The pre-plating baths of this invention and
comparative baths contai`ned 85 - 70 gNi/Q nickel sulfate,
15 -30g Zn/Q zinc sulfate and 36 g~Q sodium sulfate~ and
the pH of the baths was 2. Within the above ranges, the
[Ni ~/([Zn ] ~ ~Ni ~) ratio was varied.
The principal plating baths contained 49 gNi/Q nickel
sulfate, 25 gZn/Q zinc sulfate, 72 g~Q sodium sulfate,
1 gCo/Q cobalt sulfate, 5O2 g~l/Q alumi~num sulfate and
3 gTi/Q. The pH of the baths was 2.
It is apparent from Table 2 that coated steel sheets
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6~
1 prepared using the plated steel sheets with pre-plating
in accordance with this invention are remarkably superior
to the above-mentioned Zn-Ni plated steel sheets in both
the primary adhesion test and the secondary adhesion
test.
As has been explained above, the adhesion of Zn Ni-
alloy electroplated layer is remarkably improved by pre-
electroplating another Zn-Ni alloy in accordance with
this invention. The plated layer is fully resistant to
impulsive deformation with electrodeposition coating
thereon.
11 -
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-- 13 --
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- 14 -
Table 3
Rating Area of Peeling
____
No peeling
4.5 less than 1.0 %
~ 1 - 10 %
3 11 - 30 %
2 31 - 70 %
1 71 - 99 ~
_ ___ Whole area .
14 -
