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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved zinc phosphating method. More
particularly, it relates to an improved zinc phosphating method for
enhancing the corrosion resistance of a metal substrate at the edge
portion.
2. Statement of Related Art
In recent years, the corrosion resistance of metal substrates has been
remarkably improved by the development of zinc phosphating and the
introduction of cationic electrodeposition. In a coating film formed on
the surface of a metal substrate, however, blisters are frequently
produced on the edge portion and develop within a short period of time.
Prevention of such blister production has not been successful up to the
present time.
As the result of the microscopic observation of the surface of a metal
substrate which was zinc phosphated and then electrodeposition coated, it
has been found that at the outer edge position, a zinc phosphate coating
is present but the electrodeposition coating is almost nonexistent due to
flowing on baking. In general, a zinc phosphate coating can contribute to
enhancement of the corrosion resistance but does not have satisfactory
corrosion resistance by itself, because of its microporosity.
BRIEF DESCRIPTION OF THE DRAWINGS
Features of the invention are described in the following with reference to
the accompanying drawings wherein:
FIG. 1 shows a scanning electron microscope photograph (1500 times) of the
electric zinc phosphate coating film formed by the present invention.
FIG. 2 shows a scanning electron microscope photograph (1500 times) of a
zinc phosphate coating film obtained by conventional dipping treatment.
FIG. 3 shows, by a scanning electron microscope photograph (1500) times
that where the temperature of the acidic solution is too low, the electric
zinc phosphate coating film is not formed.
FIG. 4 shows, by a scanning electron microscope photograph (1500 times)
that when the temperature of the acidic solution is too high, the
resulting film does not contribute to improving the adhesiveness and
corrosion resistance of the coating film.
DESCRIPTION OF THE INVENTION
On the basis of the above finding, an extensive study has been made to
improve the corrosion resistance at the edge portion of a metal substrate,
and it has been found that application of direct current to a metal
surface previously zinc phosphated by a conventional procedure as a
negative electrode, in an acidic solution having a certain specific
composition, results in formation of a corrosion resistant film at the
edge portion. Distinguishable from a conventional zinc phosphate coating
film having no corrosion resistance and which is grayish white according
to this invention a corrosion resistant film is formed under the
application of direct current, and is milky white. This corrosion
resistant film will be hereinafter refered to as an "electric zinc
phosphate coating film".
According to the present invention, there is provided an improved zinc
phosphating method which comprises (B) applying direct current to a metal
surface which was (A) previously zinc phosphated as a negative electrode
in a solution comprising zinc ion in a concentration of 2 to 3 grams per
liter, phosphate ion in a concentration of 8 to 14 grams per liter and
chloride ion in a concentration of 3 to 6 grams per liter. As the result
of steps (A) and (B), an electric zinc phosphate coating film having
corrosion resistance is formed.
It is known to form a coating film on a metal surface dipped in a zinc
phosphate solution by applying electric current thereto (see Published
Japanese Patent Applications 46220/74 and 41930/80). In the foregoing
conventional method, direct or alternating current of low value is applied
to a metal surface to form a uniform coating of zinc phosphate. However,
enhancement of the corrosion resistance by the formation of such zinc
phosphate coating film can be observed only at the flat portion of the
coated substrate, and improvement in the corrosion resistance is never
observed at the edge portion.
The difference in corrosion resistance at the edge portion between the
electric phosphate zinc coating film provided by the present invention and
the zinc phosphate coating film provided by the conventional method, as
shown in the foregoing references, can be explained as follows: the zinc
phosphate coating film obtained by conventional dipping treatment is a
grayish white film, (cf. FIG. 2 of the accompanying drawing which shows
the scanning electron microscopic photograph (1,500 times)), while the
electric zinc phosphate coating film formed by the present invention is a
milky white film (cf. FIG. 1 of the accompanying drawing which shows the
scanning electron microscopic photograph (1,500 times)). On the
fluoroescent X-ray analysis, the former affords a molar proportion of P/Zn
of 1/1, while the latter gives a molar proportion of P/Zn of 1/8. Thus,
the present invention provides a zinc phosphate coating film and a coating
film comprising zinc in a greater proporiton, so that corrosion resistance
is greatly increased. As the result, corrosion resistance of a metal
substrate, such as an automobile body, is much enhanced even at the edge
portion where the coating was applied but later flows away on baking.
Formation of blisters in the coating film around the edge portion is thus
satisfactorily prevented. Still, a purely zinc plated metal surface is
poor in adhesive property with a coating film even at the flat surface,
and therefore the corrosion resistance can not be enhanced. This is also
true in the case where cationic electrodeposition is applied.
In the method of the invention, a metal substrate is previously zinc
phosphated. This zinc phosphating may be carried out by conventional
procedures. For a metal substrate such as an automobile body, cationic
electrodeposition is applied later, and in such case, dipping zinc
phosphating treatment may be preferably applied in a per se conventional
manner. A typical example of the treating solution for zinc phosphating
may be an aqueous solution comprising the following materials: Zn ion, 0.5
to 2 g/l; PO.sub.4 ion, 10 to 30 g/l; Mn ion, 0 to 2 g/l; Ni ion, 0 to 2
grams/l; NO.sub.3 ion, 0 to 10 g/l; ClO.sub.3 ion, 0 to 1 g/l; NO.sub.2
ion, 0.01 to 0.1 g/l; and F ion, 0 to 3 g/l. The temperature for treatment
may be usually from 30.degree. to 70.degree. C., and the time for
treatment may be normally from 15 to 120 seconds. Spraying of the treating
solution onto the metal surface immediately after it is taken from the
treating solution for dipping is favorable for cationic electrodeposiiton
as carried out later. The metal surface after the treatment may be, with
or without first washing with water, sujected to electric zinc
phosphating.
The metal substrate, zinc phosphated as described above is then subjected
to electric zinc phosphating in an acidic solution. The acidic solution
may be an aqueous solution comprising Zn ion in a concentration of 2 to 3
g/l, PO.sub.4 ion in a concentration of 8 to 14 g/l and Cl ion in a
concentration of 3 to 6 g/l. As the source for Zn ion, there may be used,
for example, zinc oxide, zinc carbonate, zinc nitrate, etc. When the
amount of zinc ion is too small, the coating film is not formed. When the
amount is too large, a coating film, as formed, is a zinc phosphate
coating film, not an electric zinc phosphate coating film as in our
invention. Examples of the source for PO.sub.4 ion are phosphoric acid,
sodium phosphate, zinc phosphate, nickel phosphate, etc. When the amount
of PO.sub.4 ion is too small, no coating film is formed. When the amount
is too large, no advantage is perceived and the process is economically
unfavorable. As the source of Cl ion, there may be used sodium chloride,
potassium chloride, ammonium chloride, etc. When the amount of Cl ion is
too small, the electric zinc phosphate coating film of our invention is
not formed; rather a zinc phosphate coating film is formed. When the
amount is too great, a coating film is not formed. For the practical use
of the acidic solution, it is preferred that the total acidity and the
free acidity are respectively adjusted to a point of 10 to 15 and a point
of 0.8 to 1.2.
For the electric phosphate coating, the metal surface, as zinc phosphated,
is dipped in the acidic solution as the negative electrode, and direct
current is applied thereto at a liquid temperature of 20.degree. to
40.degree. C. under a condition of 5 to 15 A/dm.sup.2 (metal surface) for
a period of 30 seconds to 3 minutes. The metal surface may be made of
iron, zinc, their alloy or the like. For the positive electrode, there may
be used stainless steel (e.g. SUS 304,316), carbon or the like. When the
liquid temperature is too low, the electric zinc phosphate coating film is
not formed (cf. FIG. 3 of the accompanying drawing which shows the
scanning electron microscopic photograph (1,500 times)). When the liquid
temperature is too high, the zinc phosphate coating film, as newly formed
and also as previously formed, are first dissolved and then redeposited.
The resulting coating film (cf. FIG. 4 of the accompanying drawing which
shows the scanning electon microscopic photograph (1,500 times)) does not
contribute to improving the adhesiveness and corrosion resistance of the
coating film. In case of the electric current being low, the electric zinc
phosphate coating film is not formed. In case of the elecric current being
high, the once formed coating film is redissolved so that the coating film
is inferior in its adhesiveness and corrosion resistance. Too short
application time does not produce the electric zinc coating film, while
too long application time causes redissolving of the once formed coating
film.
The thus treated metal substrate, i.e. the metal substrate after electric
zinc phosphating, is usually then subjected to cationic electrodeposition
coating, which may be carried out by a per se conventional procedure. The
thus formed electrodeposition coating film imparts good corrosion
resistance to and high adhesion on the metal substrate.
Practical and presently preferred embodiments of the invention are
illustratively shown in the following Examples.
EXAMPLE 1
A cleaned iron steel plate was punched to make a hole of 10 mm in diameter
having a burr of about 0.1 mm in height around the hole. This plate was
dipped in an aqueous zinc phosphate solution (comprising 1 g/l of Zn ion,
15 g/l of PO.sub.4 ion, 0.6 g/l of Ni ion, 3 g/l of NO.sub.3 ion and 0.5
g/l of ClO.sub.3 ion; total acidity, 18 point; free acidity, 0.9 point,
tonar value, 2 point) and treated at 50.degree. C. for 2 minutes. The
plate was washed with water and dipped in an aqueous acidic solution
(comprising 2.4 g/l of Zn ion, 11 g/l of PO.sub.4 ion and 4.5 g/l of Cl
ion; adjusted with NaOH to a total acidity of 12 point and a free acidity
of 1 point). Using the plate as the negative electrode and a carbon
electrode as the positive electrode, a direct current of 10 A/dm.sup.2 was
applied at a liquid temperature of 30.degree. C. for 2 minutes to make an
electric zinc phosphate coating film. The plate was then washed with tap
water and deionized water, followed by drying.
Onto the above treated plate, an amine-modified epoxy resin-containing
cationic electrode position coating composition comprising a blocked
isocyanate compound as a crosslinking agent ("Powertop U-30 black"
manufactured by Nippon Paint Co., Ltd.) was applied to make a coating film
of 20 microns in thickness, followed by baking at 180.degree. for 30
minutes.
The plate before cationic electrode position coating was subjected to salt
water spray test according to the method as described in JIS (JAPAN
INDUSTRIAL STANDARD) Z-2371, while the plate after cationic electrode
position coating was subjected to a corrosion test comprising 100 cycles,
of which each cycle consists of salt water spray test (JIS, Z-2371
35.degree. C., 2 hours), dry test (60.degree. C., 2 hours) and wet test
(50.degree. C., relative humidity of 95%, 4 hours). Then, the blister
width of the coating film from said burr was measured. The results are
shown in Table 1.
COMPARATIVE EXAMPLE 1
The same treatment as in Example 1 was repeated but omitting the
application of direct current in the acidic solution. The results are
shown in Table 1.
TABLE 1
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Example 1 Comparative Example 1
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Amount in the
zinc phosphate
coating film
Zn 2.56 g/m.sup.2
0.57 g/m.sup.2
P 0.16 g/m.sup.2
0.27 g/m.sup.2
Appearance of
Milky white Grayish white
the zinc phos-
even, dense even, dense
phate coat-
ing film
Plate before
Red rust not
Red rust
electrodeposi-
produced produced within
tion coating
within 24 2 hours
(salt spray
hours
test)
Plate after
Less than 8 mm
electrodeposi-
1 mm
tion coating
(corrosion
test)
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PREFERRED INVENTION PARAMTERS
(a) zinc ion--a range of 2 to 3 grams/liter (g/l) is both completely
operable and preferred.
(b) phosphate ion--a range of 8 to 14 g/l is completely operable, the range
of 10 to 12 g/l is preferred.
(c) chloride ion--a range of 3 to 6 g/l is completely operable, the range
of 4 to 5 g/l is preferred.
(d) electric current--a range of 5 to 15 Amperes/square decimeter
(A/dm.sup.2) is completely operable in all instances, the range of 9 to 11
A/dm.sup.2 is preferred.
(e) time of direct current application--a time of 30 seconds to 3 minutes
is completely operable.
(f) temperature of electrolyte solution--a temperature of 15.degree. to
50.degree. C. is completely operable, the range of 20.degree. to
40.degree. C. is preferred and 25.degree. to 35.degree. C. is most
preferred. It is important to maintain the above temperature ranges. The
application of electric current within the parameters of this invention
will generally result in an increase in temperature. If necessary, any
known cooling means may be applied to the electrolyte solution to keep its
temperature within a given range. Such means may include cooling coils,
film evaporation, and the like.
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