Multilayered coated corrosion resistant steel material

Multilayered coated corrosion resistant steel material having a galvanized layer on one surface thereof, an electro-deposited layer on said galvanized layer formed from a zinc-based alloy containing nickel and having a thickness of one to six microns; and a resin layer on said electrodeposited layer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a steel material having on its surface a 
multilayer protective coating which makes it resistance to wear and 
corrosion. 
2. Description of the Prior Art 
A known multilayer-coated corrosion-resistant steel material is shown by 
way of example in FIG. 3. It comprises, for example, a steel sheet 11 
having a galvanised layer 12 formed on its surface 11', a chromate film 13 
on the galvanised layer 12 and a resin layer 14 on the chromate film 13. 
Although zinc is often used to provide a protective coating on steel, the 
zinc layer must be of considerable thickness in order to protect the steel 
from corrosion effectively, since the protection of steel by means of a 
galvanised layer results from sacrificial corrosion of the zinc. 
The galvanised layer 12 must have a thickness of at least about 30 microns 
in view of the severe natural conditions to which the coated material will 
be exposed when it is, for example, to be used for making motor vehicle 
parts. The formation of such a thick layer is time-consuming which brings 
about a drastic reduction in productivity. As the galvanised layer 12 is 
so thick, it tends to crack or peel away when the coated material is 
pressed or bent into a desired shape. Moreover, the resin layer 14 tends 
to have pinholes and is also liable to crack when the coated material is 
pressed or bent. The corrosion product of the zinc tends to accumulate 
quickly in the pinholes or cracks of the resin layer 14 despite the 
presence of the chromate film 13. As a result, the resin layer 14 tends to 
peel away as the chromate film 13 loses it effectiveness, particularly 
when the coated material is used to make, for example, motor vehicle parts 
and is exposed to highly corrosive conditions including large temperature 
variations. 
SUMMARY OF THE INVENTION 
Under these circumstances, it is an object of this invention to provide an 
improved multilayer coated steel material which can maintain a high degree 
of corrosion resistance for a long period of time even under highly 
corrosive conditions. 
This invention provides a multilayer coated steel material which comprises 
a steel base, a galvanized layer formed on one surface of the steel base, 
an electrodeposited layer of a zinc-based alloy containing nickel formed 
on the galvanized layer and having a thickness of one to six microns and a 
synthetic resin layer formed on the electroplated layer. 
The multilayer coated material may further include a chromate film between 
the electroplated layer and the resin layer. 
The steel base may, for example, be in the form of a sheet, tube, bar or 
wire. 
The provision of the electrodeposited layer makes it possible to reduce the 
thickness of the galvanized layer drastically without bringing about any 
increase in the overall thickness of the coating. The reduced thickness of 
the galvanized layer enables it to be formed more quickly. The reduced 
thickness of the galvanized layer and the ductability of the electroplated 
layer which nickel imparts to it ensure that no cracking or peeling of the 
galvanized or electroplated layer occurs when the coated material is 
shaped. There is no undesirably rapid accumulation of the corrosion 
product of the zinc in the pinholes of the resin layer or in any portions 
of the resin layer which have been damaged during the mechanical shaping 
of the coated material. There is no serious peeling of the resin layer. 
The shortening of the galvanizing time means a high rate of productivity. 
The improved resistance of the coating to cracking or peeling enable the 
coated material to be mechanically processed with only a small amount of 
wastage. 
The coated material of this invention can maintain a high degree of 
corrosion resistance for a long period of time even in a highly-corrosive 
environment in which great temperature variations occur. Therefore it is 
very useful for making, for example, structural parts for motor vehicles.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, the coated material embodying this invention comprises 
a steel sheet 1 having a surface 1' which has been subjected to 
preliminary treatment including degreasing and rust removal. The surface 
1' carries thereon a galvanized layer 2 having a thickness of, say, 8 to 
20 microns. The galvanized layer 2 has a surface 2' which carries thereon 
an electrodeposited layer 3 of a zinc-based alloy containing 5 to 15% by 
weight of nickel. The layer 3 has a thickness of one to six microns, 
depending on its nickel content. If its thickness is smaller than one 
micro, it tends to display unsatisfactory corrosion resistance. If its 
thickness is greater than six microns, it lowers the workability of the 
coated material. The layer 3 has a surface 3' which carries thereon a 
layer 4 of a synthetic resin having a high degree of wear resistance. The 
layer 4 may, for example, be formed from a polyolefin resin, a 
chlorine-containing resin, a fluorine-containing resin, an epoxy resin or 
a polyamide resin. 
The coated material shown in FIG. 2 is in the shape of a steel tube and has 
a multilayer coating on its outer wall surface. The coating is essentially 
identical to that which has hereinabove been described with reference to 
FIG. 1, except that the material of FIG. 2 further includes a chromate 
film 5 between the electroplated layer 3 and the resin layer 4. 
The surface 1' may either be a plain steel surface or a surface carrying a 
thin film of copper. 
The invention will now be described more specifically with reference to a 
number of examples. 
EXAMPLE 1 
A tube of ISO 2604(2-75) steel having an outside diameter of 8.0 mm, a wall 
thickness of 0.7 mm and a length of 300 mm was subjected to degreasing and 
rust removal treatment by a customary method. 
The tube was arranged as a cathode, and a zinc plate as an anode, in an 
alkaline galvanizing bath containing mainly sodium cyanide and sodium 
hydroxide. While the bath was maintained at ambient temperature, an 
electric current was passed through it so that the cathode would have a 
current density of 3 A/dm.sup.2, and as a result a galvanized zinc layer 
having a thickness of about 13 microns was formed on the surface of the 
tube. 
The tube was immersed in a dilute aqueous solution of hydrochloric acid and 
was thereafter washed with water. The tube was arranged as a cathode in an 
electroplating bath containing mainly zinc chloride, nickel chloride, 
ammonium chloride and boric acid and at a pH of 5.8, whilst a plate of a 
zinc-based alloy containing nickel was used as an anode. While the bath 
was maintained at a temperature of 40.degree. C., an electric current was 
passed through at so that the cathode would have a current density of 2 
A/dm.sup.2, and as a result an electrodeposited layer of the alloy having 
a thickness of about five microns was formed on the galvanised layer. 
An epoxy resin primer was sprayed onto the electroplated layer and backed 
on at a temperature of 200.degree. C. After it had been cooled, a 
vinylidene fluoride resin was sprayed onto the primer and backed on at a 
temperature of 250.degree. C., as a result of which a resin layer having a 
thickness of about 35 microns was formed on the electroplated layer. 
EXAMPLE 2 
A tube of ISO 2604(2-75) steel having an outside diameter of 8.0 mm, a wall 
thickness of 0.7 mm and a length of 300 mm was subjected to degreasing and 
rust removal treatment by a conventional method. It had an outer surface 
coated with a film of copper having a thickness of three microns. 
The tube was arranged as a cathode in a galvanizing bath containing mainly 
zinc sulfate, sodium sulfate and aluminium chloride and at temperature of 
50.degree. C. An electric current was passed through the bath such that 
the cathode would have a current density of 20 A/dm.sup.2. A galvanized 
layer having a thickness of about 15 microns was formed on the surface of 
the tube. 
The electropating procedure of Example 1 was repeated for forming an 
electrodeposited layer having a thickness of about four microns on the 
galvanized layer. 
A dispersion containing vinyl fluoride was sprayed onto the electroplated 
layer and baked on at a temperature of 250.degree. C., thus forming a 
resin layer having a thickness of about 30 microns. 
COMATIVE EXAMPLE 1 
The procedure of Example 1 was repeated thus forming a galvanized layer 
having a thickness of about 20 microns on a tube of the same steel having 
the same dimensions. A conventional chromate film was formed on the 
galvanised layer. The procedure of Example 1 was repeated for forming on 
the chromate film a resin layer composed of vinylidene fluoride and having 
a thickness of about 35 microns. 
Repeated cycles of corrosion tests were conducted on the coated products of 
Examples 1 and 2 and Comparative Example 1. Each cycle consisted of four 
hours of an ISO 3768 test (neutral salt spray test for metallic coatings), 
two hours of drying at a temperature of 60.degree. C. and two hours of a 
wetting test at a temperature of 50.degree. C. and a humidity of at least 
95%. The test results are shown in the following table. 
TABLE 1 
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Cycles 100 150 
200 250 300 350 400 
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Example 1 
-- -- White 
-- Red rust 
Streaks 
More streaks 
rust of red 
of red 
rust 
rust 
Example 2 
-- -- White 
Red rust 
-- Streaks 
More streaks 
rust of red 
of red 
rust 
rust 
Comparative 
White 
-- Red Streaks 
More 
Example 1 
rust rust 
of red 
streaks 
rust of red 
rust 
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