Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED GALVANIZING PROCEDURE AND GALVANIZED
PRODUCT THEREOF
The present invention relates to the galvanizing
of ferrous surfaces.
Hot-dip galvanizing of continuous coils of steel
strip in a molten zinc bath has for years been a
standard method of providing steel with a surface
having both a pleasing appearance and a
corrosion-resistant protective layer. The steel strip
is first treated to remove oxide from the surface, so
as to enable wetting of the surface by zinc to occur in
the hot dip bath. Oxide removal has conventionally
been effected by using elevated temperature reducing
gas atmospheres, the so-called "hot" lines, or by using
chemical treatment followed by protection with fluxes,
usually zinc ammonium chloride, the so-called "cold"
lines.
The conventional zinc coatings do not possess
adequate corrosion resistance for some applications and
attempts have been made to provide a surface having
enhanced corrosion resistance. One such attempt
resides in including significant amounts of aluminum,
preferably along with small amounts of silicon, in the
hot dip zinc bath. This procedure is the subject of a
number of patents, including Canadian Patents Nos.
763,386, 802,844 and 899,729. As is set forth therein,
the coating bath comprises about 25 to about 70% by
weight of aluminum, greater than about 0.5% by weight
of the aluminum content of silicon, less than about
0.6% by weight of lead and the balance zinc.
Although potentially leading to improved corrosion
resistance, in practice the prior art process has not
provided the expected results. In accelerated and
atmospheric corrosion tests, there have been observed
rust bleed onto the sheet surface and pitting of the
sheet surface of commercial samples of products
produced by the prior art process. Further, although
Canadian Patent No. 899,729 emphasizes the necessity
for a thin uniform interfacial alloy layer between the
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ferrous substrate and the zinc-aluminum coating, a
variable thickness interfacial alloy was observed in
the above-noted commercial samples, varying from O to
about 2 microns in thickness.
In addition to the observed corrosion problems, at
the present time, the prior art process is limited to
the treatment of steel strip on hot galvanizing lines,
since the fluxes used on cold lines are unable to
remove aluminum oxides, so that the aluminum component
of the hot dip bath does not adhere properly to the
steel surface. Further, the prior art process ls
unsuitable for most wire coating operations which are
almost exclusively galvanized by cold line techniques
using fluxes.
It has now been surprisingly found that the
application of a metallic precoat to the ferrous alloy
surface prior to hot dip galvanizing in a molten
zinc-aluminum bath leads to the provision of a
galvanized surface of significantly-improved corrosion
2~ resistance and which can be applied using cold-line
techniques.
Accordingly, in an aspect of the present
invention, there is provided an improvement in a method
of coating a ferrous surface by contact with a molten
bath which contains about 25 to about 70% by weight of
aluminum, silicon in an amount of at least about 0.5%
by weight of the aluminum, and the balance by weight of
zinc, except for any minor metal constituents. The
improvement comprises forming an adherent thin coating
of a metal on the ferrous surface prior to contact with
the molten bath, the metal being one which is wettable
by and reacts with the components of the molten bath
and which is selected from the group consisting of
nickel, bismuth, antimony, manganese, cobalt, tin,
zinc, cadmium and copper.
The microstructure of the product which is
obtained using the method of the present invention is
significantly different from that obtained in the prior
art. The microstructure comprises a ferrous alloy
substrate layer, a substantially uniform thickness
intermetallic alloy layer, usually having a thickness
of about 2 to about 4 microns, on the substrate layer
and comprising an intermetallic alloy of iron, zinc,
aluminum, silicon and the precoat metal, preferably
nickel, and an outer zinc-aluminum layer comprising an
aluminum-rich phase immediately adjacent the
intermetallic alloy layer and a continuous aluminum
phase having zinc-rich aggregates and precoat
metal-rich aggregates dispersed therethrough.
Accordingly, in another aspect of the invention,
there is provided a ferrous substrate having a ductile,
adherent, corrosion resistant coating metallurgically
bonded thereto through a continuous uniform thickness
in~ermetallic alloy layer comprising iron, aluminum,
zinc, silicon and an interalloying metal which is
selected from the group consisting of nickel, bismuth,
antimony, manganese, cobalt, tin, zinc, cadmium and
copper, the coating comprising about 25 to about 70% by
weight of aluminum, silicon in an amount of at least
about 0.5~ by weight of the aluminum and the balance by
weight of zinc except for minor metal constituents and
having a region rich in aluminum immediately adjacent
the intermetallic alloy layer and aggregates of zinc
and aggregates of interalloying metal dispersed in a
continuous phase of aluminum.
The process of the invention may be effected in a
continuous on-line manner and also is applicable to the
galvanizing of a wide variety of ferrous alloy
products, such as wire, pipe and preformed structural
items, which have not heretofore been susceptible of
galvanizing treatment by zinc-aluminum baths.
Accordingly, in a further embodiment of the
invention, there is provided a continuous method for
the production of galvanized ferrous alloy strip, which
comprises feeding ferrous alloy strip from a source
thereof; subjecting the surfaces of the strip to
cleaning operations to free the surfaces from
contaminants; forming a thin adherent coating of a
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metal on the cleaned surfaces of the ferrous alloy
strip, the metal being one which may be rapidly applied
to the surfaces, is capable of being rapidly wetted by
and reacting with a zinc-aluminum alloy galvanizing
5 bath, prevents iron oxidation, and its oxide is capable
of being readily reduced by contact with the
zinc-aluminum alloy galvanizing bath, the metal being
selected from the group consisting of nickel, bismuth,
antimony, manganese, cobalt, tin, zinc, cadmium and
copper; preheating the coated ferrous alloy strip to a
temperature of about 400 to about 700F; passing the
preheated coated ferrous alloy strip into contact with
a zinc-aluminum alloy galvanizing bath having a
temperature of about 1150 to about 1200F to effect
reaction and coating formation thereon, the galvanizing
bath containing from about 25 to about 70~ by weight of
aluminum, silicon in an amount of at least about 0.5~
by weight of the aluminum, and the balance by weight of
zinc, except for minor metal constituents; controlling
the thickness of the zinc-aluminum coating formed on
the ferrous alloy strip; and cooling the zinc-aluminum
coated strip at a rate of about 50 to 75F/sec. at
least until the temperature is below about 700F.
Precoating of the ferrous alloy surface in this
invention may be effected in any convenient manner,
preferably by plating, most preferably by
electroplating. The precoat metal may be any
convenient metal which will be wetted by and react
rapidly with the zinc-aluminum alloy melt, will prevent
oxidation of the metal surface after it has been
cleaned and its oxide is readily reduced by contact
with the zinc-aluminum melt.
The invention is described further, by way of
illustration, with reference to the accompanying
drawings, in which:
Figure 1 is a schematic flow sheet of a continuous
galvanizing line modified in accordance with one
embodiment of the invention;
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Figure 2 is a photomicrograph of a steel sheet
coated in accordance with the present invention; and
Figures 3A to 3C are photomicrographs of a steel
sheet coated in accordance with the invention at
5 various stages of a Kesternich corrosion test(DIN
50018).
Referring to Figure 1, in a hot dip galvanizing
line 10, steel strip 12 to be treated is unrolled from
a coil 14 and first passes through a cleaning tank 16
which contains an alkaline solution to remove dirt from
the surfaces of the strip 12. After a cold water rinse
in a rinser 18, the steel strip 12 then passes into a
pickling tank 20. The pickling tank contains acid to
remove oxides from the steel strip surfaces. In
conventional cold line operations hydrochloric acid is
usually used, but in this invention sulfuric acid
usually is used, so as to provide common anionic
species with the anion present in the plating bath,
described below. The pickled strip is rinsed in a
rinser 22.
In conventional coid line operation, the steel
strip 12, now free from surface contaminants, is
provided with a flux coating. In accordance with the
present invention, the fluxing step is omitted and,
instead, the cleaned steel strip 12 passes into a
plating tank 24, wherein the surfaces of the steel
strip have a thin precoat metal layer applied thereto.
Preferably, the coating is applied by electroplating,
since this technique rapidly and efficiently forms a
thin adherent metal coating on the surfaces of the
steel strip. An alternative process which may be
employed is by electroless plating.
In addition, since the precoating preferably is
effected in line with the galvanizing, as illustrated,
the metal must be such that it can be applied rapidly
in a continuous mode. It is possible to effect the
precoating as a separate operation but this procedure
;~ is less preferred.
Metals which are used to provide the precoat are
nickel, bismuth, antimony, manganese, cobalt, tin,
zinc, cadmium and copper, with nickel and tin being
preferred, nickel being most preferred. Electroplating
5 of the steel strip using these metals can be
accomplished in less than about 5 seconds, typically
less than about 1 second using conventional
electroplating current densities of about 200 to about
300 amps/sq.ft. Coating thicknesses of at least about
0.05 microns are obtained by the electroplating
treatment, the actual thickness depending on the
coating conditions.
The electroplated steel strip resulting from the
plating tank 24 is subjected to a cold water rinse in
rinser 26 before passing to a preheater 28 wherein the
temperature of the steel strip 12 is raised, usually to
about 400 to about 700F. The preheater 28 may be
omitted, if desired, although it is preferred to
utilize the same so as to decrease the thermal load on
the galvanizing tank. When the preheating is effected,
the metal chosen for the precoating must be one which
will not reflow at the preheater temperature.
The preheated steel strip next enters a
galvanizing tank 30 containing a molten galvanizing
bath which essentially contains zinc, aluminum and
silicon. The aluminum content of the tank may vary
from about 25 to about 70~ by weight while the silicon
content is greater than about 0.5% by weight, usually
up to about 3% by weight, with the balance being zinc,
with the exception of minor metal constituents. It is
preferred that the lead content of the bath not exceed
about 0.6~ by weight. One particular bath composition
which has been utilized consists of about 55~ by weight
of aluminum, 43.4% by weight of zinc and about 1.6~ by
weight of silicon.
The temperature of the galvanizing bath in the
tank 30 is considerably above conventional pure zinc
galvanizing temperatures (about 850F) and usually is
about 1150 to about 1200F, preferably about 1160 to
about 1180~F. These temperatures are higher than
normally used in the prior art process using a
æinc-aluminum galvanizing bath (about 1100 to about
1120~). The contact time of the steel strip with the
5 galvanizing bath is usually about 3 to about 5 seconds.
The thickness of the coating provided on the steel
strip is controlled in conventional manner, by wiping
using air knives at the exit from the galvanizing tank
30. Any desired thickness may be provided on the steel
strip surface, but it is preferred to provide a
thickness of at least 25 microns.
The galvanized steel sheet resulting from the
galvanizing tank 30 next passes upwardly through a
cooler 32 wherein the steel strip is cooled by flowing
S air to solidify the zinc-aluminum metal coating. The
rate of cooling of the strip must be controlled in
order to develop the desired microstructure in the
zinc-aluminum coating and usually is in the range of
about 50 to about 75F/sec. The cooling in the cooler
32 is effected to result in a steel strip leaving the
cooler having a temperature of no more than about
700F.
Further cooling then may be effected under ambient
conditions. The cooled steel strip 12 next usually is
passed over levelling rollers 34 and through a
passivation tank 36 wherein it is treated with a
passivation chemical, before finally to a wind-up roll
38.
The procedure described with respect to Figure 1
is effected continuously on steel strip to form
galvanized sheet having a zinc-aluminum coating on both
surfaces. If desired, the steel sheet may be
galvanized on only one surface, using techniques known
in the art. The speed of operation of the line may be
any convenient value, for example, about 250 ft/min.
Precoating of the steel strip with nickel or other
metal enables a zinc-aluminum coating to be applied to
the steel strip in cold-line operation and the use of
fluxes to be omitted. The operation also may be
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carried out on any other ferrous alloy product, such
as, wire, pipe and preformed structural items. The
process is not limited to a cold-line operation and may
be used to advantage on a hot-line operation.
The invention is illustrated further by the
following Example:
Example:
Cold-rolled steel panels sizes 6 x 3 x 0.024 inch
were alkali cleaned using the alkali cleaner sold under
the trademark "Pennsalt 74" for 10 seconds at 180F.
After a cold water rinse, the panels were pickled in 3~
sulphuric acid at room temperature for 10 seconds, and
given a further cold water rinse.
The panels were electroplated at room temperature
with nickel using a nickel anode in an electroplating
bath composition comprising:
NiS04 6H2O 240 gpl
NiC12 6H2O 40 gpl
Boric acid 30 gpl
at an energy level of 150 coulombs/sq.ft. The plated
panels were given a cold water rinse and dried.
The plated panels were dipped for 5 seconds in a
galvanizing bath comprising, by weight, 55% aluminum,
43.5~ zinc and 1.5~ silicon and having a temperature of
1200F. After withdrawal of the plates, the
zinc-aluminum coating was cooled at a rate of about
70F/sec.
The galvanized panels had a bright, spangled
appearance, having a spangle size of about 0.5 to 1.0
mm. The panels had coating thicknesses varying from
0.7 to 1.2 mils. One sample was cross-sectioned and a
photomicrograph of the cross-section was taken, and
appears, at a magnification of 500x, as Figure 2. As
may be seen, there is a lower steel substrate, an
uniform continuous intermetallic alloy layer on the
substrate estimated as having a thickness of 2 to 4
microns, and an aluminum-zinc layer on top of the
intermetallic alloy layer. The aluminum forms a
substantially continuous phase while the zinc, showing
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up as the darker areas, forms aggregates which are
distributed within the aluminum phase. It will also be
seen that the region immediately adjacent the
intermetallic alloy layer is substantially free from
zinc.
The panels were examined at 30 times magnification
with a stereo microscope and no holes or other coating
discontinuities were observed. Panels were reverse
impacted (80 in-lb) and given a tight 90 brake bend.
No flaking was observed. The same panels were then
exposed to 92~ relative humidity and 80C for 16 days.
Adhesion of the coating was maintained in the deformed
locations. The panels were again impacted (80 in-lb)
and flaking was still not observed. As a result of
these tests, it was concluded that adhesion of the
zinc-aluminum coating to the steel substrate was
satisfactory and that the intermetallic alloy-coating
interface was electrochemically stable.
Corrosion properties of the panels were tested
("Stelco") and compared with commercial samples of
zinc-aluminum coated panels obtained from two
commercial sources ("C.S.-A" and "C.S.-B").
(a) Electrochemical Properties:
Corrosion currents were measured on 1 cm2 samples
in aerated electrolytes by the linear polarization
technique and the average of three tests were
determined. All surfaces were finely polished prior to
testing to remove any surface passivation. Galvanic
capacity was estimated by coupling with steel and
measuring the time, normalized against thickness, for
the corrosion potential to exceed -0.775 volts vs. SCE.
The results obtained are set forth in the following
Table I:
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TABLE I
Corrosion Current Galvanic Capacity
ua/cm2 (secs)
2%Na2S04 pH4 3%NaCl pH22~Na2SO4 pH2
5 Stelco 110 268 2208
C.S.-A 192 680 1470
C.S.-B 158 550 920
It will be seen from the results set forth in
Table I that the panels produced in accordance with the
1 present invention had lower corrosion rates in both
sulfate and chloride electrolytes than both the
commercial samples and had superior galvanic capacity
to both the commercial samples.
(b) Salt spray test:
Unpassivated Stelco samples and passivated
commercial samples were subjected to the standard
accelerated corrosion salt spray test for 750 hours.
White rust formed on the panels produced in accordance
with this invention after a short period of exposure,
probably due to the unpassivated nature of the surface.
However, after 750 hours, the Stelco samples were
substantially free from red rust, whereas both the
commercial samples exhibited substantial rust
formation, the C.S.-B sample somewhat less so than the
C.S.-A sample. These results demonstrate the improved
barrier protection afforded by the process of the
invention.
(c) Condensing SO2 test:
Unpassivated Stelco samples and passivated
commercial samples were subjected to the conventional
condensing SO2 test for 4 and 8 cycles. Minimal rust
bleed at the sheared edge was exhibited by the samples
produced in accordance with this invention, while
considerable rust bleed was exhibited by the commercial
samples, with less rust bleed being exhibited by the
C.S.-B sample.
Figures 3A to 3C are photomicrographs of Stelco
samples subjected to the condensing SO2 test. Figure
3A is a photograph at 200 times magnification of the
,~,D
sample after 4 cycles exposure. As would be expected,
acid attack proceeded through the zinc rich phase but
the corroding path, for the most part, did not continue
to the intermetallic alloy layer. In the isolated
instances when this penetration did occur, corrosion
did not propagate along the interface, as can be seen
in the 500 times magnification micrograph of Figure 3B.
Little change occurred in the corrosion pattern after
an additional four cycles, as can be seen from the 500x
micrograph of Figure 3C taken at the end of that
period.
It is considered that penetration to the alloy
layer did not occur in the Stelco samplos due to the
presence of the aluminum-enriched zone adjacent to the
alloy layer.
In summary of this disclosure, the present
invention relates to an improved method of producing
corrosion resistant galvanized surfaces using
zinc-aluminum coating compositions, by precoating the
2(J ferrous alloy substrate. The microstructure of the
coating which results is unique and leads to the
enhanced corrosion resistance. Modifications are
possible within the scope of this invention.
