[0001] A widely employed practice for coating ferrous metal surfaces is the hot-dip method
in which the surface to be coated is immersed in a molten bath of the coating metal.
For coatings which are employed primarily to provide corrosion protection of the underlying
ferrous base, e.g. steel sheet and strip, baths containing aluminum or baths containing
zinc are most generally employed. In virtually all corrosive environments, zinc is
anodic to steel and therefore offers sacrificial, galvanic protection to the steel;
even if the zinc barrier itself should be damaged or cut, exposing the underlying
steel surface. Aluminum, on the other hand, is cathodic to steel in many corrosive
environments. Thus, while aluminum will generally exhibit substantially lower, overall
dissolution rates (as compared with zinc), it is not capable of providing galvanic
protection to the underlying ferrous surface, if the coating should for some reason
be damaged. This lack of galvanic protection results in a tendency of commercial aluminum-coated
products, i.e., those using pure aluminum or aluminum-silicon (5 to 11% silicon) coatings,
to develop objectionable rust stain discoloration in a short time at sheared edges
or other discontinuities in the coating. Furthermore, such lack of sacrificial protection
can also lead to relatively rapid corrosion of the underlying ferrous surface, under
conditions of continual condensation or where water accumulates in ponds.
[0002] United States Patent Specification No. 3,343,930 describes a hot-dip coated article
containing a combination of zinc and aluminum which, as a result of its zinc content,
overcomes the problem of premature discoloration caused by rust-stain bleeding and,
as a result of its aluminum content, exhibits an overall "general" corrosion rate
significantly less than that of zinc coatings. While this patent discloses a coating
bath containing from 28 to 75% zinc, balance aluminum and silicon, further studies
have shown that optimum results are achieved with a bath containing about 43% zinc,
55% aluminum and 2% silicon. This optimum product is sold commercially under the trade
name Galvalume. Due to such optimization, however, in most environments the overall
or "general" corrosion rate of Galvalume is far greater than that of-commerical aluminum-coated
products.
[0003] It has now been found that hot-dip coated products can be produced which exhibit
resistance to rust staining about equal to that of Galvalume, while concomitantly
providing a "general" corrosion resistance car superior to that of Galvalume--aproaching
that of aluminum-coated steels. Such an improved combination of corrosion resistance
is achieved by utilizing a hot-dip bath consisting essentially of 12 to 24% zinc,
less than 4% silicon, 0.3 to 3.5% iron (which is an incidental impurity normally encountered
in commercial hot-dip plating baths) and balance aluminum. In addition to the superior
combination of corrosion resistance, the coatings of this invention are more ductile
in that they exhibit lower tendencies towards crazing during forming operations.
[0004] It has further been found that the use of silicon, which is a necessary constituent
in Galvalume-type baths, has a detrimental effect on the rust stain resistance of
the resulting coating; whereas baths containing less than about 18% zinc can produce
effective, adherent coatings with materially reduced amounts of silicon or essentially
no silicon.
[0005] According, therefore to the present invention, there is provided a method of producing
corrosion-resistant coatings metallurgically bonded to ferrous-base articles, comprising
dipping a clean surface of said article into a molten bath containing aluminum and
zinc for a period at least sufficient to form an aluminum-zinc coating thereon with
an interfacial alloy layer, resulting from reaction of the ferrous surface with the
bath, at least 0.01 mils (0.25 micron) thick, removing the coated surface from said
bath and cooling the molten layer adhering thereto, wherein the bath consists essentially,
by weight, of 12 to 24% zinc, 0 to 4% silicon, 0.3 to 3.5% iron and the balance aluminum.
[0006] The invention is further described, by way of example, with reference to the accompanying
drawings, in which:
Figure I; (a), (b), (c) and (d) exhibit the rust staining of Aluminum and Aluminum-Zinc
coatings after about 15 months exposure at an industrial site, and
Figure 2; (1), (b), (c), (d), (e), (f) and (g) show the rust staining on sheared edges
of Al and Al-Zn coated samples after one year of exposure.
[0007] Various coating bath compositions falling within the scope of this invention were
evaluated. For purposes of comparison, two control baths were included in this investigation;
one simulated a commercial aluminum coating composition (with 6 to 7% silicon) and
the other simulated the commercial Galvalume composition. All the baths were prepared
from commercially pure aluminum (99.9% minimum purity), special high-grade zinc (99.99%
minimum purity) and aluminum-silicon (11.7% silicon) master alloy materials. In a
manner similar to that encountered in commercial hot-dip baths, iron, which dissolved
both from the steel strip and from the submerged steel rigging components was also
present as a significant constituent. The baths were contained in an alumina-lined
stainless steel pot. A graphite coating was applied both to the alumina lining and
to the rigging components, to minimize molten metal attack on those components. The
steel base employed was representative of a commercial quality low carbon rimmed steel.
[0008] Hot-dip coating was accomplished by a procedure analogous to that shown in U. S.
Patent 3,393,089. Thus, the steel sheet was cleaned in an aqueous silicate solution,
annealed in-line under reducing conditions and cooled to a temperature slightly above
bath temperature prior to entry into the bath. Coating baths were maintained at a
temperature of 75 to 100OF (40 to 55°C) above the liquidus temperature for each bath
concentration. No changes in bath temperature were made to account for the relatively
small effect of the silicon additions on the liquidus temperature. To achieve good
wetting between the steel and the bath metal, the annealing temperatures employed
were higher than those disclosed in the above-noted U. S. Patent, that is, the annealing
cycle included heating to a temperature of 1450°F (790°C). The reducing furnace atmosphere
was maintained by introducing a hydrogen-nitrogen mixture into a snout just above
the bath surface. A baffle was located inside the snout to prevent incoming cold gases
from impinging directly onto the strip. Air-knives were used to control the thickness
of the coating on the strip. No special measures were employed to provide enhanced
cooling rates to cool the strip after it exited from the coating bath. However, because
of the low line speeds of the coating line employed in this investigation, the air
knives themselves caused a considerable degree of cooling. The cooling rate caused
by the air knives averaged about 30°F (17°C) per second within the first 8 inches
(20cm) after the strip emerged from the bath. Subsequently, cooling resulting from
normal, ambient air cooling provided a cooling rate within the range of 8 to 10°F
(4 to 5°C) per second, while the strip was at a temperature greater than 700°F (370°C).
All the baths exhibited good fluidity characteristics, in that smooth, uniformly thick
coatings of about 1 mil thick (25 microns) were readily attained.
[0009] Forming-test results - Coating adherence was evaluated in bead-forming tests, 100-inch-pound
impact tests and ASTM-A525 coating bend tests. No flaking was observed in the latter
two tests, but a considerable amount was observed on some samples in the bead-forming
tests. It is generally accepted that for a given hot-dip coated product, coating adherence
is primarily a function of the alloy-layer thickness--the thicker the alloy layer,
the poorer the adherence. However, this expected behavior was not encountered with
respect to the inventive coatings--coatings from baths with lower zinc contents generally
exhibited better adherence, even when the alloy-layer was significantly thicker.
[0010] Apparently, the ductility of the outer coating metal layer has an influence on the
overall tendency to exhibit flaking. Thus, with respect to the inventive coatings,
overall flaking tendency appears to be a complex function both of the alloy-layer
thickness and the outer coating metal ductility.
[0011] Crazing tendency was observed on the impact test samples and the 3T-bends for ASTM
A525 bend-test samples. These tests showed that crazing was generally a function of
the ductility of the outer coating metal layer -- the tendency to crazing increasing
both as the zinc content and silicon content of the outer coating increased. Thus,
the Galvalume-type coating and the coatings of this invention containing in excess
of about 1.5 percent silicon exhibited "Moderate" crazing in such tests; whereas those
containing less than 1% silicon, as well as the commercial aluminum coating exhibited
"Light" crazing.
Corrosion Behavior
[0012] Sacrificial Corrosion Characteristics -.The sacrificail properties of the coating,
i.e. the ability to resist rust stain discoloration, were evaluated in two different
atmospheric tests. Figure 1 shows the rust staining encountered after about fifteen
months exposure at a test siteinMonroeville, Pennsylvania, comparing two coatings
produced in accord with the invention: (a) 18% zince and (b) 24% zinc, with that of
(c) the Galvalume-type coating and (d) the commercial aluminum coating containing
7% silicon. As expected, the rust staining in the area adjacent to the grooves for
the aluminum coating (d) was significantly greater than that of the Galvalume sample
(c). It may be seen, however, that the discoloration exhibited by the inventive samples
is essentially the same as that of the Galvalume sample.
[0013] When little or no silicon is used (desirably less than 0.3 and preferably less than
0.1% silicon), the ability of the instant coatings to inhibit red rust formation is
further enhanced. This enhancement is shown in Figure 2, which compares the red rust
formation on sheared edges of sheet samples after one year exposure at the same Monroeville,
Pennsylvania test-site. The superiority of the 12.4% zinc and 17.8% zinc samples is
clearly evident. The seven samples depicted are: (a) aluminum - 7% silicon, (b) 12.4%
zinc - no silicon, (c) 15% zinc - 8% silicon, (d) 17.8% zinc - no silicon, (e) 43%
zinc - 2% silicon (Galvalume-Type), (f) 2% zinc - 6% silicon and (g) 33% zinc - 2%
silicon (another Galvalume-type). The detrimental effect of silicon even for a zinc
content within the scope of this invention, i.e. sample (c) containing 15% zinc, but
containing 8% silicon, is clearly evident.
[0014] These results dramatically emphasize the further benefit of employing coatings containing
from 12 to 18% zinc. As shown in U. S. Patent 3,393,089, in the production of hot-dip
aluminum-zinc coatings containing from 28 to 75% zinc; silicon is a necessity -- to
retard the growth of the interfacial alloy-layer and produce coatings with acceptable
adhesion. By contrast, for those coatings within the instant invention, but containing
less than 18% zinc, preferably less than 16% zinc, acceptable adhesion (for many commercial
applications such as roofing and siding) can be produced in silicon-free baths, without
resort to special coating techniques. Although the bath reactivity of essentially
silicon-free baths is greater than if silicon (within the range of U. S. Patent 3,393,089)
had been employed, such bath reactivity as measured by the parabolic rate constant
"a" for the silicon-free baths, varied from about 0.05 to 0.07 mil per second (32
to 45 micron per second), depending on the amount of zinc employed, and none of the
silicon-free baths exhibited a reactivity greater than that of a pure-aluminum coating
bath.
[0015] "General" Corrosion Characteristics - The general corrosion resistance provided by
coatings of the invention was evaluated by the Kesternich method-DIN 50018. This test
is a well accepted, rapid corrosion test for comparison of the resistance of similar-type
protective coatings to industrial atmospheres, particularly those rich in sulphur
dioxide. The weight loss of four different zinc concentrations within the scope of
this invention was compared with that of three different Galvalume-type zinc concentrations,
after 20 cycles of exposure. The results thereof are shown in the Table below. It
is seen that the Galvalume-type samples exhibit general corrosion rates about 2 to
3 times greater than those of the invention.
[0016] While the examples above were directed to the production of a specific overall coating
thickness of about 1 mil (25 microns), it should be understood, with respect to sheet
product, that such overall coating thicknesses will generally range from 0.2 to 2
mils (5 to 50 microns) and most often from 0.5 to 1 mils (12 to 25 microns). To achieve
such coating thicknesses, immersion times of the order 0.5 to 10 seconds will generally
be employed, preferably 1 to 5 seconds, so as to achieve an interfacial alloy layer
having a thickness of 0.01 to 0.2 mils (0.25 to 5 microns). However, for superior
deformation properties, it is preferable that the thickness of the interfacial layer
be less than 0.1 mil (2.5 micron). By contrast, when coating massive structures such
as castings, forgings, plates, bars, and preformed pipes, overall coating thicknesses
of up to 30 mils (760 microns) are often desired, therefore requiring significantly
extended immersion times. With respect to the latter structures, interfacial alloy
layers of the order of 0.25 to 1 mils (6 to 25 microns), or even greater, may result.
However, whatever the product, the thickness of the interfacial layer will generally
be significantly thinner than, preferably.less than 10% of, the overall coating thickness.
1. A method of producing corrosion-resistant coatings metallurgically bonded to ferrous-base
articles, comprising dipping a clean surface of said article into a molten bath containing
aluminum and zinc for a period at least sufficient to form an aluminum-zinc coating
thereon with an interfacial alloy layer, resulting from reaction of the ferrous surface
with the bath, at least 0.01 mils (0.25 micron) thick, removing the coated surface
from said bath and cooling the molten layer adhering thereto, characterized in that
the bath consists essentially, by weight, of 12 to 24% zinc, 0 to 4% silicon, 0.3
to 3.5% iron and the balance aluminum.
2. A method as claimed in claim 1, characterized in that said bath contains less than
1% silicon and less than 2.5% iron.
3. A method as claimed in claim 1 or claim 2, characterized in that said surface is
dipped into the bath for 0.5 to 10 seconds to produce an interfacial alloy layer 0.01
to 0.2 mils (0.25 to 5 microns) thick.
4. A method as claimed in claim 3, characterized in that said surface is dipped into
the bath for a period sufficient to form a coating having an overall thickness of
0.2 to 2 mils (5 to 50 microns).
5. A method as claimed in any preceding claim, characterized in that said bath contains
0.3 to 4% silicon.
6. A coated product produced by a method as claimed in any preceding claim.