[0001] The present invention relates to the production of corrosion-resistant coatings on
ferrous-base articles.
[0002] Hot-dip coated steel articles having a coating composed principally of various combinations
of aluminum and zinc are known to exhibit an optimum combination of general corrosion
resistance, more durable than zinc coatings, while providing more galvanic protection
to cut edges and areas of mechanical damage than do hot-dip aluminum coatings. Thus,
a coating bath containing from 25% to 70% aluminum, balance zinc and silicon is shown
in United States Patent Specification No. 3,343,930, and a coating bath containing
from 12 to 24% zinc, optional amounts of silicon and balance aluminum, is shown in
European Patent Application No. 82306407.6.
[0003] It has been shown, however, in United States Patent Specification No. 3,782,909,
under certain accelerated corrosion tests (in environments containing acidified chlorides)
that the coatings containing from 25% to 70% aluminum exhibit a tendency towards "corrosion
flaking" in which the aluminum-zinc coating separates along the interface between
the intermetallic layer and the aluminum-zinc overlay, even in the absence of any
mechanical strain such as bending. The accelerated tests comprise an initial five
minute leach with concentrated nitric acid to partially dissolve the zinc-rich materials,
rinsing, and then exposure in an acidified salt spray at a pH of 3 and a temperature
of 95°F (35°C) for 45 minutes, followed by two hours of drying with compresed air
and then 3 1/4 hours holding in the salt spray. The exposure-drying cycle is repeated
and the coating is considered stabilized if it survives 100 cycles of the test procedure
without exhibiting corrosion flaking. The initial leaching with nitric acid is deemed
so severe, that the correlation of the above tests with actual, realistic environment
is somewhat questionable. When employing a less severe test, such as ASTM B117-73,
the correlatability of which is generally more reliable (although not proven with
respect to corrosion flaking), coatings containing about an equal amount of zinc and
aluminum do not exhibit corrosion flaking even when slowly cooled and thus not stabilized.
However, flaking tendency appears to increase as the aluminum concentration of the
coating increases. Thus, coatings containing more than about 72% aluminum will exhibit
flaking, in the less drastic ASTM test, irrespective of whether they had been stabilized
by the rapid cooling. It was found that the resistance to flaking, as evaluated by
ASTM B117-73, can significantly be increased by the addition of small amounts of magnesium
to the molten aluminum-zinc alloy bath.
[0004] According, therefore, to the present invention, there is provided a method of producing
corrosion-resistant coatings metallurgically bonded to ferrous-base articles, which
comprises dipping a clean surface of said article into a molten bath consisting essentially
of, by weight, 72% to 95% Al, up to 3.5% Fe, up to 4% Si, balance Zn for a period
at least sufficient to form an aluminum-zinc coating thereon with an interfacial alloy
layer portion having a thickness greater than 0.01 mils, said layer resulting from
the reaction of the ferrous surface with the bath, removing the coated surface from
said bath and cooling the molten coating adhering thereto, wherein resistance to corrosion
flaking of the resulting coating is enhanced by adding 0.04 to 0.32% Mg to said molten
bath.
[0005] The invention is further described, by way of example, with reference to the accompanying
drawing which is a graph showing the effect of small amounts of magnesium on the corrosion
flaking of alloy coatings within the range 72 to 95% aluminum (1% Si and balance zinc).
[0006] Test samples used for evaluation of corrosion flaking, were prepared by a procedure
similar to that described in United States Patent Specification No. 3,393,089. The
steel sheet was cleaned in an aqueous silicate solution, annealed in-line under reducing
conditions by heating to a temperature of 1475°F (790°C) and cooled to a temperature
slightly above bath temperature prior to entry into the coating bath. 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
the incoming cold gases from impinging directly onto the strip. Coating baths were
maintained at a temperature of 75 to 100°F (40 to 55°C) above the liquidus temperature
for each bath concentration. Air-knives were used to control the thickness of the
coating on the strip. For each bath concentration, duplicate runs were employed: (i)
in which the average cooling rate over the range of solidification was maintained
at 25°F/sec. (14°C/sec.), and (ii) in which said average cooling rate was 15°F/sec.
(8°C/sec.) that is, below the critical rate of 20
oF/sec. (11°C/sec.) disclosed in U. S. Patent No. 3,343,930.
[0007] For coatings containing more than 72% aluminum, less than 4% Si, less than 3.5% Fe,
balance Zn, no differences in corrosion flaking tendency were detected, irrespective
of the cooling rate employed. Results of the tests performed in accord with the ASTM
B117-73 test procedure are reported graphically in the Figure. It may be seen therefrom,
with magnesium concentrations of from 0.04 to 0.32% Mg (all percentages by weight),
that the flaking tendency of such hot-dip coatings is decreased, and that immunity
(in the salt spray environment of ASTM test) is essentially achieved by the addition
of 0.07 to 0.28% Mg to such higher Al baths. In this regard, it should be noted that
the addition of somewhat similar amounts of Mg to coating baths containing lower amounts
of Al, (for example the 25 to 70% Al baths of U. S. Patent No. 3,343,930) would most
likely be beneficial in enhancing the resistance to flaking of such lower Al coatings,
as well. However, no tests were conducted in this lower Al range, since an alternative
is already available to the art, that is, the accelerated cooling procedure of U.
S. Patent No. 3,782,909.
1. A method of producing corrosion-resistant coatings metallurgically bonded to ferrous-base
articles, which comprises dipping a clean surface of said article into a molten bath
consisting essentially, of by weight, 72% to 95% Al, up to 3.5% Fe, up to 4% Si, balance
Zn for a period at least sufficient to form an aluminum-zinc coating thereon with
an interfacial alloy layer portion having a thickness greater than 0.01 mils (0.25
micron), said layer resulting from the reaction of the ferrous surface with the bath,
removing the coated surface from said bath and cooling the molten coating adhering
thereto, characterized by adding from 0.04 to 0.32% Mg to said molten bath.
2. A method as claimed in claim 1, characterized in that said ferrous-base article
is steel sheet and said sheet is dipped into the bath for a period to form an alloy
layer having a thickness less than 0.5 mils (13 microns).
3. A method as claimed in claim 1 or claim 2, characterized in that said bath contains
12 to 24% Zn, less than 1% silicon, and less than 2.5% iron.
4. A coated product produced by the method of any preceding claim.