Reduction of loss of zinc by vaporization when heating zinc-aluminum coatings on a ferrous metal base

The loss of zinc by vaporization when heating a zinc-aluminum coating to a temperature between about 427.degree. C. and 816.degree. C. (800.degree. F. and 1500.degree. F.) is significantly reduced by applying a zinc-aluminum coating containing about 30 to 75 weight percent zinc and the balance essentially aluminum to a mild carbon steel base which contains titanium in an amount sufficient to combine with all of the carbon in the steel and provide a small excess of uncombined titanium in the steel base, thereby providing a zinc-aluminum coated steel article which has improved corrosion and oxidation resistance when heated at temperatures between about 427.degree. C. and 816.degree. C. (800.degree. F. and 1500.degree. F.).

The present invention relates generally to providing a ferrous metal 
surface which is resistant to corrosion and oxidation and more 
particularly to a steel strip having a zinc-aluminum protective coating on 
a surface thereof which exhibits improved adherence and increased 
resistance to corrosion and to oxidation after being heated to a 
temperature above the melting point of zinc. 
In order to protect a ferrous metal surface against attack when the ferrous 
metal surface is exposed to a corrosive and oxidizing environment it has 
been common practice to provide a protective surface coating containing 
metallic zinc and aluminum. Metallic zinc is sacrifical toward iron and is 
particularly useful in protecting a ferrous metal surface against attack 
by corrosive metal salt solution or by an acidic environment, while 
aluminum is particularly adapted to protect a ferrous metal against 
surface oxidation and exposure to an environment containing chloride ions. 
Protective zinc-aluminum surface coatings containing relatively large 
amounts of both zinc and aluminum ranging from about 30 to 75 percent by 
weight zinc with the balance mainly aluminum provide coatings which have 
improved corrosion and oxidation resistance when subjected to the Muffler 
Condensate Test relative to conventional galvanized material (see U.S. 
Pat. No. 3,343,930). 
When a steel strip having a zinc-aluminum surface coating of the foregoing 
type is heated to a temperature above the melting point of zinc which is 
about 419.degree. C. (786.degree. F.), however, the zinc in the coating 
begins to vaporize. Thus, when a coating containing 43% zinc, 55% aluminum 
and about 2% silicon is heated at a temperature of about 482.degree. C. 
(900.degree. F.), it has been found that the zinc in the coating is lost 
by vaporization, the surface becomes very uneven and the coating weight 
and thickness is reduced proportional to the loss of zinc. Also, when the 
zinc-aluminum coating is heated at a temperature of about 650.degree. C. 
(1200.degree. F.) a continuous porous subsurface intermetallic compound 
layer is formed which further reduces the adherence of the surface coating 
and the corrosion and oxidation resistance of the coating is greatly 
reduced. Surface degradation and localized pitting of the zinc-aluminum 
coatings and oxidation of the ferrous metal surface becomes evident when 
the coated ferrous metal is heated at a temperature of about 816.degree. 
C. (1500.degree. F.). The loss of the zinc from these coatings, a 
reduction in the overall integrity of the coatings and a lowering of the 
ability of the coatings to withstand corrosion and oxidation are 
significant after heating the zinc-aluminum coatings at moderately 
elevated temperatures from about 427.degree. C. to about 816.degree. C. 
(800.degree. F.-1500.degree. F.). 
It is therefore an object of the present invention to provide a 
zinc-aluminum coated ferrous metal article which has improved resistance 
to a corrosive and oxidizing environment after being heated at a 
temperature between about 427.degree. C. (800.degree. F.) and 816.degree. 
C. (1500.degree. F.) and to the method of providing said article. 
It is a further object of the present invention to provide a zinc-aluminum 
coated ferrous metal strip which has improved resistance to a corrosive 
and oxidizing environment after being heated at a temperature between 
about 427.degree. C. (800.degree. F.) and above without requiring a large 
amount of an expensive alloying element in the protective coating or in 
the ferrous metal forming the strip and to the method of providing said 
strip.

The cross-section surfaces of the strips corresponding to the 
photomicrographs of FIGS. 1, 2, 5 and 6 were etched with a 4 percent 
solution of nital. 
It has been found that the loss of a substantial amount of zinc from 
zinc-aluminum coatings containing between about 30 and 75 wt. percent zinc 
and with the balance being mainly aluminum along with preferably a small 
amount of silicon applied to a ferrous metal strip can be prevented when 
the strip is heated at a temperature between about 427.degree. C. 
(800.degree. F.) and 816.degree. C. (1500.degree. F.) and the objects of 
the present invention achieved in an economical manner by applying the 
said zinc-aluminum coatings to a low carbon low alloy ferrous metal base, 
such as a rimmed or an aluminum killed steel strip, where the steel 
contains as the essential alloying element titanium in an amount 
sufficient to combine with all of the carbon in the steel and provide a 
slight excess of uncombined metallic titanium distributed throughout the 
steel. 
The titanium-containing plain carbon steel base preferably used in the 
present invention is a low carbon steel or mild steel having a carbon 
content of up to about 0.25 wt. percent max., usually from about 0.03 wt. 
percent to about 0.25 wt. percent and preferably from about 0.03 wt. 
percent to about 0.10 wt. percent, and having titanium added thereto in an 
amount which is sufficient to combine with all the carbon in the steel 
base and leave an excess of uncombined titanium. Typically, the plain 
carbon steel base will consist essentially of from about 0.03 wt. percent 
to about 0.25 wt. percent carbon and preferably from 0.03 wt. percent to 
0.10 wt. percent, from about 0.20 wt. percent to about 0.50 wt. percent 
manganese, about 0.05 wt. percent max. silicon, titanium in an amount 
sufficient to combine with all the carbon in the steel base and leave an 
excess of uncombined titanium, and the balance iron with the usual amounts 
of impurities and residuals. Preferably, the steel is a killed steel, such 
as an aluminum killed steel, in which case the residuals or impurities 
present will include the usual amounts of aluminum or other deoxidizers, 
such as silicon, characteristic of killed steel. Although, as explained 
hereinafter, titanium is the essential alloying element to be added to the 
plain carbon base steel to obtain the advantages of the present invention, 
it is also within the scope of the invention to add small amounts of other 
metallic alloying elements to improve the physical properties of the base 
steel. However, the amount of such other metallic alloying elements should 
not exceed about 1 percent by weight and preferably should not exceed 
about 0.5 percent by weight. Thus, the steel articles of the present 
invention are, in any case, low alloy steel articles. 
Preferably, the excess of uncombined titanium remaining in the steel is an 
amount between about 0.1 and about 0.3 percent by weight. Since the weight 
percent of titanium must be approximately four times the weight percent of 
carbon in the steel in order to combine with or precipitate essentially 
all the carbon and nitrogen in the steel in the form of titanium carbides 
or titanium carbo-nitrides. The minimum titanium content of the substrate 
steel sheet in the present invention should be four times the carbon 
content of the steel plus an additional amount of titanium sufficient to 
provide a slight excess of uncombined titanium. While the titanium content 
can be as much as ten times the weight percent of carbon in the steel, an 
amount of titanium greater than that required to provide from about 0.1 to 
about 0.3 percent by weight uncombined titanium gives no increased 
benefits and merely adds unnecessarily to the cost. And, since the amount 
of carbon in a steel conventionally used for producing zinc-aluminum 
coated steel strip is small, preferably less than 0.10 wt. percent, the 
total amount of titanium required in the present invention is small. The 
inclusion of titanium in the steel in the aforementioned amounts also 
results inherently in stabilization any nitrogen in the steel (usually not 
exceeding about 0.006 wt. percent) so that both the carbon and nitrogen 
are stabilized. The titanium carbides present provide improved high 
temperature tensile strength which prevents deformation of the steel and 
damage to the coating when the steel article is exposed to elevated 
temperatures. 
The hot-dip zinc-aluminum coating bath must contain a sufficient amount of 
silicon to prevent the formation of an objectionably thick subsurface 
intermetallic layer during hot-dip coating which results in the coating 
having poor formability. The silicon content of the zinc-aluminum coatings 
should range between about 1.5 wt. percent and 6.0 wt. percent. 
A low carbon aluminum killed titanium containing steel usable in the 
present invention can be made in a BOF or basic open hearth furnace 
according to the usual practice for producing a rimmed sheet steel. The 
steel having a low carbon content before tapping from the furnace is 
preferably killed in the ladle by adding aluminum in a conventional manner 
to effect thorough deoxidation of the steel. Thereafter, a quantity of 
titanium in the form of finely crushed commercially available 
ferro-titanium alloy containing 65 percent titanium, machined titanium 
chips or titanium sponge is added to the steel in an amount sufficient to 
combine with all of the carbon remaining in the steel and provide an 
excess of uncombined metallic titanium. For example, the above crushed 
ferro-titanium alloy is added at a rate of 175 pounds for each 24,000 
pounds ingot poured. The titanium additions must be completed before any 
oxidizing slag appears on the surface of the steel. 
The steel is then worked into sheets by the usual commercial method, as by 
the cold rolling into sheets having a thickness of about 0.04 inches with 
no special attention other than employing a low coiling temperature (i.e. 
about 1100.degree. F.) in order to suppress the formation of a large grain 
titanium carbide precipitate and reducing excessive oxidation at the 
surface of the steel sheet. 
A preferred method of coating a steel strip having the titanium content 
thereof in accordance with the present invention is by a hot-dip coating 
process generally known in the art as a Sendzimir-type process, wherein a 
continuous steel sheet or strip which is free of scale and rust is fed 
continuously from a coil through a furnace containing an oxidizing 
atmosphere maintained at a temperature between about 166.degree. C. and 
499.degree. C. (330.degree. F. and 930.degree. F.) which burns off any oil 
residue on the surface of the strip and forms a thin surface oxide film. 
The oxide coated steel sheet then passes through a furnace containing a 
reducing atmosphere, such as the hydrogen-containing HNX atmosphere, 
having a temperature between about 816.degree. C. and 982.degree. C. 
(1500.degree. F. and 1800.degree. F.), whereby the oxide coating on the 
strip is reduced to form a surface layer of metal free of non-metallic 
impurities to which molten aluminum readily adheres. Following the 
reducing step, the strip is fed into a hot-dip zinc-aluminum coating bath 
through a protective hood which prevents the reduced metal surface being 
oxidized before entering the coating bath. After leaving the hot-dip zinc 
aluminum coating bath, the coating thickness on the strip is regulated by 
a pair of oppositely disposed thickness-regulating jet wipers or rolls 
which produce a uniform thin zinc aluminum coating, and the strip is 
cooled by any suitable means. The zinc aluminum coated strip is then wound 
into a coil. Conventional Sendzimir-type process apparatus can be used in 
each of the processing steps. 
The step of burning off the oil and oxidizable combustible material on the 
surface of the steel strip before the strip is subjected to the reducing 
atmosphere can be omitted, if desired, provided the strip is otherwise 
thoroughly cleaned, immediately prior to the reducing step, as by 
conventional alkaline cleaning and pickling. 
The zinc-aluminum hot-dip coating bath used for coating the low alloy 
titanium containing low carbon steel has a melting point which ranges from 
about 524.degree. C. to 660.degree. C. (975.degree. F. to 1220.degree. F.) 
varying inversely with the concentration of zinc in the bath which can 
range from about 30% by wt. and 75% by wt. The hot-dip coating bath also 
contains between about 1.5 to 6.0 percent by weight silicon along with a 
small amount of lead and iron as incidental impurities or accumulations 
due to continuous contact with the steel strip during the hot-dip coating 
operation. 
Reproductions of photomicrographs of a zinc-aluminum coated steel strip 
prepared in the above described manner after heating at a temperature of 
about 482.degree. C. (900.degree. F.) in air for a period of 3 days are 
presented in FIGS. 1 and 3 of the drawing, and for comparison purposes 
reproductions of photomicrographs of a conventional steel strip which does 
not contain any added titanium but having substantially the same 
zinc-aluminum coating applied in the same manner and heated under the same 
conditions as the strip of FIGS. 1 and 3 are shown in FIGS. 2 and 4. 
FIGS. 1 and 2 are vertical sections (500.times.) through the respective 
coated steel strips, and it is evident that the coating of FIG. 1 is much 
more uniform than the coating of FIG. 2. 
FIGS. 3 and 4 show the surface (7.5.times.) of the strips of FIGS. 1 and 2, 
respectively, and it is evident that the strip of FIG. 3 has a uniform 
dull appearing surface, whereas the strip in FIG. 4 has an uneven bright 
appearing surface which contains numerous crators indicating a selective 
evaporation of zinc from the coating. 
FIGS. 5 and 6 show vertical sections (550.times.) of the strips of FIGS. 1 
and 2, respectively, after the strips have been heated at a temperature of 
650.degree. C. (1200.degree. F.) for 3 days. From FIG. 5 it is evident 
that the coating has generally diffused uniformly into the steel base 
without forming a significant subsurface layer of intermetallic compound 
between the surface coating and the steel base. From FIG. 6 it is evident 
that the zinc-aluminum coating which is formed on a conventional steel 
base free of added titanium has formed a distinctly dark subsurface layer 
of intermetallic compound between the surface coating and the steel base. 
The surface coating also appears to have a reduced thickness relative to 
the coating of FIG. 5. 
An important characteristic of the zinc-aluminum coated article of the 
present invention is the property of significantly reducing the loss of 
zinc from a zinc-aluminum coating by vaporization when the article is 
heated at temperatures only moderately above the melting point of zinc, 
and particularly at temperatures between about 427.degree. C. and 
538.degree. C. (800.degree. F. and 1000.degree. F.). The latter 
temperature range is the temperature range within which most of the 
components of an automotive exhaust system are repeatedly heated during 
use. 
Studies of the weight change effected in a zinc-aluminum coating.sup.(1) on 
a low titanium alloy steel strip prepared in accordance with the present 
invention (Specimen A) and a conventional steel strip which does not 
contain titanium (Specimen B) but having substantially the same 
zinc-aluminum coating.sup.(2) were made by heating the coated strips in 
air at a temperature of 482.degree. C. (900.degree. F.) for the indicated 
periods and the weight gain or loss were observed at the end of each 
period. The results observed are shown in the following Table I: 
TABLE I 
______________________________________ 
Weight Change Data (ug/cm.sup.2) 
3-Days 6-Days 10-Days 
Specimen (900.degree. F.) 
(900.degree. F.) 
(900.degree. F.) 
______________________________________ 
A +92 +51 -66 
B -258 -745 -870 
______________________________________ 
FNT (1) 43% Zn, 55% Al, 2.0% Si, all % by wt. 
FNT (2) 43.6% Zn, 55% Al, 1.6% Si, all % by wt. 
It will be evident from the data of Table I that there is a substantially 
larger reduction in the weight of the Specimen B as compared with Specimen 
A and that the weight loss in Specimen B is not offset by oxidation of the 
coating even after prolonged heating of the strip. 
The Specimens A and B were also heated at a temperature of 650.degree. C. 
(1200.degree. F.) and 816.degree. C. (1500.degree. F.) for a period of 3 
days and the weight change effected are presented in the following Table 
II: 
TABLE II 
______________________________________ 
Weight Change Data (ug/cm.sup.2) 
3-days 3-days 
Specimen (1200.degree. F.) 
(1500.degree. F.) 
______________________________________ 
A +528 +1118 
B +369 +668 
______________________________________ 
The data of Table II indicates that the weight gain which normally occurs 
due to oxidation is much less in Specimen B than in Specimen A. 
The zinc-aluminum coated steel strip or sheet of the present invention is 
particularly suited for use in fabricating components of an automotive 
exhaust system, such as exhaust mufflers, inlet pipes, tail pipes, Y-pipe 
assemblies, and catalytic converters. The zinc-aluminum coated titanium 
containing steel article made in accordance with the present invention has 
substantially improved corrosion and oxidation resistance compared with a 
similarly coated conventional rimmed steel after being heated at 
moderately elevated temperatures of 472.degree. C.-816.degree. C. 
(800.degree. F.-1500.degree. F.) in addition to exhibiting good resistance 
to corrosion when exposed to an acidic and salt environment and good 
formability at ambient temperatures and having high tensile strength at 
elevated temperatures all of which are important in the manufacture and 
operation of automotive exhaust system components. 
While the invention is in no way dependent on any theory of operation, it 
has been found that the weight loss which normally occurs when a 
zinc-aluminum coated rimmed steel strip is heated at 482.degree. C. 
(900.degree. F.) is due to the loss of zinc by vaporization, and it is 
thought that the varporization of zinc is reduced in the coated strip of 
the present invention when a zinc-aluminum coating containing a 
substantial proportion of zinc is heated at moderately elevated 
temperatures above the melting point of zinc, because the rate of 
diffusion of zinc into the titanium-containing low alloy steel which is 
used for the strip in the present invention is greater than the rate of 
vaporization of zinc at the said temperatures (the vapor pressure of zinc 
at 482.degree. C. (900.degree. F.) is 0.115 kPa (1.14.times.10.sup.-3 
atm.)) so that a substantial amount of the zinc diffuses into the low 
titanium alloy steel surface before a substantial amount of zinc is lost 
by vaporization. In a conventional rimmed steel having a like 
zinc-aluminum coating, the zinc is vaporized before there is substantial 
diffusion of zinc into the steel base, and a porous aluminum coating 
structure is formed on the surface of the strip having a reduced zinc 
concentration as well as significantly reduced adherence and resistance to 
corrosion and oxidation after prolonged heating at said moderately 
elevated temperatures. 
While the zinc-aluminum coating is preferably applied by hot-dip coating, 
the coatings can be applied by other coating means, such as plasma spray 
coating or by roll coating a zinc-aluminum powder mixture dispersed in a 
suitable liquid vehicle.