FIELD OF THE INVENTION:
The present invention relates to an Al-Mg-Mn alloy coating on a steel substrate. The
coating demonstrates excellent corrosion behaviour compared to galvanized
(GI)/galvannealed (GA) coating and excellent sacrificial corrosion properties in
comparison to commercial aluminized steels. More specifically, the invention related to
an Al-Mg-Mn alloy coating that can be applied on a steel substrate using a dip-coating
process.
BACKGROUND OF THE INVENTION:
Aluminium and Aluminium alloys represent an important category of materials due to
their high technological value and wide range of industrial applications. Presently,
aluminium alloys are used in automobile body panels, roofing enclosure and aerospace
applications. The use of aluminium alloys is not only restricted to reduce weight but
also to provide better mechanical properties. Aluminium is also used as metallic coating
for steel. In galvanized steel, the zinc coating is subjected to continuous corrosion to
provide protection, while in aluminized steel, corrosion resistance is provided mainly by
an impervious and stable thin film of aluminium oxide (AI2O3). If this film is damaged or
removed by abrasion, another layer of oxide is expected to form instantly to avoid
further corrosion.

As an alternative to Zn coating on steel, commercial aluminized steel offers attractive
properties. Aluminized steel (Type 1: Al-Si and Type 2: Pure Al) is 5-6 times more
corrosion resistant than zinc coated steel sheet in all environment (see Table 1).
Further, for the same coating thickness, Al coating is 3 times lighter than Zn coating.
Aluminized coating is high temperature oxidation resistant than Zn coated steel and
shows good formability and fair weldability.
Table 1: Corrosion rate of galvanized and commercial aluminized steel sheet at different
environments.

In Aluminized steel Type 2, the steel is coated by hot-dip process on both sides with
pure aluminium. Aluminized steel Type 2 is increasingly used for metallic drainage
components in contact with natural waters. However, corrosion is an important
durability limitation factor in these components, which are often designed for very long

service life (e.g. 75 yrs) [4]. Microscopic examination of typical aluminized steel Type 2
in cross section reveals nearly pearlite-free ferrite low carbon steel substrate with
regular grains, a partly columnar brittle inner alloy layer, and a softer outer aluminium-
rich layer. The inner alloy layer, commonly of composition Fe2Al5 [5], is an essential
ingredient of the coating protection system, supplementing the outer aluminium-rich
layer and possibly providing a second line of defence against corrosion [6]. The
composition of the outer layer is predominantly nearly pure aluminium with Fe-rich
intermetallic precipitates (6-11 wt% Fe) [7]. During manufacturing and/or handling of
the final material, discontinuities in the aluminized coating can extend to the substrate
steel, creating coating breaks. Those coating breaks exposing the steel base may result
in the formation of galvanic macro-cells. However, if the environment is not mild as
those commonly found in marine inland waters, sacrificial protection to the exposed
underlying steel may not be sufficient to prevent corrosion where aluminized coating
breaks. Similarly Aluminized Steel Type 1, which is continuously hot dip coated with
aluminium/ silicon alloy containing 5% to 11% silicon to promote better adherence,
doesn't possess sacrificial corrosion properties to protect underlying steel substrate.
To address these issues, researches are conducted on aluminium based coating on steel
to impart sufficient sacrificial properties which would protect the underlying steel
substrate. For instance, bulk Al-Mg and Al-Mg-Mn alloy sheet (3000 series of Al alloys)
have increasingly accounted for a good part of the car body and construction

application because of its good formability among Al alloys, good weldability and
corrosion resistance. Recently, Tsuru et al [8-10], Nippon Steel Corp (JP2000328217)
and Dongbu Steel Co. Ltd. (WO2009131267) have reported better corrosion resistance
properties of hot-dipped Al-Mg-Si alloy coated steel in comparison with galvanized steel.
OBJECTS OF THE INVENTION:
An object of the invention is to propose a coating that has excellent corrosion behaviour
compared to galvanized (GI)/galvannealed (GA) steel and excellent sacrificial corrosion
properties in comparison to commercial aluminized steels.
An object of the present invention is to propose an Al-Mg-Mn alloy coating that can be
applied on a steel substrate.
Another object of the invention is to produce an Aluminium alloy coated steel substrate.
Still another object of the invention is to propose an Al-Mg-Mn alloy coating that can be
applied on a steel substrate using a hot-dip process.
Another object of the invention is to manufacture Aluminium alloy coated steel
substrate for automobile body panels, roofing enclosure and aerospace applications
SUMMARY OF THE INVENTION:
Al-Mg-Mn alloy coated steel substrate was produced by hot-dipping a steel substrate
into a bath containing Al-Mg-Mn alloy without using any flux. The alloy coating as per
the current invention comprises, in weight %, 95 to 98 % of Al, 2 to 4 % of Mg, 0.75 to
1.25 % of Mn, and optionally one or more elements selected from the group consisting

of Si: 0.05-0.2%, Sn: 0.01-0.1%, Fe: 0.1-0.3%, Ni: 0.01-0.02%, Zn: 0.01-0.2%, with
the remainder consisting of unavoidable impurities. The alloy coating was bright and
adherent. This type of coated steels can be used for automobile body panels, roofing
enclosure and aerospace applications. The coating consists of broadly two layers: outer
Al-Mg-Mn alloy layer and inner finger-like Al-Fe intermetallic layer. The inner Fe-AI
intermetallic layer further consists of two layers: outer FeAI3 and Al-Mn alloy layers and
inner Fe2AI5 layer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1: Photographs of Al-Mg-Mn alloy coated steel at different dipping times: 10s, 20s
and 30s.
Fig. 2: Cross-sectional SEM micrographs of different of Al-Mg-Mn alloy coated samples
for various dipping times: 10s, 20s and 30s.
Fig. 3: Cross-sectional SEM micrographs and EDS analysis of Al-Mg-Mn alloy coating for
dipping time of 30s.
Fig. 4: Top surface XRD analysis of Al-Mg-Mn alloy coated steel substrate.
Fig. 5: Cross-sectional EDS line analysis of coating for dipping time of 30s. Elemental
intensity/analysis of Al and Fe shows two different steps for two Fe-AI intermetallic.
layers.
Fig. 6: Cross-sectional EDS line elemental intensity of Al, Fe, Mg and Mn for coating
time of 30s.
Fig. 7: Cross-sectional EDS elemental map of Al, Fe, Mg and Mn for coating time of 30s.
Fig. 8: Variation of coating thickness with hot-dipping time.

Fig. 9: Photographs of bend test samples at different hot-dipping time: 10s, 20s and
20s.
Fig. 10: Tafel curves of Al-alloy coated samples for different coating dipping times (10s,
20s, 30s), and Galvanized (GI) and Galvannealed (GA) coated steels.
Fig. 11: SEM and EDS analysis of corroded portion of Al-Mg-Mn alloy coated sample
(dipping time of 20s).
DETAILED DESCRIPTION OF THE INVENTION:
The invention proposes an Al-Mg-Mn alloy composition for hot-dip coating on a steel
substrate. Table 1 provides details of the alloy composition used as per the current
invention along with various processing parameters employed to coat a steel substrate.
In an embodiment of the invention, the steel substrate is an IF steel sheet. However,
invention is not restricted to use for IF steel sheet and can be used for any other steel
substrate as well. The steel substrate coated with the Al-Mg-Mn alloy composition was
evaluated for it corrosion properties using different tests and was observed that it
provides significant advantages compared to Galvanized (GI) and Galvannealed (GA)
steels.

Table 1: Al-Mg-Mn alloy compositions and process parameters for coating a steel sheet.

Preparation of Al-Ma-Mn coated steel:
Al-Mg-Mn alloy coated steel sheet was prepared by a hot-dip coating process using a
bath composition as specified in the table 1 above. The hot-dip coating process was
carried out in high-temperature melting furnace. The pot temperature was controlled by
thermocouple inserted into the molten bath. The bath was stabilized at desired
temperature for 2 hours before experiment. The experiments were conducted at
different temperatures and different hot-dipping times as mentioned above. After hot-
dipping the samples were cooled in forced air. The top surface and cross-section of

samples were characterized using scanning electron microscopy (SEM), energy
dispersive X-ray (EDX) and X-ray diffraction studies (XRD).
Coating adherence was measured by bending the samples at 90° in an industrial bend
test machine (Mohr and Federtuff). After testing, those samples were photographed in
high-resolution camera to observe any macroscopic crack formation as well as the
adherence of coating with the steel substrate.
The corrosion performance of Al-Mg-Mn coating on steel was measured by DC
polarization test (Tafel Test) using VersaSTAT MC®, Princeton Applied Research
instrument. The test was carried out in three electrodes system. The working electrode
was Al-alloy coated sample, counter electrode was platinum and reference electrode
was calomel electrode. The test was conducted in 3.5% sodium chloride (NaCI) solution
with scan rate of 0.5mV/s. The corrosion rate was measured by Tafel extrapolation
technique using VersaStudio® software module.
Characterization of Al-Mq-Mn coated steel:
A series of characterization tests were performed on Al-Mg-Mn coated steel sheet to
evaluate coating morphology and performance.
Coating Characterization:
Fig.l shows the photographs of top surface of Al-alloy coated samples. The coated
samples at different coating times show the bright and adherent Al-alloy coating.

Fig. 2 shows the cross-sectional SEM microstructure of Al-Mg-Mn alloy coated steel
surface at different coating times: 10s, 20s and 30s. It shows that the coating consists
of two layers- a partly columnar Fe-AI intermetallic inner layer and a softer outer Al-
alloy coating layer. The inner intermetallic layer, commonly composed of mostly Fe2AI5
is an essential ingredient of the coating protection system, supplementing the outer Al-
alloy layer and providing a second line of defence against corrosion. A higher magnified
cross-sectional SEM microstructure and EDS analysis shows the actual structure of
coating (see Fig. 3) further reveals that the inner Fe-AI intermetallic layer further
consists of two layers: outer FeAI3 and Al-Mn alloy layers and inner Fe2AI5 layer. The
EDS analysis (marked by red colour rectangles) confirmed the presence of above
mentioned Fe-AI intermetallic compounds. The formation of intermetallic layer is the
reason behind high coating adherence to the substrate for all coated samples. XRD on
top surface of coating shows the prominence of Al peak in the pattern (see Fig. 4).
For further confirmation of two Fe-AI intermetallic layers, namely FeAI3 and Fe2AI5, EDS
line analysis was carried out across the cross-section of the coating (see Fig. 5). EDS
line analysis confirmed the presence of two different steps on the intensity lines of Al
line and Fe. This confirms the presence of two different Fe-AI intermetallic compounds.
After considering the EDS composition analysis in Fig. 3, EDS line analysis in Fig. 5 and
intensity map in Fig. 6, it was evident that FeAI3 is at the outer side and Fe2AI5 layer at
the inner side of Fe-AI intermetallic layer. Furthermore, the elemental intensity of Mn in
Fig. 6 shows certain increase in the outer intermetallic layer (FeAl3). This may be due to

the presence of Al-Mn intermetallic precipitates inside the layer. The Mg intensity map
in Fig. 6 shows consistent presence throughout the Al-alloy coating and decreases
significantly at the start of Fe-AI intermetallic layers. But there is certain Mg pickup in
between FeAl3 and Fe2AI5 layers as shown in Fig. 7.
Table 2 shows the variations of Al-alloy and Al-Fe intermetallic layer thickness as a
function of hot-dipping time. It shows that the Al-alloy coating, Al-Fe intermetallic layer
and total coating thickness varies almost linearly with dipping time (see Fig. 8).
Table 2 Variations of Al-alloy coating layer, Al-Fe intermetallic layer and total coating
thickness as a function of hot-dipping time.

Bend Test
Fig.9 shows the photographs of bend test samples for different hot-dipping times. The
samples were bent at 90°. The Al-alloy coating has good adherence with substrate due
to the formation of the intermetallic layers, only a few cracks are visible at the edge of

the outer bend surface due to the uneven surface and high coating thickness. The bend
test results conclude that the coating has enough adherences for forming operation.
Corrosion Behavior
The corrosion behaviour was evaluated by Tafel experiments, which were conducted in
3.5 pet sodium chloride solution with scan rate of 0.5mV/s. The corrosion rate (mills per
year mpy) was measured by Tafel extrapolation technique using VersaStudio® software
module. The average values of each parameter are listed in Table 3 for three different
dipping time samples.
Table 3 Corrosion behaviour of samples at different hot-dipping times.

From the above Table 3, it is evident that the corrosion rate for three different samples
varies closely around 1 mpy. As the outer Al-alloy is similar for three samples (see Fig.
2), the corrosion rate of Al-alloy coating would be similar which is consistent with
experimental results. The slight variation of corrosion rate in Table 3 is due to
difference in surface roughness of samples.

Fig. 10 shows the comparison of Tafel curves of Al-alloy coated samples for different
coating dipping times (10s, 20s, 30s), with that of Galvanized (GI) and Galvannealed
(GA) steels. From Fig. 10, it was found that the Al-alloy coated samples show excellent
corrosion resistance compared to GI and GA coating since the corrosion rate decreased
compared to the GA and GI coating. A typical GA coating contains Zn and 8-10 wt% Fe
and a typical GI coating is a pure Zn coating. A quantitative comparison of corrosion
rate of different coating on steel is listed in Table 4. Icorr values of Al-alloy coating
samples are much lower compared to GI and GA coated sample indicating excellent
corrosion resistance. The corrosion rates of Al-alloy coated samples are around 1.11
mpy, compared to 6.3 mpy and 3.14 mpy for GI and GA, respectively. Hence, it can be
concluded that Al-Mg-Mn coated Steel provides better corrosion resistance than GI and
GA coating. It is conclusive from the Table 4 that Al-alloy coating on steel has almost 3
to 6 times more corrosion resistance compared to GA and GI coating, respectively.
Table 4 Corrosion properties different coatings on steel.


Furthermore, it is evident from the Table 4 that Ecorr values of Al-alloy coated steels are
more negative compared to other aluminized coating on steels. This signifies that Al-
coated steel has more sacrificial corrosion behaviour than other aluminized steels
because of the presence of Mg in the coating. Moreover, the results also show that Al-
alloy coated steel has similar sacrificial properties compared to GA and GI.
Ecorr, GI < Ecorr, Al-Mg-Mn Coating< ECorr, GA < EC0rr, Aluminized Type 1< Ecorr,
Aluminized Type 2

SEM and EDS analysis of corroded portion of Al-alloy coated samples are shown in Fig.
11 (dipping time 20s). From the EDS analysis, it is observed that the corrosion product
is mostly MgCl2, due to the sacrificial properties of magnesium and the presence of
chloride ion. Other minor chloride salts present as a corrosion products are FeCl2 and
MnCI2. Hence, it could be concluded that Al-Mg-Mn alloy coated sample has good
sacrificial corrosion properties compared to other aluminized steels.
The Steel substrate coated with Al-Mg-Mn alloy provides the following advantages over
GA/GI coating and commercial aluminized steels (Type 1 and Type 2) steel sheet:
i) Excellent sacrificial corrosion behaviour compared to aluminized steels
(Type 1 and Type 2)
ii) Comparable sacrificial corrosion behaviour compared to GA and GI
coating.
iii) 3 times more corrosion resistant than GA coating.
iv) 6 times more corrosion resistant than GI coating.
v) Very good adherence to the steel substrate

We Claim:
1. An Aluminium-Magnesium-Manganese (Al-Mg-Mn) alloy coating for a steel
substrate, the coating comprising, in terms of wt%,
Al: 95 to 98%;
Mg: 2 to 4 %; and
Mn: 0.75 to 1.25 % .
2. The coating as claimed in claim 1 further comprising optionally one or more
elements selected from the group consisting of, in terms of wt. %, Si: 0.05 to
0.2%, Sn: 0.01 to 0.1%, Fe: 0.1 to 0.3%, Ni: 0.01 to 0.02%, Zn: 0.01 to 0.2%.
3. The coating as claimed in claim 1, wherein the coating is applied on the steel
substrate by a hot-dip process.
4. The coating as claimed in claim 1, claim 2 and claim 3, wherein the coating on
the steel substrate is applied at a hot-dipping temperature varying in the range
of 650°C to 720°C.
5. The coating as claimed in claim 1 to claim 4, wherein the coating on the steel
substrate is applied for a hot-dipping duration of 2 seconds to 60 seconds.
6. The coating as claimed in claim 1 to claim 5, wherein the coating on the steel
substrate comprises two layers, an outer Al-Mg-Mn alloy layer and an inner Al-Fe
intermetallic layer.

7. The coating as claimed in claim 6, wherein the inner Al-Fe intermetallic layer
further comprises of two layers, an outer FeAb and Al-Mn alloy layer and an
inner Fe2Al5 layer.
8. An Aluminium-Magnesium-Manganese (Al-Mg-Mn) alloy coated steel substrate
produced as per the claims 1 to claim 7.

ABSTRACT

The invention is related to an Al-Mg-Mn alloy coated steel sheet having excellent
corrosion behaviour compared to galvanized (GI)/galvannealed (GA) steel and
superior sacrificial corrosion properties in comparison with commercial aluminized
steels. The alloy coating as per the current invention comprises, in weight %, 95 to
98 % of Al, 2 to 4 % of Mg, 0.75 to 1.25 % of Mn, and optionally one or more
elements selected from the group consisting of Si: 0.05-0.2%, Sn: 0.01-0.1%, Fe:
0.1-0.3%, Ni: 0.01-0.02%, Zn: 0.01-0.2%, with the remainder consisting of
unavoidable impurities. The coating consists of broadly two layers: outer Al-Mg-Mn
alloy layer and inner finger-like Al-Fe intermetallic layer. The Fe-AI intermetallic layer
further consists of two layers: outer FeAI3 and Al-Mn alloy layers and inner Fe2AI5
layer.