Title: Al-Si-Mg-Mn Alloy Coated Steel Sheet with Excellent Corrosion Behaviour
FIELD OF THE INVENTION:
The present invention relates to an Al-Si-Mg-Mn alloy coating on a steel substrate. The coating demonstrates excellent corrosion behaviour compared to galvanized (Gl)/galvannealed (GA) coating and excellent sacrificial corrosion properties in comparison to commercial aluminized steels. More specifically, the invention related to an Al-Si-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. Zinc and zinc alloy coatings on steel are extensively used in construction and automobile body parts because of their superior resistance to atmospheric corrosion and excellent sacrificial ability for underlying steel [1-6]. 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.

In galvanized steel, the zinc coating serves two objectives- barrier protection by separating steel substrate from corrosive environment and galvanic protection where the coating is sacrificially corroded to protect the steel substrate. In contrast, the corrosion resistance of aluminized steel is provided mainly by an impervious and stable thin film of aluminium oxide (Al203) which acts as a barrier. If this film is damaged or removed by abrasion, another layer of oxide forms instantaneously to avoid further corrosion.
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) [7]. During manufacturing and handling of the final material, discontinuities in the aluminized coating may happen and sometimes those can extend to the steel substrate which create coating break-up. 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 5-11 wt% silicon to reduce the hot-dip temperature and minimize iron-aluminum intermetallic thickness [8-10], does not possess sacrificial corrosion property in sulphate solution to protect underlying steel substrate but protects transiently in chloride solution [11,12].

Table 1: Corrosion rate of galvanized and commercial aluminized steel sheet at different environments.

Unfortunately, limited information exists on Al based coating on steel which imparts sufficient sacrificial property to protect the underlying steel substrate. Recently, Tsuru et al [13-15] have studied the superior corrosion resistance property and hydrogen entry behavior of hot-dipped Al-Mg-Si alloy coated steel compared to galvanized steel. Furthermore, Pradhan et al [16] reported a novel Al-Mg-Mn alloy hot-dip coating on steel surface showing promising corrosion property. It was determined that Al-Mg-Mn alloy coating has superior sacrificial properties (ECorr = -0.88V vs. -0.55V for other commercial aluminized steels). The coating has 6 times less corrosion rate than Gl coating with comparable sacrificial property. Based on the literature, the different Al-alloy coating compositions reported are listed in Table 2.


OBJECTS OF THE INVENTION:
An object of the invention is to propose a coating that has excellent corrosion behaviour
compared to galvanized (Gl)/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-Si-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-Si-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 roofing enclosure, automobile and aerospace applications.
SUMMARY OF THE INVENTION:
Al-Si-Mg-Mn alloy coated steel substrate was produced by hot-dipping a steel substrate into a bath containing Al-Si-Mg-Mn alloy without using any flux. The alloy coating as per the current invention comprises, in weight %, 82 to 85 % of Al, 11 to 12 % of Si, 3.5 to 4.5 % of Mg, 0.2 to 0.5 % of Mn, and optionally one or more elements selected from the group consisting of Sn: 0.01-0.1%, Fe: 0.1-0.2%, 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 finds application in roofing enclosure, automobile and aerospace applications.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Fig. 1: Photographs of Al-SI-Mg-Mn alloy coated steel at dipping time of 30s. Fig. 2: Cross-sectional SEM micrographs and EDS analysis of coating for dipping time of 30s. Fig. 3: Top surface XRD analysis of Al-Si-Mg-Mn alloy coated steel substrate. Fig. 4: GDOES sputter depth profile of Al-Si-Mg-Mn alloy coated sample for dipping time of 30 s. Fig. 5: Tafel curves of Al-Si-Mg-Mn alloy coated sample, Galvanized (Gl) and Galvannealed (GA) steels.

DETAILED DESCRIPTION OF THE INVENTION:
The invention proposes an Al-Si-Mg-Mn alloy composition for hot-dip coating on a steel substrate. Table 3 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-Si-Mg-Mn alloy composition was evaluated for it corrosion properties using different tests and was observed that it provides significant advantages compared to Galvanized (Gl) and Galvannealed (GA) steels.


Preparation of Al-Mg-Mn coated steel;
Al-Si-Mg-Mn alloy coating was prepared by a hot-dipping cold rolled steel sheet using a bath composition as specified in the Table 3 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). Elemental depth-profiling of coating was recorded by Glow Discharge Optical Emission Spectroscopy (GDOES) to confirm the coating thicknesses of different phases obtained by SEM on cross-sections.
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 a three-electrode electrolytic cell at room temperature. Al-alloy coated sample and platinum mesh were used as working electrode and counter electrode, respectively. Standard calomel electrode was used as reference electrode. The test was conducted in 3.5 wt% sodium chloride (NaCI) solution with a scan rate of 0.5mV/s. The corrosion current (icorr), corrosion

potential (Ecorr) and corrosion rate (mpy) was measured by Tafel extrapolation technique using VersaStudio® software module. Each measurement was repeated for three times on three different places around the sample surface to ensure repeatability. For comparison with the new coatings, polarization test was conducted on commercial grade galvanized and galvannealed steels.
Example:
An Al-Si-Mg-Mn alloy coating was developed on interstitial free (IF) steel substrate. The IF steel substrate comprises in terms of wt%, C: 0.0021, Si: 0.002, Mn: 0.10, P: 0.012, Al: 0.039, Ti: 0.033, Nb: 0.012, S: 0.008, B: 0.0001 and N: 18 ppm.). The steel substrate was dipped in Al-Si-Mg-Mn alloy coating bath at a temperature of 600°C for a hot-dipping coating for 30 seconds. The hot-dipping bath alloy comprising the elements in terms of weight%, Al: 84.26 %; Si: 11.45 %; Mg: 4.02 %; and Mn: 0.27 %. The coated layer of Al-Si-Mg-Mn alloy on steel substrate when characterised, comprising the following elements in terms of weight% - Al: 78.2 %; Si: 13.1 %; Mg: 4.4 %; Mn: 0.2%; 0:3.4 %; and Fe: 0.7%. Al-Si-Mg-Mn coated steel:
A series of characterization tests were performed on Al-Si-Mg-Mn coated steel sheet to evaluate coating morphology and corrosion performance.

Coating Characterization:
Fig.l shows the photographs of top surface of Al-Si-Mg-Mn alloy coated samples for a hot-dipping duration of 30s. The coated samples show the bright and adherent Al-alloy coating.
Fig. 2 shows the cross-sectional SEM microstructure of Al-Si-Mg-Mn alloy coated steel surface at coating time of 30s. The coating consists of Al-Si-Mg-Mn alloy layer on the outside which has composition in weight %, 78 to 80 % of Al, 11 to 13 % of Si, 3.5 to 4.5 % of Mg, 0.2 to 0.5 % of Mn. Very thin Fe-Al intermetallic layer (<3µm) was found as expected due to presence of Si in the alloy. A higher magnified cross-sectional SEM microstructure and EDS analysis shows the actual structure of coating. The EDS analysis indicates the presence of thin Fe-Al intermetallic compounds. The formation of intermetallic layer is the reason behind high coating adherence to the substrate for all coated samples.
XRD analysis was performed for phase characterization of inner Fe-Al intermetallic layers of the coating. The top surface XRD pattern of coating is shown in Fig. 3. It clearly exhibits the presence of prominent Al intensity peaks (ICDD 4-0787) along with FeAI3 peaks (ICDD 1-1265) and Fe2Al5 (ICDD 47-1435) and FeAl3 phases, which is consistent with literature [16]. Few sallow intensity peaks of Al were also appeared due to presence of Al-alloy in the coating. Although two different Fe-Al intermetallic compounds were identified, there was no distinct boundary between the two phases observed.

Fig. 4 illustrates the GDOES elemental depth profiles of Al, Si, Mg, Mn, Fe and O for the sample hot-dipped for 30s. The results show consistency with the total coating thickness and structure compared to cross-sectional SEM micrograph shown in Fig. 2. It must be noted that it is quite difficult to determine the exact individual structure of very thin Fe-AI intermetallic layers from GDOES analysis.
Corrosion Behavior
The corrosion behavior was evaluated by anodic potentiodynamic polarization experiments, which were conducted in 3.5 wt% sodium chloride solution with a scan rate of 0.5mV/s. The corrosion rate (mpy) was measured by Tafel extrapolation technique using VersaStudio® software module. Fig. 5 depicts the comparison in Tafel curves of Al-alloy coated sample along with galvanized (Gl) coating and galvannealed (GA) coating. Al-alloy coated samples show better corrosion resistance compared to Gl or GA coating.
The average values of corrosion parameters are listed in Table 3 for Al-Si-Mg-Mn alloy coated sample, galvanized steel (Gl), galvannealed (GA) coating and bare steel substrate. For comparison, two different types of aluminized steels were presented in Table 3 from literature [17]. It clearly shows that corrosion current (icorr) values of Al-alloy coated samples are much lower compared to Gl. The average corrosion rate of Al-Si-Mg-Mn alloy coated sample is around 0.51 mpy, compared to 6.38 mpy for Gl coating and 3.14 mpy for GA coating. Hence, it can be concluded that the Al-Si-Mg-Mn alloy coating on steel has almost 12 times lower corrosion rate

compared to Gl coating. Al-Si-Mg-Mn alloy coating has higher corrosion rate compared to other aluminized steels due to the presence of highly active magnesium in the coating. Furthermore, it is evident from the Table 3 that corrosion potential (Ecorr) value of Al-Si-Mg-Mn alloy coating (-1.02 V) is more negative compared to other aluminized coatings on steel (-0.52 to -0.58 V). This signifies that Al-Si-Mg-Mn alloy coated steel exhibits superior sacrificial corrosion behavior than other aluminized steels due to the presence of Mg in the coating. In comparison with steel substrate, the Al-Si-Mg-Mn alloy coating has much lower corrosion potential than that of steel substrate (-0.54 V) and hence this coating will provide cathodic protection to steel substrate efficiently. Moreover, the results also show that Al-Si-Mg-Mn alloy coated steel has better sacrificial property compared to Gl and GA coating as it's Ecorr value is more negative than Gi andGA.
Hence, the following galvanic series in 3.5 wt% NaCI can be derived:
Ecorr, Al-alloy