Claims:1. An Aluminium based alloy coating composition for a steel substrate, said composition comprising Aluminium (Al) at a concentration of about 82% to 83% (wt/wt); Silicon (Si) at a concentration of about 11% to 12% (wt/wt); Magnesium (Mg) at a concentration of about 4% to 5% (wt/wt); and Copper (Cu) at a concentration of about 1.5% to 2 %(wt/wt).
2. The coating composition as claimed in claim 1, wherein said composition further comprises element selected from a group comprising Tin (Sn) at a concentration of about 0.01% to 0.1% (wt/wt), Iron (Fe) at a concentration of about 0.1% to 0.3% (wt/wt), Nickel (Ni) at a concentration of about 0.01% to 0.02% (wt/wt) and Zinc (Zn) at a concentration of about 0.0l% to 0.2% (wt/wt) or any combination thereof.
3. The coating composition as claimed in any of the preceding claims, wherein said composition is applied on the steel substrate by hot-dip process, to form a coating on the steel substrate.
4. The coating composition as claimed in claim 3, wherein the application of the coating on the steel substrate results in formation of two layers, with microstructure comprising phases selected from Al, Si, Al2Cu, Mg2Si and Al8Fe2Si.
5. The coating composition as claimed in claim 4, wherein the two layers are an outer Al-Si-Mg-Cu alloy layer and an inner Al-Fe-Si intermetallic layer.
6. The coating composition as claimed in claim 5, wherein top surface of the outer Al-Si-Mg-Cu alloy layer comprises Al at a concentration of about 69% to 72% (wt/wt), Si at a concentration of about 20% to 22 % (wt/wt), Mg at a concentration of about 2.5% to 3% (wt/wt), Cu at a concentration of about 3% to 5% (wt/wt), Oxygen (O) at a concentration of about 1.5% to 2.5 % (wt/wt) and Fe at a concentration of about 0.4% to 0.6% (wt/wt).
7. The coating composition as claimed in claim 5, wherein the microstructure of the outer Al-Si-Mg-Cu alloy layer comprises a-Al, Mg2Si, Si and ?-Al2Cu phases.
8. The coating composition as claimed in claim 5, wherein the inner Al-Fe-Si intermetallic layer comprises Al at a concentration of about about 54% to 59% (wt/wt), Fe at a concentration of about 24% to 29% (wt/wt) and Si at a concentration of about 16% to 18% (wt/wt).
9. The coating composition as claimed in claim 5, wherein the microstructure of the inner Al-Fe-Si intermetallic layer comprises Al8Fe2Si phase.
10. The coating composition as claimed in claim 3, wherein total thickness of the coating on the steel substrate ranges from about 25 µm to 40µm.
11. The coating composition as claimed in claim 5, wherein thickness of the outer layer ranges from about 30µm to 34µm, and thickness of the inner layer ranges from about 2µm to 4µm.
12. A process for coating a steel substrate with the Aluminium based alloy coating composition as claimed in any of the preceding claims, said process comprising steps of:
preparing the alloy coating composition in a hot dip process simulator bath;
maintaining temperature of the simulator bath and coating the steel substrate by hot-dipping the substrate in the bath for a predetermined time period; and
cooling the coated substrate;
to obtain the coated steel substrate.
13. The process as claimed in claim 12, wherein the hot dip process simulator bath is maintained at a temperature ranging from about 585°C to 600°C.
14. The process as claimed in claim 12, wherein the steel substrate is subjected to hot dipping for a time period ranging from about 3 seconds to 8 seconds.
15. The process as claimed in claim 12, wherein the cooling of the coated substrate is performed in controlled atmosphere of the hot dip process simulator, wherein the controller atmosphere is N2 + 10% H2 atmosphere.
16. The process as claimed in claim 12, wherein the steel substrate is subjected to degreasing, rinsing and drying before said coating process.
17. The process as claimed in claim 16, wherein the degreasing of the steel substrate before the coating process is performed by contacting the substrate with commercial alkaline solution at a temperature of about 55°C to 65°C, for about 1 minute to 3 minutes; the rinsing is with deionized water for about 1 to 2 minutes; and the drying is at a temperature of about 40 to 50°C.
18. A steel substrate comprising at least one coat of the Aluminium based alloy coating composition as claimed in any of the preceding claims.
19. The steel substrate as claimed in claim 18, wherein thickness of the coat ranges from about 25µm to 40µm.
20. The steel substrate as claimed in claim 18, wherein corrosion rate of the steel substrate having at least one coat of the Aluminium based alloy coating composition is reduced by at least 7 times when compared to galvanized (GI) steel and 3 times when compared to galvannealed (GA) steel, in chloride solution.
21. An article comprising the coated steel substrate as claimed in claim 18.
, Description:TECHNICAL FIELD
The present invention relates to the field of material science and corrosion. Particularly, the present invention provides an Aluminium based alloy composition for coating on a substrate. Said alloy is an Al-Si-Mg-Cu alloy, suitable for 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.

BACKGROUND OF THE DISCLOSURE
Aluminium and Aluminium alloys represent an important category of materials due to their high technological values and wide range of industrial applications. Presently, Aluminium alloys are used in automobile body panels, roofing enclosures 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. Though traditionally, steel coated with zinc and its alloys have been used extensively in construction and automobile body parts, Aluminium coatings on steel is an attractive alternative to such galvanized steel. This is because such coatings provide for properties such as low weight, oxidation resistance, 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. However, in contrast, the corrosion resistance of aluminized steel is provided mainly by an impervious and stable thin film of aluminum oxide (Al2O3) which acts as a barrier. If this film is damaged or removed by abrasion, another layer of oxide forms instantaneously to avoid further corrosion. Thus, Aluminized steel makes for a good alternative to galvanized steel.
As a result, 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) [Cerlane et al]. 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 breaks. Those coating breaks exposing the steel base may lead to 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, coated with aluminum-silicon alloy containing 5-11 wt% silicon [Takeda et al.; Buehler et al.; Cheng et al.], does not possess sacrificial corrosion property in sulphate solution to protect underlying steel substrate but protects transiently in chloride solution [Vu et al.; Graeve et al.].
Unfortunately, limited information exists on Al based coating on steel which imparts sufficient sacrificial property to protect the underlying steel substrate. Therefore, it is required to develop Aluminium and Aluminium based alloys as coating materials which specifically enhance the advantages of such coating materials and successfully overcome the disadvantages that such compositions are known to be associated with.

STATEMENT OF THE DISCLOSURE
Accordingly, the present disclosure provides an Aluminium based alloy coating composition for a steel substrate, said composition comprising Aluminium (Al) at a concentration of about 82% to 83% (wt/wt); Silicon (Si) at a concentration of about 11% to 12% (wt/wt); Magnesium (Mg) at a concentration of about 4% to 5% (wt/wt); and Copper (Cu) at a concentration of about 1.5% to 2 %(wt/wt).
The disclosure further provides a process for coating a steel substrate with the Aluminium based alloy coating composition of the disclosure, wherein said process comprises steps of:
preparing the alloy coating composition in a hot dip process simulator bath;
maintaining temperature of the simulator bath and coating the steel substrate by hot-dipping the substrate in the bath for a predetermined time period; and
cooling the coated substrate;
to obtain the coated steel substrate.
Furthermore, the disclosure provides a steel substrate comprising at least one coat of the Aluminium based alloy coating composition described above.
Lastly, the present disclosure provides an article comprising the coated steel substrate.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 depicts photographs of Al-Si-Mg-Cu alloy coated steel substrate subjected to dipping time of 3s.
Figure 2 top surface SEM micrographs of the coating on the alloy coated steel substrate.
Figure 3 depicts top surface SEM micrographs and EDS analysis of the coating on the alloy coated steel substrate subjected to dipping time of 3s.
Figure 4 depicts top surface EDS elemental map analysis of the coating on the alloy coated steel substrate subjected to dipping time of 3s.
Figure 5 depicts cross-sectional SEM micrographs and EDS analysis of the coating on the alloy coated steel substrate subjected to dipping time of 3s.
Figure 6 depicts cross-sectional EDS elemental map analysis of the coating on the alloy coated steel substrate subjected to dipping time of 3s.
Figure 7 depicts top surface XRD analysis of the coating on the alloy coated steel substrate subjected to dipping time of 3s.
Figure 8 depicts GDOES sputter depth profile of the alloy coated subjected to dipping time of 3s.
Figure 9 depicts photographs of the alloy coated substrate subjected to bend test (for dipping time of 3s).
Figure 10 depicts cross-sectional SEM image shows vertical cracks formed in Al-Si-Mg-Cu alloy and Al-Fe-Si intermetallic layers during bending in the of the alloy coated substrate prepared by subjecting to dipping time of 3s.
Figure 11 depicts salt spray test (SST) results of the Al-Si-Mg-Cu alloy coated substrate and Galvanized (GI) steel sheets.
Figure 12 depicts Tafel curves of the Al-Si-Mg-Cu alloy coated steel, Galvanized (GI) steel and Galvannealed (GA) steels.

DETAILED DESCRIPTION OF THE DISCLOSURE
As used herein, the symbols ‘Al’, ‘Si’, ‘Mg’, ‘Cu’, ‘Fe’, ‘Mn’, ‘Sn’, ‘Ni’, ‘Zn’ and ‘O’ refer to elements Aluminium, Silicon, Magnesium, Copper, Iron, Manganese, Tin, Nickel, Zinc and Oxygen which may be used individually or in combination with each other to generate alloys, in the present disclosure.
As used herein, the phrases ‘Al-Si-Mg-Cu based alloy composition’, ‘Aluminium based alloy coating composition’, ‘alloy coating composition’, ‘alloy composition’ or ‘coating composition’ have been used interchangeably and refer to an alloy composition comprising Al, Si, Mg, Cu and optionally elements such as Sn, Fe, Ni and/or Zn suitable for coating on a metallic substrate, preferably a steel substrate, primarily for the purpose of preventing corrosion of the substrate.
As used herein, the terms ‘coat’ and ‘coating’ have been used interchangeably to refer to at least one layer of alloy coating composition on the steel substrate.
As used herein, the terms ‘coated substrate’, ‘coated steel substrate’ and ‘alloy coated substrate’ have been used interchangeably to refer to the steel substrate comprising a coating of the Al-Si-Mg-Cu based alloy composition on its surface.
As used herein, the abbreviation ‘GI’ refers to Galvanized and ‘GA’ refers to Galvannealed, wherein definition of both is consistent with that known in the art.
As used herein the, the abbreviation ‘SEM’ refers to the technique of Scanning Electron Microscope.
As used herein the, the abbreviation ‘EDS’ refers to the technique of Energy-dispersive X-ray Spectroscopy.
As used herein the, the abbreviation ‘XRD’ refers to the technique of X-ray Diffraction
As used herein the, the abbreviation GDOES refers to the technique of Glow Discharge Optical Emission Spectroscopy (GDOES).
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The present disclosure provides an Aluminium based alloy coating composition for a steel substrate.
More specifically, the present disclosure relates to an alloy coating composition for a steel substrate, wherein the composition comprises Aluminium, Silicon, Magnesium and Copper. The composition of the present disclosure is thus an Aluminium-Silicon-Magnesium-Copper (Al-Si-Mg-Cu) alloy.
Further, the alloy composition of the present disclosure optionally comprises one or more elements selected from a group comprising Tin (Sn), Iron (Fe), Nickel (Ni) and Zinc (Zn) or any combination thereof. Thus, in the present disclosure, the alloy coating composition for steel substrate comprises Aluminium, Silicon, Magnesium, Copper and optional element(s) selected from a group comprising Tin, Iron, Nickel and Zinc or any combination thereof.
Accordingly, the present disclosure in an exemplary embodiment provides an alloy coating composition for a steel substrate, wherein the alloy comprises Aluminium, Silicon, Magnesium, Copper and Tin. Alternatively, the composition for steel substrate comprises Aluminium, Silicon, Magnesium, Copper and Iron; or Aluminium, Silicon, Magnesium, Copper and Nickel. In a further alternate embodiment, the alloy based coating composition for steel substrate comprises Aluminium, Silicon, Magnesium, Copper and Zinc.
In all embodiments of the present disclosure, the Aluminium based alloy coating composition of the present disclosure is highly specific and provides excellent corrosion resistant properties to the substrate on which it is applied. The desired properties of the alloy are resultant of specific concentrations of elements within the composition, which significantly improve the sacrificial corrosion behavior of the steel substrate on which the coating is applied. The present disclosure thus provides the alloy coating composition having highly specific concentrations of elements and comprises Aluminium (Al) at a concentration of about 82% to 83% (wt/wt); Silicon (Si) at a concentration of about 11% to 12% (wt/wt); Magnesium (Mg) at a concentration of about 4% to 5% (wt/wt) and Copper (Cu) at a concentration of about 1.5% to 2 %(wt/wt).
As the alloy coating composition of the disclosure further comprises optional element(s), they too are provided at specific concentrations, wherein when present in the composition, Tin (Sn) is at a concentration of about 0.01% to 0.1% (wt/wt), Iron (Fe) is at a concentration of about 0.1% to 0.3% (wt/wt), Nickel (Ni) is at a concentration of about 0.01-0.02% (wt/wt) and Zinc (Zn) is at a concentration of about 0.0l% to 0.2% (wt/wt).
Accordingly, the present disclosure in an exemplary embodiment provides an alloy coating composition for steel substrate comprising Al at a concentration of about 82% to 83% (wt/wt); Si at a concentration of about 11% to 12% (wt/wt); Mg at a concentration of about 4% to 5% (wt/wt), Cu at a concentration of about 1.5% to 2 %(wt/wt) and Sn at a concentration of about 0.01% to 0.1% (wt/wt). In an alternate embodiment, the alloy coating composition for steel substrate comprises Al at a concentration of about 82% to 83% (wt/wt); Si at a concentration of about 11% to 12% (wt/wt); Mg at a concentration of about 4% to 5% (wt/wt), Cu at a concentration of about 1.5% to 2 %(wt/wt) and Fe at a concentration of about 0.1% to 0.3% (wt/wt). In yet another alternate embodiment, the alloy coating composition for steel substrate comprises Al at a concentration of about 82% to 83% (wt/wt); Si at a concentration of about 11% to 12% (wt/wt); Mg at a concentration of about 4% to 5% (wt/wt), Cu at a concentration of about 1.5% to 2 %(wt/wt) and Ni at a concentration of about 0.01% to 0.02% (wt/wt). In a further alternate embodiment, the alloy coating composition for steel substrate comprises Al at a concentration of about 82% to 83% (wt/wt); Si at a concentration of about 11% to 12% (wt/wt); Mg at a concentration of about 4% to 5% (wt/wt), Cu at a concentration of about 1.5% to 2 %(wt/wt) and Zn at a concentration of about 0.0l% to 0.2% (wt/wt).
Thus, in the present disclosure, the alloy coating composition for steel substrate comprises Al at a concentration of about 82% to 83% (wt/wt); Si at a concentration of about 11% to 12% (wt/wt); Mg at a concentration of about 4% to 5% (wt/wt), Cu at a concentration of about 1.5% to 2 %(wt/wt) and element selected from a group comprising Sn at a concentration of about 0.01% to 0.1% (wt/wt), Fe at a concentration of about 0.1% to 0.3% (wt/wt), Ni at a concentration of about 0.01-0.02% (wt/wt) and Zn at a concentration of about 0.0l% to 0.2% (wt/wt) or any combination thereof.
Further, as anticipated by a person skilled in the art, the coating composition of the disclosure may comprise unavoidable impurities that may be incorporated in the composition by virtue of their presence in the raw materials or in the environment of preparation of the alloy or coating of the substrate. However, these impurities when present, are at such significantly low concentrations, that they do not, in any manner, alter the desired properties of the coating composition of the present disclosure.
Though the coating composition of the present disclosure is highly specific in its components, the concentrations of the components and the optional elements included therein, the applicability of the coating is extremely wide. In other words, though the coating is well defined in its structural and mechanical properties, the steel on which it is coated may be of any type and grade. Regardless of the composition or inherent properties of the steel employed, the coating of the present disclosure provides the desired properties of corrosion resistance. Thus, the steel substrate referred to in the present disclosure encompasses all types of steels known to a person skilled in the art, and thus the coating composition of the present disclosure can be applied on any steel for the desired properties, as provided by the present disclosure. Accordingly, as an exemplary embodiment, the alloy coating composition of the present disclosure is coated on a steel substrate such as but not limiting to carbon steel, alloy steel, stainless steel, tool steel or any combination thereof, to form a coat on the steel substrate. Preferably, the alloy coating composition of the present disclosure is coated on interstitial free steel (IF steel) substrate.
Once the Aluminium based alloy coating composition of the present disclosure is applied on the steel substrate, morphology of the coated surface of the substrate is studied by photographing the surface and by SEM, which reveals bright and adherent nature of the coating. The top surface micrograph reflects presence of pores on the surface due to the regular phenomenon of solidification contraction, whereas the cross-sectional SEM micrograph of the Al-Si-Mg-Cu alloy coated surface of the steel substrate shows that the coating consists of multiple layers.
Thus, in all embodiments of the present disclosure, the coating results in formation of 2 layers on the steel substrate, one towards the outer periphery (or the top surface) of the coating whereas the other closer to the steel substrate. Said 2 layers of the coating formed on the steel substrate are thus an outer layer and an inner layer, wherein the outer layer comprises the Al-Si-Mg-Cu alloy and the inner layer is an Al-Fe-Si intermetallic layer.
The main component of the outer layer is aluminum which is the inherent thermodynamic active metal. above the coated surface, under atmospheric environment, 1nm natural oxide film is formed. Said layer practically show stable corrosion resistance by forming the passivation film (Al2O3). As and when the passivation film breaks, a fresh new film forms instantaneously and protects the surface from corrosion. The presence of silicon in Al alloy alters the microstructure of the aluminide layer and transforms the thicker Fe-Al intermetallic layer into a thinner Al-Fe-Si intermetallic compound. Si content in aluminum alloy also leads to diminish thickness of hard and brittle intermetallic layer Al-Fe-Si to improve the formability of coated steel. The presence of copper decreases the activity coefficient of aluminum at the interface and consequently decreases the thickness of the intermetallic layer. In addition, copper in aluminum alloy improves wettability with the steel substrate leading to decrease in the reaction layer thickness
In an embodiment of the disclosure, total thickness of the coating on the steel substrate ranges from about 25 µm to 40 µm. Amongst the two layers present in said coating, the outer layer is thicker than the inner layer, wherein while thickness of the outer layer ranges from about 30µm to 34µm, preferably about 30µm, thickness of the inner layer ranges from about 2µm to 4µm, preferably about 2µm
Further, with regard to the chemical composition, XRD pattern analysis of the top surface of the coated substrate shows presence of prominent Al intensity peaks, in line with the SEM micrograph. The outer layer comprises oxygen due to oxidation of said Aluminium, whereas the average Si content is enhanced due to higher oxidation tendency and lower atomic size of Si. Accordingly, the top surface of the alloy coated substrate shows following composition of the outer Al-Si-Mg-Cu alloy layer: Al at a concentration of about 69% to 72% (wt/wt), Si at a concentration of about 20% to 22 % (wt/wt), Mg at a concentration of about 2.5% to 3% (wt/wt), Cu at a concentration of about 3% to 5% (wt/wt), Oxygen (O) at a concentration of about 1.5% to 2.5 % (wt/wt) and Fe at a concentration of about 0.4% to 0.6% (wt/wt).
On the other hand, the EDS analysis results of the inner Al-Fe-Si intermetallic layer shows that average composition of said layer is Al: about 54% to 59% (wt/wt), Fe: about 24% to 29% (wt/wt) and Si: about 16% to 18% (wt/wt). It is this intermetallic layer which is responsible for high adherence of the coating with the substrate.
Furthermore, upon studying the microstructure of these layers, it is found that these layers comprise of uniformly distributed phases or structural elements selected from Al, Si, Al2Cu, Mg2Si and Al8Fe2Si.
In an embodiment, the outer layer of the coat comprises of multiple phases selected from a group comprising a-Al, Mg2Si, Si-segregation and ?-Al2Cu or any combination thereof. The ?-Al2Cu phase in the outer layer of the coat is a lamellar structured Cu-rich intermetallic phase. On the other hand, the inner Al-Fe-Si intermetallic layer comprises of a single Al8Fe2Si phase.
It is found that the near eutectic composition of Si in the master alloy leads to eutectic structure of (a-Al+Mg2Si) throughout the Al-Si-Mg-Cu alloy layer. The outer Al-Si-Mg-Cu alloy layer exhibits a eutectic microstructure of Al and Mg2Si, and thereby inhibits the localized corrosion on the coating surface. The Mg and Si elements (Mg2Si phase) phase is uniformly surrounded around the a-Al phase boundary, whereas in few places, Cu rich phase (?-Al2Cu) is found across the cross-section of the coating. Oxygen is also distributed evenly throughout the coating layer. This microstructure of the coating prevents horizontal cracks, traditionally responsible for flaking or peeling off of the coating layer.
While the coating composition of the present disclosure can be applied on a steel substrate by any process known to a person skilled in the art (as long as it achieves the desired composition and properties described herein), the present disclosure also provides a specific hot-dip process for the said coating.
The present disclosure thus also provides a process for coating a steel substrate with the Al-Si-Mg-Cu alloy coating composition of the disclosure, said process comprising steps of:
preparing the Al-Si-Mg-Cu based alloy coating composition in a hot dip process simulator bath;
maintaining temperature of the simulator bath and coating the steel substrate by hot-dipping the substrate in the bath; and
cooling the coated substrate
to obtain the coated steel substrate.
In an embodiment, said process for coating a steel substrate with the Al-Si-Mg-Cu alloy coating composition of the disclosure comprises steps of:
preparing the Al-Si-Mg-Cu based master alloy in a high temperature melting furnace casting said alloy to small solid pieces with precise composition;
re-melting said Al-Si-Mg-Cu master alloy casts in a hot dip process simulator bath;
maintaining temperature of the simulator bath and coating the steel substrate by hot-dipping the substrate in the bath; and
cooling the coated substrate
to obtain the coated steel substrate.
Before the steel substrate is coated by hot dipping it in bath comprising the alloy composition of the present disclosure, the composition is prepared by conventional mixing of elements in high temperature melting furnace for master alloy preparation. The master alloy is re-melted in the hot-dip simulator bath as and when required to prepare the Al-Si-Mg-Cu based alloy composition for coating. Once the coating composition is prepared, the steel substrate is dipped in the hot bath.
Accordingly, in embodiments of the present disclosure, the hot dip process simulator bath is maintained at a temperature ranging from about 585°C to 600°C, preferably about 590°C; and the steel substrate is subjected to hot dipping for a time period ranging from about 3 seconds to 8 seconds, preferably about 3 seconds.
Once dipped for the said time period, the coated substrate is cooled in controlled atmosphere of the hot dip process simulator; wherein the controller atmosphere is N2 + 10% H2 atmosphere.
In an optional embodiment, prior to subjecting a steel substrate to the coating process, the steel substrate is subjected to degreasing, rinsing and drying.
Degreasing of the steel substrate before the coating process is performed by contacting the substrate with commercial alkaline solution (Ridoline 1352 BA, of M/s Henkel Chembond India Ltd., 2.5 wt%) at a temperature of about 55°C–65°C, preferably about 60°C, for about 1 minute to 3 minutes, preferably about 2 minutes.
After said degreasing, the obtained steel substrate is subjected to rinsing with deionized water for about 1 to 2 minutes, preferably about 1 minutes, and thereafter dried at a temperature of about 40°C to 50°C, preferably about 40°C.
In an embodiment, the substrates are annealed at a temperature of about 840°C to 860°C, preferably 850°C before hot-dipping in Al-Si-Mg-Cu alloy bath using N2 + 10%H2 controlled atmosphere inside the HDPS with dew point of about -30°C to induce ductility and to produce an oxide free surface.
In an embodiment, thickness of the coating formed on the steel substrate ranges from about 25µm to 40µm, preferably about 32µm.
Further, the present disclosure provides a steel substrate coated with the Al-Si-Mg-Cu based alloy coating composition of the disclosure. Though, the present disclosure exemplifies the steel substrate with one coat of the alloy composition, it is understood by a person skilled in the art that the disclosure is not limiting to a single coat of the composition. The disclosure thus envisages more than one coat of the composition on the steel substrate.
Further, after coating, the coated steel substrate of the present disclosure is subjected to bend test to analyze adherence of the coat to the substrate, wherein the coated substrate is bent by 90°, and it is observed that-the coated substrate shows absence of horizontal cracks usually associated with flaking or peel off, on the bent substrate; proving that the coating has good adherence with substrate.
As stated above, the steel substrate coated with the alloy composition of the present disclosure exhibits excellent corrosion resistant properties. For instance, the Al-Si-Mg-Cu alloy coated steel substrate of the present disclosure resists the formation of red rust in aggressive marine environment. In an exemplary embodiment, the coated steel substrate of the present disclosure remains red rust free for more than 1200 hours on exposure to salt spray.
Additionally, the Al-Si-Mg-Cu alloy coated steel substrate of the disclosure shows better passivation behaviour when compared to GI and GA coating. Accordingly, the steel substrate has enhanced corrosion resistance compared to GI or GA coating.
In exemplary embodiments of the present disclosure, the Al-Si-Mg-Cu alloy coated steel of the disclosure has 7 times lower corrosion rate as compared to steel with GI coating and 3 times lower corrosion rate as compared to steel with GA coating.
The Al-Si-Mg-Cu alloy coated steel substrate also exhibits better sacrificial corrosion behavior (Ecorr = -0.87 V) compared to Type 1 and Type 2 aluminized steels (Ecorr = -0.52 to -0.58 V) and GA coating on steel (Ecorr = -0.848 V). Said enhancement in sacrificial corrosion behaviour is attributed to the presence of highly activate Mg (standard reduction potential of Mg is -2.36V) in the coating.
While the coated steel substrate of the present disclosure provides the said desired properties vis-à-vis enhance corrosion resistance, an article made out of or comprising the said coated steel shall also comprise of the same properties and advantages. The present disclosure thus also provides an article comprising the coated steel substrate. Such an article may be any article finding application in construction, automobile or aerospace industries. Exemplary embodiments of such articles include but are not limited to water pipes, bridges and buildings, railway cars, automobile skin panel tools etc.
In an embodiment, the foregoing descriptive matter is illustrative of the disclosure and not a limitation. Providing working examples for all possible combinations of optional elements in the composition and process parameters such as but not limiting to time and temperature of hot dipping, is considered redundant.
While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

EXAMPLES
EXAMPLE 1: Preparation of Al-Si-Mg-Cu coated steel
A cold rolled IF steel sheet (composition of steel sheet, C: 0.0021ppm, Si: 0.002ppm, Mn: 0.10ppm, P: 0.012ppm, Al: 0.039ppm, Ti: 0.033ppm, Nb: 0.012ppm, S: 0.008ppm, B: 0.0001ppm and N: 18ppm, balance Fe) is degreased in commercial alkaline solution at 50°C for 2 min and rinsed properly with deionized water and dried. The sample is then annealed at 850°C before hot-dipping in Al-Si-Mg-Cu alloy bath using N2 + 10%H2 controlled atmosphere inside the HDPS with dew point of -30°C.
The degreased IF steel sheet is then hot-dipped at 590°C for a hot-dipping time of 3s in an Iwatani Surtec® Hot Dip Process Simulator (HDPS) comprising a bath composition comprising elements (concentration in terms of wt%), as specified below-
Al: 82.30 %;
Si: 11.55 %;
Mg: 4.15 %; and
Cu: 1.85 %
Said composition is further prepared at different concentration of elements and hot dipping is conducted at different temperatures and different hot-dipping times as mentioned in Table 1.
Table 1: Al-Si-Mg-Cu alloy compositions and process parameters for coating a steel sheet.
Al-Si-Mg-Cu alloy Composition (wt%) Hot-dip Parameters
Al Si Mg Cu Others (optional) Hot-dipping Temp. (°C) Hot-dipping Time (s)
82-83 11-12 4-5 1.5-2.0 Sn: 0.01-0.1%
Fe: 0.1-0.3%, Ni: 0.01-0.02%, Zn: 0.01-0.2%, 585-600 3-8

EXAMPLE 2: Characterization of the Al-Si-Mg-Cu alloy coating on the steel substrate
The Al-Si-Mg-Cu alloy coated steel substrate characterized herein is any of the Al-Si-Mg-Cu coated steel as prepared according to example 1 above.
A. Analysis of the top surface appearance
Photographs of the top surface of the dried Al-Si-Mg-Cu alloy coated steel substrate hot dipped for duration of 3s are taken. The coated substrate shows the bright and adherent Al-alloy coating (Fig. 1).
Top surface SEM micrographs (Fig. 2) of the alloy coated substrate show no variations in coating surface morphology and lamellae size for dipping times of 3 to 8s. Further, pores are found to exist on the surface due to the solidification contraction as a usual phenomenon. The top surface of the Al-Si-Mg-Cu alloy coating from said micrographs (Fig.2a) is found to comprise in terms of average wt%:
Al: 69.83 %;
Si: 21.54 %;
Mg: 2.82 %;
Cu: 3.30 %;
O: 1.91 %; and
Fe: 0.60 %.

The higher average Si content (21.54 wt%) and its random distribution on the coating surface is due to higher oxidation tendency and lower atomic size of Si which enhance the diffusion of Si towards surface. The presence of excess oxygen on the top surface can be explained by the instantaneous oxide formation of aluminum during cooling. High resolution top surface SEM micrograph of the alloy coated substrate further depict presence of different phases of primary a-Al; Mg2Si and Si-segregation and lamellar structured Cu-rich intermetallic (?-Al2Cu) in the coat (Fig. 3) which is further confirmed by top surface EDS elemental map analysis of coating (see Fig.4) which shows a-Al phase, uniform distribution of Mg2Si and ?-Al2Cu phases. The uniform and fine distribution of Mg2Si and Al2Cu phases prevent the localized corrosion of coating layer.

B. Cross-sectional analysis of the coating layer
The Al-Si-Mg-Cu alloy coated steel substrate prepared in example 1 is further subjected to cross-sectional SEM for cross-sectional characterization. The cross-sectional SEM microstructure (Fig. 5) clearly shows that the coating on the substrate consists of broadly two layers: a thin Al-Fe-Si intermetallic inner layer (2-4µm) and an outer thick Al-Si-Mg-Cu alloy coating layer (30-34µm).
The Al-Si-Mg-Cu alloy layer shows network structure of high Si content throughout the coating layer. The near eutectic composition of Si (11.55 wt%) in the master alloy leads to eutectic structure of (a-Al+Mg2Si) throughout the Al-Si-Mg-Cu alloy layer as per the Al-Si and Al-Mg2Si phase diagrams. The outer Al-Si-Mg-Cu alloy layer exhibits a eutectic microstructure of Al and Mg2Si, which may be contributory in inhibiting localized corrosion on the coating surface.
The EDS analysis of inner Al-Fe-Si intermetallic layer (Fig. 6) shows the average composition of:
Al: 58.63 wt%,
Fe: 24.32 wt% and
Si: 17.04 wt%
The EDS analysis confirms the presence of Al-Fe-Si alloy layer and the phase present therein is Al8Fe2Si. The presence of uniformly distributed a-Al and Mg2Si phase is determined from the EDS elemental map analysis which is performed across the cross-section of the coating (Fig. 6). It clearly shows that Mg and Si elements (Mg2Si phase) phase is uniformly surrounded around the a-Al phase boundary. In few places, Cu rich phase (?-Al2Cu) is found across the cross-section of the coating (Fig.5). Oxygen is also found to be distributed evenly throughout the coating layer (Fig.6).
Further, XRD analysis of the surface of the alloy coated substrate is performed for phase characterization of the coating. The top surface XRD pattern of coating (Fig. 7) exhibits the presence of prominent Al intensity peaks along with Si, Al2Cu, Mg2Si and Al8Fe2Si phases. XRD analysis is consistent with the SEM and EDS analysis of the coating, provided above.
Lastly, GDOES elemental depth profile analysis of Al, Si, Mg, Cu, Fe and O for the alloy coated substrate hot-dipped for 3s (Fig. 8) is performed to evaluate the coating thickness and elemental distribution along the thickness of the coating. The GDOES analysis results show lower total coating thickness compared to cross-sectional SEM micrograph shown in Fig. 5. Difficulties associated with determining the exact individual structure of very thin Al-Fe-Si intermetallic layer from GDOES analysis would be obvious to those skilled in the art, hence approximate total layer thickness of about 28 µm is marked in Fig. 8.
EXAMPLE 3: Analysis of coating adherence

The alloy coated substrate subjected to dipping time of 3s in Example 1 is bent at 90° in an industrial bend test machine (Mohr and Federtuff®). After testing, the substrate is photographed in high-resolution camera and SEM to observe macroscopic cracks formed during bending as well as the adherence of coating with the steel substrate (Fig. 9).
Cross-sectional SEM image of test substrate shows only the formation of vertical cracks in the thick Al-Si-Mg-Cu alloy and thin brittle Al-Fe-Si intermetallic layers during deformation (Fig. 10). No horizontal cracks which usually cause flaking or peel off of coating layer are seen in the intermetallic or alloy coating layer. Thus, bend test results suggest that the coating has enough adherences for forming operation without flaking. The coating has good adherence with substrate due to the formation of the Al-Fe-Si intermetallic layer.
EXAMPLE 4: Evaluation of corrosion behavior: Salt Spray Test (SST)
Corrosion performance of Al-Si-Mg-Cu coating on the steel substrate is measured in a Weiss Tenik SC 450® salt spray test (SST) chamber as per ASTM B117 standard. The test is simultaneously performed on a GI steel sample for comparison.
It is evident from the photographs taken in the course of the test that there is no visible sign of red rust formation and coating delamination of Al-Si-Mg-Cu alloy coated steel surface after 350 h, whereas in case of the GI steel sample, the red rust formation starts after 300 h of test (Fig. 11). After 650 h of test, GI coating shows significant formation of red rust on the surface whereas Al-Si-Mg-Cu alloy coated sheet shows only white rust formation. These tests demonstrate the ability of the Al-Si-Mg-Cu alloy coated steel to resist corrosion even in aggressive marine environment. Even after salt exposure over 1200 h, Al-Si-Mg-Cu alloy coated steel does not show any red-rust. Hence, Al-Si-Mg-Cu alloy coated steel has life more than 1200 h of SST (Fig. 11).

EXAMPLE 5: Evaluation of corrosion behavior: Tafel Test
The corrosion performance of Al-Si-Mg-Cu coating on steel is measured by DC polarization test (Tafel Test) using VersaSTAT MC®, Princeton Applied Research instrument.
The test is carried out by anodic potentiodynamic polarization experiments in a three-electrode electrolytic cell at room temperature. Al-Si-Mg-Cu alloy coated substrate and platinum mesh are used as working electrode and counter electrode, respectively. Standard calomel electrode is used as reference electrode. The test is conducted in 3.5 wt% sodium chloride (NaCl) solution with a scan rate of 0.5mV/s. The corrosion current (icorr), corrosion potential (Ecorr) and corrosion rate (mpy) are measured by Tafel extrapolation technique using VersaStudio® software module.
Each measurement is repeated for three times on three different places around the sample surface to ensure repeatability. For comparison with the Al-Si-Mg-Cu alloy coating, polarization test is also conducted on commercial grade galvanized steel, galvannealed steel and bare steel.
Comparing the Tafel curves of Al-Si-Mg-Cu alloy coated substrate with galvanized (GI) coating and galvannealed (GA) coating, it is seen that passivation behavior is observed only in the Al-Si-Mg-Cu alloy coated substrate which shows better corrosion resistance compared to GI or GA coating (Fig. 12). The average values of corrosion parameters are listed in Table 2 for Al-Si-Mg-Cu alloy coated sample, galvanized steel (GI), galvannealed (GA) coating and bare steel substrate. For comparison, two different types of aluminized steels are presented in the table from literature [Graeve et al.].
Comparison of corrosion current (icorr) values from the TAFEL test clearly shows that icorr values of Al-Si-Mg-Cu alloy coated samples are much lower compared to GI. The average corrosion rate of Al-Si-Mg-Cu alloy coated sample is around 0.93 mpy, compared to 6.38 mpy for GI coating and 3.14 mpy for GA coating. Thus, it can be concluded that the Al-Si-Mg-Cu alloy coated steel has almost 7 times lower corrosion rate compared to GI steel.
Al-Si-Mg-Cu alloy coating has higher corrosion rate compared to other aluminized steels due to the presence of highly active magnesium in the coating. Further, comparing corrosion potential (Ecorr) values of Al-Si-Mg-Cu alloy coating with other aluminized coatings on steel, it is seen that that the Ecorr of Al-Si-Mg-Cu alloy coating (-0.87 V) is more negative compared to the other aluminized coatings on steel (-0.52 to -0.58 V). This signifies that Al-Si-Mg-Cu alloy coated steel exhibits superior sacrificial corrosion behavior than other aluminized steels due to the presence of highly active Mg (standard reduction potential of Mg is -2.36V) in the coating.
In comparison with the bare steel substrate, the Al-Si-Mg-Cu alloy coating has much lower corrosion potential (Ecorr) 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-Cu alloy coated steel has better sacrificial property compared to GA coating as its Ecorr value is more negative than that of GA.
Table 2: Corrosion properties different coatings on steel.
Sample Ecorr (vs.SCE)
(V) icorr
(A/cm2) Corrosion Rate (mpy*)
GI coating -0.989 10.80 × 10-6 6.38
GA Coating -0.848 5.31 × 10-6 3.14
Al-Si-Mg-Cu alloy coating -0.87 2.18 × 10-6 0.93
Aluminized coating (pure Al, type-2) -0.58 ~ 10-6 ~ 0.43
Aluminized coating (7% Si, type-1) -0.52 ~ 10-6 ~ 0.43
Uncoated steel substrate -0.542 8.73× 10-6 5.15
* 1 mpy = 2.54 × 10-5 m/year