Claims:1. An aluminium based alloy coating composition for a steel substrate, said composition comprising aluminium (Al) at a concentration of about 77 % to 80 % (wt/wt), Zinc (Zn) at a concentration of about 0.5% to 12.5-% (wt/wt), and Silicon (Si) at a concentration of about 9.5% to 10.5 % (wt/wt).
2. The coating composition as claimed in claim 1 or 2, wherein the composition is for coating the steel substrate before subjecting the substrate to direct hot stamping process.
3. A steel substrate coated with the aluminium based alloy coating composition as claimed in claim 1 or 2.
4. A heat treated coated steel substrate prepared by subjecting the coated steel substrate as claimed in claim 3, to a temperature equal to greater than 750°C.
5. The coated steel substrate as claimed in claim 4, wherein the temperature is ranging from about 750°C to 950°C and maintained for a time period ranging from about 4 minutes to 8 minutes.
6. The coated steel substrate as claimed in claim 3 or 4, wherein the coated steel substrate has mechanical resistance of at least 1200 MPa after thermal treatment, and the aluminum based alloy coating provides high resistance to corrosion of the steel sheet and/or prevents decarburization during thermal treatment of the steel substrate
7. The coated steel substrate as claimed in claim 3, wherein thickness of the coating on the steel substrate ranges from about 10 to 35 µm.
8. The coated steel substrate as claimed in claim 3, wherein application of the coating composition on the steel substrate results in formation of two layers; wherein the two layers are an outer alloy layer comprising at least Al-Zn-Si and an inner intermetallic layer comprising at least Al-Si-Fe; and wherein thickness of the outer layer ranges from about 9 to 35 µm, and thickness of the inner layer ranges from about 1 to 5 µm.
9. The coated steel substrate as claimed in claim 4, wherein thickness of the coating on the heat treated steel substrate ranges from about 30 to 50 µm; wherein the coating on the heat treated steel substrate comprises three layers; wherein the three layers are an outer alloy layer comprising at least Al-Zn-Si, an inner intermetallic layer comprising at least Al-Si-Fe and a ternary layer at the upper region of the outer layer primarily comprising Fe-Al-Si.
10. The coated steel substrate as claimed in claim 4 or 9, wherein the heat treated steel substrate is subjected to forming and/or quenching to form press hardened steel.
11. The coated steel substrate as claimed in any one of claims 4-6 and 9-10, wherein the heat treated steel substrate is strained to shape the steel substrate.
12. The coated steel substrate as claimed in any one of claims 4-6 and 9-11, wherein the heat treated steel substrate is cooled at a rate that produces martensitic structures; and wherein the steel substrate is cooled to temperature ranging from about 950°C to room temperature at a rate ranging from about 100 to 500°C/s.
13. The coated steel substrate as claimed in claim 12, wherein the heat treated steel substrate is predominately martensite.
14. An article comprising the coated steel substrate as claimed in claim 3 or the heat treated coated steel substrate as claimed in claim 4.
15. A process for coating a steel substrate with the aluminium based alloy coating composition as claimed in claim 1 or 2, said process comprising steps of:
pickling a steel substrate, and optionally rinsing the substrate;
annealing the substrate;
preparing the alloy coating composition in a hot dip process simulator bath;
coating the steel substrate by hot-dipping the steel substrate in bath for a predetermined time period; and
cooling the coated substrate to obtain the coated steel substrate.
16. The process as claimed in claim 15, wherein the pickling is carried out with acid selected from a group comprising sulphuric acid or hydrochloric acid; wherein the pickling is carried out at a temperature ranging from 60°C to 70°C, for a time period ranging from 1-5 min; and wherein the annealing is carried out at temperature ranging from 720°C to 830°C.
17. The process as claimed in claim 15, wherein the hot dip process simulator bath is maintained at a temperature ranging from about 580°C to 620°C; wherein the steel substrate is subjected to hot dipping for a time period ranging from about 3 seconds to 8 seconds; and wherein the cooling of the coated substrate is performed in controlled atmosphere of the hot dip process simulator, wherein the controller atmosphere is N2 atmosphere.
18. A process for producing heat treated coated steel substrate, comprising act of heating the coated steel substrate as claimed in claim 3 to a temperature equal to greater than 750°C.
19. The process as claimed in claim 18, wherein the temperature is ranging from about 750°C to 950°C and maintained for a time period ranging from about 4 minutes to 8 minutes.
20. The process as claimed in claim 18 or 19, wherein the heat treated coated steel substrate is subjected to forming and/or quenching to form press hardened steel; wherein the forming strain rate is about 0.5/s; and the forming temperature is ranging from about 750°C to 850°C.
21. The process as claimed in claim 18 or 20, wherein the heat treated coated steel substrate is cooled at a rate that produces martensitic structures.
22. The process as claimed in claim 20 or 21, wherein the quenching is at a cooling rate ranging from about 100 to 500°C/s; and wherein the substrate is cooled to temperature ranging from about 950°C to room temperature. , Description:TECHNICAL FIELD
The present disclosure relates to the field of material science and corrosion. Particularly, the present disclosure relates to an Aluminium based alloy coating composition. Said alloy is an Al-Zn-Si alloy, suitable for coating on a steel substrate. The present disclosure further relates to a steel substrate and a heat treated steel substrate coated with the Aluminium based alloy coating composition, an article obtained of the same and process for preparation thereof.

BACKGROUND OF THE DISCLOSURE
Press hardening/hot stamping of steels at high temperature gives rise to auto components having high strength owing to the presence of fully martensitic structure in the formed components like side impact beams, bumpers, A and B pillars, roof rails, cross members, and tunnels etc. The demand for reduced vehicle weight, improved safety, and crashworthiness has been the key motivator for the use of hot stamped steels. By means of heating steel sheets above the austenitization temperature, it is possible to significantly improve the formability and perform an integrated microstructural transformation subsequent to forming. This transformation from austenite to martensite by rapid cooling offers the opportunity to manufacture structural parts with high tensile strengths.

The hot stamping process can be of two types, such as, direct hot stamping and indirect hot stamping. In the direct hot stamping process, a cut-to-size blank is heated in a furnace, and held there for a desired time period. Afterwards the blank is transferred to a press and subsequently formed and quenched in the closed tool. The indirect hot stamping process steps start with cold pre-forming of the part followed by the austenitization and subsequent quenching. The use of hot forming techniques carries several advantages as compared to cold forming operations, such as reduced spring back and low press loads etc.

In the hot stamping process, both direct and indirect, the steels are usually heated in a furnace for austenitization having the atmosphere containing oxygen. This causes surface oxidation of steel during austenitization if bare steel blanks are used. Thereby coated steels are used to protect the steel surface from oxidation and maintain the surface appearance and mechanical properties of the hot stamped parts. Additionally, the corrosion protection of the steel substrate during the service life of the components is also required. So, suitable coating system for hot stamped steels is essential.

Zinc coating is used worldwide to protect steel surface from corrosion due to the sacrificial nature of the coating. However the Zinc based coatings are not found suitable for direct hot stamping process. The melting point of pure Zinc is 420oC and the boiling point is 907oC. During the high temperature conditions required for austenization in the direct hot stamping process (i.e. above 900oC), there is considerable diffusion of Iron from steel substrate in to the coating. This results in formation of different Fe-Zn phases during the hot stamping heat treatment. The Zinc and the Fe-Zn phases are in liquid phases at that high temperature. Further, during subsequent forming process at high temperature, Liquid Metal Induced Embrittlement (LMIE) may occur and the material fails to withstand high strain. The Zinc based coatings are therefore not suitable for direct hot stamping process.

Instead of pure Zinc coating, galvannealed (GA) coatings are thus tried which is an alloy coating of Iron and Zinc having overall 10-12 wt.% Iron in the coating and Fe-Zn gamma phase, Fe-Zn delta phase and Fe-Zn zeta phase from the substrate coating interface up to the coating top surface. The layer structure of the developed GA steel sheet after hot stamping consists of two layers on the a-Fe substrate. The surface layer is Zinc oxide, and second is Fe-Zn solid solution. However if the heat treatment time is small then the high melting solid solution of Fe-Zn does not form, instead the low melting Fe-Zn intermetallics (gamma, delta etc.) are formed at the interface, and causes deterioration of mechanical properties due to LMIE. So, longer heat treatment period is required for GA coating to avoid LMIE which is unrealistic from production point of view. Thus, Zinc coatings are not found suitable for direct hot stamping process and may be used for indirect hot stamping process. Indirect hot stamping process is however not favoured as forming at low temperature requires more force.

Aluminium based coatings are also used for hot stamping application. Such coatings are most widely used as it does not affect the mechanical property of the substrate blank during the direct hot stamping treatment. Aluminium based coatings are typically produced from bath composition of 87% Aluminium, 10-11% Silicon and 2-3% Iron. Such coating comprises intermetallic layer which is much thinner due to presence of Silicon and Aluminium matrix with pure Si precipitates in the overlay coating. There is presence of outer Al7Fe2Si layer and inner layer of Al3Fe and Al5Fe2 closer to the substrate. The final coating microstructure after about 5-6 min of soaking at austenitization temperature comprised of different intermetallic phases and no overlay; the overall coating thickness also increases from initial thickness. There are surface oxide layers of Al and Si oxides at the top surface. The coating microstructure consists mainly of Fe-Al3Fe solid solution at the interface and intermetallic layers above that. The intermetallic compounds are mostly Al5Fe2 and precipitates or islands of Al2Fe2Si and Al4Fe5Si. The effect of strain rate, deformation temperature on crack formation shows that effect of the deformation temperature on the flow stress is significant and the flow stress and the maximum stress increase with the decreasing temperature at a constant strain rate. The failure of the Al–Si coating originates from surface vertical cracks and follows interfacial cracking. The surface vertical cracks density firstly increases with increasing applied strain. Opening of this type of cracks causes the substrate exposure and thermal oxidation. With increasing applied strain, the macro-cracks extended and lead to the interfacial de-bonding of the cracked coating segments. It was observed that wherever there is a crack in the coating interface, the substrate gets exposed and at that position the Iron substrate in oxidized.

The brittle nature of the coating is thus a major negative issue for Aluminium Silicon coatings and it can cause coating fracture during deformation at high temperature as well as low temperature i.e. during the service life of hot stamped parts. Thereby the indirect stamping process is not suitable for this coating. Another major drawback of the Aluminium Silicon coating is that it implies that the coatings cannot offer any cathodic corrosion protection. The uncoated regions of the coated steel, such as cut edges, are therefore not protected against corrosion and there are major drawbacks of the product in terms of brittle interface of coating as well as absence of sacrificial corrosion protection.

Other types of coatings have being tried out as a potential replacement of Zinc coatings and Al-Si coatings. One such type of coating is the galvalume coating by dipping the substrate in 55 wt.% Al–Zn bath, with 1.6 wt.% Si, and at a bath temperature of 680°C. It was reported that 55 wt.% Al–Zn coated substrate was probably less susceptible to LMIE although the coating contained a large volume fraction of liquid Zn at grain boundaries at the press forming temperature. However it is required that before hot stamping the coated steel must go through a pre-conditioning stage at 550-730oC for 9-66 min which will increase the amount of Fe in the coating and decrease the probability of LMIE. However, such conditioning is not suitable to existing hot stamping line parameters and the product is not yet established successfully.

There are other types of coatings being researched like, dual Layer Zn–Al coating, Zn–Al–Mg post-process coating etc. In the first process steel is first hot-dip coated with Al–10%Si coating, and subsequently hot dipped in Zn having Al (<1 wt%). The final coating is composed of a 5µm Zn layer, a 15µm Al–Si alloy layer and a Fe2SiAl7 intermetallic layer at the coating/steel interface. In addition, a phosphating process is conducted prior to hot stamping. The Zn-Al-Mg post process coating is done on the hot stamped parts. Essentially this does not meet the requirement of surface protection during hot stamping process. Moreover during hot dip coating process of Zn-Al-Mg coating the martensite is tempered and the strength level is significantly reduced. Further research on lowering the hot dipping temperature and better adhesion of the coating to substrate is still going on.

A single solution for coating for hot stamping having right proportion of solid phases with ductility at high temperature that can be usable in both direct and indirect stamping process as well as having sacrificial corrosion protection is still not been developed. Therefore, it is required to develop a single coating solution having these properties for ultra high strength press hardenable steels.

STATEMENT OF THE DISCLOSURE
Accordingly, the present disclosure relates to an Aluminium based alloy coating composition for a steel substrate, said composition comprising Aluminium (Al) at a concentration of about 77 % to 80 % (wt/wt), Zinc (Zn) at a concentration of about 0.5% to 12.5-% (wt/wt), and Silicon (Si) at a concentration of about 9.5% to 10.5 % (wt/wt).

The disclosure further relates to a steel substrate coated with the said Aluminium based alloy coating composition.

Furthermore, the disclosure relates to a heat treated coated steel substrate prepared by subjecting the steel substrate coated with the said Aluminium based alloy coating composition, to a temperature greater than or equal to about 750°C.

The present disclosure also relates to an article comprising the aforesaid coated steel substrate or heat treated coated steel substrate.

Further, the present disclosure also relates to a process for coating a steel substrate with the Aluminium based alloy coating composition, said process comprising steps of:
pickling a steel substrate, and optionally rinsing the substrate;
annealing the substrate;
preparing the alloy coating composition in a hot dip process simulator bath;
coating the steel substrate by hot-dipping the steel substrate in bath for a predetermined time period; and
cooling the coated substrate to obtain the coated steel substrate.

The present disclosure also relates to a process for producing heat treated coated steel substrate, comprising act of heating the steel substrate coated with the said Aluminium based alloy coating composition to a temperature greater than or equal to 750°C.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with a detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where:

Fig. 1 represents the initial steel microstructure having ferrite and pearlite as observed under OM.
Fig. 2 represents the cross-sectional SEM micrograph of the as-coated steel substrate.
Fig. 3 represents the final microstructure of heat treated coated steel having martensite structure as observed under OM
Fig. 4 represents the final microstructure of heat treated coated steel along the direction of applying uniaxial strain.
Fig. 5 represents a graph depicting the engineering stress vs. engineering strain for coated steel substrate for hot stamping treatment.
Fig. 6 represents the microstructure of coated steel (IF steel.
Fig. 7 represents the microstructure of coated steel (Boron added steel).
Fig. 8 represents the Tafel curves of Al-Zn-Si alloy coated steel substrate (8s dipping time) and Galvanized (GI) steel.
DETAILED DESCRIPTION OF THE DISCLOSURE
The present disclosure relates to an aluminium based alloy coating composition for a steel substrate, said composition comprising aluminium (Al) at a concentration of about 77 % to 80 % (wt/wt), Zinc (Zn) at a concentration of about 0.5% to 12.5-% (wt/wt), and Silicon (Si) at a concentration of about 9.5% to 10.5 % (wt/wt).

In an embodiment of the present disclosure, the composition is for coating the steel substrate before subjecting the substrate to direct hot stamping process.

The present disclosure also relates to a steel substrate coated with the aforesaid aluminium based alloy coating composition.

In an embodiment of the present disclosure, thickness of the coating on the steel substrate ranges from about 10 to 35 µm. In another embodiment of the present disclosure, application of the coating composition on the steel substrate results in formation of two layers; the two layers are an outer alloy layer comprising at least Al-Zn-Si and an inner intermetallic layer comprising at least Al-Si-Fe; and thickness of the outer layer ranges from about 9 to 35 µm, and thickness of the inner layer ranges from about 1 to 5 µm.

The present disclosure also relates to a heat treated coated steel substrate prepared by subjecting the aforesaid steel substrate coated with the aluminium based alloy coating composition, to a temperature equal to greater than 750°C.

In an embodiment of the present disclosure, the temperature is ranging from about 750°C to 950°C and maintained for a time period ranging from about 4 minutes to 8 minutes.

In another embodiment of the present disclosure, the coated steel substrate has mechanical resistance of at least 1200 MPa after thermal treatment, and the aluminum based alloy coating provides high resistance to corrosion of the steel sheet and/or prevents decarburization during thermal treatment of the steel substrate

In yet another embodiment of the present disclosure, thickness of the coating on the heat treated steel substrate ranges from about 30 to 50 µm; the coating on the heat treated steel substrate comprises three layers; the three layers are an outer alloy layer comprising at least Al-Zn-Si, an inner intermetallic layer comprising at least Al-Si-Fe and a ternary layer at the upper region of the outer layer primarily comprising Fe-Al-Si.

In still another embodiment of the present disclosure, the heat treated steel substrate is subjected to forming and/or quenching to form press hardened steel.

In still another embodiment of the present disclosure, the heat treated steel substrate is strained to shape the steel substrate.

In still another embodiment of the present disclosure, the heat treated steel substrate is cooled at a rate that produces martensitic structures; and the steel substrate is cooled to temperature ranging from about 950°C to room temperature at a rate ranging from about 100 to 500°C/s.

In still another embodiment of the present disclosure, the heat treated steel substrate is predominately martensite.

The present disclosure also relates to an article comprising the coated steel substrate or the heat treated coated steel substrate.

The present disclosure also relates to a process for coating a steel substrate with the aluminium based alloy coating composition, said process comprising steps of:
pickling a steel substrate, and optionally rinsing the substrate;
annealing the substrate;
preparing the alloy coating composition in a hot dip process simulator bath;
coating the steel substrate by hot-dipping the steel substrate in bath for a predetermined time period; and
cooling the coated substrate to obtain the coated steel substrate.

In an embodiment of the present disclosure, the pickling is carried out with acid selected from a group comprising sulphuric acid or hydrochloric acid; the pickling is carried out at a temperature ranging from 60°C to 70°C, for a time period ranging from 1-5 min; and the annealing is carried out at temperature ranging from 720°C to 830°C.

In another embodiment of the present disclosure, the hot dip process simulator bath is maintained at a temperature ranging from about 580°C to 620°C; the steel substrate is subjected to hot dipping for a time period ranging from about 3 seconds to 8 seconds; and the cooling of the coated substrate is performed in controlled atmosphere of the hot dip process simulator, the controller atmosphere is N2 atmosphere.

The present disclosure also relates to a process for producing heat treated coated steel substrate, comprising act of heating the coated steel substrate to a temperature equal to greater than 750°C.

In an embodiment of the present disclosure, the temperature is ranging from about 750°C to 950°C and maintained for a time period ranging from about 4 minutes to 8 minutes.

In another embodiment of the present disclosure, the heat treated coated steel substrate is subjected to forming and/or quenching to form press hardened steel; the forming strain rate is about 0.5/s; and the forming temperature is ranging from about 750°C to 850°C.

In yet another embodiment of the present disclosure, the heat treated coated steel substrate is cooled at a rate that produces martensitic structures.

In still another embodiment of the present disclosure, the quenching is at a cooling rate ranging from about 100 to 500°C/s; and the substrate is cooled to temperature ranging from about 950°C to room temperature.

As used herein, the symbols ‘Al’, ‘Zn’, ‘Si’, ‘Sn’, ‘Fe’ and ‘Ni’, refer to elements Aluminium, Zinc, Silicon, Tin, Iron and Nickel, respectively, which may be used in any combination thereof to generate alloys, in the present disclosure.

As used herein, the phrases ‘Al-Zn-Si 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, Zn, Si, and optionally elements such as Sn, Fe and/or Ni suitable for coating on a metallic substrate, preferably a steel 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-Zn-Si based alloy coating composition on its surface.

As used herein, the term ‘heat treated coated steel substrate’ includes but is not limited to the coated steel substrate subjected to the process of press hardening / hot stamping.

As used herein, the term ‘heat treatment’ includes but is not limited to hot stamping of steel.

As used herein, the abbreviation ‘GI’ refers to Galvanized, and its definition is consistent with that known in the art.

As used herein, the terms ‘hot stamping’, ‘hot stamping of steel’, ‘press hardening’ and ‘press hardening of steel’ are used interchangeably and refer to subjecting steel substrate to the process of hot stamping to yield steel having high strength and martensitic structure.

As used herein, the terms ‘hot stamped steel’ and ‘press hardened steel’ are used interchangeably and refer to the steel substrate obtained post hot stamping of steel.

As used herein, the term ‘steel’, ‘substrate’ and ‘steel substrate’ refers to a base material made of steel on which the coating composition of the present disclosure is applied and/or the material upon which a process of the present disclosure is conducted. The present disclosure encompasses substrates of any dimensions and shape. In a non-limiting and exemplary embodiment, the steel substrate employed in the present disclosure are steel sheets having dimensions of 20 cm × 11 cm × 0.1 cm.

The characterization methods used in the present disclosure include Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) to observe the microstructure, Glow Discharge Optical Emission Spectroscopy (GDOES) to find out the alloy coating composition through the depth of the coating, Grazing angle X-ray Diffraction (GI-XRD) to identify the coating phases and Electron Probe Micro Analyzer (EPMA) coupled with Wavelength Dispersive Spectroscopy (WDS) to accurately determine the elemental distributions.

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 relates to 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 coating composition comprises Aluminium, Zinc and Silicon. The composition of the present disclosure is thus an Aluminium-Zinc-Silicon (Al-Zn-Si) alloy.

Further, the alloy composition of the present disclosure optionally comprises one or more elements selected from a group comprising Tin (Sn), Iron (Fe) and Nickel (Ni) or any combination thereof. Thus, in the present disclosure, the alloy coating composition for steel substrate comprises Aluminium, Zinc and Silicon and optional element(s) selected from a group comprising Tin, Iron and Nickel or any combination thereof.

The Aluminium based alloy coating composition of the present disclosure is employed for coating a steel substrate before subjecting the substrate to hot stamping process such as direct hot stamping process or indirect hot stamping process. It has sacrificial protection as compared to commercially available Aluminium based coatings. The coating forms suitable phases during direct hot stamping that gives rise to solid phase at the interface and does not cause any Liquid Metal Induced Embrittlement (LMIE) in the material. Furthermore, the corrosion property of the coating is sacrificial in nature and protects the steel cathodically. The coating on the steel substrate is capable of providing high resistance to corrosion of the steel substrate and/or preventing decarburization during thermal treatment of the steel substrate. Additionally, the weldability and paintability of the coated surface is adequate. The desired properties of the alloy are resultant of specific concentrations of elements within the coating composition, which significantly improve the sacrificial corrosion behavior of the steel substrate on which the coating is applied.

In an embodiment, the Aluminium based alloy coating composition is used for coating a steel substrate prior to subjecting the substrate to hot stamping process including direct hot stamping process and indirect hot stamping process.

In a preferred embodiment, the Aluminium based alloy coating composition is used for coating a steel substrate prior to subjecting the substrate to direct hot stamping process.

The present disclosure provides for the Aluminium based alloy coating composition having highly specific concentrations of elements and comprises Aluminium (Al) at a concentration of about 77% to 80% (wt/wt); Zinc at a concentration of about 0.5% to 12.5% (wt/wt), preferably at a concentration of about 11.5% to 12.5% (wt/wt); and Silicon (Si) at a concentration of about 9.5% to 10.5% (wt/wt).

As the alloy coating composition of the disclosure may further comprise optional element(s), they too are provided at specific concentrations, wherein when present in the coating 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) and Nickel (Ni) is at a concentration of about 0.01-0.02% (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 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (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 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (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 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (wt/wt), Sn at a concentration of about 0.01% to 0.1% (wt/wt) and Fe at a concentration of about 0.1% to 0.3% (wt/wt). In still another alternate embodiment, the alloy coating composition for steel substrate comprises Al at a concentration of about 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (wt/wt) and Ni at a concentration of about 0.01% to 0.02% (wt/wt) or any combination thereof. In still another alternate embodiment, the alloy coating composition for steel substrate comprises Al at a concentration of about 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (wt/wt), Sn at a concentration of about 0.01% to 0.1% (wt/wt) and Ni at a concentration of about 0.01% to 0.02% (wt/wt) or any combination thereof. In still another alternate embodiment, the alloy coating composition for steel substrate comprises Al at a concentration of about 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (wt/wt), Fe at a concentration of about 0.1% to 0.3% (wt/wt) and Ni at a concentration of about 0.01% to 0.02% (wt/wt) or any combination thereof. In a further alternate embodiment, the alloy coating composition for steel substrate comprises Al at a concentration of about 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (wt/wt), 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) and Ni at a concentration of about 0.01% to 0.02% (wt/wt) or any combination thereof.

Thus, in the present disclosure, the alloy coating composition for steel substrate comprises Al at a concentration of about 77% to 80% (wt/wt), Zn at a concentration of about 0.5% to 12.5% (wt/wt), preferably about 11.5% to 12.5% (wt/wt), Si at a concentration of about 9.5% to 10.5% (wt/wt), and optionally an 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), and Ni at a concentration of about 0.01% to 0.02% (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 coating 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. 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, 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, interstitial free steel (IF steel), Boron-added steel or any combination thereof, to form a coating on the steel substrate. In an exemplary embodiment, the alloy coating composition of the present disclosure is coated on Boron-added steel substrtae or IF steel substrate. In another embodiment, the initial microstructure of steel prior to heat treatment/hot pressing is ferrite and pearlite.

In embodiments of the present disclosure, presence of boron in the steel increases the hardenability of steel and the martensite phase forms upon quenching during hot stamping treatment.

The present disclosure also provides for a steel substrate coated with the Al-Zn-Si 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 coating composition. The disclosure thus envisages more than one coat of the coating composition on the 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 before heat stamping is studied by photographing the surface by technique such as SEM. The cross-sectional SEM micrograph of the Al-Zn-Si alloy coated surface of the steel substrate shows that the coating consists of two distinct layers.

Thus, in all embodiments of the present disclosure, application of the coating composition on the substrate results in formation of 2 layers of the coating on the steel substrate, one is an intermetallic layer closer to the steel substrate whereas the other is an overlay on top of that i.e. towards the outer periphery of the coating. Said 2 layers of the coating formed on the steel substrate are thus an inner intermetallic layer (or inner layer) and an outer alloy layer (or outer layer), wherein the inner layer comprises an Al-Fe-Si intermetallic layer and the outer layer comprises Al-Zn-Si alloy layer. The chemical composition of the intermetallic layer shows presence of Aluminium, Silicon And Iron whereas the upper layer is much leaner in Iron. In an embodiment of the disclosure, total thickness of the said coating on the steel substrate ranges from about 10 µm to 35 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 9 µm to 30 µm, preferably about 20 µm, thickness of the inner layer ranges from about 1 µm to 5 µm, preferably about 2 µm.

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 also provides a process for coating a steel substrate with the Al-Zn-Si alloy coating composition of the disclosure, said process comprising steps of:
pickling a steel substrate, and optionally rinsing the substrate;
annealing the substrate;
preparing the Al-Zn-Si alloy coating composition in a hot dip process simulator bath;
coating the steel substrate by hot-dipping the steel substrate in bath for a predetermined time period; and
cooling the coated substrate to obtain the coated steel substrate.

The pickling in the said process is carried out with acid selected from a group comprising sulphuric acid or hydrochloric acid having concentration of about 8-10% by volume. In an embodiment, the pickling is carried out at a temperature ranging from about 60°C to 70°C, preferably about 65°C, for a time period ranging from about 1-5 min, preferably about 2 min. The substrate is optionally rinsed with water, preferably deionized water, to clean carry over acid

In an optional embodiment, prior to subjecting a steel substrate to pickling in the coating process, the steel substrate is cleaned thoroughly using alkaline solution such as caustic solution at a temperature of about 55°C–65°C, preferably about 60°C, for about 2 minutes to 5 minutes, preferably about 2 minutes. Post cleaning the substrate is rinsed in water, preferably deionized water for about 1 to 2 minutes, preferably about 1 minute, and thereafter optioanlly dried at a temperature of about 40°C to 50°C, preferably about 40°C.

The substrates in the aforesaid process are annealed at a temperature ranging from about 720°C to 830°C, preferably about 800°C before hot-dipping in the Al-Zn-Si alloy bath. In an embodiment, the annealing is carrired out 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.

Before the steel substrate is coated by hot dipping it in bath comprising the coating 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-Zn-Si 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 580°C to 620°C; and the steel substrate is subjected to hot dipping for a time period ranging from about 3 seconds to 8 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 atmosphere.

In an embodiment, thickness of the coating formed on the steel substrate ranges from about 10 µm to 35 µm, preferably about 22 µm.

The present disclosure also relates to a process for producing heat treated coated steel substrate, comprising act of heating the coated steel substrate of the disclosure at a temperature greater than or equal to 750°C.

The aforesaid process for producing the heat treated coated steel substrate comprises act(s) of:
a) heating the coated steel substrate of the disclosure to a temperature greater or equal to 750°C, preferably ranging from about 750°C to 950°C; and
b) optionally cooling the heat treated coated steel substrate of step a), optionally followed by subjecting it to forming and quenching to form press hardened steel.

In an embodiment, the aforesaid process for producing the heat treated coated steel substrate comprises act(s) of:
a) heating the coated steel substrate of the disclosure to a temperature greater or equal to 750°C, preferably ranging from about 750°C to 950°C; and
b) cooling the heat treated coated steel substrate of step a), followed by subjecting it to forming and quenching to form press hardened steel.

In an embodiment, the heat treatment cycle in the aforesaid process is as given in Gleeble 1500D and Gleeble 3800D.

In an embodiment, the step of heating in the aforesaid process for producing heat treated coated steel substrate involves heating the coated steel substrate to a temperature greater or equal to about 750°C, preferably to a temperature ranging from about 750°C to 950°C, more preferably to a temperature of about 950oC and maintaining / holding it there for a time period ranging from about 4 min to 8 min, preferably about 300 s. The heat treatment is carried out under air atmosphere. The heat treatment at high temperature causes higher diffusion of Iron from the substrate to the coating and resulted in formation of different Iron-Aluminium-Silicon-Zinc phases. Post heat treatment the coating along with the substrate is subjected to cooling, strain/forming and finally the substrate is quenched.

The cooled substrate is thereafter subjected to forming by application of strain to simulate the deformation of the substrate to form press hardened steel. In an embodiment, the forming strain rate is about 0.5/s, and the forming temperature is ranging from about 750°C to 850°C. In an embodiment, prior to the forming step, the temperature drops from about 950oC to about 750-850oC during the robotic transfer of the substrate from furnace to the forming press. The substrate is finally subjected to quenching. In an embodiment, for the quenching, the substrate is cooled to about 40oC. In another embodiment the substrate is cooled at a rate that produces martensitic structures. In an embodiment, the cooling is at a rate of 100-500 oC/s

The present disclosure also relates to a heat treated coated steel substrate. The heat treated coated steel substrate is prepared by subjecting the coated steel substrate of the present disclosure, to a temperature greater than or equal to about 750°C, preferably to a temperature ranging from about 750°C to 950°C. The steel substrate is then subjected to forming, wherein the substrate is strained to shape the steel substrate, and subsequently quenching to form press hardened steel. In an alternate embodiment, the substrate is strained to shape the steel substrate prior to the heating step. In an embodiment, the forming is at a forming strain rate of about 0.1/s to 1/s, preferably about 0.5/s; and the forming temperature is ranging from about 750°C to 850°C, preferably about 800oC.

Post heating and optionally forming, the heat treated steel substrate is cooled at a rate that produces martensitic structures. In an embodiment, the steel substrate is cooled to temperature ranging from about 950°C to room temperature at a rate ranging from about 100 to 500°C/s.

The heat treated steel substrate post quenching is predominately martensite. In an embodiment, the microstructure of press hardenable steel substrate is fully martensitic after the hot stamping and quenching treatment, as observed by OM.

Morphology of the coated surface of the substrate after heat stamping is studied by photographing the surface by technique such as SEM. The cross-sectional SEM micrograph of the Al-Zn-Si alloy coated surface of the heat treated coated steel substrate shows that the coating consists of three distinct layers.

Thus, in all embodiments of the present disclosure, the heat treatment results in formation of 3 layers on the heat treated coated steel substrate. The layer close to the interface (inner layer) is primarily Iron-Aluminium-Silicon intermetallic phase. The amount of Zinc is minimal in this phase. This phase is solid at the deformation temperature and forms a continuous interface. The other phase in the upper layer is Zinc containing phase (outer layer) comprising Al-Zn-Si alloy layer. This layer containing proper amount of zinc is the source of sacrificial properties of the coating. The presence of zinc at the top surface can be beneficial in reducing the high temperature frictional coefficient of the coated surface with the die in operation. There is another discontinuous layer of Iron-Aluminium-Silicon ternary phase (ternary phase) precipitated at the upper region of outer or overlay coating. Cracks may be generated in the coating due to mismatch in thermal expansion coefficient of different phases, due to the applied strain and also the sudden quenching of the material. However no crack penetrates within the steel substrate, and there is no decarburization layer at the subsurface region due to coating cracks. Further, the coated steel substrates shows no deterioration in either strength or ductility during stress-strain when compared with bare substrate.

In an embodiment of the disclosure, total thickness of the coating on the steel substrate post heat treatment ranges from about 30 µm to 50 µm. Thickness of the inner layer ranges from about 15 µm to 25 µm, preferably about 20µm. Thickness of the other layers may be diffused.

The steel substrate coated with the alloy coating composition of the present disclosure exhibits excellent corrosion resistant properties. For instance, the corrosion behavior of the coated steel substrate is evaluated by anodic potentiodynamic polarization experiments. The corrosion rate (mpy) is measured by Tafel extrapolation technique. Al-Zn-Si coated steel substrate show better corrosion resistance compared to galvanized (GI) coating in Tafel curves. Corrosion current (icorr) value of Al-Zn-Si alloy coated steel substrate is much lower compared to GI. In exemplary embodiments of the present disclosure, Al-Zn-Si alloy coating on steel has 6-7 times higher corrosion resistance compared to GI coating and has much lower corrosion potential than that of bare steel substrate (-0.54 V), hence providing cathodic protection to steel substrate efficiently. Further, Al-Zn-Si alloy coated steel has similar sacrificial property compared to GI coating as their Ecorr value vary closely.

The Aluminium based alloy coating composition of the present disclosure thus provides for an effective coating on steel substrate for hot stamping process. It provides the right proportion of solid phases with ductility at high temperature, sacrificial corrosion protection, and can be used in both direct and indirect hot stamping process. The coated steels of the present disclosure protect the steel surface from oxidation during the hot stamping process and maintain the surface appearance and mechanical properties of the hot stamped steel. Coating the Aluminium based alloy coating composition of the present disclosure on steel prior to subjecting the steel to hot stamping process, yields ultra high strength press hardenable steels. In an embodiment, after thermal treatment the coated steel substrate has mechanical resistance of at least 1200 MPa, preferably atleast 1900 MPa.

While the coated steel substrate and heat treated coated steel substrate of the present disclosure provides the said desired properties, 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 or the heat treated coated steel substrate of the disclosure. 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 side impact beams, bumpers, A and B pillars, roof rails, cross members, and tunnels 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 is considered redundant. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.

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.

Further, the invention herein provides for examples illustrating the above described embodiments, and in order to illustrate the embodiments of the present invention, certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES
EXAMPLE 1:
EXAMPLE 1.1: Steel substrate before stamping
A Boron added steel sheet having dimensions of 20 cm × 11 cm × 0.1 cm was employed as substrate for obtaining press hardenable steel. The composition of the steel substrate is given in Table 1, the remaining is Fe. The initial microstructure of the steel substrate is ferrite and pearlite as shown in Fig. 1.

Table 1: Steel elemental composition
C Mn S P Si Al Cu Cr Ni Mo V Nb Ti N B
0.22
wt% 1.16
wt% 0.002
wt% 0.008
wt% 0.246
wt% 0.039
wt% 0.01
wt% 0.192
wt% 0.022
wt% 0.001
wt% 0.001
wt% 0.001
wt% 0.031
wt% 40
ppm 0.0029
wt%

EXAMPLE 1.2: Preparation of Al-Zn-Si coated steel
The coating on the steel substrate was prepared as per the following protocol. The steel substrate of Example 1.1 was first cleaned thoroughly using caustic solution at 60oC for 2-5 min. Post cleaning the substrate was rinsed in water. Then the substrate was pickled in either hydrochloric acid of concentration of 8-10% by volume at 65oC for 1-5 min. Further the substrate was rinsed to clean carry over acids.

The substrate were subsequently annealed at 850°C before hot-dipping in Al-alloy bath. The hot-dipping was conducted at 590°C for three different hot-dipping times of 3s, 5s and 8s using N2 controlled atmosphere inside the HDPS with dew point of -30°C. The alloy composition and parameters are given in Table 2.

Table 2: Hot dipping bath compositions and parameters
Alloy composition (wt%) Hot-dip parameters
Al Zn Si Others (optional) Hot-dipping Temp. (oC) Hot dipping Time (s)
77.65 12.05 9.95 Sn: 0.01-0.1%;
Fe:0.1-0.3%; Ni: 0.01-0.02% 590±2 3, 5, 8

The microstructure of the as-coated coating composition on the steel substrate before heat stamping and corresponding compositions are provided in Fig. 2. The cross sectional elemental distribution for the coated steel substrate is shown in Fig. 2. Two distinct layers were observed in the coating, as evident from Fig. 2, i.e. an intermetallic layer closer to the interface and another overlay on top of that. The chemical composition of the intermetallic layer showed presence of Aluminium, Silicon and Iron whereas the upper layer was found to be much leaner in Iron.

EXAMPLE 1.3: Steel substrate after stamping treatment
The coated steel substrate obtained as per Example 1.2 was further subjected to heat treatment as observed in the automotive press hardening process. Particularly, the first step of the heat treatment cycle given in Gleeble 1500D and Gleeble 3800D was heating the coated steel substrate to 950oC and holding it there for about 300 s. The heat treatment was carried out under air atmosphere. The substrate was further cooled to 850oC where the required strain was applied to simulate the deformation. The heat treatment at high temperature caused higher diffusion of Iron from the substrate to the coating and resulted in formation of different Iron-Aluminium-Silicon-Zinc phases. Post heat treatment, the coating along with the substrate was subjected to strain. The material was then quenched at a cooling rate of 100-500oC/s.

The microstructure of the press hardenable steel obtained is shown in Fig. 3. The microstructure of press hardenable steel substrate was found to be fully martensitic after the hot stamping and quenching treatment.

The final microstructure along the direction of applying uniaxial strain is presented in Fig. 4. The layer close to the interface was primarily Iron-Aluminium-Silicon comprising phase having minimal amount of Zinc. The other phase in the upper layer was Zinc containing phase. There was another discontinuous layer of Iron-Aluminium-Silicon ternary phase precipitated at the upper region of outer or overlay coating. The cracks generated in the coating were due to mismatch in thermal expansion coefficient of different phases, due to the applied strain and also the sudden quenching of the material. However no crack was observed to penetrate within the steel substrate, neither was there any decarburization layer observed at the subsurface region due to coating cracks.

Further, the stress-strain curves were developed for coated steel from the experimental data of force and stroke during the deformation at high temperature. Fig. 5 depicts the graph of the engineering stress vs. engineering strain for coated steel substrate for hot stamping treatment, at a Strain rate of 0.5/s, austenitizing temperature of 950°C, austenitizing time of 5 minute, deformation temperature of 850°C and maximum engineering strain of 40%. No deterioration in either strength or ductility was observed in the coated material when compared with bare substrate.

EXAMPLE 2: Steel substrate after stamping treatment
An IF steel sheet (composition of steel sheet, C: 0.0021 wt.%, Si: 0.002 wt.%, Mn: 0.10 wt.%, P: 0.012 wt.%, Al: 0.039 wt.%, Ti: 0.033 wt.%, Nb: 0.012 wt.%, S: 0.008 wt.%, B: 0.0001 wt.% and N: 18 ppm, balance Fe) was coated with Al-Zn-Si alloy coating as per the composition and the protocol provided in Example 1.2. The coated steel sheet/substrate thus obtained was further subjected to heat treatment observed in the automotive press hardening process, as per the protocol provided in Example 1.3.

The as-coated microstructure of IF steel after hot dipping is shown in Fig. 6. The inner layer having thin ternary phase of Fe-Al-Si and the upper overlay layer of Al-Si-Zn. The layer close to the interface was primarily Iron-Aluminium-Silicon phase having minimal amount of Zinc. The other phase in the upper layer was Zinc containing phase. There was another discontinuous layer of Iron-Aluminium-Silicon ternary phase precipitated at the upper region of outer or overlay coating. The cracks generated in the coating were due to mismatch in thermal expansion coefficient of different phases, due to the applied strain and also the sudden quenching of the material. However no crack was observed to penetrate within the steel substrate, neither was there any decarburization layer observed at the subsurface region due to coating cracks.

EXAMPLE 3: Steel substrate after stamping treatment
A Boron added steel sheet as per Example 1.1 was coated with Al-Zn-Si alloy coating as per the composition and the protocol provided in Example 1.2. The coated steel sheet/substrate thus obtained was further subjected to heat treatment observed in the automotive press hardening process, as per the protocol provided in Example 1.3.

The as-coated microstructure of IF steel after hot dipping is shown in Fig. 7 The final microstructure of the press hardened coated steel thus obtained, along the direction of applying uniaxial strain is presented in Fig. 4. The layer close to the interface was primarily Iron-Aluminium-Silicon phase having minimal amount of Zinc. The other phase in the upper layer was Zinc containing phase. There was another discontinuous layer of Iron-Aluminium-Silicon ternary phase precipitated at the upper region of outer or overlay coating. The cracks generated in the coating were due to mismatch in thermal expansion coefficient of different phases, due to the applied strain and also the sudden quenching of the material. However no crack was observed to penetrate within the steel substrate, neither was there any decarburization layer observed at the subsurface region due to coating cracks.

EXAMPLE 4: Corrosion Behavior: Tafel Test
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. 8 depicts the comparison in Tafel curves of Al-Zn-Si coated substrates along with galvanized (GI) coating. Al-Zn-Si coated steel substrate showed better corrosion resistance compared to GI coating. The average values of corrosion parameters are listed in Table 3 for Al-Zn-Si alloy coated steel substrate, galvanized (GI) coating and bare steel substrate.

Table 3: Corrosion Properties
Sample Ecorr (vs. SCE) (V) icorr (A/cm2) Corrosion rate (mpy)
1 mpy = 2.54 x E-05 m/y
Steel substrate -0.542 8.73E-05 5.15
GI coating -0.98 10.80E-5 6.38
Al-Zn-Si -0.97 2.01E-5 0.89

Corrosion current (icorr) value of Al-Zn-Si alloy coated steel substrate was found to be much lower compared to GI. The average corrosion rate of Al-Zn-Si alloy coated steel substrate was around 0.89 mpy, compared to 6.38 mpy for GI coating. Hence, the Al-Zn-Si alloy coating on steel has 6-7 times higher corrosion resistance compared to GI coating. In comparison with steel substrate, the Al-Zn-Si alloy coating has much lower corrosion potential than that of steel substrate (-0.54 V) and hence this coating provides cathodic protection to steel substrate efficiently. Moreover, the results also show that Al-Zn-Si alloy coated steel has similar sacrificial property compared to GI coating as their Ecorr value vary closely. Hence, the following galvanic series in 3.5 wt% NaCl can be derived:
ECorr, GI < ECorr, Al-Zn-Si coating < ECorr, steel