FIELD OF THE INVENTION
The invention relates to a galvanized steel. Particularly the invention relates to superhydrophobic surface on the galvanized steel and method of making it.
BACKGROUND OF THE INVENTION
Steel is one of the most commonly used materials now a day in every aspect of human life. Corrosion is a very important aspect associated with every steel industry. Corrosion of steel surfaces is mainly due to water or moisture coming into contact with steel surfaces. If one could control and minimize the water (even moisture) interaction with steel surfaces, corrosion could be prevented to a large extent. Interaction of water (and moisture) with steel surface could be minimized by decreasing its apparent surface (interfacial) energy using appropriate low surface energy material’s coating in addition to creating appropriate surface roughness. Such coatings can make steel surface superhydrophobic, with water contact angle around 160o, thus greatly reducing the chances of corrosion.
In nature, lotus leaves depicts superhydrophobic behaviour with water contact angle almost 170o due to underlying micro-nano roughness and natural wax coating. Moreover, superhydrophobic surfaces show very low roll-off angle i.e. if they are tilted even by few degrees, drops start rolling off the surface.
Many scientific groups have attempted both theoretically and experimentally to create topographically rough surfaces with extremely high water-repellency and very low flow resistance. [J. Phys. Chem., 100, 19512, 1996, Soft Matter, 4, 224, 2008, J. Mater. Chem., 22, 20112, 2012]. Since superhydrophobicity is a surface phenomenon, most of the superhydrophobic surfaces have issues regarding mechanical and environmental stability. Alternatively, they depict very poor stability against wear and tear caused by day to day interactions.
Several methodologies have been applied and patented on creating superhydrophobic surface on the surface of metals sheets.
Patent "JP8104986" discloses coating of fluoroalkylsilane composite with titanium, zirconium or aluminium to form a coating film on the steel. The solution

is applied on a substrate to form a wet coating film and baked at 200-500 °C in the atmosphere. Consequently, the concentration of alkyl groups is increased on the substrate surface, and a substrate having a superhydrophobic coating film is obtained. Since this process requires annealing the substrates at high temperatures therefore may not be practical for many application.
Patent application "US2008221009" discloses application of treated fumed silica in a solvent forming a film composite for superhydrophobic property of steel. This process uses fumed silica nanoparticles for creating surface roughness, therefore does not provide very good abrasion resistance.
Reference may also be made to a patent "JP9020983" wherein it discloses coating formation comprising a layer of a silane coupling agent on the surface of a stainless steel having surface-roughened to a roughness of 1.0 micron or more, and then an amorphous fluororesin layer. This process uses embossing of stainless steel for creating surface roughness which is a costly process and its scalability is also cumbersome.
OBJECTS OF THE INVENTION
In view of the foregoing limitations inherent in prior-art, it is an object of the invention to develop a process to make a superhydrophobic galvanized steel.
Another objective is to develop the superhydrophobic coating for GI steel being stable, resistant to corrosion, and comparatively cheap.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a process for making a superhydrophobic steel comprising steps of dipping a galvanized steel for 18 -24 hours at 60-70 C. in a solution of 91:4:5 wt.% mixture of DI water, N-N dimethylacetamide and 2-methoxyethanol for creating a vertically aligned ZnO nanorods microstructure, dipping the galvanized steel in second solution mixture of 47:2:1 wt% of toluene, polymer polydimethylsiloxane solution (PDMS) (sylgard 184) and its thermal cross-linker for producing a low surface energy polymer coating, drying the

galvanized steel in air for 1-15 mins; and annealing the galvanized steel at 100°C-120 for 30 min- 60 min.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. 1 illustrates various steps for a process to make a superhydrophobic
galvanized steel in accordance with an embodiment of the invention.
FIGS. 2a(1) and 2a(2) show scanning electron microscope (SEM) image of GI sample etched with wet oxidation technique at different magnification level in accordance with an embodiment of the invention.
FIG. 2b shows a superhydrophobic galvanized steel obtained from the process in accordance with an embodiment of the invention.
FIG. 2c and 2d show SEM image of superhydrophobic galvanized steel showing PDMS coated Zinc nano rod network at magnification level 10000 and 30000 respectively.
FIGS. 2e (1) and (2) show a contact angle of the superhydrophobic galvanized steel before and after adhesion test respectively in accordance with an embodiment of the invention.
FIG. 2f shows images the samples depicting their state on expose to supersaturated salt fog taken at different time to identify corrosion (in form of red rust) in accordance with an embodiment of the invention.
FIGS. 2g (1), (2) and (3) show the contact angle of the superhydrophobic galvanized steel after posing various environmental conditions in accordance with an embodiment of the invention.
DETAILED DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a process for making a superhydrophobic steel, the process comprising steps of dipping a galvanized

steel in a first solution mixture for 18 -24 hours at 60-70 C. for creating a vertically aligned ZnO nanorods microstructure, the first solution being 91:4:5 wt.% mixture of DI water, N-N dimethylacetamide and 2-methoxyethanol, dipping the galvanized steel in a second solution for producing a low surface energy polymer coating, the second solution being 47:2:1 wt% mixture of toluene, polymer polydimethylsiloxane solution (PDMS) (sylgard 184) and its thermal cross-linker, drying the galvanized steel in air for 1-15 mins; and annealing the galvanized steel at 100°C-120 for 30 min- 60 min.
As per the superhydrophobic principle, a surface needs to have hierarchical roughness (from nanometer to micrometer) as well as coating of low surface energy material. Thereby a wet etching route to create hierarchical surface roughness on GI steel has been adopted.
Shown in FIG. 1 is series of steps of a process (100) for making a
superhydrophobic galvanized steel made from a galvanized steel. Before the
process (100) the galvanized steel is mildly pickled by removal of oxide. The
process includes following steps:
At step (108) the galvanized steel is dipped in a first solution mixture for 18 - 24 hours at 60-70 C. for creating a vertically aligned ZnO nanorods microstructure. The first solution mixture is 91:4:5 wt.% mixture of DI water, N-N dimethylacetamide and 2-methoxyethanol, all in their highest purity. This is done for self-organized growth of ZnO nanorods. This step is also known as wet oxidation technique.
Surface roughness of the vertically aligned ZnO nanorods microstructure is 5-40 nm in the nano range and 0.5-2 micron in micron range, together give the effect of hierarchal dual roughness.
At step (112) the galvanized steel is dipped in a second solution for producing a low surface energy polymer coating. The second solution is 47:2:1 wt% mixture of toluene, polymer polydimethylsiloxane [Si(CH3)3[Si(CH3)2O]nSi(CH3)3] solution (PDMS) (sylgard 184) and its thermal cross-linker, all in their highest purity. An

extremely thin layer of PDMS is formed on the ZnO hierarchical nanorods provides low energy surface. The surface energy of the PDMS is 15 - 24 mN/m.
The thickness of the coating of low surface energy polymer of polydimethylsiloxane is 50-100 nm depending on the dipping speed. Dipping speed of 100 mm/min gives thickness of 90 nm.
At step (116) the galvanized steel sheet is dried in air for 1 -15 min.
Subsequently, at step (120) the galvanized steel is annealed at 100 - 120 C for 30-60 min.
Contact angle of the obtained superhydrophobic galvanized steel is 155-165. It is to be appreciated that if the contact angle is more than 140, it is considered to be superhydrophobic.
Water droplets when come in contact superhydrophobic surfaces, take shape of bead and roll off at very low tilt angle (< 5°) due to extremely high water contact angle. Rolling action of water droplet reduces drag significantly compared to laminar flow condition in hydrophilic samples. When water drops roll down on a tilted superhydrophobic surface, they collects all the dirt on the way along with it. Thus the superhydrophobic surface comes with inherent self-cleaning properties.
Most of the metal surfaces (including steel) are hydrophilic (θ ~ 30°) due to high surface energy caused by polar-polar interaction between water molecules and metallic surfaces. Wettability of a substrate depends on the contact angle of water on them. Higher the contact angle, lower the wettability.
Example:
The properties of the above mentioned superhydrophobic steel can be validated by the following experiments. The following experiments should not be construed to limit the scope of invention.
FIGS. 2a(1) and 2a(2) show scanning electron microscope (SEM) image of GI sample etched with the above mentioned wet oxidation technique to develop

ZnO nanorods. The magnification level for the FIGS. 2a(1) and 2a(2) is 10,000 and 50,000 respectively for wet oxidation.
Following were the parameters:
dipping a galvanized steel for 18 hours at 65 C. in a solution mixture of DI water, N-N dimethylacetamide and 2-methoxyethanol, in proportion of 91:4:5 wt.%
Subsequent to the oxidation to create surface roughness, the surface chemistry of the resulting surface was modified by coating low surface energy materials using (PDMS) coating with following parameters.
dipping the galvanized steel in a solution mixture being 47:2:1 wt% mixture of toluene, polymer polydimethylsiloxane solution (PDMS) (sylgard 184) and its thermal cross-linker. The sample was pulled out.
After pulling out samples, they were dried for 5 min in air followed by annealing at 115°C for 30 min.
Shown in FIG. 2b is a superhydrophobic galvanized steel (200) obtained from the process parameters as mentioned above. FIG. 2b shows the cross sectional view indicating a steel substrate (204), a zinc layer (208) and a ZnO nanorods (212). Top layer is a coating of low surface energy polymer of PDMS layer. The PDMS layer is very thin (in nano meters) hence not visible in the figure.
It should be appreciated that the FIG. 2a is the representative figure showing the galvanized steel surface and ZnO nanorods microstructure for the sake of easy
understanding. Whereas, in actuality the layers (middle and bottom) are over all the surfaces of the galvanized steel.
The vertically aligned ZnO nanorods (212) over the zinc layer (208) of steel substrate have optimum surface roughness required for superhydrophobic behavior. The low surface energy polymer coating of PDMS, on the rough GI surface provides desired superhydrophobicity.

FIGS. 2c and 2d show SEM image of superhydrophobic galvanized steel showing PDMS coated Zinc nano rod network at magnification level 10000 and 30000 respectively.
Wettability test:
After surface energy modification, wettability of the sample was measured by measuring the contact angle of a 2 µlit DI water drop using sessile drop method. ZnO nanorods based GI steel samples modified with PDMS provided best superhydrophobicity as contact angle 162 was found as shown in FIG. 2e (1) and was used for further investigations (adhesion, environmental and corrosion tests).
Adhesion tests:
Adhesion tests was carried out by cutting the coatings using a crosshatch cutter (ASTMD3359) followed by scotch tape peal test and the water contact angle was measured. The contact angle was found to be 161 shown in FIG. 2e (2).
From FIGS. 2e (1) and (2) it can be concluded that PDMS coated ZnO nanorods based GI steel samples provides good adhesion as the contact angle does not change and therefore no PDMS layer is peeled of the surface ZnO nanorods. So keeping superhydrophobicity and adhesion in mind, it appears that PDMS coatings on ZnO nanords GI samples provide the best result.
Corrosion protection test:
Subsequently, PDMS coated ZnO nanorods based GI steel samples were used for salt spray test (SST) to quantify the corrosion protection which is summarized in FIG. 2f. The samples were exposed to supersaturated salt fog and images were taken at different time to identify corrosion (in form of red rust).
So it is clear from images that PDMS coated ZnO nanorods on GI samples have better corrosion prevention than GI samples.

Environmental test:
Subsequently the superhydrophobic galvanized steel samples were exposed to
real environmental conditions to study their stability and longevity:
FIG. 2g (1) shows the sample kept for 31 days under the Sun.
FIG. 2g (2) shows the sample under UV radiation (254 nm, 350 µW/cm2) for 15
days.
FIG. 2g (3) shows the sample under heat (80°C) at 10-12 hours
It is clear from FIGS. 2g (1), 2g (2) and 2g (3) that superhydrophobic galvanized
steel do not degrade during Sun or UV because the top PDMS absorbs UV
radiation and the underlying ZnO nanords are not affected by it therefore
maintains the superhydrophobicity.
So it can be concluded that PDMS coated ZnO nanorods based
superhydrophobic samples depict very good adhesion, excellent corrosion
protection and stable against environmental condition.
Advantages:
It is evident that the superhydrophobic galvanized steel obtained from above said process has properties such as corrosion resistivity, self-cleaning property, stability, simple process and water drag reduction property. The process is also comparatively less costly.

WE CLAIM
1. A process for making a superhydrophobic steel, the process comprising
steps of:
dipping a galvanized steel in a first solution mixture for 18 -24 hours at 60-70 C. for creating a vertically aligned ZnO nanorods microstructure, the first solution being 91:4:5 wt.% mixture of DI water, N-N dimethylacetamide and 2-methoxyethanol;
dipping the galvanized steel in a second solution for producing a low surface energy polymer coating, the second solution being 47:2:1 wt% mixture of toluene, polymer polydimethylsiloxane solution (PDMS) (sylgard 184) and its thermal cross-linker;
drying the galvanized steel in air for 1-15 mins; and annealing the galvanized steel at 100°C-120 for 30 min- 60 min.
2. The process as claimed in claim 1, wherein surface energy of the PDMS is 15 mN/m- 24 mN/m.
3. The process as claimed in claim 1, wherein surface roughness of the vertically aligned ZnO nanorods microstructure is 5-40 nm in nano range and 0.5 – 2 micron in micron range.
4. The superhydrophobic steel having contact angle of 155-165 degrees made by process as claimed in anyone of the claims 1-3.