Description:TECHNICAL FIELD
The present disclosure relates to the field of metallurgy. Particularly, the present disclosure relates to methods for preparing a silane-coated steel substrate, wherein the method comprises coating a composition comprising hydrolysed glycidyloxypropyl trimethoxysilane (GPTMS) on a steel substrate and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
Corrosion of steel is mitigated by application of corrosion resistant coating systems or by providing external current source. The application of different coatings on steel is the most cost effective and easy technique to impart corrosion resistance. In the last two decades, a lot of research has been done on organic-inorganic hybrid coatings (mainly alkoxy silane coatings) on steel as a replacement for carcinogenic chromate coatings as well as phosphate coatings. The silica-based hydrolyzed sol gel coating is widely applied on steel and then cured to develop a uniform layer of 3-diemnsional silica network structure which has the potential to provide corrosion resistance by retarding the ingress of corrosive ions. These coatings are mainly deposited on steel by dip or roll coating methods. The electrodeposition of silane coatings has also gained a lot of research interest as this method can provide a more compact and uniform film than dip or roll coating method.

For application of silane-based coatings, alkoxy-silanes (e.g. tetra-alkoxysilanes) are first hydrolyzed in acidic water medium and then cured (at different temperatures and time) on metal surface to get the silica-based sol gel coatings. The hydrolysis and condensation process of alkoxy silane-based polymers are shown in Figure 1. The -OR (where R=CH3, C2H5 or other alkyl groups) groups of alkoxysilane convert to -OH groups due to hydrolysis as shown in Figure 1. Then condensation occurs between two silanes through the reaction between two -OH groups (which liberates water) or the reaction between -OH and -OR groups (which liberates alcohol, R-OH) as shown in Figure 1 (D. Wang and G.P. Bierwagen, “Sol–gel coatings on metals for corrosion protection,” Progress in Organic Coatings, 64 (2009) 327-338). The hydrolysis reactions accelerate in an acidic medium whereas the condensation reactions accelerate in an alkali medium.

Hu et al. (J. M. Hu et al., “Effects of electrodeposition potential on the corrosion properties of bis-1, 2-[triethoxysilyl] ethane films on aluminium alloy,” Electrochimica acta, 51 (2006) 3944-3949) have deposited Bis-1,2-[triethoxysilyl] ethane (BTSE) films on 2024-T3 aluminium alloy at different anodic and cathodic potentials with respect to open circuit potential (OCP). They proposed that the film formation at cathodic potential occurs due to the alkaline-aided condensation mechanism of silanols (Si–OH) by oxygen reduction (reaction 1) or electrolysis of water (reaction 2) on the aluminium alloy surface.
1/2 O_2+H_2 O+2e^-?2?OH?^- ………………………1
H_2 O+e^-??OH?^-+1/2 H_2 ……………………..2
Another mechanism is suggested by Sheffer et al. (M. Sheffer et al., “Electrodeposition of sol–gel films on Al for corrosion protection,” Corrosion Science, 45 (2003) 2893-2904) where they explained that the application of a negative potential on a metal substrate causes depletion of H+ ions on the metal surface and thereby increasing the local pH which accelerates the deposition and crosslinking of a sol-gel film on the substrate.

CN105462328A is directed to an electrodeposition of different functional silane-based coating compositions on steel, galvanized steel and aluminium, where the composition comprises yttrium-based corrosion inhibitor, a crosslinking agent and a catalyst. CN102230203A discloses the electrodeposition (at potentials -0.5 to -1.4V vs SCE for 30 s to 30 min) of a monosilane and a bis-silane on different metal substrates where the coating liquid composition comprises sodium carbonate, sodium silicate, etc. The coating is cured at 60 to 120? for 0.5 to 5 hours. WO2000063303A1 is directed to the application of two layers of coating on metal substrates where the first layer is made of bis-silyl aminosilanes and bis-silyl polysulfur silanes and the second layer is electrocoated paint system. CN201456490U discusses a metallic framework coated by rubber and also covered by electrodeposited silane film of 15-micron thickness. WO2018229070A1 discloses electrodeposition of a mixture of polymer, silane, graphene oxide and other particle-based corrosion resistant coating at the voltage of 1 to 50 volts between anode and cathode. CN102713021A is directed to the development of two layers of coating on stainless steel where the first layer is developed by soaking the stainless steel in an amino coupling agent followed by electrophoretic deposition of a second layer comprising resins, a coloring component, an epoxy-phosphate compound etc. IN299271B is directed to the development of coating by a film forming polymer, functional silanes, cross linking agent, catalyst along with rare earth corrosion inhibitor. CN103966646B reports the electrodeposition of silane-graphene coating on a steel plate at different deposition potentials. KR816523B1 is directed to a silane coating that contains rust preventives, additives etc. WO2019126498A1 is directed to the development of an electrodepositable coating that comprises a film forming binder, electrically conducting particles, a silane, a curing agent and a corrosion inhibitor. CN109338430A is directed to the development of a silane-epoxy hydrolysis liquid mixed with a graphene oxide suspension to form an electrodeposited composite coating on metal substrates by an electrodeposition method. JP05512940B2 is directed to an electrodeposition coating comprising silane coupling agents and surfactants.

Although various silane-coating have been developed, there is still a need to provide a simple, cost-effective silane coating that provides effective corrosion resistance. The present invention attempts to address this need.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to methods for preparing a silane-coated steel substrate, comprising coating a composition comprising hydrolysed 3-glycidyloxypropyl trimethoxysilane (GPTMS) on a steel substrate at a cathodic deposition potential.

The present disclosure also relates to silane-coated steel substrates produced by the methods described herein.

The present disclosure further relates to steel substrates comprising a coating comprising hydrolysed GPTMS.

The present disclosure also relates to a composition for use in coating a steel substrate, comprising hydrolysed GPTMS.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows an exemplary hydrolysis and condensation process of alkoxy silane-based polymers.

Figure 2, panels a-m, show scanning electron microscopic (SEM) images of steel substrates comprising electrodeposited coatings deposited at various deposition potential and time according to the invention and dip coating surfaces for different GPTMS compositions.

Figure 3 shows energy dispersive spectroscopic (EDS) images and data for coatings deposited at -1.5V at three different composition-time sets.

Figure 4, panels a-c, show potentiodynamic polarization curves for bare mild steel substrates and steel substrates coated with hydrolysed GPTMS coatings at different deposition conditions.

Figure 5, panels a-c, show electrochemical impedance spectroscopy (EIS) data for 10% GPTMS coating electrodeposited for 15 minutes (panel a), 20% GPTMS coating electrodeposited for 15 minutes (panel b), and 20% GPTMS coating electrodeposited for 30 minutes (panel c). Figure 5, panels d-e, show electrochemical equivalent circuit (EEC) models for fitting the EIS data of bare steel substrates and steel substrates with electrodeposited and dip coatings.

DETAILED DESCRIPTION OF THE DISCLOSURE
With respect to the use of 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” or “containing” or “has” or “having” 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.

Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The present disclosure provides methods for preparing a silane-coated steel substrate. In particular, the present disclosure provides methods for preparing a steel substrate coated with a composition comprising hydrolysed glycidyloxypropyl trimethoxysilane (GPTMS).

The term “hydrolysed GPTMS” as used herein refers to GPTMS comprising at least some of the alkoxy groups (the “-OR” groups) converted to -OH groups.

The term “about” as used herein encompasses variations of +/-10% and more preferably +/-5%, as such variations are appropriate for practicing the present invention.

As provided herein, a method for preparing a silane-coated steel substrate comprises coating a composition comprising hydrolysed glycidyloxypropyl trimethoxysilane (GPTMS) on a steel substrate at a cathodic deposition potential.

In some embodiments, the composition comprising hydrolysed GPTMS is prepared by mixing GPTMS with water and alcohol. In some embodiments, the composition comprises about 10% to about 20% v/v GPTMS, including values and ranges therebetween. In some embodiments, the composition comprises about 10%, about 11%, about 12%, about 13%, about 14%, about 15%, about 16%, about 17%, about 18%, about 19%, or about 20% v/v GPTMS.

In some embodiments, the composition comprises about 10% to about 20% v/v GPTMS, water, and alcohol. In some embodiments, the composition comprises about 10% to about 20% v/v GPTMS, about 70% to about 80% v/v water, and about 10% v/v alcohol. In some embodiments, the composition comprises about 10% v/v GPTMS, about 80% v/v water, and about 10% v/v alcohol. In some embodiments, the composition comprises about 15% v/v GPTMS, about 75% v/v water, and about 10% v/v alcohol. In some embodiments, the composition comprises about 20% v/v GPTMS, about 70% v/v water, and about 10% v/v alcohol. In some embodiments, water is distilled water. In some embodiments, the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, and a combination thereof. In some embodiments, the alcohol is ethanol.

In some embodiments, hydrolysis of GPTMS is accelerated by adding a few drops of an acid to any of the compositions comprising GPTMS described herein. In some embodiments, the acid is acetic acid or hydrochloric acid.

In some embodiments, the composition comprising hydrolysed GPTMS has a pH of about 4.0. For example, in some embodiments, the composition comprising hydrolysed GPTMS has a pH of about 3.8, about 3.9, about 4.0, about 4.1, or about 4.2.

Exemplary GPTMS compositions that can be employed to practice the invention are shown in Table A below.

Table A: Exemplary Compositions of GPTMS polymer solution for coating
Compositions GPTMS (V%)
(ml) Distilled water (V%)
(ml) Ethanol(V%)
(ml) Acetic acid pH
10%-GPTMS 10 80 10 Few drops 4-5
20%-GPTMS 20 70 10 Few drops 4-5

According to the methods of the present disclosure, a composition comprising hydrolysed GPTMS is coated, i.e., deposited on a steel substrate at a cathodic deposition potential.

In some embodiments, the cathodic deposition potential ranges from about -0.8V to about -2.0V, including values and ranges therebetween. In some embodiments, the cathodic deposition potential is about -0.8V, -0.9V, -1.0V, -1.1V, -1.2V, -1.3V, -1.4V, -1.5V, -1.6V, -1.7V, -1.8V, -1.9V, or -2.0V. In some embodiments, the cathodic deposition potential is about -1.0V, -1.5V, or -2.0V.

In some embodiments, a composition comprising hydrolysed GPTMS is coated on a steel substrate at a cathodic deposition potential for about 15 minutes to about 30 minutes, including values and ranges therebetween. In some embodiments, the composition comprising hydrolysed GPTMS is coated on a steel substrate at a cathodic deposition potential for about 15 minutes, 16 minutes, 17 minutes, 18 minutes, 19 minutes, 20 minutes, 21 minutes, 22 minutes, 23 minutes, 24 minutes, 25 minutes, 26 minutes, 27 minutes, 28 minutes, 29 minutes, or 30 minutes. In some embodiments, the composition comprising hydrolysed GPTMS is coated on a steel substrate at a cathodic deposition potential for about 15 minutes or about 30 minutes.

It is understood that methods of the present disclosure comprise coating any of the compositions comprising hydrolysed GPTMS described herein on a steel substrate at any of the cathodic deposition potentials for any of the time periods described herein. In some embodiments, the methods comprise coating a composition comprising hydrolysed GPTMS at a concentration of 20% v/v on a steel substrate at a cathodic deposition potential of about -1.5V or about -2.0V for about 15 or 30 minutes.

The present disclosure further provides silane-coated steel substrates produced by the methods described herein.

The present disclosure also provides steel substrates comprising a coating comprising hydrolysed GPTMS. In some embodiments, steel substrates comprise a coating comprising about 10%, about 15%, or about 20% hydrolysed GPTMS.

The present disclosure also relates to use of the compositions comprising hydrolysed GPTMS in coating a steel substrate. The compositions comprising hydrolysed GPTMS are described above. For example, in some embodiments, a composition for use in coating a steel substrate comprises about 10% to about 20% v/v GPTMS, water, and alcohol. In some embodiments, a composition for use in coating a steel substrate comprises about 10% to about 20% v/v GPTMS, about 70% to about 80% v/v water, and about 10% v/v alcohol. In some embodiments, a composition for use in coating a steel substrate comprises about 10% v/v GPTMS, about 80% v/v water, and about 10% v/v alcohol. In some embodiments, a composition for use in coating a steel substrate comprises about 15% v/v GPTMS, about 75% v/v water, and about 10% v/v alcohol. In some embodiments, a composition for use in coating a steel substrate comprises about 20% v/v GPTMS, about 70% v/v water, and about 10% v/v alcohol. In some embodiments, water is distilled water. In some embodiments, the alcohol is selected from the group consisting of methanol, ethanol, propanol, and butanol. In some embodiments, the alcohol is ethanol. In some embodiments, the compositions comprise a small amount, e.g., a few drops of an acid such as acetic acid or hydrochloric acid.

The methods of the present disclosure provide steel substrates with improved corrosion resistance compared to uncoated steel substrates or steel substrates prepared using other coating methods such as roll coating or dip coating.

It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. 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. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.

Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above described embodiments, and in order to illustrate the embodiments of the present disclosure 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: Preparation of silane-coated steel substrates and analysis of corrosion resistance

Cold rolled closed annealed (CRCA) Interstitial Free (IF) steel (i.e. mild steel) was collected from Tata Steel cold roll mill. The as-received steel substrate was cut into coupons of 5×6 cm2 size. These steel coupons were degreased with the help of 2.5 wt% alkali solution at 60? for 5 to 10 seconds followed by rinsing with distilled water. 3-Glycidyloxypropyl trimethoxysilane (GPTMS) was purchased from Sigma Aldrich. GPTMS was hydrolysed in water-ethanol medium. Few drops of glacial acetic acid were added to the solution to accelerate the hydrolyzation of GPTMS. The details of composition of the hydrolysed GPTMS is mentioned in Table 1.
Table 1: Composition of GPTMS polymer solution used for coating
Compositions GPTMS (V%)
(ml) Distilled water (V%)
(ml) Ethanol(V%)
(ml) Acetic acid pH
10%-GPTMS 10 80 10 Few drops 4-5
20%-GPTMS 20 70 10 Few drops 4-5

Coating deposition:
An electrochemical cell of three electrode system was used to develop the electrodeposited coating on a mild steel surface. The mild steel was used as working electrode, saturated calomel electrode as reference electrode and graphite plate as counter electrode. The exposed surface area of working electrode was 40 cm2. The coating deposition was done by varying the concentration of GPTMS polymer, deposition potential and time. The details for coating deposition parameters are shown in Table 2. A few dip coated steel samples were also prepared at different time and compositions (mentioned in Table 2) to compare with the electrodeposited coating substrates.
Table 2: Coating deposition parameters
Compositions Sample ID Potential (V) Deposition time (min)
10%-GPTMS Dip 0 (only dipping) 15
-0.8V -0.8
-1.0V -1.0
-1.5V -1.5
-2.0V -2.0
20%-GPTMS Dip 0 (only dipping) 15
-0.8V -0.8
-1.0V -1.0
-1.5V -1.5
-2.0V -2.0
20%-GPTMS Dip 0 (only dipping) 30
-1.0V -1.0
-1.5V -1.5

1. Characterization of coating
1.1 Analysis of surface microstructure
Field Emission Gun Scanning Electron Microscopy (FEG-SEM) was used to understand the coating surface morphology and evolution of microstructure. Energy Dispersive Spectroscopy (EDS) was used to understand the variation of different elemental fraction throughout the coating surface and the uniformity of coating over the substrate.
1.2 Corrosion evaluation
Potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) was performed on all coated samples and bare steel to understand the corrosion performances and the mechanism of corrosion. All the corrosion tests were performed in three-electrode electrochemical cell with the help of Gamry potentiostat. In this three-electrode cell, the coated and bare steel was used as working electrode (WE), saturated calomel electrode as reference electrode (RE) and platinum mesh as counter electrode (CE). The exposed area of working electrode was 1 cm2 for all corrosion tests. All the corrosion tests were accomplished in aqueous 3.5% NaCl solution at room temperature after stabilizing the open circuit potential (OCP) for 15 minutes.
All potentiodynamic polarization tests were performed in the potential range of 250 mV more negative than the corrosion potential (Ecorr) to 600 mV more positive than the corrosion potential (Ecorr) to get the Tafel plots. The scan rate was 0.5 mV/sec.
All electrochemical impedance spectroscopy (EIS) test was carried out at very low AC perturbation potential of ±10 mV with 10 points per decade after stabilizing the OCP.
1.3 Results and Discussion
Morphology and microstructure of coating
The SEM (all are in SE mode) images of electrodeposited (at various deposition potential and time) and dip coating surfaces at different compositions are shown in Figure 2, panels a-m. It is evident that the steel surface is covered by silane coating (GPTMS). The coating surface is mainly composed of two different regions which can be distinguished by black and white contrast. The EDS results of coatings deposited at -1.5V at three different composition-time set are shown in Figure 3 as representative EDS result from each coating set. It shows that the black area is rich in silicon and the white area is rich in iron. The 10% GPTMS coatings deposited for 15 minutes (Figure 2 (a-e)) show less silicon rich areas and more open areas (iron rich area) for dip coated surface (Figure 2a) than the surfaces deposited at different cathodic potentials (Figure 2(b-e)). It suggests that the only dip coating method is not able to uniformly cover the whole surface by silicon due to ununiform crosslinking condensation reaction of silanols (hydrolysed GPTMS) with all over the steel surface. However, the coatings deposited at different cathodic potentials are showing more surface coverage by silicon rich areas than the dip coating which could be attributed to the accelerated crosslinking reaction of silanols due to local increase of pH at different cathodic deposition potentials. The local increase in potential can be attributed to the hydrogen reduction reaction (reaction 3) at different cathodic potentials as well as oxygen reduction reaction (reaction 1).
2H^(+ )+ 2e^-=H_2……………….(3)
It was observed that there is no significant difference in coating coverage by silicon rich areas with decreasing the cathodic deposition potentials from -0.8V to -2.0V (Figure 2b-e). This indicates that local increase of pH due to decreasing deposition potentials up to -2.0V is not sufficient enough for making a significant difference (in terms of coating coverage by silicon rich areas) among coatings deposited at different potentials from 10% GPTMS polymer solution.
The effect of coating microstructure due to increase in polymer concentration from 10% to 20% has been shown in Figure 2(f-j). Figure 2(h-j) reveals that the coatings obtained in lower cathodic deposition potentials (-1.0V to -2.0V) show better extent of surface coverage by silicon rich areas than the dip coating (Figure 2f) and the coating deposited at -0.8V (Figure 2g).
The microstructure evolution due to increasing the coating deposition time (up to 30 min for 20% GPTMS solution) is shown in SEM images in Figure 2(k-m). It shows that almost all the surface has been covered by silicon rich areas and significantly higher than the other coating surfaces. There is no significant difference in surface coverage between dip coating and cathodic electrodeposited coatings.

2. Potentiodynamic poralization study
The comparison of potentiodynamic polarization curves for bare mild steel and coated substrates at different deposition conditions are shown in Figure 4(a-c). The corrosion potential (Ecorr), corrosion current density (icorr), corrosion rate(CR), anodic (ßa) and cathodic (ßc) Tafel slopes were calculated by Tafel extrapolation method with the help of Gamry software. All the electrochemical values obtained from Tafel fitting are shown in Table 3.
Table 3: Electrochemical parameters obtained from the potentiodynamic polarization curves by Tafel fitting
Compositions Deposition time (min) Sample ID Ecorr (mV) icorr
(A cm-2) ßa (V/decade) ßc
(V/decade) CR (mpy)
Bare steel NA Bare -689 16×10-6 0.05 0.48 7
10%-GPTMS 15 Dip -619 22×10-6 0.08 4 10
-0.8V -622 24×10-6 0.09 0.6 11
-1.0V -603 27×10-6 0.08 1.8 12
-1.5V -633 18×10-6 0.08 1 8
-2.0V -594 20×10-6 0.07 2 9
20%-GPTMS 15 Dip -681 25×10-6 0.06 1.5 11
-0.8V -649 12×10-6 0.06 0.85 6
-1.0V -706 12×10-6 0.06 1.35 5.5
-1.5V -693 5.3×10-6 0.05 0.4 2.5
-2.0V -702 5.2×10-6 0.06 0.7 2.4
20%-GPTMS 30 Dip -463 26×10-6 0.2 8 12
-1.0V -717 15×10-6 0.07 9 7
-1.5V -678 6×10-6 0.05 0.3 3

The cathodic Tafel slopes of all the samples revealed mixed control kinetics (combination of diffusion and activation controlled). Cathodic electrodeposited (deposited from potentials -0.8V to 2.0V) and dip coatings of 10% GPTMS composition deposited for 15 minutes showed inferior corrosion resistance even than the bare mild steel (Figure 4a). The corrosion of steel has increased after application of coatings. This can be explained due to the non-uniform surface coverage by silicon rich areas. Silicon comes from Si-O-Si uniform film. This Si-O-Si film is the main reason of giving barrier corrosion protection to steel. So, the corrosion resistance of GPTMS coated steel significantly depends on the extent of surface coverage by this film. SEM analysis showed that the coatings deposited for 15 minutes from 10% GPTMS solution through dip and cathodic electrodepostion technique have non-uniform silicon coverage (film coverage). There is significant amount of bare iron surface which is open to corrosive environment. This develops a good galvanic coupling between silicon rich and iron reach areas which increases the corrosion rate of coated steels even than the bare mild steel.
The corrosion resistance of electrodeposited coatings has increased upon increasing the concentration of GPTMS polymer (Figure 4b) to 20% when the deposition time is kept fixed at 15 minutes. Table 3 clearly shows that the electrodeposited coatings from 20% GPTMS solution provide better corrosion resistance than the 10% GPTMS solution. The corrosion resistance of coatings with 20% GPTMS polymer is increased due to the increased polymer concentration. The electrodeposited coatings from 20% GPTMS showed better corrosion resistance than the dip coating obtained from the same 20% GPTMS solution. Also, the corrosion resistance is much higher at lower deposition potentials (-1.5V and -2.0V) than the higher deposition potentials (-0.8V and -1.0V). This phenomenon indicates that there is a significant effect of deposition potential on the corrosion resistivity at higher GPTMS polymer concentration. The higher polymer concentration made the polymer molecules more available than low concentration solution to condense under accelerating reaction condition which makes more uniform film and provides better corrosion protection at lower deposition potentials (-1.5V and -2.0V).
The effect of increasing deposition time on corrosion resistance has also been studied which is shown in Figure 4c. Here also, the corrosion rate diminishes as the deposition potential decreases from -1.0V to -1.5V. Corrosion rate of all electrodeposited coating at 30 minutes are significantly less than dip coated steel as well as mild steel.

3. Electrochemical impedance spectroscopy (EIS) study
EIS has been used to measure the difference in barrier property and mechanism of corrosive ion penetration through different electrodeposited and dip coatings and compared with the corrosion mechanism of the bare mild steel substrate. The Bode impedance plot of 10% GPTMS coatings for 15 minutes shows (Figure 5a) that there is no significant difference in impedance (at lowest frequency) among dip, electrodeposited coatings and bare steel. This suggests that the barrier property of bare steel has not been changed due to deposition of different coatings from hydrolysed 10% GPTMS. This matches with the Tafel data where it is found that the corrosion resistance has not been improved for coated steels than the bare steel. The 20% GPTMS, coated for 15 minutes (Figure 5b) at lower deposition potentials (-1.5V and -2.0V) showed higher impedance (at lowest frequency) than the other coatings (deposited from same composition and same time duration) and bare steel which indicates their better barrier property than others. Also, the 20% GPTMS coating deposited for 30 minutes (Figure 5c) at lowest deposition potential of -1.5V showed higher impedance than other coatings deposited using the same composition for the same time but at different potentials.
The EIS data of bare steel has been fitted with electrochemical equivalent circuit (EEC) model of single time constant as shown in Fig. 5(d). This is a simple Randles circuit, where constant phase element (CPEct) and charge transfer resistance (Rct) are in parallel combination, which is in series with solution resistance (Rslon). All the electrodeposited and dip coatings were fitted with the EEC model of two time constants as shown in Fig. 5(e). This circuit consists of CPEcRpore combination in addition to the Rslon (CPEctRct) combination. This additional CPEcRpore combination has been introduced to explain the pore resistance of coatings and coating capacitance during its exposure to the electrolyte solution, whereas the (CPEctRct) combination represents the corrosion process. Here Rpore, Rct and Rslon are the pore resistance of coating, charge transfer resistance and solution resistance respectively whereas CPEc and CPEct are the constant phase elements due to coating and double layer capacitance, respectively. The constant phase elements (CPEs) has been introduced to represent the inhomogeneous electrochemical surfaces of the coatings and the bare mild steel surfaces which has deviated from the ideal capacitor. The CPE is defined by the equation (4) as follows-
Z_CPE=1/(?(j?)?^n C)………..(4)
where, j=v(-1), C is the capacitance, ? is called angular frequency (?=2pf rad s-1) and n is the exponent (0=n=1). The Gamry software was used to fit all the EIS data by respective equivalent circuits. The obtained values of various electrochemical parameters are shown in Table 4. The total resistance to corrosion (Rt) of electrodeposited and dip coatings (i.e. 10% GPTMS coatings for 15 minutes) are comparable with each other (Table 4). The insignificant increase in Rt value of coatings than bare steel can be attributed to the formation of ununiform coating over the steel surface at lower concentration.
However, the significant increase in Rt value for the coatings deposited at lower potentials (-1.5V and -2.0V) for 20% GPTMS coating (deposited for 15 minutes as well as 30 minutes) than the other coatings can be attributed to the formation of uniform and compact barrier layer of Si-O-Si on the steel substrate which is retarding the penetration of corrosive ions.

Table 4: Electrochemical parameters obtained from the fitting results of EIS curves by different equivalent circuits
Compositions Deposition time (min) Sample ID Rsoln (Ohm.cm2) Rpore
(Ohm.cm2) Rct
(Ohm.cm2)
Rt= (Rpore+ Rct)
(Ohm.cm2) Goodness of Fit
Bare steel NA Bare 20 NA 803 803 1.59E-03
10%-GPTMS 15 Dip 14 900 63 963 1.01E-02
-0.8V 14 160 907 1067 5.35E-04
-1.0V 15 473 1140 1613 1.04E-02
-1.5V 16 572 948 1520 1.40E-02
-2.0V 16 760 302 1062 5.74E-03
20%-GPTMS 15 Dip 16 282 584 866 6.69E-04
-0.8V 16 142 720 862 2.36E-02
-1.0V 16 226 537 799 1.31E-02
-1.5V 18 400 2740 3140 3.09E-03
-2.0V 12 450 2530 2980 6.21E-04
20%-GPTMS 30 Dip 14 24 1670 1694 1.21E-03
-1.0V 15 80 1130 1210 2.14E-02
-1.5V 16 2190 116 2306 3.01E-03

The above experiments demonstrate that the hydrolysed GPTMS polymer solution (silanol) can be deposited on mild steel surface effectively by electrodeposition technique. The Si-O-Si film formation becomes more homogeneous and compact at lower deposition potentials (-1.5V and -2.0V) for 20% hydrolysed GPTMS composition. The corrosion resistance improved for the coatings deposited at lower deposition potentials (-1.5V and -2.0V) for 20% hydrolysed GPTMS composition.

INCORPORATION BY REFERENCE
All references, articles, publications, patents, patent publications, and patent applications cited herein are incorporated by reference in their entireties for all purposes. However, mention of any reference, article, publication, patent, patent publication, and patent application cited herein is not, and should not be taken as, an acknowledgment or any form of suggestion that they constitute valid prior art or form part of the common general knowledge in any country in the world.

Claims:We Claim:
1. A method for preparing a silane-coated steel substrate, comprising coating a composition comprising hydrolysed glycidyloxypropyl trimethoxysilane (GPTMS) on a steel substrate at a cathodic deposition potential.
2. The method as claimed in claim 1, wherein the GPTMS is present at a concentration ranging from about 10% to about 20% v/v.
3. The method as claimed in claim 2, wherein the concentration of the GPTMS in the composition is 10% v/v.
4. The method as claimed in claim 2, wherein the concentration of the GPTMS in the composition is 20% v/v.
5. The method as claimed in claim 1, wherein the cathodic deposition potential ranges from -0.8V to -2.0V.
6. The method as claimed in claim 5, wherein the cathodic deposition potential is -1.5V.
7. The method as claimed in claim 5, wherein the cathodic deposition potential is -2.0V.
8. The method as claimed in claim 1, wherein the composition is coated at the cathodic deposition potential from about 15 minutes to about 30 minutes.
9. The method as claimed in claim 8, wherein the composition is coated at the cathodic deposition potential for 15 minutes.
10. The method as claimed in claim 8, wherein the composition is coated at the cathodic deposition potential for 30 minutes.
11. The method as claimed in claim 1, wherein the GPTMS is present in the composition at a concentration of 20% v/v, the cathodic deposition potential is -1.5V or -2.0V, and the composition is coated at the cathodic deposition potential for 15 or 30 minutes.
12. The method as claimed in claim 1, wherein the hydrolysed GPTMS is obtained by mixing GPTMS with water and alcohol.
13. The method as claimed in claim 12, wherein the alcohol is selected from the group consisting of methanol, ethanol, propanol, butanol, and a combination thereof.
14. The method as claimed in claim 12, wherein the alcohol is ethanol.
15. The method as claimed in claim 12, wherein the alcohol is present at a concentration of about 10% v/v and the water is present at a concentration ranging from about 70% v/v to about 80% v/v.
16. The method as claimed in claim 12, wherein an acid is added to a mixture of GPTMS, water, and alcohol to accelerate hydrolysis.
17. The method of claim 16, wherein the acid is selected from acetic acid or hydrochloric acid.
18. A silane-coated steel substrate produced by the method as claimed in claim 1.
19. A steel substrate comprising a coating comprising hydrolysed glycidyloxypropyl trimethoxysilane (GPTMS).
20. A composition for use in coating a steel substrate, comprising hydrolysed glycidyloxypropyl trimethoxysilane (GPTMS).
21. The composition as claimed in claim 0, wherein the GPTMS is present at a concentration ranging from about 10% to about 20% v/v.
22. The composition as claimed in claim 0, wherein the GPTMS is present at a concentration of 10% v/v or 20% v/v.