Title of Invention	A PROCESS FOR SURFACE PRETREATMENT OF FERROUS SUBSTRATES FOR SOL-GEL CERAMIC COATING
Abstract	The present invention discloses the development of a surface pretreatment process prior to sol-gel coating on plain carbon steel (mild steel) and ferrous objects. This invention has special significance to the metal/ alloys that forms poor and non-adhered oxide on the surface and results into poor corrosion resistant coating in conventional sol-gel process. This process encompasses a treatment method for metal/alloy components in metal hydroxide sol prepared from metal salts prior to sol-gel coating and curing of coated substrate at a temperature suitable to form corrosion resistant layer for subsequent coating in acidic sol. The method covered in this invention is useful to enhance the corrosion and wear resistance of components made from above mentioned type metals /alloys.

Title of Invention

A PROCESS FOR SURFACE PRETREATMENT OF FERROUS SUBSTRATES FOR SOL-GEL CERAMIC COATING

Abstract

The present invention discloses the development of a surface pretreatment process prior to sol-gel coating on plain carbon steel (mild steel) and ferrous objects. This invention has special significance to the metal/ alloys that forms poor and non-adhered oxide on the surface and results into poor corrosion resistant coating in conventional sol-gel process. This process encompasses a treatment method for metal/alloy components in metal hydroxide sol prepared from metal salts prior to sol-gel coating and curing of coated substrate at a temperature suitable to form corrosion resistant layer for subsequent coating in acidic sol. The method covered in this invention is useful to enhance the corrosion and wear resistance of components made from above mentioned type metals /alloys.

Full Text	FIELD OF INVENTION The present invention relates to a process for surface pretreatment of ferrous substrates for sol-gel ceramic coating. The invention particularly relates to a surface pretreatment process for sol-gel coating on a plain carbon steel (mild steel) and ferrous objects that form poor and non-adherent oxide on the surface and results into poor corrosion resistant coating in conventional sol-gel process. The developed pretreatment method is able to extend the life of components not only against corrosion but also abrasion and wear failures and therefore is better than the existing paints and coatings coupled to its lower cost. The process developed herein is useful for industries which use low alloy steels/ mild steel such as auto sectors, sugar industries, power plants, oil transportation, surface finishing, coating and ceramic industries building materials, etc. BACKGROUND OF THE INVENTION Due to increasing industrialization and demand for longer life of components, enhanced corrosion resistance of surfaces is extremely important. The surface properties of metals, such as micro-hardness, wear, wear-corrosion resistance and electrical insulation can be improved by using various surface engineering techniques. Ceramic coatings in recent years have been increasingly adopted for hot and wet corrosion protection of several engineering components. The coatings are being tried out using various methods including thermal spray, PVD, CVD, plasma, laser surface coating etc. However, these coatings have certain limitations such as huge equipment cost, low coating adherence, large porosity, requirement of controlled environment (such as vacuum), and line of site etc. Sol-gel coating offers a promising alternative being a low cost, easy operational, provide high adherence, and pore-free coating, permits coating of intricate geometries, requires low processing temperature, and non-line of site method. The other important advantage of the sol-gel coating method is easy control of sol-gel chemistry by varying for example, concentration of precursor and catalyst type, pH etc. (US Patent 5869141). Also the density, porosity, and microstructure can be tailored by controlling the chemistry of the sol. The key factor to obtain the protective and adherent coating is the state of interphase between coating and metal/alloy. If the surface is not clean, that is it contains dirt, debris, smut, loose oxides, etc. the bonding between coating and the substrate remains weak and affects the coating performance. The interphase here is not limited to pre-existed dirt/ smut on the surface, but includes electrochemically and chemically generated surface when object is dipped in the acidic sol. The pretreatment of the surface of the substrate to receive good sol-gel coating, therefore, is an important process and remained to be subject of development particularly for metals/alloys that form undesirable oxide in the aqueous acidic sol. US patent 5869141 describes how the oxide or smut is undesirable for good bonding between substrate and coating and discloses the development of pretreatment of titanium for subsequent sol-gel coatings. However, coating produced by this process has number of limitations, which restricts the use of such coating for corrosion and wear protection. These include lower thickness, cracking, weak bonding between coating and substrate, and permeability to gases and liquids. US Patent 6284682 discloses a novel process of preparing thick chemically bonded composite sol-gel coating. The coating is conducted using slurry prepared by dispersing nanometer scale alpha-alumina powder into a water based alumina sol. The slurry is sprayed using conventional pressurized air spray gun, followed by curing at 300 to 400 degree C to give a low cost ceramic coating. However, the challenge remains to coat plain carbon steel or low alloy steel which gets rusted (forming porous and non-adhered oxide) during coating process. Generally, the sol for obtaining ceramic coating is peptized with HCl/HNOs and maintained at acidic pH (below 4). However, it does not work for low alloy steels such as mild steel primarily due to the formation of non-protective and loose oxides that result in poorly bonded ceramic coating. The need therefore has always been felt to formulate appropriate pretreatment methods before ceramic coating on low-alloyed steel and iron. Gazala Ruhi et al. [in Surface & Coating Technology, Vol. 201, No. 3-4, pp. 1866-187, 2006] have recently described sol-gel alumina coating on chemically pretreated mild steel in order to improve the bonding characteristics. The sol-gel alumina coating was obtained on mild steel that was polished and shot-peened with steel shots (diameter 0.6 mm) followed by zinc phosphating. The coating obtained showed shift in corrosion potentials to anodic direction but corrosion current indicated only slight improvement in corrosion resistance in NaCl solution. Generally, compact ceramic coating during electrochemical polarization tests should illustrate barrier type behavior which was not apparent from the efforts made by above mentioned authors. US patent 5,776,542 describes the coating on mild steel or iron objects wherein several inorganic oxides and metal powders were mixed to form slurry for the coating. The slurry coated object was dried to remove water content completely and finally heated at a sufficiently high temperature to obtain corrosion resistant coating on the ferrous substrate. While the method stated in US 5,776,542 is complicated containing several components, the present invention is extremely simple in the sense that it requires easily available inorganics and no metal powder. To the best of our knowledge such pretreatment method for mild steel or iron specimens has not been reported elsewhere. This process will overcome the shortcomings such as insignificant improvement in corrosion resistance, high process cost, complicated methodology etc. by the known methods. OBJECTS OF THE INVENTION The main object of the present invention is thus to provide a process for the surface pretreatment of ferrous substrates and low-alloyed steel for sol-gel ceramic coatings which obviates the drawbacks of the hitherto known prior art. Another object of the present invention is to develop an inert and impermeable layer over the surface of mild steel and iron objects, during pretreatment, in order to provide template for ceramic sol-gel coating. Still another object of the present invention is to optimize the pretreatment parameters such as pH of the precipitation process, curing temperature, and time to allow diffusion and reaction of chemical species and thereby the formation of corrosion resistant layer. SUMMARY OF THE INVENTION The present invention recites a pretreatment process for surfaces such as low-alloyed steel and ferrous objects. The process encompasses a treatment method for metal/alloy components in metal hydroxide sol prepared from metal salts prior to sol-gel coating and curing of coated substrate at a temperature suitable to form corrosion resistant layer for subsequent coating in acidic sol. The pretreatment process of the present invention results in an adherent, impermeable and protective layer on the said surfaces. Subsequent sol-gel coating on the thus pre-treated surfaces offers an excellent corrosion resistance evident from very low corrosion current and barrier or insulating behaviour of coating. Such a resistance of these alloys has not been shown earlier by any known sol-gel process reported so far. Accordingly, the present invention provides a process for the surface pretreatment of ferrous substrates for sol-gel ceramic coating comprising the steps of: (a) preparing an aqueous metal salt solution by dissolving 0.05 to 1.0 moles of a metal salt in water; (b) precipitating the aqueous metal salt solution obtained from step [a] at pH between 5.5 to 10.0 and temperature ranging from 25 to 80 degree C; (c) filtering, washing and drying the precipitate as obtained in step [b] in ambient atmosphere; (d) dispersing the washed and dried precipitate as obtained in step [c] in distilled water under stirring for 48 to 72 hours to obtain a solution; (e) coating the pre-cleaned and degreased ferrous surfaces to be pretreated with the solution obtained from step [d]; (f) heating the coated surface of step [e] at temperatures ranging from 150 to 700 degree C for about 1 to 2 hours followed by furnace cooling; (g) removing the loose oxide product from the coated surface as obtained in step [fj with acetone, methyl alcohol, ethyl alcohol and the like to obtain the desired pretreated ferrous surface. In an embodiment of the present invention the metal salt may contain aluminum, titanium, chromium, zirconium or molybdenum having nitrate, chloride or acetate anions. In another embodiment of the present invention the precipitating agent may be selected from KOH, NaOH, or NH4OH and the like. In still another embodiment of the present invention, the quantity of precipitating agent added may be such that the precipitation occurs in the pH ranging between 5.5 to 10.0. In yet another embodiment of the present invention, the precipitation may be carried out in the temperature range 25 to 80 degree C. In a further embodiment of the present invention, the precipitate may be dried at room temperature and dispersed in distilled water so as to make the final concentration of the solution in the range 0.05 to 1.0 mole/litre. In still another embodiment of the present invention, the precipitate which is dispersed in water may be stirred for 48 to 72 hours. In yet another embodiment of the present invention, polished or sand blasted and cleaned substrate may be coated by dipping, spraying or brushing the above solution. In another embodiment of the present invention, the coated substrate may be dried in ambient atmosphere for 30 to 90 minutes and then heated at a temperature ranging from 400 to 700 degree C. DETAILED DESCRIPTION OF THE INVENTION State of the surface prior to coating is extremely important for assurance of good quality of final coating. The strong bonding between coating and substrate is essentially required for longevity with regards to corrosion and wear protection of substrate materials. Most often the pre-existing dirt, debris, or smut is responsible for weakening the bond between substrate and coating and thus reduce the quality of coating. There are well-established methods available to clean pre-existing debris/ oxide/ dust particle from the surface of substrate before coating. However, surface oxidation of low alloy steel or ferrous substrates due to chemical reaction with acidic sol on subsequent drying/heating interferes with coating adherence. This is of major concern when thus formed oxide is non-protective in nature such as on mild steel and iron. The sol-gel coating on stainless steels does not require any pretreatment because the nanometer thick oxide formed on stainless steel during the contact of sol is passive and impermeable. Also it receives alumina coating in a pore free fashion. It, however, does not work for mild steel as it forms loose and porous oxide when comes in contact with the sol and result in poor coating adherence. This in the present invention is overcome by pretreatment of substrate before sol-gel ceramic coating. By doing so, the composite layer comprised of oxide and hydroxide is formed which is protective, adherent and strengthens the bonding between substrate and subsequent ceramic coating. The process of the present invention begins with the preparation of solution from metal salts containing aluminium, titanium, chromium, zirconium, or molybdenum. The solution is then precipitated with KOH, NaOH, or NHiOH and like bases. The reaction conditions are maintained with pH ranging from 5.5 to 10.0 at temperature ranging from 25 to 80 degree C. The precipitate thus obtained may be dried at room temperature and dispersed in distilled water. The precipitate is then de-clustered in water by stirring for about 48 to 72 hours. Polished substrate may be dip-coated/ sprayed with the above solution and dried in ambient atmosphere for about 30 to 90 minutes and heated further at temperature ranging from 400 to 700 degree C. This results in a novel interphase for further coating or painting for corrosion and wear resistance. EXAMPLES The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. Example 1 0.5 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 7.5 at temperature of 30 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water so as to keep the concentration 0.05 moles/1 and stirred for 72 hours. The surface-cleaned mild steel specimen was dip coated at a speed of 3 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 150 degree C followed by heating at 500 degree C for 2 hours and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced corrosion current 2.3 x 10'10 A/ cm2 against 1.2 x 10"4 A/ cm2 on bare mild steel. Thus, the coating could drastically reduce the rate of corrosion by 6 order of magnitude. Experiments clearly showed barrier type protection upto 1.1V vs SCE whereas no such protection was observed on bare substrate. The barrier current density produced by coated substrate remained Example 2 1.0 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 8.5 at temperature of 80 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water so as to keep the concentration 1 moles/1 and stirred for 72. The surface-cleaned mild steel specimen was dip coated at a speed of 10 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 250 degree C followed by heating at 600 degree C for 2 hours and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced corrosion current 1.18 x 10 9 A/ cm2, about 5 order less than the base metal. The coated substrate showed barrier type protection upto 1.1V vs SCE with the barrier current density Example 3 0.75 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 6.5 at temperature of 50 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water and stirred continuously for 48 hours. The surface-cleaned mild steel specimen was dip coated at a speed of 7 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 200 degree C followed by heating at 500 degree C for 2 hours and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced corrosion current 8.5 x 10"9 A/ cm2. The coated substrate showed barrier type protection upto 1.1V vs SCE with current density below 117 nA/cm2 Example 4 0.9 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 7.0 at temperature of 70 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water and stirred continuously for 72 hours. The surface-cleaned mild steel specimen was dip coated at a speed of 8 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 225 degree C followed by heating at 500 degree C for 1 hour and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced very low corrosion current 5.9 x 10~9 A/ cm2 as compared to base metal. The coated substrate showed barrier type protection upto 1.1V vs SCE where the current density was below 173 nA/cm2. Main advantages of the present invention are: 1. To provide suitable solution to pre-treat ferrous substrates that form poor and non-adhered oxide in conventional sol-gel process and result into poor ceramic coating. 2. Preparation of solution from easily available inorganic metal salts to treat the mild steel and iron objects prior to ceramic coating. 3. Optimized process parameters such as pH, temperature, and time for dip-coating of the said surfaces in the above sol. 4. Novel method for producing stable and protective interphase on mild steel and iron objects essentially required for subsequent coatings/ painting. 5. Enhances the corrosion and wear resistance of mild steel and iron by manifolds. We claim: 1. A process for the surface pretreatment of ferrous substrates for sol-gel ceramic coating comprising the steps of: (h) preparing an aqueous metal salt solution by dissolving 0.05 to 1.0 moles of a metal salt in water; (i) precipitating the aqueous metal salt solution obtained from step [a] at pH between 5.5 to 10.0 and temperature ranging from 25 to 80 degree C; (j) filtering, washing and drying the precipitate as obtained in step [b] in ambient atmosphere; (k) dispersing the washed and dried precipitate as obtained in step [c] in distilled water under stirring for 48 to 72 hours to obtain a solution; (1) coating the pre-cleaned and degreased ferrous surfaces to be pretreated with the solution obtained from step [d]; (m) heating the coated surface of step [e] at temperatures ranging from 150 to 700 degree C for about 1 to 2 hours followed by furnace cooling; (n) removing the .loose oxide product from the coated surface as obtained in step [f] with acetone, methyl alcohol, ethyl alcohol and the like to obtain the desired pre treated ferrous surface. 2. A process as claimed in claim 1, wherein the metal salt contains aluminum, chromium, titanium, zirconium or molybdenum having nitrate, chloride or acetate as anion. 3. A process as claimed in any preceding claims, wherein the precipitating agent is a base selected from the group consisting of KOH, NaOH, or NH4OH and the like. 4. A process as claimed in any preceding claims, wherein the quantity of the base added is such that the precipitation occurs in the pH range 5.5-10.0. 5. A process as claimed in any preceding claims, wherein the precipitate is dispersed in distilled water so as to make the final concentration of the solution in the range 0.05-1.00 moles/liter. 6. A process as claimed in any preceding claims, wherein the surface is coated either by dipping, spraying, brushing and like in ambient atmosphere. 7. A process for surface pretreatment of ferrous substrates for sol-gel ceramic coating substantially as herein described with reference to the foregoing examples.

Full Text

FIELD OF INVENTION
The present invention relates to a process for surface pretreatment of ferrous substrates for sol-gel ceramic coating. The invention particularly relates to a surface pretreatment process for sol-gel coating on a plain carbon steel (mild steel) and ferrous objects that form poor and non-adherent oxide on the surface and results into poor corrosion resistant coating in conventional sol-gel process. The developed pretreatment method is able to extend the life of components not only against corrosion but also abrasion and wear failures and therefore is better than the existing paints and coatings coupled to its lower cost. The process developed herein is useful for industries which use low alloy steels/ mild steel such as auto sectors, sugar industries, power plants, oil transportation, surface finishing, coating and ceramic industries building materials, etc.
BACKGROUND OF THE INVENTION
Due to increasing industrialization and demand for longer life of components, enhanced corrosion resistance of surfaces is extremely important. The surface properties of metals, such as micro-hardness, wear, wear-corrosion resistance and electrical insulation can be improved by using various surface engineering techniques. Ceramic coatings in recent years have been increasingly adopted for hot and wet corrosion protection of several engineering components. The coatings are being tried out using various methods including thermal spray, PVD, CVD, plasma, laser surface coating etc. However, these coatings have certain limitations such as huge equipment cost, low coating adherence, large porosity, requirement of controlled environment (such as vacuum), and line of site etc.
Sol-gel coating offers a promising alternative being a low cost, easy operational, provide high adherence, and pore-free coating, permits coating of intricate geometries, requires low processing temperature, and non-line of site method. The other important advantage of the sol-gel coating method is

easy control of sol-gel chemistry by varying for example, concentration of precursor and catalyst type, pH etc. (US Patent 5869141). Also the density, porosity, and microstructure can be tailored by controlling the chemistry of the sol.
The key factor to obtain the protective and adherent coating is the state of interphase between coating and metal/alloy. If the surface is not clean, that is it contains dirt, debris, smut, loose oxides, etc. the bonding between coating and the substrate remains weak and affects the coating performance. The interphase here is not limited to pre-existed dirt/ smut on the surface, but includes electrochemically and chemically generated surface when object is dipped in the acidic sol. The pretreatment of the surface of the substrate to receive good sol-gel coating, therefore, is an important process and remained to be subject of development particularly for metals/alloys that form undesirable oxide in the aqueous acidic sol.
US patent 5869141 describes how the oxide or smut is undesirable for good bonding between substrate and coating and discloses the development of pretreatment of titanium for subsequent sol-gel coatings. However, coating produced by this process has number of limitations, which restricts the use of such coating for corrosion and wear protection. These include lower thickness, cracking, weak bonding between coating and substrate, and permeability to gases and liquids.
US Patent 6284682 discloses a novel process of preparing thick chemically bonded composite sol-gel coating. The coating is conducted using slurry prepared by dispersing nanometer scale alpha-alumina powder into a water based alumina sol. The slurry is sprayed using conventional pressurized air spray gun, followed by curing at 300 to 400 degree C to give a low cost ceramic coating. However, the challenge remains to coat plain carbon steel
or low alloy steel which gets rusted (forming porous and non-adhered oxide) during coating process.
Generally, the sol for obtaining ceramic coating is peptized with HCl/HNOs and maintained at acidic pH (below 4). However, it does not work for low alloy steels such as mild steel primarily due to the formation of non-protective and loose oxides that result in poorly bonded ceramic coating. The need therefore has always been felt to formulate appropriate pretreatment methods before ceramic coating on low-alloyed steel and iron.
Gazala Ruhi et al. [in Surface & Coating Technology, Vol. 201, No. 3-4, pp. 1866-187, 2006] have recently described sol-gel alumina coating on chemically pretreated mild steel in order to improve the bonding characteristics. The sol-gel alumina coating was obtained on mild steel that was polished and shot-peened with steel shots (diameter 0.6 mm) followed by zinc phosphating. The coating obtained showed shift in corrosion potentials to anodic direction but corrosion current indicated only slight improvement in corrosion resistance in NaCl solution.
Generally, compact ceramic coating during electrochemical polarization tests should illustrate barrier type behavior which was not apparent from the efforts made by above mentioned authors. US patent 5,776,542 describes the coating on mild steel or iron objects wherein several inorganic oxides and metal powders were mixed to form slurry for the coating. The slurry coated object was dried to remove water content completely and finally heated at a sufficiently high temperature to obtain corrosion resistant coating on the ferrous substrate.
While the method stated in US 5,776,542 is complicated containing several components, the present invention is extremely simple in the sense that it requires easily available inorganics and no metal powder. To the best of our
knowledge such pretreatment method for mild steel or iron specimens has not been reported elsewhere. This process will overcome the shortcomings such as insignificant improvement in corrosion resistance, high process cost, complicated methodology etc. by the known methods.
OBJECTS OF THE INVENTION
The main object of the present invention is thus to provide a process for the surface pretreatment of ferrous substrates and low-alloyed steel for sol-gel ceramic coatings which obviates the drawbacks of the hitherto known prior art.
Another object of the present invention is to develop an inert and impermeable layer over the surface of mild steel and iron objects, during pretreatment, in order to provide template for ceramic sol-gel coating.
Still another object of the present invention is to optimize the pretreatment parameters such as pH of the precipitation process, curing temperature, and time to allow diffusion and reaction of chemical species and thereby the formation of corrosion resistant layer.
SUMMARY OF THE INVENTION
The present invention recites a pretreatment process for surfaces such as low-alloyed steel and ferrous objects. The process encompasses a treatment method for metal/alloy components in metal hydroxide sol prepared from metal salts prior to sol-gel coating and curing of coated substrate at a temperature suitable to form corrosion resistant layer for subsequent coating in acidic sol. The pretreatment process of the present invention results in an adherent, impermeable and protective layer on the said surfaces. Subsequent sol-gel coating on the thus pre-treated surfaces offers an excellent corrosion resistance evident from very low corrosion current and
barrier or insulating behaviour of coating. Such a resistance of these alloys has not been shown earlier by any known sol-gel process reported so far.
Accordingly, the present invention provides a process for the surface pretreatment of ferrous substrates for sol-gel ceramic coating comprising the steps of:
(a) preparing an aqueous metal salt solution by dissolving 0.05 to 1.0
moles of a metal salt in water;
(b) precipitating the aqueous metal salt solution obtained from step
[a] at pH between 5.5 to 10.0 and temperature ranging from 25 to
80 degree C;
(c) filtering, washing and drying the precipitate as obtained in step
[b] in ambient atmosphere;
(d) dispersing the washed and dried precipitate as obtained in step [c]
in distilled water under stirring for 48 to 72 hours to obtain a
solution;
(e) coating the pre-cleaned and degreased ferrous surfaces to be
pretreated with the solution obtained from step [d];
(f) heating the coated surface of step [e] at temperatures ranging
from 150 to 700 degree C for about 1 to 2 hours followed by
furnace cooling;
(g) removing the loose oxide product from the coated surface as
obtained in step [fj with acetone, methyl alcohol, ethyl alcohol and
the like to obtain the desired pretreated ferrous surface.
In an embodiment of the present invention the metal salt may contain aluminum, titanium, chromium, zirconium or molybdenum having nitrate, chloride or acetate anions.
In another embodiment of the present invention the precipitating agent may be selected from KOH, NaOH, or NH4OH and the like.
In still another embodiment of the present invention, the quantity of precipitating agent added may be such that the precipitation occurs in the pH ranging between 5.5 to 10.0.
In yet another embodiment of the present invention, the precipitation may be carried out in the temperature range 25 to 80 degree C.
In a further embodiment of the present invention, the precipitate may be dried at room temperature and dispersed in distilled water so as to make the final concentration of the solution in the range 0.05 to 1.0 mole/litre.
In still another embodiment of the present invention, the precipitate which is dispersed in water may be stirred for 48 to 72 hours.
In yet another embodiment of the present invention, polished or sand blasted and cleaned substrate may be coated by dipping, spraying or brushing the above solution.
In another embodiment of the present invention, the coated substrate may be dried in ambient atmosphere for 30 to 90 minutes and then heated at a temperature ranging from 400 to 700 degree C.
DETAILED DESCRIPTION OF THE INVENTION
State of the surface prior to coating is extremely important for assurance of good quality of final coating. The strong bonding between coating and substrate is essentially required for longevity with regards to corrosion and wear protection of substrate materials. Most often the pre-existing dirt, debris, or smut is responsible for weakening the bond between substrate and coating and thus reduce the quality of coating. There are well-established methods available to clean pre-existing debris/ oxide/ dust particle from the surface of substrate before coating. However, surface oxidation of low alloy steel or ferrous substrates due to chemical reaction with acidic sol on subsequent drying/heating interferes with coating adherence. This is of major concern when thus formed oxide is non-protective in nature such as on mild steel and iron. The sol-gel coating on stainless steels does not require any pretreatment because the nanometer thick oxide formed on stainless steel during the contact of sol is passive and impermeable. Also it receives alumina coating in a pore free fashion. It, however, does not work for mild steel as it forms loose and porous oxide when comes in contact with the sol and result in poor coating adherence.
This in the present invention is overcome by pretreatment of substrate before sol-gel ceramic coating. By doing so, the composite layer comprised of oxide and hydroxide is formed which is protective, adherent and strengthens the bonding between substrate and subsequent ceramic coating.
The process of the present invention begins with the preparation of solution from metal salts containing aluminium, titanium, chromium, zirconium, or molybdenum. The solution is then precipitated with KOH, NaOH, or NHiOH and like bases. The reaction conditions are maintained with pH ranging from 5.5 to 10.0 at temperature ranging from 25 to 80 degree C. The precipitate thus obtained may be dried at room temperature and dispersed in distilled
water. The precipitate is then de-clustered in water by stirring for about 48 to 72 hours. Polished substrate may be dip-coated/ sprayed with the above solution and dried in ambient atmosphere for about 30 to 90 minutes and heated further at temperature ranging from 400 to 700 degree C. This results in a novel interphase for further coating or painting for corrosion and wear resistance.
EXAMPLES
The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention.
Example 1
0.5 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 7.5 at temperature of 30 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water so as to keep the concentration 0.05 moles/1 and stirred for 72 hours. The surface-cleaned mild steel specimen was dip coated at a speed of 3 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 150 degree C followed by heating at 500 degree C for 2 hours and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced corrosion current 2.3 x 10'10 A/ cm2 against 1.2 x 10"4 A/ cm2 on bare mild steel. Thus, the coating could drastically reduce the rate of corrosion by 6 order of magnitude. Experiments clearly showed barrier type protection upto 1.1V vs SCE whereas no such protection was observed on bare substrate. The barrier current density produced by coated substrate remained Example 2
1.0 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 8.5 at temperature of 80 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water so as to keep the concentration 1 moles/1 and stirred for 72. The surface-cleaned mild steel specimen was dip coated at a speed of 10 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 250 degree C followed by heating at 600 degree C for 2 hours and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced corrosion current 1.18 x 10 9 A/ cm2, about 5 order less than the base metal. The coated substrate showed barrier type protection upto 1.1V vs SCE with the barrier current density Example 3
0.75 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 6.5 at temperature of 50 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water and stirred continuously for 48 hours. The surface-cleaned mild steel specimen was dip coated at a speed of 7 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 200 degree C followed by heating at 500 degree C for 2 hours and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced corrosion current 8.5 x 10"9 A/ cm2. The coated substrate showed barrier type protection upto 1.1V vs SCE with current density below 117 nA/cm2
Example 4
0.9 mol of the metal salt was dissolved in distilled water and precipitated with a base having pH 7.0 at temperature of 70 degree C. Solution was filtered, washed and dried at room temperature. The residue was then dispersed in distilled water and stirred continuously for 72 hours. The surface-cleaned mild steel specimen was dip coated at a speed of 8 mm/s and dried at room temperature. Thus coated specimen was cured in the muffle furnace at 225 degree C followed by heating at 500 degree C for 1 hour and furnace cooled. Loose oxide from the specimen's surface was removed with alcohol and then subjected to alumina coating according to reported methods. The potentiodynamic polarization of coated electrode (according to ASTM G5) in 3.5 % NaCl solution produced very low corrosion current 5.9 x 10~9 A/ cm2 as compared to base metal. The coated substrate showed barrier type protection upto 1.1V vs SCE where the current density was below 173 nA/cm2.
Main advantages of the present invention are:
1. To provide suitable solution to pre-treat ferrous substrates that form
poor and non-adhered oxide in conventional sol-gel process and result
into poor ceramic coating.
2. Preparation of solution from easily available inorganic metal salts to treat the mild steel and iron objects prior to ceramic coating.
3. Optimized process parameters such as pH, temperature, and time for dip-coating of the said surfaces in the above sol.
4. Novel method for producing stable and protective interphase on mild
steel and iron objects essentially required for subsequent coatings/
painting.
5. Enhances the corrosion and wear resistance of mild steel and iron by
manifolds.

We claim:
1. A process for the surface pretreatment of ferrous substrates for sol-gel ceramic coating comprising the steps of: (h) preparing an aqueous metal salt solution by dissolving 0.05 to 1.0
moles of a metal salt in water; (i) precipitating the aqueous metal salt solution obtained from step
[a] at pH between 5.5 to 10.0 and temperature ranging from 25 to
80 degree C;
(j) filtering, washing and drying the precipitate as obtained in step
[b] in ambient atmosphere;
(k) dispersing the washed and dried precipitate as obtained in step [c]
in distilled water under stirring for 48 to 72 hours to obtain a
solution; (1) coating the pre-cleaned and degreased ferrous surfaces to be
pretreated with the solution obtained from step [d]; (m) heating the coated surface of step [e] at temperatures ranging
from 150 to 700 degree C for about 1 to 2 hours followed by
furnace cooling; (n) removing the .loose oxide product from the coated surface as
obtained in step [f] with acetone, methyl alcohol, ethyl alcohol and
the like to obtain the desired pre treated ferrous surface.
2. A process as claimed in claim 1, wherein the metal salt contains
aluminum, chromium, titanium, zirconium or molybdenum having
nitrate, chloride or acetate as anion.
3. A process as claimed in any preceding claims, wherein the
precipitating agent is a base selected from the group consisting of
KOH, NaOH, or NH4OH and the like.
4. A process as claimed in any preceding claims, wherein the quantity of
the base added is such that the precipitation occurs in the pH range
5.5-10.0.
5. A process as claimed in any preceding claims, wherein the precipitate
is dispersed in distilled water so as to make the final concentration of
the solution in the range 0.05-1.00 moles/liter.
6. A process as claimed in any preceding claims, wherein the surface is
coated either by dipping, spraying, brushing and like in ambient
atmosphere.
7. A process for surface pretreatment of ferrous substrates for sol-gel
ceramic coating substantially as herein described with reference to the
foregoing examples.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=4N/p+Gtxyy2wydUHURCvAQ==&loc=+mN2fYxnTC4l0fUd8W4CAA==

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Patent Number

268363

Indian Patent Application Number

208/DEL/2008

PG Journal Number

35/2015

Publication Date

28-Aug-2015

Grant Date

27-Aug-2015

Date of Filing

25-Jan-2008

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, RAFI MARG, NEW DELHI-110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	TIWARI SHASHI KANT	ACC DIVISION NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR-831007 (JHARKHAND), INDIA.
2	SINGH RAGHUVIR	NATIONAL METALLURGICAL LABORATORY, JAMSHEDPUR-831007 (JHARKHAND),INDIA

PCT International Classification Number

C23C 22/00

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA