Title of Invention

A PROCESS FOR THE PREPARATION OF CORROSION RESISTANT SILICA COATINGS ON METAL

Abstract

The invention describes a process for the preparation of silica coatings by passing steam over tetra ethyl ortho silicate (TEOS) wherein TEOS in alcohol medium is heated to obtain concentrated TEOS TEOS so obtained is subjected to steam to hydroiyse to silicon tetra-hydroxide simultaneously diluting the concentrated solution to a viscosity level suitable for coating. Metal substrates such as aluminium polished and pre-treated with pickling solutions was dipped into the above coating solution. The substrate was then dried in the temperature range of 60-110°C followed by sintering / heat treating to obtain uniform coatings of silica. The process of the present invention such as control of viscosity, uniformity of the final coatings removes the major drawbacks of the prior art / existing process in which TEOS is also used as coating precursor. The silica coated metal substrates find application such as in corrosion resistance.

Full Text

The present invention is related to a process for the preparation of corrosion resistant silica coatings on metal The present invention particularly relates to a process for the preparation of corrosion resistant silica coatings on metals particularly aluminium Aluminium is one of the most widely used metals due to its lightness and known for its various structural and functional applications. However, the metal is prone to corrosion in acidic and basic atmosphere. Therefore, ceramic oxides coatings such as silica, alumina, chromia are coated on Al substrate in order to protect the metal from corrosion. These oxides are generally coated by different techniques such as Plasma Spraying, Physical Vapour Deposition (PVD), Chemical Vapour Deposition (CVD), Sol-gel etc. In plasma spraying process, the free flowing powder is passed through a plasma and directed towards a substrate where it cools on contact. The main drawback of this process is the higher porosity of the resultant coatings (10% porosity is considered good), therefore, the coatings are subjected to additional heat-treatment. Another drawback of the plasma spray technique is that only line of sight geometries can be successfully coated. In PVD, expensive vacuum systems are requed to coat high quality ceramic films. In addition, this technique is also limited to line of sight geometries. An alternative method, which has gained considerable interest in recent years is sol-gel processing. The process involves hydrolyisis of organo-metallic precursor compounds of the desired ceramic oxides to form a structured solution or gel containing organo-metallic polymers or macro-clusters. Additives can be added to control the viscosity and surface tension of the sol-gel solution. Coatings are prepared by either spin, dip or spray coating or painting onto an appropriate substrate. The coated substrate is then fired/heat treated to remove the organic material and to deveoop a strong bond between the coating material and the substrate. The sol gel process has several advantages over other fabrication methods. It is simple, economically feasible and permits coatings of complex geometries, not necessarily Sine of sight Reference may be made to US Patent, Ref.no.4,865,649 wherein silica coating solution for semiconductor device applications were prepared by taking two or more varieties of Alkoxy Silanes and co-hydrolysing them by addition of water. The main drawbacks of the above processes are (a) the concentration of solid matter is as low as 10% by weight and (b) the duration of tne coating solution preparation is very large of the order of 5 days Reference may be made to US Patent, Ref. no.6.303,229, wherein silicic acid of 16 % by weight was depolymerised at specific temperature and pH which was used as coating material. The main drawback of the above process are the coating was less dense, difficult to control the coating thickness due to the high particle sized silica Reference may also be made to US patent no 5.404,706 wherein TEOS (of 5% wt) was used for silica coating to the skin o; a hypersonic flight vehicle. The main drawbacks of the above process are to obtain the required thickness and more coating time due to low concentration of precursors. Reference may be made to US patent no 4,186,026 wherein 20-50 wt % TEOS, 37-60 wt % of certain silanes and 15-35 wt % formalin was used and hydroiysed by water. The main drawback of this method was to obtain the required viscosity of coating solution suitable for coating which takes 5 days Reference may be made to De Sanctis et al. O.De Sanctis, L.Gomez. N.Pellegri, C.Parodi, A.Marajoesky and A.Duran. Protective giass coatings on metallic substrates". Journal of Non-Crystalline Solids. 121 1990. 338-343 prepared silica coatings of 0.4 microns by dip coating method for oxidation and acid corrosion of steel. Oxidation weight gain was an order magnitude iess than uncoated steel and acid attack was reduced by 35%. Reference may be made to M.Atik and M.A.aegerter, "Corrosion resistance ZrO;. Sol-gei coatings on stainless steel, Materials Research Society Symposium Proceedings. 271 1992. 471-476 prepared sol-gel silica coatings of 0.4 micron on 316 L stainless steei. In conventional method, silica coatings are earned out fay soi-gei process in which silicon alkoxides, tetra ethyl ortho silicate (TEOSi. Si(OC2H5)4 is hydrolysed in a solution of water and alcohol which subsequently polymerises to larger molecular structures known as gels by condensation Coating solutions are formulated so that gelation does not occur before the iiquia is deposited. The main drawbacks of the sol-gel process in the hitherto known prior art are (i) difficulty in producing uniform coatings due to high loss of water and alcohol which shrinks the coatings, thereby, the coating oecomes porous and sometime develop cracks also (ii) the coatings sometimes results with nigher particle size due to condensation of TEOS chains and produces coarse particles, hence, results in coarse or thick coatings, (iii) in maintaining the viscosity of the coating solution which depends upon the hydrolysis and condensation of TEOS and is very much sensitive to pH, the ratio of TEOS to alcohol and water, (iv) the coating time is very long ranging from few hours to few days. From the details and drawbacks of the hitherto known processes for providing silica coatings on metal, it is clear that there is a need and scope for improvement The main object of the present invention is to provide a process for the preparation of silica coatings on metals which obviates the drawbacks as detailed above Another object of the present invention is to provide a process for the preparation of corrosion resistant silica coatings on metai substrates such as aluminium Yet another object of the present invention is to provide uniform silica coatings on metal substrates such as aluminium Still another object of the present invention is to provide a very thin coatings of silica. Still yet another object of the present invention is to reduce the time of preparation of silica coatings. A further object of the present invention is to reduce the rate of hydrolysis Still yet another object of the present invention is to control the viscosity of the coating solution In the present invention, there ss provided a process for the preparation of ceramic..Coatings..by passing steam over tetra ethyl ortho silicate (TEOS) wherein TEOS in alcohol medium is heated to obtain concentrated TEOS. TEOS so obtained is subjected to steam to hydrolyse to silicon tetrahydroxide simultaneously diluting the concentrated solution to a viscosity level suitable for coating. Metal substrates such as aluminium polished and pre-treated, with pickling solutions was dipped into the above coating solution The substrate was then dried in the temperature range of 60-110°C followed by sintering / heat treating in the temperature range of 350-450°C for 1-4hrs to obtain uniform coatings of silica. The process of the present invention such as control of viscosity, uniformity of the final coatings removes the major drawbacks of the prior art / existing process in which TEOS is also used as coating precursor. Accordingly, the present invention provides a process for the preparation of corrosion resistant silica coatings, Which comprises heating tetra ethyl ortho silicate (TBOS) solution in alcohol at a temperature in the range of 60-110°C to obtain near alcbhol free concentrated TETOS, passing steam ove the Accordingly, the present invention provides a process for the preparation of corrosion resistant silica coatings on metal which comprises heating tetra ethyl ortho silicate (TEOS) solution in alcohol at a temperature in the range of 60-110°C to obtain near alcohol free concentrated TEOS, passing steam over the concentrated TEOS so obtained till TEOS turns whitish and viscosity reaches coating consistency, dipping a pre-treated substrate to be coated in the said TEOS coating splution, drying the coated substrate such as herein described followed by sintering. In an embodiment of the present invention the TEOS solution in alcohol used is of any concentration In an another embodiment of the present invention, the metal substrates used are such as metals in the form of sheets, blocks or any other complicated shape. In yet another embodiment of the present invention, the substrate to be coated is selected from metals such as aluminium, stainless steel, aluminium alloys. In still another embodiment of the present invention, the coating applied is such as silica, alumina, zirconia, chromia and other oxides. In still yet another embodiment of the present invention, the precursor used is alkoxides of the metals whose oxides is to be coated. In a further embodiment of the present invention, the TEOS is concentrated till nearly free of alcohol resulting in increase of viscosity of the coating solution. In a still further embodiment of the present invention, the controlled till nearly free of alcohol resulting in increase of viscosity of the coating solution In a still further embodiment of the present invention, the controlled coating solution with minimum time was achieved by concentrating the TEOS and by reaction of steam In another embodiment of the present invention, the water used for preparation of steam is de-ionised (D.I) water In yet another embodiment of the present invention, the coating thickness is in the range of 2-30 microns obtained by varying the ratio of steam to TEOS solution in the range of (1:10-1:5). In still another embodiment of the present invention, the substrate to be coated is selected from any material capable of bonding with silica. In still yet another embodiment of the present invention, the pre-treatment of the metal substrate such as aluminium is effected by conventional methods of degreasing, polishing and subjecting to a conventional pickling solution In a further embodiment of the present invention the drying of the coated substrate is carried out in the presure of air at a temperature in the range of 60-110°C for a period of 6-12 hours In a still further embodiment of the present invention, the dried-coated substrate is sintered at a temperature range of 350-450°C for a period of 1-4 hours The details of the process steps of the present invention are 1. Tetra Ethyl Ortho Silicate (TEOS) solution in alcohol (40% by vol.) was used as the starting precursor for silica coating and A! metal was used as the coating substrate. 2. TEOS was heated gently to remove alcohol to concentrate TEOS and to increase the viscosity. Steam was passed over this concentrated TEOS solution for quick hydrolysis and the viscosity of this solution was adjusted by the amount of steam. 3. The surface of the metal substrate was de-greased, polished and chemically treated with pickling solution to expose a fresh uniform surface for coating. This substrate was dipped into the coating solution prepared earlier The coating was dried out in the pressure of air at 60-110°C for 6-12 hrs and then heat treated in the temperature range of 350-450°C for 14 hrs to obtain silica coatings The novelty of the present invention lies mainly (1) to obtain uniform coating (2) to reduce the time of the coating solution preparation and its coating (3) to obtain dense, fine textured i.e. low particle sized coating (4) to control the coating composition viscosity to obtain coating consistency The novel features mentioned above have been made possible by the non-obvious inventive steps of (i) concentrating TEOS to near alcohol free (li) passing steam, having high surface area and high enthalpy The following examples are given by way of illustrations and therefore should not be construed to limit the scope of the present invention Example-1 Circular Al samples of diameter 25mm and thickness of 3 mm were used as the coating substrates. Al substrates were well polished by using 100 grit sand paper. Finally, they were polished by using 5µm sized silicon carbide powder to obtain smooth surface. The polished discs were cleaned in an ultrasonic cleaner in water to get rid of loose particles generated during polishing The fresh polished surface was subjected to a standard pickling solution of 10% sodium hydroxide solution for 10-15 minutes. The substrates after pickling were washed thoroughly with distilled water and then dried 75 cc of 40% Tetra Ethyl Ortho Silicate (TEOS) solution in alcohol medium was taken in a beaker and was warmed gently to evaporate the alcohoi to a final volume of 45 cc. The viscosity of this concentrated TEOS was found very high. The steam was prepared by boiling distilled water in a hard glass test tube attached with a U type bend glass pipe whose opening end was placed either inside the concentrated TEOS solution or just above the TEOS sol. As the steam was passed into the TEOS solution, the hydrolysis and condensation of TEOS takes place resulting in whitish precipitate look. The solution was stirred during steaming to obtain a homogeneous coating precursor. The steam was continued till the viscosity was optimum for coating. This solution was used for coating The pre-treated Al discs were dipped vertically in to the coating solution. After few minutes, the sample was withdrawn uniformly at a constant rate of 1 cm/sec and was kept vertically over-night for air drying. The coating was then dried in an oven at about 60°C for 12 hours. The sample was then heat treated in a furnace at a maximum temperature of 400°C with a soaking period of 2 hours and the furnace was cooled slowly to room temperature. The sample was removed from the furnace and the thickness of the coating was measured to be 2 µm. The corrosion resistance was measured by potentio-dynamic polarisation technique using 3.5% NaCI and 5% H2SO4 solutions. The observed corrosion rate of silica coatings in sodium chloride was 0.473 (mils per year, mpy) compared to 6.02 mpy of bare Al metal, similarly, the corrosion rate in sulphuric acid medium was 4.73 mpy compared to 15.05 mpy of bare A! metal Therefore, the improvements in corrosion resistance was 92% and 70% for 3.5% NaCI and 5% H2SO4 solution respectively, Example-2 Circular Al samples of diameter 25mm and thickness of 3 mm were used as the coating substrates. Al substrates were well polished by using 100 grit sand paper. Finally, they were polished by using 5µm sized silicon carbide powder to obtain smooth surface. The polished discs were cleaned in an ultrasonic cleaner in water to get rid of loose particles generated during polishing The fresh polished surface was subjected to a standard pickling solution of 10% sodium hydroxide solution for 10-15 minutes. The substrates after pickling treatment were washed thoroughly with distilled water and then dried 75 cc of 40% Tetra Ethyl Ortho Silicate (TEOS) solution in alcohol medium was taken in a beaker and was warmed gently to evaporate the alcohol to a final volume of 45 cc. The viscosity of this concentrated TEOS was found very high The steam was prepared by boiling distilled water in a hard glass test tube attached with a U type bend glass pipe whose opening end was placed either inside the concentrated TEOS solution or just above the TEOS sol. As the steam was passed into the TEOS solution, the hydrolysis and condensation of TEOS takes place resulting in whitish precipitate look. The solution was stirred during steaming to obtain a homogeneous coating precursor. The steam was continued till the viscosity was optimum for coating. This solution was used for coating The pre-treated Al discs were dip-coated vertically in to the coating solution. After few minutes, the sample was withdrawn uniformly at a constant rate of 1 cm/sec and was kept vertically over-night for air drying. The coating was then dried in an oven at about 60°C for 12 hours. The sample was then heat treated in a furnace at a maximum temperature of 450°C with a soaking period of 2 hours and the furnace was cooled slowly to room temperature The sample was removed from the furnace and the thickness of the coating was measured to be 10 µm The corrosion resistance was measured by potentio-dynamic polarisation technique using 3.5% NaCi and 5% HoS04 solutions The observed corrosion rate of silica coatings in sodium chloride was 0.043 (mils per year, mpy) compared to 6.02 mpy of bare Al metal, similarly the corrosion rate in sulphuric acid medium was 2.752 mpy compared to 15.05 mpy of bare Al metal. Therefore, the improvements in corrosion resistance was 99% and 82% for 3.5% NaCI and 5% H2SO4 solution respectively The main advantages of the improved process of the present invention are: 1 Uniform silica coating was obtained on Al substrate. 2. The viscosity of the coating solution was controlled effectively 3. The loss due to sintering was minimised. 4. The time of the coating preparation was reduced considerably approximately 70-90% less time compared to normal sol-gel process 5. The process is economicai compared to earlier methods 6. The coating morphology in the form of particie size and denseness was improved 7. Minimum infrastructure required We claim : 1. A process for the preparation of corrosion resistant silica coatings on metal which comprises heating tetra ethyl ortho silicate (TEOS) solution in alcohol at a temperature in the range of 60-110°C to obtain near alcohol free concentrated TEOS, passing steam over the concentrated TEOS so obtained in the ratio of steam to TEOS solution in the range of 1:10-1:5 till TEOS turns whitish and viscosity reaches coating consistency, dipping a pre-treated substrate as herein described to be coated in the said TEOS coating solution, drying the coated substrate at a temperature in the range of 60 to 110°C for a period of 6 to 12 hours followed by sintering at a temperature range of 350 to 450°C for a period of 1 to 4 hours 2. A process as claimed in claim 1 wherein, the TEOS solution in alcohol used is of 40%w/v. 3. A process as claimed in claims 1-2 wherein the substrates used are metals in the form of sheets, blocks or any other complicated shape. 4. A process as claimed in claims 1-3 wherein the substrate to be coated is selected from aluminium, stainless steel, aluminium alloys. 5. A process as cklaimed in claims 1-4 wherein the coating applied is silica, alumina, zirconia, chromia and other oxides. 6. A process as claimed in claims 1-5 wherein the TEOS is concentrated till nearly free of alcohol resulting in increase of viscosity of the coating solution.

Documents:

455-del-2003-abstract.pdf

455-del-2003-claims.pdf

455-del-2003-complete specification (granted).pdf

455-del-2003-correspondence-others.pdf

455-del-2003-correspondence-po.pdf

455-del-2003-description (complete).pdf

455-del-2003-form-1.pdf

455-del-2003-form-19.pdf

455-del-2003-form-2.pdf

455-del-2003-form-3.pdf