WO1999032234A1

WO1999032234A1 - Process for sealing coatings

Info

Publication number: WO1999032234A1
Application number: PCT/US1998/026903
Authority: WO
Inventors: Andrew Kelsey Birchenall; Paul Douglas Brothers; Jeffrey A. Hrivnak; Richard James Merigold; Richard Alan Morgan
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1997-12-22
Filing date: 1998-12-18
Publication date: 1999-07-01
Also published as: EP1042078A1

Abstract

Disclosed is a process of coating a substrate comprising the steps of: applying to said substrate a porous coating; and then applying a selected fluoropolymer solution, resulting in a durable non-porous coated substrate.

Description

TITLE PROCESS FOR SEALING COATINGS FIELD OF THE INVENTION The present invention relates to a process and material for sealing the pores of a coating, which material may be applied by spraying, dipping or brushing. The material can also be used to coat the first coating, as well as provide for attachment of a fluoropolymer coating. One material useful in the process is .an amorphous fluoropolymer solution.

TECHNICAL BACKGROUND Thermal spray deposition of one material on the surface of another is well- known to those skilled in the .art. One common technique, for example, is known as the high velocity oxy-fuel (HVOF) technique. See generally R. Irving, et al., Welding Journal 72(7), 1993, pp. 25-30. Such thermal spray techniques can be used to deposit corrosion-resistant materials on corrosion-susceptible substrates in a low-cost approach to corrosion control. Generally, HVOF guns consist essentially of an internal combustion chamber into which a gaseous or liquid fuel •and oxygen .are injected at high pressures (0.5-3.5 Mpa, 80-500 lb/in²) and high flow rates, up to 0.016 m³/s (~2000 surface ft³/h). These products .are then ignited and continuously combusted, the resulting fl.ame being allowed to expand supersonically and exit to atmosphere through a long nozzle simile to that of a rocket motor, as indicated by the presence of characteristic "shock diamonds" in the flame. Powder is injected into, or downstre.am of, the combustion chamber and particles are heated and accelerated by the flame. The high "back pressures" generated by most HVOF systems necessitates the use of pressurized powder feeders in order to convey the powder into the hot gas jets. The accelerated particles exit the gun and impinge onto the substrate, eventually forming a coating on the substrate.

This approach suffers from the problem that the sprayed layer has inherent porosity, with interconnected pathways along the boundaries of the sprayed material deposited as semi-molten "splats" which impinge first the substrate and then on previous layers of "splats". A "splat" is herein defined as a metal droplet which spreads upon impact with the substrate. Reactive or corrosive liquids or vapors, such as would be encountered in chemical processing vessels, valves and the like, can penetrate through these connected pores and reach the substrate/coating interface, causing corrosion of the substrate, blistering at the substrate/coating interface, and spalling of the thermally sprayed layer. A better way to enhance the corrosion resistance of such structures is needed. Copolymers of tetrafluoroethylene and perfluoro(ethyl vinyl ether) disclosed heretofore have been crystalline copolymers. See, for example, U.S. Pat. Nos. 3,635,926 .and 5,461,129. Amorphous copolymers of tetrafluoroethylene and perfluoro(ethyl vinyl ether) .are disclosed in U.S. Pat. Nos. 5,478,905, 5,637,663 and 5,663,255, and in commonly owned, co-pending application Serial No. 08/929,213, which are incorporated herein by reference. It is known in the .art to coat porous metal coatings with fluoropolymer dispersions. See, for example, U.S. Pat. No. 4,500,405. German Patent Application DE 19530194 A 1 also discloses the use of fluoropolymer dispersions to coat kitchen appliances. Spanish Patent Application ES 8605591 A similarly discloses the use of fluoropolymer dispersions.

U.S. Pat. No. 4,684,677 discloses the use of a thermosetting solvent solution primer comprising, in part, a fluorocarbon polymer, to coat a substrate, but there is no mention of its use to seal pores. U.S. Pat. No. 5,178,916 discloses the use of a low molecular weight fluorocarbon solution to seal the porosity in a gold plated article, but this plating layer is not applied by thermal spray. Additionally, the molecular weight of the fluoropolymer used in the solution of the present invention is in the range of 200,000 to 400,000, much higher than disclosed in the '916 patent. These materials (sold as Zonyl's by DuPont Co., Wilmington, DE) generally have carbon chain lengths of about 14 and therefore molecular weights in the hundreds. U.S. Pat. No. 5,122,441 discloses an apparatus and method for fabricating objects which have release agents included therein, including fluoropolymers. However, no mention is made of corrosion protection or thermal-spray-applied coatings.

U.S. Pat. No. 5,238,471 discloses the use of a spray-applied fluoropolymer film onto a porous membrane, but no solid substrate or thermal-spray coating is disclosed.

U.S. Pat. No. 5,660,934 discloses a high temperature thermal sprayable material which adheres to the surface of a thermal sprayable plastic particle to form a cladding layer thereon. The plastic particle may be comprised of a fluoropolymer.

JP H9-300361 A discloses, in relation to golf ball production technology, the coating of a substrate with a first metal coated by electroplating and than joining a second, non-metal coating, to that first coating. It is not clear from this publication whether there is any porosity in the first coating. SUMMARY OF THE INVENTION The present invention concerns a process for coating a substrate comprising the following steps: a) applying to said substrate a coating that contains pores; and b) applying a fluoropolymer solution to said coating to seal said pores.

The invention further concerns .an improvement in a process for the coating of a substrate, wherein a first coating having porous regions has been applied to the substrate to form a first coated substrate, the improvement comprising applying to said first coated substrate a second coating comprising a fluoropolymer solution, such that the second coating seals up porosity of the first coating, resulting in a substantially non-porous second coated substrate. Where the fluoropolymer coating solution is applied to a metal coating, for example, the porosity is decreased from about 0.25% to less than about 0.03%, in the areas penetrated by the solution. The invention also concerns coated articles, comprising substrates coated with the sealed, non-porous coatings made as described above with materials as described below.

The coatings described herein are useful for coating process vessels and electronic parts, where non-porous surfaces are desirable. The invention further comprises a process of coating a substrate comprising the steps of: a) applying to said substrate a coating that may contain pores, said coating comprising a polymeric material in a range of about 5 weight % to about 30 weight %; and b) effecting removal of said polymeric material, thereby forming voids in said coating. In a preferred embodiment said polymeric material is a fluoropolymer. The fluoropolymer may be dissolved in a solvent which flows through the coating to fill voids at the coating-substrate interface. The porous coating may additionally be comprised of a second coating of fluoropolymer solution which is in contact with said fluoropolymer in said porous coating.

The process above may be further comprised of the steps of coating said porous coating with a fluoropolymer dispersion, and sintering said fluoropolymer to effect joining of the resulting fluoropolymer layer to said fluoropolymer in said porous coating. Various materials may be used as coating herein and are described below.

One sort of material usable as a preferred coating in the present invention is an amorphous fluoropolymer, comprising copolymerized units of tetrafluoroethylene (TFE) and perfluoro(ethyl vinyl ether) (PEVE). One embodiment of this amorphous fluoropolymer includes units of one or more additional fluorinated monomers. Preferred additional monomers include perfluoro(methyl vinyl ether) (PMVE). When perfluoro(methyl vinyl ether) is present in the fluoropolymer, perfluoro(ethyl vinyl ether) is at least 15% of the combined weight of the combined perfluoro(ethyl vinyl ether) and perfluoro(methyl vinyl ether).

Further coatings suitable for use in the present process include amorphous fluoropolymers containing units of functional fluorinated comonomers or nonfunctional comonomers.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a substrate coated with a thermal sprayed inner coating having pores and adhered to a fluoropolymer solution sealant.

Figure 2 shows a substrate coated with an adherent, sintered fluorochemical coating on a thermal spray composite containing a fluorochemical. Figure 3 shows a substrate coated with an adherent solution-deposited soluble fluorochemical coating on a thermal spray composite containing soluble fluorochemical.

Figure 4(a) shows a substrate coated with a void-filled coating made by heating thermally sprayed polymer containing a composite. Figure 4(b) shows the s-ame material after heating to 500°C. Figure 5(a) shows a substrate coated with a void-filled coating.

Figure 5(b) shows the same material after exposure to a solvent.

Figure 6(a) shows an electron micrograph of a cross-section of a thermal spray coating (Inconel®) having a pore, which has a soluble fluoropolymer coating applied. Figure 6(b) shows an x-ray image of the same cross-section, identifying the nickel in the Inconel®. Figure 6(c) shows the same cross-section, but identifies the fluoropolymer on the surface and in the pore.

DETAILS OF THE INVENTION As noted above, thermal spray technology is used throughout the industry to apply protective coatings, e.g., relatively low temperature aqueous corrosion protection of corrosion susceptible substrates. The technology is particularly attractive because it can be used to rework existing equipment that may experience corrosion and/or wear problems. However, the results have often been less than optimum, because corrosion still takes place and the thermal spray coatings tend to blister. Sealing the inherent porosity of a thermal spray coating is an important step in the production of a composite coating. By composite coating herein is meant a coating comprising more than one layer or more th.an one material. One method of sealing the pores is to use an organic fluid which flows into the pores. Numerous materials such as epoxies and furan have been used for this purpose, as well as poly (tetrafluoroethylene) (PTFE) and related fluoropolymer dispersions. Dispersions of these fluoropolymers, which are comprised of fine particles, are generally very useful at operating conditions which allow the particles to sinter or after a particle sintering heat treatment. The use of solutions of fluoropolymers with molecular weights in the range of 200,000 to 400,000 are an improvement in that the heat treatment requirement is avoided . These fluoropolymers are known to have excellent chemical resistance. These solutions generally have relatively low viscosities (on the order of about 60 to 300 centipoise at shear rates from about 50 to 300 sec^-1) which enables them to flow into the pores. This effect is shown in Figure 6. The location of the fluoropolymer in the pores is also important because, unlike surface films, the material is not easily abraded or worn away. Rather, the fluoropolymer is protected from abrasion by the surrounding porous coating. A variety of amorphous fluoropolymers may be used as the sealant of this invention. This includes fluoropolymers and co-polymers which are soluble at 0.5% by weight or greater in a solvent, generally a perfluorinated one such as perfluorooctane (3M, Minneapolis, MN). An amorphous polymer is one which does not contain crystallinity when me-asured by DSC, or whose heat of melting is less than 2 J/g.

The fluoropolymers include but are not limited to copolymers of TFE with functional or non-functional monomers such as fluoroolefins having 2-8 carbon atoms and fluorinated alkyl vinyl ether in which the alkyl group contains 1 or 3 to 5 carbon atoms. Examples of the non-functional monomers include hexafluoro- propylene (HFP), chlorotrifluoro ethylene (CTFE), PEVE, PMVE .and perfluoro- (propylene vinyl ether) (PPVE). Functional monomers include perfluoroethyl vinyl ether (EVE), CF₂CFOCF2CFCF3OCF₂CF₂COOCH3 (EVE-carbamate), CF₂CFOCF₂CFCF3θCF₂CF₂Sθ2F (PSEPVE), CF₂CFOCF₂CFCF₃OCF₂CF₂CN (8CNVE), N₃(CF₂CFOCF₂CFCF₃OCF₂CF₂)3 (EVE-triazine), CF₂CFOCF₂CFCF₃OCF₂CF₂CN (EVE-CN), CF₂CFOCF₂CFCF₃OCF₂CF₂ CH₂OH (EVE-OH), CF₂CFOCF₂CFCF₃OCF₂CF₂CH₂PO₂(OH)₂ (EVE-P) and CF₂CFOCF₂CFCF₃OCF₂CF₂CH₂COOH (EVE-COOH). Commercially available materials include those from DuPont, Wilmington, DE: Teflon® SF60 (TFE/PMVE/PEVE, DuPont, Wilmington DE), Teflon® SF61 (TFE/PMVE/PEVE/EVE-P), Teflon® SF50 (TFE/HFP), Teflon® AF 1600

(PDD/TFE), and Teflon® AF2130 (PDD/CTFE); and those from Asahi Glass, Japan: Cytop®. The fluoropolymer solutions may be applied to the porous coating and articles comprising these coatings by common coating methods, including but not limited to spray application, dipping and brushing.

Any structural material can be used as a substrate in this invention, such as metals, ceramics and composites Such metals include carbon steel, stainless steel, aluminum, copper .and the like and ceramics include alumina and silica. Preferred substrates include carbon steel, aluminum and copper, as these metals generally are the least expensive. However, most of these are corrosion-prone.

A variety of materials can be applied to substrates via thermal spray deposition to form a porous coating. These materials include metals such as Alloy 625 (Inconel® 625, Inco, Inc. Huntingdon, WV), Alloy C (Hastelloy® C, Haynes, Inc., Kokomo, IN), .and type 316 stainless steel; ceramics such as tungsten carbide .and chrome oxide; polymers such as Teflon® and Tefzel^® (both from DuPont Co., Wilmington, DE); and composites such as tungsten carbide in a cobalt matrix.

Additionally, various metals may be coated onto a substrate via electroplating. One of these metals is gold, which is often used for electronic components.

The distribution of pores in the coating which must be sealed is an important par-ameter. Pore distributions are generally a function of the thermal spray material and thermal spray process parameters. One method of measuring the pore distribution is to use mercury porosimetry. This technique measures pore distribution as the pressure is increased on a reservoir of mercury. At higher and higher pressure, the mercury is forced into smaller and smaller pores, indicating the pore distribution. Once the initial porosity is measured, the fluoropolymer solution is applied, and the porosity remeasured, which indicates how well the pores have been sealed by the coating.

Another important consideration in selecting the method of sealant application is to determine the penetration depth of the sealant into the pores below the surface. This is important because the surface film itself may be subject to corrosion or wear and the sealing action provided by the film on the surface may not prove to be durable in some applications. One way to measure the penetration depth is to use x-ray fluorescense .analysis (this is done with a scanning electron microscope (SEM) with a microprobe on metallographic cross sections). This is shown in Figure 6, that is further described below. Penetration depth is dependent on parameters such as pore distribution, sealant application technique and fluoropolymer molecular weight and solution concentration. It is generally preferred that the porosity of the porous metal coating be decreased to less than about 0.025% of the original amount after coating with the fluoropolymer solution.

One benefit of this invention is the relative increase in corrosion resistance seen in sealed articles as compared to those articles which are not sealed. As shown in the examples below, the increase in corrosion-resistant lifetime is at least by a factor of 8. This becomes significant when the .articles treated by the process are process vessels, pipes, valves and the like, as are often found in chemical industry plants. When these process .articles corrode, they usually must be replaced, involving plant downtime and thus lost revenues, as well as causing potential safety considerations. This invention allows already existing plant components to be coated and sealed in situ, thus obviating the problems mentioned above.

Substrates which have been prepared by thermally spraying with a composite layer generated by the present invention, in which the final coating is a non-stick polymer resin, may be used in bakeware and top of the stove cookware. Other uses of such coated and sealed substrates include saws, hot plates, show molds, snow shovels, plows, ship bottoms, dies and tools.

In some applications, such as abradable coatings for seals in turbine engines and fiber handling rolls, where finish is being added, it may be desirable to have voids in the coating. A polymer-filled composite coating is first deposited by thermal spray. The coating is then heated to a relatively high temperature (for example about 500°C) to vaporize the polymer constituent of the composite. This leaves the high melting point constituent on the substrate with voids where the polymer was previously, before vaporization. Alternatively, a polymer soluble in selected solvents (e.g., alkanes and perfluoralkanes) may be included in the composite, and thermally sprayed. The soluble polymer is then dissolved, producing voids. The voids produced by either method which are on the surface of the coated article can hold additional finish, for example, during processing steps. Figures 1 through 5 show various configurations for the layers of the coating. Figure 1 shows a thermal sprayed inner coating 7 adhered to an outer coating of fluoropolymer solution 8, e.g. Teflon®, E.I. du Pont de Nemours .and Company, Wilmington, Delaware.

Figure 2 is a dual layer coating. The inner layer can be produced by electroplating or by thermal spray technology. The outer coating is produced by sintering a dispersion of fluorochemical particles. It is preferred to use thermal spray, rather than electroplating, for producing the inner layer as thermal spray can deliver non-conducting ceramic type materials as a high melting point constituent of the composite, while electroplating can only deliver metals. In Fig. 2, outer fluorochemical sintered coating 1 is joined to surface particles if thermal spray high melting point material 2 and a thermal spray fluorochemical consituent 3. The substrate can be, for example, carbon steel. In Fig. 3, thermal spray technology has deposited a soluble inner coating layer 4, e.g. a soluble fluorochemical, along with a high melting point constituent 5. The outer layer 6 is deposited from a solution of soluble polymer. Outer layer 6 joins to surface particles 4 and 5 of the inner layer.

Fig. 4(a) shows a substrate coated with a mixture of thermal sprayed fluoropolymer 9 and high melting point constituent 10, which is then heated to 500 degrees C for about 30 minutes. Upon heating voids 11 form. These voids may then serve to hold finish on fiber finish rolls, for example.

Figure 5(a) shows a substrate coated with a mixture of thermal sprayed fluoropolymer 12 and high melting point constituent 13. Upon exposure to a solvent voids are created by removal of soluble polymer particles as shown in Figure 5(b). Useful solvents include .any perfluorinated solvent, such as perfluorooctane (PF-5080, 3M Co., Minneapolis, MN). Penetration of the solution is determined by the viscosity of the solution .and the size and shape of pore distribution. The solvent can form a seal deep within the coating. A seal that remains deep within the composite, is protected from erosion or corrosion. Figures 6(a), 6(b) and 6(c) show three images of a large pore at the external surface of an Inconel® 625 thermal spray coating produced by the HVOF technique which was subsequently coated with a soluble fluoropolymer. The images were obtained using an electron microprobe with two types of spectrophotometric detectors, which both discriminate x-ray information, from Cameca Instruments, Inc., Trumbull, CT (model SU30). The instrument operated at 15 KV with a beam current of 4nA. The first detector was a wavelength dispersive spectometer analyzing crystal, PCI, 2d spacing of 61.20 nm, which was peaked on the fluorine Ka. The second detector was an energy dispersive spectrometer window peaked on 7477 eV, the Ka line of nickel. The images and maps were acquired with software from Princeton Gamma-Tech, Princeton, NJ. This was accomplished by selecting a Ni Ka region of interest from the EDS spectra and also an external source referencing the first detector. The software scans the electron beam across the area to be analyzed. The beam dwell time is 0.05 seconds per point at a resolution of 256 points full width. This horizontal scan is repeated with tan appropriate offset to generate an image iwth a vertical resolution of 200 points. The maps pictured in Figures 6(a), 6(b) and 6(c) are 256x200 points. Figure 6(a) shows an image of the surface derived from the energy dispersive scan. Figure 6(b) shows the s.ame surface, showing only the collection of nickel x-rays by the microprobe. Nickel is the major constituent of the alloy, and thus this image shows the location of the metal in the coating. Figure 6(c) shows the same surface, but this time a collection of fluorine x-rays is shown, obtained with the wavelength dispersive spectrometer. A deposit of fluoropolymer of about 7 μm thick is shown on the top, external surface of the coating, as shown by the lighter areas. Additionally, it can be seen by the lighter areas that the fluoropolymer has penetrated into the large pore to a depth of about 70 μm below the surface.

Definitions The following materials were used in the Examples below: TFE tetr.afluoroethylene PMVE perfluoro(methyl vinyl ether) PEVE perfluoro(ethylene vinyl ether)

EVE-P perfluoro(ethylene vinyl ether) phosphate HVOF high velocity oxy fuel

EXAMPLES Copolvmer Preparation The copolymers used in the ex.ample below were prep.ared as disclosed in commonly-assigned, copending application, Ser. No. 08/929,213. The TFE/PMVE/PEVE copolymer was prepared as follows. In a horizontal 1-gal (3.8-L) autoclave equipped with a paddle agitator, 2200 mL of demineralized water were deaerated by evacuation and purging with nitrogen. With the reactor at atmospheric pressure, 5 g of .ammonium perfluoronon-anoate (C-9) were added. The agitator was turned on at 100 rpm, the temperature was increased to 90°C, and the pressure was increased to 400 psig (2.86 MPa) by addition of a mixture of 27.2 wt% TFE, 51 wt% perfluoro(methyl vinyl ether) (PMVE), and 21.8 wt% PEVE. An initial charge of 30 mL of a 1.5 g/L solution of ammonium persulfate (APS) in water was added. At kickoff, as determined by a 10 psi (0.07 MPa) pressure drop, the same initiator solution was fed at the rate of 2 mL/min and a monomer mixture having the composition TFE/PMVE/PEVE = 62/23/15 by weight was fed to maintain pressure at 400 psig. After about 600 g of monomer had been added after kickoff, all feeds were stopped. When the pressure dropped to 250 psig (1.83 MPa), the reactor was vented and the product dispersion was collected. Solids content of the dispersion was 22.3 wt%. The polymer was isolated by vigorously mixing the dispersion with approximately 40 mL of 70% nitric acid solution, and filtering with a filter cloth. The wet resin was rinsed with demineralized water three times with vigorous agitation, and dried at 80°C under vacuum. The composition of the product resin was TFE/PMVE/PEVE = 56.3/29.9/13.8 by weight as determined by ¹⁹F NMR. No crystalline melting point was detected by DSC. The weight average molecular weight as measured by size exclusion chromatography (SEC) with universal calibration using on-line viscometry, using a Zorbax® silica column at 145°C, Cj4F₂₄ as solvent, a flow rate of 0.2 mL/min and an evaporative light scattering detector, was 279,000.

The TFE/PMVE/PEVE/EVE-P copolymer was prepared as the TFE/PMVE/PEVE copolymer described above, except that 15 mL of EVE-P were ch-arged along with the C-9, the APS initiator solution concentration was 6.0 g/L, and the initiator solution pumping rate after kickoff was 3.25 mL/min. Solids content of the raw dispersion was 24.3 wt%. The product resin contained 54.0 wt% of TFE, 30.4 wt% of PMVE, and 15.6 wt% of PEVE and EVE-P combined, as determined by ¹⁹F NMR. The amount of EVE-P in the copolymer was less than 1 wt%. No crystalline melting point was detected by DSC. The weight average molecular weight was 205,000. HVOF Application Parameters

In the example below, the HVOF thermal spray process had the following parameters. The alloy coating was deposited using a Jet Kote® IIA sprayer (Stellite Coatings, Inc.).

Fuel Hydrogen Nozzle 1/4" 0 x 9" L G-as Injector #40 Fuel Flow 1100 scfh (120 psi) Oxygen Flow 450 scfh (120 psi) Powder feeder Plasmadyne Powder Inconel® 625, -270 mesh, (Stellite, Nistelle 625)

Powder Carrier Gas Argon CCaarrririeerr GGaass FFllooww 40 cfh (80 psi) Powder Feed Rate 2 rpm Spray Distance 11" Part Speed 17% rpm @ 5% dX/dt, at a radius of 4" Spray Time 46 passes CCoooolliinngg None Substrate 3" x 3" Carbon Steel Substrate Thickness 0.195" Surface Preparation Grit Blast with #12 Al₂O₃, Ultrasonic

Degrease in Alcohol Coating Thickness 0.0114" - 0.0122"

Corrosion Testing Corrosion tests were run using a 4-neck cylindrical cell ("atlas cell") generally as described in ASTM C-868 with 3-in (7.6 cm) square test panels, and with the cell half full of 1% sulfuric acid. The test temperature was 60°C. The test panels were coated plates, with Inconel Alloy 625, a corrosion resistant alloy, applied by the HVOF procedure outlined above onto a carbon steel substrate. Panels were tested with the alloy side toward the acid solution. Both unsealed panels and panels sealed with PEVE copolymers were exposed. Amorphous PEVE copolymers used were a TFE/PMVE/PEVE copolymer and a TFE/PMVE/PEVE/EVE-P copolymer, generally prepared by the procedure described in the copending application described above. Coatings were applied from 3 wt % solutions in perfluorooctane (PF-5080, 3M Co., Minneapolis, MN). Panels were cleaned, first with acetone, then with CF-113. The clean panels were dipped twice in the copolymer solutions, drying at 150°C after each dip. The unsealed panels failed by blistering at the liquid/vapor interface after 3 weeks of exposure. The panels coated with both PEVE copolymers survived 24 weeks of exposure before blistering, showing that the coating of the present invention effectively sealed the pores and crevices of the thermally-sprayed coating, thereby inhibiting transport of reactive or corrosive agents through the pores to the substrate/coating interface and thus inhibiting corrosion and consequent blistering.

Claims

CLAIMS What is claimed is:

1. A process of coating a substrate comprising the steps of: a) applying to said substrate a first coating that contains pores; and b) applying a fluoropolymer solution second coating to said first coating to seal said pores.

2. In a process for the coating of a substrate, wherein a first coating having porous regions is applied to the substrate to form a first coated substrate, the improvement comprising applying to said first coated substrate a second coating comprising a fluoropolymer solution, such that the second coating seals up porosity of the first coating, resulting in a substantially non-porous second coated substrate.

3. The process as recited in Claim 1 or 2, wherein said substrate is selected from the group consisting of metal, ceramic and composite.

4. The process as recited in Claim 3 wherein the substrate is a metal selected from the group consisting of carbon steel, stainless steel, aluminum and copper.

5. The process of Claim 3 wherein the substrate is a ceramic selected from the group consisting of alumina and silica.

6. The process of Claim 1 or 2 wherein said porous coating is comprised of at least one member of the group consisting of metal, ceramic, polymer and composite.

7. The process as recited in Claim 1 or 2 wherein said first porous coating is applied to the substrate by thermal spray deposition.

8. The process as recited in Claim 1 or 2 wherein said first porous coating is applied to the substrate by electroplating.

9. The process as recited in Claim 1 or 2 wherein said fluoropolymer solution comprises a fluoropolymer of molecular weight between about 200,000 and 400,000.

10. The process as recited in Cl-aim 9 wherein said fluoropolymer is a copolymer of tetrafluoroethylene and perfluoro(ethyl vinyl ether).

11. The process as recited in Claim 9 wherein said fluoropolymer is a copolymer of TFE with functional or non-functional comonomers.

12. The process of Claim 11 wherein the copolymer is a fluoroolefins having 2-8 carbon atoms and fluorinated alkyl vinyl ether in which the alkyl group contains 1 or 3 to 5 carbon atoms.

13. The process of Claim 11 wherein the comonomer is a non-functional comonomer selected from the group consisting of hexafluoropropylene (HFP), chlorotrifluoro ethylene (CTFE), PEVE, PMVE .and perfluoro(propylene vinyl ether) (PPVE).

14. The process of Claim 11 wherein the comonomer is a functional comonomer selected from the group consisting of perfluoroethyl vinyl ether (EVE), CF₂CFOCF₂CFCF₃OCF₂CF₂COOCH₃ (EVE-carbamate),

CF₂CFOCF₂CFCF₃OCF₂CF₂SO₂F (PSEPVE), CF₂CFOCF₂CFCF₃OCF₂CF₂CN (8CNVE), N₃(CF₂CFOCF₂CFCF₃OCF₂CF₂)3 (EVE-triazine), CF₂CFOCF₂CFCF₃OCF₂CF₂CN (EVE-CN), CF₂CFOCF₂CFCF₃OCF₂CF₂ CH₂OH (EVE-OH), CF₂CFOCF₂CFCF₃OCF₂CF₂CH₂PO₂(OH)₂ (EVE-P) and CF₂CFOCF₂CFCF₃OCF₂CF₂CH₂COOH (EVE-COOH).

15. The process as recited in Claim 11 wherein the porosity of the porous metal coating is decreased to less than 0.025% after coating with said fluoropolymer solution, in the areas that are penetrated by the solution.

16. The process as recited in Claim 11 wherein the corrosion resistance of said coated substrate is improved by at least a factor of 8 time units.

17. A coated article, comprising a substrate coated with a coating as recited in Claim 1 or Claim 11.

18. The coated article as recited in Claim 17, wherein said substrate comprises metal.

19. The coated article as recited in Claim 17, wherein said substrate is a composite, comprising metal and a porous surface layer, said coating sealing the pores of said porous surface layer.

20. The coated article of Claim 17 wherein the substrate is a process vessel.

21. The coated article of Claim 17 wherein the substrate is an electronic component.

22. A process of coating a substrate comprising the steps of: a) applying to said substrate a coating that contains pores, said coating comprising a polymeric material in a range of about 5 weight % to about 30 weight %; and b) effecting removal of said polymeric material, thereby forming voids in said coating.

23. The process as recited in Claim 22, wherein said polymeric material is a fluoropolymer.

24. The process as recited in Claim 22, wherein removal of said polymeric material is effected by heating the coated substrate to 300┬░C for about 30 minutes.

25. The process as recited in Claim 23, wherein said fluoropolymer is soluble in perfluorinated solvents

26. The process as recited in Claim 25, wherein removal of said soluble fluoropolymer is dissolved in a perfluorinated solvent, and said fluoropolymer- solvent mix flows through said coating to fill the voids at a coating-substrate interface.

27. The process as recited in Claim 6, wherein said porous coating is additionally comprised of a fluoropolymer and said second coating of fluoropolymer solution is in contact with said fluoropolymer in said porous coating.

28. The process as recited in Claim 27, further comprising the steps of coating said porous coating with a fluoropolymer dispersion, and sintering said fluoropolymer to effect joining of the resulting fluoropolymer layer to said fluoropolymer in said porous coating.