BACKGROUND

[0001] The present invention relates to methods of providing a metal workpiece with a corrosion resistant coating, such as a steel sheet with a porcelain enamel coating.

SUMMARY

[0002] The invention provides a method of processing a workpiece to provide a coating along an edge of the workpiece that inhibits corrosion or degradation of the edge. The coating may be a porcelain enamel coating and the workpiece may be a steel sheet.

[0003] The method includes forming a curved surface having a radius along the edge of the workpiece. The curved surface may be formed by passing the edge of the workpiece through a series of rollers roll forming the edge to form the curved surface and define the radius, by passing the edge of the workpiece through a series of grinders removing material along the edge to form the curved surface and define the radius, or by otherwise forming the edge by a series of machining operations to form the curved surface and define the radius. A curved surface may also be created by machining the edge. After forming the curved surface, all debris is removed from the edge of the workpiece with an abrasion process until the edge has a white metal finish or better. The workpiece is washed and cleaned to remove debris, oils, and/or greases, and then a primer coat is applied to the edge of the workpiece. Glass frit may then be applied to the edge of the workpiece to fully encapsulate the edge before firing the workpiece to fuse the glass frit and form a corrosion resistant enamel coating bonded to the edge of the workpiece.

[0004] In one embodiment, the invention provides a method of processing a workpiece. The method includes forming a curved surface along an edge of the workpiece. The curved surface has a radius in a plane perpendicular to a length of the edge. The method also includes applying glass frit to the edge of the workpiece to encapsulate the edge. The method further includes firing the workpiece to fuse the glass frit and form a porcelain enamel coating bonded to the edge of the workpiece.

[0005] In another embodiment the invention provides a workpiece including a panel composed of steel and having a first surface, a second surface opposite the first surface, and an edge defining a thickness between the first surface and the second surface. The edge is a curved surface having a radius in a plane perpendicular to a length of the edge. The workpiece also includes a porcelain enamel coating bonded to the edge.

[0006] Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Fig. 1 is a partial cross-section view of a workpiece.

[0008] Fig. 2 is a partial cross-section view of the workpiece with a roller, a grinder, or a machine forming a curved surface.

[0009] Fig. 3 is a partial cross-section view of the workpiece with a radius formed along an edge.

[0010] Fig. 4 is a partial cross-section view of the workpiece with a primer coating.

[0011] Fig. 5 is a partial cross-section view of the workpiece with spray nozzles applying glass frit.

[0012] Fig. 6 is a partial cross-section view of the workpiece with a porcelain enamel coating.

[0013] Fig. 7 is a schematic of the workpiece with the porcelain enamel coating formed on the edge.

[0014] Fig. 8 is another schematic of the workpiece with the porcelain enamel coating formed on the edge.

[0015] Fig. 9 is a flow chart illustrating a method of processing the workpiece to form the porcelain enamel coating on the edge. DETAILED DESCRIPTION

[0016] Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

[0017] Typically, porcelain enamel coatings are used on steel and other metals in order to prevent corrosion. Often porcelain enamel coated steel is used in the manufacturing of ovens, interior hot water heaters, plumbing ware (e.g., sinks, toilets, bathtubs, or pipes), bolted storage tanks for water, municipal wastewater systems, manure slurry, and animal feed silos. Other applications of porcelain enamel coated metal include cookware (e.g., pots, pans, cooktops, or kitchen appliances), burners, laboratory equipment, and other surfaces (e.g., walls, countertops, tunnels, or escalators).

[0018] However, porcelain enamel coatings of large structural steel components (e.g., sheet, plate, or beam) present unique challenges. Structural steel may be used as construction material, for example, in buildings and storage tanks. In some embodiments, structural steel is a category of steel known in the art.

[0019] Porcelain enamel coatings are substantially vitreous (i.e., glassy), inorganic coatings that are bonded to metal at a fusion temperature. For steel, the fusion temperature is typically above 800 degrees Fahrenheit (°F). Since porcelain enamel is inorganic, it does not contain organic carbon, but rather includes minerals, such as rock and clay. In addition, various porcelain enamel coatings may include various specific materials and chemicals that are formulated to bond to a specific metal substrate. Some of the materials commonly included in porcelain enamel are glass frits, clays, refractories, opacifiers, coloring oxides, and electrolytes. Specific examples of these materials may include silica, quartz, borax, soda ash, fluorspar, sodium silica fluoride, titanium dioxide, and nickel oxide. Depending on the metal substrate material and application, each of these materials is specifically selected to provide optimum adhesion, durability, and corrosion resistance.

[0020] Porcelain enamel coatings start as glass frit that is applied to the surface of the workpiece and is fired in a kiln to fuse together and bond to the workpiece to form a hardened enamel coating on the workpiece. The glass frit is a ceramic composition that is fused and quenched to form a glass, and then granulated. The other materials and chemicals of the porcelain enamel are mixed in with the granulated glass frit.

[0021] While the workpiece coated with the glass frit is being fired, the enamel and metal (e.g., steel) at an interface thereof fuses. Specifically, iron oxide at the surface of the workpiece dissolves into the glass enamel, thereby forming a layer in which the steel and enamel are intertwined. This provides for optimum adhesion of the porcelain enamel to the workpiece. In some embodiments, the enamel coating includes a ground coat bound to the metal substrate and a cover coat bound to the ground coat.

[0022] Porcelain enamel once hardened lacks porosity on the surface. As such, porcelain enamel forms an impervious moisture barrier, inhibiting water molecules and organic solvents from reaching the metal substrate. Additionally, porcelain enamel coatings have increased long term durability and corrosion resistance over steel substrate coating alternatives.

[0023] The porcelain enamel coating goes through a melt phase when heated to a temperature where fusing to the substrate occurs. During this phase, the coating flows and levels itself to form a smooth and even film. At sharp edges or corners, the coating is unable to maintain a consistent thickness resulting in an uneven or no coating at the edges or corners, especially for structural steel components. Accordingly, corrosion is most susceptible at the edges or corners of a workpiece. Corrosion along the edges or corners of a workpiece is aesthetically unpleasant and can greatly affect product life.

[0024] Fig. 1 illustrates a workpiece 10 having an edge 14 defining a thickness Tl between a top surface 18 and a bottom surface 22. The thickness Tl may be between approximately 0.08 inches and approximately 0.75 inches. In other embodiments, the thickness Tl may be less than 0.08 inches or greater than 0.75 inches. In the illustrated embodiment, the workpiece 10 is a sheet or panel, and top and bottom surfaces 18, 22 are planar surfaces. The sheet 10 may be formed from steel or another suitable metal (e.g., aluminum, copper, or cast iron).

[0025] In yet other embodiments, the thickness Tl may be between approximately

0.1 and 1 inches, 0.1 and 0.9 inches, 0.1 and 0.8 inches, 0.1 and 0.7 inches, 0.1 and 0.6 inches, 0.1 and 0.5 inches, 0.1 and 0.4 inches, 0.1 and 0.3 inches, or 0.1 and 0.2 inches. The thickness Tl may be approximately 0.01 inches, 0.02 inches, 0.03 inches, 0.04 inches, 0.05 inches, 0.06 inches, 0.07 inches, 0.08 inches, 0.09 inches, 0.1 inches, 0.15 inches, 0.2 inches, 0.25 inches, 0.3 inches, 0.35 inches, 0.4 inches, 0.45 inches, 0.5 inches, 0.55 inches, 0.6 inches, 0.65 inches, 0.7 inches, 0.75 inches, 0.8 inches, 0.85 inches, 0.9 inches, 0.95 inches, or 1 inch.

[0026] Fig. 3 illustrates the sheet 10 after the edge 14 has been formed to define a curved surface 26 along the height of the edge 14. The curved surface 26 defines a radius R extending between the top and bottom surfaces 18, 22 in a plane perpendicular to the length of the edge 14. The curved surface 26 is formed by passing the sheet 10 through a radius forming operation, illustrated in Fig. 2. The operation may include passing the sheet 10 through a series of rollers 24 that roll form the edge 14 to form the curved surface 26 along the length of the edge 14. Alternatively, the operation may include passing the edge 14 through a series of grinders 24 that remove material from the edge 14 to form the curved surface 26 along the length of the edge 14. Alternately, the operation may include putting the sheet 10 through a series of machines 24 to form the curved surface 26 along the length of the edge 14 by known machining operations.

[0027] Without being limited to theory, the inventors have surprisingly discovered that the dimension of the radius R is dependent on the thickness Tl of the sheet 10 and the Porcelain Enameling Institute radius guidelines in PEI-101 Design & Fabrication of Metal of Porcelain Enamel. For example, if the thickness Tl is less than approximately 0.25 inches, the radius R should be between approximately three-tenths and seven-tenths of the thickness Tl (e.g., the radius R is equal to approximately half of the thickness Tl). The chosen radius may enable the porcelain enamel coating to adhere to and not pull away from a surface of the workpiece, particularly a curved surface or a corner.

[0028] In some embodiments, the radius R may be between approximately one-tenth and nine-tenths, one-fifth and nine-tenths, three-tenths and nine-tenths, three-tenths and four-fifths, three-tenths and seven-tenths, three-tenths and three-fifths, three-tenths and one-half, three-tenths and two-fifths, two-fifths and seven-tenths, one-half and seven- tenths, or three-fifths and seven-tenths of the thickness Tl . In some other embodiments, the radius R may be less than three-tenths of the thickness Tl or greater than seven-tenths of the thickness Tl . The radius R may be approximately 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 70%, 75%, 80%, 85%, 90%, 95%, or 100% of the thickness Tl .

[0029] After the radius forming operation, an abrasion process is performed to remove all debris (e.g., dust, dirt, mill scale, rust, coatings, or oxides) from the edge 14 and surfaces 18, 22, 26 of the sheet 10. Preferably, the resulting finish of the surfaces 18, 22, 26 will be at least a white metal finish (e.g., SSPC-SP 10). In some embodiments, the abrasion process is a blasting process, such as sand blasting, shot blasting, or glass bead blasting. Alternately or additionally, the abrasion process may include a chemical treatment, such as etching, pickling, or passivation.

[0030] After the abrasion process, the sheet 10 is prepped for coating by cleaning and washing the edge 14 and surfaces 18, 22, 26 of all remaining debris, oils, and greases. After cleaning and washing, a primer coating 28 is applied to the edge 14 and surfaces 18, 22, 26 (Fig. 4). In some embodiments, the primer coating 28 is an acrylic coating, an alkyd coating, an epoxy coating, a zinc-rich coating, or a nickel oxide coating.

[0031] After the primer coating 28, glass frit is applied to the surfaces 18, 22, 26 to encapsulate the edge 14. A predetermined amount of glass frit is chosen to achieve a desired post-fire enamel thickness. As shown in Fig. 5, the glass frit is applied to the edge 14 by at least one directional nozzle 29 oriented perpendicular to a length of the edge 14, or by at least one directional nozzle 29 oriented parallel to the length of the edge 14, or both.

[0032] Once the glass frit is applied, the sheet 10 is fired (i.e., heated) in a kiln or oven to a temperature dependent on the glass frit used and the material of the sheet 10. For steel, the temperature is greater than approximately 800 degrees Fahrenheit (°F). The glass frit melts in the oven and flows to form a thin film on the surfaces 18, 22, 26. The radius R of the curved surface 26 of the edge 14 allows for a consistent, uniform, and smooth film of melted frit to form on the curved surface 26 and helps to adhere the coating to the surfaces of the sheet 10. Without being limited to theory, the design of radius R based on the thickness Tl of the sheet 10 and the Porcelain Enameling Institute radius guidelines in PEI-101 Design & Fabrication of Metal of Porcelain Enamel allows for a consistent, uniform, and smooth film of melted frit to form on the curved surface 26. The inventors have surprisingly discovered that the PEI radius guidelines may be applied to the porcelain enameling of structural steel.

[0033] In some embodiments, the firing temperature may be greater than

approximately 800 °F, 900 °F, 1000 °F, 1100 °F, 1200 °F, 1250 °F, 1300 °F, 1350 °F, 1400 °F, 1450 °F, 1500 °F, 1550 °F, 1600 °F, 1650 °F, 1700 °F, 1800 °F, 1900 °F, or 2000 °F. The firing temperature may be less than approximately 2000 °F, 1900 °F, 1800 °F, 1700 °F, 1650 °F, 1600 °F, 1550 °F, 1500 °F, 1450 °F, 1400 °F, 1350 °F, 1300 °F, 1250 °F, 1200 °F, 1100 °F, 1000 °F, 900 °F, or 800 °F. The firing temperature may be approximately 1300 °F, 1350 °F, 1400 °F, 1450 °F, 1500 °F, 1550 °F, or 1600 °F. Some embodiments may include more than one firing temperature. Some embodiments may include one or more cooling temperatures. A cooling temperature may be less than a firing temperature.

[0034] In some embodiments, the porcelain coating bonded to the metal substrate may be held at the firing temperature for greater than approximately 0.25 hours, 0.5 hours, 0.75 hours, 1 hour, 1.25 hours, 1.5 hours, 1.75 hours, 2 hours, 2.25 hours, 2.5 hours, 2.75 hours, 3 hours, 3.25 hours, 3.5 hours, 3.75 hours, 4 hours, 4.25 hours, 4.5 hours, 4.75 hours, 5 hours, 5.25 hours, 5.5 hours, 5.75 hours, or 6 hours. The porcelain coating may be held at the firing temperature for less than approximately 6 hours, 5.75 hours, 5.5 hours, 5.25 hours, 5 hours, 4.75 hours, 4.5 hours, 4.25 hours, 4 hours, 3.75 hours, 3.5 hours, 3.25 hours, 3 hours, 2.75 hours, 2.5 hours, 2.25 hours, 2 hours, 1.75 hours, 1.5 hours, 1.25 hours, 1 hour, 0.75 hours, 0.5 hours, or 0.25 hours. The porcelain coating may be held at the firing temperature for approximately 0.25 hours, 0.5 hours, 0.75 hours, 1 hour, 1.25 hours, 1.5 hours, 1.75 hours, 2 hours, 2.25 hours, 2.5 hours, 2.75 hours, 3 hours, 3.25 hours, 3.5 hours, 3.75 hours, 4 hours, 4.25 hours, 4.5 hours, 4.75 hours, 5 hours, 5.25 hours, 5.5 hours, 5.75 hours, or 6 hours.

[0035] In some embodiments, the porcelain coating bonded to the metal substrate may be cooled down for greater than approximately 0.25 hours, 0.5 hours, 0.75 hours, 1 hour, 1.25 hours, 1.5 hours, 1.75 hours, 2 hours, 2.25 hours, 2.5 hours, 2.75 hours, 3 hours, 3.25 hours, 3.5 hours, 3.75 hours, 4 hours, 4.25 hours, 4.5 hours, 4.75 hours, 5 hours, 5.25 hours, 5.5 hours, 5.75 hours, or 6 hours. The porcelain coating may be cooled down for less than approximately 6 hours, 5.75 hours, 5.5 hours, 5.25 hours, 5 hours, 4.75 hours, 4.5 hours, 4.25 hours, 4 hours, 3.75 hours, 3.5 hours, 3.25 hours, 3 hours, 2.75 hours, 2.5 hours, 2.25 hours, 2 hours, 1.75 hours, 1.5 hours, 1.25 hours, 1 hour, 0.75 hours, 0.5 hours, or 0.25 hours. The porcelain coating may be cooled down for approximately 0.25 hours, 0.5 hours, 0.75 hours, 1 hour, 1.25 hours, 1.5 hours, 1.75 hours, 2 hours, 2.25 hours, 2.5 hours, 2.75 hours, 3 hours, 3.25 hours, 3.5 hours, 3.75 hours, 4 hours, 4.25 hours, 4.5 hours, 4.75 hours, 5 hours, 5.25 hours, 5.5 hours, 5.75 hours, or 6 hours.

[0036] After firing the sheet 10, the thin film is allowed to cool to form a hardened porcelain enamel coating 30 having a thickness T2, as shown in FIGS. 6-8. The thickness T2 of the porcelain enamel coating 30 depends on the amount of glass frit applied. In some embodiments, the thickness T2 is between approximately 3 mils and approximately 7 mils (e.g., approximately 5 mils). In other embodiments, the thickness T2 may be any amount. In addition, depending on the desired thickness T2 of the porcelain enamel coating 30, a different radius R of the curved surface 26 may be formed. Thinner thicknesses can tolerate a tighter radius, while larger thicknesses (e.g., greater than 12 mils) require a larger radius R.

[0037] In some embodiments, the thickness T2 may be between approximately 1 mil and 25 mils, 2 mils and 25 mils, 3 mils and 25 mils, 4 mils and 25 mils, 5 mils and 25 mils, 6 mils and 25 mils, 7 mils and 25 mils, 8 mils and 25 mils, 8 mils and 24 mils, 8 mils and 23 mils, 8 mils and 22 mils, 8 mils and 21 mils, 8 mils and 20 mils, 8 mils and 19 mils, 8 mils and 18 mils, 9 mils and 18 mils, 10 mils and 18 mils, 11 mils and 18 mils, 12 mils and 18 mils, 13 mils and 18 mils, 14 mils and 18 mils, 15 mils and 18 mils, 16 mils and 18 mils, 17 mils and 18 mils, 8 mils and 17 mils, 8 mils and 16 mils, 8 mils and 15 mils, 8 mils and 14 mils, 8 mils and 13 mils, 8 mils and 12 mils, 8 mils and 11 mils, 8 mils and 10 mils, 8 mils and 9 mils, 9 mils and 17 mils, 10 mils and 16 mils, 11 mils and 15 mils, 12 mils and 14 mils, 12 mils and 13 mils, or 13 mils and 14 mils. The thickness T2 may be less than approximately the thickness Tl .

[0038] In some embodiments, the difference between the largest thickness T2 _max and the smallest thickness T2 _min may be less than approximately 0.05 mils, 0.1 mils, 0.15 mils, 0.2 mils, 0.25 mils, 0.3 mils, 0.35 mils, 0.4 mils, 0.45 mils, 0.5 mils, 0.55 mils, 0.6 mils, 0.65 mils, 0.7 mils, 0.75 mils, 0.8 mils, 0.85 mils, 0.9 mils, 0.95 mils, 1 mil, 1.1 mils, 1.2 mils, 1.3 mils, 1.4 mils, or 1.5 mils. The difference between the largest thickness T2 _max and the smallest thickness T2 _m i _n may be approximately 0.1 mils, 0.2 mils, 0.3 mils, 0.4 mils, 0.5 mils, 0.6 mils, 0.7 mils, 0.8 mils, 0.9 mils, or 1 mil.

[0039] Although only a single edge 14 is described above, each edge of the sheet 10 may be processed and coated such that the sheet 10 is completely encapsulated in the porcelain enamel coating 30 on all edges and the top and bottom surfaces 18, 22 with a uniform thickness T2.

[0040] Fig. 9 is a flowchart illustrating a method 100 of processing the workpiece 10 to provide a coating along the edge 14 of the workpiece 10 to inhibit corrosion or degradation of the edge 14. In step 110, a radius R is formed along the edge 14 of the workpiece 10 by passing the workpiece through a forming operation. In step 120, an abrasion process is performed to remove all debris from the edge 14 of the workpiece 10 until the edge 14 has a white metal finish or better. In step 130, the workpiece 10 is washed and cleaned to remove debris, oils, and/or greases. In step 140, a primer coat 28 is applied to the edge 14 of the workpiece 10. In step 150, glass frit is applied to the edge 14 of the workpiece 10 to encapsulate the edge 14 of the workpiece 10. In step 160, the workpiece 10 is fired to fuse the glass frit and form a hardened porcelain enamel coating 30 bonded to the edge 14 of the workpiece 10.

[0041] In some embodiments, the method 100 of processing the workpiece 10 may include more or less steps. The method 100 may also be practiced in a different order than illustrated in Fig. 9. The edge 14 of workpiece 10 may be partially or wholly encapsulated in porcelain enamel coating 30.

[0042] Thus, the invention provides, among other things, a method of processing a workpiece to provide a coating along an edge of the workpiece that inhibits corrosion or degradation of the edge. Various features and advantages of the invention are set forth in the following claims.