Title of Invention

A METHOD OF PRODUCING HIGH CHROMIUM COATED GALVANIZED/GALVANNEALED STEEL SHEET FOR FUEL TANK APPLICATION .

Abstract

This invention relates to a method of producing high chromium coated galvanized/galvannealed steel sheet for fuel tank application, said method comprising the steps of using a steel sheet of required weldability and press formability; depositing a layer of zinc or zinc alloy on both sides of the sheet steel for making zinc coated sheet and depositing a chromium based chemical conversion coating on said zinc coated sheet.

Full Text

FIELD OF APPLICATION The present invention relates to a method of producing high chromium galvanized / galvannealed steel sheet for fuel tank application. The steel sheet for fuel tank of the present invention is particularly useful when it is used as a steel sheet for fabricating a fuel tank wherein an alcohol or alcohot blended fuel is used. A steel sheet for fuel tank is required to meet various requirements such as corrosion resistance to the fuel, weldability, and press formability etc. In many developed countries departure from petroleum dependency has been declared to be a national energy policy. Such countries use alternative motor vehicle fuels such as a mixture of gasoline with 5 to 10 % alcohol (methanol, ethanol). In India, the central government has introduced ethanol-blended petrol (EBP) in nine states across the country in a phased manner in order to reduce pollution levels. Alcoholic fuels have corrosivity markedly higher than the ordinary gasoline fuels since they tend to contain water and undergo phase separation when water content increases or temperature decrease. Moreover, organic acid methanol is converted info formic acid, and ethanol is converted into acetic acid which leads to the phase separation and formation of the lower layer mainly comprising alcohol, the organic acid and water. As already stated sheet steel for motor vehicle fuel tanks should have good wettability as the tanks are required to have no seam welding defects; to undergo no outer or inner surface corrosion; and to generate no floating corrosion product with may result in the blockage of the filter in the fuel circulation system. Metal powder containing organic films can be used as post treatment coating of zinc or zinc alloy coated sheets. When a steel sheet having metal powder-containing organic resin film is used for fabricating a gasoline tank, the inner surface of the tank suffers from insufficient inner surface corrosion resistance in the part wherein the steel sheet has experienced the damage of the metal powder falling off or the plated layer peeling off. The undamaged flat region of the tank also suffers form insufficient inner surface corrosion resistance since the corrosive solution is likely to become settled between the resin and the metal powder in the film. Moreover, the organic (resin) film on the surface corresponding to either the outer surface or the inner surface of the tank contains a curing agent as an essential component. When the degree of curing is too high, the organic layer is not readily heat melted, and in the welding, removal of the film will be difficult in the nugget formation process and seam weldability will be deteriorated. More illustratively, even though current passage points are provided by the metal powder, the film remaining unmelted resists the welding, and as a consequence, the nuggets formed do not sufficiently overlap with each other to result in fuel leakage. In some cases, the insufficient welding strength invites peeling. On the other hand, when the film has cured to an insufficient degree and the curing agent which did not react is present in the film, such part suffers form insufficient aggregation and high hydrophilicity of the unreacted reagent to invite invasion of the corrosive elements (acid, chorine ion etc) into the film to thereby adversely affect the outer/inner surface corrosion resistance of the tank. Moreover, the organic resin coating are of film formation type and are used in different coating thickness range varies from 1-5 µm. These organic coating are applied through the roller coat followed by high heating facilities of 80 - 150 C. Since the conversion coating is applied by spray or dip coating followed by drying at a pick metal temperature range of 50 - 65° C. In adopting the organic coating in the plant, an investment is required for roller coating and high heating facilities. Therefore, there is a need for developing a galvanized / galvannealed sheet steel which can be easily adapted for fuel tank application after giving a chemical conversion coating that can be applied on the conventional post treatment line without any investment in post treatment facilities. SUMMARY OF THE INVENTION Thus an object of the present invention is to provide a highly corrosion resistant steel sheet adapted for use in a fuel tank which has excellent inner surface corrosion resistance to the alcohol-blended gasoline. Another objective of the present invention is to develop the fuel tank material by using the existing post treatment facilities of conversion coating i.e., the spray coating and drying facilities up to 65 °C. These objects are achieved by giving the zinc coated sheet a high chromium chromate conversion coating that can be applied by spray or dip method followed by squeeze rolling and drying. The fuel tank materials of the present invention are hot-dip galvanized and galvanneaied sheet commonly known as "zinc coated sheet". Since the bare zinc coated sheet is not enough to give protection in fuel medium some post treatment is required. The conventional colourless chromate coating is not sufficient for fuel tank medium because the coating contains chromate in the range of 10 - 20 mg / sq. meter per side and hence its protection life is short. Therefore, we formulate chromium based solution which can give the chromate coating on sheet surface in range of 80 - ISO gm / sq meter per side on zinc coated sheet. The chromating solution consists of 150 - 200 - gm / I sodium dichromate, 4 - 7 ml of sulphuric acid, 5 - 20 ml of fluorosilicic acid and 1 - 2 gm of cobalt nitrate. The resultant solution pH is in the range of 0.5 to 0.9. This low pH range of 0.5 - 0.9 gives the proper etching and conversion coating deposition in shorter time period of 0.5 - 3 second. The chromate coating applied in 'dry - in place" type means the chromate conversion coating applied in continuous line by spray and without washing with water, the coating dried. In the present invention, only one side of the zinc or zinc alloy coated sheet is coated with high chromium containing solution for fuel tank material. The high chromium coated side becomes the inner part of the fuel tank and is in contact with fuel, known as inner side. Single side high chromate coated galvanized or galvanneaied sheets can be made by using existing line conditions by closing the upper side nozzle of the spray system. Thus the present invention provides a method of producing high chromium coated galvanized / galvanneaied steel sheet for fuel tank application said method comprising the steps of using a steel sheet of required wettability and press form ability; depositing a layer of zinc or zinc alloy on both sides of the sheet steel for making zinc coated sheet; and depositing a chromium based chemical conversion coating on said zinc coated sheet. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The invention can now be described in detail with the help of the figures of the accompanying drawing in which: Figure 1 shows the cycle of the humidity chamber used to evaluate the sample. Figure 2 shows galvarmealed samples after salt spray test. Figure 3 shows unchromated and chromated galvarmealed sheets. Figure 4 shows GA samples after 16 days exposure to cycle corrosion test. DETAILED DESCRIPTION OF THE INVENTION The invention will now be described in detail. The chrornate coating formulation and the application conditions are given below. The present invention provides a highly corrosion resistant steel sheet for fuel tank. It comprises a lowermost layer of hot dip zinc coating or hot dip zinc alloy coating layer deposit on each side of the steel sheet. The hot dip zinc coating layer known as galvanized material whereas the hot dip zinc coating followed by continuous annealing known as galvanneaied sheet. The zinc coated sheet is a common name for both the galvanized and galvannealed sheet and hereafter would be referred to as 'zinc coated sheet'. The zinc coated sheets are given a chemical conversion coating. The chemical conversion coating is a high chromium chromate coating containing 80 to 150 mg / sq. meter of total chromium on the chromated surface of zinc coated material. A chemical conversion coating deposit is given on either both side or single side on any of the zinc coated sheet materials. The chromium conversion coating on zinc coated surface is of 150 - 200 gm / I sodium dichromate, 4 - 7 ml of sulphuric acid, 5 - 20 ml of fluorosilicic acid and 1 - 2 gm of cobalt nitrate. The resultant solution pH is in the range of 0.5 to 0.9. The chromate conversion coating can be applied on zinc coated sheet in a continuous galvanizing line as a post treatment of zinc coated sheet. The conversion coating is applied in the temperature range from 30 - 60° C either by spray or dip coating method. The time required to apply the high chromium conversion coating on zinc coated sheet varies from 1 to 3 second depending upon the speed of the line. The conversion coating applied on the zinc coated sheet materials in continuous galvanizing line is "dry - in place" type. "The dry - in place" type means the chromate conversion coating applied in continuous line by spray or dip and without washing with water. For fuel tank application of zinc coated sheet requires only one side chromate coating, the side which is in contact of liquid fuel. Special effort was put to make one side chromate coating. Chromate coating on one side can be applied by spray coating system by closing the upper side nozzle of the spray. The top side nozzles may give a band of coating on the edge of the other side due to carryover of liquid chromate solution. In that case the bottom side nozzle of the spray system can be modified. The modification may consists of closing some of the nozzles of spray system which spray at the edge of the sheet material. This nozzle closing was up to 10 cm from both the edge of the sheet. Thus, both the galvanized and galvannealed sheet can be chromated by high chromium coating on single side or both sides depending on the customer requirements. Cyclic Corrosion test; Specimens of size 4*6 inch were cut from the unchromate and high chromate galvannealed sheet produced at the Plant. The edges of the sheet were covered by lacquer to prevent the galvanic corrosion from the edges. The samples were kept in humidity chamber to perform the cyclic test as per the cycle given in Figure 1. After 16 days of cycle corrosion test, surface appearances of chromated and unchromated GA were observed. Results are shown in Figure 4 respectively on the left and right hand sides. Test results The galvannealed steel sheet with no passivation and high chromate passivation were obtained (shown in Figure 3). The high chromate coated galvannealed surface shown on the right hand side has light yellow colour due to chromate coating. The following performance evaluation was employed to test the high chromate sheet of galvannealed material. Specimens of size 4*6 inch were cut from the single side chromate galvannealed sheet produced at the Plant. The edges of the sheet were covered by lacquer to prevent the galvanic corrosion from the edges. Separate samples were taken form unchromate and high chromate coated Galvannealed sheet for salt spray test. Salt spray chamber was done in 5% sodium chloride solution as per ASTM B117. The sample evaluation was done after an interval of 24 hours till the red rust appeared on the galvannealed surface. The unchromate GA surface became 100% red rusted in 48 hours where as the high chromium coated surface passed 168 hours in salt spray test without any red rust (as shown respectively on the left and right hand side in Figure 2). In salt spray test, high chromated galvannealed sheet passed 250 hours without any red rust. Fuel medium test The galvannealed sheet was taken for the petrol medium test. The GA sheet was processed to make a SO litre fuel tank in automobile plant. The above made fuel tank was then cut and the samples were taken from the curved area of fuel tank. The corrosion resistance was evaluated for the flat region and formed (curved) regions. In the evaluation of flat and curved region, the test samples was subjected to a corrosion test by immersing the samples in a fuel of 1/1 (on volume basis) petrol / 500 ppm aqueous formic acid at normal temperature for 1 months to determine the % of rust occurrence. Due to non solubility of petrol and water, two separate phases were formed in the above mentioned formulation. The lower phase consists of the water and the formic acid where as the upper phase is of petrol. The test result of the GA samples after one month exposure in the formic acid blended petro.l medium is given in Table 1. WE CLAIM 1. A method of producing high chromium coated galvanized/ galvannealed steel sheet for fuel tank applications comprising the steps of: - coating a layer of zinc or zinc alloy on both sides of a steel sheet of required weldability and formability; - providing a chromate conversion coating on one side of the galvanized/galvannealed steel sheet in a continuous line by spraying or dip coating a chromating solution consisting of 150-200 gm/l sodium dichromate, 4-7 ml of sulphuric caid, 5-20 ml of flurosilicic acid of concentration 20-40% and 1-5 gm of cobalt nitrate in the temperature range of ambient to 65° C. 2. The method as claimed in claim 1, wherein said deposition of zinc or zinc alloy is done by hot dip zinc coating. 3. The method as claimed in claim 1, wherein said step of hot dip zinc coating is followed by continuous annealing to make a zinc alloy coating known as galvannealed and here preferably used for developing fuel tank material. 4. The method as claimed in claim 1, wherein the one side chromate coating develop on galvannealed steel sheet by spraying chromate solution on the bottom side of the sheet which is in horizontal position during the spraying chromate coating application process in a continuous line. 5. The method as claimed in claim 1, wherein the chromate coated side of the galvannealed sheet become the inner part of the fuel tank and in contact with the liquid fuel during application. 6. The method as claimed in claim 1, wherein said high chromium chromate conversion coating contains 80 to 150 mg/sq. meter per side of zinc coated sheet. 7. The method as claimed in claim 1, wherein the resultant chromating solution has pH value of 0.5 to 1.0 which low range of pH enhance proper etching and conversion coating deposition in shorter time period of 0.5-5 seconds. 8. The method as claimed in preceding claims, wherein said chromate conversion coating is applied "dry-in-place", without washing with water. 9. A high chromium coated galvanized/galvannealed steel sheet for fuel tank application as produced according to method of claim 1. 10.The high chromium coated galvanized/galvannealed sheet as claimed in claim 7, wherein the chromium coated steel sheet is characterized evaluated on preparing cut specimens of 4X6 inch size cut from single side high chromate galvannealed sheet and unchromated galvannealed sheet on covering the edges of the sheets by lacquer to prevent galvanic corrosion from the edges, spraying the unchromated and chromated galvannealed specimens in 5% sodium chloride solution in salt spray chamber (SSC) according to ASTM B117 test procedures and it was observed that unchromated GA surface of the test specimen became 100% red rusted in 48 hrs where as the high chromium coated GA sheet passed 250 hrs without any formation of red rust. This invention relates to a method of producing high chromium coated galvanized/galvannealed steel sheet for fuel tank application, said method comprising the steps of using a steel sheet of required weldability and press formability; depositing a layer of zinc or zinc alloy on both sides of the sheet steel for making zinc coated sheet and depositing a chromium based chemical conversion coating on said zinc coated sheet.

Documents:

00028-kol-2005-abstract.pdf

00028-kol-2005-assignment.pdf

00028-kol-2005-claims.pdf

00028-kol-2005-correspondence-1.1.pdf

00028-kol-2005-correspondence-1.2.pdf

00028-kol-2005-correspondence-1.3.pdf

00028-kol-2005-correspondence.pdf

00028-kol-2005-description(complete).pdf

00028-kol-2005-description(provisional).pdf

00028-kol-2005-drawings-1.1.pdf

00028-kol-2005-drawings.pdf

00028-kol-2005-form-1.pdf

00028-kol-2005-form-13.pdf

00028-kol-2005-form-18.pdf

00028-kol-2005-form-2-1.1.pdf

00028-kol-2005-form-2.pdf

00028-kol-2005-form-3.pdf

00028-kol-2005-form-5.pdf

00028-kol-2005-g.p.a.pdf

00028-kol-2005-other document.pdf

28-KOL-2005-FORM 15.pdf

28-kol-2005-granted-abstract.pdf

28-kol-2005-granted-claims.pdf

28-kol-2005-granted-correspondence.pdf

28-kol-2005-granted-description (complete).pdf

28-kol-2005-granted-drawings.pdf

28-kol-2005-granted-examination report.pdf

28-kol-2005-granted-form 1.pdf

28-kol-2005-granted-form 13.pdf

28-kol-2005-granted-form 18.pdf

28-kol-2005-granted-form 2.pdf

28-kol-2005-granted-form 3.pdf

28-kol-2005-granted-form 5.pdf

28-kol-2005-granted-gpa.pdf

28-kol-2005-granted-reply to examination report.pdf

28-kol-2005-granted-specification.pdf