Title of Invention	REACTIVE CHROMATE PASSIVATION FOR GALVANIZED MATERIAL
Abstract	A reactive chromate passivation solution for galvanized sheet surface to decorate protecting coating, the chromate passivation solution comprising: - total chromium 3.4-17.0 grm/liter; total nitrate ion 1.3 to 6.7 grm/liter; total fiuorosilicic acid 0.3 to 1.5 grm/liter; and - the pH of chromate soln is 0.7-2.5.

Full Text

-2- TITLE: REACTIVE CH ROM ATE PASSIVATION FOR GALVANIZED MATERIAL This present invention relates to development of a reactive-chromate-based passivation formulation which can be applied as a post treatment coating on the galvanized sheet surface in continuous galvanizing line, This passivation enhances the white rust resistance as well as gives an excellent resistance to darkening and fretting corrosion on galvanized surface. BACKGROUND OF THE INVENTION: In the engineering and construction sectors, hot dip galvanized steel sheets are preferred by the user for their appearance and long term corrosion protection. However, the freshly produced galvanized sheets are highly susceptible to external factors that lead to surface degradation during storage and transport. These surface defects are mainly in the form of white rust, darkening and fretting corrosion. The presence of any one or their combination causes poor acceptance of tfie material by the customer. For these reasons, practically all galvanized sheets are given a cbromate passivation treatment after galvanizing in a continuous line. A common passivating agent used is tiie chromic acid leading to a complex chromate film of Cr (IV) and Cr (IE) in compound form on a galvanized surface. Although the generation of white rust is effectively prevented by subjecting galvanized steel sheets to a chromate treatment but tine other surface defects like blackening and / or fretting corrosion occurs in a relatively short time during storage, usage or transport. To overcome these surface defects, we need to understand these defects. -3- White rust: This is found particularly on fresh galvanized, bright surface and especially in crevices between closely packed sheets. White rust is a white, crumbly, and porous coating consisting of 2ZnC03.3Zn(OH)2 together with ZnO and voluminous B - Zn(OH)z When zinc coating corrode in open air, zinc oxide and zinc hydroxide are normally formed. If the supply of air to the surface of the zinc is restricted, as in a narrow crevice, there is not enough carbobn dioxide for the subsequent formation of a zinc carbonate layer. The layer of zinc oxide and zinc hydroxide is voluminous and porous and adheres only loosely to the zinc surface. Consequently, it does not protect the zinc surface against oxygen in the water. Corrosion can, therefore, proceed as long as there is moisture left on the surface. Darkening; Darkening is more liable to develop when GI sheets are stored in stacked form after cutting into pieces or in coiled form than when these are left standing in bare form in a room. It blackens on usages as well in the open, or in enclosed of normal air. It has been recognized that this phenomenon occurs more readily in steel sheets in which the coating surface has been activated by skin pass rolling (usually 1-3% draft) following zinc coating, and in steel sheets coated with a molten zinc^aluminum alloy containing tcvt-ij! % of akiu'mum. Also on hot-dipped galvanized steel sheets (non- skin-passed sheets), black tarnish due to the spangles is occasionally observed in spotty form during ling-term storage. Literature suggests that the larger impurity diffusion in dull spangle is responsible for its lower resistance to corrosion attack and the blackening is produced by diffusion and oxidation of antimony /lead that is used as surfactant. -4- The galvanized sheet looks black, apparently because the basic zinc carbonate of which it is composed, and which is represented as (ZnCO.sub.3)x.[Zn(OH).sub.2]y, like white rust, has a particle diameter felling within the visible light wavelength range of 400 to 700 nm, and is, therefore, very likely to scatter and absorb light. The black patina is considered as a product of corrosion formed in an environment lacking oxygen, and particularly with the progress of corrosion from the grain boundary. Fretting Corrosion: The fretting corrosion appears as light to dark black spots / marks distributed over the surface. The mechanism of fretting corrosion is that asperities rubbing together cause attrition and the debris so formed further aids the process. The conditions necessary are that the small surface shall be in finer contact or shall move only over very small amplitude. This condition occurs during transporting tiie coils as tiiey normally stand on tiie circumference that initiate the fretting to take place. It has been recognized that this phenomenon occurs more readily in steel sheets coated with a molten zinc-aluminum alloy containing several % of aluminum. The surface is entirely covered by a tiiin film containing aluminium and oxygen in approximately a 2:3 atom ratio and the film tiiiekness was of the order of 5nm. This aluminium oxide layer aggravates the fretting corrosion during the transportation. PRIOR ART; The above problems are experienced by most of the hot dip galvanized coil producer and solved by their own way. A large number of literatures and patents are already made in tiiis area and few are mentioned here. -5- Japanese Patient Application Laid-Open No. 114695/82 discloses a process for producing galvanized steel sheets superior resistance to black tarnishing and intergranular corrosion. This technique comprises treating the galvanized sheets with an alkali metal carbonate solution, followed by electroplating the treated sheets with zinc. According to this patent application, the oxides of Zn, Al, and Fe formed on the hot-dipped galvanized surface are completely dissolved by the treatment with an alkali metal carbonate solution and this permits the black tarnishing and the intergranular corrosion to be prevented even when the subsequent zinc electroplating produced a thin film. Japanese Patent Application Laid-Open No. Sho 59-177381 proposed flashing treatment with an aqueous solution containing Ni or Co ions as a method of preventing a black patina from appearing after ehromate treatment. The surface of a zinc or zinc-alloy plated steel sheet is given flashing treatment with an aqueous solution having a pH of 1 to 4, orll to 13, and containing Ni or Co ions, or both prior to its ehromate treatment, after it is washed with water, a ehromate film is formed thereon. A similar theory is patented by the USA patent 4663245, a steel sheet coated by hot dipping with zinc or with a zinc alloy containing low concentrations of Al and Mg is immersed for a short period of time in a solution of Ni ions and / or Co ions or sprayed witfi ttiis solution and is subjected to the usual chromating, thereby achieving a superior effect of preventing black tarnish. -6- USA patient 6280535 discusses about a chromating process which combines both the flashing process and chromating process as a single step process by incorporating tiie metal ion which is used in tfie flashing treatment (Ni or Co) into the chromatining bath. OBJECTS OF THE INVENTION It is therefore, an object of the present invention to propose a chromate treatment coating on the galvanized sheet surface which eliminates tfie disadvantage of prior Art. Another object of the present invention is to propose a chromate treatment coating on the galvanized sheet surface which reduces the formation of white rust on the galvanized sheet surfaee. A further object of the present invention is to propose a chromate treatment coating on the galvanized sheet surface which reduces the formation of darkening on the galvanized sheet. A still further object of the present invention is to propose a chromate treatment coating on the galvanized sheet surface which reduces the formation of fretting corrosion. As yet further object present invention is to propose a chromate treatment coating on the galvanized sheet surface which is eco-fnendly. -7- PETAIL DESCRIPTION OF THE INVENTION: The chromate treatment was given on the galvanized sheet in a continuous hot-dip galvanizing line. Regular spangles galvanized sheet material was produced in commercial line and were chromated by the invented chromate solution. The invented chromate solution comprising: - Chromium - 8.76 g/l. Nitrate irons - 3.3 g/l Cobalt - 0.2 g/l Ruorosilicic acid - 0.7 g/l The chromic acid was used to make chromium formulation, the nitrate ions produced from nitric acid and cobalt nitrate, ttie cobalt ions produces from cobalt nitrate and the 30% concentration of flurosilic acid were used. For comparison study, the galvanized sheet material treated by commercially available chromate passivation solution was taken. The pH of the invented chromate solution is maintained in the range of 0.7 to 2.5 m6 the temperature of the chromate solution is maintained is range of 25 to 65 degree centigrade. The chromate film formed on the galvanized surface having a coating weight of 5 - 40 mg/m2 in term of total chromium formation. The duration of wet coating on the surface is in the range of 1-5 second for the running strip before drying. -8- The Chromate passivation Performance was measured by salt spray test as per ASTM B 117. According to this specification, 5% sodium chloride solution is atomized in a salt spray chamber at 35°C with the solution pH around 7. the test panels are placed at an angle of 45° in the chamber, exposed to the salt for a certain period and the initiation of white rust formation on the sample was recorded. The 5% white rust on the galyanized surface was considered as a cut-off value for the chromate passivation performance evaluation. The aluminium leaching from the galvanizing surface after the chromate passivation was considered as effectiveness of the chromate film in reducing the fretting corrosion. This galvanized surface analysis was conducted by Scanning Electron Microscope (SEM) attached with Energy Dispersive Spectroscope (EDS) detector to measure the aluminium wt % present in the surface. It was observed ttiat the aluminium % was less on surface in invented chromate treated product compared to commercially available chromate treatment. The aluminium leaching value depends on the aluminium present on the galvanized surface therefore it was not quantified. However, the reduction in aluminium has been qualitatively evaluated. -9- TABLE1: COMPARATIVE STUDY OF galvanized sheets treatment with commercial and Invented chromate solution. Sample detail Coating performance test Surface appearace Salt spray result (hours) Aluminium leaching Black ening Fretting corrosion White rust resistane GI-+ chromated passivation done by commercial chromate solution Colourless 72 Moderate 5-10% 10-15% of total consigment OK GI+ chromated passivation done by invented chromate solution Colourless 90 Excellent Not reported 1-3% OK -10- WE CLAIM 1. A reactive chromate passivation solution for galvanized sheet surface to decorate protecting coating, the chromate passivation solution comprising: - total chromium 3.4-17.0 grm/liter; total nitrate ion 1.3 to 6.7 grm/liter; total fiuorosilicic acid 0.3 to 1.5 grm/liter; and - the pH of chromate soln is 0.7-2.5. 2. The chromate solution as claimed in claim 1 wherein the chromate ion present in tiie form of hexavalent and bivalent and present in chromate solution originated from soluble chromate compounds such as chromic acid, sodium dichromate etc. 3. The chromate solution as claimed in claim 1 wherein working temperature maintain in the range of 25-65°C. 4. The chromate solution as claimed in claim 1 wherein chromate film formed on the galvanized surface having a coating weight of 5-40 mg/m2 in term of total chromium formation. 5. The chromate solution as claimed in claim 1 wherein the duration of wet coating on the surface is in range of 1-5 second for the running strip before drying. A reactive chromate passivation solution for galvanized sheet surface to decorate protecting coating, the chromate passivation solution comprising: - total chromium 3.4-17.0 grm/liter; total nitrate ion 1.3 to 6.7 grm/liter; total fiuorosilicic acid 0.3 to 1.5 grm/liter; and - the pH of chromate soln is 0.7-2.5.

Documents:

00523-kol-2008-abstract.pdf

00523-kol-2008-claims.pdf

00523-kol-2008-correspondence others.pdf

00523-kol-2008-description complete.pdf

00523-kol-2008-form 1.pdf

00523-kol-2008-form 2.pdf

00523-kol-2008-form 3.pdf

00523-kol-2008-gpa.pdf

523-KOL-2008-(10-02-2012)-CORRESPONDENCE.pdf

523-KOL-2008-(25-11-2011)-ABSTRACT.pdf

523-KOL-2008-(25-11-2011)-AMANDED CLAIMS.pdf

523-KOL-2008-(25-11-2011)-AMANDED PAGES OF SPECIFICATION.pdf

523-KOL-2008-(25-11-2011)-CORRESPONDENCE.pdf

523-KOL-2008-(25-11-2011)-DESCRIPTION (COMPLETE).pdf

523-KOL-2008-(25-11-2011)-EXAMINATION REPORT REPLY RECEIVED.pdf

523-KOL-2008-(25-11-2011)-FORM-1.pdf

523-KOL-2008-(25-11-2011)-FORM-13.pdf

523-KOL-2008-(25-11-2011)-FORM-2.pdf

523-KOL-2008-(25-11-2011)-OTHERS.pdf

523-kol-2008-form 18.pdf