Title of Invention

"A PROCESS FOR PREPARATION OF IMPROVED CORROSION RESISTANCE REINFORCED ZINC COMPOSITES"

Abstract

The present invention relates to a process for preparation of improved corrosion resistance reinforced zinc composites. Zinc alloys containing particulates to reinforce the metallic matrix had been widely employed in appliances when high strength was deemed to be essential, for example, roofing, household appliances and door and window frames and components like aldrops, hinges etc. The prolonged lower temperature treatment and cryogenic processing has been used to deactivate the metallic surface of the reinforced zinc composites, without resorting to any coating or other electrochemical techniques. This avoids any possible alteration or damage of the structure that may occur during the conventional stress relieving.

Full Text

The present invention relates to a process for preparation of improved corrosion resistance reinforced zinc composites. Zinc alloys containing particulates to reinforce the metallic matrix had been widely employed in appliances when high strength was deemed to be essential, for example, roofing, household appliances and door and window frames and components like aldrops, hinges etc. Hitherto the reference may be made in this connection of composites in alloys of lead, zinc, aluminium and copper. [D.Mukherjee, Lower concentration particle dispersion in ultrafine mode improves corrosion resistance; Invention Intelligence, Vol.29, No.8, August 1994, P.420]. D.Mukherjee, K.Ravichandran and R. Mahalingam, Tools and Alloy Steel, Vol.27, No.9, Sept. 1993, P.297] wherein it has been shown that incorporation of ultrafine ceramic particulate in lower concentration has been found to reduce the surface leaching by virtue of their selective distribution in and around anodic defect sites, e.g. voids, grain boundaries etc. However, full benefit from this technique cannot be achieved due to the dispersion of the particle within the grain proper and also due to the superimpositional effect of particulate on the anodic sites. These alloys are also subjected to rolling, forging etc. during fabrication. Such processing aggravate the already active stress raiser sites present in the matrix. However, the fabricated alloy is employed with or without further heat treatment, depending on the end use. The present invention has introduced a cryogenic step for a short duration and also separately a prolonged lower temperature step, prior to application in service, during the stage of processing of these fabricated alloys itself, which results in the increased corrosion resistance of these alloys bycontrolling the dissolution and leaching at the anodic grain boundaries and by deactivating the stress raiser sites, generated during the forming process. This invention has overcome the drawbacks of the lower concentration ultrafine ceramic particles reinforced wrought zinc alloys as far as corrosion resistance in chloride containing media are concerned. The main objective of the present invention is to provide a short duration cryogenic step and also a prolonged lower temperature step during the stage of processing of the lower concentration finer ceramic particles reinforced wrought zinc alloy composites to improve the corrosion resistances of these composites. Another objective of the present invention is to take care of the preferential anodic leaching at grain boundaries and other defect sites of these composites. Still another objective of the present invention is to deactivate the stress raiser sites introduced on the surface of the reinforced composites during the fabrication process. Accordingly present invention provides a process for preparation of improved corrosion resistance reinforced zinc composites which comprises; cleaning the surface of the reinforced alloys with saturated ammonium acetate, washing in running water, drying and degreasing for removing oils and greases from the polished surfaces characterized in that followed by the prolonged lower temperature at 0°C, followed by cryogenic treatment in the temperature range of 180 to 200 °C by cleaning of the metallic surface, introducing the said cleaned panel into the cryo- chamber, evaucating the chamber, filling up of the chamber with liquid nitrogen, maintaining the metallic panels at liquid N2 temperature, evaucating the chamber and heating the treated panels back to the room temperature, purging the chamber with suitable inert gas like argon and removing the cryotreated panels for end applications. A method of testing of theses cryogenically treated zinc composites has also been developed which comprises of degreasing with trichloroethylene and subsequent impedance measurements in 3 % sodium chloride solution. This process can be adopted for improving the corrosion resistance of a wide variety of zinc alloys.The only limitation is that the ductile brittle transition temperature of alloys must be less than that of liquid nitrogen temperature. In the embodiment of the present invention, a process has been developed which comprises of cleaning the surface of the lower conccentration ultrafine particles reinforced zinc alloys with saturated ammonium acetate, washing in running water, drying and degreasing for removing oils and greases from the polished surfaces which is followed by the prolonged lower temperature (0°C) treatment and 5 hours exposure to cryogenic temperature (-180°C). The lower temperature operation is conducted by abruptly bringing down the temperature of the panels from room temperature (30°C) to 0°C, while the cryogenic treatment has been conducted by applying a negative temperature gradient of 30°C/hr from room temperature (30°C) to -180°C, then at -180°C for 5 hours, after which the temperature is increased to 30°C, during a period of 3 hours, using a positive temperature gradient of 60°C/hr. In another embodiment of the present invention a detailed pretreatment schedule under cryogenic environment have been formulated which comprises of cleaning the metallic surfaces (pre treatment), introduction of the pretreated samples in the chamber, maintaining at 0°C for three weeks, removing the panels for cryo treatment after lower temperature treatment at 0°C ' Subjecting them to cryO-treatment and re- heating them to room temperature such that the complete cryogenic treatment may be classified into nine major steps, as given below: Cleaning of the metallic surfaces (mechanical and / or chemical pretreatment), introduction of the cleaned (pre-treated) panels into the cryo chamber, evacuation of the chamber, filling up of the chamber with liquid nitrogen, maintaining the metallic panels at liquid nitrogen temperature for specified duration (-200 + 20°C, 5 + 1 Hr.), evacuating the chamber, heating the treated panels back to the room temperature, purging the chamber with suitable inert gas like argon and removing the cryo-treated panels for end applications. In yet another embodiment of the present invention a method of testing of these cryogenically treated zinc composites have been developed which comprises of degreasing the cryogenically treated lower concentration ultra-fine particles reinforced zinc composites with tnchloroethylene and subsequent impedance measurements in 3% sodium chloride solution using a conventional impedance set up incorporated with an electrochemical interface along with lock in amplifier and computer controlled printing facilities. The novelty of the invention lies in the fact that it suits well for selective applications of components with intricate sizes and shapes, where heat treatment and/or other protection methods are not possible. Both the prolonged lower temperature treatment and cryogenic processing are used to deactivate the metallic surface of the reinforced zinc composites, without resorting to any coating or inhibitor or other electrochemical techniques. It also avoids any possible alteration and damage of the surface that may occur during conventional stress relieving. Moreover, it is a clean and dry process. The prolonged lower temperature treatment can also be performed using a clean deep freezer. Reinforced zinc alloys find extensive applications in coating industry and in other electrochemical control processes. Incorporation of ultra-fine ceramic particulates in lower concentration range has been found to reduce the surface leaching by the selective coverage of anodic sites in and around grain boundaries, voids, etc. [Ref: D.Mukherjee, K.Ravichandran, R.Mahalingam, Tools and Alloy-Steel, Vol.27, No.9, Sept. 1993, P.297 and D.Mukherjee; Lower concentration ultra-fine particles improve the surface properties of metals and alloys; Anticorrosion Methods and Materials, Vol.45, No.2, March-April 1998, P.95]. However, full benefit from this technique cannot be achieved due to the dispersion of the particles within the grain proper and mismatches due to superimposition of the cathodic particulates over the anodic sites. These can be fully achieved by employing cryogenic treatment that deactivates the residual galvanic activities thereby getting the full benefit of improved corrosion resistance. Any process that increases the corrosion resistance of reinforced zinc alloys would increase their service life. Hence, this invention aims at improving the corrosion resistance of zinc alloys containing lower concentration of ultra-fine ceramic particulates, by incorporation of suitable modification in the processing of the alloys. The following examples are given by way of illustration and should not be construed to limit the scope of the invention: Example 1 The surface of the lower concentration ultra-fine particles reinforced zinc alloy Zn94 A13 SiC3, was cleaned with saturated ammonium acetate, washed in running water, dried and degreased for removing oils and greases from the polished surfaces. It was subjected to prolonged lower temperature (0°C) treatment and 5 + 0.25 hours exposure to cryogenic temperature (-185 + 10°C). The lower temperature operation was conducted by abruptly bringing down the temperature of the panels from room temperature (30°C) to 0°C, while the cryogenic treatment was conducted by applying a negative temperature gradient of 30°C/hr from room temperature (30°C) to -185 ± 10°C. Then from -185+ 10°C, the temperature was increased to 30°C, during a period of 3 hours, using a positive temperature gradient of 60°C/hr. The chamber was purged with Argon and the samples were removed for analysis. Impedance measurements were done with the above sample as working electrode, Platinum as auxiliary electrode and Saturated Calomel Electrode (SCE) as the reference. The measurements were carried out in 3 % Sodium chloride, over a frequency range of 10 K Hz to 10 m Hz. From the Nyquist plot, (real vs. imaginary impedance), the charge transfer resistance (Ret) was determined. The Ret for this treated alloy sample was found to be 320 + 5 Ohm. Similarly, the Ret for the untreated alloy sample was found to be 240 + 5 Ohm. Potentiodynamic polarization was done with the treated alloy sample as working electrode, Platinum as auxiliary electrode and SCE as the reference in 3 % Sodium chloride. The corrosion current density (Icorr), which is inversely proportional to the corrosion rate, was determined. The Icorr for the untreated alloy sample was also determined. The Icorr for this treated alloy sample was found to be 18 + 5 jiA/cm^. Similarly the Icorr for the untreated alloy sample was found to be 125 + 5 Example 2 The surface of the lower concentration ultra-fine particles reinforced zinc alloy Zn94 A13 (TiO2)3, was cleaned with saturated ammonium acetate, washed in running water, dried and degreased for removing oils and greases from the polished surfaces. It was subjected to prolonged lower temperature (0°C) treatment and 5 + 0.25 hours exposure to cryogenic temperature (-190 + 10°C). The lower temperature operation was conducted by abruptly bringing down the temperature of the panels from room temperature (30°C) to 0°C, while the cryogenic treatment was conducted by applying a negative temperature gradient of 30°C/hr from room temperature (30°C) to -190 + 10°C. Then from -190 + 10°C, the temperature was increased to 30°C, during a period of 3 hours, using a positive temperature gradient of 60°C/hr. The chamber was purged with Argon and the samples were removed for analysis. Impedance measurements were done with the above sample as working electrode, Platinum as auxiliary electrode and SCE as the reference. The measurements were carried out in 3 % Sodium chloride, over a frequency range of 10 K Hz to 10 m Hz. From the Nyquist plot, (real vs. imaginary impedance), the charge transfer resistance (Ret) was determined. The Ret for this treated alloy sample was found to be 100 + 5 Ohm. Similarly, the Ret for the untreated alloy sample was found to be 40 + 5 Ohm. Potentiodynamic polarization was done with the treated alloy sample as working electrode, Platinum as auxiliary electrode and SCE as the reference in 3 % Sodium chloride. The corrosion current density (Icorr), which is inversely proportional to the corrosion rate, was determined. The Icorr for the untreated alloy sample was also determined. The Icorr for this treated alloy sample was found to be 10 + 5 nA/cm^. Similarly the Icorr for the untreated alloy sample was found to be 120 + 5 The following table consolidates the results of the above examples: (Table Removed) The results from the above examples clearly reveal that lower temperature and cryogenic processing lead to considerable increase of charge transfer resistance and reduction in corrosion current density, and hence, an increase in corrosion resistance. The main advantages of the present invention are 1) Selective application of composites where heat treatment and / or other protection methods are not possible. 2) The prolonged lower temperature treatment and cryogenic processing may be used to deactivate the metalhc surface of the reinforced zinc composites, without resorting to any coating or other electrochemical techniques. 3) Any possible alteration or damage of the structure that may occur during the conventional stress relieving may be avoided. 4) It is a clean and dry process, such that the prolonged lower temperature treatment can also be performed using a clean deep freezer. 5) The technique under invention takes care of the possible work hardening (stress raiser sites) of the surface during the conventional fabrication of the reinforced zinc composites. 6) The cryogenic treatment also takes care to completely annul the preferential leaching of the composite surface at the anodic sites like grain boundaries, voids etc. We Claim: 1. A process for preparation of improved corrosion resistance reinforced zinc composites which comprises; cleaning the surface of the reinforced alloys with saturated ammonium acetate, washing in running water, drying and degreasing for removing oils and greases from the polished surfaces characterized in that followed by the prolonged lower temperature at 0°C, followed by cryogenic treatment in the temperature range of 180 to 200 °C by cleaning of the metallic surface, introducing the said cleaned panel into the cryo- chamber, evaucating the chamber, filling up of the chamber with liquid nitrogen, maintaining the metallic panels at liquid N2 temperature, evaucating the chamber and heating the treated panels back to the room temperature, purging the chamber with suitable inert gas like argon and removing the cryotreated panels for end applications. 2. A process as claimed in claim 1, wherein resistance of lower concentration ultrafine particles reinforced zinc alloy composites by the application of a prolonged lower temperature treatment at 0°C. 3. A process as claimed in claim 1,wherein the duration of cryogenic treatment is continued for for 5 hours. 4. A process for preparation of improved corrosion resistance reinforced zinc composites substantially as herein described with reference to the examples.

Documents:

220-DEL-2003-Abstract-(07-01-2010).pdf

220-del-2003-abstract.pdf

220-DEL-2003-Claims-(07-01-2010).pdf

220-DEL-2003-Claims-(07-10-2010).pdf

220-DEL-2003-Claims-(08-10-2010).pdf

220-del-2003-claims.pdf

220-DEL-2003-Correspondence-Others-(07-01-2010).pdf

220-DEL-2003-Correspondence-Others-(07-10-2010).pdf

220-DEL-2003-Correspondence-Others-(08-10-2010).pdf

220-del-2003-correspondence-others.pdf

220-del-2003-correspondence-po.pdf

220-DEL-2003-Description (Complete)-(07-01-2010).pdf

220-DEL-2003-Description (Complete)-(07-10-2010).pdf

220-DEL-2003-Description (Complete)-(08-10-2010).pdf

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

220-DEL-2003-Form-1-(07-01-2010).pdf

220-del-2003-form-1.pdf

220-del-2003-form-18.pdf

220-DEL-2003-Form-2-(07-01-2010).pdf

220-del-2003-form-2.pdf

220-DEL-2003-Form-3-(07-01-2010).pdf

220-del-2003-form-3.pdf