Title of Invention

A PROCESS OF FORMING PHOSPHATE CONVERSION COATING LAYER ON AN ACTIVE METAL SURFACE

Abstract

This invention relates to an improved process for forming a zinc containing phosphate conversion coating on an active metal surface comprising the steps of: taking thermo mechanical treated (TMT) rebar and cold rolled sheets, which have formation of high temperature red scale on their bright surface for forming a zinc containing phosphate conversion coating; applying a zinc containing phosphate conversion coating solution on the red scale formed surface without any additional process operation, said phosphate conversion coating selected from a phosphate solution comprising Zn- Ni phosphate, Zn-Mn phosphate and Zn-Ni-Mn Phosphate; and drying said phosphate coated surface to obtain a uniform black coating on said TMT rebar or cold rolled sheet.

Full Text

FIELD OF APPLICATION This invention relates generally to an improved process of forming a zinc containing phosphate conversion coating layer on an active metal surface. In particular it relates to a process of forming a zinc-nickel, zinc-manganese and zinc-nickel-manganese phosphate conversion coating layer on an active metal surface, like thermo mechanical treated (TMT) rebar, TMT cold rolled sheets and cold twisted rebar (CTD). BACKGROUND OF THE INVENTION Recent years have seen a trend in which, because of environmental concerns, chromate treated steel sheet, in particular, is out of favor due to the possibility of hexavalent chromium being contained therein, and hence the demands on phosphate treatment have increased. The drawbacks associated with the application of a zinc phosphate coating solution include additional process steps which are required in its application. The additional process steps include pickling, rinsing and heating the surface before and after application of the zinc phosphate solution. The rinsing and heating processes may require a considerable amount of time. The prior art includes solutions and methods for reducing the additional steps associated with the phosphating process and the costs associated with these processes. For example, U.S. Patent No. 4,659,395 to Sugama et al., provides a process for applying a zinc phosphating solution which does not require subsequent rinsing. However, the prior art of Sugama et al. does require a heating process to dry the zinc phosphating solution from the surface being treated. U.S. Patent No. 6,117,251 is directed to the use of polyvinyl alcohol, along with zinc oxide, phosphoric acid, and a metal salt in an aqueous solution. Although this composition offers advantages, it has been found that, in some situations, the presence of polyvinyl alcohol tends to cause foaming, solubility problems, and causes the coating to be too tacky. There was therefore a need to develop phosphate coating solutions on rebar metal surface which are capable of providing desired adhesion and corrosion resistance without requiring any additional process operations directed towards pickling, rinsing and heating the treated surface. SUMMARY OF THE INVENTION: In view of such situation, the object of the present invention is to provide temporary corrosion resistance to rebar to avoid early discoloration during transportation and storage. Another object of the present invention is to develop the passivation system which is capable of desired adhesion and corrosion resistance without requiring any process operations directed towards separate pickling arrangement, rinsing and heating the treated surface. These and other objects can be achieved by developing a zinc-nickel, zinc-manganese and zinc-nickel-manganese phosphate coating solutions on rebar metal surface. Thus the present invention provides an improved process for forming a zinc containing phosphate conversion coating on an active metal surface comprising the steps of: taking thermo mechanical treated (TMT) rebar and cold rolled sheets, which have formation of high temperature red scale on their bright surface for forming a zinc containing phosphate conversion coating; applying a zinc containing phosphate conversion coating solution on the red scale formed surface without any additional process operation, said phosphate conversion coating selected from a phosphate solution comprising Zn- Ni phosphate, Zn-Mn phosphate and Zn-Ni-Mn Phosphate; and drying said phosphate coated surface to obtain a uniform black coating on said TMT rebar or. cold rolled sheet. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The invention can now be described with reference to the figures of the drawing where. Figure 1 shows an untreated thermo mechanical treated rebar. Figure 2 shows a passivated TMT rebar. The material used in the present invention are thermo mechanical treated rebar commonly known as "TMT rebar" and cold twisted rebar commonly known as "CTD rebar. TMT rebars of 8-14 mm diameter are produced in coil form and 16-42 mm are produced in straight form. As desired length of straight rebar is required to be used during straightening operation, protective oxide scale is removed from the surface and shiny bright surface comes in contact with atmosphere. Due to the electrochemical reaction, red rust appears on the rebar surface. Although, it is not very detrimental from corrosion point of view, but aesthetically it is not acceptable to the customer. Hence, some post treatment of the rebar is required to prevent this early rusting process. The conventional nitrite or chromate coating is not sufficient to avoid early discoloration during transportation and storage. Therefore, a phosphate based solution has been formulated which provides the coating on rebar surface without separate pickling operation. The conversion coating developed in the present invention is applicable for thermo mechanical treated (TMT) rebar, thermo mechanical treated cold rolled sheet (TMT CRS) as well as cold twisted (CTD) rebar. Rebar surface having a high temperature oxide like hematite and magnetite is formed on the outer surface. It is confirmed by Raman spectroscopic study. Both oxides are stable and adherent with the steel substrate. They provide good corrosion protection to steel substrate in open atmosphere. High temperature oxide scale formed on the rebar surface gives good atmospheric corrosion resistance, but red rust formed on the surface can be prevented by the said chemical conversion coating with different applications of zinc, nickel and manganese. No separate pickling treatment is necessary in the present invention. Three types of phosphate solutions were used with requisite quantity (gram per liter solution) of different chemicals as mentioned below. Composition of the phosphate solutions (gram per liter of solution): Zn-Ni phosphate system contained different chemicals per liter of solution as mentioned below: ZnO: 4.0-7.5 gm Ni(NO3)2 6 H2O:30-100 gm H3PO4: 40-80 ml Zn-Mn phosphate system contained different chemicals per liter solution as mentioned below: ZnO: 4.0-7.5 gm Mn(NO3)2 6 H2O: 30-100 gm H3PO4: 40-80 ml Zn-Ni-Mn phosphate system contained different chemicals per liter solution as mentioned below: ZnO: 4.0-7.5 gm Ni(NO3)2 6 H2O:15-50 gm Mn(NO3)2 6 H2O: 15-50 gm H3PO4: 40-80 ml This low pH range (0.75-1.0) solution provided the proper etching and conversion coating deposition in shorter time period within few minutes of say 0.2 to 20 minutes. The phosphating temperature was maintained at 50-90° C. The process parameter like temperature, dipping time and drying after treatment played an important role. After phosphating treatment, immediate surface drying is very critical to obtain a good and uniform black coating on the rebar surface. The same passivation process were also applied on straight rebar of sections (16 mm-42 mm). Process parameters like temperature (50-90°C), dipping time (0.2-20 minutes) pH (0.75-1.0) of the solution, phosphate/nitrite ratio and immediate drying after treated are critical process parameters for the coating process. After phosphate treatment, immediate surface drying was obtained since it is very critical for a good and uniform black coating on the rebar surface. Zn-Mn bi-cation phosphate treated rebar showed superior atmospheric corrosion resistance (no red rust up to 60 days) compared to the Zn-Ni bi-cation phosphate treated rebar (no red rust up to 45 days). All treated samples showed satisfactory result for high humidity test (up to 45 cycles). Zn-Ni bi-cation phosphate treated rebars showed more or less same bond strength with concrete as untreated rebar. The Zn-Mn bi-cation phosphate treated rebar showed only marginally lower bond strength with concrete compared to untreated rebar. The optimum bond strength and atmospheric corrosion resistance property of the rebars were achieved through Zn-Ni-Mn phosphate treated process as described above. In the present invention it has been found that with proper control of composition, bath temperature, dipping time and pH of the solution, fully satisfactory temporary corrosion protection can be obtained from the conversion coatings. By developing the optimum passivation process of the invention it has been possible to eliminate the pickling process as followed in the existing system. Test results TMT rebars without passivation and with phosphate passivation are shown in Figures 1 and 2 respectively. The phosphate treatment process of the present invention produced black rebar as shown in Figure 2 without the conventional pickling process. This invention provides a temporary corrosion protection of rebar during transportation and storage. This invention describes the effective surface treatment process to obtain a black surface on the rebar without separate pickling process. The black conversion coating obtained on the rebar surface is dense and of uniform thickness. Phosphate coating showed satisfactory result of corrosion resistance in open atmosphere up to two months. It also showed good result in high humidity and salt spray corrosion test. WE CLAIM: 1. An improved process for forming a zinc containing phosphate conversion coating on an active metal surface comprising the steps of: -taking thermo mechanical treated (TMT) rebar and cold rolled sheets, which have formation of high temperature red scale on their bright surface for forming a zinc containing phosphate conversion coating; -applying a zinc containing phosphate conversion coating solution on the red scale formed surface without any additional process operation, said phosphate conversion coating selected from a phosphate solution comprising Zn-Ni phosphate, Zn-Mn phosphate and Zn-Ni-Mn Phosphate; and -drying said phosphate coated surface to obtain a uniform black coating on said TMT rebar or cold rolled sheet. 2. The process as claimed in claim 1, wherein said TMT rebar in sizes 8.14 mm in diameter are taken in coil form. 3. The process as claimed in claim 1, wherein said TMT rebar having diameter in the range of 16-42 mm, are taken in straight form. 4. The process as claimed in claim 1, wherein said phosphate conversion coating solution is applied by dipping in the solution at a temperature of 50 to 90°C. 5. The process as claimed in claim 4, wherein said dipping for phosphate conversion coating is carried out for 0.2 to 20 minutes. 6. The process as claimed in claim 1, wherein one litre of said Zn-Ni phosphate system comprises 4.0-7.5 gm of ZnO, 30-100 gm of Ni (NO3)2 6H2O and 40-80 ml of H3PO4. 7. The process as claimed in claim 1, wherein one litre of said Zn-Mn phosphate system* comprises 4.0-7.5 gm of ZnO, 30-100 gm of Mn (NO3)2 6H2O and 40-80 ml of H3PO4. 8. The process as claimed in claim 1, wherein one litre of said Zn-Ni- Mn phosphate system comprises 4.0-7.5 gm of ZnO, 15-50 gm of Ni(N03)2 6H2O, 15-50 gm of Mn(N03)2 6H2O and 40-80 ml of H3PO4. 9. The process as claimed in claim 1, wherein the pH value of said phosphate coating solution is 0.75-1.0. 10. An improved process for forming a zinc containing phosphate conversion coating on an active metal surface, substantially as herein described and illustrated in the accompanying drawings. This invention relates to an improved process for forming a zinc containing phosphate conversion coating on an active metal surface comprising the steps of: taking thermo mechanical treated (TMT) rebar and cold rolled sheets, which have formation of high temperature red scale on their bright surface for forming a zinc containing phosphate conversion coating; applying a zinc containing phosphate conversion coating solution on the red scale formed surface without any additional process operation, said phosphate conversion coating selected from a phosphate solution comprising Zn- Ni phosphate, Zn-Mn phosphate and Zn-Ni-Mn Phosphate; and drying said phosphate coated surface to obtain a uniform black coating on said TMT rebar or cold rolled sheet.

Documents:

00408-kol-2005-description provisional.pdf

00408-kol-2005-drawings.pdf

00408-kol-2005-form 1.pdf

00408-kol-2005-form 2.pdf

00408-kol-2005-form 3.pdf

408-kol-2005-abstract.pdf

408-kol-2005-amanded claims.pdf

408-KOL-2005-CORRESPONDENCE 1..pdf

408-KOL-2005-CORRESPONDENCE.pdf

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

408-kol-2005-drawings.pdf

408-kol-2005-examination report reply recieved.pdf

408-KOL-2005-EXAMINATION REPORT.pdf

408-kol-2005-form 1.pdf

408-KOL-2005-FORM 13.pdf

408-KOL-2005-FORM 18.pdf

408-kol-2005-form 2.pdf

408-KOL-2005-FORM 3 1.1.pdf

408-kol-2005-form 3.pdf

408-KOL-2005-FORM 5 1.1.pdf

408-kol-2005-form 5.pdf

408-KOL-2005-GPA.pdf

408-KOL-2005-GRANTED-ABSTRACT.pdf

408-KOL-2005-GRANTED-CLAIMS.pdf

408-KOL-2005-GRANTED-DESCRIPTION (COMPLETE).pdf

408-KOL-2005-GRANTED-DRAWINGS.pdf

408-KOL-2005-GRANTED-FORM 1.pdf

408-KOL-2005-GRANTED-FORM 2.pdf

408-KOL-2005-GRANTED-SPECIFICATION.pdf

408-kol-2005-others.pdf

408-KOL-2005-REPLY TO EXAMINATION REPORT 1.1.pdf