TECHNICAL FIELD
The present disclosure generally relates to a field of material science and metallurgy. Particularly, but not exclusively the present disclosure relates to plasma zinc coated rebar. Further, embodiments of the present disclosure disclosepassivation coating on plasma zinc coated rebar.
BACKGROUND OF THE DISCLOSURE
Generally, fresh produced rebars are covered with oxide scale and has blackish lustre. But, it disappears due to electrochemical reaction with open atmospheric and becomes rusty due to corrosion. The solutions available in market to make rebar free from rust are costly and involves tedious process.
A few examples of such products are epoxy coated rebar, galvanized rebar, stainless steel rebar etc. But these products suffers from drawback like epoxy rebars have pin hole problem, galvanized rebars are produced by batch process and make coating thick and is costly, stainless steel is also very costly.
An Electro Plasma Process Technology (EPT) for coating rebar is has the advantage of being economical andinvolves comparatively easy process with respect to the convenbonal above processfor applying a metal coating to an electrically conductive surface of steel.
In EPT process, plasma is produced at the metal surface by application of high voltage and generated plasma can be used for coating deposition. After cleaning, the rebar goes through zinc ion electrolytic bath and deposits about 10 urn zinc coating on rebar surface using by EPT. The detail process on EPT is given in the literatures and ASTM standard A1093/A1093M-15 [1-3].

The purpose of plasma zinc coating on carbon steel rebar is to enhance its service life during storage and in application. But, like any other galvanized material, the plasma zinc coating reacts itself with atmosphere and form white rust and dullness during storage and transportation.
Indian patent no. 221739 discloses the elimination of the darkening problem of galvanized sheet by selectively dissolution of lead from the galvanized surface and producing a colourless or a slight tinge passive layer on galvanized surface. This patented formulation is not suitable for plasma zinc coated rebar because it contains low concentration of chromium compound. Further, chromate concentration, pH and reaction time are not found suitable for plasma zinc coated rebar.
Another Indian patent no. 231443 is formulated mainly for galvannealed (Zinc-iron alloy) sheet for fuel tank application, where a very high concentration of chromium compound is used and sodium dichromate is selected as chromium compound that generate sodium ion. Because of the above mentioned reasons, the second developed formulation is not suitable for plasma zinc rebar.
Other chromate passivation formulations were also studied but they were not found suitable for our plasma zinc coated rebar product.
OBJECTIVES OF THE DISCLOSURE
In view of the foregoing limitations inherent in the prior-art, it is an object to develop a new passivation composition for plasma zinc coated rebar.
Another object is to develop such composition that when applied over the zinc coated rebar that prevents white rust and red rust on the rebar.
Another object is to develop such composition that when applied over the zinc coated rebar that prevents pin hole problem on the rebar.

Still another object is to develop such composition that when applied over the zinc coated rebar gives cost effective solution from red rust, white rust, dullness, pin holes etc.
SUMMARY OF THE DISCLOSURE
In one aspect, the disclosure provides an a passivation composition for plasma zinc coated rebar comprising chromium ion from chromium compound 5-40 (g/l), sulphate ion from a sulphate compound 2-10 (g/l), fluoride ion from a fluoride compound 0.05 to 1 (g/l) and rest water to make one litre solution, all being mixed together.
In other aspect,the disclosure provides process for coating a rebar comprising steps of mechanically surface cleaning and/or pickling of the rebar, further cleaning the rebar using an electrolytic plasma technology, zinc coating on the rebar by electrolytic plasma technology, and applying a passivation composition over the zinc coated rebar. The passivation solution comprising of chromium ion from chromium compound 5-40 (g/l), sulphate ion from a sulphate compound 2-10 (g/l), fluoride ion from a fluoride compound: 0.05 to 1 (g/l), and rest water to make one litre solution, all being mixed together,and drying the rebar.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIG. lillustrates flow chart showing a process for coating and passivating rebar in accordance with an embodiment of the present disclosure.
FIG. 2 illustrates a schematic view of a set-up for coating and passivating the rebarin accordance with an embodiment of the present disclosure.
FIG. 3a & 3bshowsphoto of the rebars with non-passivated and passivated after 120 hrs of salt spray test respectively based on experimental analysis.
FIG. 4a & 4bshows photo of the rebars with non-passivated and passivated after 216 hrs of salt spray test respectivelybased on experimental analysis.

FIGS. 5a, 5b, & 5cshows photo of the rebars with passivated after 936, 984 and 1032 hrs of salt spray test respectivelybased on experimental analysis.
DETAILED DESCRIPTION OF THE DISCLOSURE
Various embodiments of the disclosure provides a passivation composition for plasma zinc coated rebar, the passivation composition comprising of chromium ion from chromium compound 5-40 (g/l), sulphate ion from a sulphate compound 2-10 (g/l), fluoride ion from a fluoride compound: 0.05 to 1 (g/l) and rest water to make one litre solution, all beingmixed together.
Another embodiments of the disclosure provides a process for coating a rebar, the process comprising steps of mechanically surface cleaning and/or pickling of the rebar, further cleaning the rebar using an electrolytic plasma technology, zinc coating on the rebar by electrolytic plasma technology, and applying a passivation composition over the zinc coated rebar. The passivation solution comprising of chromium ion from chromium compound 5-40 (g/l), sulphate ion from a sulphate compound 2-10 (g/l), fluoride ion from a fluoride compound: 0.05 to 1 (g/l), and rest water to make one litre solution and drying the rebar, all beingmixed together.
The present disclosure discloses a passivation composition for plasma zinc coated steel rebarfor preventing the rebar developing white rust, red rust and dullness.
Thepassivation composition comprises chromium ion from chromium compound 5-40 (g/l), a sulphate ion from a sulphate compound 2-10 (g/l), a fluoride ion from a fluoride compound 0.05 to 1 (g/l) and rest water to make one litre solution, all being mixed together.
The pH of the passivation composition is 0.5 to 2.0.The pH of the passivation composition is maintained by the addition of the sulphuric acid.

The chromium compound is selected from a group consisting of chromic anhydride, sodium chromate, potassium chromate, ammonium chromate, zinc chromate and other water soluble compounds of chromium in pH range 0.3 to 2.0.
Similarly the sulphate compound is selected from a group consisting of sulphuric acid, sodium sulphate, potassium sulphate, zinc sulphate, magnesium sulphate, ammonium sulphate and other water soluble compounds of sulphate in pH range 0.3 to 2.0.
Also the fluoride compound is selected from a group consisting of hydrofluoric acid, fluorosilicic acid and other water soluble compounds of fluoride in pH range 0.3 to 2.0.
Further no special sequence needs to be maintained while adding the components.
The colour of the passivation composition is deep brown and is the same when it is applied over the zinc coated the rebar.
Shown in FIG. 1 is the process (100) comprising steps for coating also including applying the passivation composition on the rebar.FIG. 2showsa schematic view of a set-up (200) for coating and passivating the rebar.
At step (104), mechanically surface cleaning and/or pickling of the rebar is done. Rebars are passed through a metal brushes (202) to remove the scale and loosely adherent dirt.
At step (108), the rebars are entered into a cleaning reactor (204) for further cleaning via plasma technology. Here sodium bicarbonate is used as cleaning electrolyte. The rebars act as cathode and two sets of anodes are provided. A large potential is set between the anode and the cathode (rebar), normally in the range of 25 to 250 V. The luminous plasma is generated at the rebar surface and the rebar is cleaned of contaminants and oxides. The plasma formed on the rebar in a liquid electrolyte results in a unique micro-roughness for coating adherence.
In accordance with other embodiment of the disclosure, the sodium bicarbonate can be sprayed / dipped over the rebars for cleaning.

At step (112), the rebars are passed through a coating reactor (208) for plasma coating. The reactor (208) is supplied with electrolytes containing the Zn ions to be deposited in a controlled method. The electrolyte used is zinc sulphate (10-30 %). A large potential is set between the anode and the cathode (rebar), normally in the range of 25 to 250 V. The luminous plasma is generated at the rebar and the rebar is coated with the zinc metal ions contained in the electrolyte.
Moreover the steps (108) and step (112) are guided by ASTM standard A1093/A1093M-15.
At step (116) the zinc coated rebars are applied with the passivation composition by passing through a passivation tank (212). The passivation tank (212) is supplied with the passivation composition as discussed above. In accordance with an embodiment of the disclosure, the passivation composition can be sprayed or the rebar can be dipped in the passivation composition.
The temperature of the passivation composition solution is in range of 25-50°C while applying on the zinc coated rebar.
At step (120), the rebar is dried by various heating methods such as hot air at a heating station (216) and collected at discharge end (not shown).
The passivation time from the step of applying the passivation composition on the zinc coated rebar (116) to drying the rebar (120) is 10 - 60 sec.
It should be appreciated the movement of the rebar from one station to other is carried over the roller tables (not shown).
Reaction Mechanism of passivation composition on zinc coated rebar:
The passivation composition primarily constitutes of chromic acid and hexavalent chromium salt. The chromic acid is itself a very strong oxidising agent which quickly passivates the plasma

zinc surface thereby forming zinc chromate. Therefore, to inhibit the passivation of zinc rebar surface by chromic acid, activators such as sulphate, nitrate,fluoride, etc., are added in the chromate solution. The chromate film is formed on rebar surface by the following proposed mechanism:

Shownin the above reaction mechanism, chromate coating is a conversion coating. The zinc dissolve due to anodic reaction and subsequently, the chromate compound precipitated as a corrosion product by electrochemical reaction. This corrosion product build by chromate coating gives secondary barrier action in the form of Cr2O3 . CrO3 . nH2Oon Plasma rebarsurface. The secondary barrier reduces the active surface area on zinc and delay the transport of oxidising agent, which inhibits the rate of corrosion reaction.


The chromate passivation composition was applied on rebar samples by dipping/spraying/passivation tank, and kept for drying as in accordance with process (100). The electrolyte for cleaning used is sodium bicarbonate and zinc sulphate was used for coating the
rebar.
All samples were uniform had brownish appearance. There was no visual difference among passivated samples.
Characterization of rebar: Periodically, the rebar samples were collected and were characterized. In this process, rebar from different lots were characterized.Rebar samples were tested for total Cr and Zn weight and the test results are in Table 3.


The corrosion resistance property of the sample was evaluated by salt spray test (SST). The samples were observed every day for white rust (WR) and red rust (RR) formation. The SST results are compiled and presented in Table 4.

Please note the samples were considered fail if the RR%≥10.
With reference to the detail shown in Table 3 and Table 4, the hexavalent chromium compound present in the chromate film shows the inhibiting action during atmospheric exposure or under salt spray test.
Shown in FIGS 3a & 3b is the photo of the rebars with non-passivated and passivated after
120 hrs of salt spray test respectively. This clearly shows that the non-passivated rebar
attracting rust, whereas the passivated rebars still resist corrosion.
Similarly shown in FIGS 4a & 4b is the photo of the rebars with non-passivated and passivated
after 216 hrs of salt spray test respectively. This clearly shows that the non-passivated rebar
attracting more and more rust and unfit to use, whereas the passivated rebars still resist
corrosion.
Subsequently shown in FIGS 5a, 5b & 5c is the photo of the rebars with passivated after 936, 984 and 1032 hrs of salt spray test respectively. This clearly shows that the passivated rebars still has a long life and good resistance against corrosion as compared to non-passivated rebars.
It is to be noted that the chromium ion concentration on rebar surface lies in range of 500-3500 mg/m2 which signifies the better resistance againstthe corrosion.
i"

More particularly, in salt spray test, the brown colour changes to colourless coating that indicate that the hexavalent chromium is reduced to insoluble trivalent chromium. These insoluble chromium are absorbed by the defects and therefore, further delay the corrosion reaction.
Similarly, when chromate coated rebars is exposed to atmosphere, the hexavalent chromium leaches slowly and hence the colour of the product changes from the brown to clear colour. Therefore, after leaching, the chromate film is mainly composed of the insoluble trivalent chromium and the oxides of zinc surface.
The longer time-of-wetness of chromated plasma rebar reduces its service life because the leaching of hexavalent chromium starts in moist atmosphere. Therefore, higher the hexavalent chromium in chromate film gives higher service life. The high chromate film thickness gives the high chromate concentration and therefore, the service life of the chromate coating depends on the chromate thickness.
Advantages:
The life of the plasma zinc coated rebar is increased to great extent after applying the chromate passivation. It is also evident from the salt spray test (SST) as it achieves 100-1000 hrs. corrosion resistance to develop 10% red rust.The composition when applied over the zinc coated rebar gives cost effective solution from red rust, white rust, and dullness.

we claim:
1. A passivation composition for plasma zinc coated rebar, the passivation composition
comprising:
chromium ion from chromium compound 5-40 (g/l); sulphate ion from a sulphate compound 2-10 (g/l); fluoride ion from a fluoride compound: 0.05 to 1 (g/l); and rest water, to make one litre solution, all beingmixed together.
2. The passivation composition as claimed in 1, wherein the pH of the passivation composition is 0.5 to 2.0.
3. The passivation composition as claimed in 2, wherein the sulphuric acid is added to maintain the pH.
4. The passivation composition as claimed in 1, wherein the hexavalent chromium compound is selected from a group consisting of chromic anhydride, sodium chromate, potassium chromate, ammonium chromate, zinc chromate and other water soluble compounds of chromium in pH range 0.3 to 2.0.
5. The passivation composition as claimed in 1, wherein the sulphate compound is selected from a group consisting of sulphuric acid, sodium sulphate, potassium sulphate, zinc sulphate, magnesium sulphate, ammonium sulphate and other water soluble compounds of sulphate in pH range 0.3 to 2.0.
6. The passivation composition as claimed in 1, wherein the fluoride compound is selected from a group consisting of hydrofluoric acid, fluorosilicic acid and other water soluble compounds of fluoride in pH range 0.3 to 2.0.

7. The passivation composition as claimed in 1, wherein the colour of the passivation composition is brown.
8. A process for coating a rebar, the process comprising steps of:
mechanically surface cleaning and/or pickling of the rebar;
further cleaning the rebar using an electrolytic plasma technology;
zinc coating on the rebar by electrolytic plasma technology;
applying a passivation composition over the zinc coated rebar; the passivation composition comprising of chromium ion from chromium compound 5-40 (g/l), sulphate ion from a sulphate compound 2-10 (g/l), fluoride ion from a fluoride compound 0.05 to 1 (g/l), and rest water to make one litre solution, all beingmixed together, and
drying the rebar.
9. The process as claimed in claim 8, wherein the electrolyte for further cleaning is sodium bicarbonate.
10. The process as claimed in claim 8, wherein the passivation composition solution temp, is in range of 25-50°C while applying over the zinc coated rebar.
11. The process as claimed in claim 8, wherein the passivation timing is 10-60 sec.

12. The process as claimed in claim 8, wherein the chromium ion concentration on rebar surface is in range of 500-3500 mg/m2.
13. Rebar having salt spray test (SST) life of 100-1000 hrs.processed by the process as claimed in claims 8-12.