TITLE:
A green passivation composition for plasma zinc coated rebar.
FIELD OF THE INVENTION:
This invention relates to a green passivation composition for plasma zinc coated
rebar and a process for preparing the same.
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 INVENTION:
Generally, fresh produced rebars are covered with oxide scale and has blackish
luster. 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 conventional 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 µm 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.
OBJECTS OF THE INVENTION:
An object of the present invention is 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.
Still another object is to develop such composition that when applied over the
zinc coated rebar that prevents pin hole problem on the rebar.
Further 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.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided passivation composition for plasma
zinc coated rebar comprising chromium ion 5-25 (g/l), sulphate ion 5-10 (g/l),
fluoride ion 0.05 to 1 (g/l), nitrate ion 1-10 (g/l), phosphate ion 5-30 (g/l); and rest
water to make one liter solution.
Another embodiment of the disclosure provides a process for coating steel rebar
comprising steps of mechanically surface cleaning and/or acid pickling of steel
rebar, further cleaning of steel rebar by electrolytic plasma technology, zinc
coating on steel rebar by electrolytic plasma technology, applying the said
passivation composition over zinc coated steel rebar drying steel rebar.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
FIG. 1 illustrates flow chart showing a process for coating and passivizingsteel
rebar in accordance with embodiments of the present disclosure.
FIG. 2 illustrates a schematic view of a set-up for Zinc Coating and chromate
passivation on the steel rebarin accordance with embodiments of the present
disclosure.
FIG. 3shows photos of the rebars with non-passivized and passivized after 240h
of salt spray test for experimental analysis accordance with embodiments of the
present disclosure.
DETAILED DESCRIPTION OF THE INVENTION:
Various embodiments of the disclosure provides a passivation composition for
plasma zinc coated rebar, the passivation composition comprising:chromium ion
from a chromium compound 5-25 (g/l); sulphate ion from a sulphate compound 5-
10 (g/l);fluoride ion from a fluoride compound: 0.05 to 1 (g/l); nitrate ion from a
nitrate compound: 1-10 (g/l);phosphate ion from a phosphate compound: 5-30
(g/l); andrest water, to make one liter solution, all being mixed together.

Another embodiment of the disclosure providea process for coating steel rebar,
the process comprising steps of: mechanically surface cleaning and/or acid
pickling of steel rebar;further cleaning of steel rebar by electrolytic plasma
technology;zinc coating on steel rebar by electrolytic plasma technology;applying
a passivation composition over zinc coated steel rebar; the passivation
composition comprising chromium ion from a chromium compound 5-25 (g/l),
sulphate ion from a sulphate compound 5-10 (g/l), fluoride ion from a fluoride
compound 0.05 to 1 (g/l), nitrate ion from a nitrate compound: 1-10 (g/l),
phosphate ion from a phosphate compound: 5-30 (g/l) and rest water to make
one liter solution, all being mixed together, anddrying steel rebar.
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 a chromium
compound 5-25 (g/l), sulphate ion from a sulphate compound 5-10 (g/l), fluoride
ion from a fluoride compound 0.05- 1 (g/l), nitrate ion from a nitrate compound: 1-
10 (g/l), phosphate ion from a phosphate compound: 5-30 (g/l) and rest water to

make one liter solution, all being mixed together.The pH of the passivation
composition is 0.3 to 2.0.
Sulfuric acid is added to maintain pH.
The chromium compound is selected from a group 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 of sulfuric 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.
The nitrate ion is selected from a group of nitric acid, sodium nitrate, potassium
nitrate, zinc nitrate, magnesium nitrate, ammonium nitrate and other water
soluble compounds of nitrate in pH range 0.3 to 2.0.
Also the fluoride compound is selected from a group of hydrofluoric acid,
fluorosilic acid and other water soluble compounds of fluoride in pH range 0.3 to
2.0.

The phosphate compound is selected from a group of phosphoric acid, sodium
phosphate, potassium phosphate, zinc phosphate, magnesium phosphate,
ammonium phosphate and other water soluble compounds of phosphate in pH
range 0.3 to 2.0
Further no special sequence needs to be maintained while adding the
components.
Shown in FIG. 1 is a process (100) comprising steps for coating and applying the
passivation composition on the rebar.FIG. 2 shows a schematic view of a set-up
(200) for zinc coating and chromate passivation on the rebar.
At step (104), mechanically surface cleaning and/or pickling of the steel 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 electrolytic 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 one of the embodiment of the disclosure, the sodium
bicarbonate can be sprayed / dipped over the rebars for further cleaning.
At step (112), the rebars are passed through a coating reactor (208) for
electrolytic 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 % by wt). 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.
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 said 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 25-60°C
while applying on the zinc coated rebar.
At step (120), the rebar is dried. Drying can be done by various heating methods
such as hot air at a heating station (216), inspected 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 5 - 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 oxidizing agent which
quickly passivizes 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:
(i) Corrosion of metal by sulfuric acid
Anodic reaction: Zn -n+ + ne
Cathodic reaction: 2H + + 2e -► H2
(ii) Dissociation of H2Cr2O 7 ions (reduction of Cr +6 to Cr + )
2HCr2O"7 -► 2CrO4 2- + 2H +
CrO4" + 8H + + 3e -► Cr+3 +4H2O
3H2 + HCr2O"7 -► 2Cr(OH)3 + OH -
(iii) Precipitation of Cr +6 and Cr + compounds
2Cr(OH)3 + CrO4 2- + 2H + -► Cr(OH)3 . Cr(OH) CrO4
Cr2O3. CrO3. nH2O
(iv) Phosphate ions in formulation
PO43- -> CrPO4. 4H2O +Cr2O3. nH2O

Shown in 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 on plasma rebarsurface. The secondary barrier reduces the active
surface area on zinc and delay the transport of oxidizing agent, which inhibits the
rate of corrosion reaction.
Chromate based conversion coating develops a thin film but has good corrosion
resistance properties; the color depends on chromium concentration in chromate
solution. The phosphate based conversion coating develops a thick film but has
poor corrosion resistance. The developed coating is thick and corrosion resistant
that may be due to synergic effect.
It forms green passive film over plasma rebar which is maintained by periodic
addition of phosphoric and nitric acid that acts as activators in the chromate-
phosphate bath. This presence of chromium and phosphorus along with fluoride
increases the film thickness due to increase in mobility of Zn2+ ion. This favors
the formulation of more trivalent and lesser hexavalent chromium compound on
coating.

Experimental Analysis
Fivetypes of passivation formulation were prepared (given in Table 1) with
varying concentration of chromium compound and phosphate compound. Its
objective was to develop a green passivation layer with high corrosion
resistance.
The chromate passivation compositions were applied on rebar samples by
dipping/spraying/passivation tank in accordance with process (100), and kept for
drying. The formulation 2, 3, &4 develop greenish appearance whereas brown
appearance was developed in formulation 1 and 5.
It was observed that the high chromate concentration has high corrosion
resistance. The corrosion resistance decrease with decreasing the chromate ion
concentration. Therefore, the chromium ion concentration was optimized to about
5-25 (g/l) to get green passivation with desired corrosion resistance. The
chromate-phosphate film was comprised of mostly trivalent compound with small
amount of hexavalent chromium. The trivalent chromium is present as an
insoluble hydrated oxide whereas the hexavalent chromium gives self-healing
properties during atmospheric exposure or chloride attack. In addition, a
chromate-phosphate complex compound in the film also gets formed that
enhances the barrier protection.


The chromate coated samples were characterized in terms of zinc coating
weight, total chromium and salt spray life as shown in Table 2. The salt spray life
is considered fail when the samples got about 10 % red rust on whole surface in
visual examination.
Table 2: Sample characterization forzinc coating weight, total chromium and salt
spray life


The corrosion resistance properties of the samples were evaluated by salt spray
test (SST). The samples were observed after 240 hfor red rust (RR) formation.
The SST results are compiled and presented in FIGS. 3for sample 1-6.
The photos clearly show that the non-passivized rebar sample 6 gets rusted
easily, whereas the passivized rebars resist corrosion.Higher the chromate
concentration higher is the red rust resistance. It can be noted that there is only
one difference between formulation 1 and 5; Formulation 1 contained 31 g/l
chromium ion whereas formulation 5 contained 5.2 g/l chromium ion. The higher
concentration of chromium in formulation 1 resulted in higher concentration of
total chromium on rebar surface in sample 1 with respect to sample 5 by

formulation 5. Hence, the salt spray life of sample 1 and 5 are 456 & 216 hours
respectively.
The fixed amount of phosphoric and nitric acids were added in formulation 2,3
and 4 but with varying in concentration of chromium trioxide. The formulation 2,3
and 4 are used for rebar passivation and their samples were name as sample 2,3
and 4 respectively. It can be seen that the sample 2 and 3 are not rusted after
240 h but sample 4 got red rusted, which contained least chromium trioxide
compound. It indicates that green color passivation required appropriate
combination of chromate and phosphate compound to develop a corrosion
resistance film as found in formulations 2 and 3.
It is to be noted that the chromium ion concentration on rebar surface lies in
range of 500-3000 mg/m2 which signifies the better resistance against the
corrosion. But, lower the chromium lesser the corrosion resistance.

Advantages:
The life of the plasma zinc coated rebar is increased to great extent after
applying the green chromate passivation. It is also evident from the salt spray
test (SST) as it achieves more than 240 h 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 a chromium compound 5-25 (g/l);
sulphate ion from a sulphate compound 5-10 (g/l);
fluoride ion from a fluoride compound: 0.05 to 1 (g/l);
nitrate ion from a nitrate compound: 1-10 (g/l);
phosphate ion from a phosphate compound: 5-30 (g/l); and
rest water to make one liter solution, all beingmixed together.
2. The passivation composition as claimed in 1, wherein pH of the passivation
composition is 0.3 to 2.0.
3. The passivation composition as claimed in 2, wherein sulfuric acid is added to
maintain pH.
4. The passivation composition as claimed in 1, wherein the chromium compound is
selected from a group comprising 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 comprising 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 comprising of hydrofluoric acid, fluorosilic 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 phosphate compound
is selected from a group comprising of phosphoric acid, sodium phosphate,
potassium phosphate, zinc phosphate, magnesium phosphate, ammonium
phosphate and other water soluble compounds of phosphate in pH range 0.3 to
2.0
8. The passivation composition claimed in 1, wherein the nitrate ion is selected from
a groupcomprising of nitric acid, sodium nitrate, potassium nitrate, zinc nitrate,
magnesium nitrate, ammonium nitrate and other water soluble compounds of
nitrate in pH range 0.3 to 2.0.

9. A process for coating steel rebar, the process comprising steps of:
mechanically surface cleaning and/or acid pickling of steel rebar;
further cleaning of steel rebar by electrolytic plasma technology;
zinc coating on steel rebar by electrolytic plasma technology;
applying a passivation composition over zinc coated steel rebar; the passivation
composition comprising chromium ion from a chromium compound 5-25 (g/l),
sulphate ion from a sulphate compound 5-10 (g/l), fluoride ion from a fluoride
compound 0.05 to 1 (g/l), nitrate ion from a nitrate compound: 1-10 (g/l),
phosphate ion from a phosphate compound: 5-30 (g/l) and rest water to make
one liter solution, all beingmixed together, and
drying steel rebar.
10. The process as claimed in claim 9, wherein electrolyte for further cleaning by
electrolytic plasma technology of steel rebar is sodium bicarbonate.
11. The process as claimed in claim 9, wherein the passivation composition
solution temp. is in range of 25-60°C while applying over zinc coated steel rebar.
12. The process as claimed in claim 9, wherein passivation timing is 5-60 sec.

13. The process as claimed in claim 9, wherein chromium ion concentration on
steel rebar is in range of 500-3000 mg/m2.
14. A zinc coated rebar passivized with the composition as claimed in any of the
claims 9-13 is green in color.