FIELD OF THE INVENTION:
This invention relates to an environment friendly, water based temporary
protection coating composition for thermo mechanically treated steel rebars.
Further, the invention specifically relates to a coating that can provide
corrosion resistance against saline and humid environment.
BACKGROUND OF THE INVENTION:
Thermo mechanically treated rebars are commonly known as "TMT rebars"
and are produced in a coil form. The consumers have requirements of
straight TMT rebars of different lengths. During straightening operation of the
rebars, some protective oxide scale get removed from the surface and the
shiny bright surface is exposed to open atmosphere. Due to electrochemical
reaction with the environment, a red rust is formed on the rebar surface.
Although, it is not very detrimental from corrosion point of view, but
aesthetically it is not acceptable to the consumers. 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 of the rebars during transportation and storage. Different
methods are available to reduce the corrosion rate of steel surface for a
shorter period of time. Some of the well accepted and economical methods
are as follows:-
1. Thermoplastic coating: This can be applied by dissolving thermoplastic
resin in a solvent medium. After evaporation of the solvent, a polymeric film
is developed on the surface of steel. Depending upon the chemistry and
thickness of thermoplastic coating, a steel substrate can be protected for
some period of time. There are two major difficulties associated with this
method, firstly, the removal of thermoplastic coating before any further
application or fabrication of the steel substrate and secondly the inherent
solvent hazard associated with this method.

2. Use of rust preventing (RP) oils: Different types of rust preventive oils are
available, which can be applied on the steel surface by known dipping or
spraying method. Depending upon the chemistry and inhibitors of RP oils, a
steel substrate can be protected from corrosion for a period of 2-3 months.
This method also needs proper protection from water and moisture,
otherwise saponification and emulsification of RP oils will damage the
surface quality of the steel substrate.
3. Application of different inhibitors: There are different inhibitors for
example.volatile corrosion inhibitors (VCI) and volatile protective
inhibitors(VPI), which can continuously release anticorrosive vapor. When
the metal comes in contact with the vapor, a protective film is on formed
metal substrate. However, this method is not economical for protection of
large scale the materials such as rebar and coils.
Corrosion on rebar is a long lasting problem. A lot of research work has
been done to improve the corrosion resistance property of the rebars by
applying different types of coatings. Research work has been done on
different types of phospating, plating and polymeric compound based
coatings.
U.S. Pat. No. 4,659,395 to Sugama et al, teaches 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. Pat. No. 6,117,251 discloses 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 become too tacky.

During electro less process ("electro less plating solutions"), a metal can be
coated over a catalytically active surface by chemical reduction method. The
reduction and deposition process is carried out without external electric
circuit. Electro less nickel is used in a wide range of industrial applications.
Ni-B, Ni-P and Ni-W coatings are recognized in the art for their corrosion
resistance, hardness and associated wear-resistance. See, for example,
U.S. Pat. Nos. 2,726,170; 3,045,334; 3,378,400; 3,738,849; 3,674,447;
3,432,338; 3,918,137; 4,395,442; 4,567,066; 4,706616; 4,724,819;
5,019,163 5,148,780; and 6,178,306.
Electroplating of nickel was also tried in the art for temporary protection of
the rebars. Typical electro less nickel plating solutions generally comprise an
aqueous nickel salt, an aqueous alloying salt (if an alloy is present), a
reducing agent, and a chelating or complexing agent along with some buffer.
Different functional additives may also be added in relatively low
concentrations to enhance various characteristics of the solution of plated
article. The pH sensitive, temperature specific and stability of the nickel bath
and economic factors associated with the process do allow the process for
continuous coating of steel substrate like coils or rebars..
Prior art also describe coating of rebars for temporary protection by
inorganic or organic inhibitor coating, some of the prior art are
CN102491668, CN103304178, IN2003DE01187, JP02159390,
CN102070787, CN102173639, CN102199014 and CN102584077. In the
cited documents, the inventors tried to develop a temporary protection
coating on rebar surface using different inorganic acid, sodium hydroxide,
chromium salt, sodium stannate salt along with various organic amine at
high concentration.
The above specified prior art includes use of one or more corrosive and
hazardous chemicals. Further, the disclosed coatings need multiple steps for
coating application and are dried at higher temperature. However, some of
the polymer coatings are responsible for decreasing the bond strength of the
coated rebar with concrete surface.

In light of the state of prior-art as discussed hereinabove, it is evident that
there is need to develop a temporary coating on rebars that can be applied
easily, economically and without any damage to the environment. Further,
the coating should be such that the coated rebars can be used without any
further treatment or cleaning. There should not be any adverse effect during
adhesion of the rebars with concrete because of use of the anticorrosive
coating.
OBJECTS OF THE INVENTION:
It is therefore an object of the present invention to propose an eco-friendly
water based corrosion resistance coating for thermo-mechanically treated
(TMT) rebar substrate which can provide temporary protection up to three
months during storage and transportation.
Another object of the present invention is to propose an eco-friendly water
based corrosion resistance coating for thermo-mechanically treated (TMT)
rebar substrate, which can provide resistance to the rebars under saline and
humid environment.
Still another object of the invention is to propose an eco-friendly water based
corrosion resistance coating for thermo-mechanically treated (TMT) rebar
substrate, that provides a uniform coating on the steel surface when applied
by spray or dipping process.
Yet another object of this invention is to propose an eco-friendly water based
corrosion resistance coating for thermo-mechanically treated (TMT) rebar
substrate, which gets dried-up fast on rebar surface..
A further object of the invention is to propose an eco-friendly water based
corrosion resistance coating for thermo-mechanically treated (TMT) rebar
substrate, which can be applied without any surface preparation such as
acid pickling, soak cleaning, and water rinsing.

A still further object of the invention is to propose an eco-friendly water
based corrosion resistance coating for thermo-mechanically treated (TMT)
rebar substrate, which provides a temporary protection on the rebar without
reducing the bonding strength of the rebar with concrete material.
DETAILED DESCRIPTION OF THE INVENTION:
According to this invention, there is provided an eco-friendly corrosion
resistance coating for steel or metallic substrates that comprises 10 to 50%
by weight of water based silicate resin, 0.25 to 0.75% by weight of a flash
rust inhibitor, 0.5 to 1.5% by weight of a curing agent, 0.5 to 1.5 % by weight
of a film forming agent, The remaining portion of the coating is water based
upon 100 percent by weight of the total coating composition.
According to the invention, one or more of the objectives of the invention
may be achieved by applying the eco-friendly coating on TMT rebar
substrate. The eco-friendly coating comprising at least one silicate group is
applied on the TMT substrate. The present invention uses compounds such
as sodium silicate or its derivatives. In an embodiment of the invention, a
sodium silicate based resin is used. In an other embodiment of the present
invention, the water based silicate solution preferably varies in the range of
25 to 35 weight %. Thereafter, the coating mixture is cured so as to develop
a thermosetting network structure of coating for protection of the substrate
from corrosion.
The coating mixture comprises an aqueous solution of flash rust inhibitor.
The flash rust inhibitor is used to restrict or delay the onset of under film
corrosion or pre cure corrosion. Preferred examples of the flash rust
inhibitors that can be used as per the present invention include compounds
such as sodium nitrite, benzotriazole, and sodium gluconate or any
combination thereof.

Curing agent is used to accelerate the rate of reaction and to reduce the
curing temperature. The present invention teaches a curing agent such as
imidazole group or its derivatives or butyl di-ethanol amine or any
combination thereof.
The present invention shows use of a film forming agent, which gets
adsorbed or absorbed on the steel substrate and enhances the film
thickness of the applied coating. This film forming agent includes ethylene
glycol or poly vinyl alcohol or ethyl silicate or any combination thereof.
The eco-friendly corrosion resistant coating of the invention can be applied
to steel substrates such as TMT rebars using any of well-known coating
methods such as dipping or spraying. However, the coating is applied so as
to maintain a coating thickness in the range of 1 to 10 um (when measured
by weight difference method).

The coating formulation of example 1 when applied on TMT steel rebar
substrate provides a uniform coating to be dried within 10 minutes at 35°C.
The coating formulation provides a dry film thickness of 1-2 um(when
measured by weight difference method) and less than 24 hours red rust
resistance life as per ASTM B117 standard.


The coating formulation of example 2 when applied on a TMT rebar
substrate provides a uniform coating with dry film thickness of 8-10 um(
when measured by weight difference method) and gets dried up within 10
minutes at 35°C (tested as per ASTM D5859). The coating formulation
provides more than 48 h red rust resistance life as per ASTM B117
standard.
The disclosed eco-friendly coating of the present invention provides a very
good adhesion (showing 5B rating as per ASTM D3359 test) and temporary
corrosion protection to TMT rebars substrate. Further, the coated TMT rebar
is not reduced in terms of the bond strength with concrete, as compared to
bond strength of uncoated rebar with same concrete (as shown in
figure1).The new coating on TMT rebar can be applied by dipping or
spraying process to be dried within 10 minutes at 35°C. Above preferred
examples of coatings on TMT rebars are given to illustrate the invention
however, the inventive coating formulation can be used on any other steel
substrates as well as where temporary corrosion resistance is needed. The
given examples should not be construed as limiting but rather enabling the
invention.

WE CLAIM:
1. An eco-friendly corrosion resistance water based coating formulation
for steel substrates, the coating formulation comprising:
10 to 50% by weight of a water based silicate solution;
0.25 to 0.75% by weight of a flash rust inhibitor;
0.5 to 1.5% by weight of a curing agent;
0.5 to 1.5% by weight of a film forming agent; and
the remaining part is de-mineralized water.
2. The coating formulation as claimed in claim 1, wherein said water
based silicate solution is sodium silicate and or its derivative.
3. The coating formulation as claimed in claim 1, wherein said curing
agent is selected from a group consisting of imidazole group or its
derivatives, butyl diethanol amine and a mixture thereof.
4. The coating formulation as claimed in claim 1, wherein the said flash
rust inhibitor is selected from a group consisting of sodium nitrite,
benzotriazole, and sodium gluconate.
5. The coating formulation as claimed in claim 1, wherein the film
forming agent is selected from a group consisting of ethylene glycol,
poly vinyl alcohol, ethyl silicate and a combination thereof.

6. The coating formulation as claimed in claim 1, wherein the coating
formulation provides corrosion resistance of up to 48 hours in a salt
spray chamber.
7. The coating formulation as claimed in claim 1, wherein the steel
substrate is a thermo mechanically treated TMT rebar.

8. The coating formulation as claimed in claim 1, wherein the water
based silicate solution preferably varies in the range of 25 to 35
weight %.
9. The coating formulation as claimed in claim 1, wherein the coating
gets dried-up on steel substrate within 10 minutes at 35°C.

ABSTRACT

A water based environment friendly coating for coating a rebar
surface.
This invention relates to an environment friendly, water based temporary
protection coating composition for treating thermo mechanically treated steel
rebars. The silicate based coating provides corrosion resistance to TMT
rebars up to at least three months of storage and transportation. The coating
formulation of the present invention can be easily applied on rebar surface
and gets dried up within 10 minutes at 35°C.