TITLE: A chromium-free eco-friendly coating for Metal Substrates.
FIELD OF INVENTION:
This invention relates to an eco-friendly, chromium-free corrosion resistance
polymer coating composition for metallic surfaces such as galvanealed steel
substrate.
Further, the invention specifically relates to coating that can provide corrosion
resistance against a fuel composition.
BACKGROUND OF THE INVENTION:
Corrosion is a well known problem relating to metallic structure, for example
steel. Different approaches may be involved in order to prevent or reduce
corrosion. Out of different approaches, protective coating on metal surface plays
an important role. Protective coating for steel sections is typically based upon
paints, organic layers, ceramic and inorganic layers, plastic lining and
electroplating. Auto industry use different types of steel grade as major raw
material and corrosion protection is a major problem for this industry. Various
steel components used in the automotive industry are exposed in different
environments. Fuel tank is one of the components continuously exposed in petrol
and acidic corrosive environment.

The gasoline (commonly known as petrol in India) is the main fuel material for
two wheelers. The gasoline turns acidic on storage and accelerates corrosion in a
metallic fuel tank and hence, selecting a right material for the fuel tank is of very
high importance. Tata steel has been providing one side hexavalent chrome
passivated galvannealed material as fuel tank material for two wheeler
components. Galvannealed sheet is a carbon steel sheet coated with zinc on both
sides by continuous hot dip process. As the strip exits the coating bath, the zinc
coating is immediately subjected to an in-line heat treatment that converts the
entire coating to a zinc-iron alloy. The intermetallic layer of Fe+ Zn (with an iron
content of 8-12%), as a result of the diffusion of iron into zinc, imparts better
weldability compared to plain galvanized steel and also better corrosion
resistance. Conversion to the alloy results in a non-spangle matte finish which
makes the sheet suitable for painting after fabrication. After painting with proper
selected primers and paints, the combined paint/galvannealed coating offers
excellent resistance to peeling and/or blistering of the paint in addition to long
lasting corrosion resistance compared with conventional galvanized sheet. The
zinc-iron alloy coated surface also has an improved welding behavior. However,
the coating is more brittle than a regular galvanized coating, as the harder
coating is susceptible to powdering if the sheet is severely formed during

fabrication. After hexavalent chrome passivation (80-120mg Cr/m2) galvannealed
sheet provides good corrosion and gasoline resistance. But hexavalent chromium
is hazardous material and due to different environmental regulation, use of
chrome passivation is going to be banned in most of the states. Hence, there is a
need of a chrome free polymer coating for galvannealed substrate which can
provide good corrosion and gasoline resistance.
The JFE invention JP2007070572 discloses a coating material composition that
has improved rust prevention property in the heat affected zone. The
composition includes an acryl amino type thermosetting resin, a silicate
compound and thiazole type compound, and/or silane coupling agent. The Posco
patent KR2Q090070024 discloses a chromium-free processing liquid based upon
a binder resin, silicate, silane, titanium, urethane resin and epoxy resin, titanium
carbonate, and titanium phosphate. The invention claims chromium-free surface-
treated steel plate for use in fuel tank and comprises chromium-free layer and
electro-galvanized zinc-based metal plated steel sheet.
Patent applications KR20090065020 and WO2007075050 also disclose surface
treated Cr-free steel sheet for use in a fuel tank. All these inventions claim good
corrosion resistance and adhesion property.
In light of the above discussed prior art, there is a need of the chrome free
coating which has high corrosion resistance and good adhesion property.

Further, the coating should be easily applied on galvaneealed substrate by well
known techniques such as spray, brush and roll coater. Further, the chrome free
coating should have good forming properties on the metallic substrate and dry
fast.
OBJECTS OF THE INVENTION:
An object of the present invention is to prepare an eco-friendly corrosion
resistance coating for steel substrate;
Another object of the present invention is to prepare a coating which can provide
gasoline resistance to coated steel sheet;
Still another object of this invention is to develop an eco-friendly coating that
provides a uniform coating on steel surface, when applied by spray, brush or roll
coater;
Further object of this invention is to provide an eco-friendly coating composition
which can dry below 90° Centigrade within 10 to 30 seconds;

Another object of the invention is to provide a steel sheet coated with an eco-
friendly, chromium free and gasoline resistance coating composition.
DETAILED DESCRIPTION OF THE INVENTION:
According to this invention, there is provided an eco-friendly corrosion resistance
coating for steel substrates that comprises 10-60% by weight of water based
poly condensed silane solution, 0.5-2% by weight of a neutralizing agent, 0.2-
2% by weight of a flash rust inhibitor, 0.25-3% by weight of a chelating agent,
1-10% by weight of a flexibiliser, 0.25-2% by weight of a defoamer and a
leveling 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 steel substrate. The eco-
friendly coating comprising at least one organofunctional silanes group is applied
on to the steel substrate. Thereafter, the coating mixture is cured so as to
provide a dense network structure of coating for protection of the substrate from
corrosion and gasoline environment.

The primary organofunctional silane compound used in the embodiments of the
invention includes hydrolised water soluble sol-gel solution prepared from
solution, modified with organofunctional silanes like
glycidoxypropyltrimethoxysilane(GPTMS) tetraethoxy silane(TEOS), vinyl
trimethoxy silane(VTMS) and aminopropyl triethoxy silane. Beside
organofunctional group, it also contains organic functionality based on Si bond
epoxy group. Ammonia or amine derivative are used for the adjustment of pH
value of the water-borne sol-gel solution.
The water-borne sol-gel solution has following physical properties,
Density (200C): approx. 1.14 gm/ cm3
viscosity (200C): approx. 7 mPa.s
boiling point: approx 960C
pH-value: approx 4.3
solid content: approx 35%
Chelating agents are used to enhance the bond strength between polymer and
metal. The present invention includes a Afunctional organosilane chelating agent
having a reactive organic mercapto and a hydrolysable inorganic methoxysilyl
functional group. The preferred example of the chelating agent include, but not
limited to, 3-mercaptopropyltrimethoxysilane.

In order to increase the flexibility of the coating, an additional additive like
polyester urethane may be added in the coating formulation. Any polyester
urethane with following properties can be used as flexibiliser:
Density (20°C): approx. 1.04 gm/ cm3
viscosity (20°C): approx. < 500 mPa.s
pH-value: approx 9.5
solid content: approx 35%
The preferred example of the flexibiliser, but not limited to, is a reaction product
of triethyl amine and n-ethyl-2-pyrrolidone.
Amine or its derivative is used as a neutralizing agent. The said neutralizing
agent maintains the pH-value of the coating composition in the range of 7 to 8.
Any neutralizing agent having the following properties can be used in the present
invention:
Density (200C): approx. 0.968 gm/ cm3
viscosity (200C): approx. 18 mPa.s
boiling point: approx 960C
pH-value: approx 12.6
The preferred example of the neutralizing agent includes, but not limited to,
beta-hydroxyethyl dimethyl amine.

The coating mixture may comprise 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 may be used as per the present invention include compounds such as
sodium nitrite, benzotriazole and a mixture of one or more of 10-25% of C12-C14
(2-benzothiazolylthio) succinic acid tertiary amine salts, 10-25% of ethoxylated
tridecylalcohol phosphate-comprising monoethanolamine salts, 10-25% of zinc
salts of branched (C6-C19) fatty acids, <2.5% of zinc salts of naphthenic acid, 10-
25% morpholine benzoate. The flash rust inhibitor used in the present invention
has the following physical properties:
Density (200C): approx. 1.04 gm/ cm3
viscosity (200C): < 200 mPa.s
pH-value: 8-10
A 1% silicone solution or similar commercial product can be used as a leveling
and defoamer.
In an embodiment of the invention, the coating mixture may comprise a
corrosion resistance pigment. Preferred examples of the corrosion resistance
pigments as per the present invention include nano zinc oxide, nano silica, nano
alumina, nano cerium oxide, zinc phosphate, zinc phosphate derivatives or any
combination thereof.

The eco-friendly corrosion resistant coating of the invention can be applied to
steel substrates using any of well-known coating methods such as dipping,
spraying, roll coating and brush coating. The coating weight of the composition
is not particularly limited. However, the coating is applied so as to give a coating
thickness in the range of 500nm to 20 micron.
The curing temperature of the coating can be reduced by including additives
containing a polyimide derivative.
EXAMPLES:
The present invention will be described with reference to the following examples.
These examples are provided only for illustrating the present invention and
should not be construed as limiting the scope of the present invention.
Commercially available galvanealed steel substrate was used as the metal
material for applying the coating of the present invention.
Examplel


The coating formulation of example 1 when applied on galvanealed steel
substrate provides uniform coating with 0.5 to 2 micron dry film thickness. The
coating formulation provided more than 300 hours SST life as per ASTM B117
standard.

The coating formulation of example 2 when applied on galvanealed steel
substrate provides uniform coating with 0.5 to 2 micron dry film thickness. The
coating formulation provided more than 500 hours SST life as per ASTM B117

standard. Also, the coated galvanealed steel sheet provides more than one year
resistance to petrol in static exposure conditions.
The disclosed eco-friendly coating of the present invention provides a very good
adhesion and corrosion resistance to galvanealed steel substrate. Further, the
coated galvanealed steel substrate is weldable and formable and can withstand
corrosion resistance to petrol and acidic environment.

WE CLAIM:
1. A chromium-free coating composition for treating a metal-surface, the coating
composition comprising:
10-60 percentage by weight of a water based poly condensed silane
solution comprising of silica solution, modified with organo-functional
silanes;
0.25-3 percentage by weight of a bifunctional organosilane chelating
agent having a reactive organic mercapto and a hydrolysable inorganic
methoxysilyl functional group;
1-10 percentage by weight of an aliphatic polyester polyurethane resin
flexibiliser;
0.5-2 percentage by weight of a neutralizing agent;
0.2-2 percentage by weight of a flash rust inhibitor;
0.25-2 percentage by weight of a defoamer and a leveling agent; and
the balance being water based upon 100 percent by weight of the total
coating composition.
2. The coating composition as claimed in claim 1, wherein the neutralizing agent
is an amine or its derivative.
3. The coating composition as claimed in claim 1, wherein the neutralizing agent
is beta-hydroxyethyl dimethyl amine.

4. The coating composition as claimed in claim 1, wherein the flash rust inhibitor
is selected from the group consisting of sodium nitrite, benzotriazole and a
mixture of one or more of 10-25% of C12-C14 (2-benzothiazolylthio) succinic acid
tertiary amine salts, 10-25% of ethoxylated tridecylalcohol phosphate-comprising
monoethanolamine salts, 10-25% of zinc salts of branched (C6-C19) fatty acids,
<2.5% of zinc salts of naphthenic acid, 10-25% morpholine benzoate.
5. The coating composition as claimed in claim 1, wherein the chelating agent is
3-mercaptopropyltrimethoxysilane.
6. The coating composition as claimed in claim 1, wherein the flexibiliser is
reaction product of triethyl amine and n-ethyl-2-pyrrolidone.

7. The coating composition as claimed in claim 1, wherein said leveling and
defoamer is silicon solution or its derivatives.
8. The coating composition as claimed in claim 1, wherein the coating
composition further comprises of a nano-pigment.
9. The coating composition as claimed in claim 1, wherein the nano-pigment is
selected from the group consisting of zinc oxide, silica, alumina, cerium oxide or
any combination thereof.
10. A steel substrate which is coated with the chromium-free coating composition
of claim 1.

11. A fuel tank which is coated with the chromium-free coating composition of
claim 1.
12. The steel substrate as claimed in claim 10, wherein coating thickness of the
coating composition is in the range of 500 nm to 20 micron.
13. The coating composition as claimed in claim 1, wherein the pH of the coating
composition is in the range of 7 to 8.