FIELD OF THE INVENTION:
The present invention discloses the composition of self-healing and self-cleaning enamel
and method of depositing enamel powder over steel substrate. The method involves
preparation of the surface of the metal substrate is by multistep cleaning and
sandblasting; wet flow deposition of enamel frit and its firing; electrostatic spray
application of surface modified vitreous enamel; and temperature controlled heat
treatment and cooled down of the substrate. The result of these processes enables the
steel substrate to have the water repelling ability, corrosion resistance, self-healing
ability and a slick finish that can withstand severe environments.
BACKGROUND OF THE INVENTION AND PRIOR ARTS:
The enameling technique has long been used to protect metallic substrates from
corrosion and to improve aesthetic functions. The vitreous or porcelain enamel mainly
consist of alkaline alumina-borosilicate with other inorganic substances may be added
to provide desirable physical properties such as acid-alkaline resistance, heat corrosion,
and abrasion. The enamel coated steel is a composite material comprising a base metal
and a glassy coating bonded chemically by fusion. Since enamel coating provides a
smooth appearance, a high order of clean-ability, resistance to vegetative growth and

long service life, it has been widely employed for domestic and industrial applications
like domestic cookers, stoves, refrigerators, heaters, geysers, etc.
Various approaches have been suggested to deposit enamel on metal substrates
thereon and to modify its properties to make it defect free. US patent, US3282685, for
“low carbon steel alloy for vitreous enameling” is directed to a very low carbon vitreous
enameling steel having a controlled etching rate as a result of a controlled range of
residual alloy content.
US patent, US2755210, discloses a method of treating iron or mild steel to promote the
adherence of porcelain enamel and stock so produced. The invention provides a process
of treatment for promoting the adherence of porcelain enamels to ferrous metals. It
provides the method for an improved enameling stock made of iron or mild steel in
sheet form and having the property of accepting single fired-on porcelain enamel
coatings of white or light color without surface defects and with significant improvement
in adhesion.
US patent, US3930062, for “composition and method for electrostatic deposition of dry
porcelain enamel frit” discloses composition and method for electrostatic deposition of
dry porcelain enamel frit wherein the frit are reacted with a flow improving additive
consisting of an alkoxysilane and combined with an adhesion promoting composition
comprising a silazane.

US patent, US4082860, titled “Electrostatic deposition of fine vitreous enamel” provides
a method for preparing a powder of fine semiconducting vitreous enameling particles,
such as alkali-boro silicate glass frit, especially applicable to an electrostatic powder
coating process, for providing improved electrostatic adhesion to a base metallic
surface. The dried powder is exposed to tri-chloro-silane vapor which reacts with the
hydroxyl surface groups on the particles for form a surface layer which is then exposed
to water vapor producing a hydrophobic cross-linked silicone structure. In the final ring
stage of the enameling process excess organic groups are driven off, and the enamel is
not contaminated.
US patent, US6177201, for “porcelain enamel coating for high carbon steel” discloses a
porcelain enamel coating which includes a ground coat layer for coating directly onto
the steel oxide dispersed substantially uniformly throughout. The resulting porcelain
enamel coating provides good resistance to boiling defects such as pinholes.
US patent, US006375789B1, titled “powder coatings employing silanes” pertains to
powder coating and adhesive formulations utilizing organosilane compounds or their
hydrolyzates or condensates as cross-linkers or adhesion promoters.
US patent, US8316924, for “heat exchanger having powder coated elements” provides
invention of powder coated heat exchange elements for protection of surfaces of
elements.

US patent, US20120181725A1, relates to a self-healing vitreous composition containing
a particulate vanadium additive, to a method for preparing same, and to the use
thereof as a self-healing material, in particular for making seals in devices, operating at
a high temperature such as fuel oils and steam electrolyzers.
It has been noted; however, prior art exhibit coating of enamel on steel surface with
low water-contact angle and tend to crack exposing steel surface to corrosive
environment in which coatings are operated. The fish-scaling of enamel coat over steel
substrate is another problem which prior art have addressed with complicated methods
and additives. The method disclosed hereafter provides a solution to the fish-scaling,
pits, blow holes, craters, bubbles, and oxide spots.
OBJECTS OF THE INVENTION:
Object of the invention is to produce a novel enamel composition for coating the metal
surface especially steel to provide self-healing ability and low surface wettability.
Another object of the invention is to develop a method for deposition of enamel on a
steel substrate.
Further object of the invention is to provide certain physical properties to the metal
surface like resistance to acid-alkaline, heat corrosion and abrasion.

SUMMARY OF THE INVENTION:
The present invention discloses the composition of water repelling vitreous enamel and
method to deposit it on metal, especially steel substrate. The disclosed composition
consist of two-coat composition: ground coat and top coat. The ground coat consists of
silica, fluxing agents like borax, refractory agents like alumina, adhesion agents like
oxides of cobalt or copper or nickel or molybdenum. The cover coat consists of silica,
fluxing agent, refractory agent, opacifiers like titania or zirconia and surface modifiers.
The disclosed invention also provides a method for preparation of the surface of the
metal substrate by multistep cleaning and sandblasting; wet flow deposition of enamel
frit and its firing; electrostatic spray application of surface modified vitreous enamel;
and temperature controlled heat treatment and cooled down of the substrate. The
result of these processes enables the steel substrate to have reduced surface
wettability, increased corrosion resistance and a smooth surface finish that can
withstand severe environments.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 is a flow chart illustrating the steps involved in a deposition by disclosed
method.
Figure 2 is a flow chart showing the steps involved in the surface preparation of
substrate by disclosed method.
Figure 3 shows water contact angle of 50° on surface of enamel containing surface
additives deposited by disclosed method and 30° on surface of standard enamel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF
THE INVENTION:
The present invention provides the vitreous composition having lower surface
wettability than conventional enamel and method for coating the same on a steel
substrate. In a preferred embodiment, the vitreous enamel composition consists of two-
coat composition: ground coat and top coat. The ground coat consists of all or one of
the following components: silica, fluxing agent, refractory agent and adhesion agent.
The cover coat consists one and all of the silica, fluxing agent, refractory agent,
opacifiers and surface modifier.
In the preferred embodiment, silica is the main component of enamel and its properties
are modified to coat it on the steel substrate. As silica has very high melting point, very
low coefficient of thermal expansion and zero adhesion to steel. In disclosed invention,
various constituents are added to silica to obtain an enamel composition which adheres
to the surface of steel and produce low surface wettability and reduce crack
propagation.
The additives used for modifying property of silica to form disclosed enamel
composition are categorized into four main groups, depending upon the properties they
impart to the enamel: refractory materials, fluxing agents, adhesion agents,
opacifiers/coloring agents and surface property modifier.

Refractories are materials which give enamel an amorphous structure and hence
mechanical strength. These may include but is not limited to alumina which decreases
the coefficient of expansion, enhanced resistance to temperature, chemicals, and
abrasion and facilitates the action constituent of enamel.
Fluxes, which reduce the melting point and firing temperature and increase the
coefficient of expansion. In disclosed enamel composition, flux may include, but is not
limited to mixture of all or few or one of the following: anhydrous borax (sodium
tetraborate), hydrated borax, alkaline oxides such as oxides of sodium (Na2O), lithium
(Li2O), potassium (K2O), calcium (CaO), magnesium (MgO), strontium (SrO), fluorine
(F2) and boron trioxide (B2O3). These oxides produce borosilicates of sodium,
potassium, lithium, calcium, magnesium or strontium and reduce melting point lower
than silica (approximately 1720°C). These alkaline oxides, increase the coefficient of
expansion by filling the “voids” in the silica structure. Fluorine (F2) or boron trioxide
(B2O3) can also be used for decreasing the melting point of silica.
Adhesion agents promote adhesion between steel surface and enamel coating by
chemical reactions. These reactions involve the adhesion agents, the iron and carbon in
steel, as well as atmospheric oxygen. Adhesion agents are present in disclosed ground
coat enamel composition. Adhesion agent may include but is not limited to a mixture of

all or few or one of the following: nickel oxide (NiO), molybdenum oxide (MoO), cobalt
oxide (CoO), cupric oxide (CuO), manganese oxide (MnO2) and chromic oxide (Cr2O3).
Opacifiers and coloring agents, which contribute visual and tactile qualities to enameled
parts. Opacifiers serve to increase the opacity of enamel and are present in cover coat
enamel composition. Coloring agents were obtained by using one or combination of
different metal oxides including, but not limited to titanium dioxide (TiO2), antimony
oxide (Sb2O5), zirconium oxide (ZrO2) and tin oxide (SnO). In disclosed invention, the
concentration of coloring agent in the enamel, the chemical composition of the enamel
and the firing conditions in the enameling furnace were used to adjust the color of
enamel.
Surface property modifiers contribute to achieving desired properties of low surface
wetting angle and restricting crack propagation. The surface modifiers include but not
limited to oxides, sulfides, and halides of transition and alkali metals.
A uniform layer of water repelling enamel over the steel substrate is achieved by
disclosed method. The flow chart illustrating the method described in the present
invention is shown in figure 1. Various constituents of disclosed enamel composition for
ground coat and top coat are weighed and mixed. The mixture is then fused by melting
to form the glass between 1100°C and 1300°C. The glass is then rapidly quenched and

crushed to from enamel frit. The enamel frit is then milled with suitable additives
including but not limited to china clay. The surface of steel substrate is prepared for
enamel coating by steps shown in figure 2. The substrate is blasted with grit/shot to
achieve a roughness of few micrometers. This roughness provides a better bonding of
the enamel by promoting enamel-metal chemical reactions due to greater contact
surface area. Degreasing of the substrate is then carried out to remove exogenous
matter present on the steel surface. It is followed by rinsing of the substrate with hot
demineralised water, acid pickling, and acid rinsing. The pickling with acid increases the
microroughness and reactivity of steel surface and hence promotes the adhesion of
enamel. Nickel is then deposited surface of the substrate by precipitation of metallic
nickel by displacement of iron ions. The substrate is finally rinsed with demineralised
water and dried. The purpose of the final rinse is to eliminate all traces of acid still
present on the surface.
In the preferred embodiment, the ground coat is applied by wet coating methods
including but not limited to dip coating, flow coating or airless spraying. The ground
coated substrate is then dried between 70°C and 120°C to remove moisture, which
represents 40-50% of the mass deposited and can cause localized withdrawal of the
enamel during firing. Dried substrates are then fired between 750°C and 900°C in an
oxidizing atmosphere for 3 to 15 minutes. The top coat enamel is then applied using
wet or dry coating methods including but not limited to electrostatic powder spraying,

dip coating, flow coating, electrostatic solution spraying or airless spraying. The coating
is followed by drying at 70°C and 120°C and firing between 720°C and 850°C in an
oxidizing atmosphere for 1 to 10 minutes.
Example:
In a preferred embodiment, a mild steel substrate was coated with enamel containing
surface property modifier to decrease surface wettability compared to standard enamel.
The ground enamel frit was prepared by mixing 9% alumina, 40% silica, 30% borax,
0.50% cobalt oxide, 0.50% nickel oxide, 11% sodium oxide, 4% potassium oxide and
5% calcium oxide. The frit was ground with china clay to form a slurry. The enamel
slurry was coated on the surface of steel substrate by dip coating. The coating was
dried in an oven at 90°C for 15 minutes. It was then fired for 5 minutes at 870°C. Top
coat slurry containing 2% titanium oxide, 0.4% vanadium oxide, 0.6% titanium boride,
11% sodium oxide, 4% potassium oxide and 5% calcium oxide was prepared. The
surface of steel substrate was prepared by grit blasting to obtain a surface roughness of
4 µm, degreasing with alkali, rinsing with hot water, sulfuric acid pickling, nickel
deposition, deionized water rinsing, and drying. The prepared slurry was then coated on
the steel substrate having a layer of ground coat. The electrostatic spraying of liquid
was used for coating top coat which was then dried in an oven at 100°C for 10 minutes.
The dried coating was then fired at 780°C for 3 minutes in a furnace. The water contact
angle of coated substrate was then measured and found to be around 50° compared to
standard enamel which had a contact angle of around 30° as shown in figure 3.

Although the present invention has been described with reference to preferred
embodiments thereof, the disclosed method with few modifications without departing
from the scope of the present invention can be used for other applications. The claims
of present invention are appended.

WE CLAIM:
1) Vitreous enamel compositions for ground coating and top coating of the steel
substrate surface, characteristically different in composition for ground and top,
comprising of silica, refractory material, flux and adhesive agents.
2) The enamel compositions as claimed in claim 1, wherein the silica as being the
main component, characterized by very high melting point, low coefficient of
thermal expansion for both ground and top coating.
3) The enamel composition as claimed in claim 1, wherein the refractory material
for both ground and top coating include but not limited to alumina, characterized
by lowering the coefficient of expansion, increasing resistance to temperature,
chemicals and abrasion of the mixture.
4) The enamel composition as claimed in claim 1, wherein the flux for both ground
and top coating include but not limited to the mixture of all or few or any of the
materials as anhydrous borax (sodium tetraborate), hydrated borax, alkaline
oxides such as oxides of sodium (Na2O), potassium (K2O), lithium (Li2O), calcium
(CaO), magnesium (MgO), strontium (SrO), fluorine (F2) and boron trioxide

(B2O3), characterized by reducing the melting point and increasing the coefficient
of expansion of the mixture.
5) The enamel composition as claimed in claim 1, wherein the adhesive agents for
both ground and top coating include but not limited to mixture of all or few or
any of the materials as cobalt oxide (CoO), nickel oxide (NiO), molybdenum
oxide (MoO), cupric oxide (CuO), manganese dioxide (MnO2) and chromic oxide
(Cr2O3), characterized by cohesive bonding of the mixture.
6) The enamel composition as claimed in claim 1, wherein opacifiers and coloring
agents for top coating includes but not limited to one or combination of different
metal oxides as titanium dioxide (TiO2), zirconium oxide (ZrO2) and tin oxide
(SnO), characterized by , contributing towards visual and tactile qualities to the
enameled parts and opacity.
7) The enamel composition as claimed in claim 1, wherein additives for top coating
include but not limited to titanium boride for decreasing surface wettability,
vanadium oxide for self healing, characterized by surface modification.
8) A method of coating, wherein the constituents of the ground coat and the top
coat are mixed, melted, fused and quenched to form enamel frits.

9) The method as claimed in claim 8, wherein the frit obtained is milled with
suspension agents, refractory agents, coloring agents, electrolytes, opacifiers, in
a solvent not limited to water to form a slurry.
10) The method as claimed in claim 8, wherein the surface of the substrate is
prepared by all or combination of few of the processes including grit/shot
blasting, degreasing, rinsing, acid pickling, nickel deposition and drying.
11) The method as claimed in claim 8, wherein ground enamel slurry is coated by dip
coating, spray coating or flow coating and top enamel slurry is coated by
electrostatic spraying, dip coating, spray coating or flow coating.