FORM 2
THE PATENTS ACT, 1970
(Act 39 of 1970)
COMPLETE SPECIFICATION
(See Section 10; Rule 13)
Title: "VITREOUS ENAMEL COATING FOR METALS"
Applicant: PATEL HARDIKKUMAR SURYAKANT
Address: 12, Sahajanand Society, B/H Ravi Society, Nr.
Akshar Purshottam Chhatralaya, Anand-Vidyanagar Road, Dist: Anand, Vallabh Vidyanagar-388120, Gujarat, India.
Nationality: Indian
The following specification describes the nature of the invention AND the manner in which it is to be performed:

FIELD OF THE PRESENT INVENTION
The present invention relates to metals and metal engineering products having vitreous enamel coating thickness of 0.8 mm to 2.5 mm onto it. Further, the present invention relates to glass frit compositions, a process for the preparation of glass frit compositions and a process of vitreous enamel coating (enamelling) onto the metals and metal engineering products having enamel coating thickness of 0.8 mm to 2.5 mm. Vitreous enamels are glass like protective coating which are fired onto the metal to provide good aesthetics, colour or gloss surface, enhance corrosion resistance, mechanical shock resistance, impact resistance, abrasion resistance and thermal resistance properties. Among various coating system for industrial and engineering application, glass & glass ceramic coating have advantage of chemical inertness, high temperature stability and superior mechanical properties such as abrasion, impact as compared to other materials applied by spraying, dipping & pouring.
BACKGROUND OF THE PRESENT INVENTION
Applicants are aware of the following references which disclose various cermet coating compositions intended for use on various metallic surfaces in different service conditions, viz:
US2974051 indicates that serious disadvantages attend the flame spraying method of applying a protective material to a metal surface. This reference discloses a cermet coating consisting of 30 parts by weight frit (44% BaO, 37.5% Si02, 6.5% B2O3, 5.0% ZnO, 3.5% CaO, 2.5% Zr02, 1% Al2O3), 70 parts metal (65-75% Ni, 13-20% Cr, 3-5% B, <10% Fe+Si+C), 5 parts clay, and 40 parts water. The base metal or substrates coated with this composition include low carbon steel, molybdenum and numerous nickel-chromium alloys and nickel-chromium-iron alloys. The coating is fired at temperatures in the range

1850F to 2000F and is said to provide protection against temperatures up to 1200F.
US2977251 discloses a cermet coating composition comprising 20-50 parts by weight of frit (65-70 Parts B2O3, 10-20 parts BeO, 15-25 parts Li2O by weight), 50-80 parts powdered Al or Al-alloy, 5-10 parts suspension agent (ball clay) and a liquid vehicle (water).
US3203815 discloses a cermet coating composition comprising 15-60 parts by weight of frit (45-55% B2O3, 3-7% PbO, 5-12% CaO, 0.5% Si02, 8-13% Na20, and 8-13% A12O3) and 20-80 parts powdered Al. This coating provides protection for low-alloy steels, stainless steels, cast iron, cast steel, and titanium against oxidation, corrosion, and chemical attack at temperatures in the range 1450F to 1500F.
US3508938 discloses a cermet coating composition comprising ceramic and metallic ingredients within the following ranges: 0-10 AI2O3, 45-55 B2O3, 0-10 Cr2O3, 35-45 SiO2, 0-10 ZrO2, 5-30 colloidal alumina, 25-150 water, 0-30 Ni and 0-30 Fe — all parts by weight. This coating is intended for use on refractory metal and has high strength (>2000 psi both in shear and tension) at temperature on the order of 2200F.
US3597241 discloses a metallo- ceramic (i.e. cermet) coating composition containing 10-40% of a ceramic oxide having a melting point > 1900°C, 15-60% Cr and 10-50% Ni. This coating protects ferrous and non-ferrous metals against oxidation at temperatures of about 1470F.
US3772043 discloses the composition in slip form that consists on a weight basis of approximately 25 to 55 parts powdered aluminum, approximately 20 to 50 parts special homogeneous glass frit, approximately 14 to 27 parts special mill addition having both refractory compound and reducible compound constituents, approximately 0 to 10 parts flux, and water vehicle with added

buffering agent; the special homogeneous glass frit has a calculated oxide content on a 100 parts weight basis consisting of 25 to 50 parts boric oxide, 10 to 50 parts silicon dioxide, 10 to 20 parts alkali metal oxide selected from the group consisting of the oxides of lithium, potassium, and sodium, and 10 to 40 parts of oxide selected from the group consisting of the oxides of calcium, magnesium, and zinc. Such glass frit is characterized by the fact that upon re-firing to the coating firing temperature in the range of approximately 1,250F. to 1,600F., both an alkaline earth or zinc-modified boro-silicate type wetting glass system liquid phase and an alkali metal-modified boro-silicate type sealing glass system liquid phase of greater viscosity are advantageously provided in the composition. Because of the prescribed composition characteristics, the coating in slip form has a prolonged shelf life in comparison to known cermet protective coating slips.
US3850647 dated Sept 29, 1972 discloses a cermet protective coating composition for alloys like mild steels and enameling iron is disclosed. The coating protects the base metal from corrosion in aqueous, saline, and alkaline environments and from thermal degradation. The coating of the invention also serves as a protective ground coat for enamel cover coat applications.
US6001494 dated February 18, 1997 discloses metal-ceramic composite coatings. The said metal-ceramic composite coatings provide refractory anti-corrosion flexible enamel coated products. Frit is made of silicon dioxide, sodium or potassium oxide, boron oxide, calcium oxide and zinc oxide, with possible other components. The frit is wet milled with clay and bentonite as a suspending agent, magnesium carbonate and sodium nitrates as electrolyte, boric acid a buffer, and water. Finely divided aluminum is blended after milling. A substrate is coated with the frit aluminum and clay mixture, which is heated and fused. The metal component coats the substrate and provides galvanic protection. The resulting coating is flexible and deforms with the

substrate without delaminating or chipping. The coating is prepared by crushing, sieving, mixing and melting the frit components. The melt is tested by pulling, cooling and feeling a thread. The melt is fritted by pouring into water. The frit is dried, ground and sieved, and then wet milled with water, clay, bentonite and pH adjustors. Fine metal powder is added, and flowability is adjusted with water for spraying. Metal panels are cleaned and spray coated. The coating is dried, and then fired or baked, to provide the metal-ceramic composite flexible corrosion prevention coating.
Though the above said prior art has suggested several ways of coating, none of the above has disclosed enamel coating having thickness of 0.8 to 2.5 mm onto the metals or metal engineering products. In today's industrial development, especially in the chemical engineering field, it is now demanded by the industry to have the solution of the problem faced by the industry. Present invention has suggested a solution of the problem faced by the industry by providing enamel coating having thickness of 0.8mm to 2.5mm onto the metals and metal engineering products.
OBJECTS OF THE PRESENT INVENTION
A primary object of the present invention is to provide a process of vitreous enamel coating (enamelling) onto the metals and metal engineering products from glass frit compositions, the said process is less expensive and more effective than the prior art.
Another object is to provide glass frit compositions for the purpose of vitreous enamel coating onto the metal and metal engineering products, the said glass frit compositions form an integral part of the coating and wherein the glass frit compositions excel in its corrosion resistance and thermal behaviour characteristic of glass coatings, and the high ductility exhibited by metallic coatings.

Yet another object is to provide a process for the preparation of glass frit compositions.
Still another object is to provide a low-cost and intrinsically noncritical metal and metal engineering products with extended serviceability and prolonged life.
BRIEF DESCRIPTION OF DRAWINGS
Table-1 shows the oxide component alongwith their quantity to be used
in the preparation of glass frit composition.
Table-2 shows the raw materials used in preparation of Mill
compositions.
Table-3 shows the specification of selected types of mild steel and
stainless steel to be used for glass enamelling.
Table-4 shows the specification of selected types of cast iron to be used
for glass enamelling.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
The objects of the present invention may be accomplished by providing metal and metal engineering products wherein the thickness of vitreous enamel coating onto the metal and metal engineering products is between 0.8 mm to 2.5 mm. Further, glass frit compositions used in the vitreous enamel coating are also disclosed in the present invention. A process for the preparation of glass frit compositions is also provided.
In an embodiment, glass frit compositions are disclosed wherein SiO2, Na2O and K2O are used as essential components and optionally one or more components selected from CaO, CoO, BaO, Fe2O3, LiO2, SrO, Zr02, AI2O3, B2O3, Cr2O7, TiO2, CaF2, NaF, MnO2, NiO have been used for the preparation of Glass frit compositions. All the above components are also known as oxide components and the same is obtained from the appropriate raw materials selected from silica, feldspar, borax, soda ash, calcium carbonate, barium carbonate, boric acid, aluminium

oxide, fluorspar, sodium titanate, sodium nitrate, potassium nitrate, potassium carbonate, titanium dioxide, lithium carbonate, sodium silico-fluoride, zinc oxide, zircon flour, manganese dioxide, iron oxide, cobalt oxide, nickel oxide, chromium oxide, strontium carbonate, magnesium carbonate, cerium oxide and the other raw material which can generate oxide components after loss on ignition of required raw materials.
Thus, the glass frit composition of the present invention comprises: SiO2, Na2O and K2O as essential components; and one or more components selected from CaO, CoO, BaO, Fe2O3, LiO2, SrO, Zr02, Al2O3, B2O3, Cr207, TiO2, CaF2> NaF, MnO2, NiO.
Wherein, Weight percentage range of each oxide component is about 45-72% Si02, 6-24% Na20, 1-12% K20, 0.5-8% CaO, 0.5-5% CoO, 2-14% BaO, 0.1-3% Fe2O3, 1-8% Li02, 2-5% SrO, 2-8% ZrO2, 1-6% A12O3, 3-20% B2O3, 2-6% Cr2O7, 3-8% TiO2, 2-8% CaF2, 1-4% NaF, 0.5-2% MnO2, 0.2-1.5% NiO.
Range of the quantity of each oxide component used in the above said composition is shown in Table-1.
In the second embodiment, a process for the preparation of Glass frit composition is disclosed wherein a process for the preparation of glass frit composition comprises the steps of:
Mixing the essential oxide components Si02, Na2O and K2O;
optionally mixing atleast one oxide component selected from CaO,
CoO, BaO, Fe2O3, Li02, SrO, ZrO2, Al2O3, B2O3, Cr207, TiO2, CaF2,
NaF, Mn02, NiO;
heating it in smelting rotary furnace at a temperature between
1450°C to 1600°C to give molten liquid;
quenching the above molten liquid into water to obtain glass frit
composition.

Wherein,
Weight percentage range of each oxide component is about 45-72% Si02, 6-24% Na2O, 1-12% K2O, 0.5-8% CaO, 0.5-5% CoO, 2-14% BaO, 0.1-3% Fe2O3, 1-8% LiO2, 2-5% SrO, 2-8% ZrO2, 1-6% Al2O3, 3-20% B203, 2-6% Cr2O7; 3-8% TiO2, 2-8% CaF2, 1-4% NaF, 0.5-2% Mn02, 0.2-1.5% NiO. and
the said oxide components obtained from the appropriate raw material selected from silica, feldspar, borax, soda ash, calcium carbonate, barium carbonate, boric acid, aluminium oxide, fluorspar, sodium titanate, sodium nitrate, potassium nitrate, potassium carbonate, titanium dioxide, lithium carbonate, sodium silico-fluoride, zinc oxide, zircon flour, manganese dioxide, iron oxide, cobalt oxide, nickel oxide, chromium oxide, strontium carbonate, magnesium carbonate, cerium oxide and the other raw material which can generate oxide components after loss on ignition of required raw materials.
Range of the quantity of each oxide component used in the above said composition is shown in Table-1.
In the third embodiment, a process of vitreous glass enameling onto the metal or metal engineering product is disclosed wherein the process comprises the steps of:
a. Mixing the desired glass frit composition and milling additives;
b. carrying out the desired coating process selected from ground coat
or cover coat onto the metal or metal engineering product;
c. repeating the step (b) till the glass coating thickness between 0.8
to 2.5 mm of the glass enameling onto the metal or metal
engineering product is obtained.
Wherein
Glass frit composition comprises:

SiO2, Na2Oand K2O as essential oxide components; and
one or more components selected from CaO, CoO, BaO, Fe2O3,
LiO2, SrO, ZrO2, Al2O3, B2O3, Cr207, TiO2, CaF2, NaF, Mn02, NiO.
and
Milling additives comprises:
Quartz and water as essential raw materials; and
atleast one chemical selected from syloid, titanium dioxide, borax,
aluminium oxide, sodium nitrite, potassium chloride, lithium
fluoride, cobalt oxide, chromium oxide, magnesium carbonate,
clay and cerium oxide.
The quantity of oxide components used in the above process is shown in table-1. While the quantity of each chemical or raw material used in mill additives as per the table-2 is provided as follows. 40-70 parts Water, 1-50 parts quartz, 2-5 parts syloid, 3-8 parts titanium dioxide, 0.5-1.5 parts borax, 1-5 parts aluminium oxide, 1-4 parts sodium nitrite, 0.5-5 parts potassium chloride, 1-5 parts lithium fluoride, 0.1-0.5 parts cobalt oxide, 1-6 parts chromium oxide, 1-3 parts magnesium carbonate, 0.5-7 parts clay and 5-11 parts cerium oxide The quantity of each chemicals/ raw material is considered against the 100 parts by weight of glass frit composition.
Thus, raw materials and/or chemicals used in the glass frit
composition and milling additives are divided into six different groups,
namely, refractories, fluxes, opacifires, colours, floating agents, and
electrolytes.
The refractories include quartz, feldspar, and clay. Which contribute
the acidic part of the melt and give body to glass.
The fluxes include borax, soda ash, sodium nitrate, fluorspar, barium carbonate, Lithium fluoride, Strontium carbonate, potassium carbonate, potassium nitrate, magnesium carbonate, zinc oxide, and sodium silicon fluoride. Fluxes are having basic pH in characters and

react with the acidic part of refractories to form the glass. Fluxes help in reducing the melting point and firing temperature of the materials.
The opacifires include Titanium dioxide, Zirconium dioxide, zirconium silicate and Cerium oxide which increase white opacity of enamels.
The colours are oxides include cobalt oxide, Iron oxide, manganese dioxide and Nickel oxide. They may act either as refractories or fluxes. They work as adhesive agents and involved in chemical reaction to provide adhesion between ground enamel and base metal substrate materials. Colour of the enamel depends on the type of the colouring agent, its concentration in enamels, the chemical composition of the enamels and firing condition of the enamelling, furnace. Some of the colouring agents are added during enamel frit composition manufacturing process or during the grinding stage before application to metal or metal engineering product (substrate).
The floating agent includes clay, bentonite and syloide. The floating agents are used to suspend the enamel in water.
The electrolytes include borax, soda ash, magnesium carbonate, sodium nitrite, Barium chloride and sodium aluminates.
The enamelling onto the metal or metal engineering products is designed to achieve the desired properties for most application like colour or gloss surface, corrosion resistance, mechanical shock resistance, impact resistance, abrasion resistance and coefficient of thermal expansion properties. To achieve these properties calculated values are evaluated from weight percent glass frit compositions. If the value is not within the limit, one or more of the composition levels are changed to recalculate a new composition until the property values are in within the acceptable limit. Thus, the characteristics of the coated metal and metal engineering products dependent on the below check points:

(i) Type of glass frit composition used in enamelling (ii) mill additives used in enamelling and (iii) the type of coating i.e. ground coat or cover coat and (iv) number of coating carried out onto the metal or metal engineering products.
Further, selection of the metal or metal engineering products for the purpose of vitreous enamelling is also play an important role wherein the said metal engineering product must be made from the metal selected from mild steel, stainless steel, cast iron and the metal the like wherein the characteristics or properties of metal or metal engineering products must match with the specification of ASME Sec II, Part-A, Edition 2010, Addenda 2011 as shown in table 4(A) and 4(B).
As per the Table 3 and 4, metals with the following specification may be used for the purpose of vitreous enamel coating.
1. Carbon steel plate with: SA 516M Gr. 380, SA 285 Gr. C, IS 2041:2009 Gr. R220 or SA 516 Gr.70. Thickness of carbon steel plate must be within 8 mm to 50 mm.
2. Stainless steel plate of: SA 240 TP 310, SA 312 TP 304L or SA 312 TP 340. Thickness of stainless steel must be within 8 mm to 20 mm.
3. Carbon steel forging of: SA 105M, SA 181M Class 60 or SA 836M.
4. Carbon steel pipes of: SA 106 Gr. B.
5. Cast iron as per IS 210 of the grades: 100, 150, 200 and 260
Further, for the purpose of vitreous enamel coating, metal or metal engineering products must contain following characteristics.
■ Low carbon content ( not mare than 20 % )
■ Free from harmful gases in the metal ( CO2, SO2)
■ Low hydrogen collection during coating and enamelling.
■ Uniform composition and structure and free from solid solution.
■ Uniform surface texture.
■ Requisite drawing and welding properties.

■ Slag inclusion, lamination of carbide segregation should be absent in the metal structure.
Now, according to the characteristics and the industrial applicability, the coated metal or metal engineering products are divided into following groups:
Acid resistance glass coated mild steel & stainless steel (Cover coat): Colour of enamel coating is blue;
Alkaline resistance glass coated mild steel & stainless steel (Cover Coat): Colour of enamel coating is blue;
Photochromic glass coated mild steel & stainless steel (Cover coat): Colour of enamel coating is white;
Pharmaceutical purpose glass coated mild steel & stainless steel (Cover coat): Colour of enamel coating is light blue/sky blue;
High temperature resistance glass coated mild steel & stainless steel (Cover coat): Colour of enamel coating is green/creamish.
Glass coated (enamelled) cast iron (Cover coat): Colour of enamel coating is blue/white/light blue or sky blue/ green or creamish. Glass coated (enamelled) mild steel (Ground coat): Colour of enamel coating is gray;
Glass coated (enamelled enamelled stainless steel (Ground coat): Colour of enamel coating is brown;
Glass coated (enamelled cast iron (Ground coat): Colour of enamel coating is gray.
In general, (i) a process for the preparation of glass frit composition and (ii) a process of vitreous glass enamelling onto the metal or metal engineering products using glass frit composition and mill additives are described as follows:
As per the enamelling requirement and as shown in table-1, the appropriate raw materials (oxide components quantity to be calculated after loss of ignition of the required raw material) are selected and carefully weighed, which is shown in table-1. The raw materials which

are dried, mixed and transferred to smelting rotary furnace at a temperature 1450°C to 1600°C. The smelting time is decided by periodically pulling a thread. The molten glass is tested by pulling, cooling and feeling a thread, in that process a steel road is dipped into the surface of molten glass until the glass is adheres, then withdrawn smoothly and rapidly so that thread of solidified glass is obtained. After cooling, the thread is examined visually and making loop. If the glass is fully melted, the thread will be uniform and transparent and will feel smooth without knots or lumps. Then the molten glass is quenched in water. After quenching, the materials gets crystallised in small lumps to obtain glass frit composition.
This glass frit is milled by either wet or dry grinding a suitable fineness in a ball mill. If the frit is wet-milled, it can be charged directly to grinding mill without drying by adding water, colouring agents and electrolytes (mill additives). The resulting glass particles suspension solution is called slip.
Frit can be used for enamelling or glass lining of mild steel, stainless steel and cast iron at relatively low temperature range of 700 to 950°C by wet and dry process as shown below.
WET PROCESS:
If the glass frit obtained by the above said process is wet-milled, it can be charged directly to grinding mill without drying by adding water, and the required mill additives as shown in table-2. The resulting glass particles suspension solution is called slip which is then applied on the metal surface by spraying, dipping or pouring to form a uniform and adherent layer of required thickness. This coated article is dried and finally fired in the furnace (electric or gas fired) to obtain a glassy and strongly adherent coating. Two or more coatings are usually applied. Whereas the vitreous enamel or glass lining refers to coating 0.8 mm to 2.5 mm thickness.

DRY PROCESS:
In the dry process, the powder frit is sprinkled or dusted directly on a metal or metal engineering product after bringing the letter to a red heat temperature in furnace (electric or gar fired). The glass dust adheres to the surface by partial fussing. The coated articles are subsequently fired in the furnace to secure complete and uniform fusion of the coat. Multiple coats are added in the same way to build up required thickness of the lining. Whereas the vitreous enamel or glass lining refers to coating 0.8 mm to 2.5 mm thickness.
The glass lining has been produced by applying one or two ground coat (base coat) on a metal or metal engineering products with a thickness of 0.3 to 0.5 mm, which is fired onto the subtract in the electric or gas fired furnace at the temperature range 890 to 950°C for 15 minutes soaking periods.
Ground coat introduces some metallic oxide such as cobalt oxide, iron oxide, nickel oxide during frit formulation to achieve strongly adhesion with metals by producing physio-chemical bond. After ground coat, thereby applying four or more cover coat with thickness of 0.8 to 2.5 mm, which is also fired in electric or gas fired furnace at the temperature range 700 to 850°C for 15 minutes soaking periods.
Cover coat introduces some opacifying and colouring agents during frit formulation batch to provided glossy, high corrosion resistance coating. Cover coat makes chemical bond with ground coat.
Among the various prior-art for industrial and engineering application, the present invention have advantage of chemical inertness, high temperature stability and superior mechanical properties such as abrasion, impact as compared to other materials applied by spraying, dipping & pouring.

The present invention demonstrated examples cited below, which are provided as illustration only and therefore should not be construed as limitation of the possible and future invention.
EXAMPLES
GROUND COAT:
Pre-examining stage:
Take a test metal specimen for glass lining purpose. Blasted the test
specimen and make it free from foreign contamination. Check the
blasted surface to be glass lined thoroughly for any surface defects
such as pin holes, minor cracks, welding spatter, uneven surface,
welding porosity, sharp radius/ curvature etc. and ensure that these
defects are removed by appropriate methods such as grinding. Insure
that surface shall be cleaned and made free of oil, dust, moisture and
other contaminants.
Process of Ground coat onto the metal specimen: Ground Coat 1:
Spray a layer of approximately 0.2 to 0.4mm thick ground coat 1. After
drying it at room temperature put the sample in electric or gas fired furnace for baking at temperature 920°C for 15 min soaking and 10 minute cooling in the furnace at closed environment. Then specimen is removed from furnace and allowed to cool to ambient temperature.
Ground coat 2:
Check the ground coat 1 glass for visual defects such as under firing,
over firing. Remove visual defects if any, by grinding and spray a layer of approximately 0.3 to 0.5 mm thick ground coat 2. After drying put the job in electric or gas fired furnace for baking at temperature 910°C for 10 min soaking and 5 minute cooling in the furnace at closed environment. Then remove specimen from furnace and allowed to cool to ambient temperature.

COVER COAT (Wet process):
Check the ground coat 2 glass for visual defect such as over fire or
repair. Repair that portion by repair ground and then 1st cover coat is spray a layer approximately 0.5 to 0.8 mm thick. After drying put the job in electric or gas fired furnace for baking at temperature 830°C for 20 min soaking and 10 minute cooling in the furnace at closed environment. Then specimen is removed from furnace and allowed to cool to ambient temperature.
Repeat this cover coat spray process for 5 to 8 times until the acceptable quality and thickness (0.8mm to 2.5mm) of corrosion resistant glass is achieved. After each baking of cover coat check the glass lining surface visually and scanning by hand with the help of tube light for slag inclusion, stress lines & hair lines on the glass lined surface, surface unevenness, bubbles, over thickness, chipping, under firing, over firing, glass draining, discoloration etc. Check the lining thickness with thickness gauge. It also check spark test at 20 KV thoroughly for bubbles. Check the surface & maintain record of observations.
Carry out a Glass Lining Continuity test (Spark test) after thickness of approximately 0.8 mm. (after 2nd cover coat) is achieved. Spray a layer of cover coat approximately 0.3 to 0.4 mm of each coat.
Spark test is carried out as per DIN 51163 or DIN ISO 2746 or DIN EN 14430:2004
Table-A shows the type of coat, glass coating thickness on metal or metal engineering products after each coat and backing condition of metal or metal engineering products.

Table: A

Type of coat Thickness Backing Soaking Cooling
(mm) Temp. Time Time
(°C) (min) (min)
Ground coat 1 0.2-0.3 920 10 10
Ground coat 2 0.3-0.5 910 10 5
Cover coat 1 0.5-0.9 840 20 10
Cover coat 2 0.8- 1.2 830 20 10
Cover coat 3 1.2- 1.6 830 15 10
Cover coat 4 1.6- 1.8 820 20 10
Cover coat 5 1.8-2.1 820 15 10
Cover coat 6 2.1-2.3 810 15 10
Cover coat 7 2.3-2.5 800 10 10
Final Thickness of Cover coat: 1.9mm- 2.5mm
Below are provided the specific examples of glass frit compositions and mill additives for enamelling of metal and metal engineering product. It is to be noted that the process of preparation glass frit composition and enamelling of metal or metal engineering products using glass frit composition and mill additives are as per the process shown in the above paragraphs. However, these examples are not intended to limit the scope of the present invention.
(A) Preparation of Glass frit composition for Acid resistance enamelling (cover coat) onto metal or metal engineering product made from mild steel (MSI and/or stainless steel (SS):

Oxide component Weight percent Oxide component Weight percent
SiO2 61 Fe2O3 0.5
Na2O 11 Li2O 5.2
K2O 4.5 SrO 3.2
CaO 5.5 ZrO2 3.5
CoO 0.8 A12O3 2
BaO 3.7
(B) Preparation of Glass frit composition for Alkaline resistance enamelling (cover coat) onto metal or metal engineering product made from mild steel (MS) and/or stainless steel (SS):

Oxide component Weight percent Oxide component Weight percent
SiO2 58 Fe2O3 0.82
Na2O 9.2 Li2O 4.6
K2O 2 SrO 2.3
CaO 5.5 Zr02 5.2

CoO BaO 3.5 2.5 AI2O3. B2O3 1.5 4.9
Mill Additives: (For Acid & Alkaline resistant glass enamelling):
Chemical/Raw Material 100 parts by weight percent of frit
Water
Quartz flour
Potassium chloride
Syloid
Clay 50 litre
15
0.8
1
3
The glass coated steel is having dark blue colour, which provides high corrosion resistance in acidic as well as alkaline media, and is a thermal shock resistant and impact resistant. Application/Use: in food, dye and chemical industries.
(C) Preparation of Glass frit composition for Photochromic enamelling (cover coat) onto metal or metal engineering product made from mild steel (MS) and/or stainless steel (SS):

Oxide component Weight percent Oxide component Weight percent
S1O2 62.5 — —
Na2O 10 Li2O 6.4
K20 4.5 SrO 3.2
CaO 4.5 ZrO2 4.5
Al2O3 1.2 BaO 3.2
Mill Additives (For Photochromic enamelling):
Chemical/Raw Material 100 parts by weight percent of frit
Water 50 litre
Quartz flour 8
Potassium chloride 2
Titanium dioxide 4.5
Cerium oxide 6.5
Clay 4
The white coloured glass enamelled steel is having Photochromic characteristics that provides aesthetic and smother surface compare to conventional glass enamelled steel.
Application/Use: For crystallization photosynthesis process or manufacture of some specific products in chemical or any other suitable industries. Their white colour increase their reflecting power considerably thus making them very suitable for photosynthesis

process. In addition, because of the increased visibility it can be used in the process where it is required to observe the changes in the colour of the product.
(D) Preparation of Glass frit composition (cover coat) for pharmaceutical industry purpose glass enamelling onto the metal or metal engineering product made from mild steel and/or stainless steel:

Oxide component Weight percent Oxide component Weight percent
Si02 61.7 CoO 1.5
Na20 9.5 Li2O 6.2
K20 4.5 SrO 3.5
CaO 3.5 ZrO2 4.6
AI2O3. 1.8 BaO 3.2
Mill Additives (for Pharmaceutical industry purpose glass enamelling):

Raw Material 100 parts by weight percent of frit
Water 50 litre
Quartz flour 8
Potassium chloride 1.5
Cobalt oxide 0.3
Cerium oxide 5
Clay 3
The pharma purpose glass enamelled steel is cover-coated steel having light blue or sky blue colour that provides aesthetic and smother surface compare to conventional glass.
Application/Use: This enamelled steel is used in Pharmaceutical industry as it has low surface tension & low roughness average so increase the glass non sticky property which prevents the cleaning of the product to the glass. Wet solid matters does not adhere or cake, this properties greatly facilitate inter batch cleaning with low cost low pressure system. Clean glass wall can drastically reduce heat transfer problem and provide batch to batch temperature control.
(E) Preparation of Glass frit composition for High temperature resistance glass enamelling (cover coat) onto the metal or metal engineering product made from mild steel and/or stainless steel:

Oxide component Weight percent Oxide component Weight percent
S1O2 60 Cr207 4.6

Na2O 8 Li2O 5.5
K2O 6 SrO 3.2
CaO 5 ZrO2 3.5
Al2O3. 1.5 BaO 2.7
Mill Additives (For high temperature resistant glass enamelling):

Raw Material 100 parts by weight percent of frit
Water 50 litre
Quartz flour 4
Potassium chloride 1.5
Chromium oxide 3
Cerium oxide 5
Clay 3
The cover-coated steel obtained with this composition is high
temperature resistant or thermal shock resistant steel having green
colour.
Application/Use: in the reaction wherein high temperature is required.
(F) Preparation of Glass frit composition for the purpose of Opaque white (cover coat) glass enamelling onto the metal or metal engineering product: For mild steel (MS) and/or stainless steel (SS):
Oxide component Weight percent Oxide component Weight percent
SiO2 60 Na2O 9.5
Li2O 10.3 K2O 3.2
T1O2 3.5 Zr02 5.5
B2O3 8
Mill Additives (for the purpose of Opaque white (cover coat) enamelling:
Raw Material 100 parts by weight percent of frit
Water
Quartz flour Potassium chloride Titanium dioxide Clay 50 litre
6
3
4
3
(G) Preparation of Glass frit composition (cover coat) for glass enamelling onto the Cast iron:
Oxide component Weight percent Oxide component Weight percent
SiO2 54 CaF2 4.6
Na2O 18 NaF 1.8
K2O 4.5 CoO 1.5
Al2O3. 1.5 B2O3 14.1

This blue coloured cover-coated enamelled cast iron is manufactured by a dry process from the above glass frit composition.
(H) Preparation of Glass frit composition for ground coat glass enamelling onto metal or metal engineering product made from mild steel: (glass colour is Gray) Type-1:

Oxide component Weight percent Oxide component Weight percent
S1O2 51 Fe2O3 0.5
Na2O 18 Mn2O 0.8
K2O 8.3 NiO 0.9
CaO 1.5 CoO 1.5
B2O3 15 A12O3. 2.5
Type-2:

Oxide component Weight percent Oxide component Weight percent
SiO2 53 Fe2O3 0.5
Na2O 8 Li2O 1.2
K2O 8.5 NiO 0.5
CaO 4.5 Mn2O 0.8
CoO 1.5 A12O3. 2
BaO 6 B2O3 12
CaF2 1.5
Mill Additives: For ground coat enamelling onto mild steel (For Type-1 &2):

Raw Material 100 parts by weight percent of frit
Water 50 litre
Quartz flour 30
Borax 1
Sodium nitrite 1.5
Aluminium oxide 3
Magnesium carbonate 1.2
Clay 1.5
(I) Preparation of Glass frit composition for ground coat enamelling onto
metal or metal engineering product made from Stainless steel:
(glass colour is Brown)

Oxide component Weight percent Oxide component Weight percent
SiO2 55 Fe2O3 0.3
Na2O 8.2 TiO2 3.5
K2O 8 Mn2O 1
CoO 1.5 BaO 6
CaF2 5.4 B2O3 9.5
Li2O 1.6

Mill Additives: For ground coat enamelling on stainless steel:
Raw Material 100 parts by weight percent of frit
Water 50 litre
Quartz flour 30
Sodium nitrite 3
Lithium flouride 4
Magnesium carbonate 1.5
Clay 4.5
RESULTS: Analysis of the glass coated metal and/or metal engineering products:
(A) Corrosion Procedure:
The test shall be carried out on 50 cm2 X 1.5 mm thick enamel quality plates with a glass lining of 1.2 to 1.6 mm thickness. It must be free from rust and foreign contaminants. For test two test circle samples shall be carried out. The tests shall be at different concentration/temperature combinations for following different acids and alkalis to arrive at proper corrosion resistance levels.
Following steps shall be followed:
The test circle samples shall be heated in an oven for two hours at 120°C ± 5°C and transferred to a desiccators for one 30 min. to ensure that all moisture has been removed. On completion, the test circle samples are thoroughly cleaned with Acetone. The test circles samples are weighed accurately and recorded. The samples shall be covered with neoprene/chloroprene rubber protective sheath and clamped in to the test apparatus for acid as per acid test standard and for alkali resistance as per alkali test standard. The samples shall be placed and clamping bolts shall be tightened enough to ensure that the test apparatus is leak free.
For Acid resistance test, the test apparatus is connected to a reflux condenser. 450 ml of acid test solution as described shall be filled into

the test apparatus and heated with the help of heating coils. When the contents of the test apparatus start boiling, the current supply to the heating coils shall be regulated in a manner ensuring that the temperature is maintained at 110°C ± 5°C for a period of 48 hours. On completion of 48 hours, test apparatus allowed to cool to ambient temperature, test apparatus dismantled, test circle samples washed thoroughly with distilled water, acetone and finally again with distilled water. The test circle samples shall be once again treated as mentioned before testing.
Acid corrosion resistance test carried out (for HC1) as per standard DIN-51157/ DIN ISO-2743/DIN EN ISO-28706-2 are 0.01 mm/year in liquid phase and 0.04 mm/year in vapour phase and Acid corrosion resistance test apparatus to be taken as per standard DIN ISO 2733. For Alkali resistance test, 320ml of alkali test solution as described prepared and heated at 80°C shall be filled in to the test apparatus and the opening of the test apparatus closed off with a rubber plug. The test apparatus shall be then placed in a hot water bath maintained at 80°C + 1°C for 48 hours. It shall be ensured that the level of water immerses the test apparatus till only the neck portion is visible. After 48 hours the test apparatus shall be removed from the hot water bath, test circle samples remove and washed thoroughly with distilled water, acetic acid and finally again with distilled water. The test circle samples shall be once again treated as mentioned before testing.
Alkaline corrosion resistance test carried out (for NaOH) as per standard DIN-51156/ISO 2744/DIN EN ISO 28706-4 is 0.18 mm/year in liquid phase and alkaline corrosion resistance test apparatus is taken as per standard DIN ISO 2734.
Chemicals Preparation:
For Acid Resistance Test:

20% hydrochloric acid (Weight/Volume).
For Alkali Resistance Test: Alkaline (NaOH) solution for each set of two samples shall be 320ml of solution of 40gms NaOH dissolved in 1000 ml distilled water.
Calculations: (for both Acid and Alkaline resistance test):
The main quantitative criteria for this test is weight loss, which by knowing the glass density and corroded surface area, can be readily converted to a linear corrosion rate.
D = M/V where, D = density of glass, M = loss of weight, V = volume
i.e. V = M/D
i.e. Rate of corrosion = M/D

= weight loss X 29.2 mm/year for 1 day = weight loss X 14.6 mm/year for 2 day
Rate of corrosion = 14.6 X weight loss = mm/year.
(B) Surface roughness testing procedure:
Sample preparation:
The test shall be carried out on 100 mm X 100 mm X 10 mm thick
enamel quality plates with a glass lining of 1.2 to 1.6 mm thickness.
Instrument:
Roughness Tester (RA meter) of model no SJ 201F of MITUTOYO
JAPAN.
Surface roughness value for different types of glasses:
Acid / Alkaline resistant glass (colour of enamel is Blue):
Reading: 0.22 microns 0.31 microns
0.50 microns Average: 0.36 micron
0.32 microns

0.31 microns
Pharma glass (colour of enamel is Light blue/sky blue):
Reading: 0.09 microns 0.26 microns
0.18 microns Average: 0.15 micron
0.13 microns 0.10 microns
Photochromic glass (colour of enamel is White):
Reading: 0.15 microns 0.22 microns
0.25 microns Average: 0,20 micron 0.18 microns 0.20 microns
High temperature resistance glass (colour of enamel is Green):
Reading: 0.30 microns 0.25 microns
0.20 microns Average: 0.25 micron
0.28 microns 0.23 microns
(C) Impact Test:
265 gm weight ball is dropped from 600,700, 800, 900 & 1000 mm
Height.
i.e. For 600 mm height, 0.265 kgs X 0.6 meter = 0.165 M kg (no
surface crack observed ).Standard for glass lined is 0.104 M kg
(D) Thermal Shock Test:
The test shall be carried out on 100 mm X 100 mm X 10 mm plate or 50 cm2 (circle) X 1.5mm thick enamel quality plates with a glass lining of 1.2 to 1.6 mm thickness. It must be free from rust and foreign contaminants. The test Stainless glassed glass enamelled plates are heated to a pre-determined temperature (200°C) for a selected time. On completion of the heating period the specimen is quenched for 30 seconds in a jet of cold water (20°C), then heated again in the oven. The process is repeated for a number of cycles. No surface cracks observed.

This process is carried out for different temperature like 150, 180, 200, 220, 230, 240, 250, 260, 270 and 280 degree centigrade with 20°C water as above mentioned process until no surface crack observed. Thermal shock resistance test carried out as per DIN EN ISO 28721-2.
INDUSTRIAL APPLICABILITY OF THE PRESENT IVNENTION
Glass frit compositions and vitreous enamelling process using glass frit compositions and mill additives are used in manufacture of glass lined Pressure vessel, Pressure tank, Columns, Condensers, RCVD (Rotary Cone Vacuum Drier), Nautch filters. Pipes, Tee, and it's spares items like Valve body (Flush bottom outlet valve, Diaphragm valve, Bellow valve), Spindles, Blind flanges, Pressure flanges, Reducing flanges, Reducers, Protection ring,tManhole covers, Hand hole covers, Distance pieces and many other glass lined items.
These glass lined metal and their metal engineering products are used in Chemicals, Pharmaceuticals, Dye and Food industries for Transport of liquids to gases, Evaporation, Distillation, Extraction, Halogenations (Chlorination, Bromination), Nitration, Condensation, Sulphonation, Oxidation, Reduction, Ammonolysis, Diazotation, Esterification, Hydrolysis, Alkylation, Friedel-crafts reactions, Polymerization, Crystallization, Suspension, Dispersion and for many other reactions.

We claim:
1. A process of vitreous glass enameling onto the metal or metal
engineering product wherein the process comprises the steps of:
a. Mixing the desired glass frit composition and milling
additives;
b. carrying out the desired coating process selected from
ground coat or cover coat onto the metal or metal
engineering product;
c. repeating the step (b) till the glass coating thickness
between 0.8 to 2.5 mm of the glass enameling onto the
metal or metal engineering product is obtained.
Wherein
Glass frit composition comprises:
SiCb, Na2O and K2O as essential oxide components; and one or more components selected from CaO, CoO, BaO, Fe2O3, LiO2, SrO, ZrO2, Al2O3, B2O3) Cr2O7) TiO2, CaF2, NaF, Mn02, NiO and Milling additives comprises:
Quartz and water as essential raw materials; and atleast one chemical selected from syloid, titanium dioxide, borax, aluminium oxide, sodium nitrite, potassium chloride, lithium fluoride, cobalt oxide, chromium oxide, magnesium carbonate, clay and cerium oxide.
2. The process as claimed in claim 1 wherein oxide component is
obtained from the raw material selected from silica, feldspar,
borax, soda ash, calcium carbonate, barium carbonate, boric
acid, aluminium oxide, fluorspar, sodium titanate, sodium
nitrate, potassium nitrate, potassium carbonate, titanium
dioxide, lithium carbonate, sodium silico-fluoride, zinc oxide,
zircon flour, manganese dioxide, iron oxide, cobalt oxide, nickel

oxide, chromium oxide, strontium carbonate, magnesium carbonate, cerium oxide and the other raw material which can generate oxide components after loss on ignition of required raw materials.
3. The process as claimed in claim 1 wherein range of oxide component used in the glass frit composition is about 45-72% Si02, 6-24% Na2O, 1-12% K2O, 0.5-8% CaO, 0.5-5% CoO, 2-14% BaO, 0.1-3% Fe2O3, 1-8% LiO2, 2-5% SrO, 2-8% ZrO2, 1-6% A12O3> 3-20% B2O3, 2-6% Cr2O7, 3-8% TiO2, 2-8% CaF2, 1-4% NaF, 0.5-2% MnO2, 0.2-1.5% NiO.
4. The process as claimed in claim 1 wherein metal or metal engineering product for the purpose of glass enameling is selected from mild steel, stainless steel or cast iron.
5. The process as claimed in claim 1 wherein 1 to 2 ground coat is carried out.
6. The process as claimed in claim 1 wherein 3 to 8 cover coat is carried out.
7. The process as claimed in claim 1 wherein range of the chemical/ raw material used in the mill additives are between 1% to 50% by weight of Quartz; between 40% to 70% by weight of water; between 2% to 5% by weight of syloid; between 3% to 8% by weight of Titanium dioxide; between 0.1% to 0.5% by weight of Cobalt oxide; between 1% to 6% by weight of Chromium oxide; between 0.5% to 1.5% by weight of Borax; between 1% to 4% by weight of Sodium Nitrite; between 1% to 3% by weight of Magnesium carbonate; between 1% to 5% by weight of Aluminum oxide; between 1% to 5% by weight of Lithium fluoride; between 0.5% to 5% by weight of Potassium chloride,

between 0.5% to 7% by weight of Clay; between 5% to 11% by weight of Cerium Oxide.
8. An acid resistant glass coated metal or metal engineering product obtained by the process claimed in claim 1.
9. An alkali resistant glass coated metal or metal engineering product obtained by the process claimed in claim 1.
10. A Photochromic glass coated metal or metal engineering product as claimed in claim 1.
11. A pharmaceutical industry purpose glass coated metal or metal engineering product obtained by the process claimed in claim 1.
12. A high temperature resistant glass coated metal or metal engineering product obtained by the process claimed in claim 1.
13. An opaque white coloured glass coated metal or metal engineering product obtained by the process claimed in claim 1.
14. A cover coated glass lined cast iron metal or metal engineering product obtained by the process claimed in claim 1.
15. Glass frit composition comprises:
SiO2, Na2O and K2O as essential oxide components; and one or more components selected from CaO, CoO, BaO, Fe203, LiO2, SrO, ZrO2, A12O3, B2O3, Cr2O7, TiO2, CaF2, NaF, MnO2; NiO.
16. Glass frit composition as claimed in claim 15 wherein oxide
component is obtained from the raw material selected from
silica, feldspar, borax, soda ash, calcium carbonate, barium
carbonate, boric acid, aluminium oxide, fluorspar, sodium
titanate, sodium nitrate, potassium nitrate, potassium

carbonate, titanium dioxide, lithium carbonate, sodium silico-fluoride, zinc oxide, zircon flour, manganese dioxide, iron oxide, cobalt oxide, nickel oxide, chromium oxide, strontium carbonate, magnesium carbonate, cerium oxide and the other raw material which can generate oxide components after loss on ignition of required raw materials.
17. The glass frit composition as claimed in claim 15 wherein range
of oxide component used in the glass frit composition is about
45-72% SiO2, 6-24% Na20, 1-12% K20, 0.5-8% CaO, 0.5-5%
CoO, 2-14% BaO, 0.1-3% Fe2O3,1-8% LiO2, 2-5% SrO, 2-8%
ZrO2, 1-6% Al2O3, 3-20% B2O3, 2-6% Cr2O7, 3-8% TiO2, 2-8%
CaF2, 1-4% NaF, 0.5-2% MnO2, 0.2-1.5% NiO.
18. A process for the preparation of Glass frit composition comprises
the steps of:
Mixing the essential oxide components Si02, Na20 and K20; optionally mixing atleast one oxide component selected from CaO, CoO, BaO, Fe203, Li02, SrO, Zr02, A1203, B203, Cr207, Ti02, CaF2, NaF, Mn02, NiO;
heating it in smelting rotary furnace at a temperature between 1450°C to 1600°C to give molten liquid; quenching the above molten liquid into water to obtain glass frit composition.
19. The process as claimed in claim 18 wherein range of oxide component used in the glass frit composition is about 45-72% Si02, 6-24% Na2O, 1-12% K2O, 0.5-8% CaO, 0.5-5% CoO, 2-14% BaO, 0.1-3% Fe2O3, 1-8% LiO2, 2-5% SrO, 2-8% ZrO2, 1-6% Al2O3, 3-20% B2O3, 2-6% Cr2O7, 3-8% TiO2, 2-8% CaF2, 1-4% NaF, 0.5-2% MnO2, 0.2-1.5% NiO.

20. The process as claimed in claim 18 wherein oxide component is obtained from the appropriate raw material selected from silica, feldspar, borax, soda ash, calcium carbonate, barium carbonate, boric acid, aluminium oxide, fluorspar, sodium titanate, sodium nitrate, potassium nitrate, potassium carbonate, titanium dioxide, lithium carbonate, sodium silico-fluoride, zinc oxide, zircon flour, manganese dioxide, iron oxide, cobalt oxide, nickel oxide, chromium oxide, strontium carbonate, magnesium carbonate, cerium oxide and the other raw material which can generate oxide components after loss on ignition of required raw materials.