FIELD OF INVENTION:
This invention relates to a method of applying thin organic coating on galvanized
tubes to enhance corrosion resistance of the tubes.
BACKGROUND OF THE INVENTION:
Galvanized tubes are commercially produced by hot-dip zinc coating that gives
excellent corrosion resistance in the atmosphere and in scale forming water. The
zinc coating forms a protective layer consisting of a mixture of zinc oxide, zinc
hydroxide and various basic salts, depending on the nature of the environment.
Once the protective layers formed and completely cover the surface of the metal,
corrosion proceeds at a greatly reduced rate.
However, freshly prepared galvanized tube is highly susceptible to staining,
discoloration and corrosive attack in both indoor and outdoor environments. The
one form of general corrosion is not protective is "white rust" more properly called
wet storage stain. The white rust is found particularly on new galvanized surface
during storage in a narrow crevice where there is access for water but limited supply
of oxygen and carbon dioxide for the subsequent formation of a zinc carbonate
layer. The layer of zinc oxide and zinc hydroxide is a voluminous and porous and
adheres only loosely to the zinc surface. Consequently, it does not protect the zinc
surface against oxygen in the water. Corrrosion can, therefore, proceed as long as
there is moisture left on the surface.
White rust is a white, crumbly, and porous coating consisting of 2ZnCO3. 3Zn (OH)2
together with ZnO and voluminous B- Zn (OH) 2. The presence of chloride and
sulphur accelerates white rust formation. It is best avoided by preventing new zinc
surface from coming into contact with rain or condensate water during storage and
transport. Material stores outdoors should be arranged so that all surfaces are well
ventilated and water can easily run off the surface. However, it has a practical
application because of large space requirement both during storage and
transportation. Therefore, post treatment coating are widely used to combat the
white rust formation. The conventional post treatment coating includes chromate or
phosphate coating or painting or their combination.
OBJECTS OF THE INEVENTION:
An object of this invention is to propose a method of applying thin organic coating
on galvanized tubes.
Another object of this invention is to propose a method of applying coating on
galvanized tubes to enhance corrosion resistance of the tubes.
Further object of this invention is to propose a coating which has excellent corrosion
resistance, paint adhesion lubrication, anti finger print properties.
Still another object of this invention is to propose a thin organic coating application
process in which the full length Galvanized tube poasses in sequence through the
organic coating set-up which consists of mainly wet application, air wiper and
induction heater drier.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention, there is provided a method of applying this organic
coating on galvanized tubes to enhance corrosion resistance of the tubes
comprising
applying the TOC formulation in a continuous process on the full length of the Gl
tube
subjecting the coated Gl tube to the step of wiping
and
drying the Gl tube in a continuous process.
BRIEF DESCRIPTION OF THE ACCOMPYING DRAWING:
FIGURE 1: Photograph showing set-up made to study the TOC application and
wiping process.
FIGURE 2: Photograph showing set-up made to study the TOC application,
wiping process and drying.
FIGURE 3: Schematic line diagram for proposing for TOC coating application on GI
tube.
FIGURE 4: Photograph of developed transparent as well as coloured TOC GI Tube.
FIGURE 5: Photograph of TOC and conventional GI samples after 120 and 240 of
salt spray test.
DETAILED DESCRIPTION OF THE INVENTION:
The objective of this invention is to increase the corrosion resistance property of GI
tube by applying organic coating on outer surface of GI tube. The further objective
of the invention is to develop TOC application process in which the full length GI
tube passes in sequence through the organic coating set-up which consisted of
mainly wet application, air wiper and induction heater drier. As per the requirement,
TOC film on GI tube can be produced in the thickness range of 1 -40 µm dry film
that improve the corrosion resistance performance of the product.
Furthermore, the developed TOC film not only enhances the corrosion resistance
performance but also gives the lubrication, paint adhesion, anti-fingerprint
resistance of the galvanized tube product.
The TOC formulation was made from the above modified constituents given in (a) to
(f). The constituents were added as per the sequence mentioned below under high
stirring for one hour and then filtered it by filter cloth. This formulation is covered in
earlier filed patent application No. 702 K0L09 dated 5.5.2009. The formulation
composition is mentioned below:
a) 65 parts of volume of styrene acrylic copolymer of 30 % solid content
b) 30 parts of by volume of polyurethane resin of 35 % solid content.
c) 4 parts by volume of polyethylene wax of 34 % solid content
d) 4 parts by volume of colloidal silica of 30 % solid content.
e) 0.5 parts by volume of nano zinc oxide of 45 % solid content.
f) 0.2 parts by weight of sodium phosphate dibasic dodecahydrate
g) Other surfaces active agent used to further tailor made the formulation as
per the requirements such as the antifoaming agent, flash rust inhibitors,
colouring agent, biocides, etc. They are used either in single or in
combination with other surface active agents.
The study of TOC application on Gl tubes was carried out in three steps. First, a
TOC coating was prepared in the R&D lab and applied on Gl tubes by dip
coating, air wiping and the hot air drying. It enhanced the corrosion resistance
property of Gl tube measured by salt spray test where TOC coated Gl tube took
120 hours for 10 % white rust formation (superior to the conventional 24
hours).More trials were conducted by passing horizontally the full length Gl tubes
of 25 mm diameter on rolls where organic liquids was poured on the Gl tubes,
circular air wiper used to squeeze extra organic liquid and radiation heater oven
used for drying the organic liquid that resulted in uniform coating on Gl tube.
The above trials showed that the polymer application system gave uniform
coating but did not dry properly in high speed such as 40 m/minute. Based on
the above trials, an organic liquid pouring system on Gl tube was developed and
was installed at Tube Division plant. Subsequently, plant trials were taken where
Gl tubes of 25 mm diameter were fed automatically into the line and passed
through the developed polymer application and air wiping systems. The trials
were taken on full length (6 meter) tubes at different line speeds. Figure 1
shows the photograph of plant trial set-up to study the TOC application and
wiping process.
In order to overcome the drying difficulty, further plant trials using induction
heating system were carried out at two different induction heating
manufacturers' plants. Figure 2 shows that photographs of plant trial for TOC
coating application on Gl tube in which induction heater was used for coating
drying. It was observed that the organic coating dried properly after coming out
from induction heater at peak metal temperature range of 80 - 120 ° C.
Figure 3 shows the schematic line diagram of polymer application set- up that
consisted of wet polymer application then air wiping followed by induction
heater drying. The TOC coated tube gave similar corrosion resistance
performance in salt spray test and passed 120 hours of white rust resistance.
The above study showed that the TOC film or Gl tube is possible by suitable
application set-up mainly consisted of wet film application then air wiper and
induction drying system as shown in Figure 3. Any colour of TOC film can be
produced by adding suitable pigment in TOC formulation. Figure 4 shows the
photograph of Gl tubes having transparent as well as colour film of TOC. The
developed Cr-free Thin Organic Coating (TOC) was found suitable to combat the
white rust problem as well as to increase the corrosion resistance properties of
Gl tubes. Figure 5 shows the photographs of TOC Gl and conventional Gl tube
samples after 120 hours ad 240 hours of salt spray test. It was observed that
TOC coating enhanced the corrosion resistance properties of Gl tube and less
white rust appeared on TOC Gl surface compared to conventional Gl.
WE CLAIM:
1. A method of applying thin organic coating on galvanized tubes to enhance
corrosion resistance of the tubes comprising
applying the TOC formulation in a continuous process on the full length of the
Gl tube
subjecting the coated Gl tube to the step of wiping
and
drying the Gl tube in a continuous process.
2. The method as claimed in Claim 1 wherein the step of applying the TOC
formulation in mainly wet application.
3. The method as claimed in claim 2 wherein the wet application method of
organic coating consisted of sprays or dips or their combination in which
outer surface of tube cover by liquid.
4. The method as claimed in claim 1, wherein the step of wiping
is performed by using suitable air wiper/physical wiper to remove the extra
coating from the surface to get desired coating film thickness.
5. The method as claimed in claim 1 where in the wet organic coating on Gl
outer surface is dried at peak metal temperature of 60 to 150 degree C by
induction heater of 10 to 350 KW and 3 to 30 KHz frequency.
6. The method as claimed in claim 1 where in the dry film thickness of the
organic film on Gl tube is in range of 1 to 40 µm.
7. The method as claimed in claim 1, where in the full length Gl tube put as a
input material passed over the rollers at speed of 10 to 80 meters per minute
and came out as a coated Gl tube.

According to this invention, there is provided a method of applying this organic
coating on galvanized tubes to enhance corrosion resistance of the tubes
comprising
applying the TOC formulation in a continuous process on the full length of the Gl
tube
subjecting the coated Gl tube to the step of wiping
and
drying the Gl tube in a continuous process.