FIELD OF THE INVENTION
This invention relates to a twin-layer coating methods for improving sea water corrosion resistance of condenser tube sheet made of carbon steel.
BACKGROUND OF THE INVENTION
Thermal power plant situated near the sea coastal area uses sea water as a coolant for cooling the machinery as the sea water is easily accessible. But the sea water solution is highly corrosive in nature, and corrode most of the metal /alloys except some of the corrosion resistant metal like titanium, etc. Due to high corrosion resistance of titanium, it is used in sea water corrosive atmosphere. But because of its high cost, the use of titanium is limited. The condenser tube sheet plate is made of carbon steel and the steel plate is cladded with titanium sheet to protect the tube sheet plate from sea water corrosion. Cladding of such a large piece is a difficult job due to lack of commonly available economic facilities. Hence, a complete cladded piece is imported. Leading to high cost and increased cycle time.
Sea water is used in condenser of thermal power plant for cooling the water. As sea water is highly corrosive in nature and it corrodes the entire tube sheet at a fast rate. A sample of corroded tube sheet is shown in Fig.1. In order to avoid corrosion according to prior art, the tube sheet is cladded with titanium plate. Titanium is well known for their excellent corrosion resistant material under highly corrosive atmosphere. A comparison of corrosion resistance of various materials is given in Table-1.


From the above Table-1 it can be seen that titanium is the most corrosion resistant metal in comparison to other metals such as stainless steel, monel, aluminum, etc. Due to this reason, condenser tube sheet is protected by cladding with a titanium sheet of 5 mm thickness. Cladding of large titanium plate is complicated process and such technology is not available in our country. Because of this titanium cladded tube sheet is imported. Importing cladded tube sheet is a costly process and it is also time consuming.

OBJECTS OF THE INVENTION
It is therefore an object of the present invention is to propose a twin-layer coating methods for improving sea water corrosion resistance of condenser tube sheet made of carbon steel.
Another object of the invention is to propose a twin-layer coating methods for improving sea water corrosion resistance of condenser tube sheet made of carbon steel, in which the costly process of titanium cladding on condenser tube sheet is replaced with polymer coating.
SUMMARY OF THE INVENTION
A twin-layer coating methods for improving sea water corrosion resistance of condenser tube sheet made of carbon steel. The present inventions derived that Polymer coating (epoxy) is most satisfactory as polymers are highly corrosion resistant due to presence of covalent bond in the compound as well as its low cost. In order to improve a better adhesion of the coating a bond coat of stainless steel wire to a thickness of 300-400 microns is provided followed by polymer (epoxy) coating to a thickness of 2.0 to 5.0 mm. The bond coat is applied by wire arc spray process whereas epoxy coating is applied by a brush.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS AND TABLES
Figure 1 Shows a photographic view of a corroded condenser tube sheet
Figure 2 Shows various samples of condenser tube sheet components after
60 days from the date of twin layer coating applied according to the inventive method.
Table 1 Shows the corrosion resistance properties of different metals and
alloys.
Table 2 Shows different types of samples prepared according to possible
coating methods to compare and decide the best method of solving the prior art problems.
Table 3 Shows the salt-spray test conditions applied on the condenser
components to validate the results of the inventive process.
DESCRIPTION OF THE INVENTION
According to the invention, a known arc spray system is used for spraying various coating wires. Arc spray systems are relatively simpler than other thermal spray process and it can adopted at the manufacturing site/units easily. This is based on electrical arc which is generated when two wires meet each other. Once the arc is generated , it produces a lot of heat which melts the wire. Melted

wires is spread using compressed air on the substrate. Arc spray gun is mounted on a 6 axis robot for carrying out the coating. This coating is carried out inside an acoustic chamber because the coating process produces a high noise. The coating thickness is maintained around 300-400 microns for all the samples. Some of these coated samples are coated with the epoxy. Epoxy coating thickness is selected between 2000 – 5000 microns.

Epoxy coating is done by mixing the epoxy resin and hardener at a ratio of 2: 1 and mixing the mixture for 15 minutes. The mixed solution is applied manually using a brush. All these samples are grit blasted for cleaning the samples from

dirt’s, oxide scale, oily marks, etc. Grit blasting helps to clean the surface as well as it creates uneven surface which is required for better adhesion of the coatings.
All these samples as shown in Table-3 were subjected to salt spray test . Salt spray test is used for evaluating corrosion resistance under sea water atmosphere. The relevant ASTM standard is ASTM B 117. Salt spray tested samples after 60 days of exposure is shown in Fig. 2.


WE CLAIM :
1. A twin-layer coating methods for improving sea water corrosion resistance of condenser tube sheet made of carbon steel, comprising the steps of :-
- grid-blasting of the condenser tube sheet using 12-16 mess alumina grit powders to prepare the surface;
- conducting an arc spray coating using metallic wires by a robotic gun in a spraying system; and
- carrying-out polymer coating manually by a brush using resin and hardener at a ratio of 2:1 for a period of about 15 minutes;
wherein the coating thickness of the metallic layer and the polymer layer respectively maintained at 300 to 400 microns and 2000 to 5000 microns.