Babar Nadeem

Topic: Fusion Bonded Epoxy

Fusion Bonded Epoxy

 

INTRODUCTION

 

Fusion Bonded Epoxy coating has been used for the corrosion protection. It acts as a barrier to corrosive chemicals and moisture, which are essential components of the corrosion process. It is applied to preheated steel as a dry powder, which melts and cures to a uniform coating thickness. The FBE system was first introduced in Europe in 1953 for coating electrical equipment by fluidized bed dipping process. Where as the electrostatic spray system was introduced in 1962, this technology was introduced for coating pipelines for oil, gas, and water transportation. During the late 70ís FBE became the most widely used pipeline coating in USA, Canada, Saudi Arabia, and UK. Today it is successfully used in each country for corrosion protection system for underground pipelines.

 

 

It had been established that the corrosion control property of the coating is dependent on its ability to be an excellent barrier against water, oxygen, chloride, and other aggressive elements permeating to the metal surface. However, for a coating to be an effective long-term corrosion protection system, it is essential that it stays bonded to the substrate during the design life of the structure. However, polar elements such as water, once permeated to the interface, can reduce the overall adhesive strength. Therefore, epoxy coatings, including FBE, exposed to wet environment will not have the same adhesive strength as the newly applied one or the one that was exposed to only dry conditions. Since reduction in adhesive strength is inherent during its service life, the important question is whether a reduction in adhesive strength signals the end of its performance. It is important to know how much adhesive strength is needed for a coating to continue to perform. Another corrosion control mechanism of a coating is related to its electrical properties. Protective coatings are designed to offer a high resistance between the cathode and the anode of the corrosion cells and to minimize the number of micro cells on the substrate. The success depends on its ability to place itself between the cathode and the anode, which are separated only by microns. Known as the wetting property, this controls the sites where water gets accumulated and leads to coating blisters and poor adhesion. Although there are other critical properties that are required for corrosion protection coatings, adhesion and wetting properties are critically important to epoxy coatings. A clear understanding of the basic corrosion process is helpful to understand the role of these properties in corrosion control of epoxy.

 

 

 

Three-Layer Application Parameters of FBE

 

Three-layer-coating systems provide specified performance properties for the external protection of steel pipes designed for safety and long life transmission service.

The multi-layer pipe coating combines the benefits of two major advantages:

Ø  The fusion bond epoxy film, which has excellent adhesion and chemical resistance properties.

Ø  Extruded intermediate adhesive copolymer layer and the extruded multi-layer polyethylene top layer providing in addition to the corrosion protection strong physical and mechanical performance even at elevated temperatures.

FBE is very fast curing, thermosetting Powder Coating based on especially selected Epoxy resins and hardeners and formulated in order to meet the specifications related to external protection of underground steel pipelines and particularly as an anticorrosion primer prior to polyethylene coating application, according to the “3-layer” technique.

                                                                  

 Coating inspection and testing is carried out in accordance with international coating standards and test methods which include:

1-      Testing of Specific gravity, Gel time, Particle size distribution, melt flow and moisture of the FBE.

2-      Inspection of the pipe prior to shot blasting.

3-      Checking of the pipe temperature before blasting.

4-      Checking the blasted profile and depth of the anchor pattern.

5-      Temperature checking before acid wash

6-      Monitoring the chromate wash.

7-      Adjustment of the pipe speed and temperature before FBE application.

8-      Adjustment of the time carefully between the FBE application and adhesive applications

 

 

Surface preparation

 

Surface preparation is essential to the ability of the coating to bond to the pipe substrate. This bonding is important to eliminate the environmental fluid migration between the substrate and the pipe coating. It is therefore very important to understand the surface preparation requirements of the coating system to be selected. There is no shortcut here indeed, because poor surface preparation always results in poor bonding strength of the coating to the pipe.

The surface is cleaned using steel grit to obtain surface profiles of 60-110 microns. The surface is then washed with phosphoric acid and then with deionized water for the removal of water-soluble salts and organic contaminants. If left on the surface salts especially chlorides and sulfates can initiate water absorption by osmosis and lead to coating failure. It has been reported that chloride contamination will seriously affect adhesion and cathodic disbondment properties. Chromate is applied to enhance the adhesive strength of Fusion Bonded Epoxy system.

  FBE Powder Application

At a temperature of 190-250°C the pipes move through the spray booth. Some 3-6 electrostatic powder spray guns blow fluidized FBE on the pipe surface. A small amount of excess powder is recovered and conditioned before being returned. FBE melts when it contacts the hot surface. The melted epoxy resin reacts with the curing agent present in the FBE and bonds to the substrate, providing a highly cross linked polymer with a sophisticated network of covalent and coordinate bonds. These high-energy bonds provide excellent adhesion between the coating and the metallic substrate. The amongst three adhesion forces between FBE and substrate, the two of them i.e. chemical and polar polar adhesion are directly related to the number of bondable sites available on the substrate. Thereofore to provide the maximum bondable site to FBE the high peak heights are obtained by abrasive cleaning. It also depends upon the viscosity of the FBE system used and the application temperature.

A general flow chart of the pipe speed, FBE application temperatures and the intercoat timings between the FBE and adhesive applications depending upon the distance between them is as follows

 

LINE SPEED

 

PREHEATING

TEMPERATURES

 

ADHESIVE WRAPPING DISTANCE

 

 

60CM              80CM         100CM

 

 

INTERCOAT   TIMINGS

1m

165-185170-190

36sec

48sec

55sec

1.5m

185-190 190-195

24sec

32sec

40sec

2m

190-195 195-200

18sec

24sec

30sec

3m

195-205 200-210

Powder Coating Manufacturing Technology Consultant

Karachi Pakistan

Email: powder_coatingindustry@yahoo.com

Web: www.powder-coating.webs.com

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