Flow Chart of Powder Coating Process

Process Flow Chart

THE COATING PROCESS

The application of this process involves in the following steps:

Degreasing

First cleaned Reinforcement bars from surface contaminations such as oil and grease etc. by solvent cleaning.

Surface preparation

Secondly, blast clean the Reinforcement bars using abrasive grit or by shot blasting to near white metal finish. Mill scale and rust may be removes by shot blasting. It also roughens the surface to give it a textured anchor profile.

Heating

Heating the Reinforcement bars, under the temperature ranging from 200 C to 240 C using furnace.

Powder application

Dry epoxy powder is then sprayed on the heated steel rebar through spray gun. As the powder leaves the spray gun, When the dry powder hits the hot steel, it melts and flows into the anchor profile. It also bonds with the steel. The heated steel cause chemical reaction to form complex cross linked polymers. Coating thickness in the range of 50 to 150 micrometers are usually obtained.

Curing and cooling

The molten powder becomes a cure after few seconds after coating application.

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The resin part cause cross linking, which is known as ‘curing’ under the hot condition. Complete curing is attained with the help of residual heat on the steel. Full can be attained in less than one minute to a few minutes.

Inspection and testing

Coating of FBE reinforcement bars are tested as per the requirements specified in standard. The adhesion of coated bars is also tested frequently by bending of the bar. Besides this, various other tests are conducted in laboratory such as chemical resistance, resistance in continuous boiling water, abrasion resistance and impact resistance etc.

Handling, transportation and working

FBE coated reinforcement bars are to be handled with extraordinary care so that coating is not damaged meanwhile during storage. These reinforcement bars require cover contacts during transportation, stacking, handling till they are used in concrete.

CORROSION PRINCIPLE

This epoxy coating is deliberately employed to serve as barrier to protect the rebar. Concrete acts as passive but chlorine in the presence of water and oxygen, penetrate the concrete, Ph is reduced and the passive film has been destroyed and cathodic and anodic reaction take place.

Anodic Reaction

At the anode, iron atom lose electron because of the broken passive layer, hence oxidation is carried out. Fe › Fe2 + 2e-The iron ions further reacts with the hydroxides to produce ferrous hydroxide. Fe2 + 2OH- › Fe(OH)2 Ferrous hydroxides is then reacted with diffused oxygen to form rust 4Fe(OH)2 + O2 › 2Fe2O3(rust) + 4H2O

Cathodic Reaction

At the cathode, gain of electron take place through electron flow through the steel where they react with water and oxygen to form hydroxides ions. 4e- + 2H2O + O2 › 4OH-Oxygen atom are reduced and form hydroxides ions that further persist the anodic reaction.

HOW FUSION BONDED COAED REBAR (FBECR) PROTECTS

When Chlorine ions penetrate through the concrete, the unprotected steel is then corrodes. Epoxy coating on rebar acts as a barrier against chlorine ion which is easily penetrated. Steel can corrode at any hole but the extant of corrosion is suppressed by the epoxy layer which satisfies the cathodic reaction and prevents macrocell corrosion.

Performance of Fusion Based Epoxy Coating

The epoxy coating performs well and minimizes the rate of degradation in high chlorine levels. Furthermore a comparison of research work has been carried out of performance of carbon steel, galvanized steel and stainless steel rebar under high chlorine environment but FBECR performs well than above discussed rebar but less well than austenitic stainless steel.

Advantages of FBE coating

FBE coating on reinforcement bars has the following advantages.

• Since the coating is conducted on the coating assembly lines, better quality control is achieved. The process permit uniform coating thickness.
• Excellent bonding and adhesive properties is attained on hot steel rebar.
• The coating does not get destroyed when the straight bar is bent, Because of flexibility.
• FBE coating has excellent corrosion resistance in salt containing environment.

Disadvantages of FBE coating

FBE coating on reinforcement bars has the following disadvantages.

• Less adhesion between coated rebars and concrete.
• Extraordinary Handling are required.
• Corrosion can initiate at small damage area of FBE coating.
• Being a barrier type coating, it facilitates localized pitting corrosion through pinholes.
• Prolong sunlight cause degradation..
• FBE coated bar having poor alkali resistance.

APPLICATION

Epoxy coated rebar or corrosion resistant rebar is employed instead of conventional reinforcing bar to provide strengthen the concrete and provide protection against corrosion. Epoxy coating is applied to factory In order to ensure corrosion resistance. Epoxy coated rebar is utilized in the following structures

• Bridges
• Parking structures
• Pavement
• Marine structures
• Repair

Factor Affecting Fusion Bonded Epoxy Coating

1. The primary problem s caused by inorganic salt induced corrosion of steel in concrete primarily Chlorine.
2. Improper handling and transportation of steel can cause damage.
3. The initiation of macrocell due to differential aeration and chloride absorption.
4. Presence of oxygen to accelerate the corrosion process.
5. pH of concrete is very important.
6. Thickness of coating is yet another important parameter.

TESTING OF REBARS

Rockwell Hardness Test

The Rockwell hardness test method, is widely used hardness test method. The test specimen is indented by an indenter and the hardness value is obtained at the gauge.

Procedure:

• Apply a minor load of 10kgf
• Then the dial is set to zero and then major load is applied
• Then apply major load 60 to 150 according to the scale used for 4 to 5 seconds
• Release the major load only
• Machine will show the Rockwell hardness number HR on the machine
• All these operations will be done by machine automatically.

Tensile & Bend Test

Universal testing machine which applies a tensile force in opposite direction to a specimen and material is stretched or pulling by tensile loading until it fractures and then measures that force and also the elongation. The following properties can be determined by tensile testing

• Strength
• Ductility
• Elasticity
• Stiffness
• Malleability
• Modulus of toughness
• Modulus of resilience

The bend test is also known as flexure test measures the force required to bend a beam. The bend test is employed for assessing the workability of sheet and plate ductility test which is employed to evaluate the ability of metallic materials undergo plastic deformation under bending. Bend test for ductility provide a simple way to evaluate the quality of materials by their ability to resist cracking or other surface irregularities during one continuous bend. No reversal of the bend force shall be employed when conducting these tests.

Procedure:

• The test specimen is placed on the supporting pins
• The loading force is applied in the middle by means of loading pin

Fatigue Test

A fatigue test , metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause failure on a single applicat.ion of load. Failure occurring under a condition of dynamic loading are called fatigue.