Concrete structures deteriorate primarily due to corrosion of steel, which can be controlled by applying different kinds of coatings, like Fusion Bonded Epoxy.
Whenever you see a construction site of a bridge or a Metro rail, you will normally notice a green colouring in the steel used in these structures. This colouring indicates that the steel has been made resistant to corrosion by using a technique called Fusion Bonded Epoxy (EBE).
Weathering of concrete is a usual process. But if the surroundings are aggressive, the concrete undergoes degradation. The decay first attacks the steel inside the concrete, and then initiates the process of corrosion of rebars. Therefore, steel is coated with a protective layer to ward off corrosion. Presently, all infrastructure projects - bridges, viaducts, Metro tracks - use steel which has to be corrosion resistant.
There are many methods of protecting steel, but FBE coating has been the most popular one. FBE coating principally protects against corrosion by serving as an electrochemical and a physical barrier that isolates the steel from the oxygen, moisture and chloride ions that cause corrosion. Epoxy coating has high electrical resistance, which blocks the flow of electrons that make up the electrochemical process of corrosion. In addition to serving as a circuit breaker, the coating reduces the size and number of potential cathodic sites, which limits the rate of corrosion reaction that could occur.
As a matter of fact, for macrocell corrosion to take place, a large area of the steel surface is needed to serve as the cathode where oxygen reduction can occur. The coating almost eliminates such cathodic reaction.
Since their introduction as a protective coating in the early 1960s, FBE coating formulations have gone through vast improvements and developments. Today, various types of FBE coatings, which are tailor made to meet various requirements, are available. FBEs are available as standalone coatings as well as a part in multi-layers. FBE coatings with different properties are available to suit coating application on the main body of pipes, internal surfaces, girth welds as well as on fittings.
Chemistry of FBE coatings
Essential components of a powder coating are resin, hardener or curing agent, fillers, extenders and colour pigments.
The resin and hardener are together known as the binder. As the name indicates, in FBE coatings, the resin part is an epoxy type resin. The epoxy or oxirane structure contains a three-membered cyclic ring - one oxygen atom connected to two carbon atoms -in the resin molecule. This part is the most reactive group in the epoxy resins. Most commonly used FBE resins are derivatives of Bisphenol A and epichlorohydrin. However, other types of resins (for example Bisphenol F type) are also commonly used in FBE formulations to achieve various properties, combinations or additions. Resins are also available in various molecular lengths, to provide unique properties to the final coating.
The second most important part of FBE coatings is the curing agent or hardener. Curing agents react either with the epoxy ring or with the hydroxyl groups, along the epoxy molecular chain. Various types of curing agents used in FBE manufacture include dicyandiamide, aromatic amines, aliphatic diamines, etc. The selected curing agent determines the nature of the final FBE product - its cross-linking density, chemical resistance, brittleness, flexibility, etc. The ratio of epoxy resins and curing agents in a formulation is determined by their relative equivalent weights.
In addition to these two major components, FBE coatings include fillers, pigments, extenders and various additives, to provide desired properties. These components control characteristics such as permeability, hardness, colour, thickness, gouge resistance, etc. All of these components are normally dry solids, even though small quantities of liquid additives may be used in some FBE formulations. If used, these liquid components are sprayed into the formulation mix during pre-blending in the manufacturing process.
Surface preparation: Blast cleaning
Blast cleaning is the most commonly used method for preparation of steel surfaces. This effectively removes rust, scale, slats, etc., from the surface and produces an industrial grade cleaning and a rough surface finish. The roughness of the steel achieved after blasting is referred to as profile, which is measured in micrometers or mils. Commonly used profile ranges for FBE coatings are 37 to 100 micrometers (1.5 to 4 mils).
It is important to remove grease or oil contamination prior to blast cleaning. Solvent cleaning, burn-off, etc., are commonly used for this purpose. In the blast cleaning process, compressed air (90 to 110 psi/610 to 760 kPa) is used to force an abrasive onto the surface to be cleaned. Aluminum oxide, steel grit, steel shot, garnet, coal slag, etc., are the frequently used abrasives.
Another method of blast cleaning is centrifugal blast cleaning, which is especially used in cleaning the exterior of pipes. In this method, the abrasive is thrown to the rotating pipe body, using a specially designed wheel, which is rotated at high speed, while the abrasive is fed from the centre of the wheel.
Heating and FBE powder application Heating of the object can be achieved by using several methods, but the most commonly used ones are induction heating or oven heating. The steel part is passed through a high frequency alternating current magnetic field, which heats the metal part to the required FBE coating application temperature. Typical application temperature for a standalone FBE is 225°C to 245°C. When used as a primer in a multi-layer polyolifine system, application temperature may be dropped based on the FBE manufacturer´s recommendations, in order to meet the ´inter-coat adhesion´ parameters. Special grade FBE coatings which can be applied at temperatures as low as 175°C have been developed recently by certain FBE powder manufacturers.
Other methods of heating are oven heating, infrared heating, etc. The FBE powder is placed on a fluidisation bed. In a fluidisation bed, the powder particles are suspended in a stream of air, in which the powder will ´behave´ like a fluid. Once the air supply is turned off, the powder will remain in its original form. The fluidised powder is sprayed on to the hot substrate using suitable spray guns. An electrostatic spray gun incorporates an ionizer electrode on it, which gives the powder particles a positive electric charge. The steel to be coated is´grounded´ through the conveyor. The charged powder particles uniformly wrap around the substrate, and melt into a liquid form. Internal surfaces of pipes are coated using spraying lances, which travel from one end to the other end of the heated pipe at a uniform speed, while the pipe is being rotated in its longitudinal axis.
Standard coating thickness
range of standalone FBE coatings is between 250 and 500 micrometers, even though lower or higher thickness ranges might be specified, depending on service conditions. The molten powder flows into the profile and bonds with the steel. The molten powder will become a solid coating, when the gel time is over, which usually occurs within a few seconds after coating application. The resin part of the coating will undergo cross-linking, which is known as ´curing´ under hot conditions.
Complete curing is achieved either by the residual heat on the steel, or by the help of additional heating sources. Depending on the FBE coating system, full cure can be achieved in less than one minute to a few minutes in case of long cure FBEs, which are used for internal pipe coating applications.
Rebars are coated in a similar manner as coating applications, on the exterior of pipes. For FBE coating application on the interior of a pipe surface, a lance is used. The lance enters the preheated pipe, and starts spraying the powder from the opposite end, while the pipe is being rotated on its axis and the lance pulls out in a predetermined speed.
On fittings such as Tees, elbows, bends, etc., powder can be sprayed using handheld spray guns.
Since the process of coating is quite elaborate and cannot be carried out at a construction site, additional cost is incurred on logistics to send the material and get it back from the factory.
While bending the steel, the coating breaks at sharp bends, making these locations prone to corrosion.