Fusion bonded epoxy coating
FBE coating application process
Regardless of the shape and type of steel surface to be coated, the FBE powder coating application has three essential stages:
- the steel surface is thoroughly cleaned,
- the cleaned metal part is heated to the recommended FBE powder application temperature, and
- the application and curing stage.
The advantage of pipe and rebar is that their round shape allows continuous linear application over the exterior surface, while the parts are moved in a conveyor through the powder application booth, ensuring high throughput. On fittings, etc., the coating is applied by manual spray guns. Another method of application is "fluid-dip" process, in which the heated components are dipped in a fluidized powder bed (see below).
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 . Commonly used to profile ranges for FBE coatings are 37 to 100 micrometers (1.5 to 4 mils). Profile increases the effective surface area of the steel. The cleanliness achieved is assessed to ISO 8501-1 grades: these originated from a set of photographic slides in a Swedish standard (SIS) showing exemplars of the common terminology of white-metal, near white-metal, etc. Typically, SA 2½ is used for pipelines (equivalent to NACE N°2).
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 dan 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 pipe. In this method, 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 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 stand alone FBE is 225° to 245°C. When used as a primer in a multi-layer polyolifine system, application temperature may be dropped based on 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 has been developed recently by certain FBE powder manufacturers. Other methods of heating are ‘’oven heating’’, ‘’infra-red heating’’, etc. The FBE powder is placed on a “fluidization bed”. In a fluidization 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 fluidized powder is sprayed onto 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 wraps around the substrate, and melts 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 stand-alone 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 few seconds after coating application. The resin part of coating will undergo cross-linking, which is known as “curing” under the hot condition. 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 few minutes in case of long cure FBE’s, which are used for internal pipe coating applications.
Rebars are coated in a similar manner as coating application, on the exterior of pipes. For FBE coating application on the interior of pipe surface, a lance is used. The lance enters into the pre-heated 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 pre-determined speed.
On fittings such as Tee's, elbows, bends, etc., powder can be sprayed using hand held spray guns. Small sized fittings can also be coated by dipping in a fluidized bed of powder, after heating the steel to the required powder application temperature. After field welding of the pipe ends, FBE can be applied on the weld area as well.