Fusion bonded epoxy coating

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 'Fusion bonded epoxy coating ', also known as  'fusion-bond epoxy powder coating ' and commonly referred to as  'FBE coating ', is an epoxy powder coating that is widely used to protect steel pipes, rebars and valves from corrosion. FBE coatings are thermoset polymer coatings. They come under the 'protective coatings' in paints and coating nomenclature. The name 'fusion-bond epoxy' derived from the way of resin cross-linking and their method of application which is different from that of conventional liquid coatings. FBE coatings are in the form of dry powder at normal atmospheric temperatures. The resin and hardner parts in the dry powder remain unreacted at normal storage conditions. At typical coating application temperatures, usually in the range of 180 to 250 °C, the contents of the powder melt and transform to a liquid form. The liquid FBE film 'wets and flows on to' the steel surface on which it is applied and soon becomes a solid coating by chemical cross-linking assisted by heat. This process is known as “fusion bonding”. Since their introduction as a protective coating in early 60’s, FBE coating formulations had gone through vast improvements and developments. Today, various types of FBE coatings, which are tailor made to meet the varying requirements of the industry are available in the market. Following recent research works by various FBE manufacturers, FBE’s are available as stand-alone coatings as well as in multi-layers. FBE coatings with different chemical and physical properties are available to suit coating application on main body of the pipes, internal surface, girth welds as well as fittings. Similarly, variations are also available to match different ranges of pipeline service conditions.

A barrier coating for steel

One of the main usage areas for FBE coatings is currently in the pipeline industry. Many pipeline operating companies worldwide has adopted FBE — either in stand alone form or as a primer in multi-layer coating systems - as a standard corrosion protection coating for steel tubular and related fittings. Owning to their ability to adhere well to the steel in molecular levels, FBE offer excellent resistance to steel components against corrosion attack for long time under severe environmental conditions. The FBE coating isolates the steel surfaces which are prone to corrosion attack if exposed to moisture and oxygen - two basic factors for corrosion to start- effectively as a barrier. They are also proven to be fully compatible with Cathodic protection System; another vital system used for corrosion protection of steel structures. Another area where FBE coatings are widely used is the concrete reinforcing rebar industry. FBE coated rebars are proven to be very effective in concrete structures, preventing corrosion attacks on reinforcing steel leading to eventual failure of the structure.

Chemistry of FBE coatings

:  See also Epoxy#Chemistry  “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, to achieve various properties, combinations or additions with other type of resins are also commonly used in FBE formulations. 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.

FBE powder manufacturing process

Essential parts of a powder coating manufacturing plant are (1) weighting station (2) pre-blending station (3) an extruder and (4) a classifier or grinding unit. The components of the FBE formulation are accurately weighed and are pre-blended in high speed mixers. The mix is then transferred to a high-shear extruder. FBE extruders incorporate a single or dual screw setup rotating within a fixed clamshell barrel. A temperature gradient between 50 °C to 100ºC is used within the extruder barrel. This setup compresses the FBE blend, while heating and melting it to a semi-liquid form. During this process, the ingredients of the molten mix get dispersed thoroughly. Because of the fast operation of the extruder and relatively low temperature within the barrel, the epoxy and hardener components will not undergo significant chemical reaction. The molten extrudate then passes between cold-rollers and become a solid, but highly brittle sheet. It further moves to a “Kibbler” which chops it to smaller chips. These chips are ground using high speed grinders (classifiers) to a particle size of less than 150 micrometres (standard specifications requires 100% pass through in 250 micrometre sieves and maximum 3% retains in 150 micrometre sieve). The final product is packaged in closed containers — particular care is given to avoid moisture ingression. Normal storage temperatures of FBE powder coatings are below 25 °C in air-conditioned warehouses.

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. First, the steel surface is cleaned to a high grade of cleaning, second, the cleaned metal part is heated to the recommended FBE powder application temperature and finally 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 linear conveyor through the powder application booth. This method ensures high production rates. On fittings etc. the coating is applied by manual spraying 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 method removes rust, scale, slats etc from the metal surface in an effective manner 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 micrometres or mils. Commonly used profile ranges for FBE coatings are 37 to 100 micrometres (1.5 to 4 mils). Profile increases the effective surface area of the steel. The cleanliness achieved is assessed as NACE grades or in accordance with Swedish standard (SIS) terminology of “white-metal, near white-metal” etc. It is very 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) is used to force an abrasive onto the surface to be cleaned. 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 the pipes. 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. 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, incorporate 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 travels 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 to 500 micrometres 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 ‘geltime’ is over, which is 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 start spraying the powder from the opposite end, while the pipe is being rotated on its axis and the lance pulled 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. Advantages of FBE application over conventional liquid coating application are ease of application, less waste of material and rapid application and cure schedules which means faster production rates. Finished coated piece can be moved to storage area within minutes after application.

See also

• Epoxy
• Rebar
• Thermoset powder coatings
• Corrosion
• Cathodic protection
• Powder coating

References / External links

• Fusion-Bonded Epoxy (FBE): A Foundation for Pipeline Corrosion Protection - book by J. Alan Kehr (NACE International Publication)http://www.nace.org/nacestore/product.asp?SKU=37580&ProdName=Fusion+Bonded+Epoxy+%28FBE%29%3A+A+Foundation+for+Pipeli
• http://www.nace.org/nacestore/assets/freestandardsreports/RP039402.pdf
• http://www.pfonline.com/articles/010002.html
• http://www.madisonchemical.com/pdf_tech_papers/AUCSC2001_The_Corrosion_Protection.pdf
• http://www.stormingmedia.us/67/6781/A678192.html
  
  This article is licensed under the GNU Free Documentation License.It uses material from the Wikipedia article 'Fusion bonded epoxy coating' (click for full Wikipedia text) Hide