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Fusion Bonded Epoxy Coating of Steel

• satyendra
• July 13, 2019
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• adhesion, Epoxy type resin, Fusion bonded epoxy coating, lamination, reinforcement bars, Spray guns, Thermoset coating,

Fusion Bonded Epoxy Coating of Steel

Fusion bonded epoxy (FBE) coating of steel materials is primer less, one-part, heat curable, thermosetting powdered epoxy coating which is designed to provide maximum corrosion protection to the substrate steel. It is a coating of very fast curing, thermosetting protective powder which utilizes heat to melt and adhere the coating material to the steel substrate. It is based on specially selected epoxy resins and hardeners. The epoxy is formulated in order to meet the specifications related to protection of steel as an anti-corrosion coating. Heat cured FBE coatings are 100 % solids consisting of thermosetting materials which achieve a high bond to metal surface as a result of a heat generated chemical reaction. The FBE coatings can be applied by fluidized bed, flocking (air spray), or electrostatic spray.

FBE coating is widely used for coating of steel pipes, pipe fittings, pumps and valves used for the transmission of oil, gas, slurry, and water. Typical FBE coated products are shown in Fig 1. The FBE coating has been used for underground pipelines since the 1960s. It has good track record for underground piping applications. It is also being used for the coating of steel reinforcement bars used in bridge, road, and building construction to help prevent corrosion when embedded in concrete.

Fig 1 FBE coated products

Features and benefits of FBE coatings include (i) corrosion protection in harsh environments, (ii) productive application due to fast curing, (iii)  does not sag, has no cold flow, and does not become soft in storage thus allowing for long term storage, (iv) has light weight, (v) has good chemical resistance, (vi) is environmentally friendly since there are no volatile organic compounds (VOCs), (vii) resists cathodic disbondment, (viii) has high adhesion and toughness, and (ix) can be easily repaired.

FBE coating system is a heat-activated, chemically cured coating system which is applied to preheated steel material to be coated. The typical formulation for FBE coatings consist of the epoxy resin, curing agent, catalyst, accelerator, reinforcing pigment, and control agents which regulate flow and stability. In FBE coating, the resin category is an ‘epoxy’ type resin. Permeability, hardness, colour, thickness, gouge resistance etc. and other characteristics are controlled by these components. Standard coating thickness range of FBE coatings is between 250 micrometers and 500 micrometers which can be varied depending on the service conditions. The molten powder becomes a solid coating within few seconds after coating application. FBE coatings are normally used in conjunction with cathodic protection. In most cases dis-bonded areas under FBE coating is protected by cathodic protection.

FBE coating materials

FBE coatings are thermoset polymer coatings. The name ‘fusion-bond epoxy’ is because of the resin cross-linking and the application method, which is different from a conventional paint. The resin and hardener components in the dry powder FBE stock remain unreacted at normal storage conditions. At typical coating application temperatures, which are generally in the range of 180 deg C to 250 deg C, the contents of the powder melt and transform to a liquid form. The liquid FBE film wets and flows onto 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. The chemical cross-linking reaction taking place in this case is irreversible. Once the curing takes place, the coating cannot be returned to its original form by any means. Application of further heating also does not melt the coating and thus it is known as a thermoset coating.

New technologies are under continual development to optimize the properties of the FBE coatings to improve coating utility. The stoichiometric ratio is needed to be controlled by the equilibrium between the curing group and the epoxy group. As an example, increasing the level of curing agent can reduce the cross-link density and increase flexibility, while decreasing chemical resistance.

Impact resistance or hardness is a function of the cross-link density. Higher densities can be achieved using low molecular weight curing agents that show tightly cross-linked structures. Adding non-reactive diluents can interfere with this structure, providing the end product with more flexibility but less toughness.

Mechanical adhesion is the gripping force which results from the roughness of the steel substrate, (i.e. peaks and valleys). Changing from a round to angular surface profile and increasing the depth of the valleys can improve this type of adhesion. Polar adhesion is the hydrogen bonding which occurs between the steel substrate and epoxy coating.

Chemical bonds are formed through electron sharing by groups on the steel substrate and epoxy resin. These bonds are by far the strongest and contribute most to adhesion. Groups such as nitrogen and oxygen can bond with iron and silica.

FBE coating and corrosion

FBE coatings generally reduce the corrosion of a steel substrate subject to an electrolyte in two ways namely (i) they act as a physical barrier layer to control the ingress of deleterious species, and (ii) they can serve as a reservoir for corrosion inhibitors to aid the steel surface in resisting attack by aggressive species such as chloride anions.

FBE coating provides a physical barrier and thus prevents the steel substrate from the contact of moisture, oxygen and chloride ions. Furthermore being a dielectric coating, FBE coating resists electron and ion flow between the metal and the electrolyte, hence impeding the charge transfer between anode and cathode.

Corrosion control of the FBE coating is a function of the coating’s ability to provide a barrier against water, oxygen, chloride, and other aggressive elements which prevent permeation through the coating film to attack the steel substrate. There are critical properties required for corrosion protection in FBE coatings which include adhesion and wetting ability to the coated steel. Reduction in adhesive strength increases the delamination process rate. A study into delamination of FBE coatings in a simulated pore solution environment has suggested the delamination mechanism as given below.

• Delay time before initiation of observable delamination process can be a function of water penetration through the FBE coating to the interfacial or interphasial coating/substrate region.
• Delamination of FBE coating from steel substrate is predominantly caused by hydroxyl ions.
• Rate of FBE delamination is controlled by transport processes from a pore in the coating and along the delaminated coating/substrate interface to the disbondment front.
• The locality of failure of coating adhesion is in the interfacial or interphasial coating/substrate region.
• The rate of FBE delamination in near-passive conditions is controlled by hydroxyl ion migration from the bulk external solution to the coating/substrate disbondment front.
• The rate of FBE delamination in the condition of under film corrosion is controlled by hydrated cation movement to the cathode site.

Process of application of FBE coatings

The important steps which are to be controlled during the process of FBE coatings (Fig 2) include (i) surface preparation and cleanliness, (ii) blast cleaning, (iii) cleaning procedure, (iv) final cleaning and inspection, (v) surface conditioning, (vi) preheating, (vii) FBE coating application, (viii) post treatment, (ix) final inspection and quality control, (x) repair procedures, (xi) handling of FBE coated product. Total elapsed time between surface preparation activities consisting of steps (i) to (v) is to be kept to a minimum to avoid the formation of oxides on the surface. Oxidation of the steel prior to coating in any of its apparent forms is not acceptable. Visual formation of such oxides results into the repetition of the surface preparation activities for the steel prior to coating.

Fig 2 Important steps for controlling FBE coating process

Surface preparation and cleanliness – Proper attention to the cleaning and preparation of the steel substrate surface prior to abrasive cleaning has a considerable effect on the eventual quality of finished FBE coating. The basic elements of pre-cleaning are (i) removal of surface contaminants, (ii) loosening of mill scale (on newly rolled steel), and (iii) removal of frost and moisture.

Steel to be coated with FBE can be contaminated by salts, grease, oil, and other deleterious materials. These visible and non‐visible surface contaminations of the steel material can take place during transport, handling, and storage. It is important that all of these contaminants are removed prior to the first abrasive cleaning step. Failure to remove contaminants can lead to contamination of the abrasive media which causes poor performance of the subsequently applied FBE coating. Deeply embedded salts and certain organic contaminants, if not completely removed, causes adhesion failures and film formation issues. Hence, these materials need to be removed by solvent cleaning or by detergent washing or steam cleaning. No residue which can affect adhesion is to be left on the steel surface. It is desirable to preheat the steel material prior to blast cleaning to a temperature at least 5 degrees in excess of the dew point or higher.

Blast cleaning – The purpose of abrasive blast cleaning is to achieve a clean surface, having an angular surface profile with an average profile depth between 50 micro-metres  to 100 micro metres. The surface is to be cleaned to a minimum of ‘near white metal’ finish. This can be achieved most effectively with centrifugal type blasting equipment using steel grit as the abrasive media. Abrasive residues are to be removed with compressed air or by any other suitable means. The working abrasive mix is to be maintained clean of contaminants. Steel grit is to have a hardness of 50 Rockwell C to 60 Rockwell C. Particle hardness and size distribution of the steel grit employed is required to be continually controlled by screening to ensure the surface profile after cleaning. For consistent surface finish, a stabilized working mix is to be maintained by frequent small additions of new abrasive commensurate with consumption with infrequent large additions is to be avoided.

Cleaning procedure – If there are two blast chambers available then the shot can be used in the first chamber for pre-cleaning and grit in the second chamber. It is not desirable to mix shot and grit in the same chamber. Where a single blast unit is in operation, it is desirable to use steel grit only.

The first cleaning step is to establish the basic cleanliness. It also uncovers the material defects such as slivers, burrs, laminations, scabs and gouges. Disc grinding or other suitable methods are to be employed to correct these defects. If serious defects exist, the steel material is to be rejected at this stage.

The next step is a second abrasive cleaning process using steel grit as medium, having a hardness of 50 Rockwell C to 60 Rockwell C. The main purpose of this step is to achieve the final desired cleanliness and the desired anchor profile. Frequently only a light blasting is necessary for best performance, if the first stage has been performed efficiently. Regardless of the type of operation, it is important that the centrifugal wheels are to have adequate horsepower and be positioned correctly to achieve high quality and efficient cleaning.  Abrasive residues are to be removed with compressed air or by other suitable means. A good quality abrasive medium is to be used and replenished regularly to ensure a balanced working mix.

Final cleaning and inspection – After abrasive cleaning and before coating, the surface to be coated is to be carefully inspected for metal defects which may affect coating application, i.e., scabs, slivers, gouges, or laminations. All abrasive dust needs to be removed, usually by vacuum or air knife and the level of cleanliness is checked periodically by pressing a clear adhesive tape onto the steel surface and examining the underside for dirt particles. It is very important that the required anchor profile is achieved. The profile is required to be angular but is not to have ‘undercuts’ which can result from over blasting, wrong abrasive or improper positioning of the centrifugal wheels. All FBE coating operations require a ‘near white metal’ blast quality. In most cases, a skilled operator can visually recognize the required standard using visual standards.

Surface conditioning – Sometimes it is required to use a chemical pre-treatment on the surface of the steel. This is needed due to the presence of soluble salts remaining on the steel surface. Chemical pre-treatment has a secondary benefit of washing off dust residues.  A weak phosphoric acid solution in water has been found to perform well. It is highly important and critical to the success of this pre-treatment that remaining acid is removed by thorough water rinsing immediately following the treatment. The rinse water is to be either of reverse osmosis (RO) quality or de-ionized water.

Acid washing is mandatory, if the steel has been subjected to corrosion attack in the presence of chloride or sulphate ions prior to processing at the coating plant. A common cause of this is salt-water exposure during the storage of the steel in the coastal area or in an atmosphere containing SO2 or industrial CO2, or because of sea transport of the steel from the steel plant to the coating plant. Under these conditions, ferrous salts form and they are retained on the steel surface, particularly in pits, even after normal abrasive cleaning.

A good test to establish the presence of ferrous salts is by using potassium ferricyanide or Phenanthrolin tests. Any steel showing presences of ferrous salts is needed to be appropriately treated. Proper surface treatment ensures that the steel surface is free of harmful contaminants arising from transportation or coating plant operations. Additional coating performance enhancement can be achieved by treating the clean steel surfaces with a chromate solution prior to the final preheat. A chromate solution in water is applied by spreading the solution uniformly over the steel surface. Spreading of the solution is done with a rubber ‘squeegee’ or brush. If the process is well controlled, there is no run-off material. It is important that any waste material is collected for appropriate disposal as required by local regulations.

Preheating

Correct heating of the steel is one of the most important steps in the successful application of FBE coatings. The steel is required to reach the appropriate application temperature recommended by the FBE coating material supplier for the achievement of the optimum performance of the FBE coating. The preheating temperature can vary according to the grade of material.  At no time the metal temperature is to exceed 275deg C as this can cause metallurgical or surface defects. A strong blueing or darkening of the steel surface is one indication of excessive heating. With the introduction of steels of high grades, there are further restrictions on maximum heating temperature.

Acceptable heat sources are (i) gas-fired radiant heat, (ii) gas-fired direct flame, and (iii) electrical induction. It is important that gas-fired heating systems are well adjusted so that products from incomplete combustion of fuel are not deposited on the steel surface. The furnace atmosphere is to be such that the clean steel surface is not contaminated. With induction heating it is important that the appropriate frequency is used to ensure deep heating. Intense skin heating is to be avoided. Multiple induction coils are normally needed for stable heating, especially in the case of thick steel materials.

Uniform temperature of steel at the specified levels is to be maintained for the best results. The temperature is to be controlled at the entrance to the coating chamber. Temperature sticks (Tempilstiks) are most commonly used and can be very effective when used by experienced operators. Infrared pyrometer is a satisfactory control tool, but it requires regular calibration for ensuring the accuracy of the measurement.

FBE coating application

Application of FBE coating powder is best accomplished by electrostatic spraying with spraying guns. It is important that a fluidizing powder feed and a suitable reclaim system is used. The number of application guns in use can vary depending upon the required film thickness and steel material dimensions. When the guns are properly set up then normally there is relatively little overspray in the powder chamber. The important points to be considered are (i) steel material is to be well grounded during its entire travel through the coating chamber, (ii) proper charge on the sprayed powder is to be maintained (generally in the range of 50 kV to 100 kV), and (iii) the spraying guns are to be positioned appropriately in the coating chamber to give a uniform powder deposition.

The spraying guns are to be at such a distance from the surface of steel material to make optimum use of the electrostatic properties and give minimum overspray. This distance is likely to be 125 mm to 250 mm from the surface of the steel material and is, to some extent, dependent upon the pressure necessary to uniformly transport powder through the line. As a starting point, the spraying guns are positioned at a distance of 200 mm from the steel material. Then adjustment is done according to film thickness requirements, size of the steel material and line speed.

Powder delivery pipe is to be of suitable diameter (generally 12 mm or higher) and the length to be as short as practical with minimum restrictions between the spraying guns and fluidized bed. The gravity effect on powder flow through the pipe is to be avoided by careful routing of the supply pipe. Improper adjustments can also result in the more serious problem of the clogging of the spray gun. This can also be caused by either the spray guns being too near the hot surface, or partially cured overspray entering the diffusers. Proper design and selection of diffusers can minimize the problems.

The best positioning of the spray guns is at the side of the rotating steel material with the steel surface travelling in an upward direction. This minimizes detrimental effects from radiant heat and gun clogging. The deposition rate is required to be adjusted to give a steady build-up of the required film and not flood the steel material. A test is to be made to measure the necessary powder delivery for a given material size and speed. This information can be used to determine deposition efficiency and effectiveness of equipment settings.

It is important that the air used in the coating chamber and supporting systems is dry and clean. Moisture can cause both deposition problems and coating deficiencies such as porosity and pinholes. Serious problems can result from air contaminated with oil. In addition to causing coating defects oil contamination is a major cause of impact fusion, which can cause system clogging and erratic spray patterns. Excess porosity can also be caused by a high powder deposition rate. Excessive electrostatic charge is also to be avoided since this can cause back ionization and possible film defects. Poor positioning can result in spirals or striping on the surface.

A 60 mesh size or 80 mesh size screen is desirable in recovery systems for the elimination of oversized particles. For screening of new powder a 50 mesh size or 60 mesh size screen is more desirable. Magnetic separators are to be used in the powder feed system to help remove metallic contaminants. The coating chamber is to be equipped with appropriate fire and explosion detection systems.

Post treatment – For achieving optimum mechanical and protective properties, quenching is to occur after the coating is fully cured. The minimum time requirement depends on the preheat temperature, and the material size. Wetting of the conveying wheels to minimize ‘tracking’ is to be done.

Final inspection and quality control – Exhaustive inspection and coordination with the other application steps are necessary for a quality coating. Inspection is to be considered as part of the process control operation and not just a decision point for approving or rejecting coatings. If each processing step is done correctly, a high quality coating is assured. Regular quality control tests to be carried out during application include film thickness, holiday detection and cure.

Curing of the epoxy layer is usually assessed by MEK rub test (a solvent resistance rub test) for a quick online evaluation and confirmed by DSC (differential scanning colorimeter) evaluation of the glass transition temperature. Longer term tests are also be carried out periodically to assure that the system is performing optimally. These include, but are not restricted to (i) hot water soak, (ii) impact test, (iii) cathodic disbondment test, and (iv) flexibility test

Repair procedures – All coating defects noticed during inspection need to be repaired. Where the steel is exposed, it is to be repaired using a two-component 100 % solids epoxy repair compound. Thermoplastic patch sticks are not to be used.

Pinholes (defects less than 1 mm in diameter) need no more surface preparation. The material to be repaired is to be cleaned to remove all dirt and damaged or disbonded coating. The edges of the original coating is to be abraded around the area to be coated 15 mm to 25 mm out from the pinhole and all dust wiped off before applying the patch coating. Files are not to be used.

Large holidays (to a maximum size of 80 mm) require surface preparation of the steel. Any exposed metal is to be treated to remove contaminants such as corrosion products, salts, dirt, etc., using abrasive blast or other means. The FBE coating is also to be abraded around the areas to be coated 15 mm to 25 mm out from the edge of the holiday and all dust removed before applying the patch coating. The patch coating is to be applied as per the recommendations of the FBE coating material supplier to a minimum thickness of 0.65 mm with an overlap with the existing sound coating by a minimum of 25 mm.

The freshly patch‐coated areas are to be allowed to cure fully according to the FBE coating material supplier specifications prior to handling those areas.  After curing, all patches are to be visually inspected and jeeped with a wand electrode of fine brass whiskers at a voltage of not less than 4000 volts/mm and tested for adhesion by knife lifting. The use of a wet sponge detector set at the FBE coating supplier’s recommended parameters is also acceptable. The repaired material is to be holiday‐free and is not to disbond when lifted with a knife.

Handling of FBE coated product – Careful handling of the coated steel is necessary to avoid mechanical damage during stacking, loading, transportation, stringing, and lowering. All booms, hooks, clamps, forks, supports, and skids used in handling or storing coated material is to be designed and maintained in such a manner as to prevent any damage to the material or to the coating The basic requirements are (i) all contact points of handling are to be padded and a load spreader beam is to be used for the lifting of the coated steel material, (ii) separators are to be used when stacking, (iii) coated steel material is always to be lifted and not dragged, and (iv) slamming together of the material ends is to be avoided.

Advantages and disadvantages of FBE coatings

The advantages of FBE coating include (i) since the coating is done on the coating lines, better quality control is achieved, (ii) the process gives uniform coating thickness, (iii) there is good bonding of coating with the steel as FBE has very good adhesive properties, (iv) because of flexibility, the coating does not get damaged when the straight steel material is bent during fabrication on a special mandrill, (v) FBE coating acts as insulator for electro chemical cells and offer barrier protection to steel which prevents chloride ions through it, (vi) there are well established criteria for acceptance for FBE coating in different standards, and (vii) FBE coating bars provide the very effective corrosion protection to the steel materials.

The disadvantages of FBE coating on steel materials are (i) there is reduction in the bond strength between coated material and concrete in case of FBE coating on reinforcement bars, (ii) as the technology is plant based, there is need for double handling and transportation of the steel materials, (iii) handling of coated materials is to be done with utmost care to avoid damage to the coating, (iv) performance of the FBE coated materials is heavily dependent upon least defect in the coating since the patching in the defective area is not always effective, (v) even a small damage in the coating can initiate corrosion in severe environment, when the coating has no cathodic protection and due to it corrosion cells are set up in the damaged area of the material which leads to first de-lamination of the FBE coating and then rusting, (vi) being a barrier type coating, it facilitates localized pitting corrosion through pinholes, (vii) FBE coated material undergoes degradation on long term exposure to sunlight, and (viii) FBE coated reinforcement bars shows generally poor alkali resistance.