Fusion Bonded Epoxy is very fast curing, thermosetting Protective Powder Coating based on especially selected Epoxy resins and hardeners and formulated in order to meet the specifications related to protection of underground steel pipelines and particularly as an anticorrosion coating. FBE is widely used in Pipeline and Rebar coating and is the preferred over other protective systems in this segment, due to their several advantages over other protective systems. Fusion Bonded Epoxy coating has been used for the corrosion protection. It acts as a barrier to corrosive chemicals and moisture, which are essential components of the corrosion process. It is applied to preheated steel as a dry powder, which melts and cures to a uniform coating thickness. The FBE system was first introduced in Europe in 1953 for coating electrical equipment by fluidized bed dipping process. Where as the electrostatic spray system was introduced in 1962, this technology was introduced for coating pipelines for oil, gas, and water transportation. During the late 70ís FBE became the most widely used pipeline coating in USA, Canada, Saudi Arabia, and UK. Today it is successfully used in each country for corrosion protection system for underground pipelines.
It had been established that the corrosion control property of the coating is dependent on its ability to be an excellent barrier against water, oxygen, chloride, and other aggressive elements permeating to the metal surface. However, for a coating to be an effective long-term corrosion protection system, it is essential that it stays bonded to the substrate during the design life of the structure. However, polar elements such as water, once permeated to the interface, can reduce the overall adhesive strength. Therefore, epoxy coatings, including FBE, exposed to wet environment will not have the same adhesive strength as the newly applied one or the one that was exposed to only dry conditions. Since reduction in adhesive strength is inherent during its service life, the important question is whether a reduction in adhesive strength signals the end of its performance. It is important to know how much adhesive strength is needed for a coating to continue to perform. Another corrosion control mechanism of a coating is related to its electrical properties. Protective coatings are designed to offer a high resistance between the cathode and the anode of the corrosion cells and to minimize the number of micro cells on the substrate. The success depends on its ability to place itself between the cathode and the anode, which are separated only by microns. Known as the wetting property, this controls the sites where water gets accumulated and leads to coating blisters and poor adhesion. Although there are other critical properties that are required for corrosion protection coatings, adhesion and wetting properties are critically important to epoxy coatings. A clear understanding of the basic corrosion process is helpful to understand the role of these properties in corrosion control of epoxy.
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
¾ FBE for Single layer stand alone coating of different gel times & properties.¾ FBE for 3-layer polyethylene coatings in different gel times.¾ FBE for High Service Temperature stand alone coating, concrete weight coating
and three layer polyethylene/polypropylene coating.¾ FBE for Low application temperature stand alone, three layer and girth weld
coating.¾ FBE in double layer with external tougher layer to protect from mechanical
damages and abrasion resistance.¾ FBE in grit anti slip finish top coat for concrete weight coating.¾ FBE in three layer system with inner reactive layer, middle flexible layer and outer
tougher layer.¾ FBE for internal pipe coating and girth weld coating¾ FBE for high temperature internal pipe coating.¾ FBE for Rebar coating in different gel times and properties
ADVANTAGES OF FUSION BONDED EPOXY
The most important requirements in the coating system for steel pipes are the following:
¾ Strong adhesion to the surface. ¾ Long term chemical and mechanical resistance of coating materials at operational
temperatures. ¾ High mechanical impact strength and penetration resistance.
The pipe coating system is equipped with a wide range of performance properties:
¾ Low permeability to water vapour and gases. ¾ Mechanical protection against handling and transportation damage. ¾ Superior resistance to cathodic disbonding. ¾ Outstanding dielectric properties. ¾ Heat aging resistance because of stabilization against oxidation damage. ¾ Strong resistance to ambient conditions such as corrosive soils, salt water,
microorganisms and spreading plant roots.
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Surface preparationSurface preparation is essential to the ability of the coating to bond to the pipe substrate. This bonding is important to eliminate the environmental fluid migration between the substrate and the pipe coating. It is therefore very important to understand the surface preparation requirements of the coating system to be selected. There is no shortcut here indeed, because poor surface preparation always results in poor bonding strength of the coating to the pipe.
The surface is cleaned using steel grit to obtain surface profiles of 60-110 microns. During blast-cleaning the pipe surface temperature shall be more than 3 °C above the dew point. The pipe surface temperature shall always be more than 5 °C. The relative humidity shall not be greater than 85%. Abrasives shall be stored and used dry. Expendable abrasives shall not be recycled. Immediately after blast-cleaning, all remaining weld spatter and irregularities shall be removed from the pipe surface by chiseling and/or grinding. Any treated surface with an area larger than 25 cm2 shall be re-blasted to the cleanliness and roughness as specified above. After any grinding or mechanical repairs the remaining wall thickness shall be checked and compared with the minimum requirements of the line pipe specification. Pipes not meeting the minimum wall thickness shall be rejected. Before coating, the pipe surface shall be cleaned of all dust and foreign matter using clean dry compressed air or vacuum cleaning. The compressed air shall be free of any trace of oil.
The surface is then washed with phosphoric acid and then with deionized water for the removal of water-soluble salts and organic contaminants. If left on the surface salts especially chlorides and sulfates can initiate water absorption by osmosis and lead to coating failure. It has been reported that chloride contamination will seriously affect adhesion and cathodic disbondment properties. Chromate is applied to enhance the adhesive strength of Fusion Bonded Epoxy system
Induction Pre-Heating
The pipes proceed at a scheduled speed. Uniform heating is of paramount importance for good coating properties. Induction heating is the only method for this coating process and it is essential to serve a clean oxide free surface to the heater in order to obtain a constant application temperature.
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
Let us first review the basics of induction heating.Induction heating is the process where an electrically conducting work piece is placed in a copper coil through which an alternating current flows. The magnetic field created by this alternating current causes current to flow in the work piece. Figure illustrates this principle.
It can be said then, that induction heating follows the well understood transformer effectwhere an AC voltage impressed on one winding will produce an AC voltage on the other winding. The work coil in an induction heater is the primary winding of the transformer while the pipe acts as a short-circuited secondary winding. Since the pipe has an inherent electrical resistance to the current flow, heat is generated.Induction heating is unique in that the heat is developed in the pipe without any physical contact, without a radiated heat source and without producing any products of combustion. Since induction heating has no source related temperature limitations, its usage is commonly at energy levels or power densities much higher than those associated with indirect electric or fuel-fired heating methods and it is accurately controllable.
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
The transmission of heat energy throughout a body is due to temperature differential and to the second law of thermodynamics that states that heat energy migrates in a direction from a body at a higher temperature to a body at a lower temperature. During the induction heating process, the majority of heat energy is generated within the pipe at a specific depth. Heat flows from this band in 2 directions, inward toward the center of the pipe wall and outward from the pipe surface by radiation. The speed of this heat transfer is dependent upon its thermal conductivity, specific heat and density.
Frequency Selection
In order to specify an induction heating system for a hot working application, we need to define Depth of current Penetration. This factor determines the efficiency of the induction-heating coil. The frequency of the current, the relative magnetic permeability of the pipe and the resistivity of the pipe determine the Depth of Penetration. The formula for the Depth of Penetration or, as it is sometimes called, reference depth, is as follows: Depth of Penetration:
Below Figure gives a table of current penetration depths for both above and below the Curie or magnetic transformation point.
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
From this formula we know that the current penetration increases as the resistivity increases and decreases as either the permeability or frequency increases. We see then that the higher the frequency, the shallower the heating effect
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For the application of induction heating for pipe coating the line diagram in Figure 7 shows the induction system portion of the coating plant. This comprises of a solid-state medium frequency induction power supply that ensures the correct voltage is applied to the coil, which in turn heats the steel pipe to the pre-set coating temperature. Interconnection is made to the heating coil by flexible water-cooled leads.
Coated pipes may be manufactured with varying lengths for the pipeline installation, but for long distance runs they are typically 12m in length. One can imagine with a large pipeline how many pipe lengths need to be manufactured, stored, coated and shipped out to the installation site. A 500km pipeline would require 41,666 pipes of 12m in length.
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At a temperature of 170-250°C the pipes move through the spray booth. Several electrostatic powder spray guns blow fluidized FBE on the pipe surface. A small amount of excess powder is recovered and conditioned before being returned. FBE melts when it contacts the hot surface. The melted epoxy resin reacts with the curing agent present in the FBE and bonds to the substrate, providing a highly cross linked polymer with a sophisticated network of covalent and coordinate bonds. These high-energy bonds provide excellent adhesion between the coating and the metallic substrate. The amongst three adhesion forces between FBE and substrate, the two of them i.e. chemical and polar polar adhesion are directly related to the number of bondable sites available on the substrate. Thereofore to provide the maximum bondable site to FBE the high peak heights are obtained by abrasive cleaning. It also depends upon the viscosity of the FBE system used and the application temperature.
FBE has to achieve at least 97% cross-linking (cure) in order to have the optimum properties. The curing process is a heat related phenomenon. It depends on the initial steel temperature and the duration that the pipe retains the heat. This means that even if the initial steel temperature is low, you can reduce the pipe travel speed in the coating line to allow enough heat energy for the curing process to be completed. The FBE manufacturers normally provide cure data for the FBE system, which will give the relationship between the steel temperature and the time needed to achieve optimum cure. It is important that FBE achieves full cure, since the unreacted epoxies and curing agents in the FBE system can absorb water leading to a loss of adhesion. Lack of full cure can also affect the cohesive strength of the coating.
Three-Layer Application Parameters of Fusion Bonded Epoxy
Three-layer-coating systems provide specified performance properties for the external protection of steel pipes designed for safety and long life transmission service.
The multi-layer pipe coating combines the benefits of two major advantages:
¾ The fusion bond epoxy film, which has excellent adhesion and chemical resistance properties.
¾ Extruded intermediate adhesive copolymer layer and the extruded multi-layer polyethylene top layer providing in addition to the corrosion protection strong physical and mechanical performance even at elevated temperatures.
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Extruded Adhesive and Polyethylene Wrapping Technique
Two single screw extruders produce thin film through die-slots with constant thickness. A system of finely-tuned pressure rolls are set up geometrically in accordance with the pipe movement to guide the film toward the pipe surface. The system builds up multiple layers and prevents air pockets or wrinkles occurring and assures the formation of the final homogeneous layer.
Bonding between the FBE and Adhesive involve a chemical Reaction in which the Epoxide group of the FBE powder reacts with the N-hydride functional group of the adhesive. Therefore, for the reaction to occur properly the reactivity of the FBE powder primer has to be correctly controlled at the time of application of the adhesive.
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The intercoat adhesion of coatings cured using cross-linkers depends on both temperature and humidity. The conversion of the amine-to-amine carbamate salts at or near the surface, resulting in incomplete curing at the interface, is responsible for intercoat adhesion failure. The rate of reaction between the amine and the epoxy and the humidity level, are key factors in the intercoat adhesion of epoxy coatings. At appropriate temperatures of application, the rate of reaction between the amine and the epoxy is rapid, causing the formation of coatings with good intercoat adhesion. However, at lower temperatures, the rate of the cross-linking reaction is decreased, allowing moisture to permeate the coating and solubilize the amine. In its solubilized form, the amine reacts with carbon dioxide to form stable carbamate salts incapable of reacting with the epoxy. In addition, the degree of cross-linking also depends on the RH level to determine the degree of solubilization of the amine that can be converted to the carbamate salt. The glass transition temperature generally determines the appropriate level of applying the coating. The relationship between application temperature and coating performance needs to be established.
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Visual appearanceThe coating shall be free from blisters, visual holidays, scratches or any other irregularities and shall have a uniform colour and gloss.
Coating thicknessThe thickness of the cured FBE coatings are different for stand alone, double and 3-layer FRDWLQJV���7KH�QRUPDO�WKLFNQHVV�UDQJH�LV�����WR�����ȝP�IRU�standalone coatings. For pipes which are to be coated later with concrete weight coating the thickness of the cured FBE coating shall be on higher side. Similarly its also on higher side for High Service Temperature. The thickness of the cured FBE coating applied on bends and fittings shall be QRW�OHVV�WKDQ�����ȝP�DQG�VKDOO�EH�QRW�PRUH�WKDQ������ȝP��
HolidaysThe coating system shall be free from holidays which are indicated when tested inaccordance with Holiday tester as per specified standards.
AdhesionThe adhesion of the coating system shall be such that any attempt to remove the coating shall result in a cohesive break in the coating material and not in an adhesive failure of the coating/substrate interface.
Impact resistanceThe impact resistance of the coating system at ambient temperature shall be more than 1.5 Joules when tested in accordance with the procedure.
FlexibilityThe flexibility of the coating shall be such that holidays do not appear when tested inaccordance with the procedure and as per specification provided by the coating manufacturer.
Hot water resistanceAfter exposure to hot water in accordance with the procedure the coating shall show no evidence of blistering or disbonding and shall show no failure of adhesion when tested.
Cathodic disbonding resistanceCathodic disbonding tests are carried out in accordance to the standards at a testtemperature related to the operating temperature. The durations of the test for coating qualification are different in different standards. After the test, the maximum radius of disbonding shall be less than the value given in the respective value.
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Degree of cureThe degree of cure shall be determined by differential scanning calorimetry in accordance with the standards. The Tg value must be between -2 °C and +3 °C.
Microscopic examinationA sample of the applied coating shall be examined for the presence of foaming, voids and contamination in accordance with the standards. The maximum degree offoaming shall be 2 on the scale both through-film and across-film. The coating shall be free of contamination by foreign matter s.
CORROSION
Corrosion is the electro chemical reaction of a metallic material with its environment. For pipe this environment can include soil, water, air or even the contents with in the pipe. In all electrolytes the metal atoms from the pipe go in to the solution as electrically charged ions. The movement of ions causes a flow of electrical current from the metal pipe to the electrolyte. This process causes loss of metal from its surface and is commonly know as rust.The first step in the corrosion process is the conversion of metallic iron (Fe) to ferrous ion and electrons, which is represented in equation 1.
Fe Fe ++ + 2e- (equation 1)
Due to the differences in the internal energy levels of Fe and Fe++, and Fe being of higher energy, this is a natural and spontaneous process. However, the rate of this reaction depends on the concentrations of Fe, Fe++ and the electrons. Since the removal of any of the products accelerates the forward reaction in systems, which are in equilibrium, the removal of either Fe++ or electrons can accelerate the above corrosion reaction. Conversely, if the removal of either of the above products can be stopped, corrosion can be prevented. Fe++
and electrons can be removed by reactants such as oxygen, water, and chloride ion. Reactions involving these molecules are given in equations 2, 3, and 4.
Providing a barrier against the passage of these reacting ions to the metal interface can effectively control corrosion.
Also, notice the role of electrons in equation 2. Electrons are needed to convert oxygen and water to hydroxyls. These electrons are coming from the oxidation reaction of Fe to Fe++. If there are other easier sources for electrons, those provided by the oxidation of Fe will not be used. In other words, if the pipe surface has an excess concentration of electrons, equation 1 shows us that corrosion can be slowed or prevented.
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
When steel corrodes, the corrosion rate is usually governed by the cathodic reaction of the corrosion process, and oxygen is an important factor. In neutral waters free from dissolved oxygen, corrosion is usually negligible. The presence of dissolve oxygen in the water accelerates the cathodic reaction; and consequently the corrosion rate increases in proportion to the amount of oxygen available for diffusion to the cathode. Where oxygendiffusion is the controlling factor, the corrosion rate tends to increase also with rise in temperature. In acid waters (pH <4), corrosion can occur even without the presence of oxygen.
Corrosion of steel under a droplet of water
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Electrochemical corrosion can be stimulated from not only differences in the metal surface, but also from variations in the electrolyte. The above is effected to some degree by this mechanism, as oxygen diffuses into the water drop a concentration gradient is set up, where the oxygen content at the extremities is the highest and the lowest being at the center where the anode forms. Cavities in metal surfaces and metal surfaces partially covered by another material are prone to this type of attack. The diffusion of oxygen into cavities or crevices is impeded and results in these areas becoming anodic to the surrounding metal to which oxygen can easily reach (oxidation-concentration cell or differential aeration cell). The metal ions formed in the cavity migrate outwards and react with the hydroxide ions flowing in the opposite direction to form a corrosion product (rust) at the mouth of the cavity or crevice. This position of the corrosion product accentuates the corrosion by making the diffusion of oxygen to the anode more difficult, and if the cathodic area is large severe pitting may occur. Also when dry conditions prevail moisture can be trapped in the cavities allowing corrosion to continue.
CATHODIC PROTECTION SYSTEM
This principle is used in another corrosion control method known as the Cathodic Protection system. This system is used to prevent corrosion from occurring on the exterior surface of the pipe. The CP systems change the source of electrons from a buried pipeline metal to another source commonly know as either sacrificial anode or impressed current anode. CP is applied by using either galvanic sacrificial nodes or an impressed current system. Both systems operate by imparting a direct current onto the buried pipeline, typically using devices called rectifiers. As long as the current is sufficient the corrosion can be prevented. In most cases the coatings on the exterior on the pipe are used in conjunction with the CP in order to reduce the current required to protect the pipe by insulating the pipe from the electrolyte. Coatings have a high dielectric strength, which prevents the flow of electrons to the pipe’s
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
surroundings, thus interrupting the electro-chemical reaction of the metal with its environment. CP is used to protect buried pipelines, ship hulls, underground tanks, offshore platforms and any other metal surface which may come in contact with the ground or with water. CP cannot be used to prevent atmospheric corrosion, or corrosion that may occur inside the pipe due to its contents.
Epoxy coating acts as a barrier against oxygen, water, and chloride to prevent their passage to the metal surface. Therefore, the barrier property of the coating is extremely important in ensuring corrosion protection. However, the long-term performance of the coating depends on its adhesive strength and the ability to stay on the surface until its design life. It has been calculated that an extremely small concentration of oxygen and water can start the corrosion process. Therefore, to minimize corrosion, materials that have extremely low permeability have to be used in the formulation.
Underground pipelines protected with impressed current CP use voltages higher than the corrosion cell voltage. Although the CP system is designed to provide continuous electricity with a slightly higher voltage than the corrosion cell voltage, it is highly possible that changes in the soil resistivity will result in voltage fluctuations. Normally, coatings will lose adhesion and disbond from the pipe surface when it is subjected to excessive current densities. Therefore, the powder coating's ability to resist disbondment when it is subjected to extensive current through voltage fluctuations is crucial.
In many cases, however, pipe coating is supplemented by cathodic protection. The two protective methods work synergistically: the coating greatly reduces costs of the cathodic protection system, while cathodic protection substantially extends the useful life of the coating. A coating used with cathodic protection must have good dielectric strength so that both cathodic protection potentials and current flows would not affect its ability to act as a corrosion protective barrier. Coatings with a low dielectric strength, or those that will allow some current flow, often allow the buildup of cathodic deposits on the surface or under the coating, causing coating breakdown. ASTM D149 can be used to evaluate the dielectric strength of a coating.
It is also necessary for the coating to withstand cathodic disbondment. Experience in the oil and gas pipeline industry has clearly shown that coatings with better cathodic disbondment resistance have better corrosion resistance and greater longevity. The coating systems with good adhesion to the steel substrate tend to have a similar resistance to cathodic disbondment. If a coating is able to adhere to the steel substrate, it will therefore tend to resist the undercutting damage of corrosion, thereby offering a longer service life. There are several standard testing methods, which can be used, such as G42, G8, and CSA Z245.
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The cathodic disbondment (CD) property of a powder coating is one of the key indicators of its corrosion prevention capabilities. It is defined as the ability of the coating to resist disbondment and delamination when it is subjected to electrical stress in a highly conductive electrolyte. The cathodic hydrogen evolution rate and anodic iron dissolution rates were both found to affect the pH. It is predicted that formation of iron carbonate, observed extensively in some pipeline failures, occurs under a specific combination of iron dissolution rate and hydrogen evolution rate. This happens when the coating manufacturers specifications are exceeded. Cathodic protection current passing onto the metal causes the release of hydrogen, which disbonds the coating. In reality this is rarely a problem.
The current will only pass onto the metal at a coating fault, and the density of the current will depend on the size of the coating fault and the current locally available. As the current blows the coating from the metal, the volts drop at the interface will decrease, and equilibrium will be reached with a very small increase in additional disbondment.
It is logical to deduce that if cathodic disbondment is caused by current and that if all current is prevented by a perfect coating, then no disbondment will take place. This is not common sense, however, as many excavations have been dug in areas where high 'pipe- to-soil potentials' have caused concern about cathodic disbondment. In the event, it has proved the logic (above) and no disbondment have been found.
In one particular example voltages of over 5 volts had been recorded when the electrode was place on the surface above the buried pipeline, at several spots, for examination. A coating fault was found at one location but no disbondment. The current passing onto the metal at this coating fault caused a drop in the voltage of the electrode as it got nearer to the pipe. Whereas at the surface the reading had been over 5 volts, this reduced to 0.950 volts when the electrode could be placed close to the actual interface between the metal and the earth.
This simple drawing shows that the earth at the surface has a higher potential than the earth close to the pipeline at the coating fault, due to the current passing from 'mass earth' into the pipe metal.
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The success of the FBE coating as the best corrosion control system for underground pipelines lies in its ability to limit oxygen and water transport to the pipe surface and compatibility with the alternate CP system. The properties of FBE are designed such that it will work in conjunction with the CP system, not interfere with it. However, the application parameters including surface cleanliness, removal of contaminants, profile shape and densities, initial application temperature and curing temperature and time play a critical role in ensuring these important properties.
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The polymer holds the coating cohesively (internally) together and adhesively (externally) to the substrate to which it is applied. Proper polymer selection can be the most important factor in determining the physical and chemical properties, as well as protective capabilities, of the coating.
Adhesive strengthThe force needed to pull the coating away from the substrate, is the sum total of several components. However, the major contributions are from mechanical, polar polar, and chemical adhesions. Since one of the key steps in improving long-term performance is to ensure that the coating stays on the substrate, every available option to increase the adhesive strength is to be considered. Therefore, a good understanding of the major components of this important property is helpful.
Mechanical Adhesion.By definition, mechanical adhesion is the gripping force of the coating onto the surface. This is equal to the force needed to resist the movement of the coating from its position in the absence of other adhesion components. Increasing the area of contact between the coating and the substrate can increase mechanical adhesion. One way to accomplish this is by increasing the number of surface peaks and also the depth of the profile. It has been observed that by changing the round shape of the profile to an angular profile, there is considerable increase in the adhesive strength.
Polar-Polar Adhesion.The polar-polar adhesion is the force of attraction between the positive and negative poles of the substrate and the coating. It is the combined total of all hydrogen bonding between the coating and the metal. The polar-polar adhesion can be improved by increasing the polar components of the coating. It is possible to select resin systems, curing agents, and other constituents that have more polar groups. By selecting proper systems it is possible to decrease the sterric hindrances between the constituents and thereby increase the polar-polar attractions. Other methods by which polar polar adhesion can be increased are by increasing the strength of the dipoles and also by decreasing the distance between the centers of the dipoles. An enhancement in polar polar adhesion can also be achieved by increasing the application temperature. The higher application temperatures increase the cross-link density of the FBE system, allowing more interactions between the dipoles. Fusion Bonded Epoxies consist of resins and curing agents that have sufficient polar components, which can enhance the adhesion between coating and substrate.
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Chemical Adhesion.Chemical adhesion is due to the chemical bonds established between the organic coating and the metal substrate as represented below.
Fe - OH + R - OH Fe - O - R + H2O (equation 5)
Where Fe - OH is the hydrated metal oxide and R- OH is the epoxy coating.
An increase in the number of chemical bonds can be achieved by increasing the application temperature. At application temperatures between (243-254C0) the hydrogen of the hydrated metal surface can enter into a dehydration reaction with the hydroxyls of the epoxy coating. This can establish a direct bond between the metal and the coating. However, this reaction depends on the reactivity of the coating system and also the number of the hydrated oxide metal surface and the surface temperature.
Although mechanical and polar-polar adhesions are two major contributors of the total adhesive strength, chemical adhesion is the strongest one. It has been calculated that thepolar-polar bond strength seldom exceeds 5k cal/mole while the chemical bond strength values are nearly 100 kcal/mole. It has been reported that the strength of one chemical bond is 20 to 1000 times greater than one polar- polar bond. Therefore, a small increase in the number of chemical bonds can dramatically affect the total adhesive strength.
Due to the great strength of chemical adhesion, one should strongly consider ways to increase the number of chemical bonds in order to improve adhesive strength. Providing more bondable sites on the substrate can increase chemical bonds. A better abrasive cleaning that increases the peak density and depths. Another method is to increase the number of bondable sites on the polymer by incorporating highly reactive phenolic and/or hydroxyl groups. FBE is especially designed which provide maximum bondable sites to the substrate. Chemical adhesion can also be increased by chemical pre-treatments of the surface. The chemical pre-treatment can modify the surface to provide more reactive sites for coating to chemically bond with the substrate.
Maintaining proper adhesion throughout the service life of the pipeline is one of the key factors in evaluating the success of the coating. But the initial adhesive strength can diminish during the service time for several reasons. Therefore, having a high initial adhesion is extremely important for pipelines designed for a long service life.
The cohesive strength of a powder coating is a measure of the forces of attraction between the molecules, or the force required to tear the coating apart. This property is dependent on the coating's components, especially the base resins. Powder coatings come in a wide range of cohesive strengths. Although coatings with higher cohesive strengths are preferable, the ability to preserve their initial cohesive strength is perhaps more important, especially for pipelines designed for long service life. A coating with greater cohesive strength can minimize damage during transportation and pipeline construction.
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Flexibility and ImpactFBE systems require high energy to achieve cross-linking between the epoxy molecules and the curing agent. Improper energy levels can leave powder coating components unreacted. This will adversely affect the applied film properties. One of the properties that will be seriously affected is flexibility, since under-cured coatings will crack during field bending of the coated pipe. Another property that will be affected is adhesion. For chemical adhesion, an excited metal surface is needed. By increasing the application temperature, the chance for excitation of iron molecules on the substrate will be increased. This will lead to more chemical adhesion sites and increased adhesive strength of the applied coating.
Among a pipeline coating's mechanical properties, flexibility is one of the most important. It affects the construction activities in terms of expense, time and installed coating integrity. Flexibility is a measure of the powder coating's ability to resist mechanical damage when stretched. This property is critical in pipeline construction because it will decide the field bending limits of the coated pipe. This ability of the coated pipe to withstand bending during pipeline construction influences the number of prefabricated fittings. This will affect cost because fittings are among the most expensive items in the pipeline. Impact resistance is as important as flexibility among the mechanical properties. The value of impact resistance is an indication of the powder coating's ability to survive damages from impact.
Adhesive Strength of Coating in Wet and Dry Conditions
A reduction in adhesive strength of the coating has been reported on pipelines with FBE system, operating successfully in a hot, wet environment. On one of these lines, when examined after 12 years of service, the wet area showed lower adhesive strength compared to the dry area of the same line. However, it was also reported that the coating showed adequate adhesion and stayed on the pipe surface until it was mechanically removed. This observed difference in the adhesive strengths of wet and dry sections can be explained by expanding the adhesion theories discussed earlier. In dry areas, the total adhesive strength is equal to the sum total of all components, including mechanical, polar-polar, and chemical adhesions. However, when water enters the coating, as in the case of the wet area, the polar-polar bonds between the coating and the metal hydroxyls at the interface can be reoriented towards the water molecules. This will eliminate the polar-polar adhesion at individual sites and reduce the total adhesive strength. In wet condition, the adhesive strength depends on only mechanical and chemical adhesions. It is possible to recover most of the polar-polar adhesion, once water leaves the substrate, though a total recovery is unlikely to occur due to the possible physical changes of the molecules caused by the introduction of water.
For those FBE systems that depend on chemical and mechanical bonds for major portion of the total adhesive strength the loss of polar-polar adhesion may not pose a serious threat to total adhesion. However, the effect is significant for systems that depend mostly on the polar-polar adhesion for the total adhesive strength. This will be a great concern for FBE coatings, which are improperly applied, since a reduction in the number of chemical bonds is probable.
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Wetting Properties. Another mechanism by which coatings control the corrosion process is by providing an electrical barrier between the microscopic cathodes and the anodes of the electrical cell on the metal surface. This greatly depends on its ability to occupy the entire metal surface. This property, known as the wetting property, is particularly important to coatings applied over blast-cleaned surfaces having profiles between 50 to 100 microns. To perform as a good surface site to electrical barrier, the coating should be in intimate contact with the metal surface, without leaving any air pockets.
Powder coatings essentially control corrosion by separating the cathode and anode. This can only be achieved if the coating can wet out the surface completely on the micro level. To have excellent wetting properties, the powder coating should have low viscosity. Low viscosity can be achieved by using proper resin systems. However, the viscosity depends to a greater extent on the pipe surface temperature. A higher temperature will result in a low melt viscosity, allowing the powder coating to wet out the surface completely.
Coatings exhibit excellent wetting characteristics with low viscosity. In the case of FBE systems, which are solids at temperatures below 50C0, the initial melt viscosity is the important factor. A low initial melt viscosity is needed in order to achieve the complete wetting of the surface. A high-viscosity liquid may not flow into the angular profiles of the substrate, thus leaving air pockets. This can lead to micro corrosion cell set up.
High Temperature FBE:In some applications, one of the critical properties of external organic coatings is resistance to high temperature. It has been found that most organic coatings have problems at temperatures higher than 80C0. There is a need for high-temperature performance in oil and gas pipelines, especially near compressor stations for natural gas transmission and in the transport of higher viscosity crude oils. The operating temperatures of pipelines extend to 150C0. To overcome this problem high Tg Fusion Bonded Epoxy coatings are produced. Powders of different Tg levels are available.
It is recognized that conventional test methods, such as cathodic disbondment, may not be appropriate. The primary challenge is to obtain adequate flexibility with high temperature performance. For this reason, design criteria for high temperature test methods and for life prediction need to be established. The criteria for testing coatings for higher temperature applications are not the same as those for lower temperature application. For example, coatings with good cathodic performance, adhesion, barrier properties, impact resistance, and flexibility will protect the pipeline over the lifetime. At elevated temperatures, cathodic disbondment performance may not be relevant if the coated pipe is insulated. But good adhesion, barrier properties, flexibility, and resistance to movement at higher temperatures are necessary.
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
� Accelerated corrosion testMethod designed to approximate, in a short time, the deteriorating effect under normal long-term service conditions.
� AcidA chemical substance that yields hydrogen ions (H+) when dissolved in water.
� Active metalA metal ready to corrode, or being corroded.
� AdhesionThe firm attachment of a coating to a substrate or another coating.
� AlkalineHaving properties of an alkali. (2) Having a pH greater than 7.
� AmmeterAn instrument for measuring the magnitude of electric current flow.
� AnionAn ion or radical, which is attracted to the anode because of the negative charge.
� AnodeThe electrode at which oxidation or corrosion of some component occurs (opposite of cathode). Electrons flow away from the anode in the external circuit.
� Anodic reactionElectrode reaction equivalent to a transfer of positive charge from the electronic to the ionic conductor. An anodic reaction is an oxidation process.
� Back Ionization An excessive buildup of charged powder particles during electrostatic application which limits the ability of additional powder to be deposited onto the substrate; can neutralize the electrical charge of subsequently sprayed powder particles.
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� Brine electrolysisElectrolysis of an aqueous solution of common table salt (sodium chloride), also called "brine," results in the production of chlorine gas at the anode and hydrogen gas at the cathode. Since the hydrogen is produced by breaking up water molecule, the solution is becoming basic around the cathode and a solution of sodium hydroxide (also called "caustic" or "alkali") is produced. If the electrolysis is carried out in a divided cell, the products are chlorine, caustic, and hydrogen. If the electrolysis is carried out in an undivided cell and the chlorine gas and the caustic are allowed to mix and react with each other, sodium hypo chlorite (bleach) is produced if the cell operates close to room temperature, and sodium chlorate is produced if the cell is operated near the boiling point of water. The overall cell reaction is:
2NaCl + 2H2O Cl2 + H2 +2NaOH
Chlorine gas and sodium hydroxide react to form sodium hypochlorite and sodium chloride
Cl2 + 2NaOH NaOCl + NaCl + H2O
Sodium hypochlorite will react further at high temperature to form sodium chlorate and sodium chloride:
� Calomel electrodeA commonly used reference electrode. It is very similar to the silver/silver-chloride electrode both in construction and in theory of operation. The silver metal is replaced by mercury (electrical connection is made by an inert metal wire), the salt is mercury chloride, and the solution is saturated potassium chloride. Abbreviated as "SCE," for: "saturated calomel electrode." The equilibrium electrode potential is a function of the chloride concentration of the internal electrolyte ("filling solution"). The electrolyte is practically always saturated potassium chloride (hence the name: "saturated calomel electrode," SCE, "calomel" is an old name for mercurous chloride), producing a potential of 0.244 volt against the standard hydrogen electrode at 25C0.
� CathodeThe electrode of an electrolytic cell at which reduction is the principal reaction.
� Cathodic (corrosion) protectionA corrosion protection technique whereby a structure to be protected is made the cathode of an electrochemical cell. This can be achieved in two ways. In the "impressed current" technique, a cathodic current of sufficient magnitude is forced on the structure from a power source. For example, an underground pipeline is
Functional Powder CoatingFusion Bonded Epoxy Coatings for Pipeline and Rebar Industries… ……
connected to the negative terminal of a power source, while the positive terminal is connected to a nearby-buried non-corroding electrode. In the "sacrificial protection" technique, the purposeful corrosion of a less desirable metal protects a preferred metal. For example, a steel structure can be protected in seawater by contacting with a piece of zinc. The zinc (the more active metal) will become the anode and the steel (the more noble metal) the cathode of the resulting corrosion cell. Consequently, the zinc will be oxidized while the steel will be protected (as long as the zinc lasts).
� Cathodic reactionElectrode reaction equivalent to a transfer of negative charge from the electronic to the ionic conductor. A cathodic reaction is a reduction process. An example common in corrosion is: O2 (g) + 4H+ + 4e- ® 2H2O
� CationA positively charged ion that migrates through the electrolyte toward the cathode under the influence of a potential gradient. See also anion and ion.
� CellElectrochemical system consisting of an anode and a cathode immersed in an electrolyte. The anode and cathode may be separate metals or dissimilar areas on the same metal. The cell includes the external circuit, which permits the flow of electrons from the anode toward the cathode.
� Chalking - degradation of a coating due to UV exposure, which results in loss of color and gloss.
� CorrosionThe chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the material and its properties.
� Corrosion resistanceAbility of a metal to withstand corrosion in a given corrosion system.
� CorrosivityTendency of an environment to cause corrosion in a given corrosion system.
� Compatibilitythe capacity of coating powders from either different sources or of different
compositions when combined and applied which yield no visible or mechanically measurable differences in the cured film or application properties.
� Delamination - separation between two layers of coating, or a coating and the substrate.
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� Density (of solids and liquids)The mass of unit volume of a material at a specified temperature.
� Dielectric Strength Dielectric Strength is a measure of the electrical strength of a material as an insulator. Dielectric strength is defined as the maximum voltage required to produce a dielectric breakdown through the material and is expressed as Volts per unit thickness. The higher the dielectric strength of a material the better its quality as an insulator.
� Dielectric Constant.Dielectric Constant is used to determine the ability of an insulator to store electrical energy. The dielectric constant is the ratio of the capacitance induced by two metallic plates with an insulator between them to the capacitance of the same plates with air or a vacuum between them. Dissipation factor is defined as the reciprocal of the ratio between the insulating materials capacitive reactance to its resistance at a specified frequency. It measures the inefficiency of an insulating material. If a material were to be used for strictly insulating purposes, it would be better to have a lower dielectric constant.When a material is to be used in electric applications where high capacitance is needed, a higher dielectric constant is required. The test can be conducted at different frequencies, often between the 10Hz and 2MHz range.
� Faraday's lawThe amount of any substance dissolved or deposited in electrolysis is proportional to the total electric charge passed.
� Gel TimeAmount of time it takes for a resin to set (stop flowing) at a given temperature.
� Glass Transition TemperatureAssume that you have a polymer in the molten state, and you are cooling the polymer. As the temperature drops, it passes through the glass transition temperature, , and it's mechanical properties change from those of a rubber (elastic) to those of a glass (brittle.)
� OxidationA reaction in which there is an increase in valence resulting from a loss of electrons. Contrast with reduction. (2) A corrosion reaction in which the corroded metal forms an oxide; usually applied to reaction with a gas containing elemental oxygen, such as air.
� Oxidizing agentA compound that causes oxidation, thereby itself being reduced.
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� Particle Size Distribution - the overall range of particles (from coarse to fine) resulting from the grinding process; measured in microns; varies with product.
� Pretreatment - the preparation of a part prior to the application of a coating powder in order to improve adhesion and corrosion resistance.
� PHA measure of the acidity or alkalinity of a solution; the negative logarithm of the hydrogen-ion activity; it denotes the degree of acidity or basicity of a solution. At 25 ºC (77 ºF), 7.0 is the neutral value. Decreasing values below 7.0 indicate increasing acidity; increasing values above 7.0, increasing basicity.
� Phosphating: Surface pretreatment used on ferrous parts that provide a very thin crystalline film that enhances both corrosion resistance and adhesion.
� QuenchingRapid cooling of metals (often steels) from a suitable elevated temperature. This generally is accomplished by immersion in water.
� Reference electrodeA nonpolarizable electrode with a known and highly reproducible potential used for potentiometer and Volta metric analyses.
� Surface ResistivitySurface resistivity is the resistance to leakage current along the surface of an insulating material. The electrical resistance between two parallel electrodes in contact with the specimen surface and separated by a distance equal to the contact length of the electrodes. The resistivity is therefore the quotient of the potential gradient, in V/m, and the current per unit of electrode length, A/m. Since the four ends of the electrodes define a square, the lengths in the quotient cancel and surface resistivities are reported in ohms, although it is common to see the more descriptive unit of ohms per square
� Thermoplastic - a coating powder, which will repeatedly melt when subjected to heat and solidify when cooled.
� Thermoset A coating powder which, when subjected to heat, undergoes an irreversible chemical reaction during the cure cycle.
� Transfer Efficiencythe amount of powder attracted to the part compared to the amount of powder sprayed; measured as a percentage.
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� Volume ResistivityVolume resistivity is the resistance to leakage current through the body of an insulating material. The ratio of the potential gradient parallel to the current in a material to the current density. In SI, volume resistivity is numerically equal to the direct-current resistance between opposite faces of a one-meter cube of the material (Ohm-m). The higher the surface/Volume resistivity, the lower the leakage current and less conductive the material is.
Surface resistivity is expressed in ohms per squire
