Improving Functional Powder Coatings' Corrosion Protection

Fusion bonded epoxy (FBE) was introduced in Europe in 1953 for the coating of electrical equipment by the fluidized bed dipping method.¹ In the early '60s it was introduced to the pipeline industry for the protection of small diameter water and oil field production piping. The first FBE for large diameter pipe was supplied in the mid'60s. By the late '70s FBE became the most widely used pipeline coating in the U.S., Canada, Saudi Arabia and the U.K. Presently, FBE is used on every continent for the protection of line pipe, production tubing and drill pipe for the oil and gas industries. It has also gained acceptance for the protection of reinforcing steel (rebar) in the U.S., Canada, the U.K. and the Middle East.²

Although FBE is extremely successful as a corrosion protection system for underground pipelines, there are some inherent limitations, which make it difficult to achieve total corrosion protection with coatings alone. Some of these limitations are due to the chemical nature of organic materials in the coating and some are related to coating application procedures. To overcome the deficiencies and achieve total corrosion protection, another alternative method, cathodic protection (CP), is used in conjunction with the coating. Presently, FBE with the CP system is the most effective and economical corrosion control system for underground pipelines, but the success depends on the coating's ability to become an integral part of the "CPcoating" combination system.

There are several critical properties that dictate the performance of the coating and its ability to become a part of the CP system. The chemical composition of the product plays a major role in its performance, but the application parameters are equally important. In order to understand the critical role that they play in coating performance, a review of corrosion fundamentals and FBE application procedure is in order.

Corrosion fundamentals

The first step in the corrosion process is the conversion of metallic iron (Fe⁰) to ferrous ion and electrons:

Fe Ô Fe⁰ + 2e^- (equation 1).

Due to the differences in the internal energy levels of Fe⁰ and Fe⁺⁺ (the energy of Fe⁰ being higher), this is a natural and spontaneous process. However, the rate of this reaction depends on the concentrations of Fe⁰, Fe⁺⁺ and the electrons. Since the removal of any of the products accelerates the forward reaction in systems that are in equilibrium, the removal of either Fe⁺⁺ or electrons can accelerate the above corrosion process. 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. Reactions involving oxygen and water are:

2H₂O + O₂ + 4e Ô 4OH (equation 2)

2Fe⁺⁺ + 4OH^- Ô 2Fe(OH)₂ (equation 3).

Corrosion can be effectively controlled by providing a barrier against the passage of these reacting ions to the metal interface. This principle is applied in the corrosion control method using protective coatings.

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 Fe⁰ to Fe⁺⁺. If there are other easier sources for electrons, those provided by the oxidation of Fe⁰ 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. This principle is used in another corrosion control method known as the CP system. In this system, the pipe is connected to the negative terminal of an electrical supply of sufficient voltage to provide excess electrons to the pipe surface.

The effectiveness of both systems depends on various factors. In the case of the CP system, the key is providing sufficient electrons when and where they are needed. Any obstacle, such as loose coatings, rocks, etc., that prevents electrons from reaching the areas where they are required can result in serious corrosion and pipeline failures.

Coatings, on the other hand, are more complicated. Several properties of the coating dictate the corrosion control efficiency of the system, including corrosion protection properties and mechanical properties. A complete treatment of the critical properties of a coating that affect the corrosion control of pipe can be found in another paper I co-authored.³ However, a brief description of some of these critical properties and an explanation of how the formulation and application parameters affect these properties are worth discussing.

Corrosion protection properties

Protective coatings control corrosion by providing a barrier against oxygen and water. Its effectiveness is directly related to the oxygen and water permeability coefficients. It has been calculated that an extremely small concentration of oxygen and water (10^28.4 M) can start the corrosion process.⁴ Therefore, to minimize corrosion, materials that have extremely low permeability have to be used in the formulation.

A powder coating's ability to be in intimate contact with the metal surface is known as the wetting property. This is one of the most important factors that affect the success of powder coatings. Protective coatings are designed to offer high electrical resistance between the cathode and anode of the corrosion cell. On the pipe surface, since the cathode and the anode are usually separated only by micrometers, the success of breaking the electrical conductivity between these electrodes depends on the ability of the coating to place itself between them. To achieve this, powder coatings should have a very low viscosity. This is particularly important for coatings applied over a blastcleaned surface with deep profiles (between 2.5 and 4.5 mil).

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. 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.

The adhesion property of a powder coating is integral to the coating's corrosion prevention capabilities. In layman's terms, adhesive strength is the force required to pull the coating from the substrate. The adhesive strength is the sum of several components. The major portion comes from three components: mechanical, polarpolar and chemical. Mechanical adhesion is the dripping force of the coating onto the substrate. Polarpolar adhesion is the force of the attraction between the positive and negative poles of the coating and the substrate. Chemical adhesion is the force needed to remove the coating molecules that have chemically reacted with the metal 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, both thermoset and thermoplastic, 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. But, this highly desirable property can be an obstacle in the performance of the CP-coating combination system if it exceeds the adhesive strength of the coating. A detailed explanation of how cohesive strength and adhesive strength are related in the corrosion protection of underground pipelines has been given by G. Mills.⁴

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.⁵ 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.

Abrasion resistance and gouge resistance are two other mechanical properties of great importance. These properties also influence the amount of powder coating damage during transportation and pipeline construction.

FBE (fusion bonded epoxy) application procedure

The conventional application procedure involves abrasive cleaning of pipe and fittings. The surface is cleaned to NACE level2 using steel grit and shot of the proper size to obtain a 2.5- to 4.5-mil profile. The surface is then checked for salt contamination and, if necessary, washed with phosphoric acid and then deionized water. Chromate is applied to enhance the adhesive strength of some FBE formulations. This surface is then heated to 450 to 488F. The finely powdered, unreacted FBE is then fluidized with cold, dry air and conveyed to an electrostatic spray system. The fluidized FBE powder is then sprayed onto the hot pipe surface using a series of spray guns. FBE, which is solid at ambient temperature, melts when it contacts the hot surface. The melted epoxy resin reacts with the curing agent contained in the FBE system and bonds to the substrate, providing a highly crosslinked polymer with a sophisticated network of covalent and coordinate bonds. These highenergy bonds provide excellent adhesion between the coating and the metallic substrate.

Influence of surface preparation and application temperature. Most of the high performance powder coating systems require an extremely clean surface. For FBE, the minimum required cleanliness is Sa 2.5 per ISO 8501-1/SIF or NACE level2. The importance of this requirement can be easily understood by reviewing the role of adhesion in the coating's performance. The two important components of adhesion, polarpolar and chemical, are directly linked to the bonding hydroxyls of the substrate surface. The absence of these hydroxyl groups can adversely affect the overall adhesive strength of the coating. An improperly cleaned surface can limit the number of hydroxyls available for bonding.

Another important step is the removal of watersoluble salts and organic contaminants. If left on the surface, salts, especially chlorides and sulfates, can initiate water absorption by osmosis and lead to coating blisters. It has been reported that chloride contamination will seriously affect adhesion and cathodic disbondment properties.⁶

International standards such as NACE and CSA address this issue in their FBE application specifications. The standard procedure to remove soluble salts, such as chloride, involves washing the pipe with phosphoric acid followed by deionized water. The maximum allowable chloride concentration is 2.0 mg/m², according to NACE standards.⁷

The required profile for FBE application is 2.5 to 4.5 mil. The peak heights and densities of the profile have a profound effect on the corrosion protection performance of the coating.

Proper initial adhesion and the powder coating's ability to provide adequate adhesion throughout the design life of the pipeline are the key factors in providing adequate corrosion protection. Profile is a key factor in deciding the total adhesive strength of the powder coating. As stated earlier, the major adhesion components are mechanical, polarpolar and chemical. All of these are greatly influenced by the shape, peak heights and density of the anchor profile. By increasing the depth and density of the profile, the available bonding area is increased.

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. Because FBE is a solid, the melt viscosity has to be low enough to fill the profile without leaving any air pockets, which are potential areas for pit corrosion. 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.

FBE systems require high energy to achieve crosslinking 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.

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.