CORRESION CONTROL UNDER INSULATION

1. INTRODUCTION

              Corrosion under insulation is a major problem. When insulation becomes wet, due to’ poor insulation, subsequent abuse or failure to specify good vapor barrier and water ‘proofing materials, it creates   the potentials for corrosive failure of pipings. Whether pipes are above ground or buried, proper design and insulation techniques can control corrosion.

               Metal corrosion requires four elements: An anode, a cathode, an electrolyte (e.g. moisture) and an electrical path (e.g. metal) joining the anode and cathode.

The anode is the part of the Meta where corrosion occurs. It actually scarifies itself as it releases positively charged metal ions into the electrolyte and electrons are left behind in the metal. These electrons flow through the metal to protect the cathode. The cathode is protected because various ions or compounds in the electrolyte consume electrons.
               An electrolyte is a solution capable of conducting electrical current in the form of ionic flow. An electrical path is a connection between the anode and cathode where current in the form of electrons can flow. Free electrons do not flow in the electrolyte, only in a metal path.

               Corrosion occurs because anodes and cathodes are inherent anodes and cathodes are inherent in all metals, and all metals are electron conductors. Anodic and cathodic components can be microscopic in size or rather large in some cases. Therefore, thereof the four requirements for the corrosion process are inherently present in every metal. The only remaining element required for the corrosion process to begin is an electrolyte.

               Anodic and cathodic areas develop where chemicals have deposited, where there are temperature differences, and where there are temperature differences, and where there are damp areas. Chlorides and other industrial contaminants in the electrolyte can cause an area to become anodic. The contamination may be present on the metal surface before it is coated or insulated. Once these areas become wet, corrosion begins.

The corrosion of metals requires the following conditions

Ø    An anode, a cathode, an electrical path, and an electrolyte must all be present.

Ø   The anode and cathode must be in contact with the same electrolyte.

Ø   The metal must electrically connect the anode and cathode for electrons to flow; and

Ø   The anodic (oxidation) and cathodic (reduction) reactions must be equivalent and simultaneous.

Fig 1. Shows the requirements for corrosion process.

2. BASIC CORROSION CONTROL

        The sidebar discusses the four necessary elements for the corrosion process to occur. However, certain external factors will cause variance in the corrosion rate after the corrosion process begins. Examples of factors affecting corrosion rate would be hot or cold cycling expansions and contractions (stress cracking) and wet or dry cycles.

        One of the most effective ways to control corrosion is to use a properly selected and applied corrosion coating, since 1971, U.S. government regulation require oil and gas companies to coat ( with a corrosion control coating ) all pipelines before they are buried or submerged . Cathodic protection (CP) also is required on all barriers and submerged transmissions and gathering pipelines to protect most areas of coating defects. Cathodic protection is the installation of external galvanic anodes or anodes with a power source to provide protective current to the cathode. The oil gas industry has used pipe coatings and tapes successfully for many   years. These vary in type, from   the first coal tar brush on coatings covered with a spiral wrapped bitumen tape, to today’s epoxy, urethane, urea and FBE (fusion bonded epoxy) coatings.

       The pipeline industry (loosely defined as the oil and gas industry and the uninsullated piping used as transmission lines) and insulation industry (dealing with insulated piping systems) differ in their practices in some respects, but the success and the growth in the pipeline coating industry raises the question of why other industries are not using this knowledge to protect pipes and process systems. Pipeline coating obviates the need to replace piping systems every few years, avoiding the cost of such expensive projects.

3. COATING SELECTION CONSIDERATIONS

       The proper selection of coating material is important. When selecting coating metal under insulation, consider:

·               System operating temperatures,

·               Application and site requirements,

·               Surface preparation requirements,

·               Environmental requirements during surface preparation and application, and

·               Compatibility with insulating materials.

3.1 System Operating Temperatures:

       A coating has to be flexible enough to withstand the expansion and contraction of the piping system when   temperatures cycles. Temperatures fluctuations can cause a loss of adhesion between the coating and the metal, which allows water to reach the pipe. High temperatures can cause coating types to flow, crack, or sag.  Low temperatures can cause some coatings to become flexible or brittle.

       Some coatings work well at high temperatures, while other at cold temperatures.

New entries into metal protection industry have achieved temperature ranges between -320 degree F and 350 degree F (-196 degree C and 177 degree C) ,with prospects of coatings up to 900 degree F (149 degree C ) for specialized applications on equipment that cycles from cold or ambient up to higher  temperatures. This cycle accelerates that corrosion process.

3.2 Application Requirements:

       Some types of coatings require extensive surface preparation, even heating the pipe before applying the coating. Other coatings can be applied with minimal surface preparation and application equipment. Field application becomes more difficult of confined space, safety, and environmental concerns. Inspection to ensure proper coating average and thickness is cumbersome.

                Liquid coatings can be applied by brush, glove, or spray method (e.g. air, airless or plural component). Tape coatings can be applied in a “cigarette wrap” or by spiral wrapping by hand or machine. On insulated systems, tapes will affect the inner diameter (I D) and the fit of the insulation. Some types of tapes are applied using heat, usually a propane torch. Powder coatings, such as fusion-bonded epoxies (FBE) are applied to a hot (normally 450 degree F to 488 degree F [232 degree C to 253 degree C]) pipe surface and are applied in a specialized pipe coating plant with temperature end humidity control.

3.3 Surface Preparation:

              Surface Preparation is the most critical part of any coating process. Care must be taken to perform the best possible surface preparation for the application. Experts in the coating industry advise that two-thirds of the cost of any good coating job should go into surface preparation.

               Blasting with an abrasive helps to clean the surface and provides the proper anchor pattern to which the coating will adhere. Before blasting, any oil, grease, or other debris must be properly removed. Blasting only spreads oil and grease contamination – it does not remove it. Contaminants on the metal surface, such as chlorides and other salts, must be removed by proper washing and rinsing techniques. Main scale, rust and other surface contaminants can usually be removed by proper blasting. Wire brushing by hand or machine is acceptable for some types of coatings. Water blasting, with and without abrasives, may be used in other situations. This must be performed with proper methods and equipments.

              The introduction of mineralization surface conversion technology in the insulation industry reduces the amount of surface   preparation required before installing the compound. Sand blasting is not necessary, even on rusted pipes. The main requirements are to remove oil or salt films that may be on the pipe surface and the removal of scale rust with a wire brush, or water blasting.

3.4 Environmental requirements during surface preparation and application:

The type of surface to be coated dictates the type of surface preparation and coating to be used. New carbon steel can be cleaned and blasted easily, compared with corroded or pitted steel in used systems. Corroded or used metal systems may have surface contaminants such as chlorides or salts that must be properly removed before blasting. Stainless steel surfaces are very hard, making it difficult to create an anchor pattern. Certain types of stainless steel should not be blasted with steel grit or shot because the carbon in the blast material itself can cause corrosion problems. Non - carbon materials, brushes, or grinding disks should be used instead.

               Unique circumstances will dictate exceptions to any of these recommendations. Nevertheless, preparing the substrate in some way is always important. Other types of metal not mentioned here should be studied and tested to determine the most effective method of surface preparation.

3.5 Compatibility With Insulating Material:

       Some types of insulating materials may be abrasive (e.g. cellular glass) and as the pipe moves, the coating material will be damaged. Other insulating materials may cause the coating to deteriorate, become soft or brittle, or loose other properties (especially if the insulation becomes wet). The thickness of most coatings is within ID tolerance of insulation products. However, insulation fabricators may have to alter some tolerances of the ID of their insulation to provide products that fit over certain coatings. Coating selection and insulation characteristics should be considered jointly when specifying a system. In their wet state, some corrosion coatings react with fabricated foam insulations; therefore, coatings must be allowed to cure completely before installing insulation.

  

4. TYPES OF COATINGS

       Types of coating choices on the market include:  

·                     Liquid coatings: epoxies, urethanes and polyureas,

·                     Tapes and shrink sleeves,

·                     Brushable coal tar or asphalt- based corrosion coatings,

·                     Mineralization coatings, and

·                     Fusion- bonded epoxies and multilayer coating.

4.1 Liquid coatings:

       Liquid epoxies are excellent choices for coating pipes. Basic epoxies are two- component materials that are mixed and normally applied by spray or brush. Some epoxies are applied with multycomponent equipment that mixes the components at the applicator gun. Epoxies must be mixed in the proper ratio. If the mixture is not correct, the epoxy does not cure properly or perform well. Specific environmental concerns need to be addressed since fumes and cleanup are important criteria.

“Phenolic epoxies are excellent coatings for higher- temperature applications up to 300 degree F (149 degree C). Modified epoxy phenolic coatings offer good abrasion resistance and are more flexible than most epoxy phenolics.”  Phenolic epoxies are applied by experienced applicators using heated multicomponent equipment.

       Urethanes and polyureas are excellent coatings for cold process piping and vessels. Most urethanes have limited use for higher temperature operations at greater than 150 degree F (66 degree C). Urethanes are flexible and can be applied in one thick coat with a heated spray system. Moisture can be detrimental to some urethanes application, but moisture cure urethanes perform well for application moisture is a problem during application.

  4.2 Tapes and shrink sleeves:  

       Certain types of corrosion control tapes provide excellent corrosion protection for pipes on systems that operate at temperatures of 150 degree F (66 degree C) or less. Tapes are relatively easy to apply. Most require a primer in order to adhere properly to the pipe, and the primer must cure properly before tape is applied. Tapes vary from 25 to 100 mils in thickness, thus, insulation material must have the ID to enable the insulation to fit over the tape.

  4.3 Brushable coal tar or asphalt- based corrosion coatings:

       Asphalt or coal tar-based materials do not require mixing, thinning or other special treatment. They are easily applied by brush or paint glove in or two coats. Surface preparation requirements are not as stringent for these materials as they are for many other coatings. These coatings are good for irregular shapes, are flexible, adhere well to the steel, are resistant to most chemicals and impact resistance. They do require a certain amount of time to cure properly.

4.4 Mineralization Conversion Compounds:

                Mineralization conversion compounds are new to the insulation industry. As the name         implies, these coatings create a mineralization bond with a pipe, a “new” surface.    Excess coating from the application acts as a reservoir in the event of mechanical damage to the mineral layer or the intrusion of an electrolyte. Corrosion cannot takes place in the mineralized layer of pipe.

       The greatest benefits of this type of technology are if the vapor barrier is compromised and moisture enters the system, it can travel directly to the pipe’s surface and remain there without corrosion taking place. The excess coating from the installation process chemically binds the water so that it can not corrode the pipe. Only removal of excess coating with cleaner can alter the protection provided to the piping system.

       Mineralization surface conversion compounds can be applied to all types of pipes. They not only prevent corrosion but also prevent stress cracking when applied to copper or stainless steel systems.

       Mineralization surface conversion compounds are effective on new piping, valves, tanks, vessels. Conversion compounds can obviate the need to replace piping that is somewhat corroded but will remain intact if no further corroded. In past, this was not an option. If the metal retains sufficient physical integrity for the pressures it contains, it can be wire brushed or water blasted to remove any loose rust scale or corrosion scale before the pipe is treated with the conversion compound and reinsulated. This type of application will create the same mineral bond in old piping as new.

       The use of surface conversion compounds will revolutionize corrosion control on below ambient systems under insulation. The life expectancies of the systems will be increased and with proper insulation, vapor barrier and mechanical protection. Corrosion is not a problem only on below ambient piping systems; however the 250 degree F (121 degree C) temperature limits of conversion compounds limits them primarily to below ambient systems where moisture drive to the pipe can occur.

5. REHABILITATION OF EXISTING SYSTEMS

            Rehabilitation of insulated piping systems can be performed with many of these coatings. For systems where surface preparation is difficult and minimal surface preparation is performed, mineralization conversation compounds are the best option. Tapes are an option if the fabricator can supply insulation with proper ID. Brushable coal tar and asphalt- based coatings can be considered. Retrofit conditions usually do not permit spray application, limiting your options.

       The most cost-effective method for rehabilitation without replacement is the minimal cleaning required by mineralization surface conversion technology. Removal of loose scale with a brush or water jet is sufficient preparation prior to installation of mineralization coatings. Though this technology may appear new to many, it has been used in military and automotive applications used such as anti-seize on flange bolts, anti-corrosion on moving parts such as bridge rockers and anti-corrosion of protected, yet concealed steel cable.

5.1 Insulation:

               The insulations used for above freezing yet below ambient piping are quite broad including fiberglass, electrometric rubber, mineral wool. It also can include sub-zero insulations such as polyisosyanurates, polystyrenes, cellular glass and phenolics. Surface conversion compounds are less attractive for chilled water systems as failure of these systems due to corrosion is not an environmental issue as much as a possible property damage issue. Failures chilled water systems can occur within five years due to advance corrosion activity. The use of surface conversion compounds would eliminate such early demise of the piping system; however, conversion gel manufactures will not put a figure on it as the choice and installation of all components in the system contribute to the life of the system.

               Insulation types are fairly limited for below freezing piping systems; cellular glass, polyisosyanurates, polystyrenes and phenolics are normally the insulations of choice. The manufacturers of these products public technical literature that describes their uses, temperature limitations and design criteria. The job criteria (atmospheric conditions, process system, and plant environment) should determine the insulation best suited to a specific job. The fact that an “ammonia system” is being designed is not enough information on which to base an insulation specification. Although the basic design criteria of the system may be known, the environment in which the system is being installed must be considered before specifying types of insulation and insulation thicknesses.

               Insulation thickness is critical to the success of the system. Worst-case atmospheric job conditions should be used when calculating insulation thickness. This condition may only exist one week per year, but that single worst case could create a corrosive environment that eventually could cause the premature demise of a system.

Installation of the insulation is a critical as the choice of insulation itself. Improperly sealed insulation can allow moisture to migrate to the pipe, providing the electrolyte necessary to begin the corrosion process.

               Insulation manufacturer’s literature normally suggests perm rated joint sealants for their insulations. It is important to use joint sealants to slow the migration of water vapor to the pipe if a breach in the vapor barrier occurs. Since there are atmospheric conditions present during all insulations, water vapor will be trapped in the system when the vapor barrier is sealed. Consulting with an insulation fabricator is helpful given how closely fabricators work with the insulation manufacturers and how frequently they deal with insulation specifications on cold piping systems.

52 Surface Conversion Technology:

       Surface conversion technology, or minetics, is the ability to grow very thin minerals on metal surfaces for useful purposes. Engineered surfaces from when mineral-forming reactants are delivered to the surfaces of a metal or metal containing article via novel modifications of known base formulations (e.g. paint, gel, aqueous suspension)

          

5.3 Surface Conversion Application:

       Mineralized surface conversion compounds are designed to be bead applied to the bore of the inside layer of insulation by the insulation fabricator to minimize installation cost. The numbers of beads on the ID of the insulation depends on the pipe size being insulated. A bead is also applied at one end of the insulation piece to act as “insurance” at every joint. Due to the physical properties of the compound, it can be applied and shipped to the job of site without sagging and skinning over.

       The installation process is simple. The insulator installs the pipe cover and rotates the insulation around the pipe and longitudinally to distribute the compound onto the pipe. The beauty of the buffering system is that 100% coverage is not critical. Using this installation method, testing shows that coverage of better than 98% is achieved with this rotation and sliding of the insulation.

5.4 Vapor Retarders and Protective Jackets:

       One of the most critical components on cold systems is the vapor barrier or vapor retarder as commonly stated. Vapor barriers or retarers, comes in various forms. They   include products such as, ASJ papers, FSK paper, vapor barrier mastic, Mylar’s, proprietary and laminated self-adhesive membranes. The recent introduction of low- perm, peel and stick, self healing vapor barrier membranes are just what the doctor ordered! Although the cost is approx. three times that of ASJ paper (a retarder), cost should not deter the specifications these membranes! These new membranes are UV

Stable indoors and can eliminate the need for an additional mechanical jacket,( making these comparatively less expensive) where mechanical protection is not required.

       For exterior applications, laminated rubberized bitumen self-sealing membranes have been produced with various” skins “to make them impervious to the elements. These membranes eliminate the need for expansion sleep joints by providing 400% elongations before rupture; they expand and contract with the system. Insulation can be applied, and then one peel and stick jacket provides an excellent perm ultraviolet stability, excellent emissivity and sealed weather protection.

       Currently, the standard specification is for a vapor retarder jacket to be installed under a protective mechanical jacket, such as PVC or Aluminum. The vapor retarder of choice most commonly ASJ paper, which, if “crinkled” has lost his vapor retarder properties. Furthermore, the seal on a metal or PVC jacket is only as good as the installer is with the glue gun or caulking gun. Engineers also are finding that expansion and contraction of piping systems with PVC or metal jacketing can generate friction between the vapor retarder and jacket, eventually wearing through the vapor retarder.

              For a below ambient insulation system to be successful and not contribute to the corrosion process, it must be specified in the following manner:

·                 Using  a corrosion control coating ;or treatment,

·                 Tailoring the insulation to the application and environment,

·                 Calculating adequate insulation thickness, Properly sealing the joints,

·                 Applying a superior vapor barrier, and

·                 If necessary, install a mechanical jacket to protect vapor barrier from physical abuse.

To be specific, the ultimate system available today would be a joint sealed, closed cell foam insulation, bore coated with a mineralization conversion compound, sealed with a self healing low perm vapor barrier and mechanically protected with an aluminum or PVC jacket where necessary for mechanical abuse protection. Exterior systems would be jacketed with self healing, vapor barrier membrane without a mechanical jacket. The cost of such system would be approx. 30% higher than typical systems being installed today.

       Comments have been made about the insulation industry and how difficult it is to change the status quo, if the industry wants to solve this serious corrosion under insulation problem; it has to change the specifications and installation processes. Engineers must work closely with fabricators to find solutions to this aspect of the problem; if it is not beyond a fabricator’s capability. Communication levels among Engineering entities and installation fabricators, installers and manufactures must increase. Alternate innovative products are not always submitted under the guise of the increasing a contractors profit margin. Better, alternative measure in the methods industry must be considered on their merit rather than on the perceived intent of the presenter.

CONCLUSIONS

       Coating the pipe before insulation is applied will not solve all of the corrosion problem that exists in this industry, but it will result in definite improvements. Through testing, proper selection of materials and methods, and well- written and detailed specifications, tremendous improvements can be made in controlling corrosion problems. The following suggestions are

   recommended for industry:

·                   The industry must be committed to preventing corrosion ( primers or no coatings are unacceptable );

·                   Testing and insulation specifications must be developed and must outline specific coating system for each application; and must outline specific coating systems for each applications;

·                   Inspection and testing must be performed; and

·                   Only the best vapor barrier, water proofing materials and insulation should be used.

       On must projects, the initial costs to properly, prepare, coat and install insulation and vapor barriers is minimal compared with the overall project cost and invaluable when considering the cost of repair and replacement and re-insulation of corroded systems ( not even considering the losses in the production due to systems downtimes ). Systems installed today that are “value engineered” make obsolescence an integral part of systems. Reducing corrosion failures results in significant long-terms financial benefits. The safety and environmental issues related to these failures must be factored in to the cost savings.