Corrosion under insulation (CUI) of steel operating equipment and piping is recognized as an important problem in the ammonia refrigeration, chilled water, chemical and petroleum industries. A systems approach to CUI includes the correct material selection and design of the jacketing, insulation and coating. This approach will help prevent CUI, but many corrosion engineers would agree that electrolytes will eventually find their way into even the best system. Selecting the right coating is extremely important, as the coating is the last line of defense for keeping the electrolyte from the metal surface and preventing corrosion.
Historically, many different types of products have been used to prevent CUI. One of the most common is the use of a synthetic fabric impregnated with petroleum jelly that is spiral wrapped around bare piping prior to the installation of the insulation. Known as “grease tapes,” these products do not typically offer long-term performance and can add to the overall insulation system cost by requiring insulation with a larger inside diameter to cover the tape. These tapes are inexpensive and fairly easy to apply, but do not provide long-term performance.
There are numerous paints and coatings available using silicon and epoxy technologies. Some are available on new pipe. Some require multiple coatings. When used in retrofit applications, they require a degree of surface preparation of the existing pipe that adds significant cost and time to project completion. Although more effective than grease tapes, paints and epoxies must be installed with great care to prevent damage to the surface, and in retrofit applications, often require the system itself to be shut down.
Gel-type pipe coatings have gained in popularity over the last few years in the food process industry due to their high-quality performance and ease of application. Pipe gel coatings can generally be split into two categories—reactive gel (RG) technology and synthetic petroleum hydrocarbons with thickeners (SPHT). There are significant differences in these types of products and are made using different technologies. In fact, their performance characteristics, performance histories, laboratory testing and field application trials are significantly different. Here’s how it works.
SPHT are typically used as gear and chain grease-type lubricants. They are not commonly used as a CUI coating.
RGs are specially formulated and patented anti-corrosion products that are primarily designed to be used in CUI applications. RG products have the ability to form a mineralization layer as an integral part of the surface of the pipe. This prevents undercutting where moisture can get under the coating and begin the corrosion process.
A CUI coating needs to have various properties in order to be successful at mitigating corrosion. A few basic characteristics of a good CUI coating are thermal stability, the ability to resist water wash off and the ability to protect the substrate from corrosion.
Thermal stability. An independent lab tested both types of products at 240°F. This test was conducted on cold rolled steel (CRS) panels with the product applied at 25-27 mils thick (within the manufacturers recommended application thickness range). The pictures taken after 24 hours at 240°F show the degradation of the SPHT while the RG was shown to have little to no change in physical appearance.
Water wash off resistance. SPHT can be cleaned up using dishwashing liquid and water. RGs are formulated to resist moisture/water and are designed to be cleaned up with a citrus-based cleaner. Because the SPHT product has a much lower specific gravity than RG, it will be much less resistant to water wash off.
Anti-corrosion testing. ASTM B-117 is the most effective test in assessing the laboratory performance of anti-corrosion coatings and is used by corrosion engineers worldwide to compare product performance. An independent lab tested both RG and SPHT side by side at a thickness of 25-27 mils with an “X” scribe for 504 hours. An “X” is commonly added to this type of test to show resistance (or lack of resistance) to edge undercutting.
A leading SPHT manufacturer says that the corrosion properties have been previously verified by exposing a pipe to the Florida coastal elements for one month. No corrosion formed. SPHT products have no documented record of long-term success in mitigating corrosion in general and specifically CUI.
RGs were tested in a salt water immersion test where pipes were coated, covered with fiberglass insulation and submerged in 5% aerated salt water for two years. In addition, RG technology has over 10 years of specific experience in mitigating CUI in the food and beverage industry.
Lab testing shows that there is a significant difference between RG technology and SPHT in thermal stability, resistance to water wash off and anti-corrosion properties. In addition, RG technology has significantly longer field experience in various applications and significant direct experience in CIU applications.
In ammonia refrigeration and other very low-temperature applications where system integrity and worker safety are important considerations, engineers will need to closely evaluate the above factors in choosing the proper gel technology to use.