CUI can account for as much as 40 to 60 percent of a company’s piping maintenance costs, can result in repairs in the millions, and lead to significant downtime, says Dr. Hira S. Ahluwalia, president of Material Selection Resources (MSR) Inc., a materials engineering and corrosion consulting organization serving the chemical, pharmaceutical, fabrication, and metal industries. Though based in New Jersey, Ahluwalia says the issues that contribute to CUI are the same on both sides of the border.

Carbon steel and 300 series austenitic stainless steels are the common materials of construction that are found to be susceptible to CUI. Carbon steel corrodes under wet insulation as non-uniform, general corrosion, and / or highly localized pitting, while the primary types of corrosion in austenitic stainless steels are pitting and stress corrosion cracking caused by chlorides. While degradation of almost any piping, either internally or externally, is possible, that which is exposed to marine conditions or water or chemical spray are more susceptible and in a shorter amount of time.

General corrosion is the most common form of corrosion and results in relatively even surface material loss, which gradually thins the metal, compromising the piping’s wall thickness. When an electrochemical potential difference exists between two dissimilar metal couples that are electrically connected in an electrolyte, galvanic corrosion occurs. An electric current between the two conductors results in the anode member corroding more quickly, while protecting the cathode.

Pitting, crevice attack, and microbiologically influenced corrosion are all considered localized corrosion – referring to corrosive attacks that occur is a specific location or under specific conditions. Pitting is the formation of quickly propagating holes on the surface of a metal; crevice corrosion usually occurs where a tight gap exists between two surfaces; and microbiologically influenced corrosion happens when certain types of bacteria form dome-shaped colonies on a metallic surface, producing a corrosive environment within the colony, causing a crevice attack.

Certain synergistic effects of a chemical environment and the mechanical condition of a metal can cause environmentally assisted cracking, personified by corrosion fatigue and stress corrosion cracking – a combination of normal fatigue and corrosion that causes failure below the normal endurance limit of the metal involved; and the brittle fracture of a susceptible material under tensile stress in a specific environment over time, respectively. Intergranular corrosion is the selective attack of a metallic component at the grain boundaries by a corrosive medium.

Ron King, consultant to the National Insulation Association (NIA) says in his article, “Does Insulation Cause Corrosion?” published in the June 2009 issue of Insulation Outlook, that corrosion can occur almost anywhere, but cautions against blaming mechanical insulation.

“[Pipes] don’t crack or corrode because they are insulated, but because they have come in contact with moisture,” says King. “Insulation does not cause corrosion.”
Despite the various forms CUI takes, a specific set of variables must be present – carbon steel must be exposed to oxygen, moisture, and warm temperature, and for austenitic stainless steel surfaces, chlorides or similar halides must also be present at the substrate.

Insulation provides an annular space where moisture that could contain additional contaminants can be retained. Depending on the type of insulation, it can add additional contaminates that, in the presence of moisture, can speed up corrosion, and some insulation wicks moisture faster than others. King notes, however, that the common thread in these scenarios is moisture – the number one contributor to the problem.

Moisture intrusion to an insulation system can be attributed to a number of factors including rainwater, the wash down process, water vapour, ice formation and potential freeze-thaw situations, fire detection system system activation, equipment or piping leaks, and flooding, says King.

“The prevention of CUI begins with the facility design and extends through the design and installation of the mechanical insulation system.”

A well-designed mechanical insulation system gives adequate consideration to the complexity and difficulty of insulating, sealing, and maintaining the seal to prevent penetration. A common design flaw, says King, occurs in the selection of a protective and / or vapour / moisture barrier for the insulation system.

“The characteristics of the core insulation material are important,” he writes, “but the selection of the vapour barrier and protective coating or jacket appropriate for the service temperature and environment is critical.      

Maintenance is another key issue. Ten to 30 percent of installed mechanical insulation systems are damaged or missing within a few years of installation due to lack of maintenance. This along with failure to immediately and correctly repair an insulation system after non-destructive testing is another contributing factor.

Ahluwalia also says maintenance of the mechanical insulation is important, but emphasizes prevention methods developed to address circumstances in which CUI is likely due to other factors.
“The physical characteristics of thermal insulation materials can vary widely,” he says. “Some insulation materials contain a leachable inhibitor to neutralize the pH of the water in contact with the metal surface. For some systems, the coefficient of thermal expansion will influence the system design. For example, cellular glass insulation expands about the same as carbon steel, whereas cellular foam expands nine times more than carbon steel and therefore requires expansion joints.

“General industry experience over the last 20 years indicates that corrosion is possible under all types of insulation.”  

The application of organic coatings beneath mechanical insulation creates a physical barrier to corrosive electrolytes on both carbon and stainless steel. When selecting a coating system, coating type, surface-preparation requirements, environmental requirements, compatibility with insulating material, coating tests, coating vendor selection, specifications, inspection, and selection of a coating applicator must be considered.

Epoxies, urethanes, and polyurethanes; fusion-bonded coatings; brushable coal tar or asphalt-based coatings; mineralization coatings; and tapes have all been used successfully in the process industries. Thermal spray aluminum is an ideal choice in services where temperature cycling exceeds and falls below 149 degrees C.

Aluminum foil wrapping of stainless steel pipe is not widely accepted in North America, but has been used successfully in European chemical companies by providing electrochemical protection by undergoing corrosion and maintaining a safe potential. Another option is using higher nickel, chromium, and molybdenum-containing alloys.

The previous list is by no means exhaustive. Factors relating to environment, piping materials, and insulation type are endless, but the right set of solutions does exist for any application.