When Does Corrosion Under Insulation Occur?

The problem occurs on carbon steels and 300 series stainless steels. On the carbon steels it manifests as generalized or localized wall loss. With the stainless pipes it is often pitting and corrosion induced stress corrosion cracking (CISCC). Though failure can occur in a broad band of temperatures, corrosion becomes a significant concern in steel at temperatures between 32 F (0 C) and 300 F(149C). Corrosion under insulation is caused by the ingress of water into the insulation, which traps the water like a sponge in contact with the metal surface. The water can come from rain water, leakage, deluge system water, wash water, or swearing from temperature cycling or low temperature operation such as refrigeration units.

Systems Susceptible to CUI

API 570 specifies to the following areas as susceptible to CUI:

• Areas exposed to mist overspray from cooling water towers.
• Areas exposed to steam vents
• Areas exposed to deulge systems.
• Areas subject to process spills, ingress of moisture, or acid vapors.
• Carbon steel piping systems, including those insulated for personnel protection operating between 25 F and 250 F. CUI is particularly aggressive where operating temperatures cause frequent condensation and re-evaporation of atmospheric moisture.
• Carbon steel piping systems that normally operate in-service above 250 F (120 C) but are in intermittent service.
• Deadlegs and attachments that protrude from insulated piping and operate at a temperature different than the active line.
• Austenitic stainless steel piping systems that operate between 150 F and 400 F (60 C and 204C). These systems are susceptible to chloride stress corrosion cracking.
• Vibrating piping systems that have a tendency to inflict damage to insulation jacketing providing a path for water ingress.
• Steam traced piping systems that may experience tracing leaks, especially at the tubing fittings beneath the insulation
• Piping systems with deteriorated coatings and/or wrappings.
• Locations were insulation plugs have been removed to permit thickness measurements on insulated piping should receive particular attention

All equipment will be shut down at some time or other. The length of time and the frequency of the downtime spent at ambient temperature may well contribute to the amount of corrosion under insulation that occurs in the equipment. It would be a daunting task to muster the resoures needed to tackle this extensive list of piping with the traditional inspection methods. This is where real-time X-ray offers a real advantage. Once the damaged areas are indentified, follow-up X-rays and ultrasonics can measure the loss by external corrosion. These techniques will not detect CISCC in stainless steels.

Alternative inspection methods will be discussed in our next blog...

Source:https://inspectioneering.com/content/1996-11-01/116/inspection-techniques-for-dete

For more information about Corrosion and how to prevent it, contact us via our website at www.leopad.com