Corrosion Under Insulation

CUI is defined as the external corrosion of piping and pressure vessels that occurs when water gets trapped beneath insulation. CUI damage takes the form of localized external corrosion in carbon and low alloy steels, while taking the form of external chloride stress corrosion cracking (ECSCC) in austenitic and duplex stainless steels.

CUI damage in carbon and alloy steels, is characterized by either general metal wastage or pitting due to the localized breakdown of passivity, when exposed to moisture and oxygen. Damage occurs when water is absorbed by or collected beneath the insulation due to breaks in the insulation or jacketing (cladding) and cannot dissipate from the system, therefore the moisture contacts and stays on the underlying exposed steel at metal temperatures between 32 °F (0 °C) and 212 °F (100 °C) for an extended period. Water may come from numerous sources such as rainwater, a deluge system, spillage from process operations, leaking steam tracing, or condensation on the metal surface in humid environments.

Figure 1. Illustrates the corrosion under insulation in carbon steel process piping

CUI damage in austenitic and duplex stainless steels is a form of external chloride SCC. As with all forms of stress corrosion cracking (SCC), cracking occurs when a susceptible metallurgy is exposed to the combined action of a corrosive environment and an applied or residual tensile stress. Susceptible materials include Type 300 series austenitic stainless steels. Duplex stainless steels, though more resistant than austenitic stainless steels, are not immune. A corrosive environment occurs when chloride concentrate under the insulation at the surface of austenitic stainless or duplex steels when insulation becomes wet. Most CUI damages in austenitic stainless steels occurs at metal temperatures between 140 °F and 350 °F (60 °C and 175 °C) although exceptions have been reported at lower temperatures.

Figure 2. Illustrates CL SCC due to CUI in insulated stainless steel pressure vessel

Below Video explain how corrosion under insulation occurs and what type of corrosion damages expected on CS and SS materials.