Corrosion Under Insulation

Insulating pipelines and equipment is a standard practice for conserving energy in process units. It serves various functions, including fireproofing, ensuring personnel protection, heat tracing, and acoustic protection. Dry insulation is generally neutral to the corrosion of the protected metal surface; however, when wet, it can result in slow but inevitable metal degradation. After remaining undetected for decades, corrosion under insulation (CUI) may be unveiled through catastrophic failures.

General Information

Corrosion under insulation (CUI) stands out as a widely acknowledged issue across various industries; paradoxically, it remains one of the most inconspicuous challenges. This problem manifests beneath the layers of insulation, making it elusive and often undetected until substantial damage has occurred.

The primary driver of CUI is the presence of water – essentially, where there is no water, there is no CUI. Unfortunately, it is nearly impossible to prevent water ingress into the insulation. Even seemingly dry hot insulation systems may experience moisture ingress, for example, during equipment cooldown, physical damage to insulation jacketing, or joint caulk seal failures. The severity of Corrosion Under Insulation (CUI) is influenced by several factors, including the duration of moisture exposure, the insulation’s capacity to absorb and retain moisture, the cyclic nature of moisture, and the temperature of the substrate.¹ ² ³ ⁴ ⁵

When insulation comes into contact with moisture, it forms convection cells within its structure. This involves water evaporation near hot metal surfaces and subsequent condensation at the cooler jacket wall for re-absorption by the insulation material. This convection cell phenomenon, termed ‘refluxing,’ results in high energy loss. Moreover, it has the potential to increase the concentration of corrosive elements, like chlorides, in close proximity to the pipe wall, resulting in corrosion rates higher than anticipated. Figure 1 illustrates the Corrosion Under Insulation (CUI) process within a hot service context.¹ ² ³

Figure 1: Schematic of CUI mechanism in hot service.

Corrosion under insulation (CUI) is prone to developing on the external surfaces of insulated process piping or equipment within a temperature range of -4°C to 175°C (25°F - 347°F).² The actual rates of metal loss hinge on various factors, including the duration and frequency of wetness, the presence of halides and other contaminants, leachability of corroding agents from the insulation, operating temperature, wet-dry cyclic operation, and the type of coatings used.

CUI rates exhibit considerable variability, often exceeding expectations. It is not uncommon to observe corrosion ranging from very low to nonexistent under dry conditions, with rates reaching up to 2 mm/y under refluxing regime. An elevated concentration of halides, primarily chlorides, may also induce external chloride stress corrosion cracking on insulated austenitic stainless steels.⁶ ⁷ ⁸

Further insights into the impact of insulation type, design, coating, and other factors on Corrosion Under Insulation (CUI) will come soon

References

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