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Corrosion Under Insulation(CUI)

Inspection & Monitoring
mechanicalchemical

Definition

External corrosion of piping and equipment that occurs beneath thermal insulation, fireproofing, or other external coatings where moisture becomes trapped against the metal surface. CUI is one of the most prevalent and damaging integrity threats in process industries because it is hidden from visual inspection, can progress rapidly, and is difficult to detect without removing insulation or using specialized inspection techniques.

Context

CUI is a major focus of API 570 and API 571. It is most prevalent in the temperature range of -12C to 175C (where water can exist as liquid) and in environments with high humidity, rainfall, or deluge systems. In Reliatic's RBI assessments, CUI susceptibility is a key factor in external damage factor calculation.

Examples

  1. CUI is discovered during a turnaround when insulation is removed from a 150C carbon steel steam line — localized pitting has reduced the wall from 7.6mm to 2.1mm beneath a damaged insulation jacketing seam.
  2. A proactive CUI inspection program using profile radiography (RT through insulation) detects significant external metal loss on a cold service piping circuit without requiring insulation removal.

Applicable Standards

  • API 571
  • API 583
  • NACE SP0198

Related Terms

External Corrosion

Corrosion that attacks the outer surface of equipment and piping, caused by exposure to atmospheric moisture, chemicals, soil (for buried piping), or water trapped under insulation. External corrosion can be uniform (general atmospheric corrosion) or localized (under insulation, at soil-to-air interfaces, under deposits). It is distinct from internal corrosion, which is caused by the process fluid.

Damage Mechanism

A specific process or phenomenon that causes degradation of material properties, leading to eventual failure of equipment. Damage mechanisms include corrosion (uniform, localized, galvanic), erosion, fatigue, creep, hydrogen damage, stress corrosion cracking, embrittlement, and other metallurgical or environmental attack modes. API 571 catalogues over 60 damage mechanisms relevant to refining and petrochemical equipment.

API 571

Damage Mechanisms Affecting Fixed Equipment in the Refining Industry — a comprehensive catalogue of over 60 damage mechanisms that can affect refinery and petrochemical equipment. Each mechanism is described with its appearance, critical factors, affected materials, prevention methods, and inspection techniques.

Non-Destructive Examination(NDE/NDT)

A group of analysis techniques used to evaluate the condition of materials, components, and assemblies without causing damage or altering their serviceability. NDE methods detect surface and subsurface flaws, measure wall thickness, characterize material properties, and assess the integrity of welds and joints. Common NDE methods include ultrasonic testing (UT), radiographic testing (RT), magnetic particle inspection (MPI/MT), liquid penetrant testing (PT), visual testing (VT), eddy current testing (ECT), and acoustic emission testing (AE).

Corrosion Rate

The rate of material loss due to corrosion or erosion, typically expressed in millimeters per year (mm/yr) or mils per year (mpy). Short-term corrosion rate is calculated between the two most recent thickness measurements; long-term corrosion rate is calculated from the original (or earliest) thickness to the most recent measurement. The more conservative (higher) rate is typically used for remaining life calculations.

Where this appears in Reliatic

This concept is used in the Inspections module.