Damage Control: CUI Detection

Editor’s Note:  This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

Corrosion under insulation (CUI) is a form of external corrosion that can be widespread or localized, caused by trapped water/moisture that can occur on surfaces covered with insulation. Because these surfaces are not generally available/accessible for visual examination, the onset of corrosion cannot be easily identified, and in extreme cases, severe corrosion with consequential impairment of system integrity can occur. Furthermore, CUI is a prevalent industry problem affecting thermally insulated equipment in the onshore and offshore oil and gas industries, as well as the petrochemical, specialty chemical, fertilizer, and related industries. In general, external thermal insulation is a necessity in process design for any or a combination of the following reasons ^[1]:

• Heat conservation [operating temperatures generally >200°F (93°C)]
• Cold conservation (e.g., refrigeration/cryogenic systems) [usually <40°F (10°C)]
• Personnel protection [usually >140°F (60°C)]
• Freeze protection (e.g., heat tracing)
• Condensation control
• Acoustic (noise) reduction
• Fire protection
• Process control

External Chloride Stress Corrosion Cracking (ECSCC) is a surface-initiated cracking mechanism in austenitic and duplex stainless steels and some nickel base alloys under the combined action of tensile stress, temperature, and an aqueous chloride environment. ECSCC is often considered to be a special form of CUI where the damage morphology and failure modes involve the propagation of crack-like flaws. 

Additionally, passive fireproofing can lead to corrosion under fireproofing (CUF). Fireproofing is commonly used on structural steel (most often carbon steel) to minimize the impact of high temperatures (e.g., changes in microstructure and subsequent loss of strength) generated during a fire. These extreme temperatures can damage structural supports for pressure vessels (i.e., support skirts) or piping systems (I-beams). Despite their different applications, CUI and CUF are similar degradation mechanisms in that corrosion of the carbon or low-alloy steel substrate may occur when water accumulates at the underlying steel surface. CUF is typically considered to be analogous to CUI in terms of damage mechanism classification and damage morphology.