External Corrosion

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.

Corrosion that attacks the internal surface of equipment and piping, caused by the process fluid and operating conditions. Internal corrosion mechanisms include uniform thinning (sulfidation, oxidation, naphthenic acid), localized attack (pitting, mesa corrosion, crevice corrosion), flow-assisted corrosion (erosion-corrosion, flow-accelerated corrosion), and environmentally-assisted cracking (SCC, hydrogen damage). Internal corrosion is typically the primary degradation mechanism monitored through CML programs.

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.

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.