Internal Corrosion
Definition
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.
Context
Internal corrosion is the primary focus of fixed equipment integrity monitoring. The rate and mechanism of internal corrosion determine the inspection technique, CML placement, inspection interval, and material selection. Process chemistry, temperature, velocity, and phase behavior are the key drivers of internal corrosion.
Examples
- Internal sulfidation corrosion of a carbon steel crude unit side-cut piping circuit is monitored at 8 CMLs, with a long-term corrosion rate of 0.3 mm/yr correlating with total acid number (TAN) of the crude oil.
- Internal erosion-corrosion at a piping elbow downstream of a control valve creates a localized thinning pattern at the impact zone, requiring targeted CML placement at the 6 o'clock extrados.
Applicable Standards
- API 571
- API 581
- API 510
- API 570
Related Terms
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.
Corrosion RateThe 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.
Damage MechanismA 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.
Condition Monitoring Location(CML)A designated, permanently marked point on equipment where thickness measurements or other condition assessments are taken at regular intervals. CMLs (also called TMLs — Thickness Measurement Locations) are positioned at locations where degradation is expected to be most severe or most representative — areas of high velocity, turbulence, impingement, deadlegs, injection points, and heat-affected zones. Each CML has a unique identifier and documented coordinates.
API 571Damage 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.
Where this appears in Reliatic
This concept is used in the Inspections module.