Damage Mechanism
Definition
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.
Context
Identifying active and potential damage mechanisms is the foundation of inspection planning. Each damage mechanism has characteristic morphology, affected materials, critical operating parameters, and appropriate inspection techniques. Damage mechanisms drive the selection of inspection methods, the placement of CMLs, and the calculation of damage factors in RBI.
Examples
- High-temperature hydrogen attack (HTHA) is an active damage mechanism in a reactor operating above the Nelson curve threshold, requiring specialized inspection with advanced UT techniques.
- Chloride stress corrosion cracking (Cl-SCC) is identified as a credible damage mechanism for austenitic stainless steel piping operating above 60C in the presence of chloride-containing insulation.
Applicable Standards
- API 571
- API 581
- MTI Publication 196
Related Terms
The specific pattern or form of material deterioration resulting from active damage mechanisms. Degradation modes describe the observable physical change — general thinning, localized pitting, cracking, blistering, deformation, metallurgical changes — that occurs as a damage mechanism progresses. A single damage mechanism may produce multiple degradation modes depending on conditions.
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.
Damage FactorA numerical modifier in the API 581 RBI methodology that quantifies the increased probability of failure due to specific active damage mechanisms. Damage factors account for the type, rate, and extent of degradation relative to the equipment's design margins. Higher damage factors indicate more aggressive degradation and higher probability of failure. Total damage factor is the sum of individual damage factors for all active mechanisms.
Failure CauseThe underlying reason or mechanism that initiates the process leading to a failure mode. A failure cause is the root-level factor — design deficiency, material defect, manufacturing flaw, operational condition, degradation mechanism, or human error — that creates the conditions for the failure mode to manifest. Each failure mode may have multiple independent causes.
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.
SusceptibilityThe degree to which an equipment item is vulnerable to a specific damage mechanism, based on material of construction, process conditions (temperature, pressure, chemistry), design features, and operating history. Susceptibility is typically rated as None, Low, Medium, or High, and drives the selection of damage factors and inspection techniques in RBI assessments.
Where this appears in Reliatic
This concept is used in the FMEA & Risk module.