Damage mechanisms

You cannot assess a risk you have not identified. Screening the credible damage mechanisms, the specific ways a material degrades in a given service, is the foundation of every RBI assessment. API RP 571 is the reference catalogue.

The families

Thinning	General and local metal loss: sulfidation, naphthenic-acid and CO₂ corrosion, sour water and amine corrosion, erosion-corrosion, microbially induced corrosion and corrosion under insulation.
Environmental cracking	Stress corrosion cracking: chloride SCC of austenitic stainless, caustic, amine and carbonate cracking, wet H₂S damage (sulfide stress cracking, HIC and SOHIC) and polythionic-acid SCC.
High-temperature	High-temperature hydrogen attack (to the Nelson curves of API 941), creep and stress rupture, oxidation, high-temperature sulfidation, temper embrittlement, spheroidisation and graphitisation.
Metallurgical and mechanical	Brittle fracture, mechanical and thermal fatigue, vibration-induced fatigue, 885 embrittlement and sigma-phase embrittlement of stainless steels.

Screening: material meets service

A damage mechanism is credible only where the material, the fluid, the temperature and the stress line up. Chloride SCC needs an austenitic stainless, chlorides, tensile stress and temperature; HTHA needs hydrogen partial pressure and temperature above the Nelson-curve limit for the steel; creep needs metal temperature above roughly a third of the melting point. Screening is the disciplined pass that pairs each item material and service with the mechanisms that can actually occur, and rules out the rest.

The damage factor

API 581 turns a mechanism into a number through the damage factor. It captures how susceptible the item is and how fast the damage runs, how long since it was last inspected, and how effective that inspection was at finding the expected damage. A high damage factor multiplies the probability of failure; an effective inspection lowers it. Where several mechanisms are active, their damage factors combine, and the governing one drives the plan.

Matching the technique to the mechanism

The inspection only reduces risk if it can see the damage, so technique follows mechanism:

General and local thinning	Ultrasonic thickness and scanning, radiographic profiling, permanent thickness sensors.
Surface cracking (SCC)	Magnetic-particle and dye-penetrant, eddy current and ACFM, wet fluorescent MT for wet H₂S.
Sub-surface cracking (HIC, SOHIC)	Ultrasonics: phased array and time-of-flight diffraction, C-scan mapping.
High-temperature hydrogen attack	Advanced ultrasonics: TOFD, velocity ratio and backscatter, attenuation.
Creep	Surface replication, dimensional and hardness checks, ultrasonics for cavitation.

Where OptimalIntegrity Studio™ fits

OptimalIntegrity Studio™ screens the damage mechanisms automatically from the material and the process conditions of each corrosion circuit, proposes the credible set for engineering review, and carries the resulting damage factors straight into the RBI risk and the inspection technique. The catalogue is applied consistently across the estate rather than depending on who did the study.

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