Introduction to Plant Aging Mechanisms

Power, petrochemical and other industrial plants continually face plant aging concerns

These mechanisms rarely act in isolation — vibration can accelerate fatigue, corrosion can compound cracking, and elevated temperature can degrade material properties already under stress — so accurate identification through inspection, testing, and analysis is the first step toward an effective mitigation plan.

Intergranular and Transgranular Stress Corrosion Cracking — Environmentally assisted cracking that propagates along grain boundaries (intergranular) or through grains (transgranular) under the combined action of tensile stress and a corrosive environment.
General Corrosion — Uniform material loss across the exposed surface from chemical or electrochemical reaction with the process environment, gradually reducing wall thickness below the design minimum.
Pitting Corrosion — Localized breakdown of a protective oxide film that produces small-diameter, deep cavities, often missed by general thickness surveys and capable of through-wall penetration well before general corrosion limits are reached.
Crevice Corrosion and Denting — Localized attack in shielded areas such as gasket faces, tube-to-tubesheet joints, and under deposits, where stagnant chemistry differs from the bulk fluid; denting refers specifically to the resulting deformation at heat exchanger tube supports.
Intergranular Corrosion Attack — Preferential corrosion along grain boundaries driven by microstructural changes such as chromium carbide precipitation (sensitization), independent of applied stress.
Fretting and Wear — Surface damage from repeated small-amplitude relative motion between contacting components, common at tube supports and rotating equipment bearing surfaces.
Microbiologically Influenced Corrosion — Corrosion accelerated or initiated by microbial activity, typically producing localized pitting under biofilms in stagnant or low-flow water systems.
Dynamic Loading (Vibration, Water Hammer and Unstable Fluids) — Cyclic or transient mechanical loading from flow-induced vibration, water hammer events, or two-phase flow instabilities, capable of driving fatigue cracking independent of thermal or corrosive mechanisms.
Irradiation Assisted Stress Corrosion Cracking — A form of stress corrosion cracking in which neutron irradiation alters microstructure and grain boundary chemistry, increasing cracking susceptibility in reactor internals exposed to high fluence.
Erosion and Erosion-Corrosion — Material loss from mechanical impingement of solid particles, liquid droplets, or high-velocity flow, often coupled with corrosion where protective films are removed faster than they can reform.
Corrosion Fatigue and Crack Growth — Cyclic loading combined with a corrosive environment, producing crack growth rates and threshold stresses that differ from either mechanical fatigue or corrosion acting alone.
Thermal Aging Embrittlement — Long-term exposure to elevated temperature that changes microstructure and reduces fracture toughness, particularly relevant to cast austenitic stainless steels and low alloy steels in extended service.
Thermal Fatigue — Metallurgical crack growth caused by fluctuating thermal stresses, driven by through-wall gradients, flow stratification, or constraint of thermal expansion.
Radiation Embrittlement — Neutron irradiation-induced rise in ductile-to-brittle transition temperature and reduction in fracture toughness, most significant in reactor pressure vessel steels.
Hydrogen Damage and Embrittlement — Absorption of atomic hydrogen into the metal lattice, producing loss of ductility, blistering, or hydrogen-induced cracking, often associated with certain corrosion reactions or cathodic protection.
Creep and Creep Tensile Instability — Time-dependent deformation under sustained stress at elevated temperature, governing design life for high-temperature components such as boiler tubes and steam piping, and ultimately leading to tensile instability and rupture if unmitigated.