API 571 key terms

A process that changes a material's condition or properties and can reduce the ability of equipment to perform its intended function. Examples include corrosion, cracking, creep, and embrittlement.

A structured investigation of a failed component to identify the root cause, contributing conditions, and sequence of events. It often uses visual, metallurgical, and chemical examination techniques.

The speed at which metal loss occurs, commonly expressed in mils per year or millimeters per year. It is used to estimate remaining life and set inspection intervals.

An estimate of the time a component can continue to operate safely before its thickness or properties fall below a required minimum. It is based on current condition and predicted degradation rate.

A planning method that prioritizes inspection resources by combining the probability of failure from specific damage mechanisms with the consequences of that failure. It helps focus inspection on the highest-risk equipment.

The time between planned inspections, selected to detect expected degradation before it threatens integrity. It considers corrosion rate, remaining life, and consequence of failure.

Time-dependent deformation of a material under sustained stress at elevated temperature. It can lead to bulging, cracking, or stress rupture after extended service.

Failure of a component under sustained load at high temperature after a period of creep. It is characterized by limited deformation and often intergranular cracking.

Cracking caused by cyclic temperature changes that produce alternating thermal stresses. It often occurs where hot and cold streams mix or where restraints limit expansion.

Progressive cracking under repeated or fluctuating stress, typically initiating at stress concentrations and growing until the remaining section can no longer carry the load.

Sudden failure with little or no plastic deformation. It occurs when a material's toughness is low and a crack or flaw is subjected to tensile stress, often at low temperature.

Loss of toughness in some steels caused by impurity segregation at grain boundaries during slow cooling through a temperature range, often after tempering or welding.

Formation of a hard, brittle intermetallic phase in stainless steels and some alloys during prolonged exposure at elevated temperature, reducing toughness and ductility.

Embrittlement of ferritic and duplex stainless steels caused by phase separation during long exposure near 885°F (475°C). It reduces toughness at ambient temperature.

General metal loss occurring at a relatively even rate over an exposed surface, often caused by acids, caustics, or other corrosive environments. It is usually predictable and managed by thickness monitoring.

Corrosion concentrated at discrete sites such as pits, crevices, or cracks. It can penetrate quickly and cause leaks even when overall metal loss is small.

Localized corrosion that forms small, deep cavities in a metal surface, often initiated at inclusions or breaks in a passive film. It is difficult to detect by general thickness measurements.

Localized attack within narrow gaps or shielded areas where stagnant solution accumulates, often under gaskets, deposits, or lap joints.

Corrosion occurring beneath deposits, scale, or debris where the surface is shielded from bulk flow and corrosive species concentrate. It often produces localized pitting or grooving.

Corrosion accelerated by microorganisms or their metabolic products. It often produces localized pitting, tubercles, or slime deposits in water systems and stagnant areas.

Accelerated corrosion of one metal when it is electrically coupled to a more noble metal in a conductive electrolyte. The less noble metal becomes the anode and corrodes faster.

Metal loss caused by the combined action of flowing fluid and corrosive environment. It often produces grooves, valleys, or thinning at changes in flow direction.

Cracking caused by the combined action of tensile stress and a specific corrosive environment. It often produces fine branching cracks that can lead to sudden failure.

Stress corrosion cracking of austenitic stainless steels and some alloys in chloride-containing environments. It often occurs at elevated temperature and requires tensile stress.

Stress corrosion cracking caused by exposure to caustic solutions, typically in carbon steel or stainless steel under tensile stress. It often occurs near weldments or concentrated caustic areas.

Cracking of high-strength steels in wet hydrogen sulfide service under tensile stress. It can occur quickly and requires hardness control and proper material selection.

Stepwise cracking in carbon steels caused by hydrogen atoms diffusing into the steel and accumulating at inclusions or laminations. It often occurs in wet sour service.

Formation of bulges or blisters on a steel surface when hydrogen atoms enter the metal and recombine at internal defects. It often occurs in wet sour or cathodic environments.

Stress corrosion cracking of sensitized austenitic stainless steels by polythionic acids formed during shutdown or exposure to sulfur-bearing deposits and moisture.

Chromium depletion at grain boundaries in austenitic stainless steels caused by heating in a critical range. It makes the material susceptible to intergranular corrosion and cracking.

Damage in carbon and low-alloy steels at elevated temperature and hydrogen partial pressure, where hydrogen reacts with carbides to form methane, causing internal cracking and loss of strength.

High-temperature corrosion of metals by sulfur compounds, often producing sulfide scale and metal loss. It is particularly common in refining units processing sulfur-rich streams.

High-temperature reaction of a metal with oxygen to form oxide scale. It can cause metal loss and affect heat transfer and component life.

High-temperature damage where carbon diffuses into a metal, often from hydrocarbon or carbon-rich environments. It causes embrittlement and loss of ductility.

Severe high-temperature carburization that produces pitting and disintegration of metal into fine dust. It often occurs in syngas or carbon monoxide-rich environments.

Formation of graphite in carbon or low-alloy steels during long exposure at elevated temperature. It reduces strength and toughness and can lead to cracking.

A grouping of piping or equipment with similar process conditions and damage mechanisms. It is used to plan inspections and manage risk efficiently.

A designated point on equipment where thickness measurements or other inspections are taken to track degradation and estimate remaining life.

A piping section with little or no flow where corrosive fluids can accumulate or stagnate. It increases susceptibility to localized corrosion and cracking.

A location where chemicals or process streams are introduced into a system. It is often subject to localized corrosion, erosion, or thermal fatigue due to mixing and turbulence.