Damage Mechanisms

Last update: Jan 13, 2017

Damage Mechanism (also referred to as degradation mechanism) is a general term referring to any causes of problems or failures within process equipment. These can range from corrosion, to cracking, to heat damage, and everything in between.

When assessing damage mechanisms, one must take into account the current state of the equipment, as well as any potential damage the mechanism may cause later. Equipment’s susceptibility to a particular damage mechanism is affected by a number of variables, including materials of construction, process fluids, operating conditions, external environment, etc. A good understanding of the variety of damage mechanisms that exist is a must for any mechanical integrity program.

A thorough damage mechanisms review is essential for creating an effective inspection strategy. Once damage mechanisms and morphology are understood, inspection strategists can use this information to select inspection methods with the highest probability of detecting, characterizing, and measuring potential damage. Furthermore, inspection intervals can be established using industry codes and standards such as API 510, 570, 653, API RP 579 for fitness-for-service, and API 580 and 581 for risk-based inspection.

Some of the most common damage mechanisms in the refining and chemical processing industries are:

  • Wet H2S Damage, which can occur when atomic hydrogen from wet H2S corrosion reactions enters and weakens the steel.

  • CO2 Corrosion, which is a form of degradation that occurs when dissolved CO2 in condensate forms carbonic acid, which corrodes steels.

  • Corrosion Under Insulation (CUI), which occurs when moisture builds up on the surface of insulated equipment.

  • Hydrogen Embrittlement, which happens when atomic hydrogen infuses into certain higher strength steels and causes them to become brittle.

  • Brittle Fracture, which is the sudden, very rapid fracture under stress where the material exhibits little or no evidence of ductility or plastic degradation before the fracture occurs.

  • High Temperature Hydrogen Attack (HTHA), a mechanism that can affect equipment that is exposed to hydrogen at elevated temperatures (at least 400 °F or 204 °C). 

  • Sulfidation Corrosion, a type of corrosion that occurs at temperatures above 500 °F (260 °C) due to sulfur compounds in crude.

Damage mechanisms are detailed and covered at length in API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. This document provides an in-depth look at over 60 different damage mechanisms that can occur to process equipment in refineries. It includes a general description of the damage mechanism, susceptible materials of construction, critical factors, inspection method selection guidelines, and control factors.


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