API 571

Brittle fracture is the sudden rapid fracture under stress (residual or applied) where the material exhibits little or no evidence of ductility or plastic deformation.

A) Erosion is the accelerated mechanical removal of surface material as a result of relative movement between, or impact from solids, liquids, vapor or any combination thereof.
b) Erosion-corrosion is a description for the damage that occurs when corrosion contributes to erosion by removing protective films or scales, or by exposing the metal surface to further corrosion under the combined action of erosion and corrosion.

A) Fatigue cracking is a mechanical form of degradation that occurs when a component is exposed to cyclical stresses for an extended period, often resulting in sudden, unexpected failure.
b) These stresses can arise from either mechanical loading or thermal cycling and are typically well below the yield strength of the material.

A form of corrosion that occurs from moisture associated with atmospheric conditions. Marine
environments and moist polluted industrial environments with airborne contaminants are most severe. Dry rural environments cause very little corrosion.

Corrosion of piping, pressure vessels and structural components resulting from water trapped under insulation or fireproofing.

General or localized corrosion of carbon steels and other metals caused by dissolved salts, gases, organic compounds or microbiological activity.

Localized corrosion due to the concentration of caustic or alkaline salts that usually occurs under evaporative or high heat transfer conditions. However, general corrosion can also occur depending on alkali or caustic solution strength.

Corrosion of carbon steel and other alloys resulting from their reaction with sulfur compounds in high temperature environments. The presence of hydrogen accelerates corrosion. This mechanisms is also known as sulfidic corrosion.

Surface initiated cracks caused by environmental cracking of 300 Series SS and some nickel base alloys under the combined action of tensile stress, temperature and an aqueous chloride environment. The presence of dissolved oxygen increases propensity for cracking.

Caustic embrittlement is a form of stress corrosion cracking characterized by surface-initiated cracks that occur in piping and equipment exposed to caustic, primarily adjacent to non-PWHT’d welds.

A) Hydrochloric acid (aqueous HCl) causes both general and localized corrosion and is very aggressive to most common materials of construction across a wide range of concentrations.
b) Damage in refineries is most often associated with dew point corrosion in which vapors containing water and hydrogen chloride condense from the overhead stream of a distillation, fractionation or stripping tower. The first water droplets that condense can be highly acidic (low pH) and promote high corrosion rates.

A) Corrosion of steel due to acidic sour water containing H2S at a pH between 4.5 and 7.0. Carbon dioxide (CO2) may also be present.
b) Sour waters containing significant amounts of ammonia, chlorides or cyanides may significantly affect pH but are outside the scope of this section.

A) Amine cracking is a common term applied to the cracking of steels under the combined action of tensile stress and corrosion in aqueous alkanolamine systems used to remove/absorb H2S and/or CO2 and their mixtures from various gas and liquid hydrocarbon streams.
b) Amine cracking is a form of alkaline stress corrosion cracking.
c) It is most often found at or adjacent to non-PWHT’d carbon steel weldments or in highly cold worked parts.
d) Amine cracking should not be confused with several other types of SCC that can occur in amine environments which are further described in 5.1.2.3 and 5.1.2.5.

This section describes four types of damage that result in blistering and/or cracking of carbon steel and low alloy steels in wet H2S environments.
a) Hydrogen Blistering
Hydrogen blisters may form as surface bulges on the ID, the OD or within the wall thickness of a pipe or pressure vessel. The blister results from hydrogen atoms that form during the sulfide corrosion process on the surface of the steel, that diffuse into the steel, and collect at a discontinuity in the steel such as an inclusion or lamination. The hydrogen atoms combine to form hydrogen molecules that are too large to diffuse out and the pressure builds to the point where local deformation occurs, forming a blister. Blistering results from hydrogen generated by corrosion, not hydrogen gas from the process stream. (Figure 5-34 and Figure 5-35.)
b) Hydrogen Induced Cracking (HIC)
Hydrogen blisters can form at many different depths from the surface of the steel, in the middle of the plate or near a weld. In some cases, neighboring or adjacent blisters that are at slightly different depths (planes) may develop cracks that link them together. Interconnecting cracks between the blisters often have a stair step appearance, and so HIC is sometimes referred to as “stepwise cracking” (Figure 5-36, Figure 5-37 and Figure 5-38).
c) Stress Oriented Hydrogen Induced Cracking (SOHIC) SOHIC is similar to HIC but is a potentially more damaging form of cracking which appears as arrays of cracks stacked on top of each other. The result is a through-thickness crack that is perpendicular to the surface and is driven by high levels of stress (residual or applied). They usually appear in the base metal adjacent to the weld heat affected zones where they initiate from HIC damage or other cracks or defects including sulfide stress cracks (Figure 5-39 and Figure 5-40).
d) Sulfide Stress Cracking (SSC) Sulfide Stress Cracking (SSC) is defined as cracking of metal under the combined action of tensile stress and corrosion in the presence of water and H2S. SSC is a form of hydrogen stress cracking resulting from absorption of atomic hydrogen that is produced by the sulfide corrosion process on the
metal surface. SSC can initiate on the surface of steels in highly localized zones of high hardness in the weld metal and heat affected zones. Zones of high hardness can sometimes be found in weld cover passes and attachment welds which are not tempered (softened) by subsequent passes. PWHT is beneficial in reducing the hardness and residual stresses that render steel susceptible to SSC. High strength steels are also susceptible to SSC but these are only used in limited applications in the refining industry. (Figure 5-42 and Figure 5-43). Some carbon steels contain residual elements that form hard areas in the heat affected zones that will not temper at normal stress relieving temperatures. Using preheat helps minimize these hardness problems.

A) High temperature hydrogen attack results from exposure to hydrogen at elevated temperatures and pressures. The hydrogen reacts with carbides in steel to form methane (CH4) which cannot diffuse through the steel. The loss of carbide causes an overall loss in strength.
b) Methane pressure builds up, forming bubbles or cavities, microfissures and fissures that may combine to form cracks.
c) Failure can occur when the cracks reduce the load carrying ability of the pressure containing part.