NACE Resource Center
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| Corrosion - Forms |
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Stress Corrosion Cracking (SCC)
Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses, see an example of SCC of an aircraft component . The problem itself can be quite complex. The situation with buried pipelines is a good example of such complexity. Cold deformation and forming, welding, heat treatment, machining and grinding can introduce
residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up
as a result of welding operations tend to approach the yield strength. The build-up of corrosion products in confined
spaces can also generate significant stresses and should not be overlooked. SCC usually occurs in certain specific
alloy-environment-stress combinations.
The micrograph above (X500) illustrates intergranular SCC of an Inconel heat exchanger
tube with the crack following the grain boundaries. The catastrophic nature of this severe form of corrosion attack has been repeatedly illustrated in many news worthy failures, including the following:
One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels. Caustic SCC Despite the extensive qualification of Inconel for specific applications, a number of corrosion problems have arisen with Inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620oC to 705oC, depending upon prior solution treating temperature. Other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack. The most effective means of preventing SCC are: 1) design properly with the right materials; 2) reduce stresses; 3) remove critical environmental species such as hydroxides, chlorides, and oxygen; 4) and avoid stagnant areas and crevices in heat exchangers where chloride and hydroxide might become concentrated. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions. Stress corrosion crack propagation rate Stress corrosion cracks propagate over a range of velocities from about 10-3 to 10 mm/h, depending upon the combination of alloy and environment involved. Their geometry is such that if they grow to appropriate lengths they may reach a critical size that results in a transition from the relatively slow crack growth rates associated with stress corrosion to the fast crack propagation rates associated with purely mechanical failure. reference See also: Causes of SCC, Controlling SCC, EL AL crash, & SCC, Pipeline SCC, SCC Guide, SCC definition, component, SCC Mechanism, roof collapse, Testing strategy, Williams explosions Specific materials: Aluminum alloys, Brass, High strength steel_of_HSLA), Passivated steel, Stainless steel Definitions relevant to SCC Stress: The intensity of the internally distributed forces or components of forces that resist a change in the volume or shape of a material that is or has been subjected to external forces. Stress is expressed in force per unit area and is calculated on the basis of the original dimensions of the cross section of the specimen. Stress can be either direct Environments or shear. See also residual stress. Stress concentration factor SCC of aircraft : A multiplying factor for applied stress that allows for the presence of a structural discontinuity such as a notch or hole; Kt, equals the ratio of the greatest stress in the region of the discontinuity to the nominal stress for the entire section. Also called theoretical stress concentration factor. Stress-corrosion cracking Swiss : A cracking process that requires the simultaneous action of a corrodent and sustained tensile stress. This excludes corrosion-reduced sections that fail by fast fracture. It also excludes intercrystalline or transcrystalline corrosion, which can disintegrate an alloy without applied or residual stress. Stress-corrosion cracking may occur in combination with hydrogen embrittlement. Stress-intensity factor: A scaling factor, usually denoted by the symbol K, used in linear-elastic fracture mechanics to describe the intensification of applied stress at the tip of a crack of known size and shape. At the onset of rapid crack propagation in any structure containing a crack, the factor is called the critical stress-intensity factor, or the fracture toughness. Various subscripts are used to denote different loading conditions or fracture toughnesses: Kc: Plane-stress fracture toughness. The value of stress intensity at which crack propagation becomes rapid in sections thinner than those in which plane-strain conditions prevail. KI: Stress-intensity factor for a loading condition that displaces the crack faces in a direction normal to the crack plane (also known as the opening mode of deformation). KIC: Plane-strain fracture toughness. The minimum value of KC for any given material and condition, which is attained when rapid crack propagation in the opening mode is governed by plane-strain conditions. KId: Dynamic fracture toughness. The fracture
toughness determined under dynamic loading conditions; it is used as an approximation of KIC for very tough materials.
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