Stress Corrosion Cracking SCC

This form is defined as the brittle fracture of a normally ductile metal by the conjoint action of a specific corrosive environment and a tensile stress. On underground pipelines, SCC affects only the external surface of the pipe that is exposed to 

The presence of extensive SCC may necessitate replacement or rehabilitation of a pipeline. Because SCC is dependent on unique environmental conditions, a large-scale recoating program may protect against these environmental conditions 

Some of the considerations that play a role in deciding on rehabilitation or replacement of a pipeline are terrain conditions, expected or required life, excess capacity, throughput requirements, and internal versus external corrosion (Table 4.23). 

The location of a pipeline is critical to repair considerations. For instance, a pipe in swampy clay would exclude recoating as a repair option. On the contrary, pipes in prairies are conductive to recoating, with firm footing for the equipment and good accessibility. 

If the expected life of a section of pipe is short, the operator must decide whether recoating and repair would extend the life of a pipe section to match the rest of the pipeline. Otherwise, recoating is not a recommended solution. Replacing the pipeline may be the best solution. 

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder 

Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30800-8 

Failure of prestressing steel is usually induced by atomic hydrogen that penetrates the metal lattice. The conditions required for cracking are a sensitive material, a tensile stress and an environment that produces atomic hydrogen on the steel surface. 

Evolution of atomic hydrogen on the steel surface may provoke the initiation and propagation of cracks starting from the metal surface, especially in the presence of notches or localised corrosion attack. Even in the absence of flaws on the surface, atomic hydrogen may penetrate the steel lattice, accumulate in the areas subjected to the highest tensile stress, above all at points corresponding to lattice defects, and lead to brittle failure beginning at one of these sites. 

This type of attack does not require any specific environment to take place, since it can take place simply in neutral or acidic wet environments. Failure due to hydrogen is named hydrogen embrittlement since it leads to a brittle-Uke fracture surface. Indeed, the ductility of the bulk metal does not change, but the propagation of the crack is due to the mechanical stresses induced in the lattice by hydrogen accumulated near the crack tip. If hydrogen is present in the metal lattice before the application of loads a delayed fracture may occur, i. e. the steel does not fail when the load is applied, but after a certain time. 

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Most cracking problems in cooling water systems result from one of two distinct cracking mechanisms stress-corrosion cracking (SCC) or corrosion fatigue. 

Few, if any, failure mechanisms have received as much attention as stress-corrosion cracking (SCC). Yet despite an enormous research effort over many years, an acceptable, generalized theory that satisfactorily explains all elements of the phenomenon has not been produced. SCC is a complex failure mechanism. Nevertheless, its basic characteristics are well known, and a wealth of practical experience permits at least a moderately comfortable working knowledge of the phenomenon. 

The conjoint action of a tensile stress and a specific corrodent on a material results in stress corrosion cracking (SCC) if the conditions are sufficiently severe. The tensile stress can be the residual stress in a fabricated structure, the hoop stress in a pipe containing fluid at pressures above ambient or in a vessel by virtue of the internal hydraulic pressure created by the weight of its contents. Stresses result from thermal expansion effects, the torsional stresses on a pump or agitator shaft and many more causes. 

Equally, boiler surface failure may result solely from poor operational practices or other indirect problems, although more usually it is due to a combination of waterside and operational problems such as corrosion fatigue and other stress corrosion cracking (SCC) mechanisms. 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

Where waterside deposits are evident, they provide a heat insulating effect and also permit under-deposit contaminant concentration. Under conditions of high pressure, heat flux, or stress, this combination of factors may lead to the development of embrittlement corrosion or stress corrosion cracking (SCC). 

Stress corrosion cracking (SCC) or embrittlement corrosion describes any of a number of corrosion processes where in a corrosive environment, localized stress accelerates the rate of corrosion that may occur under or within a deposit. 

Yet another problem associated with ammonia is stress corrosion cracking (SCC or caustic embrittlement) of brasses (such as brass valves and other stressed components). Stress corrosion cracking of brass may develop in systems where ammonia steadily becomes available from a suitable source (such as the breakdown of sodium nitrate when it is added to inhibit SCC of steel) because it can concentrate in the steam. 

Environmental Cracking The problem of environmental cracking of metals and their alloys is very important. Of all the failure mechanism tests, the test for stress corrosion cracking (SCC) is the most illusive. Stress corrosion is the acceleration of the rate of corrosion damage by static stress. SCC, the limiting case, is the spontaneous cracking that may result from combined effects of stress and corrosion. It is important to differentiate clearly between stress corrosion cracking and stress accelerated corrosion. Stress corro-... 

Stress anisotropy, in ferrites, 77 62-64 Stress-corrosion cracking (SCC), 7 810, 812, 23 301... 

It has been reported that hydrogen embrittlement is a form of stress corrosion cracking (SCC). Three basic elements are needed to induce SCC the first element is a susceptible material, the second element is environment, and the third element is stress (applied or residual). For hydrogen embrittlement to occur, the susceptible material is normally higher strength carbon or low-alloy steels, the environment must contain atomic hydrogen, and the stress can be either service stress and/or residual stress from fabrication. If any of the three elements are eliminated, HE cracking is prevented. 

High chloride waters will increase the risk of stress corrosion cracking (SCC) in austenitic stainless steels (e.g., 304/304L, 316/316L) and will tend to both increase the general rate of corrosion and attack in localized areas, often causing pitting-type corrosion. 

The Primary Reformer is a steam-hydrocarbon reforming tubular furnace that is typically externally fired at 25 to 35 bar and 780°C to 820°C on the process side. The reformer tubes function under an external heat flux of 75,000 W/m2 and are subject to carburization, oxidation, over-heating, stress-corrosion cracking (SCC), sulfidation and thermal cycling. Previously SS 304, SS 310 and SS 347 were used as tube materials. However these materials developed cracks that very frequently led to premature tube failures (see Table 5.10)88. 

One of the major problems encountered in the storage and transport of anhydrous liquid ammonia is the stress-corrosion-cracking (SCC) of carbon steel equipment. Cracks most often occur at the weld joints, where the leftover stress is at a maximum. The leftover stress is that which remains even after heat treatment. The hardness of the material and the presence of impurities and oxygenates in ammonia aggravate SCC88. 

Abstract. The present work deals with major technical items related to defectoscopy of metallic materials - in particular, welded joints - which can be considered as constitutive for H-storage high pressure tanks such vessels must be corrosion and pressure-resistant, especially in order to avoid any infiltration by light H-atoms and, in general, H-embrittlement and Stress Corrosion Cracking (SCC) phenomena. 

The stress-corrosion cracking (SCC) mode of failure was later observed even in the case of ferritic stainless steels. The only clear message from this is that the exact mechanism of failure by this mode is not well established. Alloys containing >34% Ni were found to prolong the time of SCC failure. Ferritic type alloys 430 and 434 are resistant to SCC both in MgCl2 and NaCl environments in the mill-annealed condition, but not in welded conditions. Also, welding impairs the ductility and their resistance to SCC. 

The aluminum cylinder is constantly under pressure and this is believed to be a contributing factor for stress corrosion cracking (SCC). 

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