STRESS CORROSION CRACK

14.0. STRESS CORROSION CRACK

The mechanism of stress corrosion cracking (SCC) involves a very complicated sequence of corrosion and fracture. It has been shown that, for some materials, fine pitting is needed to start the activity. Further progress is then a process alternating between corrosion and mechanical cracking until the material fails completely.

Transformable ferritic steels are susceptible to hydrogen-induced stress corrosion cracking (SCC) in a wide range of aqueous media. Hydrogen formed on the surface of the steel by a cathodic corrosion reaction can diffuse into the material, causing embrittlement, and, if the region is subject to tensile stress, cracking can result.

If conditions are right, almost any metal can be made to fail by stress corrosion cracking. One characteristic of stress corrosion cracking (SCC) that is very useful for diagnosis is the direction of the crack. It always follows the plane of maximum stress and, therefore, has branches in its form. These branched cracks are often visible without any other assistance. When viewed under a microscope, the branched direction of the cracks can be seen and they are practically always trans granular. Exceptions are caustic solution and improperly heat-treated stainless steels. In these cases, the path of the crack is intergranular.

There are several species that can lead to stress corrosion cracking (SCC), of which the following are relevant to refinery environments:  

It is NDE FLAW TECHNOLOGIES PVT. LTD., wealth of knowledge in corrosion and the effects of hydrogen on the properties of materials, combined with our testing facilities and broad experience in the field of metallurgy and corrosion which sets us apart from our competitors in Stress Corrosion Cracking (SCC) testing.

Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. Stress corrosion cracking (SCC) is highly chemically specific in that certain alloys are likely to undergo Stress corrosion cracking (SCC) only when exposed to a small number of chemical environments. The chemical environment that causes Stress corrosion cracking (SCC) for a given alloy is often one which is only mildly corrosive to the metal. Hence, metal parts with severe Stress corrosion cracking (SCC) can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for Stress corrosion cracking (SCC) to go undetected prior to failure. Stress corrosion cracking (SCC) often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure.

The stresses can be the result of the crevice loads due to stress concentration, or can be caused by the type of assembly or residual stresses from fabrication (e.g. cold working); the residual stresses can be relieved by annealing or other surface treatments.

14.1. MATERIAL SUSCEPTIBILITY:

Stress corrosion cracking (SCC) is the result of a combination of three factors – a susceptible material, exposure to a corrosive environment, and tensile stresses above a threshold. If any one of these factors are eliminated, Stress corrosion cracking (SCC) initiation becomes impossible.

METALS

·         Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides. This limits the usefulness of austenitic stainless steel for containing water with higher than a few parts per million content of chlorides at temperatures above 50 °C (122 °F);

·         Mild steel cracks in the presence of alkali (e.g. boiler cracking and caustic stress corrosion cracking) and nitrates;

·         Copper alloys crack in ammoniacal solutions (season cracking);

·         High-tensile steels have been known to crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially when chlorides are present.

With the possible exception of the latter, which is a special example of hydrogen cracking, all the others display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions where fracture mechanics predicts that failure should not occur. That is, in the presence of a corrodent, cracks develop and propagate well below critical stress intensity factor. 

POLYMERS

A similar process (environmental stress cracking) occurs in polymers, when products are exposed to specific solvents or aggressive chemicals such as acids and alkalis. As with metals, attack is confined to specific polymers and particular chemicals. Thus polycarbonate is sensitive to attack by alkalis, but not by acids. On the other hand, polyesters are readily degraded by acids, and SCC is a likely failure mechanism. Polymers are susceptible to environmental stress cracking where attacking agents do not necessarily degrade the materials chemically. Nylon is sensitive to degradation by acids, a process known as hydrolysis, and nylon mouldings will crack when attacked by strong acids.

Close-up of broken nylon fuel pipe connector caused by SCC

Cracks can be formed in many different elastomers by ozone attack, another form of SCC in polymers. Tiny traces of the gas in the air will attack double bonds in rubber chains, with natural rubber, styrene-butadiene rubber, and nitrile butadiene rubber being most sensitive to degradation. Ozone cracks form in products under tension, but the critical strain is very small. The cracks are always oriented at right angles to the strain axis, so will form around the circumference in a rubber tube bent over. Such cracks are dangerous when they occur in fuel pipes because the cracks will grow from the outside exposed surfaces into the bore of the pipe, so fuel leakage and fire may follow. Ozone cracking can be prevented by adding anti-ozonants to the rubber before vulcanization. Ozone cracks were commonly seen in automobile tire sidewalls, but are now seen rarely thanks to the use of these additives. On the other hand, the problem does recur in unprotected products such as rubber tubing and seals.

CERAMICS

This effect is significantly less common in ceramics which are typically more resilient to chemical attack. Although phase changes are common in ceramics under stress these usually result in toughening rather than failure (see Zirconium dioxide). Recent studies have shown that the same driving force for this toughening mechanism can also enhance oxidation of reduced cerium oxide, resulting in slow crack growth and spontaneous failure of dense ceramic bodies.

GLASS

Illustrated are regions of different crack propagation under stress corrosion cracking. In region I, crack propagation is dominated by chemical attack of strained bonds in the crack. In region II, propagation is controlled by diffusion of chemical into the crack. In region III, the stress intensity reaches its critical value and propagates independent of its environment.

Given that most glasses contain a substantial silica phase, the introduction of water can chemically weaken the bonds preventing subcritical crack propagation. Indeed, the silicon-oxygen bonds present at the tip of a crack are strained, and thus more susceptible to chemical attack. In the instance of chemical attack by water, silicon-oxygen bonds bridging the crack are separated into non-connected silicon hydroxide groups. The addition of external stress will serve to further weaken these bonds.

Subcritical crack propagation in glasses falls into three regions. In region I, the velocity of crack propagation increases with ambient humidity due to stress-enhanced chemical reaction between the glass and water. In region II, crack propagation velocity is diffusion controlled and dependent on the rate at which chemical reactants can be transported to the tip of the crack. In region III, crack propagation is independent of its environment, having reached critical stress intensity. Chemicals other than water, like ammonia, can induce subcritical crack propagation in silica glass, but they must have an electron donor site and a proton donor site.

14.2. STRESS

As one of the requirements for stress corrosion cracking is the presence of stress in the components, one method of control is to eliminate that stress, or at least reduce it below the threshold stress for SCC. This is not usually feasible for working stresses (the stress that the component is intended to support), but it may be possible where the stress causing cracking is a residual stress introduced during welding or forming.

Residual stresses can be relieved by stress-relief annealing, and this is widely used for carbon steels. These have the advantage of a relatively high threshold stress for most environments; consequently it is relatively easy to reduce the residual stresses to a low enough level.

In contrast, austenitic stainless steels have a very low threshold stress for chloride SCC. This combined with the high annealing temperatures that are necessary to avoid other problems, such as sensitization and sigma phase embrittlement, means that stress relief is rarely successful as a method of controlling SCC for this system.

For large structures, for which full stress-relief annealing is difficult or impossible, partial stress relief around welds and other critical areas may be of value. However, this must be done in a controlled way to avoid creating new regions of high residual stress, and expert advice is advisable if this approach is adopted. Stresses can also be relieved mechanically. For example, hydrostatic testing beyond yield will tend to ‘even-out’ the stresses and thereby reduce the peak residual stress.

Laser peening, shot-peening, or grit-blasting can be used to introduce a surface compressive stress that is beneficial for the control of SCC. The uniformity with which these processes are applied is important. If, for example, only the weld region is shot-peened, damaging tensile stresses may be created at the border of the peened area. The compressive residual stresses imparted by laser peening are precisely controlled both in location and intensity, and can be applied to mitigate sharp transitions into tensile regions. Laser peening imparts deep compressive residual stresses on the order of 10 to 20 times deeper than conventional shot peening making it significantly more beneficial at preventing SCC. Laser peening is widely used in the aerospace and power generation industries in gas fired turbine engines.

14.3. STRESS CORROSION CRACK RELATIVE TO NDT:

We are specialized in manufacturing of artificial stress corrosion crack. In-service inspection revealed stress corrosion cracking (SCC). The method requiring inspection quantification was a surface method, namely penetrant testing (PT), Magnetic particle testing (MPT) is physical dimensions of the smallest flaw requiring detection by NDT method was well defined for both pitting and cracking defects. Qualification was considered in terms of flaw volume together with absolute flaw linear dimensions. Inspection qualification would, therefore, determine the adequacy of the applied technique by both physical reasoning and by empirical measurement to determine the largest flaw that could escape detection. The main purpose of this artificial stress corrosion crack is like a demo or assurance to the client.