Stress corrosion cracking characterization

In stress corrosion cracking, the material breaks as the result of mechanical stress under the influence of a corrosive medium. Stress corrosion cracking is characterized by the presence of deep intergranular or intercrystalline cracks that are generally not externally evident. It can be caused by inherent stress, which can be due to cold working or arise near a welding seam. 

N. Winter, A. Atrens, W. Dietzel, V. Song, K.U. Kainer, Stress corrosion cracking in magnesium alloys characterization and prevention, JOM 59 (2007) 49-53. 

For some material-environment combinations it has been shown that accelerated anodic dissolution of yielding metal is the significant mechanism. This is the case for austenitic stainless steels in acidic chloride solutions. In these steels, plastic deformation is characterized by a dislocation pattern giving wide slip steps on the surface. For such systems, Scully [7.50] has proposed a model for initiation and development of stress corrosion cracks, which has been supported by other scientists [7.51]. The model in its simplest form is illustrated in Figure 7.52. A necessary condition is that flie surface from the beginning is covered by a passivating film (A). 

Static tests carried out with non-precracked specimens allow us to characterize, for a given environment, the sensitivity of an alloy to stress corrosion cracking and hydrogen embrittlement. TyfpicaUy, the test specimen subjected to a constant load or a constant strain is exposed to a corrosive environment and one measures the time to failure. The test specimens sometimes contain a notch—not to be confused with precracking—that fixes the location where failure will occur. In constant strain tests, the time to failure corresponds to the appearance of the first cracks. This kind of experiment thus indicates the crack initiation time. In constant load tests, on the other hand, the time to failure is the time leading to fracture of the specimen. It corresponds to the sum of the crack initiation and the crack propagation times. 

The development and use of appropriate standards for SCC response in both research and characterization for application is extremely important if continued improvements in the literature database are to be realized. ASTM has put forth the most extensive effort in this endeavor, dating to the early 1960s, with the first standard published in 1972 (ASTM G 30). Since that time, 17 standards have been published on specimens, environments, alIo3 , and classifications, but only three have been added since the first edition of Manual 20. The development of such standards deaUng with stress corrosion has been the responsibility of ASTM Subcommittee GO 1.06 on Stress Corrosion Cracking and Corrosion Fatigue, under ASTM Committee GOl on Corrosion of Metals. An additional important contribution to the standards development process is the sponsorship of technical symposia, which provide an effective technical forum for specialists to present current work in progress. The subcommittee has sponsored nine such symposia [1,41,58-64 ], which have served as catalysts for many developed standards. 

Stress corrosion cracking of brass commonly occurs when brass is subjected to an applied or residual tensional stress or while in contact with a trace of ammonia or amine in the presence of moisture and oxygen. The risk of stress corrosion cracking in brasses is greatest in industrial and urban atmospheres, characterized by high contents of sulfur dioxide and ammonia. The stress corrosion susceptibility is markedly lower in marine atmospheres. The relative resistance to stress corrosion cracking of the brasses is as follows ... 

Many types of alloy can suffer stress-corrosion cracking (SCC) if subject to external stress or residual stress when in contact with corrosive media. Typically SCC is characterized by a low-ductility or brittle fracture. For example standard austenitic stainless steels, e.g. types 1.4301 or 1.4401, are susceptible to chloride-induced stress-corrosion cracking in a chloride-containing environment at a temperature above 50 C. Depending on the specific environment cracks propagate trans- or intergranularly chloride-induced SCC is characterized by trans-granular cracks (Fig. 1-12). 

Stress corrosion is the failure of a metal resulting from the conjoint action of stress and chemical attack. It is a phenomenon associated with a combination of static tensile stress, environment and in some systems, a metallurgical condition which leads to component failure due to the initiation and propagation of a high aspect ratio crack. It is characterized by fine cracks which lead to failure of components are potentially the structure concerned. Stress corrosion cracking is abbreviated as see. The failures are more often sudden and unpredictable which may occur after as little as few months or years of previously satisfactory service. 

PB Srinivasan, R Zettler, C Blawert, W Dietzel, A study on the effect of plasma electrolytic oxidation on the stress corrosion cracking behaviour of a wrought AZ61 magnesium alloy and its friction stir weldment. Materials Characterization, 2009,... 

EIS has been applied extensively to the analysis of the corrosion mechanism of iron and other metals in aqueous solutions. To characterize a given corrosion process, it is practically advisable to obtain a full AC frequency scan of the system, including sufficient low-frequency response and small amplitude voltage perturbation with cyclic voltammetry, before acquiring the response data such as current, voltage, and polarization resistance Most corrosion kinetics studies have been done on uniformly corroding surfaces where the dissolution of the metal is uniform all over the surface in contact with the electrolyte [43]. Localized corrosion and stress corrosion cracking can also be analyzed by impedance methods such as local EIS (Section 13-4). 

This behavior is characterized by a plateau region, which prevails above a definite threshold K. It is often referred to as stress-corrosion fatigue because SCC systems usually exhibit this behavior, and the most common theory assumes that the crack growth rate results from the addition of SCC, and pure fatigue crack advance. This is a type of... 

The objective of this section is to give additional information to confirm this hypothesis. One of the important corrosion parameters is the stress corrosion susceptibility factor, . The exponent n characterizes the evolution of the crack velocity as a function of the stress intensity factor. It is determined by fatigue experiments, under the corrosive environment of interest. 

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