Stress corrosion cracking local anode

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

In the previous analysis, homogeneous current distribution has been assumed but, on many occasions, corrosion occurs with localized attack, pitting, crevice, stress corrosion cracking, etc., due to heterogeneities at the electrode surface and failure of the passivating films to protect the metal. In these types of corrosion processes with very high local current densities in small areas of attack, anodic and cathodic reactions may occur in different areas of disparate dimensions. 

The electrochemistry of corrosion is a big piece of electrochemistry. It permeates most of the surface aspects of materials science, at least for practical metal systems in contact with moist air. It influences not only the surface but often the bulk owing to its influence an embrittlement and stress corrosion cracking. So, at the beginning, we argued that a corroding metal is rather like a local fuel cell in which the corroding metal has a very large number of pairs of microsized electrodes on its surface, an equal number of them anodic and cathodic, respectively. 

The changes in the anode composition and morphology are also responsible for the erratic response of the current density vs. potential difference tendency and the decrease in the electrocatalytic activity. Thermo-corrosion and thermal stress failure, stress corrosion cracking during gas evolution, and overcritical local pressure conditions can cause extensive pitting and large ohmic voltage drops. 

A more recent theory on the mechanism of anodic stress corrosion cracking, based partly on tests, combines the electrochemical process of local metal dissolution with hydrogen embitterment at the base of the crack caused by the atomic hydrogen forming during corrosion, which may be of major significance to crack propagation. 

The SVET was utilized for the initiation of stress corrosion cracking of sensitized type 304 stainless steel in dilute thiosulfate solution (Isaacs, 1988 a). The method of potential monitoring to identify the onset of cracking was combined with the in situ measurement of local currents by means of the SVET. After solution annealing at 1100°C, the samples were sensitized at 600°C for 24 h and polished afterwards. The specimen were loaded, exposed to the electrolyte, and afterwards the potential and the spatial distribution of current were measured. Since the exposed surface area was very small, the connection and disconnection of a platinum foil were used to vary the area for cathodic reduction and thereby the corrosion potential of the specimen. By connecting and discoimecting the platinum foil, cracking could be initiated and anodic currents were observed at the respective sites, as afterwards confirmed by the application of dye penetrant. 

This is one of the most unpieasant forms of corrosion, since it occurs suddeniy and can quickiy iead to faiiure of the components. The so-caiied anodic stress corrosion cracking is caused hy the interaction of mechanical tensile stresses of sufficient height, and locally acting anodic dissolution processes. Generally, this type of corrosion originates from cracks and damage in the protective passive top layer of the material. 

Figure 4.45 Schematic of film rupture model (a) illustrates the main feature of the model—the protective film is ruptured at the crack tip by localized slip, permitting propagation by anodic dissolution. The crack tip is depicted in greater details in (b) which illustrates the view that crack advance results from a number of independent film rupture and transient dissolution events. (From Staehle, R.W. (1979). Stress Corrosion Cracking in Alloys, NATO, p. 223)...

The formation of corrosion products, the solubility of corrosion products in the surface electrolyte, and the formation of passive films affect the overall rate of the anodic metal dissolution process and cause deviations from simple rate equations. Passive films distinguish themselves from corrosion products, in the sense that these films tend to be more tightly adherent, are of lower thickness, and provide a higher degree of protection from corrosive attack. Atmospheric corrosive attack on a surface protected by a passive film tends to be of a localized nature. Surface pitting and stress corrosion cracking in aluminum and stainless alloys are examples of such attack. 

Because stress corrosion cracking is a localized corrosion process, involving the spatial separation of the local anode (in the crack) and the local cathode (on the external surfaces), the phenomenon of IGSCC is expected to fall under the theoretical umbrella of the Differential Aeration Hypothesis (DAH), as depicted in Fig. 82. The DAH, which was first postulated by Evans in the 1920s and which has since been recognized as the theoretical basis for essentially all localized corrosion phenomena requires that, in order to maintain the spatial separation between the local anode and local cathode, a positive ionic coupling current flows through... 

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