Stress corrosion cracking local cathode

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. 

Stress corrosion cracking also involves localized breakdown of the protective film. The corrosion is narrowly confined within the metal due to stress factors which may arise from either residual Internal stress or applied external stress. In some cases the stress failure can be accelerated by chemical factors, such as surface adsorption or hydrogen dissolution from cathodic hydrogen leading to embrittlement. 

Classification by the morphology of the attack The attack may be homogeneous but the attack may also be rather localized (pitting corrosion, stress corrosion cracking, intergranular corrosion, etc.), or the material may remain virtually intact but the interface to a coating may be destroyed (cathodic delamination). 

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. 

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... 

The exact mechanism of stress cracking corrosion is unknown but it is thought to be related to hydrogen embrittlement and to be another phenomenon whose intensity is due to the effect being localized. It was seen in the earlier discussion that in many corrosion situations the cathodic part of the process is hydrogen... 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

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