Onset of pitting corrosion

The onset of pitting corrosion occurs suddenly If one performs electrochemical experiments with stainless steel, e. g. by applying a constant electrical potential to a sample immersed in dilute NaCl solution, the electrical current - which is an indicator for chemical activity (corrosion) on the metal surface - is low over a wide parameter range. But if critical parameters like temperature, potential, or electrolyte concentration exceed a certain critical value, the current rises abruptly and the metal surface is severely affected by pitting corrosion. The transition to high corrosion rates is preceded by the appearance of metastable corrosion pits. 

Fig. 8.7. Onset of pitting corrosion obser >ed with optical microscopy. (A) Snapshots of a computer processed video sequence obtained with optical microscopy green stars mark the iiucleatioii of new pits. (B) Space-time plot along the line ab in A. (C) Total number of pits on a logarithmic scale, (D) Total current as a function of time. The reaction conditions were T = 20.3 C with the potential held at 615 mV nh E Reproduced from [23],...

Potentiodynamic polarization (intrusive). This method is best known for its fundamental role in electrochemistry in the measurement of Evans diagrams. A three-electrode corrosion probe is used to polarize the electrode of interest. The current response is measured as the potential is shifted away from the free corrosion potential. The basic difference from the LPR technique is that the apphed potentials for polarization are normally stepped up to levels of several hundred millivolts. These polarization levels facihtate the determination of kinetic parameters, such as the general corrosion rate and the Tafel constants. The formation of passive films and the onset of pitting corrosion can also be identified at characteristic potentials, which can assist in assessing the overall corrosion risk. 

In conclusion, the present discussion of proposed film breakdown and pit initiation mechanisms suggests that several phenomena are responsible for the loss of passivity and the onset of pitting when a metal is polarized to a high potential in presence of aggressive anions. Structural defects in the passive film reflecting those of the metal, anion adsorption on the film and the metal surfaces and the effect of anions on the kinetics of the electrochemical reactions governing oxide formation and metal dissolution are most critical. Practical consequences of these phenomena for pitting corrosion will be discussed in Section 7.3. 

As a general rule corrosion rate increases with temperature. One notable exception is the corrosion of carbon steel by water in an open container (Fig. 2) that was discussed earlier. Temperature also inQuences the onset of localized corrosion such as pitting and crevice corrosion of passive alloys. 

The strange anodic polarization behaviors (negative difference effect, lower apparent valence, low anodic dissolution efficiency, low anodic polarization resistance and poor passivity ) are closely associated with the AHE process which is further related to the onset of localized corrosion or pitting . A comprehensive anodic dissolution model can be employed to understand these. 

Surface finish, since good surface finish retards the onset of certain types of corrosion attack, including pitting and stress corrosion cracking. 

Fig. 8.4. Siniulatioii of corrosion onset with periodic boundary conditions. (Top) Siiajishuts showing the local film damage at the indicated time moments. Blue corresponds to low, orange to high film damage. (Middle) Space/tiine diagram along the line marked with ab. (Bottom) Red line Acciimutatcd total number of pitting sites. Blue line Total current.

The molybdenum (Mo) content of types 316 and 317 stainless resists the onset and development of pitting. The presence of nitrogen (N), a strong austenitic former, remarkably increases resistance to pitting and crevice corrosion. The relevant effect of Cr, Mo, and N on crevice corrosion is denoted by the widely used Pitting Resistance Equivalent Number (PREN). 

Using EN techniques, it has been claimed that it is possible to detect the onset of corrosion, determine whether it is uniform or localized, and even distinguish between pitting and crevice corrosion [729]. There is evidence, however, that this claim should be taken with some caution [722]. At its present state of development the most common use for the EN technique has been in corrosion monitoring [724]. Although EN shows great promise for use in monitoring of MIC, there have been few applications to date. 

Critical pitting temperature can be rapidly obtained using electrochemical equipment to maintain a preset potential, increase temperature and detect the onset of corrosion by monitoring corrosion current [24], In ASTM G 150, Test Method for Electrochemical Critical Pitting Temperature Testing of Stainless Steels, a pre-set potential provides consistent conditions and a potential-independent critical pitting temperature. Sensitive detection of attack using the current allows an accelerated evaluation. 

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