Propagation of pitting

Depth of localized corrosion should be reported for the actual test period and not interpolated or extrapolated to an annual rate. The rate of initiation or propagation of pits is seldom uniform. The size, shape, and distribution or pits should oe noted. A distinction should be made between those occurring underneath the supporting devices (concentration cells) and those on the surfaces that were freely exposed to the test solution. An excellent discussion of pitting corrosion has been pubhshed [Corro.sion, 25t (January 1950)]. 

Of great significance is the velocity of the sea-water, and the initiation and propagation of pits is more likely under stagnant conditions. Obviously... 

Potentiostatic methods. Once the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarizing individual samples at potentials above and below this value will indicate the validity of the chosen scan rate and give some kinetic data on the initiation and propagation of pits at different levels. Another possibility is to initiate pits above the pitting or breakdown potential and then shift to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential, one should observe current decrease until complete repassivation. 

The chloride threshold has been found to be dependent on the presence of macroscopic voids in the concrete near the steel surface [14]. Voids that can be normally found in real structures due to incomplete compaction may weaken the layer of cement hydration products deposited at the steel/concrete interface, and thus may favour local acidification that is required for sustained propagation of pits (Section 7.4). For instance, it was shown that by decreasing the volume of entrapped air in the steel-concrete interfacial zone from 1.5% to 0.2% (by volume), the chloride threshold increased from 0.2 % to 2 % by mass of cement, as illustrated in Figure 6.4 [15]. The presence of air voids, as well as crevices or microcracks, can also be an explanation for the lower values of chloride threshold that are normally found in real structures compared with those found in (usually well compacted) laboratory specimens with similar materials [16]. 

Pitting cavities may fill with corrosion products and form caps over the pit cavities sometimes creating nodules or tubercles (Fig. 6.10). While the shapes of pits vary widely (Fig.6.11) they are usually roughly saucer-shaped, conical, or hemispherical for steel and many associated alloys. The following are some factors contributing to initiation and propagation of pitting corrosion ... 

Step (1) of the mechanism shown above, describes the initiation of pitting, steps (2-5) describe the propagation of pitting, and step (6) termination of pitting. 

On steel, pitting is controlled by the initiation step (as detected by the pitting potential), and it propagates quickly. On aluminium, there will always be an initiation of a very large number of superflcial and very small pits around intermetallic phases that have a cathodic potential with respect to the matrix. However, the propagation of pits cannot be deduced from the measurement of the potential. 

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