Pitting potential distributions

It is useful to compare the resistance to pitting as a function of the critical concentration of chloride which causes pit initiation on different alloys. The appearance, morphology, distribution and depth of pits (average penetration of the 10 deepest pits and the deepest one) should be determined in parallel with pitting potential determinations. The statistical distribution of pits and morphology of the pit should be examined by different microscopic techniques. The pit depth can be... 

Passivity breakdown appears to occur preferentially at local heterogeneities, such as inclusions, grain boundaries, dislocations, and flaws on the passive metal surface. In the case of stainless steels, the passivity breakdown and pit initiation occur almost exclusively at sites of MnS inclusions, and the pitting potential was observed to decrease linearly with the increasing size of MnS inclusions [44]. With metals containing no apparent defects, however, passivity breakdown is likely to occur in the presence of sufficient concentrations of film breaking ions. It is worth noting that any of the localized phenomena is nondeterministic but somehow stochastic. For stainless steels in chloride solution, the passivity breakdown was found to obey a stochastic distribution [45]. 

The development of conunercially available multip>oten-tiostats has made measiuements of pitting conditions and the underlying factors more tractable. Lunt et al. [37] used an array of nominally identical electrodes to characterize the strengths and extent of the typies of interactions that occur among active pits. In addition, such instrumentation makes the determination of the statistical distribution of pitting potentials via the testing of a large number of specimens much less time intensive, as multiple electrodes (up to 100) [3S] can be independently tested simultaneously. 

It is important to note that the geometry associated with porous electroplated coatings can result in significant deviation from the uniform potential distribution associated with the simple mixed potential model described above [9]. In reality, the distributed nature of the pores may yield a highly nonuniform potential distribution. Furthermore, the occluded geometry of the pores, like that associated with pitting corrosion, may result in local solution chemistry markedly different from that of the bulk electrolyte. Likewise, the conductivity of the electrolytic medium is also... 

Potential distribution at sites protected by a crystalline barrier-type oxide (a) at defect-free barrier layer, (b) at defect (grain boundary), and (c) at depassivated site (pit nucleus). (From Marcus, P. et al. Corrosion Sci., 50,2698,2008.)... 

Structures or pits for water lines are mostly of steel-reinforced concrete. At the wall entrance, contact can easily arise between the pipeline and the reinforcement. In the immediate vicinity of the pit, insufficient lowering of the potential occurs despite the cathodic protection of the pipeline. Figure 12-7 shows that voltage cones caused by equalizing currents are present up to a few meters from the shaft. With protection current densities of 5 mA mr for the concrete surfaces, even for a small pit of 150 m surface area, 0.75 A is necessary. A larger distribution pit of 500 m requires 2.5 A. Such large protection currents can only be obtained with additional impressed current anodes which are installed in the immediate vicinity of the pipe entry into the concrete. The local cathodic protection is a necessary completion of the conventional protection of the pipeline, which would otherwise be lacking in the pit. 

Figure 45 Cumulative distributions of pitting breakdown potentials for the commercial purity (CP), high sulphur (HiS), and high purity (HiP) 304L steels. (From Ref. 50.)...

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