Pitting mechanism

It enables greater cathodic/anodic surface area ratios to become active in corrosion processes, thereby promoting pitting mechanisms in vulnerable materials. 

The fact that impurities do not affect the active dissolution in chloride solutions at current densities larger than 0.01mA/ cm2 shows that the inhomogeneity resulting in a pitting mechanism of dissolution is unrelated to impurities and is an inherent property of the metal. 

Electron transfer from bare iron metal exposed by pitting of the oxide layer, while the pitting mechanism involves localized corrosion and possible catalytic dissolution pathways... 

If the passive film cannot be reestablished and active corrosion occurs, a potential drop is established in the occluded region equal to IR where R is the electrical resistance of the electrolyte and any salt film in the restricted region. The IR drop lowers the electrochemical potential at the metal interface in the pit relative to that of the passivated surface. Fluctuations in corrosion current and corrosion potential (electrochemical noise) prior to stable pit initiation indicates that critical local conditions determine whether a flaw in the film will propagate as a pit or repassivate. For stable pit propagation, conditions must be established at the local environment/metal interface that prevents passive film formation. That is, the potential at the metal interface must be forced lower than the passivating potential for the metal in the environment within the pit. Mechanisms of pit initiation and propagation based on these concepts are developed in more detail in the following section. 

Of all the mechanisms the conveyor has naturally accumulated the longest running time. All the irradiated fuel discharge pit mechanisms have their design faults (each mechanism has its own design faults) although up to recent time their performance has caused no serious troubles (to provide for this the necessary measures on the periodicity and scope of the maintenance and... 

If the pitting mechanism is involved in the reduction of RX by Fe , then (in the absence of O2 and presence of RX) the major reduction half-reaction at the mouth of the pit will be dehalogenation of RX (equation 4) (9). At equilibrium. 

Figmre 1.13 a) Localized corrosion on 2195 Al-Li allc after a potentiodynamic polarization run in 3.5% NaCl. Th ealloy was aged at 190C for 0.5h. SEM photomicrograph talmn at a magnification of 2,00Qx. b) Pitting mechanism. 

Should pitting occur, a heterogeneous mechanism for metal dissolution defines a localized attack that may involve metal penetration in thin stmctural sections, hi the case of massive stmctural sections, pitting is usually of little significance. On the other hand, surface fatigue failures due to pitting mechanism are weU documented in the literature. Hence, pitting develops with surface microcracks. 

A recent study of the initial stages of pitting by STM (Maurice et al., 1998b) has revealed a high density of extremely small pits (not detectable by the classical current transient measurements), initiated along oriented steps of the surface, showing a direct relationship between the surface structure and the sites of pit initiation. Such data may modify the present view of pitting mechanisms, but more data are still needed. 

Figure 7-38. Proposed pitting mechanism for SiC hot corrosion. (Jacobson and Smialek, 1986.)...

Fundamentals of the pitting mechanisms have been discussed in Chapter 7 and the whole question of pitting corrosion was reviewed by Smialowska [la] in 1986. It is intended here to shed light on some points of practical or theoretical importance which were recently investigated or revisited and to propose a comprehensive viewpoint on some rather scattered experimental evidence. The examples are restricted to stainless steels, in weakly acid chloride-containing aqueous media whose pH is larger than the depassivation pH, so that the passive film remains stable except in the pit itself. 

Some other saUent features of the pitting mechanism suggested by various workers are ... 

L. Liu, Y. Li and F. H. Wang (2008), Pitting mechanism on an austenite stainless steel nanocrystalline coating investigated by electrochemical noise and in situ AFM... 

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