Pitting corrosion occluded regions

Stainless steels tend to pit in acid solutions. Pits form local areas of metal loss associated with breakdown of a protective oxide layer. Breakdown is stimulated by low pH as well as by the decrease of dissolved oxygen in occluded regions. Small, active pit sites form and remain stable because of the large ratio of cathodic surface area (unattacked metal surface) to the pit area. Active corrosion in the pit cathodically protects immediately adjacent areas. If conditions become very severe, pitting will give way to general attack as more and more of the surface becomes actively involved in corrosion. 

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. 

An Analysis of Pitting Corrosion in Terms of IR Potential Changes in Occluded Regions and Relationship to Polarization Curves (Ref 20)... 

These factors can be discussed with reference to the polarization curves for the initial and changing conditions within the occluded region. The combined effects of a potential drop into the pit and the effect of the lowered pH, which raises Epp and increases icrit, are also analyzed by reference to Fig. 7.6 (Ref 20). As previously assumed, the solid anodic curve is taken as representative of a stainless steel in an environment of pH = 1. The dashed extension again represents the anodic polarization behavior in the absence of a passive film. At a potential, Ecorr (or Epot if the potential is maintained potentiostatically), the passive current density would be iCOrr,pass and the active corrosion current density would be iCorr,act- Assume that a small flaw through the passive film is associated with an (IR), drop that lowers the potential in the bottom of the flaw to E,. Since this potential is higher than the passivating potential, Epp, this flaw should immediately repassivate and not propagate. 

Alkire and his coworkers [50] have demonstrated the effects of flow on the inhibition of pitting. Similar ideas can be applied to crevice corrosion. In both instances, flow would act to inhibit initiation of attack to the extent that the flow lines enter the occluded region. For pit initiation, as described in Chapter 4.2, flow at the surface is very successful at delaying or preventing aggressive solution development. In a crevice, the large length-to-gap ratio makes initiation control much less effective than for pits. 

Crevice corrosion is another form of localized corrosion that occurs in occluded regions where the electrolyte has limited access. Examples of such regions include under rivets and lap joints. The chemistry of crevice corrosion bears some similarity to pitting corrosion in that aggressive electrolyte conditions can develop in the occluded regions with a resultant acceleration of the corrosion rate. 

Pitting corrosion is an insidious type of localized corrosion in which cavities appear on a smooth passivated surface. Crevice corrosion is another type of localized attack in which the attack is on metal surface that is not exposed to the bulk solution but is immediately below the border, under a creviced or occluded region (such as under a gasket). From the environmental point of view, both types of attack depend on the concentration of halides (especially chloride), the electrochemical potential, and the temperature. Table 2-7 shows the rate of corrosion of several alloys according to the standard ASTM G 28 tests and for the green-death and yellow-death solutions (see Section 2.1.1). 

PITTING AND CREVICE CORROSION arise fix)m the creation of a localized aggressive enviion-ment that breaks down the normally corrosion-resistant passivated surface of the metal. This localized environment normally contains halide anions (e.g., chlorides) and is generally created because of differential aeration, which creates corrosion potential drops between most of the surface and occluded regions (e.g., pits or crevices) that concentrate the halide at discrete locations. 

---