Corrosion features crevices

Finally, pitting may be viewed as a special form of concentration cell corrosion. Most alloys that are susceptible to crevice corrosion also pit. However, many metals may pit but not show crevice attack. Further, although sharing many common features with concentration cell corrosion, pitting is sufficiently different to warrant a separate categorization. 

Crevice corrosion and pitting have a number of features in common, and it has been stated that pitting may be regarded as crevice corrosion in which the pit forms its own crevice however, whereas a macroscopic heterogeneity determines the site of attack in crevice corrosion, the sites of attack in pitting are determined by microscopic or sub-microscopic features in the passive film (5 Sections 1.3 and 1.5). 

One of the main factors which establishes the corrosivity of water to stainless steel is the chloride content. Also significant are oxygen content and pH, and it is also probable that other features such as hardness and the nature and concentration of other anions and cations have effects. Water temperature and flow velocity can also be important. Any corrosion takes the form of pitting or, if crevices are available, larger areas of attack within the crevice. 

Many features and much mechanistic detail on crevice corrosion can be determined using this technique (46). Here the discussion is concentrated on the... 

Some attractive features of radiography are access to interior difficult-to-reach portion, such as valves and pipes, the objects or sample covered with insulation can be tested, reference radiographs can be accessed, the nature and severity of the attack can be assessed with ease, in principle, pit depths can be obtained although laser profilometry is the preferred method in this regard. The main drawback of the method is the whole sample needs to be exposed. Some applications of the method are determination of flaws in weldments and castings to identify sites of initiation of crevice corrosion, valve... 

Over the past 40 years, automobile engineers have improved the design to reduce the extent of corrosion. The design improvements consisted of removing crevices and locations where salt and soil can accumulate. Dissimilar metal contacts were removed. The number of nose over hoods, hood louvers, tuck-under areas, and other design features that promote chipping and corrosion have been reduced. 

One important feature of cracks is their crevice type geometry with a very high aspect ratio between their depth and their opening. This has quite similar consequences to what occurs in crevice corrosion ... 

As a consequence, the polarization curves drawn on a specimen equipped with a crevice former device exhibit (Fig. 7) features very similar to those typical of pitting corrosion. Dining the upward potential scan, crevice corrosion initiates quite abruptly at a critical potential that is generally lower than the pitting potential. Then, during a subsequent backward potential scan, corrosion stops at a critical potential significantly lower than... 

Monitoring of the corrosion potential allows one to detect any increase of the corrosion rate dne to the polarization of the cathodic reaction on the free snrfaces. Thns, the initiation of active corrosion in the crevice may be detected. In addition, electrochemical noise (i.e., potential flnctnations) indicating some dissolution transient may be interpreted as piecnrsor events of passivity breakdown or at least of a low degree of passivity. The analysis of potential noise in terms of stochastic versus chaotic features has been shown [106] to allow early detection of pit initiation. This type of analysis should be checked for crevice corrosion. 

As the basic mechanisms of crevice corrosion have been understood to some extent, many models have been developed either to describe the steady state of an actively corroding crevice or to calculate the environment evolution and predict the passivity breakdown. These models are able to reproduce some major features of crevice corrosion, particularly geometric effects, but they all suffer from the lack of knowledge of the complex concentrated solutions that are formed in the crevice. 

There are large numbers of reported case histories of MIC on stainless steel in water and aqueous waste systems. They are related to different industrial applications such as freshwater storage and circulation systems in nuclear power plants [103, 113,116,142] and cooling water systems in chemical process industries [117,118]. There are basically three cases (a) crevice corrosion under unexpected deposits, (b) sensitivity of pitting and crevice corrosion to trace of H2S, and (c) crevice corrosion in natural seawater. Most of these reports are not well documented concerning the microorganisms involved in the process. However, some general features are... 

Avoid geometrical configurations that fadhtate corrosive conditions such as Features that trap dust, moisture, and water Crevices (or else fill them in) and situations where deposits can form on the metal surface Designs that lead to erosion corrosion or to cavitation damage... 

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