Crevice corrosion locations

General description. Incomplete penetration describes the condition in which the weld fails to reach the bottom of the weld joint, resulting in a notch located at the root of the weld (Fig. 15.12). This critical defect can substantially reduce the intrinsic mechanical strength of the joint and can combine with environmental factors to produce corrosion fatigue (Chap. 10), stress-corrosion cracking (Chap. 9), or crevice corrosion (Chap. 2). 

The sites for the oxidation reactions are called anodes, and the sites for the reduction reactions are called cathodes. Anodes and cathodes can be spatially separated at fixed locations associated with heterogeneities on the electrode surface. Alternatively, the locations of the anodic and cathodic reactions can fluctuate randomly across the sample surface. The former case results in a localized form of corrosion, such as pitting, crevice corrosion, intergranular corrosion, or galvanic corrosion, and the latter case results in nominally uniform corrosion. 

As with all corrosion, crevice corrosion involves concomitant anodic and cathodic reactions. The location and intensity of these reactions determines the development of the morphology both inside and outside the crevice that characterize crevice corrosion. In general, crevice corrosion occurs with a large-scale (0.1-10 mm) separation of the anodic and cathodic reactions. The scale of the separation of anodic and... 

The potential and current distribution results from the computational modeling are shown in Fig. 20. The calculated x rit values (determined by locating the position at which the potential falls into the active region) are also shown. A comparison of the data from the crevice corrosion experiments and those predicted by the model is shown in Fig. 21. Figure 22 plots the same data, but with the ordinate axis is now in terms of... 

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. 

Location of the anodic dissolution processes onto somewhat limited areas, which may result in loss of the joints as in the case of crevice corrosion, perforations as in the case of pitting corrosion, and fluid leakage or collapse of structures as in cases of SCC and corrosion fatigue... 

Single 1100 77 Top 95% of surface stained brown, with several isolated pits. Three squared sections that were in contact with the Teflon separators and the central portion, where the A1 support FRL-027-14 was located, show signs of crevice corrosion, with white deposits... 

Bottom Crevice corrosion in the central portion where the alumina and Teflon separators were located. The rest of the sample looks quite clean and free of attack... 

Crevice corrosion sites may be inherent with specimen mounting and may also be intentionally created at specimen or probe element locations. For example, a corrosion test... 

Implant design can alter the corrosion performance of alloys in vivo. A case in point of a device whose complex design has spurred much interest in its corrosion behavior is that of the cardiovascular stent [75,76]. To consider another example, many prosthetic devices and fracture fixation implants are by nature multicomponent or modular. This means they have various pieces that mate together, e.g., screws and screw holes in plates. These locations may be foci of localized corrosion processes such as crevice corrosion or (in the case of relative motion) fretting corrosion or both. Careful design of such components can minimize in vivo corrosion problems. 

The corrosion rates for uniform surface corrosion are always less than 0.002 mm/a (0,08 mpy), so that this type of corrosion can be neglected. However, for molybdenum-free steel grades, there is a risk of crevice corrosion in most cases. Both types of steel are also insensitive to damp gases containing H2S that occur in some parts of the plant. Damp chlorine vapors, which arise if there is insufficient ventilation in the vicinity of chlorine storage tanks, can also lead to pitting corrosion of the 316-type steels at locations where such vapors can collect. 

The major corrosion problems with titanium alloys appear to be crevice corrosion, which occurs in locations where the corroding media are virtually stagnant. Pits, if formed, may progress in a similar manner. A general comparison of corrosion resistance for titanium is provided in Fig. 1(a). 

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. 

Common exanples of stagnation include nondtain-ing stmctures, dead ends, badly located components, and poor assembly or maintenance practices (Fig. I). General problems include localized corrosion associated with differential aeration (oxygen concentration cells), crevice corrosion, and deposit corrosion. 

A 12-in. diameter pipe located along a gas rack was experiencing severe corrosion due to a combination of galvanic corrosion and crevice corrosion. 

An example of this is in a condenser where the corrosion probe is in a region where the temperature is lower than that at the critical condition of interest. Local scale buildup is another example of this type of situation, as is formation of a crevice at a specific location. 

Favored locations for erosion-corrosion are areas exposed to high-flow velocities or turbulence. Tees, bends, elbows (Fig. 11.5), pumps, valves (Fig. 11.6), and inlet and outlet tube ends of heat exchangers (Fig. 11.7) can be affected. Turbulence may be created downstream of crevices, ledges (Fig. 11.8), abrupt cross-section changes, deposits, corrosion products, and other obstructions that change laminar flow to turbulent flow. 

Macroscopic heterogeneities, e.g. crevices, discontinuities in surface films, bimetallic contacts etc. will have a pronounced effect on the location and the kinetics of the corrosion reaction and are considered in various sections throughout this work. Practical environments are shown schematically in Fig. 1.3, which also serves to emphasise the relationship between the detailed structure of the metal, the environment, and external factors such as stress, fatigue, velocity, impingement, etc. 

An idea of the.diktributibh bf galvanic corrosion in the atmosphere is prp vided by the location of the corrosion of magnesium exposed in intimate contact with steel in the assembly shown in Fig. 19.28 after exposure in the salt atmosphere 25 m from the ocean at Kure Beach, North Carolina, for 9 years. Except where ledges or crevices may serve to trap unusual amounts of electrolyte, it may be assumed that, even with the most incompatible metals, simple galvanic effects will not extend more than about 4-5 mm from the line of contact of the metals in the couple. 

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