Localized corrosion crevice

Oxide stabilized refers to materials, such as aluminum and the stainless steels, whose corrosion resistance depends on the formation and stability of a very thin surface oxide layer that is inert, easily healed if damaged, and tenacious. When the oxide layer has been disrupted and not healed, the material usually has little corrosion resistance. Both active and passive states sometimes exist adjacent to each other on the surface, resulting in rapid local corrosion. Crevice corrosion in stainless... 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Two types of localized corrosion are pitting and crevice corrosion. Pitting corrosion occurs on exposed metal surfaces, whereas crevice corrosion occurs within occluded areas on the surfaces of metals such as the areas under rivets or gaskets, or beneath silt or dirt deposits. Crevice corrosion is usually associated with stagnant conditions within the crevices. A common example of pitting corrosion is evident on household storm window frames made from aluminum alloys. 

Local variations in temperature and crevices that permit the accumulation of corrosion products are capable of allowing the formation of concentration cells, with the result of accelerated local corrosion. 

A final type of measurement is the detection of localized corrosion, such as pitting or crevice attack. Several corrosion-measuring probes can be used to detec t localized corrosion. Some can detect locahzed corrosion instantaneously and others only its result. These types of corrosion may contribute little to the actual mass loss, but can be devastating to equipment and piping. Detec tion and measurement of localized corrosion is one of the areas with the greatest potential for the use of some of the newest electrochemicaUy Based corrosion monitoring probes. 

Evidence of localized corrosion can be obtained from polarization methods such as potentiodynamic polarization, EIS, and electrochemical noise measurements, which are particularly well suited to providing data on localized corrosion. When evidence of localized attack is obtained, the engineer needs to perform a careful analysis of the conditions that may lead to such attack. Correlation with process conditions can provide additional data about the susceptibility of the equipment to locaHzed attack and can potentially help prevent failures due to pitting or crevice corrosion. Since pitting may have a delayed initiation phase, careful consideration of the cause of the localized attack is critical. Laboratory testing and involvement of an... 

Microstructural examinations revealed V-shaped openings along the tube seam, some extending into as much as 50% of the tube wall thickness. The incompletely closed seam provided a crevice in which differential concentration cells developed (see Chap. 2, Crevice Corrosion ). The resulting localized corrosion caused the observed pits. 

Chemical removal of deposits and corrosion products revealed the appearance of the groove (Fig. 14.5). The crevice formed by the incompletely fused weld seam fostered the establishment of differential concentration cells (see Chap. 2). This resulted in localized corrosion and eventual perforation through the greatly thinned tube wall at the bottom of the crevice. The tubercle, which is composed of corrosion products, is a simple result of the corrosion process occurring locally within the crevice. 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

Under-Deposit Corrosion In the same way that oxygen becomes depleted in a crevice, and a differential-oxygen concentration cell is established, leading to localized corrosion of the oxygen-starved anodic area, so the same phenomenon readily occurs in dirty boilers under deposits, sludge, and other foulants. 

Localized, concentration-cell corrosion (differential aeration corrosion), occurring as Tuberculation corrosion Crevice corrosion Under-deposit corrosion Pitting corrosion All forms of localized, concentration-cell corrosion are indirect attack type corrosion mechanisms. They result in severe metal wastage and can also induce other corrosion mechanisms, e.g. Stress corrosion Corrosion fatigue... 

Visible sign of metal wastage in the form of a deep crevice resulting from various forms of localized corrosion. 

Polarization techniques have also been used to determine mechanisms by which microorganisms induce localized corrosion in the forms of pitting or crevice corrosion. In most cases itpit was determined in the presence and absence of bacteria, itpit provides data as to the tendency for pitting, but not the rate for pit propagation. Salvarezza et ah " and De Mele... 

More often the passive layer is broken down locally and then the steel is said to be attacked by localized corrosion, the most important forms being pitting, crevice corrosion, and corrosion cracking. Most often the localized corrosion is caused by halogen ions such as chloride, bromide, and iodide. Pitting or pitting corrosion is seen as small pinholes on the surface of the steel. This section describes electrochemical instrumental methods to investigate and measure this form of corrosion attack. 

Crevice corrosion is another form of localized corrosion, which is easier to initiate than pitting. Since aevice corrosion will occur at lower potentials and temperatures than pitting, it is necessary to avoid crevice corrosion completely when trying to characterize resistance to pitting. Over the years, several suggestions have been presented for solving this problem. 

This is a form of localized corrosion that takes place in or around crevices or microscopic pockets. 

This type transfers heat from the process liquid to the cooling water across a large number of thinly spaced corrugated plates. They find particular application in food and beverage industry applications or for smaller heat loads in hotels or hospitals. Plate and frame types are widely used because of their compact design and availability of various construction materials. Because localized corrosion and metal wastage can occur within the crevices (crevice... 

Under these circumstances, appropriate water management objectives should probably include a requirement to take all measures to prevent or minimize localized corrosion processes occurring that could cause pitting, crevice attack, tuberculation, etc. This will undoubtedly require close attention to maintaining clean waterside metal surfaces, but may also require more tolerance of an acceptable rate of general corrosion, to, say, 4 mpy, or a little higher. 

One can provide several practical examples of localized corrosion occurring by differential aeration. Crevice attack is a common phenomenon (Fig. 12.27), or, one may mention the corrosion of partially immersed metals in sea water (Fig. 12.28). The region near the waterline provides easy access to oxygen and thus becomes an electron-source area for the lower part of the metal, which becomes an election sink because of its relative oxygen starvation. 

Fig. 12.69. Current distribution resulting from polarization of mouth of a crevice to +0.3 V (SCE). Dark represents anodic current, light represents cathodic current. (Reprinted from R. G. Kelly, Measurement and Modeling of Localized Corrosion Sites, Interface 6(2) 21, 1997, Fig. 5c. Reproduced by permission of The Electrochemical Society.)...

The goal of this chapter is to provide a basic understanding of the processes involved in localized corrosion in terms of what has been covered in Chapters 1 and 2. In addition, the different test techniques that are used to determine the resistance of alloys to pitting and crevice corrosion will be reviewed and discussed. 

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