Crevice corrosion, 6.18

Crevice corrosion is a localized type of corrosion resulting from local differences in oxygen concentration associated with deposits on the metal surface, gaskets, lap joints, or crevices under bolt or rivet heads where small amounts of liquid can collect and become stagnant. 

The material responsible for the crevice need not be metallic. Wood, plastics, rubber, glass, concrete, asbestos, wax, and living organisms have all been reported to cause crevice corrosion. Once the attack begins within the crevice, its progress is very rapid and it is frequently more intense in chloride environments. For this reason, the stainless steels containing molybdenum are often used to minimize the problem. However, the best solution to crevice corrosion is a design that eliminates crevices. 

The CCT of an alloy is that temperature at which crevice corrosion is first observed when immersed in a ferric chloride solution. The critical corrosion temperatures of several alloys in 10% ferric chloride solution are as follows  

Crevice corrosion occurs in cracks or crevices formed between mating surfaces of metal assemblies, and usually takes the form of pitting or etched patches. Both surfaces may be of the same metal or of dissimilar metals, or one smface may be a nonmetal as shown in Fig. 6.20. It can also occur imder scale and surface deposits and under loose fitting 

The series of events leading to the formation of a severe crevice can be summarized in the following three stages. Firstly, crevice corrosion is believed to initiate as the result of the differential aeration mechanism mentioned earlier. IDissolved oxygen in the liquid which is deep in the crevice is consumed by reaction with the metal [Fig. 6.21( )]. Secondly, as oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between the 

The cathodic oxygen reduction reaction cannot be sustained in the crevice area, making it the anode of a differential aeration cell. This anodic imbalance may lead to the creation of highly corrosive microenvironmental conditions in the crevice, conducive to further metal dissolution. It is also thought that subsequent pH changes at anodic and cathodic sites further stimulate local cell action [Fig. 6.21(c)]. The aggravating factors present in a fully developed crevice can be summarized in the following points  

Many mathematical models have been developed to simulate the initiation and propagation of crevice corrosion processes as a function of external electrolyte composition and potential. Such models are deemed to be quite important for predicting the behavior of otherwise benign situations that can progress into serious corrosion situations. 

One such model was applied to several experimental datasets, including crevice corrosion initiation on stainless steel and active corrosion of iron in several electrolytes [11]. The model was said to break new ground by 

High austenitic stainless steels, in the presence of corrosion inhibitors, suffer erevice corrosion attacks occurring in all locations. Table 8.11 shows the results of the erevice corrosion tests in 28% HCl at 130°C. All results of SCC tests are reported in Table 8.12. 

Laboratory tests, carried out in 28% HCl solution at 130 °C, showed the following results  

80 °C the behavior was acceptable. As a consequence, there is a need for better inhibitors for high operating temperatures. 

Crevice corrosion tests revealed susceptibility to corrosion attack in the presence of corrosion inhibitors, while in their absence no preferential attack in crevice sites was observed, see, in the presence of commercial inhibitors, did not occur for any tested materials in the absence of corrosion inhibitors, transgranular microcracks were present at the bottom of pits on high austenitic stainless steel. 

The designer should always consider the structure and equipment as a whole and should avoid regarding individnal items in isolation because this is not actually true in practice. It is imperative that all intermaterial influences are properly evaluated before any final decision is taken on a design. Compatible materials will not cause uneconomic breakdown within the utility. This section is concerned with the various types of intermaterial relations met in engineering design. 

This form of corrosion occurs in the presence of stagnant corrosive solution near a hole, under a deposit, or any geometric shape that can form a crevice. It is also known as cavernous corrosion or underdeposit corrosion. This form of corrosion results from a concentration cell formed between the electrolyte within the crevice, which is oxygen starved, and the electrolyte present outside the crevice where plenty of oxygen is present. The metal within the crevice acts as anode, and the metal outside the crevice functions as the cathode. The difference in aeration produces a different equilibrium potential, given by the Nemst equation applied to the reaction 

Using the activities of the dissolved species in water and assuming water activity to be 1, we can write 

Thus localized corrosion ensues because of the difference in potential, which is in turn a result of the difference in oxygen concentration. The more aerated surfaces act as cathodes because of their more noble potential. Differences in metal ion concentrations can result in localized corrosion where the crevice rich in ions acts as the cathode. 

This is a case for illustrating the effects of nomnetallic materials (gasket, mbber, concrete, wood, plastic and the like) in contact with a surface metal or aUoy exposed to an electrolyte (stagnant water). For instance, as Fontana [5] pointed out, crevice attack can cut a stainless steel sheet by placing a stretched mbber band around it in seawater. Thus, metal dissolution occurs in the area of contact between the alloy and mbber band. 

Stagnant solution in small holes, gasket faces, lap joints, surface deposits, cervices under bolts and rivet heads are all sources of crevice corrosion. It is also called deposit or gasket corrosion. 

A particular anodic dissolution process is that associated with crevices and in general with deep and narrow contact regions of stagnant solution under a stress state. These regions may have a local chemistry much different from that interesting the work piece in which they are present. This favors the development of an accelerated and localized corrosion process known as crevice corrosion. Crevice 

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In the following, the most typical modes of corrosion—other than the above discussed unifonn dissolution (active corrosion) and localized pitting and crevice corrosion (local active dissolution)—are briefly presented. 

Ma.rine. In the presence of an electrolyte, eg, seawater, aluminum and steel form a galvanic cell and corrosion takes place at the interface. Because the aluminum superstmcture is bolted to the steel bulkhead in a lap joint, crevice corrosion is masked and may remain uimoticed until replacement is required. By using transition-joint strips cut from explosion-welded clads, the corrosion problem can be eliminated. Because the transition is metaHurgicaHy bonded, there is no crevice in which the electrolyte can act and galvanic action caimot take place. Steel corrosion is confined to external surfaces where it can be detected easily and corrected by simple wire bmshing and painting. 

Titanium is susceptible to pitting and crevice corrosion in aqueous chloride environments. The area of susceptibiUty for several alloys is shown in Figure 7 as a function of temperature and pH. The susceptibiUty depends on pH. The susceptibiUty temperature increases paraboHcaHy from 65°C as pH is increased from 2ero. After the incorporation of noble-metal additions such as in ASTM Grades 7 or 12, crevice corrosion attack is not observed above pH 2 until ca 270°C. Noble alloying elements shift the equiUbrium potential into the passive region where a protective film is formed and maintained. 

Fig. 7. Temperature—pH limits for crevice corrosion of titanium alloys in naturally aerated sodium chloride-rich brines. The shaded areas indicate regions...

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

The best way to prevent crevice corrosion is to prevent crevices. From a cooling water standpoint, this requires the prevention of deposits on the metal surface. Deposits may be formed by suspended soHds (eg, silt, siUca) or by precipitating species, such as calcium salts. 

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. 

The stainless steels contain appreciable amounts of Cr, Ni, or both. The straight chrome steels, types 410, 416, and 430, contain about 12, 13, and 16 wt % Cr respectively. The chrome—nickel steels include type 301 (18 wt % Cr and 9 wt % Ni), type 304 (19 wt % Cr and 10 wt % Ni), and type 316 (19 wt % Cr and 12 wt % Ni). Additionally, type 316 contains 2—3 wt % Mo which gready improves resistance to crevice corrosion in seawater as well as general corrosion resistance. AH of the stainless steels offer exceptional improvement in atmospheric conditions. The corrosion resistance results from the formation of a passive film and, for this reason, these materials are susceptible to pitting corrosion and to crevice corrosion. For example, type 304 stainless has very good resistance to moving seawater but does pit in stagnant seawater. 

Crevice Corrosion Crevice corrosion occurs within or adjacent to a crevice formed by contact with another piece of the same or another metal or with a nonmetalhc material. When this occurs, the intensity of attack is usually more severe than on surrounding areas of the same surface. 

Potential differences leading to galvanic-type cells can also be set up on a single metal by differences in temperature, velocity, or concentration (see subsection Crevice Corrosion ). 

Evaluation of Results After the specimens have been reweighed, they should be examined carefully. LocaHzed attack such as pits, crevice corrosion, stress-acceleratedcorrosion, crackiug, or intergranular corrosion should be measured for depth and area affected. 

Evaluation of attack if other than general, such as crevice corrosion under suppoi t rod, pit depth and distribution, and results of microscopic examination or bend tests... 

It is now well established that the activity of pitting, crevice corrosion, and stress-corrosion cracking is strongly dependent upon the corrosion potential (i.e., the potential difference between the corrod-... 

Pitting and Crevice Corrosion The general literature for pre-dic ting pitting tendency with the slow scan reviews the use of the reverse scan if a hysteresis loop develops that comes back to the repassivation potential below the FCP (E ) the alloy will pit at... 

FIG. 28-15 Sch( niatic diagram of the (icctrochcmical cell used for crevice corrosion testing. Not shown are three hold-down screw s, gas inlet tiil)e, and external thermocouple tiil)e. 

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... 

These alloys have extensive applications in sulfuric acid systems. Because of their increased nickefand molybdenum contents they are more tolerant of chloride-ion contamination than standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. 

The three major forms of concentration cell corrosion are crevice corrosion, tuberculation, and underdeposit attack. Each form of corrosion is common in cooling systems. Many corrosion-related problems in the cooling water environment are caused by these three forms of wastage. The next three chapters—Chap. 2, Crevice Corrosion, Chap. 3, Tuberculation, and Chap. 4, Underdeposit Corrosion — will discuss cooling water system corrosion problems. 

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. 

The crevice must be filled with water. Surfaces adjacent to the crevice must also contact water. Without liquid filling the crevice, corrosion cannot occur. The crevice typically is a few thousandths of an inch wide. Corrosion may extend up to several feet into a crevice mouth in high conductivity waters. 

Figure 2.4 Crevice corrosion—initial stage in oxygenated water containing sodium chloride. (Courtesy of Mars G. Fontana and Norbert D. Greene, Corrosion Engineering, McGraw-Hill Book Company, New York City, 1967.)...

As the name implies, crevice corrosion occurs between two surfaces in close proximity, such as a crack. Table 2.1 gives a partial listing of common crevice corrosion sites in cooling water systems. 

There is often a period before corrosion starts in a crevice in passivating metals. This so-called incubation period corresponds to the time necessary to establish a crevice environment aggressive enough to dissolve the passive oxide layer. The incubation period is well known in stainless steels exposed to waters containing chloride. After a time period in which crevice corrosion is negligible, attack begins, and the rate of metal loss increases (Fig. 2.8). 

Copper alloys often show only weak crevice corrosion. This is especially the case if the copper alloy is coupled to a less noble alloy such as steel. The corrosion of the steel is stimulated by the galvanic effect caused by the coupling of dissimilar metals. Hence, the sacrificial corrosion of the steel protects the copper alloy (Fig. 2.9). See Chap. 16, Galvanic Corrosion. ... 

Crevice corrosion is markedly increased as water conductivity increases. 

Frequently, so-called crevice washers are used in coupon studies to test the environment for its ability to produce crevice corrosion (Fig. 2.22). There are several designs most consist of a small Teflon washer with radially oriented, wedge-shaped teeth. The washer is held to the coupon by a mounting bolt that passes through a central hole. The spaces between teeth form small crevice-shaped areas in which attack may occur (Fig. 2.23). The test is somewhat subjective and is not easily quantified. Using this test, attack in crevices either occurs or does not. 

Additionally, crevice corrosion can be reduced by two techniques used successfully on most aqueous corrosion—chemical inhibition and cathodic protection. However, both these techniques may be cost prohibitive. 

Wastage was caused by long-term crevice corrosion. Attack was much more severe beneath the baffle than elsewhere. Subsequent investigation revealed severe damage at many baffles. 

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