Corrosion types crevice

Fig. 1.49 Crevice corrosion, (a) Crevice resulting from the joining of two plates of steel and b) crevice due to a gasket between two flanged pipes (c), (d) and (e) show the method used by Streicher to test different steels for their propensity to crevice corrosion two plastic cylinders are held onto the sheet of metal by two rubber bands of the same size, giving rise to three different types of crevices in duplicate...

The two major types of localized corrosion discussed are pitting corrosion, and crevice corrosion including filiform corrosion. In spite of the different morphological appearance of these two types of corrosion (Figure 6.15), the electrochemical basis of these two types are almost the same. The difference may rise from different causes in the initiation step of pitting or crevice corrosion.25... 

In discussing environment, we can look at its effect on a macro scale, e.g. in the atmosphere, in the ocean, etc. and also examine effects on a micro scale, i.e. what is happening on the metal surface or over short distances. Due to the great variety of environments in which metals are put to use, the range of corrosion problems are equally numerous. Often, similar types of corrosion occur in many environments and may stem from similar mechanisms these have been given specific names which indicate how the corrosion has occurred. For example, under-deposit corrosion and crevice corrosion are related, both being due to oxygen concentration cells. 

The most widespread corrosion types are point corrosion or pitting, filiform, crevice, contact and intercrystalline corrosion. The highest danger for the metal-polymer system is presented by the crevice type of corrosion. 

Fig. 4 Example of the results of crevice corrosion. Type 304 SS exposed to 6 wt.% ferric chloride for 48 h at room temperature with castellated crevice washer applied around center hole. Pitting also occurred at several sites outside the crevice. The draining of the occluded solution from the crevice sites (and pits) led to the river pattern of...

Identical metab in contact with solutions of different concentrations The metal dissolves from the electrode immersed in a dilute solution, and is deposited on the electrode that is immersed in a more concentrated solution. The corrosion stops when the electrolyte concentration is homogeneous at the interfaces of both of electrodes. The other type of electrochemical concentration cell is known as a differential aeration cell. The electrode potential difference in this case results from different oxygen aeration of the electrodes. This type of corrosion initiates crevice corrosion in aluminum or stainless steel when exposed to a chloride environment. 

Analysis of electrochemical current noise Detection of pitting corrosion defects, signatures of specific corrosion types (pitting or crevice vs general) W(Mxl et al. (2002), Sulyma and Roy (2010h), Monticelli et al. (1998)... 

Iron(III) chloride, which is frequently added as a flocculant to treatment tanks, can lead to pitting corrosion and crevice corrosion of both types of steel. 

Differential aeration cells can be caused by crevices, lap joints, dirt and debris, and moist insulation. Under these conditions, the oxygen starved areas are anodic, while the areas with free access to oxygen are cathodic. Common terms for this type of corrosion include crevice corrosion, oxygen screening, and poultice action. 

Stress corrosion cracking, prevalent where boiling occurs, concentrates corrosion products and impurity chemicals, namely in the deep tubesheet crevices on the hot side of the steam generator and under deposits above the tubesheet. The cracking growth rates increase rapidly at both high and low pH. Either of these environments can exist depending on the type of chemical species present. 

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. 

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 ). 

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. 

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. 

The formation of crevices between dissimilar metals should be avoided. Corrosion at such connections is generally more severe than either galvanic or crevice corrosion alone. Also, crevices between metals and certain types of plastics or elastomers may induce accelerated rates of combined crevice and chemical attack. Testing is recommended prior to establishing final design specifications. 

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