Crevice corrosion and pitting

Stainless steels exposed to seawater develop deep pits within a matter of months, with the pits usually initiating at crevices or other areas of stagnant electrolyte (crevice corrosion). Susceptibility to pitting and crevice corrosion is greater in the martensitic and ferritic steels than in the austenitic steels it decreases in the latter alloys as the nickel content increases. The austenitic 18-8 alloys containing molybdenum (types 316,316L, 317) are still more resistant to seawater however, crevice corrosion and pitting of these alloys eventually develop within a period of 1-2.5 years. 

Stainless steels exposed at room temperature to chloride solutions containing active depolarizing ions, such as Fe, Cu, or Hg, develop visible pits within hours. These solutions have sometimes been used as accelerated test media to assess pitting susceptibility. 

Many extraneous anions, some more effective than others, act as pitting inhibitors when added to chloride solutions. For example, as mentioned in Section 6.6, addition of 3% NaNOs to a 10% FeClj solution completely inhibited pitting 

1 Theory of Pitting. Pitting corrosion is usually considered to consist 

Initiation, in which the integrity of the passive film is breeched at localized areas and pits form. 

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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. 

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. 

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. 

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... 

High-alloy steels with >16% Cr" (e.g. 1.4301, AISI 304) Neutral waters and soils (25°C) <0.2 <-0.1 Protection against pitting and crevice corrosion... 

In the construction of plants, titanium with 0.2% Pd is mainly used. It can be employed with advantage in nonoxidizing acid media and also has increased resistance to pitting and crevice corrosion because of its more favorable pitting potential [40]. 

Wilde, B. E. and Williams, E., The Relevance of Accelerated Electrochemical Pitting Tests to the Long Term Pitting and Crevice Corrosion Behaviour of Stainless Steels in Marine Environments , J. Electrochem. Soc., 118, 1056 (1971)... 

Uhlig, H. H., Distinguishing Characteristics of Pitting and Crevice Corrosion , Mater. Prol. 

The arbitrary division of behaviour has been made because of the extreme behaviour of some chemicals that initiate small areas of attack on a well-passivated metal surface. The form of attack may manifest itself as stress-corrosion cracking, crevice attack or pitting. At certain temperatures and pressures, minute quantities of certain chemicals can result in this form of attack. Chloride ions, in particular, are responsible for many of the failures observed, and it can be present as an impurity in a large number of raw materials. This has led to the development of metals and alloys that can withstand pitting and crevice corrosion, but on the whole these are comparatively expensive. It has become important, therefore, to be able to predict the conditions where more conventional materials may be used. The effect of an increase in concentration on pitting corrosion follows a similar relationship to the Freundlich equation where... 

In sea-water flowing at slower velocities and more especially in stagnant conditions, pitting and crevice corrosion may develop, particularly beneath deposits and marine growths at the surface of the metal. Some data for the Ni-30 Cu Alloy 400 are shown in Fig. 4.40 the corrosion was mostly pitting. 

Recommended practice for examination and evaluation of pitting corrosion Test method for determining susceptibility to stress corrosion cracking of high-strength aluminium alloy products Test method for pitting and crevice corrosion resistance of stainless steels and related alloys by the use of ferric chloride solution Recommended practice for preparation and use of direct tension stress corrosion test specimens... 

A common chemical laboratory test for corrosion resistance is a simple exposure test using metal coupons. The ASTM standard G48 —Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution—describes a simple exposure test. The material coupons (e.g., 60 x 60 mm) are placed on a glass cradle and immersed in the solution in such a way that the coupons are evenly exposed. 

Inco alloy 25-6MO —Used for its corrosion resistance in many environments, this is an austenitic nicket-jron-chromium alloy with a substantial (6%) addition of molybdenum. Especially useful for resisting pitting and crevice corrosion in media containing chlorides or other halides. Applications include equipment for handling sulfuric and phosphoric acids, offshore platforms and other marine equipment, and for bleaching circuits in pulp and paper plants. 

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. 

Figure 44b Effect of charge density on the repassivation potential for pitting and crevice corrosion.

Chromates are particularly effective inhibitors, and there appear to be several components to inhibition. Chromate in solution inhibits metal dissolution and oxygen reduction reactions. It also slows metastable pitting, the transition to stable pitting, and, when present in sufficient concentration, the growth stage of pitting and crevice corrosion. 

The resistance to general corrosion, pitting and crevice corrosion and stress-corrosion cracking of the ferritic alloy is given in Table 4.10. 

F) Pitting and crevice corrosion KMn04-NaCl Room temperature F R R R R R... 

Table 4.14 Critical temperatures in some pitting and crevice corrosion tests...

Resistance to H2SO4 and SCC used in manufacture of synthetic rubber, high-octane gasoline, solvents, explosives, plastics, synthetic fibers, chemicals, pharmaceuticals, and food processing, not resistant to pitting and crevice corrosion in low pH chloride media... 

Table 4.43 Critical pitting and crevice corrosion temperature per ASTM G-48...

The alloy Haynes 6B is resistant to corrosion in organic acids, but subject to pitting and crevice corrosion and SCC in chloride media. The corrosion rate of 0.3 mm/yr or 12mpy has been observed in 30 wt % of NaOH it is likely that caustic cracking will occur at high concentrations of NaOH and temperatures in the case of all the cobalt alloys. The nominal composition of high-temperature cobalt alloys is given in Table 4.54. 

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