Crevice corrosion development

The mechanism of crevice corrosion for ceramics is similar to that of metals. Crevice corrosion develops in an occluded area of a corroding material. Once corrosion has started within the crevice, corrosion rates within the occluded area will increase with time as a result of either (1) depletion of a passivating component within the crevice or (2) increasing acidity within the crevice, or a combination thereof. 

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... 

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. 

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

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. 

Resistance to crevice corrosion Titanium is more resistant to crevice corrosion than most conventional metals and alloys, particularly where differential aeration is involved, e.g. it is very resistant to crevice attack in sea water at normal temperatures. This form of corrosion becomes more severe when acidity develops in a crevice and this is more prone to occur under conditions of heat transfer . Under these circumstances, especially in the presence of halide, even titanium may suffer attack, and the metal should not be employed in strong aqueous halides at temperatures in excess of 130°C. This limiting temperature can be raised to 180°C by use of the Ti-0- 15Pd alloy " or by coating with noble metals. (See also Sections 1.4 and 1.6.)... 

Although important contributions in the use of electrical measurements in testing have been made by numerous workers it is appropriate here to refer to the work of Stern and his co-workerswho have developed the important concept of linear polarisation, which led to a rapid electrochemical method for determining corrosion rates, both in the laboratory and in plant. Pourbaix and his co-workers on the basis of a purely thermodynamic approach to corrosion constructed potential-pH diagrams for the majority of metal-HjO systems, and by means of a combined thermodynamic and kinetic approach developed a method of predicting the conditions under which a metal will (a) corrode uniformly, (b) pit, (c) passivate or (d) remain immune. Laboratory tests for crevice corrosion and pitting, in which electrochemical measurements are used, are discussed later. 

The above considerations show that although considerable advances have been made in developing laboratory controlled potential tests for evaluating crevice corrosion and pitting, the results must be interpreted with caution. 

Normally, square specimens are cut from 2-5-mm-thick sheet material. Two versions of the Avesta cell have been developed. One contains 120 ml of solution and uses specimens with an exposed area of 1 cm. The other contains 1200 ml of solution and uses specimens with an exposed area of 1-5-10 cm. The specimen is mounted as illustrated (Fig. 15). There is a small flow (0.1 ml/min) of distilled water through the pores of a ring of filter paper in the crevice between the specimen and the cell bottom. The filter paper ensures an even distribution of the flow of distilled water. In this way the liquid in the crevice is flushed constantly and the chloride ions do not enter the crevice and cannot initiate crevice corrosion. 

Solution flow typically enhances corrosion rates, by increasing the transport of dissolved oxygen to the metal surface, by increasing the rate of removal of protective corrosion products, and, in extreme cases, by physically removing the corrosion products or even metal (in the case of erosion by suspended particles or cavitation) (Fig. 4). In a few situations flow can be beneficial thus for stainless steels in chloride solutions, flow can prevent the development of the acidification that is necessary for pitting and crevice corrosion. 

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