Prevention of crevice corrosion

Crevice corrosion typically appears under stagnant conditions or under deposits. Crevice corrosion at stainless steels is caused by local activation and the halides chloride or bromide can stimulate crevice corrosion. Thus for prevention of crevice corrosion similar counter-measures are used, i.e. high chromium and molybdenum content. Critical crevice corrosion temper-... 

Prevention of crevice corrosion must be taken into account at different stages of the life of an industrial apparatus ... 

Where there is a perceived risk of crevice corrosion, cathodic protection can often be used to prevent its initiation. Once more a 100 mV cathodic polarisation will usually prove sufficient. However, it is doubtful whether cathodic protection can arrest crevice corrosion once started and, despite claims to the contrary, whether it could be effective in arresting stress-corrosion cracking. The problem lies in the fundamental difficulty of forcing cathodic current into an occluded area. 

CORROSION, OXYGEN DEFICIENCY - A form of crevice corrosion in which galvanic corrosion proceeds because oxygen is prevented from diffusing into the crevice. 

A three-point loaded specimen schematic is shown in Fig. 9.4b [16]. A disadvantage using this sample is the possibility of the pressure generated in the central support to create unknown biaxial stresses. When three-point loaded specimens are used, there is a possibility of crevice corrosion at the central support, which is closer to the maximum tensile stress that cathodically protects the specimen and prevents crack formation. 

Whether crevice corrosion occurs largely depends on the crevice geometry. Crevice widths are particularly critical when the distance between the crevice-forming surface is less than 1 mm. Since the reactions taking place in the crevice cannot be influenced from the outside, damage due to crevice corrosion can only be avoided through the prevention of crevices. 

If an electrolsrte can enter the crevice formed by the faying surfaces of two almninmn surfaces an oxygen concentration, and subsequently chemical concentration, cell can form and cause accelerated locahzed attack. As such, corrosion protection often is required in joints, even when not needed on the freely exposed almninmn. The severity of crevice corrosion depends on the electrolyte and how readily it is replenished. It also is influenced by the geometric shape of the crevice, and the ratio of active crevice cathode area to the adjacent external cathode area. The best protective measures are to design so that crevices will drain, and to effectively seal crevices to prevent ingress of the electrolyte. 

In this chapter, the basic mechanisms of crevice corrosion are briefly presented but most of the text is devoted to the crevice corrosion of passive alloys, particularly Fe-Ni-Cr-. .. alloys in aerated chloride environments. Phenomenological aspects, the mechanisms of initiation, the conditions of propagation, the modeling, the experimental techniques, and the possibility of prevention are successively described. 

This results in the prevention of caustic buildup beneath deposits or within a crevice where leakage is occurring. Caustic corrosion (and caustic embrittlement, discussed later) does not occur, because high caustic concentrations do not develop. 

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. 

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... 

Bates, J. F., Cathodic Protection to Prevent Crevice Corrosion of Stainless Steel in Halide Media , Corrosion, 29, 28 (1973)... 

Zero solids treatment (ZST) is a further enhancement of AVT and MT programs and, as its name suggests, the program employs extensive pre- and post-treatment equipment to ensure the highest possible FW and BW purity. ZST has been employed in the secondary circuits of nuclear-powered SGs as an aid in the prevention of SCC, crevice corrosion, and denting, especially where condenser cooling is effected by the use of brackish or estuarine waters. 

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. 

Has 21% Cr, 13% Mo, 3% W, 3% Fe, 60% Ni and suitable for use in oxidizing environments corrosion resistance better than C-276, C-4 in oxidizing media, better pitting resistance inferior to C-276, C-4 in reducing media and with respect to crevice corrosion Superior to C-22, C-276, super thermal stability attributed to the ternary system, Ni-Cr-Mo devoid of W, Cu, Ti and Ta Similar to C-276 in composition except for Cr level being 16-21% alloy is solution annealed at 1200°C and rapidly cooled to prevent precipitation of intermetallic phases thermal behavior not as good as Alloy 59 and its corrosion resistance was less than 59 1.6% Cu has been added to C59 lower corrosion resistance and thermal stability than Alloy 59... 

Design-to-prevent is the best approach to avoid pitting and crevice corrosion. Some examples of surface treatments and coatings are ... 

Greater care should be taken and proper procedures followed by the condenser manufacturer when rolling the tube ends to avoid excessive wall reduction and tube extrusion. To prevent crevice corrosion in the tube-to-tubesheet joint, the tube end should be rolled for the full thickness of the tube sheet.12... 

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