Crevice corrosion prevention

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)... 

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. 

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. 

In electrochemical measurements it is necessary to establish good insulated electrical contact with the sample and furthermore to have a well-defined exposed surface area. It is difficult to comply with these demands without creating, at the sample/mount interface, a crevice between the sample and the sample holder. If the bulk solution is not prevented from entering the crevice, crevice corrosion attack is often initiated. ... 

All joints, including deck plate to structural members, shall be continuously seal-welded to prevent crevice corrosion. Stitch welding, top or bottom, is unacceptable. 

Crevice corrosion may occur where liquid is trapped between close fitting metal surfaces, or between a metal surface and a non-metallic material such as a gasket. Attention to detail at the design and fabrication stage should be given to areas such as jointing to prevent crevice corrosion. 

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... 

In the Uhde [986], [987] and Steinmiiller [989] concept the tubesheet is anchored to and supported by the tubes to withstand the differential pressure, which imposes some restriction on the tube length. Babcock-Borsig s [988], [990], [991] tubesheet is reinforced by stiffening plates on the back side (Figure 100). Both solutions have full-penetration tube-to-tubesheet welds for the tubes to prevent crevice corrosion. Struthers [992] reduces stress by making the tubesheet-to-shell connection flexible. 

The control of corrosion in pharmaceutical product processes is largely managed through the use of stainless steel. Rust-free surfaces and cleanliness issues to prevent product contamination have been the primary corrosion concerns. Resistance to mildly aggressive cleaning solutions and saline solutions and the potential for under deposit or crevice corrosion present the most severe service conditions. The high standards of cleanliness necessary for pharmaceutical processes favor the mitigation of 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. 

Prevention—The methods outlined above to combat crevice corrosion also apply to pitting. Metals that are resistant to pitting should be used as alloying agents their passive films are more protective and more stable to halogen attack. For example, the addition of 2% molybdenum to 18-8 (type 304) stainless steel to produce 316 stainless steel significantly increases pitting resistance. 

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

Alkire and his coworkers [50] have demonstrated the effects of flow on the inhibition of pitting. Similar ideas can be applied to crevice corrosion. In both instances, flow would act to inhibit initiation of attack to the extent that the flow lines enter the occluded region. For pit initiation, as described in Chapter 4.2, flow at the surface is very successful at delaying or preventing aggressive solution development. In a crevice, the large length-to-gap ratio makes initiation control much less effective than for pits. 

Maintaining the potential of the alloy-solution interface at sufficiently low potential has been shown to prevent crevice corrosion stabilization [58]. As shown in Fig. 25, the repassivation potential for crevice corrosion becomes independent of the amount of charge passed. Kehler and coworkers [59] have demonstrated the same for Alloy 625 and Alloy 22, including the deleterious effects of increased temperature and concentration ratio of [Cl ] [other anions]. Data for AUoy 22 are shown in Fig. 26. 

Cathodic protection can be used to combat crevice corrosion [60, 61], in some cases, with sufficiently low potentials preventing it altogether. It should be noted that anodic protection is generally not effective in preventing crevice corrosion. 

In fact, anodic polarization generally increases the rate of the crevice attack. The only situation for which anodic polarization would be useful in preventing crevice corrosion is for systems that can be completely described by the ohmic drop model and for which no changes occur in either the occluded solution composition or the electrochemical behavior of the material in the occluded region. 

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