Steels continued crevice corrosion

TABLE 4.4 FERRITIC-AUSTENITIC STAINLESS STEEL-CABOT WROUGHT PRODUCTS (continued) Crevice-Corrosion Data in 10% Ferric Chloride at Room Temperature for 10 Days... 

In boiling water reactors chromium steels behave similarly to those in pressurised water reactors. 13% chromium steels withstand the effects of steam (700 °C) in continuous operation [75]. Chromium steels with 13-17% C are especially sensitive to intergranular corrosion after being welded. Also, riveted joints were seen to have pitting corrosion up to 1.7 mm in depth as well as crevice corrosion after one year [76]. 

Austenitic steels are sufficiently corrosion-resistant for chlorine concentrations below 3 mg/1, which usually occur if the waste water is chlorinated. The molybdenum-free grades are susceptible to crevice corrosion if they are in continuous contact with waters with a residual chlorine concentration of 3-5 mg/1. Austenitic steels are not suitable in chlorine discharge systems in which the chlorine concentration may reach several hundreds of mg/1. 

Figure 7 Aspects of the nucleation of SCC by localized corrosion, (a) Peak aged Al-Li-Cu-Mg alloy 8090 after unstressed preexposure in aerated 3.5% NaCl for 7 days, (b) SCC initiated from one of the fissures shown in (a), following removal of the solution and continued exposure to laboratory air under a short transverse tensile stress (courtesy of J. G Craig, unpublished data), (c) Creviced region of 316L stainless steel after a slow strain rate test in 0.6M NaCl + 0.03M Na2S203 at 80°C and an applied anodic current of 25 xA, showing unstable pitting leading to crevice corrosion and SCC initiation (courtesy of M. I. Suleiman).

For copper alloys, increase in temperature accelerates film formation. While it takes about 1 day to form a protective film at 15°C, it may take a week or more at 2°C. It is important to continue initial circulation of clean seawater long enough for initial film formation for all copper alloys. For stainless steels and other alloys that are prone to pitting and crevice corrosion, an increase in temperature tends to facilitate initiation of these types of attack. However, data on propagation rate suggest... 

TABLE 8.35 Influence of Different Alloying Additions and Microstructure on the Pitting and Crevice Corrosion Resistance of Duplex Stainless Steels (Continued)... 

When the precipitation is relatively continuous, the depletion renders the stainless steel susceptible to intergranular corrosion, which is the dissolution of the low-chromium layer or envelope surrounding each grain. Sensitization also lowers resistance to other forms of corrosion, such as pitting, crevice corrosion, and SCC. 

The crevice shape markedly affects corrosion. Crevices so tight that water may not enter are entirely immune to attack. In misting environments or alternately wet-diy environments, the crevice holds water and may allow continued attack even when neeu by surfaces eire dry. In sea water, the severity of attack in stainless steel crevices depends on the ratio of the crevice area to the cathodic surface area outside the crevice. If the cathodic area is large relative to crevice eirea, corrosion is promoted. 

Another important factor is the highly corrosive environment produced by the accumulation of dirt, vegetation, and other debris in metallic entrapment areas [7]. These areas remain wet almost continuously due to the presence of this poultice and produce a stationary electrolyte environment. The result is a concentrating effect of salts and acids similar to the phenomena observed in crevices. When steels are exposed to this poultice environment, the corrosion process becomes autocatalytic leading to rapid deterioration. 

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