Pitting corrosion continued tests

In the interim period before the new deionization equipment for the L and K basins was received, portable equipment was installed in July 1995 and used to lower the L basin water conductivity from 110 to below 8 pS/cm in 2.5 months. The equipment was then moved to the K basin, and within three months the conductivity was lowered to below 10 pS/cm. Continued deionization in both basins for two more months lowered the conductivity further, to less than 3 pS/cm, and the chlorides, nitrates and sulphates were lowered to about 0.5 ppm. The corrosion surveillance programme continued in the three reactor basins and in the RBOF while the basin and water quality improvements were being carried out, i.e. until mid-1996. Results of the component immersion tests through September 1997 (the last withdrawal) showed no pitting corrosion on any of the corrosion coupons. These coupons were exposed to a variety of conditions for 37-49 months as conditions improved in the basins. Table 1.1 presents a summary of component immersion tests for the period 1992-2000, when corrosion coupons accumulated exposure time in extremely high quahty water and withdrawal intervals were extended. 

Kurchatov Institute is continuing to expose the coupons of racks 1 and 3 to the storage basin water in order to (a) obtain more data about the low corrosion rates observed so far (b) investigate further the pitting corrosion on 6061 and 6063 test coupons and (c) clarify the reasons for the differences in corrosion behaviour between the coupons of racks 1 and 3. 

G 44 can be used for both stressed and unstressed specimens. Historically, it has been used for stress corrosion cracking testing, but often is used for other forms of corrosion, such as uniform, pitting, intergranular, and galvanic. It uses a 1-h cycle that includes a 10-min period in a aqueous solution of 3.5 % sodium chloride (NaCl) followed by a 50-min period out of the solution, during which the specimens are allowed to dry. This 1-h cycle is continued 24 h/day for the total number of days for the particular alloy being tested. 

Other tests on steels with 0-4% Cr and 0-1% A1 or Mo also showed that a steel with 2% Cr and 1% A1 represents the best compromise in resistance to uniform and local corrosion. Chromium contents above 2% continue to reduce the corrosion rates, but also increase the tendency to pitting corrosion [50]. 

Chemical and Corrosion Resistance The corrosion resistance of CCCs depends on thickness and coating age. Corrosion resistance has been observed to scale with total chromium content [153]. Some studies have found that corrosion resistance does scale with Cr(VI) content [154], while others have found no such correlation [155]. Corrosion resistance is evaluated by continuous or cyclic accelerated exposure testing and electrochemical methods. On aluminum alloys, heavy CCCs will resist pitting for as long as 400 to 1000 h [156]. CCC-coated surfaces will exhibit total impedances of 1 to 2 Mf2 cm after exposure to aerated 0.5 M NaCl solution for 24 h. Such coatings can be expected to withstand 168 h of salt spray exposure without serious pitting [157]. CCCs usually perform well in mild neutral environments, but do not fare as well under... 

In one of the tests, the corrosion potential of the sample in 9 g/1 sodium chloride is noted. The pH, initial, and final potential are noted. The current at -1-0.8 V versus SCE is noted. If localized corrosion is not simulated in the first 20 s, the polarizing currents will be small and decrease with time. If localized corrosion is not stimulated in 15 min, the test is terminated, and the sample is resistant to localized corrosion. Localized corrosion is indicated by increasing polarizing current with time (>500 pA/cm ). The potential is then returned to corrosion potential to determine if the sample will repassivate or the localized corrosion will continue to occur. Evidence of pitting and crevice corrosion should be noted in ASTM F 746 (30). 

When 1.5% sodium chloride was added to the gypsum, significant pitting occurred at the interface on partially embedded samples that were kept moist chromating (Cronak process) virtually prevented attack in tests up to one year, but phosphating allowed corrosion to continue at the interface. 

Electrical Resistance Probes—Electrical resistance probes are small in size and can be easily installed in the service environment however, the walls of the test equipment must be penetrated for the probes to be installed and consequently, care must be taken to avert leakage in the system. The usefulness of the probes is limited in that they provide a measurement of uniform corrosion and can be continuously monitored, but provide no information on localized corrosion such as pitting or crevice corrosion. In addition, errors can result in the probe data if the temperature is varied during the time of the measurement [69]. 

Tests at Miami, Florida have exposed identical specimens by continuous total immersion and by intermittent immersion during high tide. The continuously immersed specimens tended to develop fewer but deeper sites of corrosion. Tests on pilings and pipe exposed above and below water have shown that increased pitting sometimes occurs at the waterline and in the splash zone. This most likely is the result of increased oxygen content near the surface, plus concentration effects due to partial drying in the splash zone. 

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).

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