Pitting coupon testing

Coupon tests involved a number of metallurgies and were done to evaluate precipitator-plate alloys. Test stainless steel plates failed, not only because of pitting but also because stress-corrosion cracks developed. 

The rate of penetration of pits in aluminium has been shown to decrease rapidly with time. Aziz and Godard found that in field test coupons the pitting rate curve follows a cube root law [2.14] ... 

The pitting rate equation discussed in this chapter, although not accurate, gives a reasonable evaluation of the aggressiveness of the water quahty in fuel storage basins. It has been used to monitor the basin water cleanup activities at SRS, and improvements in water chemistry have been verified by corrosion coupon tests. Additional work is needed to improve this correlation. 

Because detailed information on coupon testing is available elsewhere, this section will focus on the aspects of such testing that are specific to the evaluation of pitting corrosion. When performing coupon exposures, it is important to include alloys whose corrosion behavior in the environment of interest is well characterized and reproducible to serve as an internal standard. This protocol is of particular importance when localized corrosion is studied. Because pitting has a stochastic nature, the use of these controls" increases the confidence with which final material selection choices are made. If the control materials behave as expected, it is probable that the results for the other materials reflect typical behavior for the environment of interest. 

A common chemical laboratory test for corrosion resistance is a simple exposure test using metal coupons. The ASTM standard G48 —Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution—describes a simple exposure test. The material coupons (e.g., 60 x 60 mm) are placed on a glass cradle and immersed in the solution in such a way that the coupons are evenly exposed. 

With small modifications, ASTM standard G48 can be used to determine a CPT. The test is used as a ranking parameter for the resistance to pitting of high-alloyed austenitic stainless steels. In this method, material coupons are typically exposed for 24 or 72 h to a 6% FeCl3 (=1.11 mole/liter) solution at fixed temperatures (typically with 2.5°C intervals). The CPT is defined as the lowest temperature at which the specimen is attacked by pitting corrosion. 

Black corrosion products were formed on the coupons, which is an indication of the presence of hydrogen sulfide. When the black product was tested in arsenic acid solution, a bright yeUow precipitate was formed. This indicates that the black corrosion product was iron sulfide. Also from the microscopic inspections, pitting forms of corrosion were observed. The pitting occurred in the form of deep and large pits. Due to the intensive pitting on the steel surface, the high corrosion rate was obtained for the steel specimens. 

An IPEN rack containing 1060, 6061 and 6262 alloys, used in fuel assembly manufacture, was also immersed in the basin. After 16 months of exposure, it was observed that some pitting had occurred on the uncoupled coupons, mostly on the top surfaces. The aluminium couples were stained inside the crevices but were not pitted. The stainless steel-aluminium galvanic coupons were much more severely corroded. Additional laboratory tests were conducted to determine whether increased levels of silver in the basin water could have increased the corrosion of the aluminium cladding in the IPEN basin. Results indicated no pitting but an increase in darkness of the surface oxide colour with the increase of silver concentration. 

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. 

With these factors in mind, disc type coupons were chosen. These were similar to the American Sodety for Testing and Materials (ASTM) test coupons but smaller in diameter (100 and 70 mm) to facilitate transport. Single sheet coupons were used to evaluate pitting corrosion on exposed surfaces, and couples or sandwich type coupons were used to simulate crevice and/or galvanic corrosion. The coupons were cleaned and numbered, and the alloy type was identified using a laser scriber. The coupons were stacked in a stainless steel rack. This consisted of a steel pipe welded to a stainless steel base plate and threaded at the other end to hold the coupons in place with a stainless steel nut welded to a hook to enable the rack to be suspended with a nylon rope. A... 

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. 

Probably the most widely used test for pitting resistance is salt spray. B 117 describes the expeiimentfd arrangements to be used and mandates the use of 5 wt% NaCl at slightly elevated temperature (35X). The test chamber allows the solution to be sprayed within its confines, creating a salt fog environment around the coupons. Such a constantly moist, salty environment is quite aggressive. 

Rusting, pitting, and SCC tests frequently provide results with poor reproducibility. The percent surface rusted after a salt spray test can also be quite variable. The amount of rust covering a specimen after the salt spray test varies if the rust initiates at the top of the coupon and runs down over the test surface or initiates at the bottom and covers only a small portion of the specimen (Fig. 9). Rusting is also dependent upon the surface finish smoother finishes are usually more resistant. Because of these factors, it is difficult to use the salt spray test for quality control. 

The decision to test is usually driven by unusual chemistry such as a new catalyst, reaction components, or reaction conditions. Immersion/mass loss method is primarily used in conjunction with microscopic examination. Process fluids from production or pilot runs are primarily used to best simulate potential corrosion. The test methods are custom designed based, in part, on cost and the ability to obtain sufficient quantities of test fluids, and handle the process conditions. Process conditions are tested outside the process control limits (e.g., temperatures, pH) to better accentuate the corrosion potential. These extremes have to be tempered by the stability of the products in the stream. For wall thicknesses greater than 0.250 in. (6.35 mm), a uniform loss of less than 10 mil/year (0.26 mm/year) is considered structurally acceptable. Signiflcandy lower levels of uniform loss are of concern for product or process contamination issues. Microscopic examination is used to determine potential localized corrosion concerns, such as pitting or stress cracking. Indications of pitting or stress corrosion in stamped areas of the coupon are of particular concern. U-bend tests are rarely used because of insufflcient test fluid quantities and availability. 

Coupon immersion tests confirmed the long-term predictions. Slight attack was found under the artificial crevice formers in the complete liquid exposure. The practical conclusion of this in-service study was that, since localized corrosion often takes time to develop, a few days of exposure to this chemical product could be acceptable. However, it was recommended to avoid long-term exposure since both pitting and crevice corrosion would be expected for longer exposure periods. 

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