Crevice washers

Frequently, so-called crevice washers are used in coupon studies to test the environment for its ability to produce crevice corrosion (Fig. 2.22). There are several designs most consist of a small Teflon washer with radially oriented, wedge-shaped teeth. The washer is held to the coupon by a mounting bolt that passes through a central hole. The spaces between teeth form small crevice-shaped areas in which attack may occur (Fig. 2.23). The test is somewhat subjective and is not easily quantified. Using this test, attack in crevices either occurs or does not. 

Figure 2.22 Crevice washer. The radially oriented, wedge-shaped washer teeth provide small areas between which crevices are formed.

Figure 29 Details of multiple crevice washer (not to scale).

Fig. 4 Example of the results of crevice corrosion. Type 304 SS exposed to 6 wt.% ferric chloride for 48 h at room temperature with castellated crevice washer applied around center hole. Pitting also occurred at several sites outside the crevice. The draining of the occluded solution from the crevice sites (and pits) led to the river pattern of...

Standard laboratory immersion tests described in the previous section above are used. U-bend stress corrosion [9,32] tests and multiple crevice washer tests for crevice corrosion [33] are used to evaluate localized corrosion. 

Figure 7.5 Appearance of 8090-T851 aluminum panels with crevice washers after partial immersion in seawater for 4 months. The air/water interface was near the top of the central hole.

Any region where two surfaces are loosely joined or come into close proximity qualifies as a crevice site as long as water may enter. Partially exfoliated coatings may produce crevices. Surfaces may be metallic or nonmetallic. Usually, however, at least one surface is metallic. Crevices commonly are present at gaskets, flanges, washers, bolt holes, rolled tube ends, incompletely fused welds, contact points in plate-and-... 

Figure 2.23 Shallow pitting in crevice areas on a 304 stainless steel coupon exposed to a misting atmosphere. Note the relatively clean areas where the washer teeth contacted the coupon surface. (Magnification 7.5x.)...

Crevice corrosion is yet another example of corrosion caused by a difference in oxygen concentration between two areas on the metal surface. In this case, the region of low oxygen concentration lies inside a crevice caused by the overlapping of a piece of metal or other material, e.g. the crevice which exists under a washer pressed onto a metal surface (Fig.6). Even if the washer is insulating (e.g. nylon) as is used for mounting test coupons, corrosion will still occur. 

Fig. 6. Crevice corrosion beneath a washer. Note the under-deposit corrosion due to seepage of corrosion products from the crevice.

In fact, when testing corrosion inhibitors, the major amount of corrosion may occur under the mounting washers as fresh inhibitor cannot reach such a confined space and, in addition, the low oxygen concentration leads to corrosion. As in the section above, ferric ions may seep from within the crevice and deposit around the fixture, thus giving rise to under-deposit corrosion. 

Washer of j some material Crevice as bulk specimen... 

Coupons were examined every month and discoloration was noted as the coupons developed a general oxidation film with time. No pitting was ever seen on any of the exposed surfaces. Once the coupons were disassembled, some pitting was seen under the ceramic washers of the galvanic coupons. Pitting was also observed under the washers of the crevice coupons. There was no detectable visual change in the stainless steel coupons. The water conductivity was maintained at between 0.1 and 0.7 pS/cm. 

Dull, surface exposed to water. Deposition of scale. Corrosion (general) mainly under washer White deposits, water stains and thin, irregular scale due to water seepage between crevice faces. Shining surface compared with front Dull in colour White deposits... 

Insulating washer produced a thin boundary hne without any pits. Other areas bright and unattacked. At a few sites on this line, some shallow pits 2 pm deep found. Under insulating washer, pits developed and grain boundaries were etched Scale streaks 2 pm thick formed owing to seepage of water within crevice. No pits... 

Pits under washer, with various types of opening, i.e. round and irregular. Most occurred at grain boundaries. Pit depths 8,10,10,10,12,12,12,14,14,14,18 pm Very shallow pits, 2 pm deep. Some contact areas bright. Most of the area unattacked. Water channels formed in crevice resulted in scale formation... 

Pitting under insulation washer. Pit depth maximum 20 pm. Uniform scale formed on all exposed surfaces Contact area bright. Thin scale at other areas of water entry into crevice. No pits... 

The multiple crevice assembly (MCA) consists of two "serrated washers made of TFE-fluorocarbon bolted together through a hole drilled in the specimen, as illustrated in Fig. 2(b). The bolts, made of a highly corrosion resistant alloy, are electrically isolated from the specimen. The MCA eliminates the crevices created where the rubber band contacts the specimen and facilitates the measurement of the crevice area to boldly exposed area ratio. The advantage of the MCA is that the crevice geometry can be made very severe by... 

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