Clad Alloys

In addition to the possibiUty of selecting alloys having minimum SCC susceptibiUty while retaining other properties as needed, there are other steps possible to reduce the SCC probabiUty (/) avoid designs that permit water to accumulate (2) avoid conditions in which salts, especially chlorides, can concentrate and (i) where available and otherwise acceptable, use a clad alloy (2) (see Metal surface treatments). 

More uniform results may be expected if a substantial layer of metal is removed from the specimens to ehminate variations in condition of the original metaUic surface. This can be done by chemical treatment (pickling), electrolytic removal, or grinding with a coarse abrasive paper or cloth, such as No. 50, using care not to work-harden the surface. At least 2.5 X 10 mm (0.0001 in) or 1.5 to 2.3 mg/cm (10 to 15 mg/iu") should be removed. If clad alloy specimens are to be used, specif attention must be given to ensure that excessive metal is not removed. After final preparation of the specimen surface, the speci-... 

Shape and form of the specimen support should assure free contact of the specimen with the corroding solution, the liquid line, or the vapor phase, as shown in Fig. 25-5. If clad alloys are ejq)osed, special procedures are required to ensure that only the cladding is ejq)osed (unless the purpose is to test the ability of the cladding to protect cut edges in the test solution). Some common supports are glass or... 

The tension test specimen from plate 12.7 mm (V2 in.) and thicker is machined from the core and does not include the cladding alloy therefore, the stress values listed are those for materials less than 12.7 mm (V2 in.). 

Clad Alloys. The heat-treatable alloys, in which copper or zinc are major alloying constituents, are less resistant to corrosive attack than a majority of the non-heat-treatable alloys. To increase the corrosion resistance of these alloys in sheet and plate form, they are often clad with a high-purity aluminum, a low-magnesium-silicon alloy, or an aluminum alloy containing 1% zinc. 

After decladding is completed, the zinc-cladding alloy is separated from the oxide fuel and cover salt. The zinc is recovered by distillation for recycle. 

A thorough state of the art literature review on the corrosion of aluminium alloys was compiled by the IAEA in 1998. This review was published in IAEA-TECDOC-1012, Durability of Spent Nuclear Fuels and Facility Components in Wet Storage. It covered a wide range of quantitative and semi-quantitative data on cladding alloys used in nuclear fuel elements and assemblies, and included separate sections on corrosion of aluminium, zirconium, stainless steel, carbon steels and copper alloys in a wet storage... 

Storage basins around the USA that are storing aluminium clad spent nuclear fuels without corrosion problems operate deionization equipment continuously and maintain conductivity levels of typically less than 10 gS/cm. Storage times for aluminium clad alloys of up to 25 years without corrosion in low conductivity waters have been reported [2.5],... 

Substantial quantities of nomadioactive material may be vaporized from the reactor core during this stage of an accident. In particular, control rod materials and burnable poisons may be vaporised. Constituents of structural materials such as steel and clad alloying agents as well as uranium oxides may be vaporised along with radionuclides. These nonradioactive materials add to the mass of condensable effluents from the core region and can affect the behavior of radionuclides both in the reactor coolant system and in the containment (See Chapter V). 

Thus, bonds made with 6061-T6 alloy will normally last about four times longer than those prepared with the less corrosion-resistant 2024-T3 alloy, when exposed to a marine environment. Similarly, after three weeks exposure to 5% salt spray at 95°F, joints prepared with 2026-T4 alloy had failed, those prepared with 6151-T4 alloy had weakened considerably, but had not fallen apart, and those prepared with X5085-H-111 alloy had retained about 30% of their initial strength. The effects described are for bonds formed with only minimal preparation of the adherend surface, under which conditions bondline corrosion becomes the dominant factor in determining durability. The same trends may not be apparent when anodized surfaces, or corrosion-inhibiting primers, are used. Also the same order of relative durabilities may not apply to noncorrosive exposure conditions. For example, long-term tropical exposure indicates that more durable bonds are formed with 2024-T3 clad alloy than with 6061-T6 alloy. ... 

Cotter and Kohler likewise concluded that anodized surfaces produce the most durable adhesive bonds to aluminum. Their study included a comparison of the effects of using bare and clad alloys. In humid conditions, no significant difference was observed in the durability of chromic acid-anodized specimens prepared from the different alloys. In a corrosive environment, however, specimens prepared from the bare alloy proved to... 

High operating parameters of the used cladding alloy have been verified by pilot fuel element testing in an icebreaker reactor core. 

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