Scale removal aluminium alloys

Alloys that are exposed to aggressive atmospheres at high temperatures are usually designed to be heat resistant and one of the most serious problems is how exposure to a complex gas can cause the protective behaviour of scales to change. Usually, such alloys rely on scales based on chronua or alumina formation. Consequently, they rely on sufficiently high concentrations of chromium and aluminium being available in the alloy. This can be disrupted if internal sulphidation removes the chromium or aluminium from solution in the matrix. 

Figure 8.18. The Na2S04 has penetrated into the alloy preferentially removing chromium and aluminium. Since the Na2S04 layer is thick, virtually no oxygen is supplied by the gas phase. Due to reaction with chromium and aluminium the oxygen pressure is reduced below that to oxidize nickel, and nickel sulphide particles are formed at the surface of the specimen. Figure 8.18(a) and 8.18(b). Within the reaction-product scale sulphate ions provide oxygen to oxidize aluminium and chromium and sulphur to form nickel sulphide. Consequently, the oxide-ion concentration is increased to levels at which the Cr203 and AI2O3 can react with oxide ions as described in Equations (8.14) and (8.15) ...

One of the first activities with the establishment of the safe enclosure was the disassembly of the reactor of the bum-up measurement facility. This was a graphite-moderated, air-cooled reactor with strip-shaped fuel elements made of an aluminium uranium alloy. The reactor contained 3.9 kg of high-enriched uranium (93% U-235), the thermal power output was 500 W. Because of the highly cramped conditions, the acceptable dose level and the limited number of fuel stripes, the decommissioning was executed almost exclusively manually. To reduce the collective dose of the personnel, an extensive training with a 1 1 scale mock-up was carried out prior to decommissioning. The removed fuel elements were put into special baskets and were shipped to the interim storage facility BZA in two CASTOR THTR/AVR casks. 

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