Influence of Thiosulphate Ions on the Localized Corrosion of Stainless Steel

摘要

The thiosulphate ion (S/sub 2/O/sub 3//sup 2-/) can be formed either by microbial activity or by air-oxidation of metal sulfides, e.g. in blast-furnace slag. Since blast-furnace slag may be added to concrete for nuclear waste containment, information on the corrosion effects of thiosulphate on stainless steels such as are likely to be used for waste canisters in contact with such concrete has been reviewed and areas requiring further study identified. In neutral solutions, thiosulphate is extremely corrosive towards stainless steels. Pitting corrosion has been reported for Type 304 steel in solutions containing less than 10/sup -5/ molar thiosulphate ions, provided that an appropriate amount of sulfate is present. Chloride ions are not necessary for pitting of this alloy. Two mitigating circumstances lead to the conclusion that Type 316L steel will probably not suffer from thiosulphate corrosion under disposal conditions. First, the concrete environment has a high pH (greater than or equal to 12.5), and this has been shown to inhibit pitting. Second, no pitting of Type 316L steel has so far been detected in solutions where sulfate (rather than chloride) is the predominant anion. Experimental confirmation of the good resistance of Type 316L steel has been achieved using a scratching test. (ERA citation 13:024610)