The Effect of Ruthenium on the Corrosion of Stainless Steel in Nitric Acid Liquors

Nuclear reprocessing plant liquors resulting from the PUREX process contain ruthenium present as a mixture of nitrosyl complexes in nitric acid. Ruthenium is not corrosive toward stainless steels unless the conditions are sufficiently oxidizing to promote the decomposition of one or more of the nitrosyl complexes present, in which case the production of finely-divided RuO₂ occurs. In this work, the corrosive effect of RuO₂ coatings that were produced by the thermal decomposition of RuCl₃ is reported. The corrosion potential and corrosion rate of stainless steel were found to be significantly increased by the presence of RuO₂ coatings, and depended on the temperature, but did not depend on the amount of RuO₂ present provided above a threshold which was found to lie in the range 43 μg/cm² to 86 μg/cm² of RuO₂. Corrosion potential measurements indicated that in hot 10 M nitric acid, the RuO₂ coatings formed by the thermal decomposition of RuCl₃ reproduced the effect of RuO₂ formed from the in situ oxidation of a tri-nitrato ruthenium compound. However, a stronger, but only short-term, effect was also found, which is attributed to an intermediate precursor of RuO₂, considered likely to be RuO₄. Two separate corrosion accelerating effects can therefore arise in ruthenium-bearing liquors.