Zirconium breakaway corrosion

The discovery by Fiegna and Weisgerber that noble metals are able to catalyse the breakaway corrosion of zirconium has not been built into either of the main theories. Antill et al. have also found it difficult to explain their similar observation for the oxidation of steel by COj. Reactor grade CO2 contains both water and CO as impurities CO is also produced by the reaction... 

Water as an impurity is known to promote the breakaway corrosion of a number of metals in addition to iron in CO2 the effect has been reported for magnesium (hydrocarbons have more effect on the oxidation of this metal), beryllium, zirconium and sodium. In the latter case water is known to convert the oxide to deliquescent NaOH but acceleration of beryllium oxidation probably results from hydride formation and mechanical damage to the oxide. 

Curve A shows two breakaway stages while curve B shows one breakaway point. An example of such behavior is the oxidation of zirconium. Tin is added to zirconium to minimize breakaway corrosion. 

The outstanding corrosion property of zirconium is its resistance to alkalies at all concentrations up to the boiling point, ft also resists fused sodium hydroxide. In this respect, it is distinguished from tantalum and, to a lesser extent, titanium, which are attacked by hot alkalies. Zirconium is resistant to hydrochloric and nitric acids at all concentrations and to <70% H2SO4 up to boiling temperatures. In HCI and similar media, the metal must be low in carbon (<0.06%) for optimum resistance. In boiling 20% HCI, a transition or breakaway point is observed in the corrosion rate (see below) after a specihc time of exposure. The... 

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