Rare earth element inhibition

The physical influences of rare-earth elements on rates and direction of growth and structural characteristics of oxide scales mainly result fi om the incorporation of the rare-earth compounds in the scale. Other influences relate to their capacity to nucleate more uniform fine-grained oxide, inhibit the formation of transitional oxides and promote, for instance, Cr203 formation where it would not normally be expected, as exemplified in studies by Stringer et al. (1972), Whittle et al. (1977), Rhys-Jones et al. (1987) and Rhys-Jones and Grabke (1988). 

Rare earths have been used in conjunction with anodizing of aluminium in several ways. Firstly, they have been used in fundamental investigations of the anodic oxidation of aluminium alloys where the rare earth element (cerium, hafnium, neodymium and samarium) was employed as an alloying element. These smdies revealed the distribution of the alloying element in the anodic alumina and the outward migration rate of the rare earth cations relative to aluminium ions. It has been shown also that rare earths (particularly cerium) can inhibit ejection of ions from films during anodizing in alkaline electrolytes. 

Superconductivity is a rather common phenomenon in metals at low temperatures. In the rare earth group however, the only superconducting element at normal pressure is lanthanum. Most rare earth metals are magnetic due to their incomplete 4f shell. The magnetism inhibits the formation of Cooper pairs and thus inhibits superconductivity. Similarly, magnetism prevents superconductivity in the 3d-transition period beyond vanadium (cf. fig. 10.1). 

Oxides of heavier elements, such as Mb, Hf, Ti, Zr, and Ta are very stable in their highest oxidation state. The mechanism for rare-earth inhibition seems to originate from the alkaline precipitation of protective oxide films at active cathodes. However, soluble and mobile precursors of these oxides remain difficult to stabilize in aqueous solution with the slight exception of Ce, which is the only lanthanide element that exhibits a tetra-valent oxidation state that is stable as a complex in aqueous... 

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