Corrosion protection by rare earth species

The various routes that have been examined to enhance the corrosion resistance of anodized aluminium using cerium compounds, such as nitrates or sulphates, include pre- and post-treatments and anodizing with cerium species added to the bath formulation (the addition of cerium to commercial alloys for corrosion protection has not been reported). Each of these approaches has revealed some benefits in short-term laboratory corrosion tests. However, the imderlying mechanisms of corrosion protection are imclear. Corrosion inhibition of aluminium alloys caused by cerium species is well-established, with cerium acting as a cathodic inhibitor at intermetallic particles (Amott et al, 1987). Cerium oxide/ Itydroxide precipitates in response to the increase of pH at cathodic particles as the alloy briefly corrodes following its immersion in the environment. The precipitates stifle further corrosion by hindering the reduction of oxygen and water. 

Pretreatments have involved the immersion of the alloy substrates in a suitable solution, generally at temperatures above ambient, containing rare earth ions to form a mixed oxide-hydroxide containing rare earth derivatives (Li et al, 2010a,b). In such studies, the alloy was anodized in a traditional sulphurie add bath after pretreatment. Li et al (2010a,b) used pretreatment solutions eontaining 75-95 g r of cerium or neodymium nitrates and Imol L of niekel nitrates at 

producing 4-5 thick oxide/hydioxide layers, with lamellar morphologies 

7 Surface morphologies of aluminium specimens after pretreatments in (a) nickel nitrate, (b) nickel nitrate and neodynium nitrate and (c) nickel nitrate and cerium (III) nitrate. (Reprinted from Li etai, 2010b, Copyright (2010) with permission from Elsevier.) 

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