The corrosion protection afforded by rare earth conversion coatings applied to magnesium

The corrosion protection afforded by cerium, lanthanum and praseodymium con version coatings formed on pure magnesium and a magnesium alloy, WE43, has been studied using de polarisation and ac impedance techniques. The coating s, which were formed by immersion in rare-earth salt containing solutions, reduced significantly the dissolution of magnesium in a pH 8.5 buffer solut ion. With continued immersion of the treated electrodes in the aggressive p H 8.5 solution, the coatings first appeared to become more protective, but after periods exceeding 60 min began to deteriorate. This is attributed to the formation of magnesium hydroxy corrosion products and mixed rare earth/ magnesium oxide/hydroxide coatings, which on continued immersion became con sumed by the formation of magnesium corrosion products. (C) 2000 Elsevier S cience Ltd. All rights reserved.