Rare earths coating effectiveness

Deposition effects of rare earth oxides on the surfaee of iron and stainless steels have also been reported [58]. The eorrosion rate eonstants decreased significantly by the coating in corrosion tests under isothermal eonditions. In thermal cyclic conditions, protective scale spallation completely disappeared for eoated samples. The rare earth effect is more remarkable with elements located on the left part of the lanthanide series (lighter rare earths). For example, eeria coatings strongly modify the microstructure and texture of the wustite (FeO) scale formed during low pressure oxidation of pure iron [59]. Cerium is located in the wustite matrix as a CeFeOs phase which dissolves in FeO in time. 

The addition of the rare earth improves the corrosion resistance of the laser coatings. The microstmcture becomes more compact by the refining and purifying effect of La20s, which suppresses the corrosion. 

Metals passivation in the resid FCC unit may also be accomplished through the addition of metal traps. Tin, barium titanate, strontium titanate, magnesium oxide, manganese oxide, and specialized zeolite types and contents have all been used for vanadium trapping. In addition, zeolites coated with alumina and catalyst particles coated with rare earth have also been applied to resist vanadium poisoning. Antimony, bismuth, and specialized active high crystalline aluminas have all been used successfully to counteract the negative effect of nickel in the FCC unit. 

Devianto et al. [21] investigated the poisoning effect of H2S on Ni-based anodes in MCFC at low H2S concentrations, simulating biogas impurity. A conventional Ni-Cr anode was coated with ceria using dip coating to form a rare earth metal oxide thin layer on the surface of the anode. Electrochemical studies of the Ni-based samples were performed in symmetric cells under anode atmosphere (H2, CO2, H2O, and N2 as balance) with 2, 6, 12, and 24 ppm of H2S by means of electrochemical impedance spectroscopy. 

Montemor M.F, Simoes, A.M., Ferreira, M.G.S., (2002). Composition and corrosion behaviour of galvanized steel treated with rare - earth salts the effect of the cation. Progress in Organic Coatings, Vol.44, No2, pp.111-120. ISSN 0300-9440. 

Benefits of the kind outlined above would suggest that the best resistance to sulphidation is achieved by oxide scales containing rare-earth elements (mainly Ce and Y). This being the case, one of the most cost-effective routes for enhancing resistance to sulphidation is to incorporate the rare-earth oxides by pre-oxidation of the substrates upon which rare-earth oxides have superficially been applied. The sequential development of this approach is illustrated in fig. 13, where the most effective sulphidation resistance is achieved on Incoloy 800H which has been pre-oxidized in air at 850"C and coated with Ce02 (Fransen et al. 1985). 

There is now a search for less toxic, greener alternative corrosion inhibitors. Chromates have combined effectiveness with relatively low cost. Sinko has reviewed the quality parameters needed for corrosion inhibitors in coatings, and generally concluded that inorganic non-chromate inhibitors such as phosphates, molybdates, borates and silicates are inferior to chromates. Rare earth salts have been proposed and show promise as alternative corrosion inhibitors, and we have been developing rare earth carboxylates as potential dual function inhibitors combining the roles of the rare earth and the aromatic carboxylates. If rare earth-based corrosion inhibitors are to be used, there is a need to understand the chemistry of the elements and of the inhibitors. 

As discussed in other chapters of this book, corrosion inhibition by rare-earth containing species has been investigated as an alternative system to the environmentally hazardous chromate coatings. Rare earth species have proven to be effective corrosion inhibitors for steel, " zinc and aluminium. " ... 

Abstract The incorporation of rare earth compounds as corrosion inhibitors in polymeric paint systems is presented. Specifically, uses of cerium salts in an electrolytically deposited coating commonly used in the automotive industry and praseodymium oxide in spray applied epoxy-polyamide primers for aerospace applications are discussed. Special emphasis is placed on the importance of phase of the rare earth material in relation to polymeric matrix as the effectiveness of the corrosion inhibition is strongly link to the starting materials, the formulated paint system and the ambient environment in which the coating operates in practice. 

Several studies have found that combinations of rare earths can produce synergistic effects (Markley et ai, 2007a,b Aballe et ai, 2001 Davo and de Damborenea, 2004). The inhibitors have been added to a wide range of conventional coating systems inclnding ... 

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