Forsyth / Hinton Rare Earth-Based Corrosion Inhibitors

1. Auflage 2014
ISBN: 978-0-85709-358-5
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band 61, 338 Seiten

Reihe: Woodhead Publishing Series in Metals and Surface Engineering

Corrosion inhibitors are an important method for minimizing corrosion; however traditional inhibitors such as chromates pose environmental problems. Rare earth metals provide an important, environmentally-friendly alternative. This book provides a comprehensive review of current research and examines how rare earth metals can be used to prevent corrosion and applied to protect metals in such industries as aerospace and construction. Chapter 1 begins by examining the important need to replace chromate, and then goes on to discuss the chemistry of the rare earth metals and their related compounds. Chapter 2 considers the techniques that can be used to identify corrosion inhibition mechanisms and to test the levels of protection offered to different metals by rare earth compounds. Subsequent chapters consider in more detail how rare earth elements can be used as corrosion inhibitors in different forms and for different metals. This includes discussion on the potential of rare earth elements for self-healing, tunable and multifunctional coatings. Finally, chapter 10 considers the cost and availability of the rare earths and the potential health and environmental risks associated with extracting them. ÿ - Provides a review of current research and examines how rare earth metals can be used to prevent corrosion and applied to protect metals in such industries as aerospace and construction - Includes discussion on the potential of rare earth elements for self-healing, tunable and multifunctional coatings - Considers the cost and availability of the rare earths and the potential health and environmental risks associated with extracting them

Autoren/Hrsg.

Forsyth, Maria

Hinton, Bruce

Leseproben

1 The chemistry of rare earth metals, compounds, and corrosion inhibitors
T. Behrsing,    Monash University and PrixMax Australia Pty Ltd., Australia G.B. Deacon,    Monash University, Australia P.C. Junk,    James Cook University, Australia Abstract:
The chemistry of rare earths is fundamental to an understanding of the role of rare earths as corrosion inhibitors and to the preparation of effective rare earth-containing inhibitors. This chapter defines rare earth elements and their place in the Periodic Table, including electronic configurations, oxidation states and the lanthanoid contraction, their discovery, location and abundance, and their uses other than as corrosion inhibitors. A major section is devoted to the general chemistry of rare earths elements, oxides, hydroxides, salts and coordination compounds, and includes a discussion of their separation together with a brief account of their spectra and magnetic properties. A further section deals with rare earth carboxylate complexes as these are promising inhibitors. Their synthesis and structures are described together with attempts at modelling their behaviour on iron (steel) surfaces. Ongoing problems are also considered. Key words
rare earths; lanthanoids; occurrence; uses; corrosion inhibitors; rare earth chemistry; rare earth carboxylates; synthesis; structures; modelling surface films 1.1 Introduction: the need to replace chromate
Metal chromates have a long history of use as corrosion inhibitors, both in aqueous media, for example in cooling towers and radiators, and also in paints and protective coatings.1–4 However, it has been known for many years that chromium(VI) compounds are carcinogenic.5–10 This is attributed to in vivo reduction to highly reactive CrV and CrIV species. The Centers for Disease Control and Prevention issued a series of NIOSH documents initially in 1975, indicating risks associated with hexavalent chromium.11 As a consequence, bans or limitations on use of chromium(VI) compounds are developing. Thus, the European Union severely restricted use of such compounds from 2006.12 Although corrosion inhibitors in some uses were exempted, the European Chemical Agency (ECHA) is now proposing a ban on strontium chromate, a mainstay of corrosion inhibition in organic coatings, especially in the aircraft industry.2,3 While regulation and restrictions are developed, litigation in the USA has already bridged the gap between biological risk and regulation, notably in the Pacific Gas and Electric case, the basis of the celebrated feature film, Erin Brockovich. There is now a search for less toxic, greener alternative corrosion inhibitors. Chromates have combined effectiveness with relatively low cost. Sinko3 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,13–16 and we have been developing rare earth carboxylates as potential dual function inhibitors combining the roles of the rare earth and the aromatic carboxylates.17 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. 1.2 Rare earth elements and their place in the Periodic Table
1.2.1 Electronic configurations
The rare earth elements comprise the Group 3 metals, scandium and yttrium, and the lanthanoid (IUPAC)/lanthanide (common usage) elements from lanthanum through to lutetium (Table 1.1). All 17 elements are united by many common properties (Section 1.5), especially the dominance of the +III oxidation state. All 15 elements (La-Lu) occupy one place in the Periodic Table but there is a distinction between Sc, Y, La and Ce-Lu based on the electronic configuration (Table 1.1). This is most clearly seen in the electronic configurations of the trivalent ions. For scandium, yttrium and lanthanum, the Ln3+ ion has the configuration of the preceding inert gas, whereas for Ce-Lu there is progressive filling of the 4f shell from Ce3+ (4f1) to Lu3+ (4f14). An argument has been put forward based on chemical similarities that Lu should be in Group 3 and La to Yb should constitute the lanthanoids. There are certainly close similarities between the chemistry of Sc3+, Y3+ and Lu3+, perhaps more so than between Y3+ and La3+. This similarity is based on the similar ionic radii of Y3+ and Lu3+ (and more so for Ho3+/Er3+) (Table 1.1), and is reflected in the dominance of YIII in ores rich in ‘heavy’ rare earths. Table 1.1 Rare earth elements 21 Scandium (Sc) [Ar] 3d’4s2 [Ar] 0.87 III (0, I, II) 39 Yttrium (Y) [Kr] 4d’5s2 [Kr] 1.01 III (0, II) 57 Lanthanum (La) [Xe] 5d16s2 [Xe] 1.16 III (0, II) 58 Cerium (Ce) 4f15d16s2 4f1 1.14 III, IV (II) 59 Praseodymium (Pr) 4f36s2 4f2 1.13 III, IV (0, II) 60 Neodymium (Nd) 4f46s2 4f3 1.11 III (0, II) 61 Promethium (Pm) 4f56s2 4f4 1.09 III 62 Samarium (Sm) 4f66s2 4f5 1.08 II, III (0) 63 Europium (Eu) 4f76s2 4f6 1.07 II, III 64 Gadolinium (Gd) 4f75d16s2 4f7 1.05 III (0, II) 65 Terbium (Tb) 4f96s2 4f8 1.04 III, IV (0, II) 66 Dysprosium (Dy) 4f106s2 4f9 1.03 III (0, II) 67 Holmium (Ho) 4f116s2 4f10 1.02 III (0, II) 68 Erbium (Er) 4f126s2 4f11 1.00 III (0, II) 69 Thulium (Tm) 4f136s2 4f12 0.99 III (II) 70 Ytterbium (Yb) 4f146s2 4f13 0.98 II, III 71 Lutetium (Lu) 4f145d16s2 4f14 0.98 III (0, II) aIonic radius for 8-coordination.18 bIn parenthesis: recently established divalent molecular complexes19,20 and zero valent compounds.21 1.2.2 The lanthanoid contraction
The ionic radii decrease by ca. 20% from La3+ to Lu3+ (Table 1.1, values for 8-coordination18). It arises through inadequate screening of the nuclear charge by the f electrons. As the number of f electrons increases, there is an increase in the effective nuclear charge leading to a progressive reduction in the Ln3+ size. The effect is so pronounced that with Ho3+ and Er3+, the ionic radius has decreased to that of Y3+, the rare earth of the second transition metal series. Consequently, the chemistry of Y resembles that of the heavy lanthanoids and is associated with them in the mineral xenotime. Yttrium is termed a heavy rare earth because of these similarities. A consequence of the contraction is that there is an increase in the stability of complexes with a particular ligand from La3+ to Lu3+. Complexes of Y3+ fit in according to the ionic radius of that element. The stability sequence associated with the lanthanoid contraction forms the basis for the separation of the lanthanoids by ion exchange or solvent extraction. The lanthanoid contraction can have a major effect on coordination number/structure of rare earth complexes. For example, [LnCl3(thf)n] (thf = tetrahydrofuran) complexes decline in coordination number from 8 (Ln=La) to 6 (Ln=Lu) [22a]. However, such changes are not inevitable as all 18-crown-6 complexes of LnCl3 and Ln(NCS)3 are 9-coordinate.22b,c Thus, one of the intriguing aspects of lanthanoid coordination chemistry is how complexes of a given ligand respond to the contraction. Structure/coordination number breaks can occur anywhere in the 15 elements (La-Lu), even between La and Ce. Cases are known of a structural break where a complex of the borderline element can be isolated in two forms. An example relevant to corrosion inhibitors is that [Dy(cinn)3] (cinn = cinnamate) can be isolated as both a 9- and a 7-coordinate complex.23 The lanthanoid contraction also has effects far beyond the lanthanoid series. Thus, it is responsible for why mercury and its compounds are so different from zinc and cadmium to the point where zinc and cadmium are more similar to magnesium (Group 2) than mercury. Other groups in the Periodic Table show changes between the last two elements, although not as substantial as those between cadmium and mercury. 1.2.3 Oxidation states
Rare earth metals are highly...