Hyperfine parameters of Co- and Mn- akaganeite

KAREN GARCÍA TELLEZ; ANA E. TUFO; ELSA E. SILEO; C. ARISTIZABAL; C.A. BARRERO

Medellin

Congreso; XIII Latin American Conference on the Applications of the Mössbauer Effect LACAME 2012 and International School on Recent Advances in Mössbauer Spectroscopy.; 2012

HYPERFINE PARAMETERS OF Co- AND Mn-AKAGANEITE For corrosion science is important to investigate the effects of alloying elements upon the formation and/or inhibition of iron oxides and oxyhydroxides as corrosion products in different environments. Akaganeite is a common corrosion product in steels exposed to chloride-rich environments [1]. In previous works we studied the effects of Al, Cu, Cr, Ti and urea on the formation of akaganeite when prepared by hydrolysis of FeCl3 solutions [2]. In this work, Mössbauer spectrometry is used to investigate the hyperfine parameters of akaganeite formed in the presence of urea, Mn and Co ions. Akageneites were prepared by thermal hydrolysis of FeCl3 solutions, as explained in previous work [2]. Xray diffraction and Infrarred spectroscopy demonstrate the purity of the samples. Room temperature 57Fe Mössbauer spectra of all samples consisted of only two components, doublet 1 (D1) and doublet 2 (D2), whose hyperfine parameters are reported in Table 1. Component D1 originates from those irons located close to the chloride sites, while component D2 is attributed to iron atoms located close to the chloride vacancy sites. In comparison to the pure sample, it is observed that the line width, particularly that of the D2 site, increases very slightly for those samples prepared with urea and Co (or Mn). Within the error bars the quadrupole splittings for the D2 sites are rather similar, but very slightly increases for the D1 sites, perhaps suggesting that only a very small amount of these cations may have entered into the structure if any. The lowering in the isomer shifts of both D1 and D2 sites for those akaganeites prepared in the simultaneous presence of urea and Co (or Mn) reflects a change in the s electron density at the Fe nucleus due to a change in the number or in the distribution of the valence shell electrons. These later changes can be originated by changes in the hybridization or bond polarity, perhaps due to the fact the Cl- ions are substituted by OH-. In fact the chemical formula of b-FeO(OH)1- xClx is used for akaganeite [3]. This interpretation is in line with the fact that the areas of D2 sites increase due perhaps to an increase in the Cl vacancy sites, which in turn may be due to a slight replacement of Cl- ions by OH- groups. Table I. Hyperfine parameters for those samples prepared in the presence of 0.4 M urea and Co, Mn. Estimated errors are of ± 0.01 mm/s for the centre shift, d, line broadening, G, and quadrupolar spliting, D, and of about ± 2 % for the area. Isomer shifts are quoted relative to that of α-Fe at RT. Sample Doublet δ, mm/s Γ,mm/s Δ,mm/s Area, % Pure D1 0.36 0.30 0.54 65 D2 0.37 0.30 0.97 35 Urea D1 0.36 0.28 0.54 61 D2 0.37 0.31 0.95 39 Urea,10mol%Co D1 0.28 0.31 0.56 59 D2 0.29 0.33 0.99 41 Urea,10mol%Mn D1 0.28 0.32 0.57 59 D2 0.29 0.33 0.99 41