A New Method for Describing the Atomic-scale Structure of Rusts Formed on the Iron Based Alloy Surlaces.

Abstract

Quantitative structural analysis by conventional X-ray diffraction and anomalous X-ray scattering coupled with the reverse Monte-Carlo simulation technique has been carried out, in order to characterize the atomicscale structure of rust formed on the surface of Fe, Fe-2 %Cr, Fe-3 %Ni and Fe-1.6 %Cu alloys and a weathering steel. Rust samples were prepared by filtering corrosion products formed on the surface of these alloys in salt water. X-ray diffraction patterns showed that main components of the rust consist of ferric oxyhydroxides such as g-FeOOH. Radial distribution functions obtained from both conventional X-ray diffraction and anomalous X-ray scattering clearly indicated that the fundamental local unit structure in the rust is the octahedral FeO 6 , although it is likely to be distorted. Realistic atomic arrangements in the rust were estimated by fitting of the ordinary and environmental interference functions calculated using the reverse Monte-Carlo simulation technique. These results provided that the network structure consists of the FeO 6 octahedral units and its distortion depend upon the composition of samples. These facts suggest that alloying elements in the rust, which themselves are corroded, play an important role in forming the atomic-scale structure of the rust.KEY WORDS: X-ray diffraction; reverse Monte-Carlo simulation; iron based alloys; weathering steel; rust; corrosion; atomic structure.alloys, containing chromium, nickel or copper, and weathering steel in salt water, because a few percents of these alloying elements are known to be added to iron based alloys in order to improve corrosion protection of steel. It has also been the focusing point of this work to reveal the network structure consisting of FeO 6 octahedral units and its distortion from the ideal case found in a typical ferric oxyhydroxide of g-FeOOH. This includes how the alloying elements affect on the structure of rust formed on the surface of various iron based alloys and steel. Experimental SamplesSamples of iron, iron-2.0 mass% chromium, iron-3.0 mass% nickel and iron-1.6 mass% copper alloys were prepared from pure iron, chromium and nickel by vacuum induction melting. 12) These samples were hereafter referred to as Fe, Fe-2 %Cr, Fe-3 %Ni and Fe-1.6 %Cu, respectively. A commercial weathering steel referred to as WS, of which the chemical composition is 0.10% C, 0.050% Si, 0.50% Mn, 0.096 % P, 0.016 % S, 0.15 % Ni and 0.56 % Cr in mass%, was also utilized. The iron based alloys and steel were shaped to10 mm square sheets of 1 mm in thickness. They were leached in artificial seawater with 2.5 mass% NaCl, 1.1 mass% MgCl 2 , 0.4 mass% Na 2 SO 4 and 0.07 mass% KCl at room temperature for about 15 days, so as to form colloidal corrosion products, from which powder rust samples were obtained by filtering. The structures of the corrosion products were almost independent of leaching time. Measurements And analysisConventional X-ray diffraction experiments were carried out using Cu Ka radiation, in order to identify constituents...

A Feasible SIMS Study for Characterizing Ferric Oxyhydroxides Formed on the Iron Surface Using Deuterium

“…These ions are formed during sputtering the ferric oxyhydroxides by Cs þ bombardment. Since the structure of the ferric oxyhydroxides can be described by the network structure of FeO 6 octahedral units containing hydrogen, 3,8) the bonding between Fe and O seems to be relatively strong, as observed in fragment ions of Figs. 1, 2 and 3.…”

<I>In-situ</I> X-ray Diffraction of Corrosion Products Formed on Iron Surfaces

“…Most of diffraction peaks are assigned to -FeOOH, which frequently appears as corrosion products. 19) Although the population of the peak heights is deviated from that given by JCPDS database, the deviation is considered to arise from a change of the measured position in the sample by tilting the sample stage and so on. This is because corrosion products were formed and floated in the aqueous solution.…”

Atomic-scale Structure of Ferric Oxyhydroxide Formed from Fe&ndash;Si Alloy in Aqueous Solutions Containing Different Salts

“…The realistic atomic-scale structures of g-FeOOH particles were determined by fitting the experimental interference functions with those calculated using statistical optimization; in this optimization, the atomic positions were changed from the initial configurations in an ideal structure by the RMC simulation technique. [13][14][15][24][25][26][27] In this simulation, the supercells of gFeOOH were determined with dimensions of 3.756 nmϫ 3.870 nmϫ3.684 nm in order to ensure that the crystal was composed of a sufficient number of atoms for computational accuracy. The simulation was accomplished with initial configurations of 4 320 atoms (1 440 for iron and 2 880 for oxygen) positioned in the supercell of g-FeOOH.…”

“…[13][14][15]21,22) However, the anions in corrosion products such as carbonates, silicates, and sulfates are composed of relatively light elements that cannot be easily identified by X-ray scattering. Thus, the application of the AXS measurement with synchrotron radiations is limited to some extent.…”

“…It is possible that the atomicscale structures of ferric oxyhydroxides are correlated with their morphology and stability. 13,14) It is reported that some elements strongly affect the formation and atomic-scale structures of ferric oxyhydroxides. [15][16][17][18][19][20][21][22][23] Therefore, the influence of cations and anions on the atomic-scale structures of ferric oxyhydroxides formed by iron-based alloys and steel under various environments are strongly required in order to understand the relation between the atomic-scale structures and corrosion process.…”