Mössbauer Characterization of Rust Obtained in an Accelerated Corrosion Test

Transcription

1 Hyperfine Interactions 148/149: , Kluwer Academic Publishers. Printed in the Netherlands. 177 Mössbauer Characterization of Rust Obtained in an Accelerated Corrosion Test K. E. GARCÍA 1, A.L.MORALES 1,2, C.E.ARROYAVE 1, C. A. BARRERO 1,2 and D. C. COOK 3 1 Grupo de Corrosión y Protección, Departamento de Ingeniería de Materiales, Universidad de Antioquia, A.A Medellín, Colombia 2 Grupo de Estado Sólido, Instituto de Física, Universidad de Antioquia, A.A Medellín, Colombia 3 Department of Physics, Old Dominion University, Norfolk 23529, VA, USA Abstract. We have performed drying-humectation cyclical processes (CEBELCOR) on eight A36 low carbon steel coupons in NaCl solutions containing Mand M concentrations. The main purpose of these experiments is to contribute to the understanding of the conditions for akaganeite formation. Additionally, and with the idea to perform a complete characterization of the rust, this work also considers the formation of other iron oxide phases. The corrosion products were characterized by Mössbauer spectroscopy and X-ray diffraction techniques. Gravimetric analysis demonstrates that the coupons presented high corrosion rates. Magnetite/maghemite was common in the rust stuck to the steel surface, whereas akaganeite was present only in traces. In the rust collected from the solutions, i.e., the rust that goes away from the metal surface easily, a magnetite/maghemite was not present and akaganeite showed up in larger quantities. These results support the idea that high concentrations of Cl ions are required for the akaganeite formation. We concluded that akaganeite is not easily bonded to the rust layer; this may lead to the formation of a less protective rust layer and to higher corrosion rates. Key words: corrosion rate, X-ray diffraction, Mössbauer spectroscopy, chloride ions. 1. Introduction Simulation of corrosion processes in the laboratory have had an important impact on the understanding of the mechanisms which drive corrosion on steel structures exposed to the atmosphere. The corrosion rates have been found to depend on the concentrations of the pollutants mainly chloride and sulfate ions, where the largest deterioration is found for the largest chloride concentrations [1 3]. Florez- Merino [4] has found that the corrosion rate for low carbon steel increases steadily with the concentration of chlorides. Santana Rodríguez et al. [5] have identified akaganeite as the main rust component present in carbon steels exposed to different marine sites at Canary Islands (Spain), and proposed a limit of chloride concentration over mg/m 2 /day for its formation. Oh et al. [6] have found high corrosion rates for low carbon steel at marine test sites while for weathering steels the corrosion rate is substantially smaller under the same conditions

2 178 K. E. GARCÍA ET AL. but even smaller in rural and industrial sites. The corrosion rates found in wet dry simulation experiments [1, 2] are larger than the ones found at test sites [6]. In spite of the large number of investigations reported in the literature, the actual mechanism by which the corrosion rates are considerable larger in marine atmospheres is not well known and the present study is an effort to shed light into this problem. 2. Experimental method The experimental set up consisted of two vessels containing different pollutant concentrations of Cl ions: vessel A (0.1 M NaCl) and vessel B (0.01 M NaCl). The steels were previously cleaned by sandblasting, acetone and washing with water and soap and then dried in a stove. The final solutions ph were around 6.7 for vessel A and 7.5 for vessel B during the experiment and were changed every 48 hours to avoid contamination with corrosion products. The solutions were aerated continuously during the experiment to simulate the presence of oxygen. The mm 3 samples of A36 low carbon steel of composition 0.073% C, 0.544% Mn, 0.022% Ni, 0.007% Si, 0.011% S, 0.001% Cu, and 0.09% P, rotated continuously over a period of about 41 minutes. The samples were immersed in the solution, for 13 minutes (see [2]), simulating the wet period and the rest of the time were outside at a temperature of 50 C, simulating the drying period. All samples, from each vessel, were removed after 46 days. At 22 days from the beginning of the experiment, time necessary to form a substantial rust layer [1] and where there is a decrease in the corrosion rate, we collected the rust that felt into the solution from the previous 48 hours. We call this rust the less adherent rust (LAR) and classified the rust according to the nomenclature (vessel, days); e.g., A22, B46. The experiment proceeded until 46 days were reached, and we also collected the LAR since the previous two days. Three of the samples were used to calculate the corrosion rate by means of Equation (1), the fourth sample was used to obtain the rust fixed to the steel that we call the adherent rust (AR), A46A and B46A. The corrosion rate, expressed in µm/y, was measured by means of a gravimetric test in accordance with the ISO/DIS 8407, and it is obtained from the expression: V CORR = 10 m/(a δ t), (1) where m is the mass lost in mg, A is the area exposed to corrosion in cm 2, δ is the material density in g/cm 3,andt is the exposure time in days. The results are shown in Table I. Mössbauer spectra (MS) were measured in transmission mode at room temperature (RT), with a Co-57 source in Rhodium matrix. The MS spectra were fitted with quadrupole distributions for the LAR using program DIST3E [7] and for the AR, Lorentzian line profiles were used using program MOSF [7]. X-ray diffractograms

3 MÖSSBAUER CHARACTERIZATION OF RUST 179 Table I. Corrosion rates for low carbon steel at different NaCl concentrations Concentration (M) Corrosion rate (µm/y) Vessel A Vessel B Vessel 6 (Reference [1]) Reference [5] >50 were taken with a Bruker D8 Advance equipped with either a Co or Cu source, in the range θ, step θ, time per step 1 sec. 3. Results and discussion Figure 1 shows the Mössbauer spectra (MS) for the rust produced in all samples. The LAR, that precipitated in the solution after 22 and 46 days experiment, show a doublet, while the AR that remained attached to the steel and later removed by hitting, shows sextets and one doublet. The sextets are assigned to the system magnetite/maghemite, due to the difficulty of distinguishing these components, and to a magnetic goethite component. By using XRD (see Table II), we were able to identify the doublet components as lepidocrocite, superparamagnetic goethite, and akaganeite in all cases. Table III shows the hyperfine parameters from two doublet distributions fitting the MS of the LAR. The fitting parameters are very similar in all cases, for that reason we have tried MS at lower velocity window to see more details of the distribution profiles, A22L and A46L (see Figure 1). We can see a more spread quadrupole distribution in the LAR at 22 days, from vessel with 0.1 M salt solution, which may be due to a larger percentage of akaganeite and to superparamagnetic goethite of smaller particle size as indicated by XRD in Figure 2, i.e., the akaganeite peak is larger in A22L than in A46L and the goethite feature is wider in A22L than in A46L. For the 0.01 M vessels, corresponding to samples B22 and B46, the MS hyperfine parameters are very close for both periods of time but XRD shows a different character of the oxide components, i.e., goethite shows a more peaked structure in B22 indicating particles of larger size but still superparamagnetic. Besides, akaganeite shows more clear peaks in B22 indicating a larger content, and lepidocrocite shows large and broad peaks in both samples indicating a broad distribution of particle sizes. Table IV shows the hyperfine parameters for the AR, the main feature with respect to the LAR is the presence of magnetite/maghemite, and magnetic goethite components. This fact is in agreement with previous works on carbon steels where an internal layer of magnetite/maghemite and an external layer of lepidocrocite and goethite have been found [1, 2]. XRD shows that the oxides found in the AR present narrower and more intense peaks indicating a more crys-

4 180 K. E. GARCÍA ET AL. Figure 1. Room temperature Mössbauer spectra (MS) for all studied samples. MS at lower velocity window for samples A22L and A46L are also shown. talline character. The only difference, with respect to rust obtained under smaller pollutant concentrations, is the presence of akaganeite which starts to show up at a salt concentration of 0.01 M, for lower concentrations it is not present [2]. The magnetic component is 65% for A46A and 75% for B46A suggesting that the adherent rust for low carbon steel is mainly composed of magnetic products. These results agree with those by Oh et al. [6] and Morales et al. [1, 2], while weathering steels give of the order of 27 42% magnetic products as reported by Oh et al. The XRD results in Table II show that akaganeite is mainly present in the less adherent rust while only traces show up in the more adherent rust. This indicated that this oxide does not accommodate itself in the rust layer impeding the formation of a layer with good properties to protect the steel from further corrosion. The

5 MÖSSBAUER CHARACTERIZATION OF RUST 181 Table II. XRD identification of species in rust. L, G, M stand for lepidocrocite, goethite, and magnetite/maghemite, respectively. I means the intensity as read in ICDD cards in the first column, and as obtained in a rust peak analysis in the other columns Oxide-I Co source Cu source Co source B22 B46 A22 A46 A46A B46A 2Theta-I 2Theta-I 2Theta-I 2Theta-I 2Theta-I 2Theta-I L G L M L M L L A M L Table III. Mössbauer parameters for the LAR. δ, max, A, stand for the isomer shift, maximum quadrupole splitting, and relative areas A22L A46L A46 B46 A22 B22 δ δ max, max, A A akaganeite behaviour is further supported by the corrosion rate obtained, i.e., in changing the NaCl concentration from to 0.01 the corrosion rate increases a factor of three and from to 0.1 in a factor of five. 4. Conclusions The less adherent rust, the one that goes away from the metal surface easily, and of course not taken into account in corrosion field experiments is studied here in the CEBELCOR type laboratory corrosion experiment. We find this rust to be composed of lepidocrocite, superparamagnetic goethite and akaganeite. The corrosion rates are high under these pollutant concentrations. We think that the presence of

6 182 K. E. GARCÍA ET AL. Figure 2. XRD patterns for some selected samples. This figure is complemented with the values reported in Table II. Table IV. MS hyperfine parameters for the AR. B is the hyperfine magnetic field B 1 B 2 B 3 δ 1 δ 2 δ A 1 A 2 A 3 1 δ 1 A 4 A46A B46A akaganeite is an important factor in such an increase due to its poor adherence to the rust layer. Acknowledgements Thanks go to COLCIENCIAS (project No ) and CODI (sustainability program for Corrosion and Protection Group 2002) for financial support.

7 MÖSSBAUER CHARACTERIZATION OF RUST 183 References 1. Morales, A. L., Cartagena, D., Rendón, J. L., Valencia, A., Barrero, C. A. and Dauwe, C., Hyp. Interact. (C) 2 (1997), Morales, A. L., Cartagena, D., Rendón, J. L. and Valencia, A., Phys. Stat. Sol. (b) 220 (2000), Stratmann, M. and Hoffmann, K., Corros. Sci. 29 (1989), Florez-Merino, S., Ph.D. thesis, U. Complutense, Madrid, Spain, Santana Rodríguez, J. J., Santana Hernández, F. J. and Gonzáles Gonzáles, J. E., Corros. Sci. 44 (2002), Oh, S. J., Cook, D. C. and Townsend, H. E., Corros. Sci. 41 (1999), Vandenbergue, R., de Grave, E. and de Bakker, P. M. A., Hyp. Interact. 83 (1994), 29.