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Langmuir 1999, 15, 494-499
Ellipsometric Study of Oxide Removal from Steel Surfaces in Hydrochloric Acid Solutions Robert B. Bjorklund,*,† Jonas Hedlund,‡ and Bill Carlsson§ Department of Physics and Measurement Technology, Linko¨ ping University, S-581 83 Linko¨ ping, Sweden, Chemical Technology, Luleå University of Technology, S-971 87 Luleå, Sweden, and Vadstena Varmfo¨ rzinkning AB, Kvarnbacksv. 8, S-592 41 Vadstena, Sweden Received July 3, 1998. In Final Form: October 30, 1998 Iron oxide films formed on three different steel surfaces by thermal oxidation were removed in hydrochloric acid solutions at 20 °C. The oxide removal process in flowing solutions was followed in situ by ellipsometry. Two different removal mechanisms were observed in 0.5-2 M HCl, one where undermining of the film resulted in large intact pieces of the oxide leaving the substrate surface at the end of the removal period, and one where the film scaled off the surface in small pieces during the entire removal process. Oxide films which exhibited the undermining mechanism were found to contain about 10% hematite (Fe2O3) and 90% magnetite (Fe3O4). The scaling off mechanism was observed for films which were nearly pure magnetite. Optical models were constructed using data from scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements for the thicknesses and compositions of films after different immersion times in HCl solutions. Spectra calculated from the models agreed well with the experimental spectra.
Introduction Surface-coating treatments involving melt dipping and electrochemical deposition require very clean substrate surfaces in order to ensure good adhesion of the protective coating. Removal of oxides and rust from metal surfaces is usually done by reduction at high temperature or by immersion in strong acid solutions,1 with the latter being the more cost-effective method. Sulfuric acid is the acid of choice in the United States, usually with bath temperatures of 60-80 °C, whereas hydrochloric acid is dominant in Europe. Hydrochloric acid baths are effective at ambient temperature and can even be used at temperatures near freezing if slower oxide removal can be tolerated. Although oxide removal by immersing a metal object in an acid bath appears simple, the surface chemistry involved is quite complex. In addition, while the oxidecovered steel substrates reaching the finishing section of the steel manufacturer’s process line are reasonably well characterized, the metal objects arriving at the independent surface finisher are often of unknown composition and depending on prior history can be covered by rust. Different rolling processes result in different types of oxide coatings. The rate of cooling of the sheet has a large influence on the oxide composition.2 Dissolution rates for iron oxides in acid solutions are low,3 and oxide removal often follows an undermining mechanism where interfacial ferrous ions dissolve into solution and the scale falls off the metal surface and accumulates at the bottom of the bath.1 For a commonly encountered two-layer magnetite/ hematite oxide coating on steel, the acid attack is through * Corresponding author. E-mail:
[email protected]. † Linko ¨ ping University. ‡ Luleå University of Technology. § Vadstena Varmfo ¨ rzinkning AB. (1) Hudson, R. M.; Joniec, R. J.; Shatynski, S. R. In Metals Handbook, 9th ed.; Wood, W. G., Ed.; American Society for Metals: Metals Park, OH, 1982; Vol. 5, pp 68-82. (2) Yamaguchi, S.; Yoshida, T.; Saito, T. Tokubetsu Hokokushos Nippon Tekko Kyokai 1994, 34, 670. (3) Blesa, M. A.; Morando, P. J.; Regazzoni, A. E. Chemical Dissolution of Metal Oxides; CRC Press: Boca Raton, FL, 1994.
cracks in the film with dissolution of the lower part of the magnetite layer at the metal/oxide interface consisting of a mixture of magnetite and iron.4 Additives to the acid bath have been observed to enhance scale removal.1,5 Studies of dissolution of iron oxide films have been concentrated on the passive film formed on iron in different environments. Among the more popular systems has been the passivity of iron and its breakdown in borate buffer solution at pH 8.4. Examples of studies employing optical techniques are described in refs 6 and 7. Since the passive layer is rather thin, it is difficult to separate the film properties from the characteristics of the underlying metal surface. The chemical stability of oxide films formed by chemical vapor deposition8 and by RF sputtering9 on various substrates in hydrochloric acid solution has been reported. X-ray absorption near-edge spectroscopy (XANES) has been used to study iron oxide sputtered deposited films with regard to the iron valence state and thinning rate during cathodic reduction in borate buffer and in 0.1 M H2SO4.10 The reduction was observed first to lower the valence state of the films and then to result in a reductive dissolution of the oxide. Dissolution of sputtered deposited iron oxide films in acidic solutions has also been studied by XANES, where the presence of Fe(II) in the oxides was observed to increase the dissolution rate.11 Reductive dissolution of iron oxides is also an area of current interest in geochemistry with regard to dissolution assisted by reducing agents in solution.12 (4) Evan, U. R. The Corrosion and Oxidation of Metals; Edward Arnold (Publishers) Ltd.: London, 1960. (5) Gaur, B.; Singh, T. B.; Singh, D. D. N. Corrosion 1996, 52, 154. (6) Chin, Y.-T.; Cahan, B. D. J. Electrochem. Soc. 1992, 139, 2432. (7) Bu¨chler, M.; Schmuki, P.; Bo¨hni, H. J. Electrochem. Soc. 1997, 144, 2307. (8) Sugimoto, K.; Seto, M.; Tanaka, S.; Hara, N. J. Electrochem. Soc. 1993, 140, 1586. (9) Virtanen, S.; Schmuki, P.; Bo¨hni, H.; Vuoristo, P.; Ma¨ntyla¨, T. J. Electrochem. Soc. 1995, 142, 3067. (10) Schmuki, P.; Virtanen, S.; Davenport, A. J.; Vitus, C. M. J. Electrochem. Soc. 1996, 143, 574. (11) Virtanen, S.; Schmuki, P.; Davenport, A. J.; Vitus, C. M. J. Electrochem. Soc. 1997, 144, 198. (12) Stumm, W. Adv. Chem. Ser. 1995, 244, 1.
10.1021/la9808061 CCC: $18.00 © 1999 American Chemical Society Published on Web 12/30/1998
Oxide Removal from Steel Surfaces
We report here results of a study about the removal of thermally grown iron oxide films from steel surfaces in hydrochloric acid solutions using ellipsometry to monitor the steel/solution interface. Films up to 4 µm thick were employed in the study in order to make possible the use of strong acid solutions for the film removal. This limited the utility of ellipsometry to independently characterize the films in terms of thickness and composition, and its main use was to monitor in situ the kinetics of the film removal. However, with the help of thickness and composition data for the films obtained from ex situ SEM and XRD measurements, it was possible to construct an optical model which described the structure of the films at different stages of the immersion in the HCl solutions. Three different steel surfaces were investigated, and these exhibited different removal mechanisms dependent on the different compositions of their oxide coatings. Experimental Section Three cold-rolled, dual-phase (ferrite/martensite) Docol steels, (A) Docol 600, (B) Docol 1000, and (C) Docol RP220,13 were kindly provided by SSAB Tunnplåt AB in Borla¨nge, Sweden. The 1 mm thick sheets were wet-polished first with 1000 mesh SiC paper and then with a 6 µm diamond-in-oil slurry followed by a final 2.5 µm polishing. After being cut into smaller pieces (about 1 × 2 cm2), the steel surfaces were cleaned in a NH4OH-H2O2-H2O bath at 80 °C for 5 min and then placed in an oven held at 550 °C in an oxygen flow of about 100 mL/min. Oxide films were grown for different time periods, and after oxidation the pieces were first held at 320 °C for 15 min before cooling to room temperature. A Philips XL 30 scanning electron microscope equipped with a LaB6 emission source was used to determine oxide thickness. Steel pieces were embedded in Bakelite and polished with abrasive paper. The samples were coated with a 10 nm gold film by sputter deposition, and cross-sectional micrographs were obtained. XRD data from the films were collected with a Siemens D500 diffractometer operating at a detector 2Θ scan between 15 and 70°. Film compositions were determined by comparison of three peaks each for magnetite and hematite with reference data obtained from measurements on pure powder samples of each oxide (as determined by XRD) which had been ground in a mill. Thin films of magnetite as an optical reference material were prepared on a silicon substrate by RF sputtering with a 50 V bias on the substrate using Fe3O4 targets (99.9%) at the Institute of Material Science at Tampere University of Technology.14 Ellipsometric measurements were performed at 20 °C on a variableangle spectroscopic ellipsometer (VASE) from J. A. Woollam Co. Oxide removal was followed at 800 nm with the steel pieces placed in a cuvette having a volume of about 8 mL in an experimental setup similar to that illustrated in ref 15. The angle of incidence for the light beam was 68°, and the light entered and left the measurement cell perpendicular to the walls. About 3 mL of solution filled the cell during the measurements, and the flow rate of solution upward through the cell was about 30 mL/min. This permitted the investigation of the oxide removal under essentially iron(II) chloride-free conditions in the bulk solution. In addition to the kinetic measurements, ellipsometric spectra were taken in water at 68° in the 300-1000 nm spectral range both before and after immersion of the surfaces in the HCl solutions. The measured quantity in ellipsometry is the complex reflectance ratio, which is defined as r ) Rp/Rs ) tan ψ exp(i∆), where Rp and Rs are the reflection coefficients for light polarized parallel and normal to the plane of incidence and ψ and ∆ are the ellipsometric angles.16 (13) Steel Sheet Handbook; SSAB Tunnplåt AB: Borla¨nge, 1992. More information at www.ssabtunnplat.com. (14) Vuoristo, P. Ph.D. Thesis, Tampere University of Technolgy, Tampere, 1991. (15) Bjorklund, R. B.; Arwin, H. Miner. Eng. 1993, 6, 895. (16) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North-Holland: New York, 1977.
Langmuir, Vol. 15, No. 2, 1999 495 Table 1. Steel Compositions and 1 µm Thick Oxide Removal Times in HCl Solutions composition (wt %)
removal timea (min)
steel
C
Si
Mn
0.5 M
1M
2M
A B C
0.11 0.16 0.04
0.15 0.51 0.01
0.70 1.49 0.21
6 8 20
3 6 10
2 3 6
a Mean values from at least four runs at each concentration rounded off to the nearest whole minute. The average deviation from the mean was about 0.5 min for removal times less than 7 min and about 1 min for removal times greater than 7 min.
The VASE technique has been developed to improve analysis of complex materials relative to what is possible with singlewavelength, single-angle measurements.17 In practice, the measured ψ and ∆ are compared with ψ and ∆ values calulated for a given assumed structural model where the parameters in the calculation are the thickness of each layer (for multilayer structures), the indices of refraction of each layer, and the fraction of constituents for layers containing several materials.18 A regression analysis is used to vary the parameters of the model to obtain a best fit to the data. For complex materials, it is beneficial to obtain many more ψ and ∆ measurements at different angles of incidence and wavelengths than the number of parameters being solved for in the regression analysis. To analyze the results for oxide removal, it was necessary to develop multilayer models based on the thickness and composition data obtained from SEM and XRD measurements. Optical constants for hematite were available in the WVASE software and were similar to what has been reported elsewhere.19 Optical constants for magnetite were obtained from ellipsometric spectra taken without a measurement cell at 60, 65, 70, and 75° in air (no restraints on the angle of incidence because of scattering, as is the case when the light beam passes through the walls of a measurement cell). Best fits of the refractive indices, n, and the extinction coefficients, k, as a function of wavelength for the 300-1000 nm region were done using the WVASE program. Spectra from two films, 160 and 320 nm thick as determined by SEM, were fitted simultaneously. The film thicknesses were initially held constant but were included as variables in the latter stages of the analysis.
Results and Discussion The compositions of the three steel materials used in the study are shown in Table 1 (the three additives with the highest concentrations). Chromium and nickel concentrations were on the order of 0.015-0.03 wt %. Although there was no intention for the study to be a systematic investigation of oxide removal as a function of composition, the three steel types exhibited somewhat different behavior despite the rather small variation in their compositions. A standard oxide thickness of about 1 µm was obtained by oxidizing steels A and B for 10 min at 550 °C and steel C for 6 min. The resulting oxide films were all fairly uniform and contained no large cracks, as shown in Figure 1 for steel type C. XRD measurements showed that the oxides formed on steels A and B were 88% magnetite and 12% hematite whereas steel C’s oxide contained 99% magnetite. No wu¨stite was observed in the oxides, since the 550 °C oxidation temperature was below the stability range for wu¨stite formation.4 Removal of 1 µm Thick Oxide Films. Oxidized steel samples were fastened to the back wall of the measurement cell, and HCl solutions of 0.5, 1, or 2 M concentration were flowed through the cell. The ellipsometric angles ψ and ∆ were recorded as a function of time at 800 nm, where (17) Aspnes, D. E. J. Vac. Sci. Technol. 1981, 18, 289. (18) Bu-Abbud, G. H.; Bashara, N. M.; Woollam, J. A. Thin Solid Films 1986, 38, 27. (19) Ruzakowski Athey, P.; Tabet, M. F.; Urban, F. K. J. Vac. Sci. Technol., A 1997, 15, 998.
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Figure 1. SEM micrographs of films grown at 550 °C for 6 (A) and 10 (B) min on steel C.
magnetite and hematite are nearly transparent, in order to monitor the removal of the oxide from the substrates. The phase difference between the p and s directions, ∆, is usually related to the thickness of a transparent film,16 and ∆ was observed to most closely follow the visual observations of the oxide removal. Final removal of oxide from all the steel substrates was indicated by a ∆ value of about 100°, as confirmed by visual inspection of the substrates in the HCl solutions. As shown in Figure 2 for 1 M solutions, the changes in ∆ as a function of time for steel A indicated a removal time of about 3 min (time difference between when the HCl solution was introduced at t ) 5 min and when ∆ reached 100°). The final liftoff of the film, usually in pieces several square millimeters in area, led to essentially a step change in ∆ from about 40° to 100° beginning at t ) 7 min. Thus steel A exhibited S curve kinetics for oxide removal, similar to what has been observed for scale removal during pickling of lowcarbon steels.20 For steel C, ∆ was observed to increase gradually throughout the removal process as small oxide pieces (
