“Measurement of Carbon on Cold-Rolled Steel:
A Comparative Study Using Surface Analytical and Coulometric Methodologies” Industrial Engineering Chemical Research 33 (1994): 2618-2630.
deVries James E., Coduti, Phillip L., and Larry P. Haack
Three cold-rolled steels manufactured under different mill-processing conditions were analyzed by X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS) and direct oxidation CO2 coulometry (DOCC). XPS and SIMS depth profiling were also used to characterize the steels after successive DOCC oxidative treatments at 450 and 600˚C in order to assess what type of carbon is consumed and what analysis depth is probed at each stage of the DOCC measurement. XPS experiments involving in situ oxidative treatments revealed that, for each steel, all detectable surface carbon is removed at 450˚C. SIMS depth profile analyses determined that Fe2O3 layers formed during the 450 and 600˚C DOCC combustion processes were approximately 200 and 2000 nm thick, respectively, revealing that near surface inorganic carbon is also measured during the DOCC analysis. Nevertheless, it was concluded that the DOCC analysis at 450˚C measures mostly surface organic carbon, since the near surface carbon contribution is minor. The subsequent 600˚C DOCC measurement is comprised entirely of near surface inorganic carbon and is reflective of the process control conditions used to manufacture the steel.
INTRODUCTION
The ability to manufacture and prepare surfaces free from carbon-containing contaminants is important both technically and economically to a number of industries. Measuring the amount of carbon present on sheet steel surfaces is a widely recognized way of assessing steel surface cleanliness. (Coduti and Smith, 1979; Coduti, 1980; Hospadaruk et al., 1978; Wojtkowiak and Bender, 1979; Fisher et al., 1980; Iezzi and Leidheiser, 1981; Leroy et al., 1984). The majority of surface carbon analysis methods reported in the literature consist of heating metal samples in the presence of O2, thereby converting the surface carbon to CO2, and measuring the amount of CO2 produced. Investigations have included the measurement of the CO2 by manometric (Boggs and Pellissier, 1967), conductometric (Solet, 1950), and thermal conductivity (Lee and Lewis, 1970) methodologies. Wojtkowiak and Bender (1980) reported using a stream of nitrogen and a thermal evolution analyzer to sweep volatile organic species into a flame ionization detector. Direct oxidation CO2 coulometry (DOCC) was employed in this study. In this technique carbon on steel is combusted sequentially in two separate heating zones with O2 to form CO2, which is measured by coulometry. The first heating zone, set at 450˚C, is designed to react the surface organic carbon, while the second heating zone, set at 600˚C, combusts any remaining inorganic carbon.
The DOCC technique has been used in a number of diverse applications to obtain measurements of surface carbon on a variety of steel surfaces (Coduti, 1981, 1982). Analyses have also been expanded to the investigations of suface carbon on electrogalvanized steel, hot-dipped galvanized steel, galvannealed steel, stainless steel, aluminum, catalysts, silicon wafers, ceramics, and glasses(King, 1978). This method has been also incorporated as an in-plant statistical process control tool for monitoring surface cleanliness (Pumnea and Stadnick, 1988).
X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) techniques were used to explicitly define and quantify the surface carbon and oxide present on cold-rolled steel (CRS) before and after the various stages of the DOCC experiment. XPS analyses of the steels heated in situ were also under-taken to mimic the coulometric experiments. SIMS depth profiles were obtained on samples before and after each DOCC oxidation step to determine how the surface and near surface of the steels were modified by each treatment.
EXPERIMENTAL SECTION
Materials. Three low-carbon (0.05 – 0.07 wt %) CRS samples, supplied by Inland Steel Company, East Chicago, IN, were selected from three steel coils having different mill-processing histories (i.e., tandem reduction, batch annealing, and temper rolling). Reference materials of iron carbide (Fe3C, 99% pure) and gray cast iron (pig iron containing 4.0% carbon) were obtained from Alpha Products, Danvers, MA, and Inland Steel, respectively. These materials were used for XPS and SIMS iron carbide characterization and compared with data acquired on the three steel samples.
Prior to analysis, steel test panels measuring 10 x 30 cm were laboratory spray power washed (Coduti and Earl, 1980). This procedure simulates commercial alkaline-cleaning processes and removes any loosely bound oils and surface soils, with minimal disturbance to the tenaciously bound surface carbon. Test strips measuring 1 x 10 cm, with a 0.48 cm diameter hole at one end, were punched out of the spray power washed panels using a clean punch press. The test strips were then immediately analyzed by the DOCC technique. Companion steel samples for XPS and SIMS characterization were stored in a dessicator until analysis.
