Paper AS-TuP12
Interaction of Cobalt with Weathered Steel Surfaces Studied by XPS

Tuesday, October 16, 2007, 6:00 pm, Room 4C

Researchers at the U.S. Army Edgewood Chemical Biological Center have developed and tested a solution for decontaminating surfaces from biological and chemical warfare agents: DECON GREEN^TM. Prior tests conducted at the Pacific Northwest National Laboratory (PNNL) to examine the efficacy for removal of radionuclides showed that the effectiveness of DECON GREEN^TM in removing Co from aged steel, though better than water, was considerably lower than that observed for other surfaces. Only about 50% of the Co could be removed from the steel surface using DECON GREEN^TM. In the current work, the mechanism of the Co adhesion to the steel was investigated using X-ray Photoelectron Spectroscopy (XPS). Cobalt(II) chloride (CoCl₂) aqueous solution was pipetted onto a weathered steel coupon and allowed to air-dry. The coupon was examined by XPS to determine the chemical state of the resulting cobalt deposit. An untreated, weathered steel coupon was examined as a control, and samples of cobalt metal foil and CoCl₂ powder were examined as reference compounds. The steel coupon treated with the CoCl₂ was rinsed with water after the initial XPS analysis, then re-examined to identify any changes in the cobalt chemical state. The coupon was then treated with DECON GREEN^TM and rinsed with water, and re-examined. The key observations are: 1) The cobalt on the weathered steel coupon was mainly metallic, based on the shape and position of the Co 2p_3/2 XPS peak, indicating that cobalt(II) in the applied solution was reduced to metal at the surface. The appearance of shoulders on the high binding energy side of the main peak suggested the presence of a small amount of oxidized cobalt. The iron in the weathered steel coupon was somewhat oxidized before the cobalt deposition; the deposition further oxidized the iron. 2) Rinsing with water did not significantly change the shape of the Co 2p_3/2 XPS peak; the shape and position still indicate mainly Co metal with some oxidized cobalt, probably a cobalt hydroxide or oxide. Rinsing oxidized all the iron metal at the surface, and greatly increased the fraction of oxygen on the surface. 3) Treating with DECON GREEN^TM increases the fraction of oxidized cobalt at the surface, but the majority is still metallic. The iron may undergo some additional oxidation. The DECON GREEN^TM treatment left carbon at the surface, probably from organic constituents.