Adsorption of iodine on metal coupons in humid and dry environments

Abstract

In this study, five different metal coupons were evaluated for gaseous iodine [I 2(g) ] adsorption including two stainless steels (i.e., SS304 and SS316), two Inconel alloys (i.e., 625 and 718) and pure Ni (i.e., Ni-200) within a dynamic flow-through system where temperature, iodine concentration, flow rate, atmosphere, and relative humidity were controlled. Humidity was shown to be critical to iodine adsorption on SS304, SS316, and Ni-200 at ambient temperatures. The results presented herein provide evidence that a moisture-mediated reaction is occurring. However, higher humidity levels decrease the adsorption, suggesting an ideal range of humidity for the highest corrosion potential in these metals. A comparison of the five metal substrates showed the highest I 2(g) adsorption in the following descending order: Ni-200 > SS304 > SS316 > 718 > 625. The Inconel 625 and 718 alloys were fairly inert to iodine adsorption under the conditions tested. Characterization of the coupons by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction of the Ni-200 coupon indicates that NiI 2 formed and flaked off the surface as a black powder. The SS304 and SS316 coupons showed evidence of extensive reactions with I 2(g) and formed a much more deliquescent corrosion product , which reacted with air when removed from the flow-through system for weighing on the analytical balance. A static system was also used to expose separate polished coupons to I 2(g) , which allowed for analysis of the corrosion layers via scanning electron microscopy. These findings assisted in predicting iodine adsorption behaviors on a variety of metal surfaces under near-ambient conditions.