Adsorption of iodine on metal coupons in humid and dry environments

Abstract In this study, five different metal coupons were evaluated for gaseous iodine [I2(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 I2(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 NiI2 formed and flaked off the surface as a black powder. The SS304 and SS316 coupons showed evidence of extensive reactions with I2(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 I2(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.