Deep Intergranular Fluoride Attack by High-Temperature Corrosion on Alloy 625 by LiF in Air at 600 °C

In most chemical and high-temperature processes, metals are exposed to temperature gradients which, in turn, affect the extent of corrosion phenomena. In this study, a long, continuous strip of alloy 625 was exposed to lithium fluoride in a temperature range of 50–600 °C, air environment. The hottest section of this strip was analyzed as a coupon and compared with two other coupons which were exposed isothermally. One of the isothermal exposures was carried out in a tube furnace, and the other one was in a vertical furnace. Oxygen had three different kinds of access to these three coupons, which, in turn, affected the corrosion process. In order to limit the access of oxygen, a long column of lithium fluoride was used in a vertical furnace. The results of the isothermal exposure showed that more access of oxygen in a horizontal tube furnace facilitated the fluoride ingress to a great extent. However, a long sample exposed to a temperature gradient suffered more corrosion attack than the isothermal coupon, under the same LiF load in the vertical furnace. This was associated with the reduction of oxygen at a larger cathode area reaching into colder regions in Inconel 625 strip. Increased oxygen reduction also increases the efficiency of an inner anode at the hottest section, causing the observed rapid intergranular fluoride uptake. The study proposes a mechanism explaining these observations.