The Effects of Solution pH, Temperature and Redox Environment on Corrosion and Oxide Formation on Inconel X-750
Abstract
Nickel-based superalloys have a wide range of applications in different industries due to their exceptional mechanical and corrosion-resistance properties. These alloys are widely employed in the nuclear industry. Inside a nuclear reactor, these alloys are exposed to a continuous flux of gamma-radiation. When exposed to gamma-radiation, water decomposes, resulting in the production of oxidizing and reducing species. These species have a considerable effect on the redox chemistry of the solution, which controls the overall corrosion behaviour of nickel-based superalloys.
Corrosion of metals and alloys involves several elementary steps. A comprehensive understanding of these steps and how they influence each other is key to predicting the overall corrosion behaviour of an alloy. This thesis investigates the effect of solution conditions on the corrosion of Inconel X-750, which is a nickel-based superalloy and consists of three main elements (i.e., Cr, Fe and Ni). The parameters studied in this thesis are solution pH, temperature, dissolved oxygen concentration and the presence and absence of gamma-radiation. Electrochemistry experiments were coupled with coupon-exposure tests in order to better understand the effects of these parameters on the corrosion kinetics of Inconel X-750. Solution analysis was carried out to measure the amounts of dissolved ions, while the surfaces of corroded coupons were analyzed to evaluate the morphology of the oxide films formed.
The results showed that the corrosion evolution of Inconel X-750 starts with a 1st (pseudo-) steady-state but does not remain in this steady-state. Rather, the corroding system evolves toward a 2nd (pseudo-) steady-state, which is close to the redox potential of a metal oxide/hydroxide solid species or between the redox potentials of two sets of metal oxide/hydroxide solid species. The evolution of the corroding system from the 1st to 2nd (pseudo-) steady-state was attributed to the precipitation and formation of corrosion products in the form of a hydrogel network which facilitated the formation of metal cations of higher oxidation states. Once a hydrogel network had developed, the metal cations of higher oxidation states acted as the main oxidant and controlled the interfacial reactions. In later stages, the hydrogel network could dehydrate and convert into solid crystal oxides which could further inhibit metal oxidation.