Effects of water content on the corrosion behavior of NiCu low alloy steel embedded in compacted GMZ bentonite

Buffer material and metal disposal containers are the key engineering barriers in the geological disposal of high-level radioactive waste. The durability of disposal containers largely depends on the water content in buffer material. This work focused on investigating the corrosion evolution of NiCu low alloy steel in compacted GMZ bentonite with different water contents for 270 d by using weight loss, electrochemical measurements, and various methods for analyzing corrosion products. As the water content increased from 13% to 20%, the water in the bentonite transformed from an unsaturated to a critical saturated state, and the corrosion rate of NiCu steel clearly increased. In these two systems, the oxygen could migrate to the thin liquid film on the steel surface through the air pores in the bentonite in the gas phase and undergo cathodic reduction. Meanwhile, it oxidized the ferrous hydrolysis products into ferric corrosion products and formed a rust layer, which could block the diffusion of oxygen. At that moment, the cathodic process of NiCu steel corrosion changed to rust reduction. When the water content continually increased to 30% and 40%, the compacted bentonite was in a saturation state, and the corrosion rate of NiCu steel was significantly decreased. This was because most pores among the bentonite particles were occupied by a large amount of free water, which hindered the diffusion of oxygen and inhibited its cathodic reduction. Furthermore, it restrained the oxidation of ferrous corrosion products, which greatly weakened the cathodic depolarization of rust, leading to the cathodic process being dominated by the hydrogen evolution reaction.