INFLUENCE OF pH VALUES AND CORROSION PRODUCTS ON LOW CARBON STEEL CORROSION SUSCEPTIBILITY IN BORATE BUFFER SOLUTION

Low carbon steel has been studied as one candidate material of the outer container used in the geological disposal of the high-level radioactive nuclear waste. The active/passive state of the steel is the most important factor in maintaining the service life of the container. The active state will ensure that the container achieve the designed life under general corrosion, and moreover, the passive state will degrade the container's life under stress corrosion cracking (SCC) caused by pitting corrosion. In the groundwater, a lot of chemical compositions have very complicated effects on the active/passive state of the low carbon steel. Among these factors, obviously, the pH values of the ground water and the corrosion products of the low carbon steel would be the most important factors. In this paper, the influences of pH values and corrosion products on the susceptibility of the low carbon steel in the deaerated borate buffer solutions were studied. The results showed that the corrosion product did not change the corrosion potential of the low carbon steel at a pH value of 8, which means that the steel was in the active state, while the corrosion product made the corrosion potentials of low carbon steel move to the passive state at pH values of 9 and 10. XPS and XRD analyses indicated that the corrosion products were composed of Fe6(OH)12B4O7 and γ-FeOOH. The low carbon steel with corrosion products was passivated more easily than the naked steel substrate.