Passivation Behavior of Mild Steel Used for Nuclear:Waste Disposal Package
- PDF / 529,154 Bytes
- 8 Pages / 392.58 x 623.88 pts Page_size
- 6 Downloads / 191 Views
atmospheric oxygen. The anodic current density of mild steel in the solution of pH = 8 at 901C was also measured and was found to decrease rapidly with time, leading to the absence of a critical current density for passivation. Thus, mild steel can be self-passivated in the system. An empirically determined corrosion phase diagram is provided for mild steel in 0.1 mol/L [HCO 3 2 + C0 3 ] solution at 90°C, based on these findings and some published data. INTRODUCTION Mild steel has been investigated as one of the overpack materials for geological disposal of high-level nuclear waste. The overpack which stores the canisters containing vitrified high-level nuclear waste for super-long term disposal is expected to be able to hold the canisters securely for at least 1,000 years. Mild steel undergoes general corrosion in neutral natural water environments. However, mild steel is passivated in certain conditions as stainless steel in neutral
to alkaline conditions. Mild steel is susceptible to localized corrosion as pitting corrosion, crevice corrosion and stress-corrosion cracking under certain passivity conditions. Thus,
quantitative evaluation of corrosion behavior of mild steel in terms of the critical conditions for passivation is quite important as prerequisites for the proposed application. There are examples of changes of groundwater composition in various sites being exposed to chemical buffering actions from artificial barriers, leading to the maximum concentration of [HCO 3 ] = 0.13 mol/L (7930 ppm), and that of [SO 42 ] = 0.0022 mol/L (211 ppm) [1]. In Japan, groundwater is generally soft water, and ordinary natural water contains [HCO 3-] = 25 ppm and [SO42] = 10 ppm. Thus, the basic composition of natural water is enriched as the result of the contact with artificial barriers. Furthermore, it is well known that the depassivation pH, pHd, of mild steel depends on the [HCO 3- + CO 32 ] concentration [2,3]. However, very little is reported
so far on detailed mechanisms of corrosion of mild steel in neutral solutions containing high [HC0 3- + C0 3'] concentration. Thus, the present study is aimed in evaluating passivation behavior of mild steel in neutral solutions containing high [HC03- + C0 32] concentration according to the concept of self-passivation of metals.
871 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
Concept of self-passivation The corrosion reaction of iron in aqueous environments ranging from neutral to weak alkaline conditions can be described by the following electrochemical reaction that combines the anodic reaction of equation (1) and the cathodic reactions of equation (2) or (3). Fe2÷+ 2e Fe 30 4 + 8H+ + 8e) 40H H2 + 20H-
Fe (3Fe + 4H 20 02+
=
2H 20 +4e
2H 20 + 2e
= =
(1)
(2) (3)
High values of dissolved oxygen concentration are expected to be present from the air brought at the initial stage of the disposal operation. The corrosion of mild steel under such conditions can be described according to equations (1) and (2). Fig.1 and Fig.2 show schematically
Data Loading...
