Critical Conditions for Initiation of Localized Corrosion of Mild Steels in Contact with Bentonite Used in Geological Di
- PDF / 370,727 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 28 Downloads / 137 Views
CRITICAL CONDITIONS FOR INITIATION OF LOCALIZED CORROSION OF MILD STEELS IN CONTACT WITH BENTONITE USED IN GEOLOGICAL DISPOSAL PACKAGES OF NUCLEAR WASTE Guen Nakayama and Masatsune Akashi Research Institute Ishikawajima-Harima Heavy Industries Co., Ltd. 3-1-15 Toyosu, Koto-ku, Tokyo 135 Japan ABSTRACT In the current design of geological disposal of high-level nuclear waste, the use of bentonite to stand as an artificial barrier-cum-buffer between the host rock and the packages made of mild steel is being investigated. Although mild steels commomly have been considered to be passivity in alkaline environments, under certain circumstances, they become liable to localized corrosion, e.g., pitting corrosion and crevice corrosion. Since bentonite can turn the environment alkaline to apH of approximately 10 when it is mixed with groundwater, critical conditions for the initiation of localized corrosion of mild steel must be known to evaluate the extremely long time integrity of disposal packages serving in such an environment. This paper presents and discusses the observations and results acquired in a series of critical conditions for the initiation of localized corrosion of mild steels in various groundwater-bentonite environments at 20C, with a deaerated aqueous solution of 1 mMol/L [HCO3 -] +10 ppm [CI-], simulating the natural groundwater and varying the bentonite content. INTRODUCTION The geological disposal packages that are to be disposed at a depth of approximately 1000 m containing high level nuclear wastes must be ensured of their structural integrity for a extremely long timespan that may well be a millennium. The greatest detriment to its integrity would be corrosion by groundwater. Therefore, quantitative assessment of critical conditions for initiation of localized corrosion is indispensable. If the environment of a geological disposal site remains acidic to neutral, the problem of corrosion may be solved simply by providing a sufficiently thick protective margin [1], because then mild steels undergo the general corrosion. If bentonite is used as a buffer to stand between the packages and host rock and has become saturated with groundwater, bentonite will turn the environment weakly alkaline [2].
I .
.
w M V)
(Pitting)
0.51
0i
0..5
-
(Gee~a~o~os~n)(Passivity)
0,L
6
0
0 0
0
0
>_)
:-1
.5
tLL
-0.5H®Jzzj .0 -10(Immunity)
.5 -1.5
1 6
, 10 It
7
I2
13,'
pH Figure 1
Empirical E-pH Diagram for Mild Steel in a Deaerated 1 mMol/L [HCO3-] + 10 ppm [CI-] Solution at 20C, with pH Adjusted with Na 2 CO3 for the Case of the B/S Ratio up to 0.1
Mat. Res. Soc. Symp. Proc. Vol. 294. ©1993 Materials Research Society
330
In an alkaline environment, mild steel can be passivated and become liable to develop localized corrosion, e.g., pitting corrosion and crevice corrosion, wherein the corrosion rate can be very high if conditions are met. Evaluation of the critical conditions for, and initiation of localized corrosion is a matter of great importance. Marsh et al, [3] have done research on the cri
Data Loading...
