Guen Nakayama

A Process Model for the Initiation of Stress-Corrosion Crack Growth in BWR Plant Materials

The process of initiating the stress-corrosion crack growth comprises following six elementary processes; namely, (I) the incubation period, (II) the process of nucleating the corrosion pits (or corrosion crevices), (III)the process of the growth of pits (or crevices), (IV) the process of initiating miorocracks, (V) the process of the propagation of microcracks, (VI) the process of coalescing the microcracks, leading into the process of steady propagation of the main crack. The processes I, III, and V fall into the category of the deterministic process, while the processes II, IV, and VI the stochastic process. This paper deals with the last three stochastic elementary processes by analyzing the stress-corrosion cracking behavior for each stage, and examines the models developed thereby to predict the initiation life of each. Following observations and conclu-sions have been made: (1) the critical pit depth to initiate the microcrack is approximately 20 μm for the carbon steel, and the micro-cracks are non-propagative semicircular cracks of approximately 50 μm in depth; (2) the processes II of the pit nucleation, IV of the microcrack initiation, and VI of the microcrack coalescence can all be represented to the Poissonian stochastic process; (3) inasmuch as the total crack-initiation life is determined by a stochastic process comprising these Poissonian processes concatenated in series, the probability distribution of the life leading to the onset of steady propagation of a main crack can be represented by the exponential distribution; (4) the applied stress exerts its influence mainly on the rate of proliferation of microcracks, but hardly on the propagation of microcrack, critical pit depth, and the probability of the pits gener-ating microcracks; and (5) inasmuch as the proliferation rate of microcrack is represented by a linear function in applied stress, the distribution lower limit of the main-crack initiation life is in inverse proportion to the applied stress.

Effects of Chloride, Bromide, and Thiosulfate Ions on the Critical Conditions for Crevice Corrosion of Several Stainless Alloys as a Material for Geological Disposal Packages for Nuclear Waste

In addition to mild steel, several stainless alloys are being proposed as materials for packages for geological disposal of high-level nuclear waste. When buried deep underground, the greatest detriment to the integrity of packages made of these alloys is localized corrosion, for which critical conditions for initiation of crevice corrosion in chloride environments, with or without other ions, need be precisely known. Crevice corrosion behavior of Type 304 stainless steel, Type 316 stainless steel, Alloy 825, Ti-Gr.1, and Ti-Gr.12 in solutions containing ions of chloride, bromide (these two for their ordinary presence in natural waters), or thiosulphate (this for the likelihood of microbially influenced corrosion) to varying concentrations have been empirically examined. All of these alloys exhibit much the same concentration dependency of crevice corrosion sensitivity for chloride and bromide ions, while Type 304 stainless steel is particularly sensitive to the thiosulphate ion. The region of insensitivity for chloride ion is wider in the increasing order of Type 304 stainless steel, Type 316 stainless steel, Ti-Gr. 1, and Ti-Gr. 12, with that of Alloy 825 lying somewhere in between.

The Effects of Carbonate-Bicarbonate Concentration on Empirical Corrosion Diagram of Mild Steel as a Material of Geological Disposal Package for High Level Nuclear Wastes

In the scheme for geological disposal of high level radioactive nuclear wastes, the burial pit is to be isolated from the sphere of human life by a multiple-barrier system, which consists of an artificial barrier, composed of a canister, an overpack and a bentonite cushioning layer, and a natural barrier, which is essentially the bedrock. As the greatest as well as essentially the sole detriment to its integrity would be corrosion by groundwater. The groundwater comes to it seeping through the bentonite zone, thereby attaining conceivably the pH of transition from general corrosion to passivity, pH d , the behaviors of mild steel in such a groundwater environment have been examined. It has been shown that the pH d is lowered (enlargement of the passivity domain) with rising temperature and carbonate-bicarbonate concentration, while it is raised (enlargement of the general corrosion region) with increasing concentrations of chloride and sulfate ions.

The Critical Condition for the Initiation of Localized Corrosion of Mild Steels in Contact with Bentonite Used For Nuclear Waste Package

It has been established that the mild steels which undergo the general corrosion in the acidic to neutral environments, attain the passivity status in alkaline environments, thereby becoming liable to the localized corrosion, such as pitting corrosion and crevice corrosion. Now, for the case of using bentonite as a buffer to stand between the hostrock and the geological disposal packages of high level nuclear waste, localized corrosion behaviors of mild steel as a candidate for such a package has been studied quantitatively for environments where the the otherwise neutral ground water would be turned slightly alkaline with pH = 9.5 - 10.0. In view of the lack of quantitative data on the passivity-to-localized corrosion of mild steel in natural water environments of weak alkalinity, the present authors have previously determined an empirical E- pH diagram for mild steel with a 20 °C, 1 m mol/L [HC0 3 -], 10 ppm [CI -] solution simulating the natural water environment concerned; it has been shown that the general corrosion-to-passivity transition condition was determined to be pH d =9.4, and the mild steel was shown to be liable to localized corrosion over a large portion of the passivity domain. The present paper discusses behaviors, mechanisms, and critical conditions for initiation of localized corrosion in mild steel placed in bentonitesuspending natural water environment, in terms of the critical potentials for pitting ( V c ), and crevice corrosion ( E R,CREV ). Bentonite was addid to the solution in varying amounts to give bentonite-to-solution ratios up to 0.1, while the pH value was adjusted appropriately with sodium carbonate, always keeping the bentonite particles in suspension. It is demonstrated that bentonite particles suspended in water will deposit upon the steel on receipt of Fe 2 + ions, thereby promoting pitting corrosion by preventing repassivation and promoting crevice corrosion by acting as an effective crevice, once the environment conditions become favorable for localized corrosion. We conclude therefore that disposal package made of mild steel and placed in an underground water environment with bentonite as buffer will be liable to localized corrosion.