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

ABSTRACTIn 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, pHd, the behaviors of mild steel in such a groundwater environment have been examined. It has been shown that the pHd 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.

Aluminium corrosion studies. I. Potential-pH-temperature diagrams for aluminium

The potential-pH-temperature relationships for the aluminium-water system have been calculated by the methods of de Bethune, Khodakovskiy, Criss and Cobble, and Helgeson and a critical comparison made. The Criss and Cobble method produced the most consistent results and was used to construct a corrosion diagram for the range 25-300�C. The effect of the hydrolysed ions Al(OH)2+, Al(OH)2+ and Al(OH)30 was also calculated. The work has shown up a temperature effect which should be taken into account when designing and operating aluminium circuits in which different parts are operated at different temperatures.

Electrochemical Behavior of High Purity Aluminum in Chloride Containing Solutions

The electrochemical behavior of high purity aluminum has been studied to clarify the corrosion behavior of the metal when exposed to oxygen free and oxygen saturated saline solutions of varying pH. The mixed or corrosion potential, EM, of the electropolished high purity aluminum decreases as the pH is increased except in the intermediate pH range (4 to 8) where EM increases. A local maximum in the corrosion potential curve as a function of pH is thus observed. The potentiostatic polarization data and controlled potential weight loss data obtained at pH 4.0 suggest that the rate of dissolution of high purity aluminum is independent of electrode potential over a fairly wide range, relatively insensitive to the presence of dissolved oxygen in the electrolyte, and highly sensitive to changes in pH. Based on the potentiostatic data, a corrosion diagram believed to be approximately valid from pH 0 to pH 14 has been constructed. The corrosion diagram is consistent with the observed relation between EM and pH, and also yields estimates of the rate of dissolution of aluminum as a function of pH. The rate of dissolution of high purity aluminum increases slightly between pH 0 and 4, decreases between pH 4 and 8, and increases from pH 8 to 14. The minimum at pH 8 may be the point at which the rate of formation of aluminum hydroxide on the electrode surface equals the rate of formation of the aluminate ion.