Corrosion behavior of Alloy 22 in heated surface test conditions in simulated Yucca Mountain Nuclear Repository environment

In the nuclear repository conditions, the nuclear waste package wall surfaces will be at elevated temperatures because of the heat generated by fission reactions within the waste. It is anticipated that the ground water may contain varying levels of anions such as chloride, nitrate, sulfate picked up from the rocks. The ground waters could seep through the rock faults and drip on to the waste packages. The dripped water will evaporate due to the heat from the nuclear waste leaving behind concentrated brine which eventually becomes dry salt deposit. The multi-ionic salts in the ground water are expected to be hygroscopic in nature. The next drop of water falling at the same place or the humidity in the repository will transform the hygroscopic salt deposit into a more concentrated brine. This cycle will continue for years and eventually a potentially corrosive brine will be formed on the waste package surface. Hence the waste package surface goes through the alternate wet-dry cycles. These conditions indicate that the concentration and pH of the environment in the repository vary considerably. The conventional corrosion tests hardly simulate these varying environmental conditions. Hence there has been a need to develop an electrochemical test that could closely simulate the anticipated repository conditions stated above. In this research, a new electrochemical method, called as Heated Surface Corrosion testing (HSCT) has been devised and tested. In the conventional testing the electrolyte is heated and in HSCT the working electrode is heated. The present study employs the temperature of 80°C which may be one of the temperatures of the waste package surface.; The new HSCT was validated by testing stainless steel type 304. The HSCT was observed to be more aggressive than the conventional tests. Initiation of pitting of SS 304 in chloride solution (pH 3) occurred at much shorter exposure times in the HSCT condition than the exposure time required for pitting in conventional testing. The reduced time to pitting demonstrated the capability of HSCT to impose repository more corrosive conditions. The stability of the passive film of stainless alloys under the hygroscopic salt layers could be determined using this technique.; Alloy 22, a nickel base Ni-22Cr-13Mo-3W alloy has an excellent corrosion resistance in oxidizing and reducing environments. Corrosion behavior of Alloy 22 was evaluated using the newly devised HSCT method in simulated acidified water (SAW), simulated concentrated water (SCW) and in pure chloride (pH 3 and 8) environments. In this method, the concentration of the environment varied with test duration. Alloy 22 was evaluated in four different heat treated conditions viz. (a) mill annealed, (b) 610°C/1 h-representing Cr depletion, (c) 650°C/100 h-representing Mo+Cr depletion, (d) 800°C/100 h-representing Mo depletion. The corrosion rate of mill annealed Alloy 22 was not affected by the continuous increase in ionic strength of the SAW (pH 3) environment. Passivation kinetics was faster with increase in concentration of the electrolytes. The major difference between the conventional test and HSCT was the aging characteristics of the passive film of Alloy 22. Cyclic polarization was carried out on Alloy 22 in conventional ASTM G61 and HSCT method to compare. The electrochemical response of Alloy 22 was the same by heating the electrolyte or heating the electrode.; The corrosion behavior of Alloy 22 was investigated in three different aged conditions using HSCT approach in two different electrolytes. The thermal aging conditions of the specimens introduced depletion of chromium and molybdenum near the grain boundaries/phase boundaries. Long-term exposure tests (up to 850 h) were conducted in simulated acidified water (SAW, pH 3) and simulated concentrated water (SCW, pH 8) at 80°C. Corrosion potential, corrosion current and passive current decay exponent were determined at regular intervals. The speci...