Steam generation corrosion mechanisms

Primary water stress corrosion cracking (PWSCC) has been observed in PWR steam generator tubing, control rod drive mechanism penetrations, and bolting materials. Methods to evaluate susceptibility of components, mechanisms, and methods for qualifying alternate materials for resistance to this specific form of IGSCC are required. 

The corrosion failure mechanisms specific to the steam generation industry include corrosion fatigue Gow-cycle and high-cycle) and SCC. Often, this cracking is preceded by pitting. 

The inadvertent introduction of chemical products in the primary coolant and other safety systems has caused a rapid corrosive attack of some materials. In several cases, these products were either sulphur species present in the resins of the demineralizers, released because of a mechanical failure and transported by the system flow, or the reactives used in the demineralizers generation process. Sulphur species have proven to be very aggressive against certain materials like the Inconel-600 of the steam generator tubes or other pressure boundary components. 

Experimental non-destructive techniques based on positron annihilation phenomena and the MOssbauer effect were used to study real reactor pressure vessel and steam generators. Positron annihilation spectroscopy and Mossbauer spectroscopy were applied to investigate chemical composition, irradiation and post-irradiation heat treatment effects on the microstructures of steels, in order to enhance our understanding of the mechanisms of anomalous structural degradation after irradiation. Mossbauer spectroscopy was used also for corrosion and ageing studies. 

A solution of sulfur trioxide [7446-11-9] dissolved in chlorosulfonic acid [7990-94-5] CISO H, has been used as a smoke (U.S. designation FS) but it is not a U.S. standard agent (see Chlorosulfuric acid Sulfuric acid and sulfur trioxide). When FS is atomized in air, the sulfur trioxide evaporates from the small droplets and reacts with atmospheric moisture to form sulfuric acid vapor. This vapor condenses into minute droplets that form a dense white cloud. FS produces its effect almost instantaneously upon mechanical atomization into the atmosphere, except at very low temperatures. At such temperatures, the small amount of moisture normally present in the atmosphere, requires that FS be thermally generated with the addition of steam to be effective. FS can be used as a fill for artillery and mortar shells and bombs and can be effectively dispersed from low performance aircraft spray tanks. FS is both corrosive and toxic in the presence of moisture, which imposes limitations on its storage, handling, and use. 

Facilities for separating steam from brine and for power generation were designed by mechanical, chemical, and electrical engineers. Dissolved salts in the brine cause severe scaling and corrosion in wells and pipelines. Chemists and chemical engineers developed new production techniques to overcome these problems, as well as pollution control technology for the operation. 

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