The impact of ODA microadditions into secondary system on corrosion rate reduction in VVER steam generators

Injection of film-forming corrosion inhibitors is a challenging way of suppressing erosion-corrosion and crud induced corrosion in power stations. Films of surface-active inhibitors, such as octadecylamine (ODA) provide a diffusion barrier to penetration of corrosion-aggressive ions onto the metal surface. Erosion and corrosion tests were conducted in autoclaves and on a pilot steam generator (SG) design to look into the impact of ODA. To accelerate corrosion process tests were conducted in a more aggressive environment as compared to actual operating conditions, including high chloride concentration and stress levels. It is not only important to reduce deposition growth, but also to wash out deposits previously formed on heat exchanger surfaces. This allows to reduce the risk of local corrosion and corrosion cracking development. A number of VVER plants have conducted full-scale testing that confirmed the impact of ODA microadditions on local corrosion mitigation. Some PWR plants are testing injection of surface-active dispersants to loosen SG deposits. Multiple studies proved ODA ability to remove chlorides from smooth surfaces which allowed to reduce the rate of microcrack growth. Trial testing has shown that the rate of corrosion cracking on SG tubes was reduced by 60-70% owing to ODA injections. Such effect was due to significant reduction in chlorides absorption by the metal surface during the year of ODA injection. Tests on a pilot SG design have shown that ODA could be used for partially wash out deposits from a heating surface. This also minimizes local corrosion. The tests showed that ODA microadditions remove chlorides from microcracks and crevices on SG tubing spacer grids. The ability to wash out previously formed deposits allows to reduce risk of local corrosion and cracking. The abilities of microadditions of film-forming corrosion inhibitors identified through the above mentioned testing could be used under the VVER plant life extension program. The experience of the Kola NPP showed long-term reliable SG operation owing to ODA treatment even without routine chemical cleaning. Such effect was due to significant reduction in chlorides absorption by the metal surface during ODA injection. Results of laboratory investigations of ODA effect on local corrosion processes have been validated in the course of commercial tests at Nord Unit 4 (Germany) and Kola Unit 2. (author)