Corrosion of carbon steel and Monel-400 in EDTA based steam generator cleaning formulations

Corrosion products and the scale forming impurities foul the steam generato rs of Pressurized Water Reactors (PWR) and Pressurized Heavy Water Reactors (PHWR). The impurities from the makeup water, the condenser leaks and the corrosion product oxides leached from the feed train structural materials a re carried along with the feed water and concentrate in the steam generator s. These impurities deposit/precipitate over the steam generator surfaces a nd cause several problems such as reduction in heat transfer efficiency, un der deposit attack, tube failures and production loss. In order to solve th is problem, the power utilities are resorting increasingly to chemical clea ning of the steam generators. The chemical formulation selected for cleanin g should be able to dissolve the accumulated corrosion products/sludge and at the same time should be compatible with the steam generator structural m aterials. Of the various chemical-cleaning formulations, the EDTA based for mulations are more popular and have been used in the chemical cleaning of m any nuclear steam generators. Its corrosion compatibility with the structur al-materials of the steam generators needs a thorough evaluation. In this w ork, the effect of various additives added to the EDTA such as pH additive, reducing agent, oxidizing agent and corrosion inhibitors have been investi gated. Increase in the concentration of hydrazine has been found to increas e the corrosion rate of carbon steel. The variation in corrosion release fr om carbon steel with time was found to fit a second order equation. The var iation in corrosion rate with time was attributed entirely to the increase in surface area caused by roughening of the surface. Presence of dissolved oxygen in EDTA based iron formulation enhanced the corrosion rate of carbon steel and pitted the Monel-400. The study indicated the need for strict ox ygen control and the necessity of using a suitable corrosion inhibitor duri ng the iron removal step. The role of copper in the corrosion of carbon ste el during chemical cleaning has also been studied. Problems encountered in the use of oxygen as an oxidant for copper removal in steam generators havi ng Monel-400 as tube material have been investigated. The results of experi ments to investigate the corrosion behavior of Monel-400 tubes, that has se en ten years of operation and has already undergone intergranular attack, i n EDTA based formulation has also been discussed.