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

Corrosion products and the scale forming impurities foul the steam generators 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 are carried along with the feed water and concentrate in the steam generators. These impurities deposit/precipitate over the steam generator surfaces and cause several problems such as reduction in heat transfer efficiency, under deposit attack, tube failures and production loss. In order to solve this problem, the power utilities are resorting increasingly to chemical cleaning of the steam generators. The chemical formulation selected for cleaning should be able to dissolve the accumulated corrosion products/sludge and at the same time should be compatible with the steam generator structural materials. Of the various chemical-cleaning formulations, the EDTA based formulations are more popular and have been used in the chemical cleaning of many nuclear steam generators. Its corrosion compatibility with the structural-materials of the steam generators needs a thorough evaluation. In this work, the effect of various additives added to the EDTA such as pH additive, reducing agent, oxidizing agent and corrosion inhibitors have been investigated. Increase in the concentration of hydrazine has been found to increase the corrosion rate of carbon steel. The variation in corrosion release from carbon steel with time was found to fit a second order equation. The variation 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 oxygen control and the necessity of using a suitable corrosion inhibitor during the iron removal step. The role of copper in the corrosion of carbon steel during chemical cleaning has also been studied. Problems encountered in the use of oxygen as an oxidant for copper removal in steam generators having Monel-400 as tube material have been investigated. The results of experiments to investigate the corrosion behavior of Monel-400 tubes, that has seen ten years of operation and has already undergone intergranular attack, in EDTA based formulation has also been discussed.