Chemistry and Corrosiveness of EDTA Containing Chemical Cleaning Solutions

Abstract A new inhibitor, CCI-80/1, an alkylthiopolyiminoamid, was developed for EDTA-hydrazine containing chemical cleaning solvents used in nuclear steam generator cleaning at 200 and 250°F. The inhibitor was qualified for a variety of different metals, a wide range of solvent composition and various operating parameters. A problem occurred early in the work with ASTM-533-A alloy which appeared to be difficult to inhibit against corrosion. A detailed study of this problem showed that the accelerated corrosion rates could be explained on the basis of grain structure and lattice energy. A mechanism was developed which allowed correlation of essentially all data and explains the inhibitor action in terms of chemistry occurring on the surface of the metal. The chemistry in solution and on the surface of the metal is controlled by a number of pH and temperature dependent chelating phenomena which must be in the correct balance in order to achieve effective inhibition. The ferric ion corrosion was studied in the absence of hydrazine. It was found that solvent oxidation by ferric ion occurs faster than expected from previous work. It was shown that it is, therefore, possible to dissolve magnetite from crevices at 250°F with 20% EDTA in the solvent in the absence of hydrazine. In fact, the magnetite dissolution rate is dependent on the rate of reduction of ferric ions by the solvent. Corrosion becomes minimal at 20% EDTA with CCI-80/1 at 250°F even in the presence of magnetite because ferric ion is most likely reduced as fast as it is generated in solution.