The Role of Hydrazine in EDTA Containing Chemical Cleaning Solvents

Abstract The chemistry of hydrazine in chemical cleaning solvents showed some surprising features which needed clarification for the purpose of understanding the chemical cleaning process in these solvents, specifically the magnetite dissolution and the corrosion phenomena associated with it. It was found that hydrazine suppressed ferric ion corrosion at pH 7 but only slightly at pH 4.2. Furthermore, the addition of hydrazine to EDTA at pH 7 accelerated uninhibited corrosion but remained without effect at pH 4. At temperatures above 200°F the presence of magnetite appears to reduce the corrosion in the presence of hydrazine in EDTA, thus supporting the existence of ferrous-, or ferric-hydrazine complexes suggested earlier. At pH 7 it was shown by means of rotating disc electrode measurements that hydrazine reacts very rapidly with ferric ions. Using a novel analysis technique for hydrazine it was shown that this interaction results in a ferric-hydrazine complex which is approximately 1018 times stronger than the ferrous-hydrazine and/or EDTA complexes at 200°F. In fact, the ferric-hydrazine complex is strong enough to prevent ferric ion corrosion completely. This complexing action between hydrazine and ferric ion is suggested to persist at pH 4.2, but without preventing ferric ion corrosion. Hydrazine aids in the dissolution of magnetite at pH 7. In the absence of hydrazine it was found that EDTA is being oxidized by dissolving magnetite. The reaction appears to be faster than expected from literature results. The implications of this chemistry are of importance with respect to the ferric ion corrosion problem and choice of pH of solvent, as well as with respect to the mechanism of the magnetite dissolution in these solvents.