Corrosion of steel in EDTA

Abstract

Alkaline ethylenediamine tetra-acetic acid (EDTA) is a useful chemical cleaning agent used in many heat exchange systems for removing magnetite. For steel systems, especially at temperatures in the range 70–150°C, high corrosion rates can occur. It has been assumed, to date, that this is due to chelation effects but this work has shown that it is due to the cathodic reduction of EDTA. The EDTA acts as a cathodic stimulant and high rates of corrosion can occur even in the absence of dissolved oxygen. This corrosion behaviour has been demonstrated in a two compartment cell with a steel electrode and sodium hydroxide in one vessel connected to another vessel containing sodium hydroxide and EDTA with an inert platinum electrode. After operating the cell for several hours the decomposition products of EDTA were collected and analysed by infrared and NMR spectroscopy. There was evidence of the reduction of carboxylic acid groups to aldehydes and possibly hydroxy compounds. This work will point the way to the type of inhibitor that might be used to reduce corrosion of the steel without reducing the rate of magnetite dissolution.