Development of a Dissolved Oxygen Monitoring System for Corrosion Control of Salt Producing Plants

Abstract

A dissolved oxygen monitoring system has been developed for better control of corrosion problems in salt producing plants. The limiting current density for the reduction of oxygen on a rotating Pt electrode is determined for flowing solutions sampled out from plant lines. The current density is then converted into relative oxygen concentration referring to the current densities for both deoxygenated and fully erated sample solutions. The oxygen concentration in the fully aerated sample solution can be estimated satisfactorily using alting-out parameters, which allows convenient determination of absolute concentration of dissolved oxygen in the sample. The activity of Pt electrodes can be maintained in an enhanced condition by the cyclic polarization anodically to 0.7 V (Ag/AgCl). The existence of Cu²⁺ in the synthetic brine above 1 ppm interferes with the oxygen analysis, electrodepositing on Pt during the cathodic polarization. Meanwhile, the anodic current peak for the stripping of the deposited Cu can be used for determination of Cu²⁺ in the solution. During the cathodic reduction of oxygen dissolved in the brine containing Fe²⁺ above 1 ppm, Fe(OH)₂ deposited on the Pt electrode due to the surface alkalization. It causes a serious interference with the oxygen reduction reaction. Ni²⁺ ion up to 5 ppm showed a scare effect on the electroreduction of oxygen on the rotating Pt electrode. These detrimental effects could be eliminated using a cation exchange method.