Comprehensive studies of corrosion processes of austenitic stainless steel and carbon steel in permanganic acid solutions

摘要

The original version of the AP-CITROX technology (AP: alkaline permanganate; CITROX: citric and oxalic acid) applied all over the World and also for chemical decontamination of large stainless steel surfaces at the Paks NPP (Nuclear Power Plant) was not adequately developed considering its surface chemical and corrosion effects. To replace the AP-CITROX procedure, a novel ,,soft" chemical decontamination base-technology has been developed at the Institute of Radiochemistry and Radioecology of the University of Pannonia (Hungary). The present work gives a brief overview on the corrosion and dissolution processes of austenitic stainless steel and carbon steel specimens that occur in the oxidative pretreatment step (mixture of 1,0 g·dm~(-3) HMnO_4 + 0,2 mol·dm~(-3) HNO_3, pH = approx. 0,8-1,0) of the novel base-technology. The oxidative pretreatment has been performed under laboratory conditions in a laboratory decontamination system elaborated earlier. In the course of the chemical procedure the concentrations of the main alloying components - dissolved from the surface oxide layer into the decontamination solutions - were determined by ICP-OES method. Based upon the ICP-OES results the average thickness values of the oxide layer removed from the surface into the solutions and the average corrosion rates were calculated. The morphology and chemical composition of the oxide films formed on the surfaces of steel specimens were studied by scanning electron microscopy (SEM), equipped with an energy dispersive X-ray microanalyzer (EDX). In addition, some voltammetric studies (open circuit potential and potentiostatic polarisation measurements) of the steel surfaces in permanganate solutions were carried out by the means of a PC controlled VoltaLab 40 (RADIOMETER) type electrochemical measuring system. The redox-potential of the decontamination solution was measured on-line by Consort C861 type electrochemistry meter.