Electrochemical studies of hot corrosion of type 347H stainless steel

Abstract

Electrochemical studies of the hot corrosion of AISI SA 213 TP 347H stainless steel have been carried out in a mixture of 80 wt-% V₂O₅ + 20 wt-% Na₂SO₄. The range of temperatures was 540–680°C at intervals of 20 K and the techniques employed included corrosion potential, Tafel polarisation, and electrochemical noise measurements. At 620°C the corrosion potential, measured against a platinum reference electrode (PRE), decreases from ?350 mV to ?480 mV and remains at this level during the first 8 h. Using Tafel polarisation, it was found that, with change in the temperature from 540 to 680°C, the corrosion potential decreased on initial heating to 600°C and then increased again at higher temperatures, the corrosion rate increasing continuously with increasing temperature. However, at constant temperature (620°C) the corrosion rate increased with time during the first 8 h, after which it decreased and reached a steady state after 27 h, probably owing to the formation of a surface film. Electrochemical noise measurements, of both voltage and current noise, indicated a combination of general corrosion, probably owing to the formation of a surface layer, and localised corrosion in the grain boundaries.