The concentration effect of HF on pitting corrosion behavior of 904 L SS in a phosphoric environment

Abstract

The electrochemical behavior of 904 L austenitic stainless steel is investigated in an industrial phosphoric acid, with a focus on the synergetic effect of HF and chloride and the concentration effect of HF. For this purpose, electrochemical measurements such as cyclic potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) have been used. Cyclic potentiodynamic polarization demonstrated that chlorides affect mainly the activation domain density and the presence of HF alongside chloride enhances this effect. Also, HF affects especially the passive and trans-passive domains by enhancing the appearance of the hysteresis which is a sign of the appearance of local corrosion. Whereas, the presence of HF alongside Cl− makes the passive film more destabilized, asserting the synergetic effect of these two species on weakening the protective layers. Furthermore, EIS revealed that HF promotes the dissolution of the outer layer, aggressively affecting the inner layer resistance, and resulting in a loss of protective properties. These findings are confirmed by Raman and UV–Vis spectroscopy. Scanning electron microscopy (SEM) analyses assert that in the presence of HF, a new type of large-shape pitting corrosion appeared on the surface.