Innovative agar-glycerol electrolytes for corrosion studies on irregular surfaces of stainless steels

摘要

Common cells employed in electrochemical measurements present some limitations when the resistance to localized corrosion of passive systems is evaluated. Interferences of crevices generated when delimiting the testing area are sometimes difficult to avoid. Moreover, the study of non-flat surfaces has specific problems that are often difficult to overcome with traditional cell configurations. In the current work, semi-solid, gel electrolytes were investigated to be employed in electrochemical cells for polarization tests of stainless steels. These gels are based on the use of agar (0.5-1% w/w) and glycerol (0-70% w/w). Agar acts as a gelling agent and contributes to the conductivity of the gel. Glycerol acts as plasticizer to modify the mechanical properties of the gel. NaCl was also added as depassivating agent, using always a concentration of 0.5% (w/w). Different compositions of the gel were considered in order to obtain good adaptability to irregular surfaces and suitable conductivity to carry out the polarization tests. The wettability of the different gel compositions on diverse complex AISI 304 surfaces was analyzed. Different techniques were employed to characterize gel electrolytes: texture analysis to study the mechanical properties, electrochemical impedance spectroscopy and anodic polarization curves to evaluate pitting corrosion behavior of stainless steel. Also, compansons between traditional aqueous, neutral chloride electrolyte and gel electrolyte were made to validate the use of gel electrolytes in the corrosion behavior of stainless steels. Crevice interference was never detected in any of the polarization test performed with the different studied gel electrolytes. Gels with lower stiffness were obtained with reduced agar contents. The use of plasticizers allowed to manufacture semisolid electrolytes with agar contents as low as 0.5%. The addition of glycerol improved the adaptability of gels to complex surfaces, but, on the other hand, it caused a decrease in ionic conductivity. Moreover, gels with high glycerol contents exhibited low ionic conductivity and curing difficulties. Gel electrolytes with 40% of glycerol and 0.5% of agar showed the most satisfactory results, being a suitable alternative to replace aqueous electrolyte in pitting susceptibility studies of stainless steels with irregular surfaces.