Comparison of liquid and gel electrolytes for the investigation of pitting corrosion on stainless steels

Electrochemical methods have become established for the investigation and testing of stainless steels for their resistance to pitting corrosion. In contrast to liquid electrolytes, which are normally used for this purpose, gel-like electrolytes have clear advantages regarding their usability. However, it must be clarified whether both types of electrolyte lead to the same or different results. In the investigations presented here, various agar-based gel electrolytes were characterized regarding their corrosion-relevant properties, such as chemical composition, rheo-logical properties and the electrolyte film forming on the surface. To compare the two electrolyte types, electrochemical investigations were carried out with graduated agar and NaCl concentrations on the material 1.4301 (AISI 304). Pitting and repassivation potentials were determined by dynamic polarization experiments and the varied factors were statistically evaluated regarding their influence. The results show that liquid and gel electrolytes provide comparable results. However, the pitting corrosion phenomena under external polarization are shallower with gel electrolytes and therefore their repassivation is supported. Also, the use of gel electrolytes with indicators for iron ions, which at the same time act as oxidizing agents and allow polarization into critical areas, gives comparable results. Gel electrolytes are therefore a very good alternative to liquid electrolytes if the detection of corrosion-sensitive surfaces is in focus.