3.4. Stainless steel electrodes
Stainless steel is an iron–chromium alloy containing at least 11% chromium. The grade designated 316 belongs to the austenitic family of stainless steels, and contains chromium (17%), nickel (10%), and molybdenum (2%) as major alloying elements [15]. In the food industry, stainless steels are in widespread use as food contact surfaces. The stainless steel electrodes exhibited pronounced corrosion rates (Table 2), hydrogen generation (Fig. 7), and also pH changes of the heating media (Table 3) at all the pH values. In addition to the chemical reactivity of the alloying elements, the lack of double layer capacitance, as implied by the SEM analysis, would be responsible for the pronounced electrochemical behavior. The observed adherent surface films formed on the stainless steel electrodes during ohmic heating were transparent with a light golden color and some brown rust. The films, however, did not cover the electrode surfaces uniformly, and showed several cracks. Since there were pH changes in the heating media, the hydrogen generation shown in Fig. 7 does not solely represent the electrolysis reaction (Eq. (4)). Based on the observations, assignment of predominant electrochemical reactions is attempted as follows.