Electrochemical Investigation of the Effects of Acid Concentration and Dissolved Oxygen on the Corrosion Behavior of Austenitic Low-Nickel Stainless Steels in Citric Acid

Abstract

All materials which are intended to have in contact with food and other commodities produced or processed for human consumption are called food contact materials (FCM’s). Stainless steel (SS) – a widely known metallic FCM is used mainly in processing equipment, containers and household utensils. It is known for having numerous industrial and domestic applications worldwide due to its special characteristics of having notable corrosion resistance. However, this corrosion resistance is not all-encompassing since SS may still undergo degradation when subjected to a specific corrosion-inducing environment. SS may be classified according to its microstructure. If the atoms which make up the SS can be viewed as having a face-centered cubic structure, then the alloy is said to be austenitic. This SS grades include the conventional 300-series and the newly-developed 200-series. The former has superior corrosion resistance while the latter is far cheaper. In this study, the corrosion behavior of AISI 202 SS in two different levels of dissolved oxygen (O2) and three acid concentrations was investigated using electrochemical techniques, namely, open-circuit potential (OCP) measurements and electrochemical impedance spectroscopy (EIS). As the concentration of citric acid is increased, the measured OCP values of the alloy decreased and the polarization resistance (Rp) decreased, indicating decrease in alloy stability and decline in the corrosion resistance, respectively. With regards to effects of dissolved O2, results revealed that increasing the level of dissolved O2 has consequently increased the polarization resistance and shifted the OCP to more positive values. All the generated Nyquist plots exhibited a depressed capacitive loops indicating that corrosion in the designated solution occurred with charge transfer as the rate-determining step.