Corrosion Rates of Stainless Steels in Renewable Biofuel Sources of Refined Rapeseed Oil, Waste Cooking Oil and Animal Waste Lard

The 1.4541 and the 1.4571 stainless steels and a carbon steel were subjected to immersion corrosion tests in stagnant and stirred biomass fuel sources such as rapeseed oil, waste cooking oil and animal waste lard as well as their emulsions with 5 and 50 wt.% aqueous citric acid solutions at a temperature of 80°C so as to model storage, handling and purification conditions. Passivation of carbon steel was facilitated by flow of the less acidic rapeseed and waste cooking oils and increased flux of oxygen. Carbon steel was sensitive for the higher concentration of proton donor species, acidity of the waste lard. Higher mass loss rates correlated with increased corrosion currents measured in citric acid solution by electrochemical methods. Flow of the biomasses and increased acid concentration of the emulsions were beneficial for the passivation of stainless steels. Although corrosion related mass loss, dissolution rate of the passive layers increased by flow and high acidity of the fluids, both the formation and compactness of passive layers are facilitated by the biomasses with higher concentration of oxygen donating species like water, alcohol and acids. Surface transformation of the passivating steels was reflected by decreasing electrochemical pseudo-capacity of the interfaces and increasing resistance of the passive layers derived from the results of Tafel and Stern methods as well as impedance results. Anti-correlation between mass loss results obtained by immersion in the biomasses and electrochemical data measured in dilute aqueous citric acid solution is explained by the varied compactness, resistance of the passive layers and exchange currents of the steel electrodes due to the orders of magnitude different activities of the hydrogen ion in the biomass mixtures and citric acid solution.