Electrochemical behavior of 304L stainless steel in high saline and sulphate solutions containing alga Dunaliella Salina and β-carotene

Type 304L stainless steel alloy is a main constructional material in many factories for the production of several salts from the hyper saline water. In this water, such that of Lake Quaroun in Fayioum, Egypt, the sole surviving micro-organism is almost the unicellular alga Dunaliella Salina which secretes relatively massive amount of β-carotene. The present investigation deals with the role of this alga as well as its secreted β-carotene on the corrosion and passivation behavior of the 304L steel alloy in hyper saline water and sulphate solutions. The tested β-carotene was extracted from a culture alga solution. Standard biochemical, chemical and electrochemical methods were applied. The results reveal that this compound does not affect the mechanism of the corrosion process but merely decrease its rate by making a physical barrier, indicating that β-carotene acts as an adsorption inhibitor. Various electrochemical measurements were used including open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Analysis of the EIS results based on the dispersion formula approach, allowed estimation of the electrical parameters of the proposed equivalent circuit used to simulate the alloy/passive film/solution system. From the increase of the surface film resistance, the degree of surface coverage has been calculated as a function of the added β-carotene concentration, hence specifying the appropriate adsorption isotherm. The data were found to obey the Temkin adsorption equation.